DOI: 10.1002/chem.201404294

Communication

& Domino Reactions | Hot Paper |

Domino Hydroformylation/Aldol Condensation/Hydrogenation Catalysis: Highly Selective Synthesis of Ketones from Olefins Xianjie Fang,[a] Ralf Jackstell,[a] Armin Bçrner,[a, b] and Matthias Beller*[a] Abstract: A general and highly chemo- and regioselective synthesis of ketones from olefins by domino hydroformylation/aldol condensation/hydrogenation reaction has been developed. A variety of olefins are efficiently converted into various ketones in good to excellent yields and regioselectivities in the presence of a specific rhodium phosphine/base–acid catalyst system.

Ketones constitute one of the most important classes of organic compounds. The keto group is found in numerous natural products as well as in literally thousands of synthetic compounds. Therefore, ketones are much sought-after chemical compounds on account of their many possible uses as intermediates and industrial products in fine and bulk chemistry.[1] Due to their importance, numerous methodologies have been developed for their preparation.[2] For example, oxidation of secondary alcohols is one of the most frequently used methods.[3] In addition, the Tsuji–Wacker oxidation is a powerful method for the preparation of ketones from olefins.[4] For the dehydrogenation of alcohols, ruthenium, rhodium, or iridium catalysts have been suggested as catalysts in the presence or even in the absence of hydrogen acceptors.[5] Hydration of non-terminal alkynes or conjugated diolefins in the presence of a catalyst (e.g. ruthenium) is another useful method for the preparation of ketones.[6] Recently, the efficient and selective production of methyl ketone by economically and environmentally friendly photo-irradiation of an acetone/water mixture containing olefins has been reported.[7] Most of these elegant developments are of interest for smaller-scale applications, due to the price of the starting materials, reagents, and/or catalyst performance. Thus, considerable interest remains in novel ketone processes that might be applied on bulk scale, too. Considerable improvements in the large-scale preparation of ketones might be achieved by employing relatively cheap feedstocks, such as olefins and synthesis gas (CO/H2). In this respect, the hydroformylation of olefins [a] X. Fang, Dr. R. Jackstell, Prof. Dr. A. Bçrner, Prof. Dr. M. Beller Leibniz-Institut fr Katalyse e.V. an der Universitt Rostock Albert-Einstein-Str. 29a, 18059 Rostock (Germany) E-mail: [email protected] [b] Prof. Dr. A. Bçrner Institut fr Chemie der Universitt Rostock Albert-Einstein-Str. 3a, 18059 Rostock (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404294.’ Chem. Eur. J. 2014, 20, 15692 – 15696

has been known for 75 years.[8] The reaction is well investigated and constitutes one of the largest homogeneously catalyzed processes in industry. In fact, originally this reaction was named the oxo reaction, because of the formation of both aldehydes and ketones using ethylene and synthesis gas at high temperature and high pressure. However, soon after the discovery of the reaction it was realized that it mainly delivers aldehydes. Herein, we present a general synthesis of ketones from olefins, synthesis gas and acetone or related substrates. Combination of a specific rhodium(I) phosphine complex and pyrrolidinium benzoate creates an efficient catalyst system for practical domino hydroformylation/aldol condensation/hydrogenation reactions. Both industrially and synthetically important olefins are selectively transformed into the desired ketones in good to excellent yields and regioselectivities. Following a general trend in green chemistry,[9] hydroformylation reactions have become an integral part of tandem or domino reaction sequences which avoids the formation of significant amounts of waste. Known examples combine hydroformylation[10] with subsequent aldol reaction,[11] hydrogenation[12] or amination.[13] In general, in such processes, substrates have been applied that avoid the regioselectivity problems of the hydroformylation step. With respect to ketones, it is interesting to note that Breit and co-workers also reported a procedure for their synthesis from alkenes by a domino hydroformylation/Wittig olefination/hydrogenation sequence.[14] Despite this elegant development, the overall atom economy of this process is low, due to the Wittig olefination step. Obviously, a direct coupling using ketones is more appealing, with water being the only by-product (Scheme 1). Herein, we disclose a straightforward and atom-efficient domino synthesis of a variety of alkyl ketones. Crucial for the success of this sequence is the precise control of both chemoand regioselectivity by the respective [Rh(CO)2(acac)]/Naphos (L1) catalyst. Furthermore, the combination of this organometallic catalyst with an optimal acid–base combination prevents unwanted self-condensation processes, which have often been observed in previous aldol-type reactions. To develop a selective and active catalytic system for a general synthesis of alkyl ketones from olefins, the reaction of 1octene (1 a) with acetone (2 a) to give 2-dodecanone (3 aa) was chosen as a model system. Initial attempts to identify the optimal parameters for the ketone formation under carbonylation conditions were carried out in the presence of [Rh(CO)2(acac)] and Naphos (L1). This ligand was previously use in our group and allows for highly regioselective rhodium-catalyzed hydroformylation reactions.[15] In general, catalytic experiments

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Communication

Scheme 1. Synthesis of ketones from olefins via hydroformylation reactions.

were performed at 65 8C in the presence of 0.1 mol % of [Rh(CO)2(acac)] and 0.2 mol % of L1 under 10 bar of syngas. In order to control the aldol condensation step appropriately, several bases and acids were tested. In the presence of simple NaOH, conversion of 1-octene (1 a) was low (Table 1, entry 1). Unfortunately, unlike in previous reports,[11] only 20 % of the desired product 3 aa was formed using a standard catalyst for aldol reactions (l-proline; Table 1, entry 2). The addition of pyrrolidine resulted in moderate yield of nonanal (4 a), but still a very low yield of the desired product 3 aa was observed (Table 1, entry 3). Moreover, we investigated the influence of several acids in this reaction step. As an example, benzoic acid showed no activity for the aldol condensation step (Table 1, entry 4). Since the formation of more reactive enamines is catalyzed by acids,[16] next we applied a combination of pyrrolidine and benzoic acid as a cocatalyst system. Gratifyingly, the desired saturated ketone product 3 aa was obtained in 65 % yield and with high regioselectivity (Table 1, entry 5). Using 5 mol %

Table 1. Domino hydroformylation/aldol condensation/hydrogenation of 1-octene (1 a) with acetone (2 a): Variation of reaction conditions.[a]

Entry

Cocatalyst

Ligand

Conversion[b]

1 2 3 4 5 6 7 8 9 10 11 12[d] 13[e] 14[f]

NaOH (10 mol %) L-proline (10 mol %) pyrrolidine (10 mol %) benzoic acid (10 mol %) pyrrolidine (10 mol %) and benzoic acid (10 mol %)

L1 L1 L1 L1 L1 L1 L2 L3 L4 L5 L1 L1 L1

7% 38 % 63 % 97 % 73 % 75 % 16 % 98 % 98 % 70 % 98 % 85 % 98 % 98 %

pyrrolidine (5 mol %) and benzoic acid (5 mol %)

4 a yield (n/iso)[c]

3 aa yield (n/iso)[c]

5 aa yield (E/Z)[c]

4 % (89:11) Trace 45 % (95:5) 50 % (79:21) Trace 0% 11 % (82:18) Trace 6 % (26:74) Trace 7 % (82:18) 0% 0% 0%

Trace 20 % (98:2) 8 % (98:2) 0% 65 % (98:2) 68 % (98:2) 0% 86 % (74:26) 70 % (63:37) 29 % (98:2) 65 % (75:25) 76 % (98:2) 86 % (98:2) 92 % (98:2)

Trace 4 % (> 99:1) Trace 0% 3 % (> 99:1) 2 % (> 99:1) Trace 0% 3 % (> 99:1) 18 % (> 99:1) 3 % (> 99:1) Trace 0% 0%

[a] Reaction conditions: 1 a (1.5 mmol), [Rh(CO)2(acac)] (0.1 mol %), monodentate ligand (1.0 mol %), bidentate ligand (0.2 mol %), CO/H2 (10 bar), acetone (2 mL), 65 8C, 24 h; [b] conversion determined by GC analysis using isooctane as the internal standard; [c] yields and the ratios of isomers determined by GC analysis using isooctane as the internal standard; [d] reaction temperature: 100 8C; [e] reaction conditions: 100 8C, CO/H2 (5:15 bar); [f] reaction conditions: 100 8C, CO/H2 (5:15 bar), acetone (3 mL). Chem. Eur. J. 2014, 20, 15692 – 15696

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Communication of acid–base catalyst resulted in a similar yield of 3 aa (Table 1, entry 6). Having a suitable catalyst system for the aldol condensation step in hand, we elaborated the influence of different ligands on this domino reaction. Low conversions and no corresponding ketones were observed in the absence of any phosphorus ligands (Table 1, entry 7). When standard monodentate phosphine or phosphite ligands were used, good yields of the desired product 3 aa were obtained, but unfortunately only low n-regioselectivities (Table 1, entries 8 and 9). Applying Xantphos (L4) as ligand afforded a low yield of the desired product 3 aa along with a significant amount of unsaturated ketone 5 aa (Table 1, entry 10). A good yield of 3 aa with low n-regioselectivity was obtained using the highly active and selective bidentate phosphite ligand BIPHEPHOS (L5) for the hydroformylation step, which might be explained by its instability under acidic conditions (Table 1, entry 11). Hence, only the combination of Naphos (L1) with pyrrolidine/benzoic acid allowed control of both regio- and chemoselectivity. Furthermore, we evaluated the influence of critical reaction parameters such as gas pressure, reaction temperature, and the concentration of acetone in the presence of L1 as ligand. An excellent yield of the desired product 3 aa with high n-regioselectivity was obtained by increasing the H2 pressure, reaction temperature and the amount of acetone (Table 1, entries 12–14). To understand this reaction sequence in more detail, the progress of the hydroformylation/aldol condensation/hydrogenation of 1-octene (1 a) with acetone (2 a) was examined under the optimized reaction conditions. Simultaneous decrease of 1 a and increase of the desired product 3 aa took place over the period of 200 mins reaction time (Figure 1 ). Throughout the course of the reaction, the initial hydroformylation product 4 a and the unsaturated ketone 5 aa were pres-

Figure 1. Compound distribution of the domino hydroformylation/aldol condensation/hydrogenation sequence. Reaction conditions: 1-octene (7.5 mmol), [Rh(CO)2(acac)] (0.1 mol %), L1 (0.2 mol %), pyrrolidine (5 mol %) and benzoic acid (5 mol %), CO/H2 (5:15 bar), acetone (10 mL), 100 8C, 200 min. Chem. Eur. J. 2014, 20, 15692 – 15696

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Table 2. Reaction of various olefins (1) with synthesis gas and acetone (2 a).[a]

Entry

1

3

Yield (n/iso)[b]

1

89 % (98:2)[c]

2

78 %[c]

3

71 % (> 99:1)[c]

4

89 % (98:2)[c]

5

89 % (98:2)[c]

6

84 % (98:2)

7

86 % (98:2)

8

77 % (> 99:1)

9

85 % (99:1)

10

92 % (99:1)

11

85 % (> 99:1)

12

93 % (> 99:1)[c]

[a] Reaction conditions: 1 (15 mmol), [Rh(CO)2(acac)] (0.1 mol %), Naphos (0.2 mol %), pyrrolidine (5 mol %), and benzoic acid (5 mol %), CO/H2 (5:15 bar), acetone (30 mL), 100 8C, 5 h; [b] yields of isolated product and the ratios of linear to branched isomers were determined by GC analysis; [c] yields and the ratios of linear to branched isomers were determined by GC analysis.

ent only as minor components (< 5 %). Apparently, the hydroformylation of 1-octene is the rate determining step in this hydroformylation/aldol condensation/hydrogenation reaction. With the optimized reaction conditions established (Table 1, entry 14), we examined the generality of this novel domino process using various olefins (Table 2). Both short- and longchained terminal aliphatic olefins 1 a–1 g provided the corresponding saturated ketones in good yields with high regioselectivities (n/iso ratios = > 98:2; Table 2, entries 1–7). Sterically hindered olefin 1 h was also smoothly transformed into the corresponding ketone in good yield with excellent regioselectivity (Table 2, entry 8). Gratifyingly, 1 i and 1 j, with different olefinic groups present in the substrate, showed high selectivity for the functionalization of the terminal double bond (Table 2, entries 9 and 10). Notably, these products can be easily refined further on using known hydroformylation cata-

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Communication lysts. Moreover, functional groups such as amine and ether were well tolerated and the corresponding functionalized ketones were achieved in good yields with excellent regioselectivities (Table 2, entries 11 and 12). Notably, the corresponding double alkylation of acetone products did not occur in any of these cases,[17] which is attributed to the low concentration of the initial hydroformylation product throughout the course of the reaction (Figure 1). Finally, we evaluated the possibility to alkylate different ketones using 1-octene 1 a as a standard coupling partner. Generally, various aliphatic and aromatic ketones underwent efficient transformation to afford the corresponding saturated ketones in good to excellent yields with high regioselectivities (Table 3). Using 2-butanone 2 b demonstrated the high chemoselectivity of the reaction sequence. Hence, functionalization only took place on the methyl group and not on the ethyl part (Table 3, entry 1). Excellent chemoselectivity was also observed

Table 3. Reaction of 1-octene (1 a) with synthesis gas and different ketones (2).[a]

Entry

2

3

1

for 2,5-hexanedione (2 e), which led to monofunctionalization (Table 3, entry 4). Moreover, functional groups including ester (Table 3, entry 3) and halides (Table 3, entries 6 and 7) are well tolerated, which provided useful handles for further synthetic transformations. The alkylation of easily available methyl levulinate (2 d) is also of interest as an easy tool for further valorization of biomass (Table 3, entry 3). Similarly, heterocyclic ketone 2 j proved also to be an efficient coupling partner to generate the corresponding ketone in good yield (Table 3, entry 9). In summary, we developed a highly selective intermolecular domino hydroformylation/aldol condensation/hydrogenation reaction sequence that allows for an efficient synthesis of ketones. The generality of this process was demonstrated in the synthesis of more than 20 different ketones. Key to success for this transformation was the use of a specific cooperative rhodium/phosphine and organocatalytic system and the inherently low concentration of the aldehyde formed in the hydroformylation step. Our “green” synthetic protocol is straightforward, atom-efficient, and does not need stoichiometric amounts of additives or base. Because of the importance of ketones in organic chemistry and the chemical industries, we believe this practical synthetic strategy will be of value for the preparation of laboratory building blocks as well as industrial products such as flavors and fragrances.

Yield (n/iso)[b] 87 % (98:2)

2

91 % (98:2)

3

78 % (98:2)

4

75 % (98:2)

5

95 % (98:2)

6

95 % (98:2)

Experimental Section General procedure for the preparation of compounds 3: A 100 mL steel autoclave was charged under argon atmosphere with [Rh(CO)2(acac)] (3.87 mg, 0.1 mol %), Naphos (L1; 19.52 mg, 0.2 mol %), and benzoic acid (91.5 mg, 5 mol %). Then, ketone 2 (30 mL), pyrrolidine (62.5 mL, 5 mol %), and olefin 1 (15 mmol) were added under argon atmosphere. The autoclave was pressurized with 10 bar CO/H2 (1:1), 10 bar H2 and heated to 100 8C for 5 h. After the reaction time, the autoclave was cooled with ice water and the pressure was released. The regioselectivity was measured by GC analysis of the crude reaction mixture. After removing the solvent by vacuum, the residue was directly purified by flash chromatography on silica gel (eluent: heptane/ethyl acetate, 30:1) to give the desired product 3 (3 ka and 3 ad–3 aj were purified by bulb-to-bulb distillation).

Acknowledgements 7

96 % (98:2)

8

96 % (98:2)

9

90 % (98:2)

[a] Reaction conditions: 1 a (3 mmol), [Rh(CO)2(acac)] (0.1 mol %), Naphos (0.2 mol %), pyrrolidine (5 mol %) and benzoic acid (5 mol %), CO/H2 (5:15 bar), 2 (6 mL), 100 8C, 5 h; [b] yields of isolated product and the ratios of linear to branched isomers were determined by GC analysis.

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This research was founded by the Bundesministerium fr Bildung und Forschung (BMBF) and the State of MecklenburgVorpommern. We thank Dr. Christine Fischer, Susann Buchholz and Susanne Schareina for their technical and analytical support. We are grateful to Dr. Abhishek Dutta Chowdhury for helpful discussions. Keywords: domino reactions · homogeneous catalysis · hydroformylation · ketones · olefins [1] H. Siegel, M. Eggersdorfer, “Ketones” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.

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[12] For reviews on hydroaminomethylation, see: a) P. Eilbracht, A. M. Schmidt in Transition Metals for Organic Synthesis (Eds.: M. Beller, C. Bolm), Wiley-VCH Verlag GmbH, New York, 2008, pp. 57 – 82; b) D. Crozet, M. Urrutigoı¨ty, P. Kalck, ChemCatChem 2011, 3, 1102. [13] For selected examples of hydroxymethylation, see: a) A. J. Sandee, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. van Leeuwen, J. Am. Chem. Soc. 2001, 123, 8468; b) I. I. F. Boogaerts, D. F. S. White, D. J. Cole-Hamilton, Chem. Commun. 2010, 46, 2194; c) O. Diebolt, C. Muller, D. Vogt, Catal. Sci. Technol. 2012, 2, 773; d) L. L. W. Cheung, G. Vasapollo, H. Alper, Adv. Synth. Catal. 2012, 354, 2019; e) D. Konya, K. Q. Almeida Lenero, E. Drent, Organometallics 2006, 25, 3166; f) K.-i. Tominaga, Y. Sasaki, J. Mol. Catal. A 2004, 220, 159; g) L. Diab, T. Sˇmejkal, J. Geier, B. Breit, Angew. Chem. Int. Ed. 2009, 48, 8022; Angew. Chem. 2009, 121, 8166; h) D. Fuchs, G. Rousseau, L. Diab, U. Gellrich, B. Breit, Angew. Chem. Int. Ed. 2012, 51, 2178; Angew. Chem. 2012, 124, 2220; i) K. Takahashi, M. Yamashita, T. Ichihara, K. Nakano, K. Nozaki, Angew. Chem. Int. Ed. 2010, 49, 4488; Angew. Chem. 2010, 122, 4590; j) K. Takahashi, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2012, 134, 18746; k) L. Wu, I. Fleischer, R. Jackstell, I. Profir, R. Franke, M. Beller, J. Am. Chem. Soc. 2013, 135, 14306; l) I. Fleischer, K. M. Dyballa, R. Jennerjahn, R. Jackstell, R. Franke, A. Spannenberg, M. Beller, Angew. Chem. Int. Ed. 2013, 52, 2949; Angew. Chem. 2013, 125, 3021. [14] B. Breit, S. K. Zahn, Angew. Chem. Int. Ed. 1999, 38, 969; Angew. Chem. 1999, 111, 1022. [15] a) H. Klein, R. Jackstell, K. D. Wiese, C. Borgmann, M. Beller, Angew. Chem. Int. Ed. 2001, 40, 3408; Angew. Chem. 2001, 113, 3505; b) H. Klein, R. Jackstell, M. Beller, Chem. Commun. 2005, 2283; c) A. Seayad, M. Ahmed, H. Klein, R. Jackstell, T. Gross, M. Beller, Science 2002, 297, 1676; d) M. Ahmed, A. M. Seayad, R. Jackstell, M. Beller, Angew. Chem. Int. Ed. 2003, 42, 5615; Angew. Chem. 2003, 115, 5773; e) M. Ahmed, R. Jackstell, A. M. Seayad, H. Klein, M. Beller, Tetrahedron Lett. 2004, 45, 869. [16] a) M. E. Kuehne, J. Am. Chem. Soc. 1959, 81, 5400; b) T. A. Spencer, H. S. Neel, D. C. Ward, K. L. Williamson, J. Org. Chem. 1966, 31, 434; c) R. B. Woodward, Pure Appl. Chem. 1968, 17, 519; d) Z. G. Hajos, D. R. Parrish, J. Org. Chem. 1974, 39, 1612; e) N. Mase, F. Tanaka, C. F. Barbas III, Org. Lett. 2003, 5, 4369. [17] C. Bee, S. B. Han, A. Hassan, H. Iida, M. J. Krische, J. Am. Chem. Soc. 2008, 130, 2746.

Received: July 8, 2014 Published online on October 21, 2014

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hydrogenation catalysis: highly selective synthesis of ketones from olefins.

A general and highly chemo- and regioselective synthesis of ketones from olefins by domino hydroformylation/aldol condensation/hydrogenation reaction ...
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