Unexpected Selectivities in C−H Activations of Toluene andp-Xylene at Cationic Platinum(II) Diimine Complexes. New Mechanistic Insight into Product-Determining Factors

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Abstract
The CH activation of toluene and p-xylene at cationic PtII diimine complexes (NN)Pt(CH3)(H2O)+BF4- (NN = ArNCMeCMeNAr; 1(BF4-), NfNf, Ar = 3,5-(CF3)2C6H3); 2(BF4-), N‘N‘, Ar = 2,6-(CH3)2C6H3) has been investigated. The reactions were performed at ambient temperature in 2,2,2-trifluoroethanol (TFE), and after complete conversion of the starting material to mixtures of Pt-aryl/Pt-benzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts. In the reactions with toluene, the relative amounts of products resulting from aromatic CH activation were found to decrease in the order (NN)Pt(m-tolyl)(NCMe)+ > (NN)Pt(p-tolyl)(NCMe)+ > (NN)Pt(o-tolyl)(NCMe)+ for both 1 and 2. Unlike the reaction at 1, significant amounts of the benzylic activation product (N‘N‘)Pt(benzyl)(NCMe)+ were concurrently formed in the CH activation of toluene at 2. The CH activation of p-xylene revealed an even more remarkable difference between 1 and 2. Here, the product ratios of (NN)Pt(xylyl)(NCMe)+ and (NN)Pt(p-methylbenzyl)(NCMe)+ were found to be 90:10 and 7:93 for reactions at 1 and 2, respectively. The elimination of toluene from (NfNf)Pt(Tol)2 species (3a−c; a, Tol = o-tolyl; b, Tol = m-tolyl; c, Tol = p-tolyl) after protonolysis with 1 equiv of HBF4 was investigated. Most notably, protonation in neat TFE followed by addition of acetonitrile gave a 77:23 mixture of (NfNf)Pt(m-tolyl)(NCMe)+ (4b) and (NfNf)Pt(p-tolyl)(NCMe)+ (4c) from all three isomeric bis(tolyl) complexes 3a−c. The presence of acetonitrile during the protonation reactions resulted in considerably less isomerization. This behavior is explained by an associative mechanism for the product-determining displacement of toluene by the solvent. For the CH activation reactions, our findings suggest the existence of a dynamic equilibrium between the isomeric intermediates (NN)Pt(aryl)(CH4)+ (aryl = tolyl/benzyl from 1; xylyl/p-methylbenzyl from 2). The observed selectivities might then be explained by steric and electronic effects in the pentacoordinate transition-state structures for the solvent-induced associative elimination of methane from these intermediates.