Carbon-carbon bond activation via formal β-methyl-elimination from [η5-6,6-dimethylcyclohexadienyl)Ru(dppe)(CH2Cl2)]PF6

Article abstract of DOI:10.1016/0277-5387(94)00328-C

Carbon-carbon bond activation has been observed through a formal β-methyl elimination from a 6,6-dimethylcyclohexadienyl (dmCh) ligand. Reflux of an EtOH solution of RuCl3.3H2O, (dmCh)H (15 equiv.), and Zn dust (15 equiv.) afforded (dmCh02Ru (1, 65-77%). Protonation of 1 with HBF4.Et2O in ether provided [(dmCh)2RuH][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 1in CH3CN, gave [(η5-dmCh)Ru(NCCH3)3][BF4] (3, 70%). Treatment of 3 with 2.0 equiv. PMe3 or 1.0 equiv. dppe produced [(dmCh)RuL2(NCCH3)][BF4 ] (4, L=PMe3; 5, L=dppe), which were poor precursors to halide derivatives. Treatment of 1 with 12 M aqueous HCl in acetone gegerated [(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)RuL2Cl (8, L=PMe3, 90%; 9, L2=dppe, 53%) upon addition of the appropriate phosphine. Chloride abstraction from 8 with TlPF6 afforded numerous [(dmCh)Ru(PMe3)2(solvent)]PF6 [(10-solvent), solvent=CD2Cl2, CD3NO2,THF, 2-Me-THF] derivatives, but β-methyl elimination was not obser ved in subsequent thermolyses. A similar chloride abstraction from 9 produced [(dmCh)Ru(dppe)(CD2Cl2)]PF6 ([11-CD2Cl2]PF6); thermolysis of 11-CD2Cl2 at 91°C for 12 h generated [(η6-C7H8)Ru(dppe)(CH3)]PF6 (12), presumably via the coordinatively unsaturated precursor, [(dmCh)Ru(dppe)]PF6 ([11]PF6). The molecularity of the β-methyl elimination pathway remained elusive. Addition of 1.0 equiv. of [Cp2Fe][PF6] to 1 in CD3CN gave 3-PF6, while oxidation in CD2Cl2 provided [(dmCh)Ru(η6-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 β-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.