Stereoselective S_N1-Type Reaction of Enols and Enolates

This paper is dedicated to the outstanding contribution of Herbert Mayr to physical organic chemistry

Abstract

Stereoselective alkylation of enolates represents a valuable and important procedure for accessing carbon–carbon-bond frameworks in natural and nonnatural product synthesis. Usually, activated electrophilic partners that react through an S_N2 mechanism are employed. To overcome the limitations due to reduced reactivity and steric hindrance, S_N1-type reactions can be considered a valid and practical alternative. Accessible enolates can be used in stereoselective (diastereo- or enantioselective) reactions with electrophilic carbenium ions, either used as stable reagents or generated in situ from suitable precursors. The results achieved in this active field are summarized in this review.

1 Introduction

2 Alcohols in S_N1-Type Reactions with Enolates

2.1 Enantioselective Reactions with Metal Complexes

2.2 Organocatalytic Enantioselective Reactions

3 Alcohols and Alcohol Derivatives in S_N1-Type Reactions with Enolates­: Enantioselective Reactions with Metal Enolates

4 Isolated Carbenium Ions in S_N1-Type Reactions with Enolates: Enantioselective­ Reactions with Metal Enolates

5 Miscellaneous

6 Conclusion

Key words

enolates - stereoselectivity - S_N1 reactions - carbenium ions - Mayr scale

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