Unexpected selectivities in C-H activations of toluene and p-xylene at cationic platinum(II) diimine complexes. New mechanistic insight into product-determining factors
L. Johansson et al., Unexpected selectivities in C-H activations of toluene and p-xylene at cationic platinum(II) diimine complexes. New mechanistic insight into product-determining factors, J AM CHEM S, 123(27), 2001, pp. 6579-6590
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.