Chiral ligands derived from monoterpenes: application in the synthesis of optically pure secondary alcohols via asymmetric catalysis.

DOI: 10.1002/chem.201404303

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Chiral Ligands Derived from Monoterpenes: Application in the Synthesis of Optically Pure Secondary Alcohols via Asymmetric Catalysis Mohammed Samir Ibn El Alami,[a] Mohamed Amin El Amrani,[a] Francine AgbossouNiedercorn,[b] Isabelle Suisse,*[b] and Andr Mortreux[b]

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Minireview cluding hydrogen transfer, C C bond formation via addition of organozinc compounds to aldehydes, hydrosilylation, and oxazaborolidine reduction, leading to high activities and enantioselectivities.

Abstract: The preparation of optically pure secondary alcohols in the presence of catalysts based on chiral ligands derived from monoterpenes, such as pinenes, limonenes and carenes, is reviewed. A wide variety of these ligands has been synthesized and used in several catalytic reactions, in-

Introduction

In that context, access to optically enriched secondary alcohols through asymmetric reduction of unsaturated compounds, especially C=O bonds, or C C bond formation through addition of organometallic compounds to aldehydes remains a pivotal goal for organic synthesis.[5, 6] A large variety of ligands have been reported for these enantioselective catalytic reactions, and a previous review dealt more generally with the use of monoterpenes derived from 1,2-amino alcohols as catalysts precursors for asymmetric reactions.[7] This Minireview focuses on monoterpenes used in catalytic reactions, either as precursors of chiral ligands or as the source of chiral catalysts for the formation of optically pure secondary alcohols. The first section will focus on the asymmetric transfer hydrogenation of ketones and the second section will describe C C bond formation via addition of dialkylzinc to aldehydes. The third and fourth sections will deal with the asymmetric hydrosilylation and oxazaborolidine catalytic reductions of ketones, respectively. The fifth section will be devoted to organocatalysts derived from monoterpenes and their application in alkynylation and allylation of carbonyl compounds. The final section will critically review the impact of the above transformations in organic synthesis.

One of the basic criteria for sustainable chemistry is to consider natural products as sources of raw materials for chemistry, with regard to their renewability and biodegradability. Many studies are now devoted to their use in destructive processes for the synthesis of platform molecules that are usually derived from petroleum chemistry. Another valuable option is to take advantage of the functionalized structure of natural products to produce new molecules for targeted applications. In this context, the optically pure character of natural products is worthy of consideration, and a large spectrum of studies has been devoted to the use of naturally occurring chiral compounds in asymmetric synthesis, for diastereoisomeric separations, hemisynthesis, or asymmetric catalysis. Among them, natural chiral compounds such as sugars,[1] amino acids,[2] or 1,2- and 1,3-amino alcohols,[3, 4] have been extensively studied. Terpenes, produced primarily by a wide variety of plants, particularly conifers, are some of the most interesting bioorganic molecules. This is particularly the case of a- and b-pinene, 2and 3-carene, and limonene (Figure 1). Given their occurrence in pure enantiomeric forms, this series of compounds is a convenient source of precursors for the synthesis of optically active ligands for use in asymmetric catalysis.

Asymmetric transfer hydrogenation of C=O bonds Asymmetric transfer hydrogenation (ATH) using propan-2-ol as hydrogen source represents an efficient method for the synthesis of chiral secondary alcohols with numerous advantages.[8] Asymmetric reduction by hydrogen transfer is defined as the reduction of a multiple bond with the aid of a hydrogen donor in the presence of a catalyst (Scheme 1).

Figure 1. Structure of some monoterpenes.

[a] Dr. M. S. I. El Alami, Prof. Dr. M. A. El Amrani Laboratoire de Chimie Organique Applique Universit Abdelmalek Essaadi Facult des Sciences, BP 2121-93002 Tetouan (Morocco) Fax: (+ 212) 5-39-99-45-00

Scheme 1. Asymmetric transfer hydrogenation (ATH).

A large variety of ligands has been used in this reaction,[9] but the use of chiral pool ligands has rather seldom been reported. The first example in this field was described by Singaram and co-workers.[10] The [{RuCl2(p-cymene)}2] dimer was used as precatalyst in the presence of chiral b-amino alcohols synthesized directly from carene (ligand 1) and limonene (ligands 2–6) for asymmetric reduction of acetophenone and

[b] Dr. F. Agbossou-Niedercorn, Dr. I. Suisse, Prof. Dr. A. Mortreux UMR CNRS 8181, UCCS (Unit de Catalyse et de Chimie du Solide) Universit Lille Nord de France ENSCL, Av Mendeleı¨ev, Cit Scientifique CS 90108 59652 Villeneuve d’Ascq Cedex (France) Fax: (+ 33) 3-20-43-65-85 E-mail: [email protected]

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Minireview several ketones (Scheme 2). No activity was observed using the amino alcohol 6. This result is in agreement with previous reports,[7] from which it is well established that amino alcohols bearing a tertiary amino group are not suitable for reduction of carbonyl bonds using this catalytic combination. Amino alcohols 1 and 2 gave the same result in terms of enantioselectivity (50 % ee) during reduction of acetophenone, but the amino alcohol 2 was more efficient in terms of activity. The influence of the substituent on the amino group proved to be the most important parameter and was studied for amino alcohols derived from limonene (ligands 2–5). However, no specific trend was deduced with regard to the steric bulk of the substituents. Ligands 2 and 3 provided the Scheme 2. Evaluation of ligands issued from carene and limonene in ATH of acetophenone. S = substrate; L = ligand; B = base.

most encouraging results in terms of activity and enantioselectivity and were then tested for the ATH of a wide range of ketones (Scheme 3). In the case of non-aromatic ketones, the catalyst bearing ligand 2 containing a primary amine moiety was totally inactive. Enantiopure b-amino alcohols 7–12, synthesized from apinene, were also associated to RuII catalysts for the asymmetric transfer hydrogenation of acetophenone[11] (Scheme 4). High conversions (up to 97 %) and moderate enantioselectivities (7–45 % ee) were observed, the best ee corresponding to the N-biphenylmethyl substituted amino alcohol 12. In that

Francine Agbossou-Niedercorn obtained her Ph.D. degree in the field of porphyrins chemistry at the University of Nancy. Then, she took a research fellow position with the CNRS in Villeurbanne and completed her Doctorat d’Etat at the IRCELYON on Cytochrome P450 models. She joined the Laboratoire de Chimie de Coordination of Toulouse in 1985. After 18 months’ postdoctoral stay at the University of Utah with Pr. J. A. Gladysz focusing on chiral rhenium organometallic complexes, she joined the Unit of Catalysis and Solid State Chemistry (UCCS) at Lille in 1991. Since 2001, she has been Director of Research at CNRS. She is leader of the group Catalysis, Chirality, and Fine Chemistry and of the Department of Catalysis and Molecular Chemistry at the UCCS. Her research interests focus on catalysis in general, on the synthesis and application of chiral ligands, on asymmetric catalysis, and on the preparation of optically pure intermediates of interest. Mohammed Samir Ibn El Alami was born in Tetouan, Morocco, in 1978. He graduated from the University of Abdelmalek Essaadi in 2005. From 2005 to 2006 he worked as contract researcher at the University of Cadiz (Spain) in Prof. Valerga’s group. Afterwards, he moved to the University of Lille 1 where he obtained his Ph.D. under the supervision of Prof. A. Mortreux and Prof. A. El Amrani from Abdelmalek Essaadi University. After spending 4 months at TELENE S.A.S industry, he has held a position as Temporary Associate Professor at the University of Lille for two years. His research interests are centered on the asymmetric homogeneous catalysis and the valorization of bio-sourced ligands by homogeneous catalysis.

Andr Mortreux was born in Libercourt, France, 1943 and graduated in Lille University France in 1966. He became assistant professor in 1968 in Poitiers, France, where he discovered the first homogeneous catalyst for alkyne metathesis during his Doctorat d’Etat in 1975 under the supervision of Pr Blanchard. After a postdoctoral position with Professor F. G. A. Stone in Bristol in 1976, he moved back to Lille University where he was promoted a full Professor in 1983. His research has been devoted to the search for new catalysts and new homogeneous catalytic reactions, including unsaturated hydrocarbon oligomerization and polymerization, carbon monoxide chemistry, and asymmetric catalysis, which led to him being elected the Homogeneous Catalysis chair of the Institut Universitaire de France in 2001. He is currently Emeritus Professor at Lille University.

Mohamed Amin El Amrani was born in Larache, Morocco, in 1963. He received his Ph.D. from the University of Lille (France) in 1989 under the supervision of Pr Petit† and Pr Mortreux on nickel catalyzed oligomerization of dienes. He became an Assistant Professor in 1990 and a full Professor of Chemistry at the University of Ttouan (Morocco) in 2000. His research interests involve the field of synthesis of optically pure ligands for asymmetric homogeneous catalysis by transition metal complexes.

Isabelle Suisse was born in Anthony, France. She graduated from the Engineering School of Chemistry of Lille in 1990. She earned her Ph.D. from the University of Lille in 1994 under the supervision of Prof. Mortreux on nickel-catalyzed dimerization and telomerization of butadiene. Since 1994 she has been Assistant Professor of Chemistry at the University of Lille. Her current research focuses on the development of new catalysts for asymmetric reactions and valorization of bio-sourced alcohols by homogeneous catalysis.

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Scheme 5. ATH of acetophenone on ruthenium catalysts using various arene and amino oxime ligands.

Scheme 3. ATH of various ketones using ligands 2 and 3.

the choice of the aromatic ligand of the ruthenium precursor strongly affects the ee; the best enantioselectivity (80 % ee) was obtained with [{RuCl2(benzene)}2] bearing the chiral aamino oxime 14. Based on the results obtained with acetophenone, benzene ruthenium(II) chloride dimer [{RuCl2(benzene)}2] was used as precatalyst in the presence of chiral a-amino oximes 13 and 14 for asymmetric reduction of various ketones (Scheme 6).

Scheme 4. ATH of acetophenone using Ru/amino alcohols catalysts issued from a-pinene.

case, a general trend was observed, by which the bulkiness of the substituent on the amino group enhanced the ee. Similarly, the C2-symmetric diimino and diamino alcohols from a-pinene were also synthesized but the results in the RuII-catalyzed transfer hydrogenation of acetophenone were disappointing (maximum 18 % ee).[11] Recently, our group developed a new class of catalytic systems for asymmetric hydrogen transfer reactions.[12] Several aamino oxime-type ligands derived from both enantiomers of limonene were synthesized and used as chiral ligands in the ruthenium(II)-catalyzed reduction of acetophenone and their substituted aromatic derivatives with iPrOH as hydrogen donor. These ligands (13–16) gave excellent conversions (up to 99 %) and moderate to good enantioselectivities (up to 80 % ee) were recorded (Scheme 5). In the case of acetophenone, different RuII precursors were tested and it appears that &

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Scheme 6. Scope of substrates in ATH catalyzed by [{RuCl2(benzene)}2]/ amino oxime ligands 13 and 14.

Both activities and enantioselectivities were highly dependent on the electronic properties of the substituent on the phenyl ring of the substrates. In the presence of electron-withdrawing groups, such as Cl and NO2, the hydrogen transfer was very fast (up to 99 % conversion within only 1 h), which is in agreement with former results reported by Noyori.[13] In contrast, in the presence of electron-donating substituents, such as methyl and methoxy groups, catalytic activities were similar to those observed with acetophenone (90 % conversions were obtained 4



Minireview mental data. Overall, examination of the above results indicates that there is still room for further research in that area and the development of new ligands based on monoterpenes deserves to be pursued.

after 24 h). The same trends were reported by Gladiali and coworkers.[14] Modest to good enantioselectivities (6–78 % ee) were obtained using a-amino oximes 13 and 14 (Scheme 6). Based on the Noyori group’s work in the asymmetric transfer hydrogenation process promoted by [{RuCl2(arene)}2] as precursor coordinated by N-tosyl-1,2-diphenyl-ethylenediamine (TsDPEN),[15] as well as on other results from our group on amino alcohols,[16] potential catalytic intermediates have been isolated, allowing proposal of a mechanism taking into account X-ray experimental observations. Complexes of the type [RuCl(arene)(N-tosyl-1,2-diamine)], prepared in situ by mixing [{RuCl2(arene)}2] with novel (+)-(R)-limonene-derived tosylamines were applied by Czarnocki and co-workers in the asymmetric transfer hydrogenation of ketones (Scheme 7).[17, 18] The reaction was carried out in formic

Addition of dialkylzinc to aldehydes Asymmetric addition of organometallic reagents to aldehydes has been known as one of the most valuable ways to obtain optically pure secondary alcohols via C C bond formation. Addition of diethylzinc to aromatic aldehydes represents the most studied reaction of this type (Scheme 8).[19]

Scheme 8. Addition of diethylzinc to aldehydes.

In 1983, Oguni and co-workers[20] reported the first asymmetric condensation of diethylzinc with benzaldehyde in the presence of camphor-based cobalt and palladium catalysts leading to low to moderate enantiomeric excesses (2–58 % ee). A significant advance was reported in 1986 by Noyori by using the (2S)-3-exo-(dimethylamino)isoborneol [( )-DAIB] issued from camphor as an efficient chiral catalyst for the same reaction (99 % ee of (S)-1-phenylpropan-1-ol).[21] Following these results, several authors described numerous amino alcohols for this application, such as prolinol,[22] ephedrine and derivatives,[23] and norephedrine.[24] Similarly to asymmetric transfer hydrogenation, chiral compounds derived from monoterpenes have been used for this reaction. The first evaluation of this family of chiral alcohols was described by von Zelewsky.[25] Bipyridyl alcohol derivatives 17 were synthesized from the fused a-pinene pyridine and tested as catalysts in the asymmetric addition of diethylzinc to benzaldehyde. The alcohol product was obtained in quantitative yields and good enantioselectivities (up to 86 % ee; Scheme 9). A new class of non-symmetrical N,O-functionalized catalysts 18 a–d bearing the pinene framework was synthesized in few steps and applied by Chelucci in the same reaction.[26] The cat-

Scheme 7. ATH of acetophenone using Ru/N-tosyl diamines derived from limonene.

acid/triethylamine mixtures at room temperature. In the case of acetophenone, the corresponding alcohol was obtained with excellent conversions (up to 100 %), albeit after a few days, and high enantioselectivities (up to 92 % ee). Interestingly, the results were strongly dependent on the diastereoisomer used. Substituted aryl alkyl ketone substrates were also tested but the enantioselectivities remained lower. Although amino alcohols or diamines and diimines are easily accessible from monoterpenes, their performances when applied in the asymmetric hydrogen transfer reaction remain rather disappointing. Few catalytic systems led to alcohols with high ee values (Scheme 6). A specific look to the last part of this section suggests however that the variation of the configurations of the chiral centers on the ligand may not only lead to a reactivity enhancement, but also strongly affects the enantioselectivity; a typical matched–mismatched diastereomeric behavior is observed. Furthermore, changing the arene group on ruthenium is also particularly impactful, although there is no definite rule that can be deduced from the experiChem. Eur. J. 2014, 20, 1 – 17

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Scheme 9. Evaluation of alcohol-substituted bipyridyl-a-pinene catalysts in diethylzinc addition to benzaldehyde.

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Scheme 10. Evaluation of phenolic pyridyl-a-pinene catalysts in diethylzinc addition to benzaldehyde.

Scheme 12. Evaluation of a morpholino-substituted 3-carene-derived alcohol in the addition of diethylzinc to benzaldehyde and derivatives.

alytic systems exhibited good activities with conversions up to 98 % but only moderate enantioselectivities were observed (Scheme 10). Several b-amino alcohols 19 were synthesized by diastereoselective addition of various secondary amines to a commercially available 1:1 mixture of cis and trans (R)-(+)-limonene oxide under experimental conditions that allowed the kinetic resolution of limonene oxide. These amino alcohols were evaluated as catalysts in the addition of diethylzinc to benzaldehyde (Scheme 11).[27, 28] The best enantioselectivity was obtained

clohexanecarboxaldehyde could also be transformed efficiently (Scheme 12). Chen and co-workers[30] reported the synthesis and the applications of a novel C2-symmetric diol 21, containing a central bipyridine–pinene-derived core and their use as ligand associated to TiIV as catalyst for the addition of diethylzinc to various aromatic aldehydes (Scheme 13). Using this system, reactions

Scheme 13. C2-symmetric diol ligands 21 and 22 derived from (1R)-(+)-apinene.

were carried out at temperatures between 0 8C and 100 8C and in different solvents. The best result was obtained in toluene at room temperature (up to 91 % ee). For this catalytic system, the level of enantioselectivity determined on various alcohols issued from substituted benzaldehydes was strongly dependent on the electronic properties of the aromatic moiety of the substrate. Very recently these authors described new pinene-based bipyridinediol ligands 22 with an additional carbon between the two pyridine rings (Scheme 13).[31] The skeleton modification allowed an important improvement of the enantiomeric excesses on the produced secondary alcohols (up to 97 % ee for a large series of aromatic, polycyclic aromatic and aliphatic aldehydes substrates). The same group also prepared and evaluated a new family of catalysts (23–25) prepared from (1R)-(+)-a-pinene for enantioselective addition of diethylzinc to substituted benzaldehydes.[32] Preliminary studies showed that the reaction could be realized preferentially in toluene or hexane at room temper-

Scheme 11. Evaluation of limonene-derived amino alcohol catalysts in diethylzinc addition to benzaldehyde.

using the pyrrolidine derived amino alcohol (up to 85 % on (R)1-phenylpropan-1-ol). Malhotra and co-workers[29] reported the use of the chiral bamino alcohol 20 synthesized in two easy steps from (+)-3carene in the enantioselective addition of diethylzinc to benzaldehyde and derivatives. Moderate to good conversions were obtained (55–77 %) and with good enantioselectivities for a large variety of substrates (up to 98 % ee), except for para-nitrobenzaldehyde and 2-pyridinecarboxaldehyde. Notably, cy&

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Scheme 16. Evaluation of a-pinene-derived amino alcohols in diethylzinc addition to benzaldehyde.

Scheme 14. Evaluation of bipyridyl-a-pinene alcohols and diols as catalysts in diethylzinc addition to benzaldehyde.

substituted aminodiol ligand 31 d. The selectivity in this series is clearly affected by the N-substituents, as suggested by theoretical calculations, following the order: NR1R2 > NHR > NH2. New amino alcohols as chiral auxiliaries derived from ( )-bpinene were synthesized and applied by Singaram’s group in the enantioselective addition of diethylzinc and dimethylzinc to aromatic, a,b-unsaturated and aliphatic aldehydes.[35] Secondary alcohols were obtained with high yields (up to 100 %) and enantiomeric excesses (up to 99 % ee), notably in the presence of chiral amino alcohols 2-MAP 34 and 3-MAP 35. Interestingly, these two chiral directors provided opposite enantiofacial selectivity in the diethylzinc addition to aldehydes (Scheme 17).

ature in the presence of 5 % alcohol catalyst as the optimal amount. Under these conditions, alkylation gave the corresponding alcohol in generally good conversions (up to 86 %) and moderate ee values (maximum 70 % ee; Scheme 14). This dissymmetric bipyridyl alcohol has been also identified as the most efficient in the asymmetric addition of diethylzinc to various substituted benzaldehydes (46–79 % ee). Flçp and co-workers described the use of new chiral diamine 26 and g-amino alcohols 27–30.[33] These catalysts derived from (+)- or ( )-a-pinene allowed to prepare 1-phenylpropan-1-ol with moderate asymmetric induction (up to 62 % ee). The primary amino alcohol 27 and N-methyl-substituted secondary amino alcohol 29 provided 1-phenyl-propanol with R configuration whereas the N-benzyl 28 or N,N-dimethyl 30 led to the S alcohol (Scheme 15). Molecular modeling at the

Scheme 15. Evaluation of various a-pinene-derived diamines and amino alcohols in diethylzinc addition to benzaldehyde.

Scheme 17. Evaluation of b-pinene-derived amino alcohols in diethylzinc addition to aldehydes.

ab initio level was carried out to explain this effect of the substituent-dependent enantioselectivity. The same authors also reported a series of primary, secondary, and tertiary aminodiols synthesized from ( )-a-pinene via the a-pinene oxide formation and their applications as enantioselective catalysts in this model reaction (Scheme 16).[34] Good conversions up to 92 % were attained and the best result in terms of ee was obtained with the N-benzyl-N-methyl

Trifunctional O,N,O- (36) and N,N,O-ligands (37) with pinane frameworks were synthesized and tested in the asymmetric addition of diethylzinc to benzaldehydes.[36] These compounds catalyzed the reaction with high conversions (almost 100 %), whereas low to good enantioselectivities on the alcohol product were obtained (5–80 % ee). Particularly, the best value (80 % ee) was obtained with ligand 36 b bearing a tBu moiety on the aromatic ring (Scheme 18).

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Scheme 18. Use of trifunctional catalysts in the addition of diethylzinc to benzaldehyde and derivatives.

Scheme 20. Carene-based oxazine catalysts and their use in asymmetric diethylzinc addition to aldehydes.

Scheme 19. Carene-based amino diol catalysts

In 2011, Szakonyi et al.[37] reported new chiral aminodiols 38, obtained regio- and stereoselectively from (+)-3-carene via an epoxyalcohol intermediate, and their applications as chiral catalysts in the asymmetric addition of diethylzinc to aliphatic and aromatic aldehydes (Scheme 19). Good conversions could be achieved but enantioselectivities remained low (3–37 % ee). To prepare conformationally more constrained skeletons for improved enantioselectivity, new chiral oxazine compounds were prepared from these aminodiols by regioselective ring closure and tested in the asymmetric addition of diethylzinc to substituted benzaldehydes and aliphatic aldehydes. These catalysts proved to be the most efficient in this reaction with good conversions (up to 83 %) and enantioselectivities varying between 38 % and 96 % ee. The best ee values were obtained for conversion of various aldehydes with the N-(R)-1-phenylethyl-substituted catalyst 39 (Scheme 20).[37] Recently, the same group synthesized a new library of chiral pinane-based aminodiols and their corresponding oxazolidines obtained by ring closure with excellent regioselectivity.[38] In the model reaction, conversions were up to 86 % but low to moderate enantioselectivities (1–61 % ee) were observed (Scheme 21). The N-benzyl catalyst was the most efficient in terms of enantioselectivity. The substituents influenced the enantioselectivity according to the sequence NHR > NRR > NH2. The pinane-fused oxazolidines obtained by ring closure of the aminodiols proceeded less efficiently in terms of selectivity than their regioisomeric analogues. To summarize, a great deal of efforts has been devoted to the application of diversely functionalized natural monoterpenes in the development of catalysts applied in the addition of dialkyl zinc derivatives onto aldehydes. Since the beginning &

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Scheme 21. Pinane-based aminodiol and oxazolidine catalysts.

of this research in 1983, the reported catalysts exhibited enantioselectivities varying from moderate to high even though they were generally below 90 % ee. These results also include those obtained using b- and g-amino alcohols derived from (+)-camphor and ( )-fenchone as starting terpenes, which have not been discussed in this chapter, and for which enantioselectivities remained moderate.[39] Overall, one must admit that the results obtained so far have not been so successful in terms of both activities and enantioselectivities as compared with the former results using the DAIB catalyst reported by Noyori.[21] The most relevant results from this section were reported recently with the MAP 37 and 38 catalysts (ee between 83 and 99 % on the formed secondary alcohols; Scheme 17). The enantioselectivity enhancement obtained upon ring closure of aminodiols described in Scheme 20 (vs Scheme 19) is in agreement with this statement. Indeed in these latter cases, the ee increased from 37 to 99 %. An overlook at the above results indicates that the use of highly strained amino alcohols seems to be a prerequisite to induce good enantioselectivities 8



Minireview and therefore, the synthesis of new ligands of that family seems worth to be developed. In addition, the scope of dialkylzinc compounds needs to be enlarged in the future.

step procedure from (+)-a-pinene, and its use in the asymmetric hydrosilylation of acetophenone with diphenylsilane catalyzed by rhodium complexes.[42] A satisfying conversion (98 %) but a poor enantioselectivity (15.8 % ee) were obtained. The same authors reported novel nitrogen-containing ligands with optically active pinene framework 41 and 42, which gave good conversions and enantiomeric excesses of 79.6 % and 33.8 %, respectively, for the same reaction.[43] Chelucci studied several chiral ligands with phenantroline-type nitrogen donors (45, 48, 51) including 5,6-dihydro derivatives (44, 47, 50), as well as 2,2’ bipyridines (43, 46, 49) and 2,2’:6’,6’’ terpyridines (52–54) with active pinene moieties.[44, 45] A rhodium catalyst bearing ligand 49 induced 100 % conversion but an almost racemic alcohol was obtained (1 % ee). For other catalytic systems, enantioselectivities were also very low, but values of 70–76 % ee were recorded for N,N donor ligands 46–48. The same research group reported on new chiral pinene–terpyridine-derived ligands 54 a–e and the corresponding rhodium(III) complexes 55 a–e.[46] The RhI/54 a–e catalysts were prepared in situ from [{Rh(cod)Cl}2] and the appropriate ligand at a 1:1 L/Rh ratio and the reactions were carried out for 24 h at room temperature. The reduction products were obtained with moderate conversions (18–59 %), and low enantioselectivities (7–13 % ee). In an attempt to improve the ee values, rhodium(III) complexes

Hydrosilylation of ketones The asymmetric metal-catalyzed hydrosilylation of aryl ketones by formation of transient Si O derivatives also represents a powerful alternative tool for the synthesis of secondary alcohols.[40] Various metal complexes based on copper, zinc, titanium, and iridium can be used, but rhodium remains the most frequently applied (Scheme 22). As for other catalytic asymmetric reactions, several research groups have developed numerous ligands for asymmetric hydrosilylation reactions, but only few of them derived from monoterpenes (Scheme 23).[41] Brunner described the application of diphenylphosphinopinane 40, synthesized in a two-

Scheme 22. Hydrosilylation of ketones.

Scheme 23. Terpene-derived ligands used in the hydrosilylation of acetophenone by diphenylsilane. Chem. Eur. J. 2014, 20, 1 – 17

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Minireview 55 a–e were prepared from ligands 54 a–e and RhCl3 and assessed in the presence of AgBF4 to test the ability of cationic species in the hydrosilylation reaction under the same conditions. Better levels of conversion were obtained (65–95 %) but lower enantioselectivities were attained (2–6 % ee). More recently, Kocˇovsky´ et al. described the synthesis of new pyridine– oxazoline ligands 56 a and 56 b, starting from available bpinene, and their applications in the asymmetric hydrosilylation.[47] Under standard conditions, the catalytic systems also exhibited disappointing enantioselectivities (4 % ee with 56 a and 8 % ee with 56 b). Silane derivatives were also used as reducing reagents in the work of Chelucci and co-workers, who proposed the enantioselective reduction of acetophenone with polymethylhydrosiloxane (PMHS) and tin(II) complexes bearing chiral ligands (50–54, for example) issued from pyridine derivatives and ( )-b-pinene (Scheme 24).[48] No improvement was reported for either activity or enantioselectivity.

Scheme 26. CBS catalyst for reduction of ketones.

active alcohols with a high degree of prediction and performance (Scheme 26).[52] In this section of the review, we will describe a class of oxazaborolidine compounds derived from a- or b-pinene and 3-carene, which have recently been applied in the catalytic reduction of ketones. In 1996, Masui and co-workers described new chiral oxazaborolidines, formed in situ by the reaction of b-amino alcohols synthesized from both enantiomers of a-pinene and trimethylborates, and their use as catalysts for the asymmetric reduction of prochiral ketones in the presence of the borane–dimethyl sulfide complex (Scheme 27).[53] Initial studies were carried on

Scheme 24. Reduction of acetophenone catalyzed by SnII/50–54 complexes with PMHS.

In conclusion, it appears that most of the monoterpenebased chiral ligands tested date have given disappointing results in the Rh-catalyzed hydrosilylation of acetophenone (maximum ee = 79.6 % with pyridyl-oxazolidine ligand 41; Scheme 23), although they have shown important chiral control for other asymmetric catalytic reactions.[49] Nevertheless, it appears that the presence of a second chiral center in the ligand has a strong beneficial effect on the ee, so ligands of this type would merit further study in this reaction.

Scheme 27. Pinene-based oxazaborolidine catalysts in reduction of ketones.

the asymmetric reduction of model substrate acetophenone and the reactions were carried out from 0 8C to room temperature in THF. High yields (> 90 %) and very good enantioselectivities (up to 94 % ee) were obtained. Many other ketones were also tested using 10 mol % of 57 at 0–5 8C, leading to the corresponding alcohols with excellent conversions (up to 99 %) and good enantioselectivities. The same group also applied the b-methoxy oxazaborolidine 58 (Scheme 28), which is a closely related compound to 57, with the objective to increase the Lewis acidity of the boron atom (expected to have a beneficial effect on the enantioselectivity).[54] In comparison with 57, b-methoxy oxazaborolidine 58 gave the corresponding alcohol with slightly higher yields and enantioselectivities (up to 95.5 % ee Scheme 28. Pinene-based bfor acetophenone). methoxy oxazaborolidine.

Oxazaborolidine reductions Enantioselective reductions using boron reagents, such as boron hydrides, Alpine–borane, or DIP–chloride[50] (Scheme 25), and oxazaborolidines as stoichiometric reagents have been recognized as a very valuable tool for the synthesis of optically enriched secondary alcohols.[51] The catalytic version of this reaction in the presence of borane and a chiral oxazaborolidine as catalyst, reported by Corey (Corey–Bakshi–Shibata; CBS), represents one of the most interesting way to access optically

Scheme 25. Alpine–borane and DIP-chloride.

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Scheme 29. Pinene-based b-methoxy-oxazaborolidine catalysts in reduction of ketones. Scheme 31. Asymmetric reduction of a mixture of arylketones.

New b-methoxy oxazaborolidines, prepared by the reaction of the amino alcohol 59 and trimethylborate without isolation, were tested in the asymmetric reduction of acetophenone and various aryl alkyl ketones (Scheme 29).[55] After optimization of the catalyst system, high yields could be achieved for all ketones (up to 99 %), as well as enantioselectivities (98 % ee in the case of acetophenone and 2-acetonaphthone). In 2009, Ła˛czkowski and co-workers described a convenient method for the stereoselective synthesis of b-amino alcohol 60 derived from (+)-3-carene.[56] The corresponding oxazaborolidine was prepared in situ by reaction of this b-amino alcohol with B(OiPr)3, then tested without isolation in the asymmetric reduction of prochiral ketones. Alcohols were obtained with excellent yields and enantioselectivities (Scheme 30).[7] Hobuss et al. reported a series of cis and trans b-amino alcohols synthesized from ( )-a-pinene, and their applications as chiral agents in the asymmetric reduction of a mixture of arylketones (Scheme 31).[57] Excellent conversions (up to 100 %) were obtained for various ketones during the reduction cata-

lyzed by oxazaborolidines 62 a–c, prepared in situ from compounds 61 a–c and trimethylborate, in the presence of BH3·THF. cis-61 a/B(OMe)3 gave the best enantioselectivities (96 % ee (R)-phenylethanol and 93 % ee (R)-phenylpropan-1-ol). However, the presence of alkyl substituents on the nitrogen atom of the amino alcohol (ligands 62 b and c) resulted in drastic decreases in enantioselectivity (0–22 % ee). The use of catalysts formed with trans-amino alcohols proved to be useless, giving low activities and racemic mixtures of the alcohols. Other substrates, such as ether derivatives of a-ketoaldoxime, were also reduced in the presence of BH3/oxazaborolidine 63 at 0 8C.[58] Yield and selectivity were both moderate (Scheme 32).

Scheme 32. Reduction of an a-keto aldoxime O-ether.

In 2011, Krzemin´ski and C´wiklinska described the synthesis and isolation of spiroborate-esters derived from a-pinene as new catalysts for the asymmetric reduction of ketones with borane.[59] The reaction was carried out in dry THF at room temperature and with 1, 5, and 10 mol % catalyst loadings, resulting in good yields and enantioselectivities. Catalyst 64, bearing an ethyleneglycol moiety, gave the best results in the case of acetophenone and was further tested in the reduction of various alkyl aryl ketones under the same conditions. Good activities and enantioselectivities were obtained, except for those for phenylnaphthylketone, which gave an almost racemic mixture of the alcohol (4 % ee; Scheme 33). Overall, from the results using boranes and derivatives in reduction reactions for the production of optically enriched sec-

Scheme 30. Carene-based b-methoxy-oxazaborolidine catalysts in reduction of ketones. Chem. Eur. J. 2014, 20, 1 – 17

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Scheme 35. Ligands used in the alkynylation of aldehydes.

selectivities (maximum 47 % ee with catalyst 66) were obtained. The authors proposed a transition state model in which the facial selectivity observed is controlled by the amine moiety and the (C1) methyl group. A large series of aromatic and aliphatic aldehydes was then tested with a catalytic system based on catalyst 66; conversions up to 86 % and moderate enantioselectivities were obtained (up to 60 % ee, (S) configuration; Scheme 36). A comparison has been done between 66

Scheme 33. Spiroborate esters 64 derived from a-pinene in reduction of ketones.

ondary alcohols, it is evident that is the main application of terpene-based chiral systems in asymmetric synthesis. Highly efficient is the oxazaborolidine catalyst prepared by reaction of the amino alcohol 60 with triisopropylborate, furnishing secondary alcohols with ee values close to 99 % (Scheme 30). Although the procedure is catalytic only in rare cases, the organic chemist who would like to develop a multistep synthesis of a biologically active molecule will find here a convenient, rapid, and predictive way for obtaining the desired alcohol enantiomer in its synthetic sequence.

Asymmetric alkynylation and allylation of carbonyl compounds Optically pure secondary alcohols could be obtained by asymmetric alkynylzinc addition to aldehydes.[60] Pure b-amino alcohols prepared from limonene oxide and (+)-3-carene have therefore been tested by Singaram’s group[27, 28] in the enantioselective alkynylzinc addition to aromatic and aliphatic aldehydes to produce optically active propargylic alcohols which are useful building blocks and intermediates for the synthesis of unique and biological active molecules and natural products such as steroids, prostaglandins or pheromones (Scheme 34).

Scheme 36. Enantioselectivity in alkynylation with phenylacetylene of aldehydes in the presence of catalyst 66.

and an amino alcohol synthesized from (1R,2S)-( )-2-amino1,2-diphenylethanol using the same substrates, leading to the conclusion that they were comparable, 66 providing in some cases better asymmetric inductions. Malkov and co-workers described new Lewis base heterobidentate bipyridine N-oxides 67, 69 and 70 and N,N-dioxides 68 issued from a- and b-pinenes which proved to be efficient organocatalysts for the asymmetric allylation of benzaldehyde with allyltrichlorosilane (Schemes 37 and 38).[62] The stereochemical outcome was found to be controlled by the potential axial chirality of the catalyst, which in turn was determined by the central chirality of the annulated terpene units of properly substituted derivatives (Scheme 38). With the N,N-dioxide 68 catalyst, a low yield (18 %) and a modest enan-

Scheme 34. Alkynylzinc addition of aldehydes.

Several b-amino alcohols derived from monoterpenes were first evaluated in alkynylation of benzaldehyde with phenylacetylene. Some examples of these ligands are depicted in Scheme 35.[61] All reactions used a catalyst loading of 10 mol % at 20 8C for 24 h. High conversions (up to 95 %) and moderate enantio&

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Minireview Lewis base catalysts for the asymmetric allylation of aromatic aldehydes (Scheme 39).[66] The reaction was carried out in dichloromethane at 0 8C for 3 h and gave homoallylic alcohols in high yields (97 %) and satisfying enantioselectivities (up to 86 % ee). The results showed that the best ee was obtained using substrates containing an electron-withdrawing group at the para position of the aryl moiety. Malkov and co-workers reported the synthesis of new pinene-derived organocatalysts with a set of monodentate and bidentate chiral Lewis bases featuring a pyridine N-oxide fragment for the asymmetric allylation of benzaldehyde and a few aromatic aldehydes with allyltrichlorosilane (Scheme 40).[67] Reactions were carried out using 10 mol % of catalyst as the optimized amount. In most cases, dichloromethane proved to be the solvent of choice. In the presence of catalysts 73 and 74, whose chirality originates from the annulated terpene unit, ee values remained moderate with 56 % ee as the maximum value. Unfortunately, these new bis-N-oxides with pinene moiety proved rather inefficient compared to other C2-symmetrical bis-N-oxides. Alkynylation of carbonyl compounds allows the easy synthesis of propargylic alcohols with high ee values, especially using nitrogen-containing ligands (> 90 % ee).[60] However, in the case of terpene-derived catalysts, enantioselectivites remained rather moderate with maximum values of around 50 % ee (Scheme 36). Of particular interest in this section are the results obtained by using organocatalysis for the allylation of alde-

Scheme 37. Allylation of benzaldehyde with allyltrichlorosilane.

Scheme 38. Bipyridine N-oxide and N,N-dioxide catalysts used in allylation of benzaldehyde with allyltrichlorosilane.

tiomeric excess (41 % ee) were obtained. In the same catalytic conditions, the analogue monoxide PINDOX 67 a was more active (78 % yield) and enantioselective (90 % ee). Interestingly, the introduction of two methyl groups in the bipyridine moiety furnished Me2-PINDOX catalyst 67 b, bearing an additional axial chirality leading to an even higher enantioselectivity (98 % ee). The same group also described the synthesis of novel chiral N-monoxides, R-PINDOX 69 a–d from a-pinene and their evaluation as organocatalysts in the asymmetric allylation of benzaldehyde and derivatives (Scheme 38).[63, 64] Allylic alcohols were obtained in good yield (up to 75 %) and the best results were obtained for benzaldehyde (96 % ee on the resultant alcohol). However, the enantioselectivity was clearly dependent on the size of the R group in Scheme 39. Terpyridine tri-N-oxide catalysts used in allylation of aromatic aldehydes with the pinene moiety. Due to this steric hindrance, the allyltrichlorosilane. iPr-PINDOX showed a higher level of enantioselectivity (96 % ee) than the methyl- and butyl-substituted analogues. In contrast, the unsubstituted catalyst 69 a induced a lower enantioselectivity, (46 % ee; Scheme 38). Malkov’s group also reported the enantioselective allylation of aromatic aldehydes and a,b-unsaturated aldehydes using catalyst based on chiral pinene monopyridine-type N-oxide METHOX 70. Very high yields (> 95 %) and enantioselectivities (up to 96 % ee) were obtained for a large variety of secondary alcohols.[65] A new series of terpene-derived terpyridine tri-N-oxide comScheme 40. Bipyridine N,N-dioxide catalysts used in allylation of benzaldepounds, 71 and 72 a–d, has also been developed and used as hyde with allyltrichlorosilane. Chem. Eur. J. 2014, 20, 1 – 17

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Minireview hydes with allyltrichlorosilane, by which a combination of central and axial chiralities in the organocatalyst proved a convenient way to improve the enantioselectivities (see ligands 67 b vs. 67 a; Scheme 38). Among this family of catalysts, METHOX 70 can be regarded as the most successful organocatalyst for the asymmetric allylation of numerous aldehydes. This concept could be extended to other reactions. Notably, this catalytic reaction uses no transition metal and may prove preferable to the traditional method that uses the chiral TADDOL-derived allylic titanium reagent in stoichiometric amounts.[68]

Scheme 42. Ring-opening of meso-epoxides with thiophenol catalyzed by a C2-symmetric bipyridyldiol–titanium complex derived from (+)-a-pinene.

Miscellaneous reactions

Summary and Outlook

Monoterpene-derived ligands for the production of optically secondary alcohols have also been used in the Henry reaction and in the enantioselective ring-opening of meso-epoxides using thiophenol. The copper catalyzed nitroaldol (Henry) reaction[69] was chosen by Buresˇ and co-workers as a model reaction for the evaluation of novel annulated terpene–imidazole ligands (Scheme 41).[70] However, the reaction was rather sluggish,

This review shows the scale of the efforts made to exploit the intrinsic chiral framework of monoterpenes to synthesize new compounds that can be used as chiral inducers in asymmetric catalytic reactions for the synthesis of chiral alcohols. Analyzing the above-discussed reactions leads to the main conclusion that the most successful results have been observed when using b-amino alcohol derivatives. Especially relevant are the oxazaborolidine reductions, in which a high level of enantioselectivity is obtained using catalytic amounts of the chiral agent (5–10 %). The same applies to the asymmetric alkylation of aldehydes using diethyl zinc, although in this case a more sophisticated catalyst, bearing four asymmetric centers, was used. Variations at the substituents on the nitrogen atom of the ligand and/or on the substrate itself led to strong variations in terms of enantioselectivity. Therefore, in cases where a particular chiral product was targeted, there would be some room for researchers to look at the synthesis of new ligands using the same chiral backbone, via different substitutions at nitrogen, and including an extension of the chiral array with the objective to observe a matched effect. Such attempts would be similar to the extensive work conducted in our group on aminophosphine phosphinite ligands, whose chiral backbone is also based on a chiral amino alcohol skeleton, and for which extension of the chiral array through planar chirality has been found to lead to matched and mismatched cooperative effects on ligands in rhodium-catalyzed functionalized ketone hydrogenation.[73] Extending the number of chiral centers or combining central with axial chirality when applicable could be a way to provide at least more enantioselective catalysts, although the activity and consequently the substrate/catalyst ratio would not be necessarily improved unless a careful design of those substituents would increase the reaction rate through electronic effects.

Scheme 41. Terpene–imidazole ligands used in the Henry reaction.

giving 62–96 % conversions after 24 to 72 h and modest ee values similar to those observed for similar a-amino acid-derived imidazole ligands. The titanium-catalyzed asymmetric ring opening of meso-epoxides with diverse nucleophiles is an important transformation which allows the formation of 1,2-bifunctionalized systems with a hydroxyl moiety. The chiral bipyridinyldiol 21 was prepared from a-pinene and acted as a chiral ligand for the enantioselective addition of thiophenol to various substituted stilbene oxides, allowing the formation of b-hydroxysulfides in optically active forms, generally ranging from 20 % to 69 % ee (Scheme 42).[71] These two reactions afforded b-functionalized alcohols, bearing either a nitro or a sulfide group. Few ligands have been evaluated in these catalytic reactions but limited results are obtained especially in the copper catalyzed nitroaldol reaction. Further ligands, such as hydroxyamines and hydroxyimines, should be tested to improve these results in terms of activity and enantioselectivity, with the aim of being competitive with the results already obtained using more traditional bis(oxazoline) ligands.[72] &

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Acknowledgements We thank the “Ministre des Affaires Etrangres” (programme Volubilis, AI no. 012/SM/07), the CNRS, the “Ministre de l’Enseignement Suprieur et de la Recherche” and the “Ministre de l’Education Nationale, de l’Enseignement Suprieur et de la Recherche Scientifique du Maroc” for their financial support. 14



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Keywords: alcohols · asymmetric catalysis · chiral ligands · natural product synthesis · terpenes

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Minireview

MINIREVIEW & Terpenes

The preparation of chiral secondary alcohols in the presence of catalysts based on optically pure ligands derived from monoterpenes, such as pinenes, limonenes and carenes, is reviewed. A large variety of these ligands has been synthesized and used in catalytic reactions, including hydrogen transfer, C C bond formation via addition of organozinc reagents to aldehydes, hydrosilylation, and oxazaborolidine reductions.

M. S. I. El Alami, M. A. El Amrani, F. Agbossou-Niedercorn, I. Suisse,* A. Mortreux && – && Chiral Ligands Derived from Monoterpenes: Application in the Synthesis of Optically Pure Secondary Alcohols via Asymmetric Catalysis

Chiral Ligands The preparation of optically pure secondary alcohols in the presence of catalysts based on chiral ligands derived from monoterpenes, such as pinenes, limonenes and carenes, is discussed in the Minireview by I. Suisse et al. on page && ff. A wide variety of these ligands has been synthesized and used in entantioselective catalytic reactions, including hydrogen transfer, C C bond formation via addition of organozinc compounds to aldehydes, hydrosilylation, and oxazaborolidine reduction.

Chem. Eur. J. 2014, 20, 1 – 17

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