CARBONIUM ION-SILANE HYDRIDE TRANSFER REACTIONS .3. CYCLOPROPYLMETHYL CATIONS

In methylene chloride-trifluoroacetic acid, cyclopropylcarbinols (1a-e) underwent rapid ring-opening reactions leading to 4-substituted 3-butenyl-1-trifluoroacetate esters (2a-e). With 1,1-dicyclopropylbenzyl alcohol (lb) and tricyclopropylcarbinol (1c), multiple ring opening occurred leading to bis- and tristrifluoroacetates 4 and 5, respectively. With di- and triorganosilanes present in the reaction mixture, ring opening was suppressed and hydride transfer from ≡SiH to the carbonium ion took place, in several cases to the exclusion of ester formation. Hydride transfer produced cyclopropylmethanes (3a-e) exclusively and in no case was hydride transferred to a ring-opened homoallyl cation. This fact indicates that delocalization of the positive charge by the cyclopropyl ring takes place by some mechanism which does not make the ring carbons very electrophilic. The more highly substituted cyclopropylmethyl cations exhibit a greater tendency to ring open than the less substituted ones probably because of nonbonded repulsions in the ion. The stereochemical features of ring opening are discussed in terms of preferred conformations in the carbonium ion. © 1969, American Chemical Society. All rights reserved.