Investigation of the Mechanism of Alkane Reductive Elimination and Skeletal Isomerization in Tp‘Rh(CNneopentyl)(R)H Complexes:  The Role of Alkane Complexes

Abstract

Experiments are described that provide indirect evidence for the involvement of alkane sigma-complexes in oxidative addition/reductive elimination reactions of Tp'Rh(L)(R)H complexes (Tp' = tris-3,5-dimethylpyrazolylborate, L = CNCH(2)CMe(3)). Reductive elimination rates in benzene-d(6) were determined for loss of alkane from Tp'Rh(L)(R)H, where R = methyl, ethyl, propyl, butyl, pentyl, and hexyl, to generate RH and Tp'Rh(L)(C(6)D(5))D. The isopropyl hydride complex Tp'Rh(L)(CHMe(2))H was found to rearrange to the n-propyl hydride complex Tp'Rh(L)(CH(2)CH(2)CH(3))H in an intramolecular reaction. The sec-butyl complex behaves similarly. These same reactions were studied by preparing the corresponding metal deuteride complexes, Tp'Rh(L)(R)D, and the scrambling of the deuterium label into the alpha- and omega-positions of the alkyl group monitored by (2)H NMR spectroscopy. Inverse isotope effects observed in reductive elimination are shown to be the result of an inverse equilibrium isotope effect between the alkyl hydride(deuteride) complex and the sigma-alkane complex. A kinetic model has been proposed using alkane complexes as intermediates and the selectivities available to these alkane complexes have been determined by kinetic modeling of the deuterium scrambling reactions.

The activation of alkyl cyanides using a rhodiumtrispyrazolylborate complex

“…After clean activation of acetonitrile, it was observed that 2 was stable indefinitely at room temperature in the presence of benzene. Previously, it had been reported that upon formation of alkyl hydride complexes via COH bond activation of alkanes, the complexes would undergo reductive elimination in the presence of benzene to form 6 and free alkane with half lives of Ϸ5 h (methane) to Ϸ1 h (ethane-hexane) (46). The studies investigated here show that the nitrile-containing complexes are much more stable with the stability of the complex being dependent on the chain length of the alkylnitrile.…”

“…The electron-withdrawing cyano group appears to strengthen the RhOC bond, raising the energy required to reach the transition state for COH reductive elimination. Although the detailed mechanism by which these eliminations occur has not been investigated, they are likely to proceed via initial reductive coupling to make -COH complexes, which then dissociate from the metal, as in the alkane analogs (46).…”

Structural and dynamic properties of propane coordinated to TpRh(CNR) from a confrontation between theory and experiment

“…Observation of the rearranged product indicates a unimolecular pathway that is consistent with -alkane complexes as intermediates (22). By studying the kinetics of rearrangements of a series of alkyl deuteride complexes, and by kinetic modeling of the rearrangements, including a determination of isotope effects (27), the relative barrier heights for C-H oxidative cleavage, migration from one C-H bond to an adjacent C-H bond, and dissociation of the alkane from the metal were determined (22). Although this study did not provide an indication of the energies of either the methyl or methylene -alkane complexes, it did provide energy differences between the various processes available to the -alkane complexes.…”

“…The thermodynamic preference for the former substrate was found to be 2.07 kcal⅐mol Ϫ1 (1 kcal⅐mol Ϫ1 ϭ 4.18 kJ⅐mol Ϫ1 ), which, when corrected for statistical factors, corresponds to a 95:1 preference for activation of methyl over methylene C-H bonds. § Confirmation of this conclusion comes from the independent synthesis of the isopropyl complex TpЈRh(CNR)(CHMe 2 )H, which rearranges in benzene solution to the more stable n-propyl isomer TpЈRh(CNR)(CH 2 CH 2 CH 3 )H before loss of propane (22).…”

“…If the reductive coupling to produce the C-D bond were to occur with liberation of propane-d 1 , the resulting unsaturated intermediate would have been trapped by irreversible reaction with the benzene solvent. Observation of the rearranged product indicates a unimolecular pathway that is consistent with -alkane complexes as intermediates (22). By studying the kinetics of rearrangements of a series of alkyl deuteride complexes, and by kinetic modeling of the rearrangements, including a determination of isotope effects (27), the relative barrier heights for C-H oxidative cleavage, migration from one C-H bond to an adjacent C-H bond, and dissociation of the alkane from the metal were determined (22).…”