Redox reactions are ubiquitous in organic synthesis and intrinsic to organic electrosynthesis. The language and concepts used to describe reactions in these domains are sufficiently different to create barriers that hinder broader adoption and understanding of electrochemical methods. To bridge these gaps, this Synopsis compares chemical and electrochemical redox reactions, including concepts of free energy, voltage, kinetic barriers, and overpotential. This discussion is intended to increase the accessibility of electrochemistry for organic chemists lacking formal training in this area.
Robust, highly water-soluble quinone derivatives are efficiently prepared by a scalable electrosynthetic protocol employing low-cost commercially available precursors.
Molecular oxygen is the quintessential
oxidant for organic
chemical
synthesis, but many challenges continue to limit its utility and breadth
of applications. Extensive historical research has focused on overcoming
kinetic challenges presented by the ground-state triplet electronic
structure of O2 and the various reactivity and selectivity
challenges associated with reactive oxygen species derived from O2 reduction. This Perspective will analyze thermodynamic principles
underlying catalytic aerobic oxidation reactions, borrowing concepts
from the study of the oxygen reduction reaction (ORR) in fuel cells.
This analysis is especially important for “oxidase”-type
liquid-phase catalytic aerobic oxidation reactions, which proceed
by a mechanism that couples two sequential redox half-reactions: (1)
substrate oxidation and (2) oxygen reduction, typically affording
H2O2 or H2O. The catalysts for these
reactions feature redox potentials that lie between the potentials
associated with the substrate oxidation and oxygen reduction reactions,
and changes in the catalyst potential lead to variations in effective
overpotentials for the two half reactions. Catalysts that operate
at low ORR overpotential retain a more thermodynamic driving force
for the substrate oxidation step, enabling O2 to be used
in more challenging oxidations. While catalysts that operate at high
ORR overpotential have less driving force available for substrate
oxidation, they often exhibit different or improved chemoselectivity
relative to the high-potential catalysts. The concepts are elaborated
in a series of case studies to highlight their implications for chemical
synthesis. Examples include comparisons of (a) NO
x
/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential
quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic
oxidation catalysts with or without NO
x
cocatalysts. In addition, we show how the reductive activation of
O2 provides a means to access potentials not accessible
with conventional oxidase-type mechanisms. Overall, this analysis
highlights the central role of catalyst overpotential in guiding the
development of aerobic oxidation reactions.
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