Studies on the Mechanism of Allylmetal-Acetal Additions

Abstract

The mechanism and stereochemical course of addition of allylsilanes and allylstannanes to acetals has been investigated. Using an intramolecular process to model the possible transition states the reaction stereochemistry has been shown to depend on the nature of the Lewis acid and acetal structure, but not on the allylmetal. Trimethylsilyl tritiate, triflic acid and BF3·OEt2 gave highly selective reactions while SnCl4 and TiCl4 did not. A spectroscopic investigation into the complexes formed between dimethyl acetals and various Lewis acids showed divergent results ranging from no observable complexation of the acetal with TMSOTf to complete, stoichiometry-dependent complexation with SnCl4. Finally, the duality of mechanism and its stereochemical consequences in reactions of acetals has been demonstrated. Using enol ethers as oxocarbenium ion precursors, the results show conclusively that the model dimethyl acetal does not react with TMSOTf via an oxocarbenium ion. As in classic nucleophilic aliphatic substitution, stereochemistry is used as a probe for differentiating SN1 and SN2 mechanisms in acetal reactions as well.