Roles of Base in the Pd-Catalyzed Annulative Chlorophenylene Dimerization
!"#$%&"'&()%$&*+&,-$&./01),)#23$/&4++5#),*6$& 1-#"7"8-$+2#$+$&9*:$7*3),*"+& Li-Ping Xu,a,b Brandon E. Haines,c Manjaly J. Ajitha,a Kei Murakami,d Kenichiro Itami,*,d and Djamaladdin G. Musaev*,a a Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia 30322, USA b School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255000, China c Westmont College, Department of Chemistry, Santa Barbara CA, 93108, USA d Institute of Transformative Bio-Molecules (WPI-ITbM) and Graduate School of Science, and JST-ERATO, Itami Molecular Nanocarbon Project, Nagoya University, Chikusa, Nagoya 464- 8602, Japan KEYWORDS: Palladium catalyst, polycyclic aromatic hydrocarbon, C-H functionalization, DFT, base effect ABSTRACT By using computation, the detailed mechanism of the Pd-catalyzed annulative chlorophenylene dimerization has been elucidated and the roles of the base have been identified. It is shown that the initial steps of this reaction are the active catalyst formation and the first C–Cl bond activation proceeds via the “base-assisted oxidative addition” mechanism. Overall, these steps of the reaction proceed via 16.2 kcal/mol free energy barrier for the C–Cl bond activation and are highly exergonic. Importantly, the base directly participates in the C–Cl bond cleavage introduces a mechanistic switch of the Pd(0)/Pd(II) oxidation and makes the reaction more exergonic. The following steps of the reaction are palladacycle and Pd-aryne formations, among which the palladacycle formation is found to be 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: therefore, the participation of the Pd-aryne complex, or its derivatives, in the Pd- catalyzed annulative chlorophenylene dimerization in the presence of Cs2CO3 seems to be unlikely.
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