IPSO SUBSTITUTION OF TRIARYLVINYL CATIONS BY ALKOXIDE ANIONS

Photolysis and solvolysis of triarylbromoethanes 1 in the presence of alkoxide anions in alcohols resulted in significant formation of products derived from ipso substitution by alkoxide anions. Photolysis of 1-aryl-1-bromo-2,2-diphenylethenes 1Aa and 1Ab with 10 mol equiv of alkoxide anions gave 1-alkoxy-1-aryl-2,2-diphenylethenes 2A and 3,3-dialkoxy-6-(2,2-diphenylvinylidene)-1,4-cyclohexadienes 3A (ipso adducts). However, the photolysis with weak bases such as TEA, pyridine, NaHCO3, and K2CO3 did not give ipso adducts 3A but only enol ethers 2A. Interestingly, photolysis of 2,2-bis(p-alkoxyphenyl)-1-bromo-1-phenylethenes 4a and 4b with NaOEt in ethanol afforded ipso adducts 6a and 6b, respectively, which had a 1,2-aryl-rearranged structure. Furthermore, solvolysis of 1-bromo-1-(p-methoxyphenyl)ethenes 1Aa, 1Ba, and 1Ca in ethanol containing 10 mol equiv of NaOEt at 120-130-degrees-C for 1-3 days provided p-ethoxyphenyl-substituted enol ethers 2bb, p-ethoxyphenyl-substituted bromoethenes 1b, and p-methoxyphenyl-substituted enol ethers 2ab, respectively. The major formation of p-ethoxyphenyl-substituted compounds (ipso-substituted products) suggests that there is no large difference in ipso substitution between vinyl cations generated by photolysis and solvolysis. On the basis of the results obtained above the factors affecting ipso attack on arylvinyl cations are discussed.