Mechanism of the Hydrometalation ( Insertion ) and Stoichiometric Hydrogenation Reactions of Conjugated Dienes Effected by Manganese Pentacarbonyl Hydride: Processes Involving the Radical Pair Mechanism

Article abstract of DOI:10.1021/ja00241a016

Manganese pentacarbonyl hydride (I) reacts with 1,3-butadiene, isoprene, and 2,3-dimethyl-1,3-butadiene to form predominantly the hydrometalated products of 1,4-addition of H-Mn(CO)5 to the dienes, i.e. R, R'=H, Me Monoolefins, the products of stoichiometric 1,2- and 1,4-addition of two hydrogen atoms to the dienes, are also formed as minor products, i.e. Hydrogenation is the predominant process with 1,3-cyclohexadiene.The hydrometalation reactions are first order in both I and diene, and the reaction rates are unaffected by added carbon monoxide (1 atm), while careful NMR monitoring of the early stages of the reactions under appropriate conditions reveals striking CIDNP polarizations for the 1H resonances of both the products of hydrometalation and of hydrogenation.The experimental evidence thus suggests that the reactions do not proceed via prior coordination of the olefin to the metal followed by conventional migratory insertion and reductive elimination processes but rather via hydrogen atom abstraction from I by the diene to give the corresponding radical pair.The latter can couple to give the hydrometalated compounds or diffuse apart to react with a second molecule of I to give Mn2(CO)10 and the monoolefins.For both types of products, the signs and, where measureable, the relative intensities of the CIDNP polarizations are completely consistent with the radical pair mechanism.