posted on 2022-12-28, 18:36authored byRanjeesh Thenarukandiyil, Rohit Kamte, Subhash Garhwal, Philipp Effnert, Natalia Fridman, Graham de Ruiter
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.