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Microwave-Assisted Efficient Thiolate-Catalysed Homo- and Crossed Intermolecular Tishchenko Reactions

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Microwave-Assisted Efficient Thiolate-Catalysed Homo- and Crossed Intermolecular Tishchenko Reactions CREATED USING THE RSC COMMUNICATION TEMPLATE (VER. 3.1) - SEE WWW.RSC.ORG/ELECTRONICFILES FOR DETAILS ARTICLE TYPE www.rsc.org/xxxxxx | XXXXXXXX Microwave-assisted efficient thiolate-catalysed homo- and crossed intermolecular Tishchenko reactions Cornelius J. O’ Connor, a Francesco Manoni, a Simon P. Currana and Stephen J. Connon*a Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X 5 First published on the web Xth XXXXXXXXX 200X DOI: 10.1039/b000000x Recently, the first efficient intermolecular crossed Tishchenko thiols, used in the presence of a Grignard reagent base.15 reactions were reported. The utility of these processes is curtailed 40 Homo-Tishchenko reactions involving either benzaldehyde or by long reaction times of up to 4 days (at reflux). Herein we (significantly) substituted analogues proceeded in good- 10 report that these reactions are highly susceptible to acceleration excellent yields with 5-20 mol% catalyst loading (Figure 1). by microwave irradiation – allowing fast, efficient, high-yielding Perhaps more significantly, considerable direct evidence coupling to proceed in 10-180 min. supporting the proposed reaction mechanism involving 2-4 as 45 intermediates (Figure 1) and a slow acyl-transfer step was 1,2 The Tishchenko reaction is a hydride-transfer mediated obtainable by 1H NMR spectroscopy. This mechanistic process related to (but potentially more synthetically useful information subsequently guided the development of thiol 3 15 than) the Cannizzaro reaction, where two aldehyde molecules precatalysts tailored to promote the first selective can undergo disproportionation, with subsequent coupling to intermolecular16 crossed-Tishchenko reactions between form a single ester product. This intriguing aldehyde-to-ester 50 aldehydes and activated ketones. transformation reaction has been known for over a century, The major drawback associated with this protocol was yet few practical applications of the process have been reaction rate – the reaction must be held at reflux temperature 20 developed and it is little-used as a synthetic strategy by for prolonged times: up to 4 days for cases involving contemporary chemists.4 recalcitrant substrates. Congnisant of the impact long reaction There are several limitations which have curtailed the 55 times will have on the widespread applicability and utility of utility and applicability of the reaction: several metal-based the reaction, we embarked on a programme aimed at catalytic systems for promoting the reaction have been increasing the reaction rate without resorting to the use of 25 devised,5,6,7,8,9,10 however the reaction is of poor substrate impractical catalyst loadings. scope: only ‘dimerisation’ of aldehydes was possible, yields of homo-Tishchenko esters using substituted benzaldehyde Table 1 Microwave-assisted homo-Tishchenko reactions substrates are often variable and until recently the coupling of O 8, 9 or 10 O aldehydes to ketones (in an intermolecular context) was (cat.), THF 30 unknown.4 MW irradiation O 6 110 °C 7 R SH PhMgBr H F3C N SH O O Ar O SH 1 Ar RSMgBr SH Ar O 8 9 10 5 60 BrMg slow O Entry Catalyst Loading Conc. Time Yield R a Ar S Conditions (mol%) (M) (min) (%) H 2 RSH (5-20 mol%) MgBr PhMgBr (5-20 mol%) Ar O THF, reflux 1 8 10 0.68 10 70 O 3 up to 96 h 2 9 10 0.68 10 64 1 O Ar 3 10 10 0.68 10 28 R Ar S 4 8 10 1.0 10 85 4 5 8 10 1.2 10 83 6 8 10 1.2 30 82 Figure 1. The thiolate catalysed homo-Tishchenko reaction 7 8 10 1.8 10 85 8b 8 5 1.8 30 92 (90) 9 8 5 1.8 10 85 Recently - inspired by the mode of action of the glycolytic 35 10 8 5 2.0 30 91 enzyme glyceraldehyde-3-phosphate dehydrogenase11,12,13 - a 1 we developed the first thiol(ate) catalysed Tishchenko Determined by H NMR spectroscopy using (E)-stilbene as an internal standard. bIsolated yield in parenthesis. reactions.14 The catalysts required are simple, inexpensive This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 1 The reaction is most efficient in ethereal solvents (THF was reaction time (18-20, entries 8-10). identified as the optimal medium studied in our initial 60 investigations17); thus we attempted to carry out the process in higher boiling ethers such as 2-methyltetrahydrofuran and 1,4- Table 2 Microwave-assisted intermolecular crossed-Tishchenko reactions 10 (20 mol%) 5 dioxane (at relux), without observing any rate acceleration. O O O CF3 PhMgBr (20 mol%) It was next decided to investigate the effect of microwave 1 2 1 2 R H R CF3 MW irradiation R OR radiation on the process. The results of our preliminary 110 °C experiments involving the microwave-aided homo-Tishchenko THF (1.2 M) reaction are outlined in Table 1. 10 We were very pleased to observe significant rate Entry Product Time Yield accelerations on irradiating to a maximum reaction (h) (%)a temperature of 110 °C under conditions otherwise identical to 14,18 those used in the original study. Under these conditions 4- tert-butylbenzylmercaptan (8 - at 10 mol% levels) could O CF3 1 3 82 15 promote the formation of benzyl ester 7 from benzaldehyde O (6) in good yield in only 10 minutes (entry 1). As expected, 11 catalysts 9 and 10, which previously proved useful in the O CF3 catalysis of crossed Tishchenko reactions, were less effective 2 3 81 19 O here (entries 2-3). 12 20 We next set about optimising the conditions associated with Cl the microwave-mediated process. Increasing the reaction O CF3 3 3 81 concentration (a desirable modification in its own right) O resulted a marked improvement in product yields: a 13 concentration of 1.0 – 1.2 molar allowed the formation of 7 in Br O CF3 25 yields >80% (entries 4-6), while using 1.8-2.0 M reaction 4 F 3 63 concentrations makes the isolation of 7 in >90% yield O 14 possible after 30 minutes reaction time with a reduced catalyst OCF3 loading of 5 mol% (entries 7-10). It is noteworthy that a b 5 S 3 51 minimum of 10 mol% catalyst loading was required to achieve O 30 a product yield of >90% in the corresponding thermal process 15 14 reported previously. Attempted reactions at concentrations OCF3 higher than 2.0 M led to poor solubility and inconsistent 6 3 68 O results. Attention now turned to the more challenging crossed- 16 Cl 35 Tishchenko process (Table 2). In these reactions, acyl-transfer OCF3 7 3 0 between the thioester intermediate and the ketone-derived O alkoxide (after hydride transfer) is rate-determining; therefore 17 Cl use of the substituted thiophenol catalyst 10 (which results in OCF3 a more electrophilic thioester intermediate which undergoes 8 Cl 2 73 O 40 more rapid acyl-transfer) is required. Treatment of a 1:1 mixture of benzaldehyde and 2,2,2- 18 OCF3 trifluromethylacetophenone in THF under microwave 9 Cl 3 83 irradiation in the presence of 10 (20 mol%) resulted in the O formation of the coupled product 11 in high yield (entry 1) 19 Cl 45 after only 3 hours. The reaction proceeded chemoselectively – OCF3 1H NMR spectroscopic analysis did not detect any homo- 10 3 76 O Tishchenko product (i.e. 7) in the crude reaction mixture. Benzaldehyde could also be reacted with halo-substituted F 20 Cl O CF activated ketones to furnish 12-14 without difficulty (entries 3 11 3 56c O 50 2-4). The (relatively) deactivated 2-thiophenyl substituted ketone proved a more challenging substrate which afforded 21 ester 15 in 51% yield (entry 5). Substituted benzaldehydes OCF 3 c,d also participate in the process – use of 2-naphthaldehyde gave 12 3 34 O 16 in good yield (entry 6), however the highly hindered 55 mesitaldehyde appears to be beyond the orbit of this 22 aIsolated yields. bDetermined by 1H NMR spectroscopy using (E)- methodology and failed to produce 17 under these conditions stilbene as an internal standard. cUsing 2.0 equiv. of the ketone. (entry 7). Smooth coupling was observed in cases involving 65 dIsolated as a 4:1 mixture of diastereomers. halobenzaldehydes and trifluoromethylketones inside 3 hours This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 2 Of particular interest is the coupling of an aliphatic aldehyde (entry 11) with trifluromethylacetophenone. This 4 For recent reviews see: (a) T. Seki, T. Nakajo and M. Onaka, Chem. reaction is complicated by competing deleterious thiolate- Lett., 2006, 35, 824. (b) O. P. Törmäkangas and A. M. P. Koskinen, catalysed aldol pathways, however in the presence of a modest Recent. Res. Dev. Org. Chem., 2001, 5, 225. 5 For the use of aluminium alkoxides see: (a) W. C. Child and H. 5 excess of the ketone (entry 11), a substantial yield of the Adkins, J. Am. Chem. Soc., 1925, 47, 798. (b) F. J. Villani and F. coupled product 21 could be isolated. The use of the more Nord, J. Am. Chem. Soc., 1947, 69, 2605. (c) L. Lin and A. R. Day, J. enolisable 2-phenylpropanal led to the formation of 22 with Am. Chem. Soc., 1952, 74, 5133.(d) T. Saegusa and T. Ueshima, J. reduced product yield as a micture of diastereomers (entry Org. Chem., 1968, 33, 3310. (e) T. Ooi, T. Miura, K. Takaya, K. 12). To the best of our knowledge these represent the first Maruoka, Tetrahedron Lett., 1999, 40, 7695. (f) T. Ooi, K. Ohmatsu, K. Sasaki, T. Miura and K.
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