A novel Co(II) complex, (PPHP)CoI2, was synthesized and evaluated as a pre-catalyst for the hydrogenation and hydroboration of various alkenes in the presence of KBEt3H as an activator under mild conditions. (PPHP)CoI2 is able to selectively reduce terminal alkenes in almost quantitative yields. Hydrogenation of the terminal alkene group of limonene was shown. Interestingly, saturated frameworks bearing a terminal olefin were found to produce a mixture of the fully saturated product and internal olefins via a chain-walk mechanism. This chain-walking phenomenon was not observed when pyridine was used as an additive (6 mol%). We hypothesize that a pyridine-cobalt adduct generated in-situ slows the process of catalyst deactivation and may play a part in this observed selectivity. Additionally, this catalyst effectively hydroborylate terminal alkenes with high selectivity for the anti-markovnikov products with low catalyst loading (1 mol%) in three hours. The sole metal-containing product observed upon completion of catalytic reactions, [(PPP)CoH]2, suggests a monomeric (PPP)CoIIH active species that is deactivated by dimerization when substrates are depleted.

Catalytic selective hydroelementation of various terminal alkenes by a (PPP) pincer-ligated cobalt(II) complex

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