Stereospecificity in Reactions of Allylstannanes with Aldehydes Explored by Electronic Structure Calculations

Electronic structure calculations at the 6-31G/B3LYP level have been used to identify stationary structures on the potential energy surfaces for reactions of 1-alkoxy- and 1-alkyl-alk-2-enylstannanes with aldehydes. The role of substituents at both the 1-position and at the tin center in determining the stereochemistry of the products and the size of the barriers to reaction have been explored. For alkyl substituents at the two positions, steric interactions lead to the cis-products being preferred, their preference increasing with the size of the substituents. Chloro-substitution at tin has both electronic and steric effects. The electron-deficient tin can form stronger bonding interactions, leading to barriers considerably lower than those for alkyl substitution. This is especially pronounced for 1-alkoxy substituents; only for the transition state leading to the cis-product is a six-coordinate tin arrangement found, with a correspondingly very low barrier.