Mechanism of arylation of nucleophiles by aryllead triacetates. Part 2. Support for a ligand coupling process and X-ray molecular structure of (p-methoxyphenyl)-α-methylphenacyllead (IV) diacetate

Abstract

In the presence of boron trifluoride–diethyl ether complex p-methoxyphenyllead triacetate 4 and the propiophenone silyl enol ether 6 were found to undergo a rapid reaction to give (p-methoxyphenyl)-α-methylphenacyllead diacetate 9 in high yield. Analogous products, 5, 10 and 11 respectively, were formed when the silyl enol ethers of acetophenone, butyrophenone and isobutyrophenone were treated under the same conditions with aryllead compound 4. The diorganolead diacetates 9, 10 and 11 were relatively unstable, giving a number of products when heated at 60 °C in chloroform. In each case there was significant elimination of Pb(OAc)₂, with formation of the product of ligand coupling, the deoxybenzoin. α-Methylphenacyl(o-prop-2-enyloxy)phenyllead diacetate 21, which was produced to probe the mechanism of the coupling reaction, behaved similarly to compound 9, giving no dihydrobenzofuran derivatives. Therefore, it would appear that aryl free radicals are most probably not produced in these thermal reactions, and a ligand-coupling mechanism is proposed.