Theoretical Treatment of C-H Activation

Over the past 15 years, many theoretical treatments of C-H activation have appeared. Early work by Hoffmann addressed qualitative orbital approaches to C-H activation by CpML fragments [52]. More quantitative approaches have appeared recently for the addition of methane to the [CpRh(CO)] fragment [53— 56]. These more recent calculations provide support for the presence of methane a-complexes along the reaction coordinate for methane oxidative addition, and confirm the weak nature of the interaction between the metal center and the C-H sigma bond ( 20 kj mol-1). A more detailed comparison of these results is beyond the scope of this chapter. 

One of the earliest reports of alkane C-H activation was made by Shilov in 1969 in which H/D exchange was reported between methane and a D20/CH3C00D solvent in the presence of K2PtCl4 [57]. While the mechanistic details of this exchange were not entirely clear, the work stood as an isolated example of alkane activation for many years. Alkane activation by platinum was not reported again until 1986, when Whitesides found that (Cy2PCH2CH2PCy2)Pt(neopentyl)H lost neopentane and activated a variety of alkanes at 50°C (Eq. 12) [58,59]. These reactions are believed to proceed by way of an initial reductive elimination to 

Subsequent work with a Pt11 complex containing a labile triflate ligand showed evidence for the alkyl/aryl exchange labeled as type (c) in Fig. 1. While mechanistic studies were limited, the observation of little positional selectivity in the reaction with toluene argued in favor of the oxidative addition pathway, i.e., via a PtIV intermediate (Eq. 13) [60]. 

Horvath has reported conditions under which Pt11 is used to catalyze the conversion of methane to methyl chloride. The reaction conditions employed are indicated below, and avoid the hydrolysis of the methyl chloride to methanol. While the total quantity of methyl chloride formed is less than the amount of platinum initially present, the system is catalytic in Pt11 (Eq. 14), with PtIV serving as a stoichiometric oxidant and Cl2 stabilizing the system against precipitation of Pt° [61]. 

Reaction of Cp Ir(PMe3)(CH3)OTf with NaBArF in dichloromethane leads to the precipitation of NaOTf and the formation of [Cp Ir(PMe3)(CH3)(CH2Cl2)] 

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