SYNTHESIS AND SOLVOLYSIS OF BICYCLO[3.1.1]HEPT-3-EN-2-YL, BICYCLO[3.1.1]HEPT-2-YL, AND 2-HALOGENOCYCLOHEX-2-EN-1-YL METHANESULFONATES AND P-NITROBENZOATES

As an extension of previous work bicyclo[3.1.1]hept-3-en-2-ol (5) was synthesised in four steps from benzvalene and is now much more accessible than by former routes. The 3-bromo derivative 9 of 5 was obtained from bicyclo[2.1.1]hexene by addition of dibromocarbene and hydrolysis of the resulting dibromide 8. - Methanesulfonates were prepared from 5, 9, 2-norpinanol (7) as well as from 2-bromo- (11a) and 2-chlorocyclohex-2-en-1-ol (11b). Due to its high reactivity, the bicycloheptenyl mesylate 12 could only be characterised by low-temperature NMR spectra. At 20 degrees C, 2-norpinyl mesylate (16) rearranged slowly to endo- (endo-17) and exo-2-norbornyl mesylate (exo-17) in the ratio 2:1. The formation of endo-17 was also the major process on treatment of 16 with aqueous ethanol or acetone. - Solvolyses of bicyclo[3.1.1]heptenyl mesylates 12 and 20 proceed 4-5 times more slowly than solvolyses of corresponding cyclohexenyl mesylates 21c and 21a. The beta-bromine substituent deactivates solvolyses of 21a compared with 21c, and 20 compared with 12, by a factor of 2 . 10(3). The allylic double bond accelerates solvolyses of 21c compared with cyclohexyl mesylate by a factor of 10(7). However, 12 solvolyses only 100 times faster than norpinyl mesylate 16, showing a 10(5)-fold effect due to cyclobutylcarbinyl ring expansion. From solvolysis data the energy difference between the 2-norpinyl and 2-norbornyl cation is estimated to be 16 kcal mol(-1), in fair agreement with a recent ab initio calculation.