(Cyclopentadienone)iron tricarbonyl compounds have been known since the 1950s, but their catalytic activity has only recently been explored. They are active in transfer hydrogenations of carbonyl compounds, transfer dehydrogenations of alcohols, and redox coupling processes known as borrowing hydrogen reactions. Because iron is an earth-abundant metal and simple carbonyl compounds can be used as hydrogen acceptors, these catalytic reactions are safer and more sustainable than those based on strong oxidants and/or platinum-group metals. We are interested in applying these catalysts to alcohol dehydrogenations, and the equilibrium-driven natural of the Oppenauer oxidation-type process leads to most catalysts favoring the oxidation of secondary alcohols over primary alcohols. During a recent study of diol lactonizations, we discovered that iron tricarbonyl catalysts bearing trimethylsilyl-substituted cyclopentadienones preferentially reacted with primary over secondary alcohols. Here we report the effects of increasing the steric bulk of the silyl groups on catalyst selectivity toward primary alcohol oxidations. Catalysts with different silyl groups were prepared and tested in competition reactions where both a primary and secondary alcohol were present. The conversion data obtained from gas chromatography showed that as the steric bulk increased, the selectivity toward primary alcohol oxidation also increased but the overall conversions decreased.

Ligand steric effects on the selectivity of primary alcohol oxidations using (cyclopentadienone)iron tricarbonyl catalysts

ACS Spring 2022

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