Ring-size and substituent effects in intramolecular reactions of alkylidenecarbenes (carbenoids)

Abstract

Intramolecular reaction of methylenecarbenes (carbenoids), derived by reaction of 1,1 -dibromoalkenes and methyllithium, with 5,6-related C–H bonds leads to cyclopentenes when there is an alkoxy or amino substituent on C-5, but no reaction occurs at a 4,5-related C–H bond when there is a heteroatom on C-4; when there is an alkylthio group either on C-4 or C-5 attack occurs on sulphur and the alkyl group is lost, leading to a 2,3-dihydrothiophene or a 3, 4-dihydro-2H-thiopyran respectively. If a heteroatom-hydrogen bond is present at the 5,6-position a formal insertion occurs at that bond leading to 2,3-dihydropyrroles, 2,3-dihydrofurans or 2,3-dihydrothiophenes.