Diastereoselective nucleophilic substitution reactions of oxasilacyclopentane acetals: Application of the "inside attack" model for reactions of five-membered ring oxocarbenium ions

The additions of nucleophiles to oxocarbenium ions derived from oxasilacyclopentane acetates proceeded with high diastereoselectivity in most cases. Sterically demanding nucleophiles such as the silyl enol ether of diethyl ketone add to the face opposite the C-2 substituent. These reactions establish the syn stereochemistry about the newly formed carbon-carbon bond. Small nucleophiles such as allyltrimethylsilane do not show this same stereochemical preference: they add from the same face as the substituent in C-2-substituted oxocarbenium ions. The stereoselectivities exhibited by both small and large nucleophiles can be understood by application of the "inside attack" model for five-membered ring oxocarbenium ions developed previously for tetrahydrofuran-derived cations. This stereoelectronic model requires attack of the nucleophile from the face of the cation that provides the products in their lower energy staggered conformations. Small nucleophiles add to the "inside" of the lower energy ground-state conformer of the oxocarbenium ion. In contrast, sterically demanding nucleophiles add to the inside of the envelope conformer where approach is anti to the C-2 substituent of the oxocarbenium ion, regardless of the ground-state conformer population.