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°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 β‐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.