Title: Arene C−H Bond Activation and Arene Oxidative Coupling by Cationic Palladium(II) Complexes
Abstract: N,N'-Diaryl-α-diimine-ligated Pd(II) dimethyl complexes (tBu2ArDABMe)PdMe2 and {(CF3)2ArDABMe}PdMe2 {tBu2ArDABMe: ArNC(CH3)−C(CH3)NAr, Ar = 3,5-di-tert-butylphenyl; (CF3)2ArDABMe: Ar = 3,5-bis(trifluoromethyl)phenyl} undergo protonolysis with HBF4(aq) in trifluoroethanol (TFE) to form cationic complexes [(α-diimine)Pd(CH3)(H2O)][BF4]. The cations activate benzene C−H bonds at room temperature. Kinetic analyses reveal trends similar to those observed for the analogous platinum complexes: the C−H activation step is rate-determining (KIE = 4.1 ± 0.5) and is inhibited by H2O. The kinetic data are consistent with a mechanism in which benzene substitution proceeds by a solvent- (TFE-) assisted associative pathway. Following benzene C−H activation under 1 atm O2, the products of the reaction are biphenyl and a dimeric μ-hydroxide complex, [(α-diimine)Pd(OH)]2[BF4]2. The Pd(0) formed in the reaction is reoxidized by O2 to the dimeric μ-hydroxide complex after the oxidative C−C bond formation. The regioselectivity of arene coupling was investigated with toluene and α,α,α-trifluorotoluene as substrates.