Oxidative Hydrophenylation of Ethylene Using a Cationic Ru(II) Catalyst: Styrene Production with Ethylene as the Oxidant

The complex [(MeOTTM)Ru(P(OCH2)(3)CEt)(NCMe) Ph][BAr4'] (MeOTMM= 4,4', 4 ''-(methoxymethanetriyl)-tris(1-benzyl-1H-1,2,3-triazole), BAr4'= tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) is used to catalyze the hydrophenylation of ethylene to produce styrene and ethylbenzene. The selectivity of styrene versus ethylbenzene varies as a function of ethylene pressure, and replacing the MeOTTM ligand with tris(1-phenyl-1H-1,2,3-triazol-4-yl)methanol reduces the selectivity toward styrene. For styrene production ethylene serves as the oxidant to produce ethane, as determined by both H-1 NMR spectroscopy and GC-MS. The Ru(III/II) potentials of [(MeOTTM)Ru[P(OCH2)(3)CEt](NCMe)Ph][BAr4'] (0.86 V) and [(HC(pz(5))(3))Ru[P(OCH2)(3)CEt](NCMe)Ph][BA(4)'] (0.82 V) (HC(pz(5))(3)=tris(5-methyl-pyrazolyl)methane) are nearly identical. Since catalytic conversion of ethylene and benzene by [(HC(pz(5))(3))Ru[P(OCH2)(3)CEt](NCMe)Ph][BAr4'] is known to selectively produce ethylbenzene, the formation of styrene using [(MeOTTM)Ru[P(OCH2)(3)CEt](NCMe) Ph][BAr4'] is attributed to the substituents on the triazole rings of the MeOTTM ligand.