the end methyl group, where oxidative cleavage was rapid. This hypothesis was
supported by experiments using deuterium-labeled alkyl deuterides. By monitoring
the rates at which the deuterium appeared in the α and distal positions of the alkyl
group vs. dissociation, the relative rates could be established as
k C–H cleavage > k migration > k dissociation (Eq. 3) [11].
ð3Þ
Further evidence for the intermediacy of alkane complexes came from studies of
the reductive elimination of methane from Tp
0 Rh(CNR)(Me)H, 3. Here, the rate of
reaction with C 6 D 6 to produce 2-d 6 was found to be dependent on the concentration
of C 6 D 6 in inert C 6 F 6 solvent. As with the reaction with isocyanide in Eq. 2, the rate
was first order in [C 6 D 6 ] at low concentrations but less than first order at higher
[C 6 D 6 ]. These observations were treated in terms of a reversible equilibrium
between 3 and an η
2 -methane complex that then underwent bimolecular displacement by benzene. The reaction also shows a “solvent kinetic isotope effect,” with
the rate being faster in C 6 H 6 than in C 6 D 6 (k C 6 H 6 =k C 6 D 6 ¼ 1:08). Since the ratedetermining step involves bimolecular reaction with benzene, the rate is slightly
different with C 6 D 6 vs. C 6 H 6 [12].
ð4Þ
Terminal alkynes also add to [Tp
0 Rh(CNR)]. Irradiation of the carbodiimide
complex 1 in neat 1-alkyne leads to the activation of the sp C–H bond. In cases
where other “activatable” C–H bonds were presented, competitive C–H activation
at these positions was observed. For example, t-butylacetylene and trifluoromethyl
acetylene give exclusively alkynyl hydride products, whereas 1-octyne and
trimethylsilylacetylene also give products resulting from methyl group activation.
In both of the latter cases, the sp
3 C–H activation products are unstable and convert
to the terminal alkynyl products at room temperature after a few days (Scheme 1).
Similarly, the activation of arylalkynes leads to mixtures of sp and sp
2 C–H
activation products. The unsaturated fragment [Tp
0 Rh(CNR)] was prepared either
70
W.D. Jones
supported by experiments using deuterium-labeled alkyl deuterides. By monitoring
the rates at which the deuterium appeared in the α and distal positions of the alkyl
group vs. dissociation, the relative rates could be established as
k C–H cleavage > k migration > k dissociation (Eq. 3) [11].
ð3Þ
Further evidence for the intermediacy of alkane complexes came from studies of
the reductive elimination of methane from Tp
0 Rh(CNR)(Me)H, 3. Here, the rate of
reaction with C 6 D 6 to produce 2-d 6 was found to be dependent on the concentration
of C 6 D 6 in inert C 6 F 6 solvent. As with the reaction with isocyanide in Eq. 2, the rate
was first order in [C 6 D 6 ] at low concentrations but less than first order at higher
[C 6 D 6 ]. These observations were treated in terms of a reversible equilibrium
between 3 and an η
2 -methane complex that then underwent bimolecular displacement by benzene. The reaction also shows a “solvent kinetic isotope effect,” with
the rate being faster in C 6 H 6 than in C 6 D 6 (k C 6 H 6 =k C 6 D 6 ¼ 1:08). Since the ratedetermining step involves bimolecular reaction with benzene, the rate is slightly
different with C 6 D 6 vs. C 6 H 6 [12].
ð4Þ
Terminal alkynes also add to [Tp
0 Rh(CNR)]. Irradiation of the carbodiimide
complex 1 in neat 1-alkyne leads to the activation of the sp C–H bond. In cases
where other “activatable” C–H bonds were presented, competitive C–H activation
at these positions was observed. For example, t-butylacetylene and trifluoromethyl
acetylene give exclusively alkynyl hydride products, whereas 1-octyne and
trimethylsilylacetylene also give products resulting from methyl group activation.
In both of the latter cases, the sp
3 C–H activation products are unstable and convert
to the terminal alkynyl products at room temperature after a few days (Scheme 1).
Similarly, the activation of arylalkynes leads to mixtures of sp and sp
2 C–H
activation products. The unsaturated fragment [Tp
0 Rh(CNR)] was prepared either
70
W.D. Jones