Vinylic cations from solvolysis. Part XIII. SN1 and electrophilic addition–elimination routes in the solvolysis of α-bromo- and α-chloro-4-methoxystyrenes

Abstract

The solvolysis of α-chloro-(3) and α-bromo-4-methoxystyrene (4) in 80% EtOH gives 4-methoxyacetophenone and 4-methoxyphenylacetylene and is mechanistically S_N1. The solvolysis is enhanced by two β-methyl substituents; the Winstein–Grunwald m values are 0·76–1·00 at 25 °C and k₁(4)/k₁(3)= 54. In the acetolysis of compounds (3) and (4),4′-methoxyacetophenone is formed, k₁(4)/k₁(3)= 0·46–0·56, k₁(80% EtOH)/k₁(AcOH)= 0·13 for (3) and 15 for (4), k_1AcOH/k_1AcoD= 1·94 for (3) and 1·45 for (4), and deuterium is not incorporated in the unchanged (3) and (4). The hydrolysis of α-acetoxy-4-methoxystyrene and the addition of AcOH to 4-methoxyphenylacetylene and 4-methoxystyrene in AcOH are faster than the solvolyses of (3) and (4). These data suggest that the acetolysis of the chlorostyrene (3) is by concomitant S_N1 and electrophilic addition–elimination (Ad_E–E) routes, while that of (4) is mainly S_N1. The competition between the two routes is discussed.