Vinylic cations from solvolysis. Part XV. β-Phenyl and β-p-methoxyphenyl rearrangement during the solvolysis of 2,2-diaryl-1-phenylvinyl bromides

Abstract

Triphenylvinyl bromide (1), trans- and cis-2-p-methoxyphenyl-1,2-diphenylvinyl bromides (2) and (3), and 2,2-bis-p-methoxyphenyl-1-phenylvinyl bromide (4) were solvolysed in 2,2,2-trifluoroethanol (TFE) and in 60% EtOH. In TFE, the products were the trifluoroethyl ether from (1), 15% of a 1 : 1 cis–trans mixture of the unrearranged and 85% of the β-phenyl rearranged trifluoroethyl ethers from (2) and (3), and a 1 : 1 cis–trans mixture of the β-p-methoxyphenyl rearranged trifluoroethyl ethers from (4). With toluene-p-thiolate ion, (4) gave only a rearranged sulphide, whilst the sulphides from (2) and (3) are 70% rearranged. The trifluoroethanolysis of (1)–(3) was accompanied by common ion rate depression. The relative initial k₁(120°) values were 1(1) : 3·4 (2) : 3·7 (3) : 9·5 (4). In 60% EtOH the products were mostly the unrearranged acetophenone derivative from (2) and (3) and only the rearranged ketone from (4), and the relative k₁(140°) values were 1 (1) : 2·14 (2) : 2·44 (3) : 5·45 (4). It is concluded that (a) the initial ionisation does not involve β-aryl participation, (b) the products are derived from free, open, linear vinyl cations, which return to covalent bromide (k_Br), rearrange (k_r), and react with the solvent (k_ROH) or toluene-p-thiolate ion (k_SR′), (c)β-p-methoxyphenyl (An) rearrangement is the dominant process for the ion An₂CCPh in the two solvents, (d)β-phenyl rearrangement is dominant for the ion AnC(Ph)CPh (19) in TFE (k_Br : k_r(Ph) : k_ROH= 214 : 5·7 : 1 at 120°) while in 60% EtOH k_ROH:k_r(Ph)= 19 : 1; the apparent ‘abnormal’ order of migratory aptitudes Ph > An is either due to a hidden β-p-methoxyphenyl rearrangement or to a higher energy loss in the deconjugation of the migrating aryl group with the double bond than the energy gain in the briding by the migrating aryl, and (e) the effects of β-aryl substituents on the solvolysis rate are nearly additive.