C-arenium-C-alkyl bond making and breaking: Key process in the platinum-mediated C-aryl-C-alkyl bond formation. Analogies to organic electrophilic aromatic substitution

The reaction of cationic platinum aqua complexes 2 [Pt(C6H2{CH2NMe2}(2)-E-4 )(OH2)](X') X' = SO3CF3, BF4) with alkyl halides RX gave various air-stable arenium complexes 3-5 containing a new C-C bond (R = Me, 3; Et, 4; Bn, 5). Electron-releasing oxo-substituents on the aromatic ligand (E = e.g., OH, b; OMe, c) enhance the reactivity of the aqua complex 2 and were essential for arenium formation from alkyl halides different from MeX. This process i s initiated by oxidative addition of alkyl halides to the platinum(II) cent er of 2, which affords (alkyl)(aryl) platinum(IV) complexes (e.g., 9, alkyl = benzyl) as intermediates. Spectroscopic analyses provided direct evidenc e for a subsequent reversible 1,2-sigmatropic shift of the alkyl group alon g the Pt-C-aryl bond, which is identical to repetitive C-arenium-C-alkyl, b ond making and breaking and concerted metal reduction and oxidation, Temper ature-dependent NMR spectroscopy revealed DeltaH degrees = - 1.3 (+/- 0.1) kJ mol(-1), DeltaS degrees = +3.8 (+/- 0.2) J mol(-1) K-1, and DeltaG degre es (298) = -2.4 (+/- 0.1) kJ mol(-1) for the formation of the arenium compl ex 5b from 9 involving the migration of a benzyl group. The arenium complex es were transformed to cyclohexadiene-type addition products 7 or to demeta lated alkyl-substituted arenes, 8, thus completing the platinum-mediated fo rmation of a sp(2)-sp(3) C-C bond which is analogous to the aromatic substi tution of a [PtX](+) unit by an alkyl cation R+. The formation of related t rimethylsilyl arenium complexes 6 suggests arenium complexes as key interme diates, not only in (metal-mediated) sp(2)-sp(3) C-C bond making and breaki ng but also in silyl-directed cyclometalation.