η-Arene–rhenium chemistry: activation of sp3 carbon–hydrogen bonds of alkylbenzenes by rhenium; formation of binuclear µ-alkylidene derivatives

Abstract

Co-condensation of rhenium atoms with arenes gives the binuclear compounds [(η-R)Re(µ-H)₂(µ-CHR′)Re(η-R)], where R = toluene, p-xylene, or mesitylene and R′= Ph, p-McC₆H₄, or 3,5-Me₂C₆H₃ respectively. Similarly, rhenium atoms with ethylbenzene give a mixture of the isomers [(η-C₆H₅Et)Re(µ-H)₂(µ-CHCH₂Ph)Re(η-C₆H₅Et)] and [(η-C₆H₅Et)Re(µ-H)₂(µ-CMePh)Re(η-C₆H₅Et)]. These µ-alkylidene compounds have been characterised inter alia by detailed n.m.r. studies. Variable-temperature n.m.r. data show the barriers to rotation of the aryl groups of the µ-CHR′ system to be ΔG^‡= 55.1, 64.8, and 78.0 kJ mol^–1 for R′= Ph, p-MeC₆H₄, and 3,5-Me₂C₆H₃ respectively.