Formation of a manganese alkyl complex as an intermediate in the ring-opening hydroformylation of α-cyclopropylstyrene by HMn(CO)₅

The reaction of α-clopropylstyrene (CPS) with HMn(CO)₅ can result in either hydrogenation or hydroformylation. The hydrogenation occurs by a previously established pathway involving sequential hydrogen atom transfers from the metal hydride to the organic substrate, and is the predominant reaction observed with samples of HMn(CO)₅ that have been thoroughly dried and purified using P₂O₅. With samples of HMn(CO)₅ that have been not been treated with P₂O₅, the reaction that occurs with CPS is ring-opening hydroformylation, leading to the isolation of Mn₂(CO)₉(η¹-HCOCH₂CH₂CHC(Ph)CH₃). The detailed procedures for preparation and handling of HMn(CO)₅ that are reported in this paper enable the hydroformylation reaction to be reproducible observed as the predominant reaction. An induction period of a few hours is typically observed in the hydroformylation, but addition of Mn₂(CO)₉(η¹-tolualdehyde) eliminates the induction period. The hydroformylation reaction is inhibited by CO (1 atm). The manganese alkyl complex (CO)₅MnCH₂CH₂CHC(Ph)CH₃ is an intermediate in the hydroformylation. The details of the mechanism of formation of this alkyl complex from CPS and HMn(CO)s are not completely understood, but several mechanistic possibilities are considered.