Chiral Silacycles for Enantioselective Allylation and Crotylation Reactions
By: Aaron Thornton, Chemfiles Volume 11 Article 1
The asymmetric allylation and crotylation of aldehydes and other carbonyl compounds remains one of the most fundamental reactions for the construction of chiral building blocks. While numerous methods for this challenging task have been examined previously, including the use of chiral auxiliaries, chiral reagents and catalytic systems, still today a truly convenient and broadly reaching method remains elusive. With this goal in mind, the group of Professor James Leighton has developed a versatile system that has proven to be uniquely effective. Leighton and co-workers have harnessed the power of strained silacycles for use as allylation reagents without the need for any further catalysts or reagents (Scheme 1).
Scheme 1: Leighton’s chiral allyl silanes.
These bench-stable and non-toxic reagents undergo enantioselective addition to a range of carbonyl compounds, including aldehydes, ketones,and hydrazones1 (Table 1). Notably, all of these reactions are carried out at convenient reaction temperatures without the need for external activating reagents, thereby simplifying reaction set-up and manipulation.
Table 1: Enantioselective allylation of various aldehydes and hydrazones with (706671).
In addition to enantioselective allylation reactions, Leighton and co-workers have extended this concept to the enantioselective crotylation of carbonyl compounds2 (Table 2). Importantly, these diamine derived silacycles are bench-stable crystalline solids, providing the added benefit of simple reaction setup and purification.
Table 2: Enantioselective crotylation of aldehydes.
- (a) Kinnaird, J. W. H.; Ng, P. Y.; Kubota, K.; Wang, X.; Leighton, J. L. J.Am. Chem. Soc. 2002, 124, 7920. (b) Berger, R.; Duff , K.; Leighton, J. L. J. Am. Chem. Soc. 2004, 126, 5686.
- Hackman, B. M.; Lombardi, P. J.; Leighton, J. L. Org. Lett. 2004, 23, 4375.