Triggering of Reaction Steps at Transition Metal Centers

Biological catalysts utilize steric and electronic modifications to attain a remarkable degree of substrate specificity. One mechanism by which enzymes finely tune reactivity is through allosteric regulation, in which the enzyme responds to the presence of a distant activator that induces conformational changes to alter enzyme activity. The operation of a chemical switching mechanism enables the active site to be protected in a stable resting state, which is converted to a more reactive form during catalysis. This allows many biological systems to be active only when a triggering signal is present, thus protecting them from decomposition. Although tuning reactivity via chemical triggers is common in biological systems, there are currently few analogs for synthetic catalysts. In particular, examples involving electronic triggers are rare. In an effort to identify possible triggering mechanisms that might influence electrophilicity at a metal center, several complexes featuring chelating, nitrogen-containing heterocycle ligands have been studied in collaboration with our colleague Robert Bergman. The binding of B(C6F5)3 to bipyrazine (bpyz)-platinum complexes was found to increase the rate of biaryl reductive elimination by a factor of 64,000, and with 2,2'-bipyrimidine (bpym) complexes (bpym)PtAr2 complexes, rate enhancements for biaryl elimination are as high as 108 (with Zn(C6F5)2). Many more mechanisms for regulation of reactivity in synthetic catalysts are possible.