Transition-metal complexes organic substrates

As water is immiscible with most organic substrates, most reactions involving water are done with liquid-liquid biphasic systems. The use of biphasic organometallic catalysts to catalyze aqueous-phase reactions is a novel method to address this issue. The catalyst in such reactions is a water-soluble transition metal complex with substrates that are partially water-soluble. The Ruhrchemie-Rhone-Poulenc process, which involves hydrofor-mylation of propylene to n-butanol, is an example of biphasic organometallic catalysts being used on an industrial scale (Comils and Kuntz, 1995). The catalyst employed is a water-soluble Rhodium (I) complex of trisulfonated triphenylphosphine (tppts) (see Fig. 5.3). 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

The fixation of carbon dioxide into organics may involve its activation by coordination to low-valent transition metal complexes. Carbon-carbon bonds can be thus formed by simultaneous activation at a metal center of both carbon dioxide and an organic substrate. 

In addition to catalytically active transition metal complexes, several stable, electrophilic carbene complexes have been prepared, which can be used to cyclopropanate alkenes (Figure 3.32). These complexes have to be used in stoichiometric quantities to achieve complete conversion of the substrate. Not surprisingly, this type of carbene complex has not attained such broad acceptance by organic chemists as have catalytic cyclopropanations. However, for certain applications the use of stoichiometric amounts of a transition metal carbene complex offers practical advantages such as mild reaction conditions or safer handling. 

In contrast to the biological CO2 fixation during the dark reaction of photosynthesis where very low concentrations of CO2 from air can be fixed at room temperature, most reactions with transition-metal complexes or with organic substrates either require high partial pressure of CO2 or high temperatures. An exception was published quite recently, the rapid fixation of CO2 and O2 from air at a palladium(O) complex 10 (Scheme 11) [77]. 

Complexes with Metal-carbon a-Bonds Formed in Redox Processes Between Transition Metal Complexes and Organic Substrates... 

In studies of reaction pathways, nitrosyl radicals are frequently used as spin traps to provide evidence for free radical pathways. A caution in interpretation of these results is that the probe or products will interact with the transition metal complex in the reaction and affect the reactivity of the probe with the organic substrate or free radicals produced. A number of reactions of the stable free radicals RNO and R2NO with platinum(II) complexes have been carried out which show that such reactions must indeed be considered (equations 473-... 

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