Vinylic cations from solvolysis. Part XII. Solvolysis of cis- and trans-α-bromo- and-αβ-dibromo-4,4′-dimethoxystilbenes

Abstract

Solvolysis of cis- and trans-α-bromo-4,4′-dimethoxystilbenes (3) and (4) in 80% and 90% EtOH and in AcOH gives mainly bis-4-methoxyphenylacetylene (8). The first order k_cis: k_trans reactivity ratios in aqueous ethanol are 43·5–49 in the presence of sodium acetate and 19·7 in the acetolysis. With 0·5M-NaOH in 80% EtOH k_cis:k_trans= 0·06 where k_cis is a first-order coefficient and k_trans is the second-order coefficient for the E2 reaction. Acetolysis of cis- and trans-αβ-dibromo-4,4′-dimethoxystilbenes (5) and (6) is slower than those of compounds (3) and (4). Values of k(3)/k(5)= 187, k(4)/k(6)= 3·3 and k(5)/k(6)= 0·37–0·41 were calculated. The products with sodium acetate are 4,4′-dimethoxybenzil, 4,4′-dimethoxybenzoin acetate, deoxy-4,4′-dimethoxybenzoin, and the acetylene (8), but in the presence of AgOAc, trans-α-acetoxy-β-bromo-4,4′-dimethoxystilbene (7) was formed. The acetolysis of all the compounds showed common ion rate depression and that of compound (7) gives similar products to those of (5) and (6). The mechanism of the reaction of compound (4) in the presence of sodium hydroxide is suggested to be E2 while all the other reactions are mechanistically S_N1–E1. The rate difference between compounds (3) and (4) in the E1 process is ascribed mainly to a rate enhancement of compound (3) due to ground-state steric interactions. No evidence for β-bromine participation was observed. The structure and the selectivity of the intermediate vinyl cations are discussed.