A Change in C–H Activation Mechanism: Experimental and Computational Investigations of Rh-Catalyzed Disubstituted Benzene Functionalization

Abstract

We report the ethenylation of 1,3- and 1,2-disubstituted benzenes using [(η2-C2H4)2Rh(μ-OAc)]2as a catalyst precursor and Cu(OPiv)2as the oxidant. The regioselectivity of alkenylation for 1,3-disubstituted benzenes produces 3,5-disubstituted styrene products, while the alkenylation of 1,2-disubstituted benzenes produces 3,4-disubstituted styrene products. The rate of alkenylation is influenced by steric and electronic factors based on the substituents of the benzene unit. In all cases, 1,2-disubstituted benzenes react faster than 1,3-disubstituted benzenes, with a rate difference that is from 2 times up to >70 times more rapid for 1,2-disubstituted substrates. This is likely due to the difference in the number of accessible C–H bonds based on the steric protection of C–H bonds adjacent to functionality. Furthermore, the rate of alkenylation is influenced by the arene substituent electronics. The rates of alkenylation for 1,2-disubstituted benzenes follow the trend OMe > Me > CF3> Cl, while for 1,3-disubstituted benzenes the trend is CF3> Cl > Me > OMe. Using quantum mechanics DFT calculations, we found that the C–H activation step can occur by two different mechanisms. The electronic properties of substituents on the arene ring change the preferred C–H bond-breaking mechanism for 1,2-disubstituted and 1,3-disubstituted benzenes.