Reactions of Alkyl-Substituted Rhenacyclobutadiene Complexes with Electron-Rich Alkynes

The reactivity of low-valent alkyl-substituted metallacyclobutadienes towards electron-rich alkynes was studied. Treatment of the alkyl-substituted rhenacyclobutadiene complexes Re{-C(R)=C(CO2Et)C(OEt)=}(CO)(4) (R=Me, Et) with HC equivalent to COEt produced a mixture of the cyclopentadienyl complexes Re{eta(5)-C5H(R)(CO2Et)(OEt)(2)}(CO)(3) and the new rhenacyclobutadienes Re{-C(OEt)=C[C(R)=CH(CO2Et)]C(OEt)=}(CO)(4) bearing a pendant vinyl substituent C(R)=CH(CO2Et) at the beta-carbon. The reactions proceed through initial nucleophilic attack of the terminal carbon of HC equivalent to COEt on the ReC(R) carbon of the starting rhenacyclobutadienes. The rhenacyclobutadiene complex Re{-C(Me)=C(CO2Et)C(OEt)=}(CO)(4) reacted with aminoalkynes RC equivalent to CNEt2 (R=Ph, Me) to produce the unusual coupling products Re{-C(OEt)=C(CO2Et)C[CH=C(NEt2)CH2R]=}(CO)(4), metallacyclobutadiene complexes bearing the vinyl substituent CH=C(NEt2)CH2R at an alpha-carbon. The mechanisms of the reactions are discussed.