Synthetic Implications of Spin States in the Reactivity of Ni Intermediate Complexes in Coupling Reactions

We present a mechanistic study on the Ni‐catalyzed cross‐coupling reaction of bromoalkanes with diboron compounds as reductants. Our results point to a more complex scenario than usually accepted and suggest the participation of Ni(II) triplet complexes as relevant intermediates. We have shown that activation of haloalkanes by boryl‐ and alkyl‐Ni(I) complexes involves initial homolytic cleavage of the CBr bond and leads to usually overlooked Ni(II) species having two unpaired electrons. The geometry of these complexes favors an unexpected Br‐B reductive elimination instead of the usual C‐B reductive elimination from the Ni(III) species formed after radical coordination. These results not only explain the experimental observations and the absence of intermediate alkylboronates in the cross‐coupling of haloalkanes but also suggest that consideration of the intermediacy of triplet complexes may lead to design new reactions.