CO Insertion into the Iron-Carbon ? Bond in Optically Active CpFe(CO)(L)R. Solvent Dependence and Acid Catalysis

Article abstract of DOI:10.1021/ja00322a019

The insertion of CO into the iron-carbon bond in optically active organoiron complexes CpFe(CO)(PPh3)Et (Cp=η⁵-C5H5) (1) and CpFe(CO)(mtpb)Et (mtpb=P(OCH2)3CMe) (3) in the presence of CO yields CpFe(CO)(PPh3)COEt (2) and CpFe(CO)(mtpb)COEt (4), and in the presence of cyclohexyl isocyanide (chi) yields CpFe(chi)(PPh3)COEt (5) and CpFe(chi)(mtpb)COEt (6), respectively.In nitroethane or nitromethane under CO pressure 2 and 4 are formed highly stereospecifically, and in MeCN with much reduced stereospecificity, but all with formal alkyl migration.In HMPA, Me2SO, DMF, or propylene carbonate the optical purity is moderate to low, and the configuration corresponds to formal CO migration.Mixed-solvent studies, cyclic voltammetry, and UV-vis spectra suggest that a special charge- or electron-transfer mechanism in nitroalkane solvent is unlikely.Use of 13C-labeled CO results in the expected incorporation of label only in the terminal CO position of 2.Strong catalysis results from BF3*Et2O addition, but with no change in stereochemistry.Insertion in the presence of chi yields 5 and 6 with formal alkyl migration in either nitroethane or HMPA (the configurational assignment of the latter is highly tentative) but with reduced percentee.Operation of the usual reversible first-order insertion followed by second-order ligand uptake in the presence of chi is confirmed by a kinetics study of 1 with chi in Me2SO with k₁=4.9E-4 s^-1, and k₂/k_-1=0.54 L M^-1.No intermediates could be detected by IR or NMR in nucleophilic solvents.Possible mechanisms are discussed, including the likely presence of an (η²-acyl)iron intermediate in rapid equilibrium with an unsaturated (η¹-acyl)iron species.