Reduction of carbon-carbon double (C=C) bonds remains one of the most fundamental transformations in organic synthesis. There are many examples of metal-mediated homogeneous and heterogeneous reductions of C=C bonds in the presence of hydrogen. Transfer hydrogenation (TH) is an alternative approach, which alleviates the need for high pressures of hydrogen and opens opportunity for selective functionalisation and mechanistic investigation. Recently, we reported a three-coordinate iron(II) β-diketiminate complex can catalyze the TH of C=C bonds using n-butylamine or aniline and pinacol borane (HBpin) as TH reagents. The reaction proceeds quickly at room temperature and grants access to highly regioselective deuterium incorporation. Further investigation suggested that the substitution of HBpin for a silane can lead to monodeuteration with improved selectivity. Herein, we report the development of a more benign iron-catalyzed TH methodology utilising the same iron pre-catalyst, using the cheapest available silane source (poly(methylhydrosiloxane) (PMHS)) and biorenewable n-butanol as TH reagents, proceeding at room temperature under solvent-free conditions. Ethanol and methanol also prove to be effective proton sources, allbeit in reduced yield. The TH of 1-hexene and allyl benzene with diphenylsilane and aniline-N,N-d2 leads to complete deuterium transfer to the terminal position of the C=C bond. A mechanistic study reveals our TH reaction at odds with competing dehydrocoupling and isomerisation processes. In the absence of substrate, the dehydrocoupling of PMHS and alcohol occurs, generating high yields of hydrogen within 30 minutes at room temperature.

Room temperature iron-catalyzed transfer hydrogenation using n-butanol and poly(methylhydrosiloxane)

ACS Spring 2022

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