A Case of Concerted Formation of Five and Six Member Rings; Solvolytic Behavior of 1-Aryl-1-chloro- -4,8,13,17,21-pentamethyl-4,8,12,16,20-docosapentaenes

The solvolytic behavior of benzyl chloride 1 (1-chloro-1-pheny-l4,8,13,17,21pentamethyl-4,8,12,16,20-docosapentaene) revealed inconsistency in some aspects, thus the reaction mechanism is ambiguous. The ethanolysis rate (ku) of 1 is practically the same as the rate of its saturated analog (ku / ks = 2.5), indicating that the reaction may proceed by way of an unassisted process. On the other hand, activation parameters obtained are consistent with the rate determining participation of at least one double bond, in which the high degree of order required in the transition state (large negative S) is overcompensated by a rather small H. The magnitude of S = –122 13 J K mol obtained with 1 in aqueous ethanol ( = 80%, 80E) is even larger than the entropy of activation obtained for the concerted monocyclization reaction with the referent benzylic chloride 2 ( S = –99.0 21 J K mol), but is lower than the one obtained with referent chloride 3 ( S = –194 12 J K mol). It should be emphasized that, according to all methods used CROATICA CHEMICA ACTA CCACAA 72 (1) 115¿121 (1999)