Substrate activation transition metal complexes

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 microporous supports or zeolites, catalyst immobilization is possible by steric inclusion or entrapment of the active transition metal complex. As catalyst retention requires the encapsulation of a relatively large complex into cages only accessible through windows of molecular dimensions, the term ship-in-a-bottle has been coined for this methodology. Intrinsically, the size of the window not only determines the retention of the complex, but also limits the substrate size that can be used. The sensitivity to diffusion limitations of zeolite-based catalysis remains unchanged with the ship-in-a-bottle approach. In many cases, complex deformation upon heterogenization may occur. 

In comparison with catalytic reactions in compressed CO2 alone, many transition metal complexes are much more soluble in ionic liquids without the need for special ligands. Moreover, the ionic liquid catalyst phase provides the potential to activate and tune the organometallic catalyst. Furthermore, product separation from the catalyst is now possible without exposure of the catalyst to changes of temperature, pressure, or substrate concentration. 

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... 

In the above cases, an optically active reducing agent or catalyst interacts with a prochiral substrate. Asymmetric reduction of ketones has also been achieved with an achiral reducing agent, if the ketone is complexed to an optically active transition metal Lewis acid. ... 

Two significant communications indicate the considerable potential of transition metal complexes as multifunctional homogeneous catalysts in the silane field (5, 53). Here the same catalyst activates silanes toward different substrates and it is probable that all proceed via a common metal hydrido intermediate. Both Co2(CO)8 and (Ph3P)3CoHX [X = H2, N2, or (H)Si(OEt)j] catalyze 0-silylation and hydrosilylation the hydrogen on Si may be replaced by R O, R COO, R CONH, or R3SiO [e.g., Eqs. (117)-(120)], and excellent yields of silylated product result. Phenolic groups do... 

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. 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

While the greatest percentage of PTC-aided anionic substitutions involve non-aromatic systems (7-10), a number of liquid-liquid and solid-liquid, PTC-aided SnAr reactions have been reported (32-38). These reports involve a variety of substrates [unactivated (32,33), slightly activated (M), activated (35-37), and transition metal complexed 32,38)1, nucleophiles OMe (32,38), CN ( ), SR (34) SCN (36), SO (36), OR (37)] and PTCs... 

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