How Alkali Metal Alkoxides Initiate Organic Radical Reactions

TL;DR

This paper shows that alkali metal alkoxides initiate radical reactions by deprotonating substrates to form benzynes, not through electron transfer as previously thought.

Contribution

The paper introduces a new mechanism for alkali metal alkoxide-initiated radical reactions involving benzynes and remote deprotonation.

Findings

Deprotonation of substrates forms benzynes, not electron transfer, initiating radical chemistry.

Multiple benzynes (o-, m-, p-, and remote) are formed simultaneously.

Methylation of arenes occurs via methyl radicals from tert-butoxide through a new, unreported mechanism.

Abstract

Alkali metal alkoxides have long been known to cause
hydrodehalogenation
of aryl halides; this conversion of aryl halides to arenes happens
when the reactions are conducted in appropriate solvents (with weak
C–H bonds). More recently, when aryl halides are heated with
alkoxides in arene solvents, coupling to arenes occurs. Both of these
reaction types are known to involve aryl radical intermediates. The
consensus has been that alkali metal alkoxides undergo electron transfer
to aryl halides to form radicals, but crucial evidence has been missing.
We now refute this proposal and show through deuterium isotope studies
that the deprotonation of the substrates leads to benzynes that initiate
radical chemistry. Surprisingly, o-, m-, p- and, in appropriate cases, r- (remote) benzynes are simultaneously formed. During reactions with
potassium tert-butoxide, we observed for the first
time low-level methylation of arenes, resulting from methyl radicals
derived from tert-butoxide. Although methyl radicals
could, in principle, arise by electron transfer from tert-butoxide ions, followed by known radical fragmentation, we show
that a different, previously unreported mechanism applies.

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