Functionalization carbon-hydrogen bond activation

Carbon-hydrogen bond activation in hydridotris(pyrazolyl)borate platinum(IV) complexes Comparison of density functionals, basis sets, and bonding patterns... 

Carbon-hydrogen bond activation by transition metals has found its way to becoming an important aspect of organic synthesis. Metals have been found to break C-H bonds and then participate in follow-up reactions, oftentimes insertions of olefins or alkynes, that permit functionalization of a substrate or the formation of fused-ring systems. As this chemistry is developed, it is clear that selectivity in C-H bond activation is a critical issue that must be controlled to make a given functionalization reaction valuable. 

Carbon-hydrogen bond activation is formally the oxidative addition of a hydrocarbon to a metal complex, as shown in reaction (5.47). It is a potentially important reaction because it represents the initial step in a possible route to functionalize hydrocarbons. 

Figures 7-9 show the fractional conversion of methanol in the pulse as a function of temperature for the three catalysts and the three methanol feeds. Evidently the kinetic isotope effect is present on all three catalysts and over the complete temperature range, indicating that the rate limiting step is the breaking of a carbon-hydrogen bond under all conditions. From these experiments, the effect cannot be determined quantitatively as in the case of the continuous flow experiments, but to obtain the same conversion of CD,0D, the temperature needs to be 50-60° higher. This corresponds to a factor of about three in reaction rate. The difference in activity between PfoCL and Fe.(MoO.), is larger in the pulse experiments compared to tHe steady stateJ results.

The feasibility of hydrogen abstraction at the peptidyl a-carbon hydrogen bond by 1,4-aryl diradicals has been determined by examining a model reaction, i.e. abstraction of deuterium from dideuterioglycine by aryl radicals. The results have biological implications for the reactivity of the enediyne anti-tumour antibiotics with proteins. The non-Arrhenius behaviour of hydrogen-abstraction reactions by radicals has been investigated. For a number of reactions studied the enthalpy of activation was found either to increase or to decrease as a function of temperature. 

MixecT-function oxidases are ubiquitously distributed throughout the plant and animal kingdoms as well as in many aerobic procaryotes (3). They play a number of important biological roles and, indeed, are biochemically unique in their ability to catalyze the activation of non-activated carbon-hydrogen bonds. 

C. L. Hill, in C. L. Hill (Ed.), Activation and Functionalization of Alkanes, Catalytic Oxygenation of unactivated carbon-hydrogen bonds-. Superior oxo transfer catalysts and the inorganic metallopor-phyrin, Wiley, New York, 1989, p. 243. 

Cytochrome P-450 also catalyzes the insertion of an oxygen atom in aromatic and aliphatic carbon-hydrogen bonds. Thus, in the case of benzene the primary product from reaction with activated oxygen is phenol. The functional goal is to solubilize foreign substances in order that they may be excreted. However, in this case, the solubilized substance is extremely toxic. In contrast, toluene is converted to benzoic acid by cytochrome P-450. Hence, the toxic response in human liver to the ingestion of benzene and toluene is substantially different. 

Copper and silver complexes bearing trispyrazolylborate ligands have shown catalytic activity towards several organic transformations involving the functionalization of unsaturated carbon-carbon bonds or several saturated E—H bonds, particularly of carbon-hydrogen bonds. The tunability of this class of ligands from both steric and electronic perspectives allows the control of those catalytic capabilities. On the basis of the work already described, the potential for their use on other, yet unreported transformations seems feasible. 

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