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

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AbstractAs 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 · 103. The allylic double bond accelerates solvolyses of 21c compared with cyclohexyl mesylate by a factor of 107. However, 12 solvolyses only 100 times faster than norpinyl mesylate 16, showing a 105‐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.