α-Methylation of Ketones and Indoles Catalyzed by a Manganese(I) PCNHCP Pincer Complex with Methanol as a C1 Source

Abstract

Acceptorless alcohol dehydrogenation is a powerful reaction in sustainable synthesis. When combined in a tandem reaction with dehydrogenative coupling or hydrogen borrowing, acceptorless dehydrogenation can be used for the environmentally benign construction of C–C, C–N, and C–O bonds. While many of these reactions rely on using precious metals, in the past decade the use of earth-abundant metals has become more prevalent. If a green and renewable feedstock could be introduced as well, the sustainability of the acceptorless dehydrogenation could further be increased. Methanol would be such a substrate and could serve as a renewable C1 source when used for the methylation of a wide variety of substrates. Herein, we report the efficient manganese-catalyzed α-methylation of ketones and indoles. The manganese catalyst is based on a PCNHCP pincer platform containing a rare central carbene donor. The reaction supports a diverse set of functional groups, occurs at moderate temperatures (110 °C), and provides the corresponding methylated ketones and indoles in excellent yields. In contrast to previously reported mechanisms, the herein reported mechanism does not depend on metal–ligand cooperativity but rather proceeds via (i) a metal-based mechanism featuring a manganese hydride or (ii) via a ligand-centered mechanism where a manganese-carbonyl acts as catalytic center, depending on the used additive.