DOI: 10.1002/chem.201404994
Communication
& Synthetic Methods
A Well-Defined Monomeric Aluminum Complex as an Efficient and General Catalyst in the Meerwein–Ponndorf–Verley Reduction Brian McNerney,[a] Bruce Whittlesey,[a] David B. Cordes,[b] and Clemens Krempner*[a] Dedicated to Prof. Gerhard Roewer on the occasion of his 75th birthday as an inexpensive, safe and nontoxic reducing agent.[5] The MPV reduction proceeds via hydride transfer from a secondary alcohol to a carbonyl compound mediated by coordination to a Lewis acidic metal center,[6] most often aluminum[7] but also other metals have been employed[8] (Scheme 1).
Abstract: The metal-catalyzed Meerwein–Ponndorf–Verley (MPV) reduction allows for the mild and sustainable reduction of aldehydes and ketones but has not found widespread application in organic synthesis due to the high catalyst loading often required to obtain satisfactory yields of the reduced product. We report here on the synthesis and structure of a sterically extremely overloaded siloxide-supported aluminum isopropoxide capable of catalytically reducing a wide range of aldehydes and ketones (52 examples) in excellent yields under mild conditions and with low catalyst loadings. The unseen activity of the developed catalyst system in MPV reductions is due to its unique monomeric nature and the neutral donor isopropanol weakly coordinating to the aluminum center. The present work implies that monomeric aluminum alkoxide catalysts may be attractive alternatives to transition-metalbased systems for the selective reduction of aldehydes and ketones to primary and secondary alcohols.
Scheme 1. Commonly accepted mechanism of the MPV reaction.
The recent years have witnessed tremendous research efforts in sustainable catalysis primarily aimed at the use of readily available feedstocks, the development of metal-catalyzed atom-economic reactions, the reduction of chemical waste and most importantly the replacement of precious metals with more abundant and less toxic main group or first-row transition metals. Classic examples of this development are iron-catalyzed hydrogenations,[1] transfer hydrogenations[2] and hydrosilylations[3] of aldehydes and ketones to primary and secondary alcohols, respectively, which are important synthetic intermediates in the pharmaceutical and fine chemical industry.[4] Less explored in this regard is the classical and highly sustainable metal-catalyzed Meerwein–Ponndorf–Verley (MPV) reduction of ketones and aldehydes, which exhibits exceptional chemoselectivity, is operationally simple and uses isopropanol [a] Dr. B. McNerney, Prof. Dr. B. Whittlesey, Prof. Dr. C. Krempner Department of Chemistry and Biochemistry, Texas Tech University Box 1061, Lubbock, Texas, 79409-1061 (USA) E-mail: [email protected] [b] Dr. D. B. Cordes School of Chemistry, St. Andrew University Purdie Building Fife KY16 9ST, St. Andrews (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404994. Chem. Eur. J. 2014, 20, 14959 – 14964
From the prospective of sustainability aluminum is an attractive catalyst component as it is the most abundant metal on earth (only challenged by iron), environmentally benign and of relatively low toxicity. Many of its inorganic and organometallic compounds are commercially available at low cost. Despite these advantages, the homogeneous aluminum alkoxide mediated MPV reduction has not found widespread utility in synthetic organic chemistry and natural product synthesis,[9] primarily due to the high catalyst loadings required to obtain satisfactory yields of the reduction product and the rather narrow substrate scope.[7, 8, 10] The low activity of simple aluminum alkoxides and aryloxides can largely be attributed to the formation of Al O bridged aggregated structures. These very stable aggregates seem to persist even in solution resulting in coordinatively saturated aluminum centers of reduced Lewis acidity. We envisioned that a sterically encumbered bidentate disiloxide ligand, which we developed recently,[11] would prevent the resulting cyclic aluminum complex from being aggregated and further enhance the electrophilicity of aluminum center owing to the electron-withdrawing properties of the siloxide groups.[12] Herein, we will demonstrate for the first time that this conceptually new approach, leads to a well-defined and thermally robust aluminum isopropoxide that is monomeric and thereby an exceptionally active catalyst in the MPV reduction of ketones and aldehydes. The straightforward synthesis of the aluminum methyl complex 2 and the aluminum isopropoxide 3, is illustrated in Scheme 2. Compound 3 can conveniently be prepared in a one-pot procedure starting from the sterically encumbered 1,4-disilane-diol 1[11] in yields of 70–80 %. Isopropoxide 3 is
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Communication a thermally stable crystalline material that is moisture-sensitive but air-stable. Note that the alcoholysis of the Al CH3 bond of 2 occurs only very slowly to produce the isopropoxide 3. The half life of 2 in dry isopropanol is about 24 h at 25 8C and 2 h at 50 8C demonstrating the very effective steric protection of the aluminum center of 2. Diffusion experiments (1H DOSY NMR spectroscopy) of monomeric 2 and 3 at 25 8C in C6D6 reveal both compounds to have very similar diffusion coefficients clearly indicating a monomeric structure for 3 in solution.
Scheme 2. Synthesis of aluminum disiloxide 3.
The results of an X-ray data analysis of 3 further confirm its monomeric nature and the tremendous steric protection of the tetracoordinated aluminum center (Figure 1). Single crystals of two monomeric forms, 3 a and 3 b, were grown from isopropanol. Both contain in the solid state one molecule of isopropanol weakly coordinating to the aluminum center [3 a: Al1 O3 182.4 pm; 3 b: Al1 O4 182.1 pm]. Form 3 a contains two additional highly disordered isopropanol molecules located in the periphery, while in 3 b the two isopropanol molecules are in proximity to the aluminum center, connected via an array of hydrogen bonds. Note that intermolecular donor adducts of neutral, monomeric aluminum isopropoxides are extremely rare,[13] and there are no examples of monomeric isopropanol adducts. Next, 3 and the commercially available simple alkoxides Al(OiPr)3, Al(OsecBu)3, Al(OtBu)3 and AlMe3 were tested as catalysts in the MPV reduction. Cyclohexanone as a model substrate and 5 equiv of dry iPrOH as the reducing agent (Table 1) were used under equilibrium conditions (sealed scintillation vial in a glove box). Catalyst 3 (0.5 mol %) catalyzed the quantitative reduction of cyclohexanone within 4–6 h, while 1 mol % of Al(OiPr)3, Al(OsecBu)3, Al(OtBu)3 and AlMe3, respectively, reduced only about 30–80 % of cyclohexanone after 24 h. The observation that 2 only slowly converts into 3 (t1/2 ~ 24 h) at room temperature and that by far not all of catalyst 3 was dissolved in iPrOH encouraged us to further reduce the catalyst loading under otherwise similar conditions. The results revealed exceptional activity of 3 even at loadings of 0.05 mol %; TOF’s of up to 103 were achieved at 60 8C (entry 10). To demonstrate the efficiency and scope of the MPV reduction with 3 as the catalyst a range of aromatic and aliphatic aldehydes were reduced in the presence of excess isopropanol (Table 2). Excellent catalyst activities were noted for most of Chem. Eur. J. 2014, 20, 14959 – 14964
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Figure 1. Solid-state structures of 3 a (top, disordered iPrOH molecules are not shown) and 3 b (bottom). All H atoms are omitted for clarity (black = carbon, dark gray = silicon). Selected distances [pm] and angles [8]: 3 a: Si1 O1 162.5(2), Si8 O2 163.3(2), Al1 O1 171.0(2), Al1 O2 171.0(2), Al1 O3 182.4(2), Al1 O4 174.5(2), O3 C31 145.5(4), O4 C34 143.1(4), O1-Al1-O2 115.4(1), O4Al1-O3 104.4(1), Si1-O1-Al1 144.2(1), Si8-O2-Al1 140.1(1); 3 b: Si1 O1 163.6(2), Si8 O2 163.5(2), Al1 O1 170.8(2), Al1 O2 170.6(2), Al1 O3 176.6(2), Al1 O4 182.1(2), O2-Al1-O1 113.1(1), O3-Al1-O4 97.4(1), Si1-O1-Al1 142.0(1), Si8-O2Al1 143.9(1).
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Table 1. Aluminum-catalyzed MPV reduction of cyclohexanone.[a]
Entry
Catalyst
Loading [mol %]
Yield [%][b]
t [h]
TON
TOF [h 1]
Yield [%][c]
1 2 3 4 5 6 7 8 9 10
Al(OiPr)3 Al(OsecBu)3 Al(OtBu)3 AlMe3 AlMe3 3 3 3 3 3
1 1 1 1 0.5 0.5 0.1 0.1[d] 0.05 0.05[d]
10 32 32 35 22 96 90 98 25 94
10 5 5 4.5 4.5 4.5 4.5 2 2 2
10 32 32 35 44 192 900 980 500 1880
1 6 6 8 10 43 200 490 250 940
20 48 50 80 35 > 99 > 99 > 99 > 99 > 99
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner); conditions: room temperature, 5 equiv iPrOH, no co-solvent; [b] determined by NMR spectroscopy; [c] final yields determined by NMR spectroscopy after 24 h; [d] 60 8C.
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Communication the aldehydes and almost all of them could fully be converted to the corresponding primary alcohols. Astonishingly, undesired side reactions such as Tishchenko and aldol reactions were not seen with these reduction conditions, which is remarkable given the susceptibility of aliphatic and aromatic aldehydes to readily undergo ester formation via Tishcenko reaction. As expected, the more electron-rich 4-methoxy-, 2-methoxyand 2,3-dimethoxy substituted benzaldehydes (entries 16–18) reacted more slowly than 4-nitro- and 4-cyano-benzaldehydes (entries 12, 13). Other functional groups were also tolerated as demonstrated for the quantitative reductions of thiophene-, furan- and 3-pyridine-carboxaldehydes (entries 14, 19–21). The fact that 2-pyridine-carbaldehyde was not reduced at all appears to be due to its excellent chelating properties, leading to the deactivation of the catalyst by irreversible binding to the aluminum center. Astonishingly, challenging a,b-unsaturated aldehydes such as cinnamaldehyde, myrtenal and citral (entries 23–25) can selectively be reduced, albeit under more forcing conditions and with slightly higher catalyst loadings. Under classical MPV conditions, reductions of most aromatic ketones to secondary alcohols are more difficult to achieve, thus often requiring elevated temperatures to readily equilibrate and a large excess of reducing agent to shift the equilibrium in favor of the products. Increasing the iPrOH to catalyst ratio, on the other hand, may suppress substrate binding and hence lowers the reaction rate. Therefore, we were pleased to see that a variety of differently substituted acetophenones
could almost completely be reduced with 3 as the catalyst system (Table 3). Again, the electron-withdrawing 4-nitro and 4-CF3 substituted acetophenones (entries 26, 27) reacted faster than the electron-rich 2-OMe and 4-OMe substituted acetophenones as well as acetyl-thiophene and -furan (entries 32, 41, 45, 46). Astonishingly, even the notoriously difficult 2,6-dimethoxy-acetophenone (entry 42)[8g] could also quantitatively be reduced. The bulky a,a,a-trifluoro- and a,a,a-trimethyl-acetophenones (entry 37 and 38) were more challenging and required extensive heating to achieve moderate to good yields of the corresponding alcohols. The ability to conveniently reduce cyclic and acyclic aliphatic ketones to secondary alcohols under mild conditions is of high importance in natural product synthesis. Therefore, a range of cyclic ketones of various size and cyclohexanone derivatives that contain functional groups were screened as these structural motifs are incorporated in many drugs and natural products (Table 4). S-, O- and N-containing cyclohexanones can smoothly and quantitatively be reduced (entries 47–51). Various aliphatic ketones of different steric profile could also readily be reduced (entries 52–57). Note that the sterically hindered pinacolone (entry 55) converted quantitatively to the corresponding alcohol, while at least 50 % of the extremely bulky diisopropylketone (entry 56) converted to the corresponding alcohol clearly demonstrating that even sterically challenging substrates can be reduced with the MPV methodology. Furthermore, 5a-cholestane-3-one, a synthetic intermediate in the chemistry of steroids, was tested and also found to be
Table 2. Aluminum siloxide catalyzed MPV reduction of aromatic and aliphatic aldehydes to primary alcohols.[a] Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
11
0.2
5
24
50
> 99
19
0.5
25
6
50
> 99
12
0.2
5
24
40
> 99
20
0.5
25
6
50
> 99
13
0.2
5
24
40
> 97
21
0.7
25
24
60
> 99
14
0.5
5
24
40
> 99
22
0.5
25
24
25
> 99
15
0.5
5
24
50
> 99
23
1
3
24
80
> 99
16
0.5
25
6
50
> 99
24
0.5
5
24
80
> 99
17
0.5
25
6
50
> 99
25
0.5
5
24
80
90
18
0.5
25
6
50
> 99
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by NMR spectroscopy.
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Communication Table 3. Aluminum siloxide catalyzed MPV reduction of aromatic ketones to secondary alcohols.[a] Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
26
0.5
25
24
50
98
27
0.5
25
24
50
28
0.5
25
24
29
0.5
25
30
0.5
31
Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
37
0.5
25
48
80
61
98
38
0.5
25
24
80
62
50
93
39
0.5
25
24
80
93
24
50
96
40
0.7
25
48
80
94
25
24
50
98
41
0.5
25
24
80
98
0.5
25
24
50
93
42
0.5
25
24
80
98
32
0.5
25
48
80
85
43
0.5
25
24
50
95
33
0.7
25
24
60
> 99
44
0.5
25
24
80
93
34
0.7
25
24
60
93
45
0.5
25
21
50
90
35
0.5
25
48
50
95
46
0.5
25
24
80
83
36
0.5
25
24
50
85
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by NMR spectroscopy.
reduced to the respective a,b-cholesterol (ratio a/b = 67:33) in almost quantitative yields under very mild conditions and with low catalyst loadings (Scheme 3). In contrast, under flow conditions with ZrO2 as catalyst at 130 8C only 60 % of the product was obtained.[8g] Encouraged by these results, the reduction of oxo-lanosterol was attempted as well. Although by far more sterically demanding than 5a-cholestane-3-one, complete reduction could be achieved at slightly higher temperature (ratio a/b = 25:75) to yield a,b-lanosterol, an important precursor in the enzymatic biosynthesis of cholesterol.[14] To further demonstrate the applicability of the newly developed catalyst system, 5a-cholestane-3-one, 2,3-dimethoxy-benzaldehyde, 2,6-dimethoxy-acetophenone, 1-boc-4-piperidone, citral and decanal were quantitatively reduced on a 1 g scale. The formed alcohols were isolated simply by vacuum distillation with isolated yields ranging from to 86 to 97 %; tedious purification by column chromatography was not necessary. In conclusion, the unprecedented activity and selectivity of a sterically encumbered and monomeric aluminum siloxide Chem. Eur. J. 2014, 20, 14959 – 14964
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Scheme 3. Catalytic MPV reduction of 5a-cholestan-3-one to cholestan-3-ol (top) and 3-oxo-lanosterol to a/b-lanosterol (bottom).
catalyst in the MPV reduction of a wide variety of aliphatic and aromatic aldehydes and ketones under mild conditions was demonstrated (52 examples). Reductions occurred with low
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Communication Table 4. Aluminum siloxide catalyzed MPV reduction of aliphatic ketones to secondary alcohols.[a] Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
Entry
47
0.5
25
17
50
> 99
48
0.5
25
28
50
49
0.2
5
24
50
0.5
25
51
0.5
52
53
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
54
0.5
25
24
60
> 99
> 99
55
0.5
25
48
50
94
50
> 99
56
1.0
5
24
80
53
48
50
98
57
0.5
25
21
50
89
25
6
50
> 99
58
0.5
25
24
50
94
0.2
5
24
50
> 99
59
0.5
25
24
50
97
0.5
25
24
50
90
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by NMR spectroscopy.
catalyst loadings (0.1–1.0 mol %) under solvent-“free” conditions, and, moreover, the great majority of substrates could be utilized as received and without distillation prior to use. Clearly, the high activity of catalyst 3 is a result of its monomeric nature and the fact that the weakly binding isopropanol is readily replaced by the incoming carbonyl substrate, facilitating the rate determining hydride transfer. Finally, the present work suggests that monomeric aluminum isopropoxide catalysts may be attractive alternatives to transition metal based catalyst systems for the selective reduction of aldehydes and ketones to primary and secondary alcohols.
Acknowledgements The authors would like to acknowledge Texas Tech University for support and the NSF for funding the acquisition of a Jeol Eclipse 400 MHz NMR spectrometer (CRIF MU grant CHE1048553). We would like to thank Prof. Dr. W. David Nes (Texas Tech University) for kindly providing us with a sample of 3oxo-lanosterol. Keywords: aldehydes · aluminum · ketones · MPV reduction · siloxide [1] Y. Li, S. Yu, X. Wu, J. Xiao, W. Shen, Z. Dong, J. Gao, J. Am. Chem. Soc. 2014, 136, 4031 – 4039; b) P. O. Lagaditis, P. E. Sues, J. F. Sonnenberg, K. Y. Wan, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2014, 136, 1367 – 1380; c) S. Fleischer, S. Zhou, K. Junge, M. Beller, Angew. Chem. Int. Ed. 2013, 52, 5120 – 5124; Angew. Chem. 2013, 125, 5224 – 5228; d) G. Bauer, K. A. Kirchner, Angew. Chem. Int. Ed. 2011, 50, 5798 – 5800; Angew. Chem. Eur. J. 2014, 20, 14959 – 14964
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Received: August 25, 2014 Published online on October 5, 2014
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Communication
& Synthetic Methods
A Well-Defined Monomeric Aluminum Complex as an Efficient and General Catalyst in the Meerwein–Ponndorf–Verley Reduction Brian McNerney,[a] Bruce Whittlesey,[a] David B. Cordes,[b] and Clemens Krempner*[a] Dedicated to Prof. Gerhard Roewer on the occasion of his 75th birthday as an inexpensive, safe and nontoxic reducing agent.[5] The MPV reduction proceeds via hydride transfer from a secondary alcohol to a carbonyl compound mediated by coordination to a Lewis acidic metal center,[6] most often aluminum[7] but also other metals have been employed[8] (Scheme 1).
Abstract: The metal-catalyzed Meerwein–Ponndorf–Verley (MPV) reduction allows for the mild and sustainable reduction of aldehydes and ketones but has not found widespread application in organic synthesis due to the high catalyst loading often required to obtain satisfactory yields of the reduced product. We report here on the synthesis and structure of a sterically extremely overloaded siloxide-supported aluminum isopropoxide capable of catalytically reducing a wide range of aldehydes and ketones (52 examples) in excellent yields under mild conditions and with low catalyst loadings. The unseen activity of the developed catalyst system in MPV reductions is due to its unique monomeric nature and the neutral donor isopropanol weakly coordinating to the aluminum center. The present work implies that monomeric aluminum alkoxide catalysts may be attractive alternatives to transition-metalbased systems for the selective reduction of aldehydes and ketones to primary and secondary alcohols.
Scheme 1. Commonly accepted mechanism of the MPV reaction.
The recent years have witnessed tremendous research efforts in sustainable catalysis primarily aimed at the use of readily available feedstocks, the development of metal-catalyzed atom-economic reactions, the reduction of chemical waste and most importantly the replacement of precious metals with more abundant and less toxic main group or first-row transition metals. Classic examples of this development are iron-catalyzed hydrogenations,[1] transfer hydrogenations[2] and hydrosilylations[3] of aldehydes and ketones to primary and secondary alcohols, respectively, which are important synthetic intermediates in the pharmaceutical and fine chemical industry.[4] Less explored in this regard is the classical and highly sustainable metal-catalyzed Meerwein–Ponndorf–Verley (MPV) reduction of ketones and aldehydes, which exhibits exceptional chemoselectivity, is operationally simple and uses isopropanol [a] Dr. B. McNerney, Prof. Dr. B. Whittlesey, Prof. Dr. C. Krempner Department of Chemistry and Biochemistry, Texas Tech University Box 1061, Lubbock, Texas, 79409-1061 (USA) E-mail: [email protected] [b] Dr. D. B. Cordes School of Chemistry, St. Andrew University Purdie Building Fife KY16 9ST, St. Andrews (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404994. Chem. Eur. J. 2014, 20, 14959 – 14964
From the prospective of sustainability aluminum is an attractive catalyst component as it is the most abundant metal on earth (only challenged by iron), environmentally benign and of relatively low toxicity. Many of its inorganic and organometallic compounds are commercially available at low cost. Despite these advantages, the homogeneous aluminum alkoxide mediated MPV reduction has not found widespread utility in synthetic organic chemistry and natural product synthesis,[9] primarily due to the high catalyst loadings required to obtain satisfactory yields of the reduction product and the rather narrow substrate scope.[7, 8, 10] The low activity of simple aluminum alkoxides and aryloxides can largely be attributed to the formation of Al O bridged aggregated structures. These very stable aggregates seem to persist even in solution resulting in coordinatively saturated aluminum centers of reduced Lewis acidity. We envisioned that a sterically encumbered bidentate disiloxide ligand, which we developed recently,[11] would prevent the resulting cyclic aluminum complex from being aggregated and further enhance the electrophilicity of aluminum center owing to the electron-withdrawing properties of the siloxide groups.[12] Herein, we will demonstrate for the first time that this conceptually new approach, leads to a well-defined and thermally robust aluminum isopropoxide that is monomeric and thereby an exceptionally active catalyst in the MPV reduction of ketones and aldehydes. The straightforward synthesis of the aluminum methyl complex 2 and the aluminum isopropoxide 3, is illustrated in Scheme 2. Compound 3 can conveniently be prepared in a one-pot procedure starting from the sterically encumbered 1,4-disilane-diol 1[11] in yields of 70–80 %. Isopropoxide 3 is
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Communication a thermally stable crystalline material that is moisture-sensitive but air-stable. Note that the alcoholysis of the Al CH3 bond of 2 occurs only very slowly to produce the isopropoxide 3. The half life of 2 in dry isopropanol is about 24 h at 25 8C and 2 h at 50 8C demonstrating the very effective steric protection of the aluminum center of 2. Diffusion experiments (1H DOSY NMR spectroscopy) of monomeric 2 and 3 at 25 8C in C6D6 reveal both compounds to have very similar diffusion coefficients clearly indicating a monomeric structure for 3 in solution.
Scheme 2. Synthesis of aluminum disiloxide 3.
The results of an X-ray data analysis of 3 further confirm its monomeric nature and the tremendous steric protection of the tetracoordinated aluminum center (Figure 1). Single crystals of two monomeric forms, 3 a and 3 b, were grown from isopropanol. Both contain in the solid state one molecule of isopropanol weakly coordinating to the aluminum center [3 a: Al1 O3 182.4 pm; 3 b: Al1 O4 182.1 pm]. Form 3 a contains two additional highly disordered isopropanol molecules located in the periphery, while in 3 b the two isopropanol molecules are in proximity to the aluminum center, connected via an array of hydrogen bonds. Note that intermolecular donor adducts of neutral, monomeric aluminum isopropoxides are extremely rare,[13] and there are no examples of monomeric isopropanol adducts. Next, 3 and the commercially available simple alkoxides Al(OiPr)3, Al(OsecBu)3, Al(OtBu)3 and AlMe3 were tested as catalysts in the MPV reduction. Cyclohexanone as a model substrate and 5 equiv of dry iPrOH as the reducing agent (Table 1) were used under equilibrium conditions (sealed scintillation vial in a glove box). Catalyst 3 (0.5 mol %) catalyzed the quantitative reduction of cyclohexanone within 4–6 h, while 1 mol % of Al(OiPr)3, Al(OsecBu)3, Al(OtBu)3 and AlMe3, respectively, reduced only about 30–80 % of cyclohexanone after 24 h. The observation that 2 only slowly converts into 3 (t1/2 ~ 24 h) at room temperature and that by far not all of catalyst 3 was dissolved in iPrOH encouraged us to further reduce the catalyst loading under otherwise similar conditions. The results revealed exceptional activity of 3 even at loadings of 0.05 mol %; TOF’s of up to 103 were achieved at 60 8C (entry 10). To demonstrate the efficiency and scope of the MPV reduction with 3 as the catalyst a range of aromatic and aliphatic aldehydes were reduced in the presence of excess isopropanol (Table 2). Excellent catalyst activities were noted for most of Chem. Eur. J. 2014, 20, 14959 – 14964
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Figure 1. Solid-state structures of 3 a (top, disordered iPrOH molecules are not shown) and 3 b (bottom). All H atoms are omitted for clarity (black = carbon, dark gray = silicon). Selected distances [pm] and angles [8]: 3 a: Si1 O1 162.5(2), Si8 O2 163.3(2), Al1 O1 171.0(2), Al1 O2 171.0(2), Al1 O3 182.4(2), Al1 O4 174.5(2), O3 C31 145.5(4), O4 C34 143.1(4), O1-Al1-O2 115.4(1), O4Al1-O3 104.4(1), Si1-O1-Al1 144.2(1), Si8-O2-Al1 140.1(1); 3 b: Si1 O1 163.6(2), Si8 O2 163.5(2), Al1 O1 170.8(2), Al1 O2 170.6(2), Al1 O3 176.6(2), Al1 O4 182.1(2), O2-Al1-O1 113.1(1), O3-Al1-O4 97.4(1), Si1-O1-Al1 142.0(1), Si8-O2Al1 143.9(1).
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Table 1. Aluminum-catalyzed MPV reduction of cyclohexanone.[a]
Entry
Catalyst
Loading [mol %]
Yield [%][b]
t [h]
TON
TOF [h 1]
Yield [%][c]
1 2 3 4 5 6 7 8 9 10
Al(OiPr)3 Al(OsecBu)3 Al(OtBu)3 AlMe3 AlMe3 3 3 3 3 3
1 1 1 1 0.5 0.5 0.1 0.1[d] 0.05 0.05[d]
10 32 32 35 22 96 90 98 25 94
10 5 5 4.5 4.5 4.5 4.5 2 2 2
10 32 32 35 44 192 900 980 500 1880
1 6 6 8 10 43 200 490 250 940
20 48 50 80 35 > 99 > 99 > 99 > 99 > 99
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner); conditions: room temperature, 5 equiv iPrOH, no co-solvent; [b] determined by NMR spectroscopy; [c] final yields determined by NMR spectroscopy after 24 h; [d] 60 8C.
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Communication the aldehydes and almost all of them could fully be converted to the corresponding primary alcohols. Astonishingly, undesired side reactions such as Tishchenko and aldol reactions were not seen with these reduction conditions, which is remarkable given the susceptibility of aliphatic and aromatic aldehydes to readily undergo ester formation via Tishcenko reaction. As expected, the more electron-rich 4-methoxy-, 2-methoxyand 2,3-dimethoxy substituted benzaldehydes (entries 16–18) reacted more slowly than 4-nitro- and 4-cyano-benzaldehydes (entries 12, 13). Other functional groups were also tolerated as demonstrated for the quantitative reductions of thiophene-, furan- and 3-pyridine-carboxaldehydes (entries 14, 19–21). The fact that 2-pyridine-carbaldehyde was not reduced at all appears to be due to its excellent chelating properties, leading to the deactivation of the catalyst by irreversible binding to the aluminum center. Astonishingly, challenging a,b-unsaturated aldehydes such as cinnamaldehyde, myrtenal and citral (entries 23–25) can selectively be reduced, albeit under more forcing conditions and with slightly higher catalyst loadings. Under classical MPV conditions, reductions of most aromatic ketones to secondary alcohols are more difficult to achieve, thus often requiring elevated temperatures to readily equilibrate and a large excess of reducing agent to shift the equilibrium in favor of the products. Increasing the iPrOH to catalyst ratio, on the other hand, may suppress substrate binding and hence lowers the reaction rate. Therefore, we were pleased to see that a variety of differently substituted acetophenones
could almost completely be reduced with 3 as the catalyst system (Table 3). Again, the electron-withdrawing 4-nitro and 4-CF3 substituted acetophenones (entries 26, 27) reacted faster than the electron-rich 2-OMe and 4-OMe substituted acetophenones as well as acetyl-thiophene and -furan (entries 32, 41, 45, 46). Astonishingly, even the notoriously difficult 2,6-dimethoxy-acetophenone (entry 42)[8g] could also quantitatively be reduced. The bulky a,a,a-trifluoro- and a,a,a-trimethyl-acetophenones (entry 37 and 38) were more challenging and required extensive heating to achieve moderate to good yields of the corresponding alcohols. The ability to conveniently reduce cyclic and acyclic aliphatic ketones to secondary alcohols under mild conditions is of high importance in natural product synthesis. Therefore, a range of cyclic ketones of various size and cyclohexanone derivatives that contain functional groups were screened as these structural motifs are incorporated in many drugs and natural products (Table 4). S-, O- and N-containing cyclohexanones can smoothly and quantitatively be reduced (entries 47–51). Various aliphatic ketones of different steric profile could also readily be reduced (entries 52–57). Note that the sterically hindered pinacolone (entry 55) converted quantitatively to the corresponding alcohol, while at least 50 % of the extremely bulky diisopropylketone (entry 56) converted to the corresponding alcohol clearly demonstrating that even sterically challenging substrates can be reduced with the MPV methodology. Furthermore, 5a-cholestane-3-one, a synthetic intermediate in the chemistry of steroids, was tested and also found to be
Table 2. Aluminum siloxide catalyzed MPV reduction of aromatic and aliphatic aldehydes to primary alcohols.[a] Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
11
0.2
5
24
50
> 99
19
0.5
25
6
50
> 99
12
0.2
5
24
40
> 99
20
0.5
25
6
50
> 99
13
0.2
5
24
40
> 97
21
0.7
25
24
60
> 99
14
0.5
5
24
40
> 99
22
0.5
25
24
25
> 99
15
0.5
5
24
50
> 99
23
1
3
24
80
> 99
16
0.5
25
6
50
> 99
24
0.5
5
24
80
> 99
17
0.5
25
6
50
> 99
25
0.5
5
24
80
90
18
0.5
25
6
50
> 99
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by NMR spectroscopy.
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Communication Table 3. Aluminum siloxide catalyzed MPV reduction of aromatic ketones to secondary alcohols.[a] Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
26
0.5
25
24
50
98
27
0.5
25
24
50
28
0.5
25
24
29
0.5
25
30
0.5
31
Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
37
0.5
25
48
80
61
98
38
0.5
25
24
80
62
50
93
39
0.5
25
24
80
93
24
50
96
40
0.7
25
48
80
94
25
24
50
98
41
0.5
25
24
80
98
0.5
25
24
50
93
42
0.5
25
24
80
98
32
0.5
25
48
80
85
43
0.5
25
24
50
95
33
0.7
25
24
60
> 99
44
0.5
25
24
80
93
34
0.7
25
24
60
93
45
0.5
25
21
50
90
35
0.5
25
48
50
95
46
0.5
25
24
80
83
36
0.5
25
24
50
85
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by NMR spectroscopy.
reduced to the respective a,b-cholesterol (ratio a/b = 67:33) in almost quantitative yields under very mild conditions and with low catalyst loadings (Scheme 3). In contrast, under flow conditions with ZrO2 as catalyst at 130 8C only 60 % of the product was obtained.[8g] Encouraged by these results, the reduction of oxo-lanosterol was attempted as well. Although by far more sterically demanding than 5a-cholestane-3-one, complete reduction could be achieved at slightly higher temperature (ratio a/b = 25:75) to yield a,b-lanosterol, an important precursor in the enzymatic biosynthesis of cholesterol.[14] To further demonstrate the applicability of the newly developed catalyst system, 5a-cholestane-3-one, 2,3-dimethoxy-benzaldehyde, 2,6-dimethoxy-acetophenone, 1-boc-4-piperidone, citral and decanal were quantitatively reduced on a 1 g scale. The formed alcohols were isolated simply by vacuum distillation with isolated yields ranging from to 86 to 97 %; tedious purification by column chromatography was not necessary. In conclusion, the unprecedented activity and selectivity of a sterically encumbered and monomeric aluminum siloxide Chem. Eur. J. 2014, 20, 14959 – 14964
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Scheme 3. Catalytic MPV reduction of 5a-cholestan-3-one to cholestan-3-ol (top) and 3-oxo-lanosterol to a/b-lanosterol (bottom).
catalyst in the MPV reduction of a wide variety of aliphatic and aromatic aldehydes and ketones under mild conditions was demonstrated (52 examples). Reductions occurred with low
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Communication Table 4. Aluminum siloxide catalyzed MPV reduction of aliphatic ketones to secondary alcohols.[a] Entry
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
Entry
47
0.5
25
17
50
> 99
48
0.5
25
28
50
49
0.2
5
24
50
0.5
25
51
0.5
52
53
Product
3 [mol %]
iPrOH [equiv]
t [h]
T [8C]
Yield [%][b]
54
0.5
25
24
60
> 99
> 99
55
0.5
25
48
50
94
50
> 99
56
1.0
5
24
80
53
48
50
98
57
0.5
25
21
50
89
25
6
50
> 99
58
0.5
25
24
50
94
0.2
5
24
50
> 99
59
0.5
25
24
50
97
0.5
25
24
50
90
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by NMR spectroscopy.
catalyst loadings (0.1–1.0 mol %) under solvent-“free” conditions, and, moreover, the great majority of substrates could be utilized as received and without distillation prior to use. Clearly, the high activity of catalyst 3 is a result of its monomeric nature and the fact that the weakly binding isopropanol is readily replaced by the incoming carbonyl substrate, facilitating the rate determining hydride transfer. Finally, the present work suggests that monomeric aluminum isopropoxide catalysts may be attractive alternatives to transition metal based catalyst systems for the selective reduction of aldehydes and ketones to primary and secondary alcohols.
Acknowledgements The authors would like to acknowledge Texas Tech University for support and the NSF for funding the acquisition of a Jeol Eclipse 400 MHz NMR spectrometer (CRIF MU grant CHE1048553). We would like to thank Prof. Dr. W. David Nes (Texas Tech University) for kindly providing us with a sample of 3oxo-lanosterol. Keywords: aldehydes · aluminum · ketones · MPV reduction · siloxide [1] Y. Li, S. Yu, X. Wu, J. Xiao, W. Shen, Z. Dong, J. Gao, J. Am. Chem. Soc. 2014, 136, 4031 – 4039; b) P. O. Lagaditis, P. E. Sues, J. F. Sonnenberg, K. Y. Wan, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2014, 136, 1367 – 1380; c) S. Fleischer, S. Zhou, K. Junge, M. Beller, Angew. Chem. Int. Ed. 2013, 52, 5120 – 5124; Angew. Chem. 2013, 125, 5224 – 5228; d) G. Bauer, K. A. Kirchner, Angew. Chem. Int. Ed. 2011, 50, 5798 – 5800; Angew. Chem. Eur. J. 2014, 20, 14959 – 14964
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Received: August 25, 2014 Published online on October 5, 2014
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