Rearrangement of Substituted Bicyclo[2.1.1]hex-2-yl Mesylates Under Solvolytic Conditions

When heated in formic acid, methyl 2-mesyloxybicyclo[2.1.1]hexane-2-carboxylate (3) ionizes significantly faster than expected and yields the rearranged isomer (6) essentially quantitatively. The latter, on the other hand, solvolyses at a much reduced rate. These data are interpreted as evidence for the presence in the ester of a favourable mesomeric interaction which largely offsets an otherwise destabilizing inductive effect when the ester is attached to a carbocationic centre. The corresponding cyano mesylates (8) and (29) were found to behave in an analogous manner when exposed to the solvolytic conditions. Heating the related acid mesylate (25) in an aqueous medium leads to the 1,2 hydroxy acid (24), whereas its sodium salt (26) is considerably more reactive and gives the 2,2 hydroxy acid (11). These observations are rationalized on the basis of the greatly enhanced ability of the COO- group relative to a COOH to stabilize an adjacent positively charged centre. The methyl substituent is also found to exert a profound effect on the nature of the solvolysis products. In hot formic acid, for example, 1-methylbicyclo[2.1.1]hex-2-yl mesylate (31) produces the formate (32) in which the substitution pattern is retained. Under milder conditions, however, the ester (31) solvolyses with rearrangement to give the 2,2 formate (33).