Selectivity Effects of Hydrogen Acceptors and Catalyst Structures in Alcohol Oxidations Using (Cyclopentadienone)iron Tricarbonyl Compounds.

Oppenauer-type oxidations are catalyzed by air- and moisture-stable, sustainable, (cyclopentadienone)iron carbonyl compounds, but the substrate scope is limited due to the low reduction potential of acetone, which is the most commonly used hydrogen acceptor. We discovered that furfural, an aldehyde derived from cellulosic biomass, is an effective hydrogen acceptor with this class of catalysts. In general, reactions using furfural as the hydrogen acceptor led to higher isolated yields of ketones and aldehydes compared to those using acetone. Importantly, primary benzylic and allylic alcohols─typically a challenging class of alcohols to oxidize with these catalysts─could be oxidized. The selectivity for primary vs secondary alcohol oxidation with (cyclopentadienone)iron carbonyl catalysts was also explored using acetone and furfural as the hydrogen acceptors. Most of the catalysts tested preferentially oxidized unhindered secondary alcohols, but catalysts with trialkylsilyl groups in the 2- and 5-positions of the cyclopentadienone preferentially oxidized primary alcohols. A combination of substrate scope experiments and kinetic studies concluded that the selectivity with the trialkylsilyl-based catalysts was kinetically derived─primary alcohols were oxidized more quickly than secondary─and the selectivity for secondary alcohol oxidation with the other catalysts arose from the equilibrium-driven nature of the Oppenauer-type oxidation.