Unexpected selectivities in C-H activations of toluene and p-xylene at cationic platinum(II) diimine complexes. New mechanistic insight into product-determining factors

The C-H activation of toluene and p-xylene at cationic Pt-II diimine comple xes (N-N)Pt(CH3)(H2O)+BF4- (N-N = Ar-N = CMe-CMe = N-Ar; 1(BF4-), N-f-N-f, Ar = 3,5-(CF3)(2)C6H3); (2)(BF4-), N'-N', Ar = 2,6-(CH3)(2)C6H3) has been i nvestigated. The reactions were performed at ambient temperature in 2,2,2-t rifluoroethanol (TFE), and after complete conversion of the starting materi al to mixtures of Pt-aryl/Pt-benzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts. In the rea ctions with toluene, the relative amounts of products resulting from aromat ic C-H activation were found to decrease in the order (N-N)Pt(m-tolyl)(NCMe )(+) > (N-N)Pt(p-tolyl)(NCMe)(+) > (N-N)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 C-H acti vation of toluene at 2. The C-H activation of p-xylene revealed an even mor e remarkable difference between 1 and 2. Here, the product ratios of (N-N)P t(xylyl)(NCMe)(+) and (N-N)Pt(p-methylbenzyl)(NCMe)(+) were found to be 90: 10 and 7:93 for reactions at 1 and 2, respectively. The elimination of tolu ene from (N-f-N-f)Pt(Tol)(2) species (3a-c; a, Tol = o-tolyl; b, Tol = m-to lyl; c, Tol = p-tolyl) after protonolysis with 1 equiv of HBF4 was investig ated. Most notably, protonation in neat TFE followed by addition of acetoni trile gave a 77:23 mixture of (N-f-N-f)Pt(mtolyl)(NCMe)(+) (4b) and (N-f-N- f)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 associ ative mechanism for the product-determining displacement of toluene by the solvent. For the C-H activation reactions, our findings suggest the existen ce of a dynamic equilibrium between the isomeric intermediates (N-N)Pt(aryl )(CH4)(+) (aryl = tolyl/benzyl from 1; xylyl/p-methylbenzyl from 2). The ob served selectivities might then be explained by steric and electronic effec ts in the pentacoordinate transition-state structures for the solvent-induc ed associative elimination of methane from these intermediates.