Highly selective formation of unsaturated esters or cascade reactions to α,ω-diesters by the methoxycarbonylation of alkynes catalysed by palladium complexes of 1,2-bis(ditertbutylphosphinomethyl)benzene

The methoxycarbonylation of phenylethyne catalysed by Pd/1,2-bis(ditertiarybutylphosphinomethyl) benzene gives the unusual linear product, methyl cinnamate with high activity (initial turnover frequency, TOFo > 1700 mol product.(mol catalyst.h)(-1)) and regioselectivity (>90%). Terminal aliphatic alkynes give alpha,beta-unsaturated esters after short reaction times or alpha,omega-diesters, including dimethyl 1,6-hexanedioate (dimethyl adipate), from 1-butyne after longer times. The diesters are formed by a cascade methoxycarbonylation-isomerisation-methoxycarbonylation sequence. Methoxycarbonylation of internal alkynes (e. g. 4-octyne) leads to the formation of the mono-carbonylated product as a result of the low propensity of the tri-substituted double bond of the product towards isomerisation. Hydroxycarbonylation of phenylethyne gives predominantly E-3-phenylpropanoic acid with smaller amounts of branched and disubstituted products as well as 3-phenylpropanoic acid. Evidence is presented that the reactions occur via a hydride mechanism.