Nickel-Catalyzed Negishi Arylations of Propargylic Bromides: A Mechanistic Investigation

Although
nickel-catalyzed stereoconvergent couplings of racemic
alkyl electrophiles are emerging as a powerful tool in organic chemistry,
to date there have been no systematic mechanistic studies of such
processes. Herein, we examine the pathway for enantioselective Negishi
arylations of secondary propargylic bromides, and we provide evidence
for an unanticipated radical chain pathway wherein oxidative addition
of the C–Br bond occurs through a bimetallic mechanism. In
particular, we have crystallographically characterized a diamagnetic
arylnickel­(II) complex, [(i-Pr-pybox)­NiIIPh]­BArF4, and furnished support for [(i-Pr-pybox)­NiIIPh]+ being the predominant
nickel-containing species formed under the catalyzed conditions as
well as a key player in the cross-coupling mechanism. On the other
hand, our observations do not require a role for an organonickel­(I)
intermediate (e.g., (i-Pr-pybox)­NiIPh),
which has previously been suggested to be an intermediate in nickel-catalyzed
cross-couplings, oxidatively adding alkyl electrophiles through a
monometallic pathway.