Abstract
The enantioselective crotylation of aldehydes with 1,2-diaminochlorocrotylsilane reagents is effectively catalyzed by Sc(OTf)3. The one significant limitation on the utility of these reagents – substrate scope – has thus been addressed. The net result is the most comprehensive and highly practical method for enantioselective aldehyde crotylation yet advanced.
Due to the direct relevance of the products to important classes of natural products, the development of diastereo- and enantioselective aldehyde crotylation reactions has been the subject of an enormous amount of effort over the last three decades. The first generally highly diastereo- and enantioselective and practicable solution was advanced by Brown in 1986,1 and, remarkably, this chiral crotylborane methodology remains the most widely employed to the present day.2 Despite what might be assumed based on its near-total dominance of the market, the Brown method does suffer both from an inability to fully or even partly overcome the inherent diastereofacial bias of certain chiral aldehydes3 and from significant practical liabilities.4 Because of this, efforts to supplant it as the method of choice have continued,5 with only limited success.
Our previously reported crotylsilanes 1 and 2 are crystalline solids that may be prepared in bulk and stored, and that react with aldehydes at 0 °C over the course of ∼20 h to consistently provide enantioselectivities (93-99% ee) among the highest ever recorded for aldehyde crotylation reactions (Scheme 1).6 While these characteristics go a long way towards realization of a more practical and user-friendly system, the reagents suffer from one highly significant limitation: the substrate scope is exceedingly narrow with aromatic, unsaturated, and sterically hindered aliphatic aldehydes all giving moderate to low yields, or even, in many cases, no product at all. For example, reagents 1 and 2 completely fail to react productively or at all with 3, α-methylcinnamaldehyde, and 4.
Scheme 1.
Because the successful crotylation of aldehydes such as 4 is relevant to several current synthetic projects in our group, we recently decided to revisit this methodology and attempt to solve the reactivity problem. We had two relevant pieces of information at the outset: 1) protonation of our aminochlorosilane Lewis acids (by way of reaction with protic nucleophiles, which displace the chloride and generate an equivalent of HCl) leads to a significant boost in their reactivity,7 and 2) Lewis acids8 (Sc(OTf)3 is particularly effective) and Brønsted acids9 may be used to catalyze the reactions of allylboronates with aldehydes by binding or protonating one of the boronate oxygen atoms. Initial experiments quickly revealed that whereas both Brønsted and Lewis acids could effectively catalyze the reactions of 1 and 2 with aldehydes, the Brønsted acid-catalyzed reactions were generally less highly enantioselective. A broad screen of Lewis acids revealed that Sc(OTf)3 provided the best combination of high enantioselectivity and effective catalysis.10 Thus, treatment of α-methylcinnamaldehyde with reagents (S,S)-1 and (S,S)-2 and 5 mol% Sc(OTf)3 in CH2Cl2 at 0 °C for 1 h led to the isolation of products 5 and 6 as single diastereomers (≥40:1 dr) in 87% yield and 94% and 91% ee, respectively (Scheme 2).
Scheme 2.
The results of a survey of the crotylation reactions of three standard aldehydes using silanes (S,S)-1 and (S,S)-2 are compiled in Table 1. As shown, the yields and enantioselectivities are excellent across the board (in all cases the diastereoselectivity was ≥40:1), whereas the corresponding reactions without Sc(OTf)3 required significantly longer reaction times (∼20 h) and provided yields that were in the range of 52-83%.6
Table 1. Sc(OTf)3-Catalyzed Aldehdye Crotylation Reactions.
 | ||||
|---|---|---|---|---|
| entry | R | silane | yield (%) | ee(%) |
| 1 | PhCH2CH2 | (S,S)-1 | 96 | 96 |
| 2 | PhCH2CH2 | (S,S)-2 | 96 | 97 |
| 3 | Ph | (S,S)-1 | 94 | 93 |
| 4 | Ph | (S,S)-2 | 96 | 91 |
| 5 | (E)-PhCH=CH | (S,S)-1 | 96 | 95 |
| 6 | (E)-PhCH=CH | (S,S)-2 | 93 | 91 |
Complex chiral aldehydes such as 3 represent one of the more important classes of aldehydes for crotylation reactions, and any method that would lay claim to being a comprehensive solution should be able to provide for high levels of reagent control for all possible stereochemical permutations. Commonly employed Roche ester-derived aldehydes 7 and 811 were treated with (S,S)-1 and (S,S)-2 using the standard conditions outlined above, and in every case the reactions proceeded with excellent diastereoselectivity (≥97:3 dr, major diastereomer:sum of all minor diastereomers) corresponding to reagent control (Scheme 3). While these results were encouraging, it is also the case that aldehydes with a chiral center at the β-position (e.g. 3) can be more challenging, and indeed most of the cases where the Brown reagents cannot override the diastereofacial bias of a chiral aldehyde fall into this category.3b-e We therefore prepared aldehyde 9 and subjected it to reaction with (S,S)-1 and (R,R)-1 using the same reaction conditions as above. Although the reactions were slower, requiring ∼4 h at ambient temperature to proceed to completion, they delivered the products of reagent control (10a and 10b, respectively) in good yields and with excellent levels of diastereoselectivity. Product 10b is of particular interest because the Brown method cannot deliver this stereochemical permutation at all with a β-TBS protecting group on the aldehyde,3d and with better than 2:1 diastereoselectivity with a β-PMB protecting group.3c Aldehyde 11 was also subjected to crotylation with (S,S)-1 leading to the isolation of 12 in 86% yield and with 94:6 diastereoselectivity. The stereochemical array in 12 is another that may not be accessed highly selectively with the Brown method,12 further supporting the claim that the present method is possessed of greater scope and an ability to override the diastereofacial biases of some of the most difficult chiral aldehydes.
Scheme 3.
Finally, we examined the crotylation of aldehyde 4, one of the more sterically and electronically deactivated aldehydes relevant to polyketide natural product synthesis imaginable. Reaction with (R,R)-2 and 5 mol% Sc(OTf)3 (CH2Cl2, 23 °C, 3 h) led to the isolation of 13 in 81% yield and 97% ee (Scheme 4). Alternatively, the unpurified reaction mixture may be submitted to a Tamao oxidation/diastereoselective tautomerization reaction13 to provide 14 in 82% yield (with 8:1 diastereoselectivity for the stereocenter α to the ketone) and 97% ee. This one pot conversion of 4 to 14 explicates our interest in aldehydes like 4 and highlights the power of the methodology rapidly to deliver stereochemically complex polyketide fragments.
Scheme 4.
Although the main focus of this study is crotylation reactions, we have confirmed that, as expected, Sc(OTf)3 is an effective catalyst for the corresponding reactions of the parent allylsilane reagent 15.14 Thus, reaction of aldehydes 7 and 8 with (S,S)-15 and 5 mol% Sc(OTf)3 (CH2Cl2, 0 °C, 1 h) gave the illustrated products in 77 and 83% yields respectively and with >99:1 diastereoselectivity in both cases (Scheme 5).
Scheme 5.
In order fully to leverage the fact that the silane reagents are crystalline solids and render the experimental procedures as straightforward as possible, we have prepared mixtures of the crotylsilanes 1 and 2 and the allylsilanes 15 with Sc(OTf)3 in a 25:1 molar ratio for which we propose the general names EZ-CrotylMix and EZ-AllylMix (Scheme 6). The use of 650 mg of an EZ-CrotylMix (or 635 mg of an EZ-AllylMix) per 1.0 mmol of aldehdye corresponds to 1.1 equiv of the silane and 4.4 mol% of Sc(OTf)3. Thus, treatment of 1.0 mmol of α-methylcinnamaldehyde with 650 mg of (S,S)-cis EZ-CrotylMix (in CH2Cl2 at ambient temperature for 30 min) resulted in the isolation of 5 in 89% yield and 92% ee.15 The EZ-CrotylMix stoichiometry (in the form of (R,R)-trans EZ-CrotylMix) also sufficed for less reactive aldehdye 4, giving 13 in 85% yield and 97% ee. Of course, additional Sc(OTf)3 or EZ-CrotylMix may be added to the reactions of particularly unreactive aldehydes, but this EZ-CrotylMix/EZ-AllylMix formulation appears to be effective for most aldehydes.
Scheme 6.
Sc(OTf)3 is an effective catalyst for the enantioselective crotylation of aldehydes using crotylsilanes 1 and 2, and this has resulted in a dramatic increase in the scope of aldehydes that may be effectively crotylated using this methodology. Indeed, based primarily on the reactions of aldehydes 4, 7, 8, 9, and 11, we may conclude that our method has a broader scope than any other asymmetric crotylation methodology. That the silanes and Sc(OTf)3 are crystalline solids has further facilitated the EZ-CrotylMix formulation,16 rendering this methodology the most comprehensive and highly practical solution to the enduring problem of enantioselective aldehyde crotylation yet advanced.
Supplementary Material
Acknowledgments
This work was supported by a grant from the National Institutes of Health (NIGMS R01 GM58133). We gratefully acknowledge the National Science Foundation for acquisition of a 400 MHz Nuclear Magnetic Resonance Spectrometer (CRIF 0840451).
Footnotes
Supporting Information Available: Experimental procedures, characterization data, and stereochemical proofs. This material is available free of charge via the internet at http://pubs.acs.org.
References
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- 16.The allyl- and crotylsilanes and the EZ-CrotylMixes are commercially available from Sigma-Aldrich and Strem. The EZ-Allylmixes are in the late stages of development and will be available soon.
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