Abstract
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Redox-neutral
methods for the functionalization of amine α-C–H
bonds are inherently efficient because they avoid external oxidants
and reductants and often do not generate unwanted byproducts. However,
most of the current methods for amine α-C–H bond functionalization
are oxidative in nature. While the most efficient variants utilize
atmospheric oxygen as the terminal oxidant, many such transformations
require the use of expensive or toxic oxidants, often coupled with
the need for transition metal catalysts.
Redox-neutral amine
α-functionalizations that involve intramolecular
hydride transfer steps provide viable alternatives to certain oxidative
reactions. These processes have been known for some time and are particularly
well suited for tertiary amine substrates. A mechanistically distinct
strategy for secondary amines has emerged only recently, despite sharing
common features with a range of classic organic transformations. Among
those are such widely used reactions as the Strecker, Mannich, Pictet–Spengler,
and Kabachnik–Fields reactions, Friedel–Crafts alkylations,
and iminium alkynylations. In these classic processes, condensation
of a secondary amine with an aldehyde (or a ketone) typically leads
to the formation of an intermediate iminium ion, which is subsequently
attacked by a nucleophile. The corresponding redox-versions of these
transformations utilize identical starting materials but incorporate
an isomerization step that enables α-C–H bond functionalization.
Intramolecular versions of these reactions include redox-neutral amine α-amination,
α-oxygenation, and α-sulfenylation. In all cases, a reductive
N-alkylation is effectively combined with an oxidative α-functionalization,
generating water as the only byproduct.
Reactions are promoted
by simple carboxylic acids and in some cases
require no additives. Azomethine ylides, dipolar species whose usage
is predominantly in [3 + 2] cycloadditions and other pericyclic processes,
have been identified as common intermediates. Extension of this chemistry
to amine α,β-difunctionalization has been shown to be
possible by way of converting the intermediate azomethine ylides into
transient enamines.
This Account details the evolution of this
general strategy and
the progress made to date. Further included is a discussion of related
decarboxylative reactions and transformations that result in the redox-neutral
aromatization of (partially) saturated cyclic amines. These processes
also involve azomethine ylides, reactive intermediates that appear
to be far more prevalent in condensation chemistry of amines and carbonyl
compounds than previously considered. In contrast, as exemplified
by some redox transformations that have been studied in greater detail,
iminium ions are not necessarily involved in all amine/aldehyde condensation
reactions.
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