Beyond organometallic in situ activation: the influence of the counter ion in base metal catalysis

Abstract

The amine-activated iron-catalysed hydrosilylation of alkenes and alkynes has been developed, and has been found to selectively provide the anti-Markovnikov product. It was established that a range of amines could be used to activate an iron(II) pre-catalyst containing triflate counter ions in situ, with diisopropylethylamine facilitating the highest reactivity (Scheme A1). Pre-catalyst and amine loadings as low as 0.25 mol% and 1 mol%, respectively, could be used for the gram-scale hydrosilylation of 1-octene with phenylsilane, in air. The activator-free iron- and cobalt-catalysed hydrosilylation of alkenes and alkynes was subsequently established, by using pre-catalysts containing tetrafluoroborate counter ions. The methodology was extended to the hydroboration and hydrogenation of olefins. The activation has been proposed to occur via the in situ formation of a hypervalent silicon ‘ate’ complex, formed as a result of the hydrolysis of the BF4 - counter ion. It was found that the iron-catalysed system provided anti-Markovnikov hydrosilylation products, whereas Markovnikov products were formed preferentially when using the cobalt pre-catalyst (Scheme A2). Consequently, the regioselectivity of hydrosilylation could be easily altered.