Reaction of metal carbonyl anions with electrophilic alkynes: Synthesis of isomeric η3-acryloyl and σ-vinyl complexes

Treatment of the metal carbonylate anions [CpMo(CO)2(L)]− (Cp = η-C5H5; L = PPh2Me, PPh2Et) with the electrophilic alkynes methyl propiolate or DMAD (RCtriple bond; length of mdashCCO2Me, where R = H or CO2Me, respectively) followed by protonation affords the η3-acryloyl (1-oxoallyl) complexes [CpMo(η3-COCRdouble bond; length as m-dashCHCO2Me)(CO)(L)] (3a–d) as the major products, together with the isomeric vinyl complexes trans-[CpMo(CRdouble bond; length as m-dashCHCO2Me)(CO)2(L)] (4a–d). On the basis of the regioselectivity of the reaction, it is proposed that nucleophilic attack of the carbonylate anion occurs at the alkyne carbon bearing R; migration of the anionic vinyl ligand to a CO followed by protonation gives 3, whereas protonation without insertion gives 4. The X-ray structures of the acryloyl complex [CpMo(η3-COCHdouble bond; length as m-dashCHCO2Me)(CO)(PPh2Me)] (3b) and its vinyl isomer [CpMo(σ-CHdouble bond; length as m-dashCHCO2Me)(CO)2(PPh2Me)] (4b) have been determined.