Control of Selectivity through Synergy between Catalysts, Silanes, and Reaction Conditions in Cobalt-Catalyzed Hydrosilylation of Dienes and Terminal Alkenes

Readily accessible (i-PrPDI)CoCl2 [i-PrPDI = 2,6-bis(2,6-diisopropylphenyliminoethyl)pyridine] reacts with 2 equiv of NaEt3BH at -78 degrees C in toluene to generate a catalyst that effects highly selective anti-Markovnikov hydrosilylation of the terminal double bond in 1,3- and 1,4-dienes. Primary and secondary silanes such as PhSiH3, Ph2SiH2, and PhSi(Me)H-2 react with a broad spectrum of terminal dienes without affecting the configuration of the other double bond. When dienes conjugated to an aromatic ring are involved, both Markovnikov and anti-Markovnikov products are formed. The reaction is tolerant of various functional groups such as an aryl bromide, aryl iodide, protected alcohol, and even a silyl enol ether. Reactions of 1-alkene under similar conditions cleanly lead to a mixture of Markovnikov and anti-Markovnikov hydrosilylation products, where the ratio of the products increasingly favors the latter, as the size of the 2,6-substituents in the iminoylaryl group becomes larger. The complex (i-PrPDI)CoCl2 gives exclusively the linear silane for a wide variety of terminal alkenes. Mechanistic studies suggest a pathway that involves a key role for an in situ generated metal hydride, (L)Co(I)-H. Exclusive reduction of the terminal double bond (vis-a-vis hydrosilylation) when (EtO)(2)Si(Me)H is used in the place of PhSiH3 is explained on the basis of an alternate silane-mediated decomposition path for the linear Co(I)-alkyl intermediate.