Generation of 2-(Trimethylsiloxy)allyl Cations and Their Reactions with 1,3-Dienes. Change in Mechanism of 3 + 4 -> 7 Cycloaddition with Solvent

Article abstract of DOI:10.1021/ja00369a031

A series of 12 different 2-(trimethylsiloxy)allyl cations 34a-l is generated from various 2-(trimethylsiloxy)allyl chlorides (3-5) with silver perchlorate.In nitromethane solution, all the cations smoothly react with furan and cyclopentadiene in a 3 + 4 -> 7 manner to give a comprehensive series of 8-oxabicyclo<3.2.1>oct-6-en-3-ones and bicyclo<3.2.1>oct-6-en-3-ones in good yields.The cycloaddition with furan proceeds in a stereospecific manner with the retention of allyl cation configurations, in accord with a concerted mechanism.The 3 + 4 -> 7 reactions in THF/ether contrast with those in nitromethane in the following ways. (1) The yield of the adducts strongly depends on the strusture of the allyl cations. (2) The reaction with furan is nonstereospecific. (3) An electrophilic substitution reaction strongly compets with the cycloaddition. (4) The cycloaddition between the cation 34a and 2-methylfuran is highly regioselective (11/12 = 19) as compared to that in nitromethane (the ratio being only 1.9).These findings in THF/ether are reasonably explained by a stepwise mechanism via an intermediate 40.Reactions with acyclic dienes (isoprene and 2,3-dimethylbutadiene), naphthalene, anisole, and methanol are also described.