Generation and solution-phase behaviour of some 2-halogeno-1,3-ring-fused cyclopropenes

Abstract

The title cyclopropenes 2b are readily formed by reaction of ring-fused β-silylated-gem-dihalogenocyclopropanes of the general type 1b with tetrabutylammonium fluoride in tetrahydrofuran solution. Those halogenocyclopropenes 2b which are fused to a seven- or eight-membered ring can be trapped with a range of 1,3-dienes and the corresponding Diels–Alder adducts 3b are produced in high yield. In contrast, those chlorocyclopropenes which are fused to a five- or six-membered ring cannot be trapped efficiently by added diene because they each undergo rapid rearrangement to an isomeric vinyl carbene. Thus, 6-chlorobicyclo[3.1.0]hex- 1(6)-ene 27 undergoes ring-expansion to carbene 28 which either inserts into the α-C–H bond of the reaction solvent tetrahydrofuran or adds to one of the double bonds of furan. In contrast, the carbene, 32, derived by ring-cleavage of 7-chlorobicyclo[4.1.0]hept-1(7)-ene 31 reacts with chloride ion to give, after protonation, (E)-2-chloro-1-(chloromethylene)cyclohexane 34. This latter result is at variance with an earlier claim that when the cyclopropene 31 is generated under vacuum gas-solid reaction conditions it rearranges, via a ring-expanded vinyl carbene 36, to 2-chlorocyclohepta-1,3-diene 37. In the present work it has been established that this claim is incorrect. X-Ray crystallographic analyses of (1′α,3′α)-(E)-3′-chloro-2′-(chloromethylene)cycloheptyl p-nitrobenzoate 54 and (1′R,2prime;S,3′S,8′S,9′S,10′S)-9′-chloro-137prime;-oxatetracyclo[8.2.1.0^2′.8′.0^2′.9′]tridec-11′-en-3′-yl p-nitrobenzoate 67 have been carried out.