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Download fileMechanistic Insight into Catalytic Redox-Neutral C–H Bond Activation Involving Manganese(I) Carbonyls: Catalyst Activation, Turnover, and Deactivation Pathways Reveal an Intricate Network of Steps
journal contribution
posted on 2019-01-29, 22:10 authored by L. Anders Hammarback, Alan Robinson, Jason M. Lynam, Ian J. S. FairlambManganese(I) carbonyl-catalyzed C–H
bond functionalization
of 2-phenylpyridine and related compounds containing suitable metal
directing groups has recently emerged as a potentially useful synthetic
methodology for the introduction of various groups to the ortho position
of a benzene ring. Preliminary mechanistic studies have highlighted
that these reactions could proceed via numerous different species
and steps and, moreover, potentially different catalytic cycles. The
primary requirement for typically 10 mol % catalyst, oftentimes the
ubiquitous precursor catalyst, BrMn(CO)5, has not yet been
questioned nor significantly improved upon, suggesting catalytic deactivation
may be a serious issue to be understood and resolved. Several critical
questions are further raised by the species responsible for providing
a source of protons in the protonation of vinyl–manganese(I)
carbonyl intermediates. In this study, using a combination of experimental
and theoretical methods, we provide comprehensive answers to the key
mechanistic questions concerning the Mn(I) carbonyl-catalyzed C–H
bond functionalization of 2-phenylpyridine and related compounds.
Our results enable the explanation of alkyne substrate dependencies,
i.e., internal versus terminal alkynes. We found that there are different
catalyst activation pathways for BrMn(CO)5, e.g., terminal
alkynes lead to the generation of MnI–acetylide
species, whose formation is reminiscent of CuI–acetylide
species proposed to be of critical importance in Sonogashira cross-coupling
processes. We have unequivocally established that alkyne, 2-phenylpyridine,
and water can facilitate hydrogen transfer in the protonation step,
leading to the liberation of protonated alkene products.