Petroleum and natural gas are important sources of inexpensive hydrocarbons for the chemical and pharmaceutical industries. The selective conversion of C-H bonds into new functional groups under mild conditions is therefore of considerable interest. High-valent non-heme iron-oxo catalysts have been developed as potent oxidants for the functionalization of unactivated C-H bonds. Previous studies have advanced our mechanistic understanding of these versatile intermediates, provided insight into the role of spin state on reactivity, and made strides in product selectivity and substrate scope. However, a significant limitation of synthetic non-heme iron-oxo catalysts is poor stability and low turnover numbers under the harsh oxidizing conditions required of C-H bond oxidation. Polydentate ligands used in previous systems typically utilize oxidatively-weak, aliphatic linking groups and alkyl amine donors that are susceptible to catalyst decomposition. In order to improve catalyst stability while preserving high reactivity, we have developed new catalysts employing oxidatively-rugged tetradentate ligands comprised of robust aromatic building blocks, which provide cis-labile coordination sites at iron. Using m-chloroperbenzoic acid as the terminal oxidant, a new iron catalyst shows an alcohol-to-ketone (A/K) ratio of 6.6 for cyclohexane oxidation with conversion percentages as high as 80%. Moreover, catalysis towards adamantane oxidation shows remarkable regioselectivity (3°/2° = 48:1) favoring tertiary C-H bonds. Results indicate a molecular Fe(IV)=O intermediate is involved in the catalytic process with a characteristic λmax at 750 nm and a half-life of 1 hour at room temperature.

Durable and highly reactive iron-oxo catalyst for hydrocarbon C-H bond functionalization

255th National Meeting (2018)

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