摘要:
N,N'-Diaryl-alpha-diimine-ligated Pd(II) dimethyl complexes ((tBu2Ar)DAB(Me))PdMe2 and {((CF3)2Ar)DAB(Me)} PdMe2 {(tBu2Ar)DAB(Me): ArN=C(CH3)-C(CH3)=NAr, Ar=3,5-di-tert-butylphenyl; ((CF3)2Ar) DAB(Me): Ar = 3,5-bis(trifluoromethyl)phenyl} undergo protonolysis with HBF4(aq) in trifluoroethanol (TFE) to form cationic complexes [(alpha-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 O-2, the products of the reaction are biphenyl and a dimeric mu-hydroxide complex, [(alpha-diimine)Pd(OH)](2)[BF4](2). The Pd(0) formed in the reaction is reoxidized by O-2 to the dimeric mu-hydroxide complex after the oxidative C-C bond formation. The regioselectivity of arene coupling was investigated with toluene and alpha,alpha,alpha-trifluorotoluene as substrates.