A systematic theoretical study on the Rh-catalyzed oxidative Heck-coupling of phenol carbamates with alkenes is carried out. Two possible mechanisms (i.e. arene activation-first and alkene activation-first mechanisms) are examined. As to the C–H activation step, four mechanisms including oxidative addition, electrophilic substitution, concerted metallation-deprotonation (CMD), and σ-bond metathesis are evaluated. The calculation results indicate that the arene activation-first mechanism is more favorable for the overall catalytic cycle. This mechanism involves three steps: arene C–H activation at the position ortho to the carbamate directing group affording a six-membered rhodiacycle intermediate, insertion of the alkene double bond into the Rh(III)–aryl bond, and a final β-H elimination step to release the product and re-generate the catalyst. The rate determining step of the overall catalytic cycle is the arene C–H activation step, which is found to proceed through the acetate-assisted CMD mechanism.
No comments:
Post a Comment