The Ni/Ir-photocatalyzed coupling of an aryl bromide
(ArBr) with
an alkyl bromide (RBr) has been analyzed using in situ LED-19F NMR spectroscopy. Four components (light, [ArBr],
[Ni], [Ir]) are found to control the rate of ArBr consumption, but
not the product selectivity, while two components ([(TMS)3SiH], [RBr]) independently control the product selectivity, but not
the rate. A major resting state of nickel has been identified as ArNiII(L)Br, and 13C-isotopic entrainment is used to
show that the complex undergoes Ir-photocatalyzed conversion to products
(Ar-R, Ar-H, Ar-solvent) in competition with the release of ArBr.
A range of competing absorption and quenching effects lead to complex
correlations between the Ir and Ni catalyst loadings and the reaction
rate. Differences in the Ir/Ni Beer–Lambert absorption profiles
allow the rate to be increased by the use of a shorter-wavelength
light source without compromising the selectivity. A minimal kinetic
model for the process allows simulation of the reaction and provides
insights for optimization of these processes in the laboratory.
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