7. Mechanistic Origin of Regioselectivity in Nickel-Catalyzed Olefin Hydroheteroarylation through C–H Activation

Jiang, Y.-Y.; Li, Z.; Shi, J.* Organometallics 2012, 31, 4356–4366. Download Link

Abstract: Ni-catalyzed addition of electron-deficient arenes and heteroarenes to olefin substrates through C−H activation provides an important method for the synthesis of diarylalkanes. This reaction usually generates Markovnikov adducts for aryl olefins, whereas anti-Markovnikov adducts are obtained for alkyl-substituted alkenes. To understand the mechanistic origin of this interesting regioselectivity, we conducted density functional theory calculations using the reactions of benzoxazole with styrene and 1-hexene as models. The calculation results are consistent with experimental observations, showing that the reaction proceeds through a mechanism involving Ar−H oxidative addition, hydronickelation, and C−C reductive elimination. Further calculations indicate that a better antiMarkovnikov regioselectivity can be obtained for olefins substituted with more bulky alkyl groups, whereas a better Markovnikov regioselectivity can be achieved for more electron-deficient para-substituted styrenes. Further analysis shows that a secondary orbital overlap exists between Ni and the aryl group of styrene in the C−C reductive elimination transition state. This secondary orbital interaction is the key factor causing Markovnikov regioselectivity for vinylarenes. On the other hand, in the absence of this secondary orbital overlap the steric effect dominates the selectivity and, therefore, leads to the anti-Markovnikov products for alkylsubstituted alkenes.