Roles of Base in the Pd-Catalyzed Annulative Chlorophenylene Dimerization

The detailed mechanism of the Pd-catalyzed annulative chlorophenylene dimerization (ACD) has been elucidated and the roles of the base have been identified. It is shown that the initial steps of this reaction-the active catalyst formation and the C-Cl bond activation-proceed via the "base-assisted oxidative addition" mechanism and require only a 16.2 kcal/mol barrier. The following steps of the reaction are palladacycle and Pd-aryne formations, among which the former is favorable: it proceeds with a moderate C-H activation barrier and is slightly exergonic. Although the Pd-aryne formation requires a slightly lower energy barrier, it is highly endergonic. It is shown that the base plays important roles also in the palladacycle formation and facilitates the driving of the reaction forward by removing a proton to the solution via the bicarbonate-to-carbonate exchange mechanism. Addition of the second C-Cl bond to the thermodynamically favorable palladacycle, i.e., Pd(II)/Pd(IV) oxidation, is a rate-limiting step of the entire Pd-catalyzed and Cs-carbonate-mediated ACD reactions: it occurs with a 35.8 kcal/mol energy barrier and is exergonic by 25.1 kcal/mol. The following polycyclic aromatic hydrocarbon (PAH) formation is a multistep process and requires a lesser energy barrier. An alternative pathway, namely, cyclooctatetraene (COT) formation, requires a higher energy barrier and is not feasible. This finding is consistent with experiments that show no COT product in the utilized conditions. The calculations also indicate that the observed diminishing of the yield of the Pd-catalyzed ACD reaction upon the use of Na₂CO₃ instead of Cs₂CO₃ is the result of not only the poor solubility of Na-carbonate in the used experimental conditions but also a prohibitively large free-energy barrier required for the second C-Cl activation.