Oxidative Addition of Chlorohydrocarbons to a Rhodium Tris(pyrazolyl)borate Complex

The
reactive fragment [Tp′Rh­(PMe3)], generated from
the thermal precursor Tp′Rh­(PMe3)­(Me)­H, is found
to cleave the C–Cl bonds of chlorohydrocarbons under mild conditions.
Reaction with chloromethane gives clean formation of an initial C–H
activation product, which rearranges to form the C–Cl activation
product at 30 °C. Reaction with dichloromethane or benzyl chloride
gives a mixture of C–Cl activation products as well as products
from chlorination. Reaction with chlorocyclohexane gives a mixture
of intermediates from C–H activation, which react further upon
heating to give a C–Cl cleavage product as well as the β-chloride
elimination product Tp′Rh­(PMe3)­(Cl)H plus cyclohexene.
Complete conversion from a C–H activation product to a C–Cl
activation product was observed in the reaction with 1,2-dichloroethylene,
where β-elimination is circumvented. Activation of 1-chlorobutane,
1,2-dichloroethane, or 1,4-dichlorobutane gives a mixture of C–Cl
activation products as well as Tp′Rh­(PMe3)­(Cl)­H
plus olefin. Similar to the case for activation of methylene chloride,
C–Cl activation and hydride/chloride exchange was observed
in the reaction with benzyl chloride, where C–H activation
was not seen. The reaction with chlorobenzene gives isomeric species
resulting from C–H activation, which react further to give
the corresponding chloride derivatives upon heating. Reaction with
pentachlorobenzene gives a cyclometalated product from C–H
bond cleavage in the phosphine ligand. These reactions are compared
and contrasted with related photoreactions with the [Tp′Rh­(CNneopentyl)]
analogue, where C–H activation is solely observed in most cases.
Mechanistic studies suggest the spectator ligand dependent reactivity
relies greatly on the dissociation energy of the Tp′Rh–L
bond.