Kinetics and Mechanism of Substitution Reactions of Some (η3-Allyl)manganese Tetracarbonyl Compounds

Article abstract of DOI:10.1021/ja00297a018

Kinetic data are reported for CO substitution of (η³-C3H4X)Mn(CO)4, where X is a substituent in the 1- or the 2-position of the allyl ligand and where X¹ = H, Me, Ph, t-Bu, and Cl.Also the anti- and the syn-(η³-C3H3(1,2-Ph2))Mn(CO)4 isomers were prepared for the fist time and their rates of substitution determined.In all cases the rates of reaction are first order in substrate concentration and zero order in entering nucleophile concentrations.Studies on decarbonylation reactions of (η¹-C3H5)Mn(CO)4L rule out an η³ --> η¹ --> η³ mechanism.It appears that CO substitution takes place by a dissociation (S_N1 process, and the kinetic parameters for (η³-C3H5)Mn(CO)4 are k(45 deg C) = 2.8 X 10^-4 s^-1, ΔH(excit.) = 26.8 kcal/mol, and ΔS(excit.) = 9.6 eu.Substituents on the 1-position of the allyl group have a small retardation effect on the rates of CO substitution.Substituents on the 2-position enhance the rates of reaction; furthermore, this rate enhancement increases with increasing bulkiness of the substituent.Still, the maximum rate observed was only 500 times greater than that for the parent comopound, and this is for the 2-tert-butylallyl compound.Unfortunately, all attempts to prepare the desired compounds with strong electron-donating and -withdrawing substituents on the allyl ligand failed.