7 Thermodynamic Determination of Rhodium–Carbon
Bond Strengths in Tp
0 Rh[P(OMe) 3 ](R)H
As with the other ligands, reductive elimination studies were carried out in C 6 D 6
solvent to generate hydrocarbon and Tp
0 Rh[P(OMe) 3 ](C 6 D 5 )D (9-d 6 ). The eliminations were carried out at temperatures between 20 and 140
C. To compare these
elimination barriers with those of benzene, reductive elimination of C 6 H 6 from
9 was carried out at 70–100
C and activation parameters measured for the reductive
elimination. An Eyring plot gave ΔH
{
¼ 30.7(6) kcal/mol and ΔS
{
¼ 10.3(3) e.u.
and permitted comparison of barriers in Fig. 1 at the same temperature. Table 3
summarizes the barrier heights measured and the temperature at which they were
measured.
Competition experiments between benzene and the hydrocarbon substrates were
examined by photolysis of 7 in a mixture of the two substrates as solvent. Examination of the NMR spectra after a short irradiation time provided the competition
data, typically by examination of the area of the hydride resonances of the products.
Scheme 4 Reactions of Tp
0 Rh[P(OMe) 3 ] with hydrocarbon substrates
84
W.D. Jones