CARBON-CARBON BOND ACTIVATION VIA FORMAL BETA-METHYL-ELIMINATION FROM [ETA(5)-6,6-DIMETHYLCYCLOHEXADIENYL)RU(DPPE)(CH2CL2)]PF6

Carbon-carbon bond activation has been observed through a formal P-methyl elimination from a 6,6-dimethylcyclohexadienyl (dmCh) ligand. Reflux of an EtOH solution of RuCl3.3H(2)O, (dmCh)H (15 equiv.), and Zn dust (15 equiv.) afforded (dmCh)(2)Ru (1, 65-77%). Protonation of 1 with HBF4.Et(2)O in ether provided [(dmCh)(2)RuH][BF4] (2) in 77% yield; NMR spectra were consistent with either a terminal hydride or rapidly equilibrated agostic ground-state structure. Addition of CH3CN to 2, or protonation of 1 in CH3CN, gave [(eta(5)-dmCh)Ru(NCCH3)(3)][BF4] (3, 70%). Treatment of 3 with 2.0 equiv. PMe(3) or 1.0 equiv. dppe produced [(dmCh)RuL(2)(NCCH3)][BF4] (4, L = PMe(3); 5, L = dppe), which were poor precursors to halide derivatives. Treatment of 1 with 12 M aqueous HCl in acetone generated [(dmCh)RuCl](n), (6) in 55% yield. Addition of excess norbornadiene to 6 in hexane yielded (dmCh)Ru(NBD)Cl (7, 90%), which proved to be a ready precursor to (dmCh)RuL(2)Cl (8, L = PMe(3), 90%; 9, L(2) = dppe, 53%) upon addition of the appropriate phosphine. Chloride abstraction from 8 with TlPF6, afforded numerous [(dmCh)Ru(PMe(3))(2)(solvent)]PF6 [(10-solvent), solvent = CD2Cl2, CD3NO2, THF, 2-MeTHF] derivatives, but beta-methyl elimination was not observed in subsequent thermolyses. A similar chloride abstraction from 9 produced [(dmCh)Ru(dppe)(CD2Cl2)]PF6 ([11-CD2Cl2]PF6); thermolysis of 11-CD2Cl2 at 91 degrees C for 12 h generated [(eta(6)-C7H8)Ru(dppe) (CH3)]PF6 (12), presumably via the coordinatively unsaturated precursor, [(dmCh)Ru (dppe)]PF6 ([11]PF6). The molecularity of the beta-methyl elimination pathway remained elusive. Addition of 1.0 equiv. of [Cp(2)Fe][PF6] to 1 in CD3CN gave 3-PF6, while oxidation in CD2Cl2 provided [(dmCh)Ru(eta(6)-toluene)]PF6 (13-PF6); cyclic voltammetry pinpointed the irreversible oxidation at +/-0.85 V vs Ag/AgCl in THF. Three critical factors are responsible for beta-methyl elimination from [11]PF6: (1) coordinative/electronic unsaturation; (2) the compatability of ruthenium to both dmCh (precursor) and toluene (product) ligation; (3) an orbital with directionality appropriate to accept the migrating methyl group.