Synthesis of Benzocyclobutenes by Palladium-Catalyzed C-H Activation of Methyl Groups: Method and Mechanistic Study

An efficient catalytic system has been developed for the synthesis of benzocyclobutenes by C-H activation of methyl groups. The optimal conditions employed a combination of Pd(OAc)(2) and (PBu3)-Bu-t as catalyst, K2CO3 as the base, and DMF as solvent. A variety of substituted BCB were obtained under these conditions with yields in the 44-92% range, including molecules that are hardly accessible by other methods. The reaction was found limited to substrates bearing a quaternary benzylic carbon, but benzocyclobutenes bearing a tertiary benzylic carbon could be obtained indirectly from diesters by decarboxylation. Reaction substrates bearing a small substituent para to bromine gave an unexpected regioisomer that likely arose from a 1,4-palladium migration process. The formation of this "abnormal" regioisomer could be suppressed by introducing a larger subsituent para to bromine. DFT(B3PW91) calculations on the reaction of 2-bromo-tart-butylbenzene with Pd((PBu3)-Bu-t) with different bases (acetate, bicarbonate, carbonate) showed the critical influence of the coordination mode of the base to induce both an easy C-H activation and to allow for a pathway for 1,4-palladium migration. Carbonate is shown to be more efficient than the two other bases because it can abstract the proton easily and at the same time maintain k(1)-coordination without extensive electronic reorganization.