posted on 2023-12-27, 16:07authored byZhipeng Lin, João C.
A. Oliveira, Alexej Scheremetjew, Lutz Ackermann
The electrochemical
transition metal-catalyzed cross-dehydrogenative
reaction has emerged as a promising platform to achieve a sustainable
and atom-economic organic synthesis that avoids hazardous oxidants
and minimizes undesired byproducts and circuitous functional group
operations. However, a poor mechanistic understanding still prevents
the widespread adoption of this strategy. In this regard, we herein
present an electrochemical palladium-catalyzed oxidative coupling
strategy to access biaryls in the absence of a stoichiometric chemical
oxidant. The robust palladaelectrocatalysis considerably suppresses
the occurrence of homocoupling and oxygenation, being compatible even
with electron-deficient arenes. Late-stage functionalization and Boscalid
precursor synthesis further highlighted the practical importance of
our electrolysis. Remarkably, mechanistic studies including the evaluation
of the reaction order of each component by variable time normalization
analysis (VTNA) and initial rate analysis, H/D exchange experiment,
kinetic isotope effect, and stoichiometric organometallic experiments
provided strong support for the involvement of transmetalation between
two organopalladium complexes in the turnover limiting step. Therefore,
matching the concentrations or lifetimes of two distinct organopalladium
intermediates is revealed to be a pivot to the success of electrooxidative
catalysis. Moreover, the presence of cationic copper(II) seems to
contribute to the stabilization of the palladium(0) catalyst instead
of playing a role in the oxidation of the catalyst.