Alkynylhalocarbenes. 4. Generation of (alk-1-ynyl)halocarbenes.from 3-substituted 1,1,1,3-tetrahalopropanes under the action of bases

When treated with KOH under phase-transfer catalysis or with (BuOK)-O-t, 3-substituted (alkyl or phenyl) 1, 1, 3-tribromo-1-fluoropropanes 1a-c exclusively generate previously unknown (alk-1-ynyl)fluorocarbenes 5a-c, which react with olefins to give 1-(alk-1-ynyl)-1-fluorocyclopropanes 6a-h in 12-69% yields. Under analogous conditions, 3-alkyl- and 3-aryl-3-bromo-1, 1, 1-trichloropropanes 2a-c selectively afford (alk-1-ynyl)chlorocarbenes 7a-c, which are trapped by olefins to form the corresponding 1-(alk-1-ynyl)-1-chlorocyclopropanes 8a-k in 35-70% yields. (Phenylethynyl)chlorocarbene 7a is also selectively generated from 1, 1, 1, 3-tetrachloro-3-phenylpropane (3a) upon its treatment with (BuOK)-O-t. With an excess of 2,3-dimethylbut-2-ene or 2-methylpropene, carbene 7a yields 1-chloro-1-(phenylethynyl)cyclopropanes 8a or 8c, respectively. In contrast, 1, 1, 1, 3-tetrachloroheptane 3b and 3-alkyl- and 3-phenyl-1, 1, 1, 3-tetrabromopropanes 4a,c,f react with bases in the presence of olefins to give, along with the corresponding 1-(alk-1-ynyl)-1-halocyclopropanes 8a,c,d and 11a-f, vinylidenecyclopropanes 12a,c-g, which suggests the generation, under these conditions; both (alk-1-ynyl)halocarbenes 7b and 9a-c and vinylidenecarbenes 10 and 11a-c. The composition and structures of intermediate products in the reactions of tetrahalides 1b, 2a, 2b, 3a, and 3b with (BuOK)-O-t were determined and the mechanisms for carbene generation in these reactions were proposed.