Selectivity in C-H Activation

If an experiment is conducted that gives a mixture of two products, there is oftentimes difficulty in determining if the observed product ratio reflects the kinetic or themodynamic selectivity of the system. If the products are known to be stable towards reductive elimination under the reaction conditions, then the product ratio reflects the kinetic selectivity. Alternatively, if the initially formed product ratio is observed to change over time to give a constant ratio of prod- 

Graham has reported that the fragment [Cp Ir(CO)] shows a 4 1 kinetic preference for cleavage of a benzene C-H bond over a cyclohexane C-H bond. Unfortunately, no other kinetic selectivities were reported for this complex [5]. 

The above systems have also been examined in some detail for their thermodynamic selectivity towards C-H bond activation. As mentioned above, irradiation of Cp Ir(PMe3)H2 in pentane gives a mixture of primary and secondary C-H activation products. Upon heating to 110°C, however, the primary activation product is observed to increase at the expense of the secondary activation products (Eq. 4). This observation indicates that the initial product ratio indeed represented a kinetic selectivity, and that the n-pentyl product is thermodynamically preferred by several kj mol-1, since no secondary products were seen at equilbrium [22]. 

For complexes of the type Cp Ir(PMe3)(R)H, equilibration between two possible alkane activation products was found to occur at 140°C (Eq. 5). The mixture of alkane solvents used and the relative quantities of the two C-H insertion products was then used to calculate Keq and AG° (Eq. 6). The results of these equilibrations are shown in Table 2. Examination of this table shows a strong preference for activation of primary over secondary C-H bonds, and a preference for less hindered primary bonds over more hindered primary bonds. This latter preference can be extreme as seen in the equilibration among the methyl groups of 2,2-dimethylbu-tane, which shows only activation of the C-H bonds on C4 [23]. 

Another system for which thermodynamic data have been obtained in some detail is the Tp Rh(CNneopentyl)(R)H system studied by Jones. Here, the relative thermodynamic stabilities of a number of adducts were obtained by measuring both the competitive kinetic selectivity for two types of C-H bond (AAGt in Fig. 2) as well as the barrier for reductive elimination of free alkane from each adduct (AG and AG in Fig. 2). The free energies for the latter were obtained from kinetic studies of the reductive elimination of hydrocarbon in benzene. A summary of the AG° values, calculated equilibrium constants, and relative metal-carbon bond strengths are given in Table 4 [26]. For DC H for benzene, see ref. 

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