Scheme 2 Palladium-catalysed reaction of 1,4-dichlorobenzene (1a ) with internal alkynes (2a ) proposed by Duan group10.
To elucidate the catalytic reaction, Duan’s group performed some mechanistic experiments and proposed possible reaction mechanisms with the substrates dialkylalkyne (2a1 ) and diarylalkyne (2a2 ), respectively. As shown in Scheme 3, L1 ligand reacts with Pd(OAc)2 catalyst to form intermediateA , followed by C-H bond cleavage of 1,4-dichlorobenzene, producing arylpalladium B . Alkyne 2a then insertes into the C(aryl)-Pd bond to obtain alkenylpalladium intermediateC , from which different reactions occur. For the reaction of2a1 , C undergoes oxidative addition with the C-Cl cleavage to form Pd (IV) species D via path i. Subsequently, the reductive elimination of D leads to aryl Pd (II) speciesE , completing the 1, 4-Cl migration. Protonolysis of Ewith acetic acid furnishes product PR11 with the dissociation of the active catalyst A . With respect to the reaction with 2a2 , C directly hydrolyzes through path ii, affording product PR22 without a 1, 4-halide shift.