Ring-expansion of Benzene by a Zinc-bromocarbenoid Reagent

Abstract

The accelerating effect of oxygen for the generation of a zinc-bromocarbenoid reagent from diethylzinc and bromoform was utilized for benzene-ring enlargement reaction; 7-ethyltrop ili dene [1a] was obtained from benzene in a 58% yield. Toluene and cumene also gave the corresponding alkyl-substituted 7-ethyltropilidenes ([1b] and [1c]) in yields of 42 and 47%respectively, the isomer distributions of which were very similar to those resulted by the reaction of zinc-iodocarbenoid reagent from diethylzinc and iodoform.
From the relative amounts of the products formed it was estimated that cyclohexene and toluene were 3.1 and 1.6 times more reactive, respectively, than benzene toward the zinc bromocarbenoid reagent.
Chloro- and b romobenzene gave the halo-substituted 7-ethyltropilidenes ([1d] and [1e])in lower yields along with dehalogeno-alkylation products, 1-ethylpropylbenzene [2], propylbenzene [3], and trace amounts of toluene [4] (eq. ( 2 )). The reaction paths for the formation of the dehalogeno-alkylation products were discussed on the basis of the results of control reaction of benzylidene dibromide or benzyl chloride with diethylzinc.