Electron catalyzed reduction

Although sulfur is unHkely to chelate the metal in this case, it is worth mentioning the axially chiral diphosphine Hgands, based on hz-thienyl systems which increase the electronic density at phosphorus such as 159 (used in Ru-catalyzed reduction of /1-keto esters with 99% ee) [llla],BITIANP 160,andTMBTP 161 (in a Pd-catalyzed Heck reaction, the regio- and enantioselectivity are high with 160 but low with 161) [mb]. 

Anson and co-workers have shown that two Co ions were not necessary for four-electron 02 reduction.266 The mew-substituted complex porphyrin Co(TPyP) (42) complex bears four active pyridyl donors which readily react with four equivalents of [Ru(NH3)5(OH2)]2+ to produce the tetra-ruthenated derivative. The four Ru centers are sufficiently remote that their RuIII/n potentials coincide. Under steady state conditions [Co(TPyP)] Ru(NH3)5 4]8+ (43) adsorbed onto a pyrolytic graphite working electrode catalyzes the reduction of dioxygen (Figure 6). 

The overall reaction catalyzed by PS I is one electron oxidation-reduction reaction. 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

For amine protection, sulfonamides such as 9 offer several advantages over urethanes 5 or amides 7. In particular, secondary amines protected as the urethane or the amide exist as mixtures of rotational isomers, confusing NMR characterization and making crystallization more difficult. The limitation has been that sulfonamides have been difficult to remove. Masanobu Uchiyama of the University of Tokyo reports (J. Am. Chem. Soc. 2004,126, 8755) the development of transition metal ate complexes that catalyze electron-transfer reduction. While the sulfonamide 10 is inert to Mg in THF, inclusion of a catalytic amount of the ate complex 11 led to 12 in quantitative yield. 

FAD alone may also catalyze reduction by acting as an electron donor. 

Several factors may limit the overall rate of enzymatic reductive reactions. First, the electron transfer to the reactive metal (e.g., Co, Fe, or Ni) may be limiting. It is also possible that access of the organic substrates to the reduced metals contained within enzyme microenvironments may be limited. Mass transfer limitation is even more important in intact bacterial cells. For example, Castro et al. (1985) found that rates of heme-catalyzed reductive dehalogenations were independent of the heme content of the cells. 

The pH dependence of the rate of formation of a nitrosyl complex shows that nitrous acid is the reactive intermediate in the reaction when the pH is in the range of 2-8. The catalysts are not deactivated during repeat cycles between their oxidized and reduced states. The catalyzed reduction appears to depend on the ability of the multiply reduced heteropolyanions to deliver electrons to the NO group bound to the iron center. 

Homolytic oxidations involve free radical intermediates and are catalyzed by first-row transition metals characterized by one-electron oxidation-reduction steps, eg. Com/Con, Mnln/Mnn. The hydrocarbon substrate is generally not coordinated to the metal and is oxidized outside the coordination sphere. Consequently, the oxidation is not very selective and does not often preserve the stereochemical configuration of the substrate. 

Iridium-catalyzed reductive coupling of acrilates and imines has been reported to provide trans (3-lactams with high diastereoselection [142], The use of electron-deficient aryl acrylates resulted in improved product yields. The mechanism, proposed by the authors, started from an in situ generated iridium hydride reacting with the acrilate to provide an iridium enolate that, then, reacted with the imine to give a (3-amido ester. Subsequent cyclization furnished the p-lactam and an iridium alcoxide. 

It is noteworthy that although DMP+ catalyzes reduction of fluorobenzene it does not affect benzene. The respective electron affinities in the gas phase are —0.89 and —1.15 eV27) and reaction of fluorobenzene with hydrated electrons is only 6 times faster than that of benzene 82). 

The coupling of independent catalytic cycles for both radical generation and reduction has been realized by the combination of the titanocene catalyzed reductive epoxide opening [36—4-0] via electron transfer and the catalytic reduction of radicals after H2 activation by Wilkinson s complex [Rh(PPh3)3Cl] as shown in Scheme 16 [41—43],... 

Fig. 22 Electroreduction of ketones or aldehydes using ADH as catalyst. Reduction system A shows the ADH-catalyzed reduction coupled with regeneration of NADPH or NADH by ferre-doxin-NADP+ reductase (FNR) or diaphorase (DP), respectively with assistance of methyl violo-gen as an electron mediator. In system B, ADH is used as sole enzyme which catalyzes both reduction of substrates and regeneration of cofactors...

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