Endo,endo- andexo,exo-Bicyclo[1.1.0]butane-2,4-dimethanol Dimesylate: Synthesis, Structure and Solvolysis

Abstract

The title compounds endo,endo-9 and exo,exo-9 were prepared from benzvalene. As determined by single-crystal X-ray diffraction, several geometrical parameters of endo,endo-9 are particularly remarkable, namely the large interflap angle of the bicyclo[1.1.0]butane system (128.2°) and the length of the C-1−C-3 bond (151.2 pm). Solvolyses of endo,endo-9 in 60% acetone/water, ethanol and 2,2,2-trifluoroethanol gave rise mainly to cis-5-substituted cyclopent-2-ene-1-methanol mesylates (cis-10, cis-11). Small quantities of the corresponding trans isomers (trans-10,trans-11) suggest a configurational conversion of the intermediate, which is proposed to be the nonclassical pseudoaxial 2-mesyloxymethyl-substituted cyclopent-3-en-1-yl cation (ax-31) formed from endo,endo-9 by heterolytic dissociation accompanied by a Wagner-Meerwein rearrangement. The solvolyses of exo,exo-9 took an entirely different course and afforded nonrearranged products (26, 27) exclusively. This is interpreted in terms of the stereochemical requirements of the Wagner−Meerwein rearrangement, which in the case of exo,exo-9 would result in a highly strained cation such as 32 or 33. In consequence, the generation of the bicyclobutylcarbinyl cation 34 seems to be the most favourable alternative. This result, together with kinetic data for solvolyses of endo,endo-9 and exo,exo-9, casts doubt on a report on solvolyses of the dimethylbicyclo[1.1.0]but-2-ylcarbinyl tosylates 6.4 Solvolyses of endo,endo-9 in several solvents are about 5 times faster than those of the ditosylate 2 (or the corresponding dimesylate), a less flexible bicyclo[1.1.0]butane derivative. That the solvolysis of exo,exo-9 takes place about 8 times more slowly than endo,endo-9 is interpreted by invoking a weaker σ-participation in the case of the former. In comparison to the stereochemically closely related exo,exo-9, the cyclobutanedimethanol ditosylate 28 solvolyses only 50 times more slowly.