Reactions of metal carbonyl anions and Fischer-type transition metal carbene complexes: An investigation of the different sites of nucleophilic attack on Fischer carbenes

Transition metal carbene complexes have received considerable attention for their involvement in reactions such as olefin cyclopropanation, olefin metathesis, and the Fischer-Tropsch synthesis. Fischer-type carbenes typically undergo nucleophilic attack at the carbene carbon. However, examples in which attack is suppressed to the cis-carbonyl carbons exist. Reactions that we have investigated between Fischer carbenes and transition metal carbonyl anions have demonstrated three different sites of nucleophilic attack on the carbene depending on the system.; Reactions between neutral carbenes, {dollar}rm M(CO)sb5({lcub}={rcub}C(OMe)Ph){dollar} (M = Cr, W) and metal carbonyl anions {dollar}rm(Mspprime=Cr(CO)sb5sp{lcub}-2{rcub}, W(CO)sb5sp{lcub}-2{rcub}, Fe(CO)sb4sp{lcub}-2{rcub}, CpFe(CO)sb2sp-, Re(CO)sb5sp-, Mn(CO)sb4PBusb3sp-, Mn(CO)sb4PPhsb3sp-, Cpsp{lcub}*{rcub}Cr(CO)sb3sp-, CpMo(CO)sb3sp-){dollar} resulted in the quantitative demethylation of the carbene complex forming the products {dollar}rm M(CO)sb5(C(O)Ph)sp-{dollar} and MeM{dollar}spprime.{dollar} A slower rate for reaction with {dollar}rm W(CO)sb5({lcub}={rcub}C(OEt)Ph){dollar} in comparison to the methyl analogue is consistent with nucleophilic attack of the metal carbonyl anion on the methyl of the methoxy group of the carbene, representing a new site of attack by metal carbonyl anions on Fischer-type carbenes.; Reactions of cationic Fischer-type carbenes, (CpFe(CO)(PCy{dollar}rmsb3)({lcub}={rcub}C(OR)Me)rbracklbrack BFsb4rbrack{dollar} (R = Me, H) with metal carbonyl monoanions, (M{dollar}rmspprime=CpFe(CO)sb2sp-, Mn(CO)sb4PPhsb3sp-, Mn(CO)sb5sp-, CpMo(CO)sb3sp-){dollar} provide products indicative of an electron transfer process (dimers, ligand exchange products) as well as products consistent with abstraction of the R group forming CpFe(CO)(PCy{dollar}sb3){dollar} (C(O)Me) and M{dollar}spprime{dollar}-R. To account for the two different types of products we have proposed that the metal carbonyl anion can attack at two different sites: (a) directly at, and abstraction of the R group, or (b) at the carbonyl carbon of the carbene, forming a bridging carbonyl complex through which an inner-sphere electron transfer process can occur.; The third site of attack was observed for the reaction between (CpFe(CO)(PCy{dollar}sb3){dollar}(=C(OMe)Me)) (BF{dollar}sb4rbrack{dollar} and Fe(CO){dollar}sb4sp{lcub}-2{rcub},{dollar} which provides {dollar}rm Fe(CO)sb3(PCysb3)({lcub}={rcub}C(OMe)Me){dollar} as the only discernable product. We have proposed that this product is a result of an initial nucleophilic attack by the anion at the carbene carbon, followed by a transfer of the carbene moiety to the more nucleophilic metal anion. In a subsequent step, it is believed that there is a phosphine substitution reaction that takes place, forming the observed product.