A Theoretical Model for the Orientation of 16-Electron [CpML] Insertion into the C-H Bond of Propane and Cyclopropane and Its Regio- and Stereoselectivity

Abstract

Complete geometry optimizations were performed with density functional theory (DFT) in order to study the potential energy surfaces of [CpM(PH 3 )] (M Rh, Ir) complexes inserted into C ± H bonds of propane and cyclopropane. The agreement between DFT and experimental results indicates that the B3LYP/LANL2DZ method can be a powerful tool for the investigation of these oxidative addition reactions. A fragment molecular orbital model suggesting the mechanistic pathway for the oxidative addition of saturated alkanes to [CpML] is described. It is shown that these oxidative addition reactions all proceed in a concerted fashion via a three-center transition state, and all lead to exothermic reactions. In particular, we show that both electronic and steric effects play a major role in the preference for a s CH 2 -type of approach, from which one may predict the formation and stabilities of the regio-and stereoselective insertion products. Our theoretical findings suggest that highly reactive [CpIr(PH 3 )] tends to be nonless discriminating and reacts randomly, while the less reactive [CpRh(PH 3 )] complex is highly selective. We also found that, for both [CpRh(PH 3 )] and [CpIr(PH 3 )], the ease of oxidative addition is in the order: secondary cyclopropane b primary propane b secondary propane. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. It is demonstrated that both the singlet ± triplet energy gap of 16-electron [CpML] and the s(C ± H) 3s*(C ± H) triplet excitation energy of hydrocarbons play a decisive role in the determination of the reactivity as well as the selectivity of [CpML] insertion.

Deoxygenation and Desulfurization of Oxiranes and Thiiranes by Carbenes: A Theoretical Study

“…Our DFT results suggest an decreasing trend in DE ss* for oxirane (188 kcal mol À1 ) to thiirane (106 kcal mol À1 ). [38] This is in good accord with the experimental and theoretical observations as noted earlier, that is, thiirane is more susceptible to a carbene-abstraction reaction than oxirane.…”

A Computational Study of Ethylene C−H Bond Activation by [Cp*Ir(PR3)]

“…The triplet is found to be the ground state, but the singlet is fairly low‐lying, with excitation energies of 6.4 and 8.4 kcal mol −1 for [CpIr(PH 3 )] at the B3LYP/LanL2DZ [This value is much smaller than that published in ref. 6c, although they were both derived using the same program, method, and basis set. We have checked our result very thoroughly, and attempted to otherwise explain the discrepancy, without success.]…”

Using the Microcyte Flow Cytometer To Monitor Cell Number, Viability, and Apoptosis in Mammalian Cell Culture

“…Relative to the triplet ground state of the 16‐electron fragment, the oxidative addition process is highly endoergic for Co, only slightly exoergic for Rh, and strongly exoergic for Ir. For this reason, the oxidative addition of hydrocarbons occurs for the Ir and Rh systems (with a greater bond selectivity for the Rh system, as previously discussed),29 whereas it has not been observed for the Co system.…”

“…Methane CH oxidative addition to 16‐electron [CpM(PH 3 )] : This is the first comparative study of this reaction for all three Group 9 metal complexes and the first reporting all critical points along the reaction coordinate, including the MECP. Previous investigations have addressed the thermodynamics of methane addition to the Rh and Ir fragments by using LCAO‐HFS,26 RHF,27 MP227, 28 and MP4(SDTQ),28 as well as selectivity issues for the oxidative addition of propane and cyclopropane to the same fragments using B3LYP 29. [CpCo(PH 3 )] has also been previously examined theoretically by B3LYP, but the reaction with methane was not addressed 30.…”

“…19 These species are typically generated in situ by photolysis of CpM(PR 3 )H 2 or by thermolysis of [CpM(PR 3 )(R)(H)] precursors with elimination of H 2 or RH, respectively 20–25. This process has also been extensively investigated by computations for the Rh and Ir systems,26–29 which indicate that the 16‐electron intermediates undergoing oxidative addition adopt a triplet ground state. None of the previously reported investigations, however, have examined the details of the crossing region and how these may affect the reaction rate.…”