Nickel Hydroxo Complexes as Intermediates in Nickel-Catalyzed
Suzuki–Miyaura Cross-Coupling
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Abstract
The
synthesis, characterization, and reactivity of intermediates
formed in the Ni-catalyzed Suzuki–Miyaura cross-coupling (SMC)
of an aryl chloride are described. Oxidative addition of 1-chloro-4-trifluoromethylbenzene
(<b>1</b>) to a mixture of Ni(cod)<sub>2</sub> and PCy<sub>3</sub> afforded NiCl(4-CF<sub>3</sub>Ph)(PCy<sub>3</sub>)<sub>2</sub> (<b>2</b>), which then cleanly provided dimeric [Ni(4-CF<sub>3</sub>Ph)(μ–OH)(PCy<sub>3</sub>)]<sub>2</sub> (<b>3</b>) by reaction with aqueous KOH. Reactivity studies of <b>2</b> and <b>3</b> with phenylboronic acid (<b>4</b>) revealed
that, while <b>2</b> affords only traces of the biphenyl coupling
product after 24 h, the same reaction with <b>3</b> is complete
within minutes at room temperature. In contrast, the reaction of <b>3</b> with potassium phenyltrihydroxyborate (<b>6</b>) is
much slower than that with boronic acid <b>4</b>, and significantly
lower yields of the cross-coupling product are obtained. We show that
formation of the hydroxo species <b>3</b> is the rate-determining
step in the present SMC
