Solvolysis of 2-Methylene Bicyclic Bridgehead Derivatives: A Model for Gradual Variation of ?-Conjugation in Carbocations

Article abstract of DOI:10.1021/jo00027a050

The rates of solvolysis in ethanol or 80percent ethanol at 25 deg C have been determined on 2-methylenebicyclo<2,2,2>oct-1-yl triflate (4a), 2-methylenebicyclo<3.2.1>oct-1-yl triflate (5a), 2-methylenebicyclo<3.2.2>non-1-yl mesylate (6a), 2-methylenebicyclo<3.3.1>non-1-yl mesylate (7a-OMs) and heptafluorobutyrate (7a-OHFB), 1-chloro-2-methylenebicyclo<4.2.2>decane (8a), 2-methylenebicyclo<4.3.1>dec-1-yl trifluoroacetate (9a), and 4-methylene-3-homoadamantyl heptafluorobutyrate (10a) and on their corresponding parent 1-bicycloalkyl and 3-homoadamantyl derivatives 4b-10b containing the respective leaving group.The rate ratios for 4a/4b, 5a/5b, 10a/10b, 6a/6b, 7a/7b, 8a/8b, and 9a/9b are 10^-3.9, 10^-1.9, 10^-1.1, 10^-0.8, 10^0.9 (for mesylate), 10^-0.2, and 10^0.7, respectively.A plot of the logarithms of the rate ratios against olefinic strain energies of their corresponding unsubstituted bridgehead olefins shows that the smaller the olefinic strain energy, the greater the rate ratio, providing a methodology to gradually change the conjugative ability of bridgehead carbocations.The enhancement of allylic conjugation with increasing skeletal flexibility has been further verified by the enhanced solvolysis rate of (E)-2-ethylidenebicyclo<3.2.2>non-1-yl mesylate ((E)-6e) relative to 6a by a factor of 259.A similar study on much more rigid (E)-2-ethylidenebicyclo<2.2.2>oct-1-yl triflate ((E)-4e) gave a (E)-4e/4a rate ratio of 6.3.AM1 semiempirical molecular orbital calculations on pertinent 2-methylene and (E)-2-ethylidene bridgehead carbocations and corresponding hydrocarbons (L = hydrogen) also supported the increase in the conjugation with increasing skeletal flexibility.The solvolysis products were solely bridgehead substitution products, no indication for the formation of bridgehead olefin via an S_N1' mechanism having been obtained.