Cyclometallation steric congestion

The competition between intra- and intermolecular C-H activation has been discussed by Crabtree and his main conclusions are summarized here (i) alkane activation is kinetically favored, while cyclometallation is usually the thermodynamic preferred pathway (ii) higher steric congestion favors cyclometallation, whereas intermolecular activation is more likely in less congested systems. Let us illustrate these points by using the unsaturated complexes IrlCp XPMes) and IrlCp KPPhs). Both activate benzene (reactions (75) and (76)), but the products of the latter are a 1 1 mixture of the phenyl hydride complex and a four-membered metallacycle ... 

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