Selective reductions

Selective reduction of a benzene ring (W. Grimme, 1970) or a C C double bond (J.E. Cole, 1962) in the presence of protected carbonyl groups (acetals or enol ethers) has been achieved by Birch reduction. Selective reduction of the C—C double bond of an a,ft-unsaturated ketone in the presence of a benzene ring is also possible in aprotic solution, because the benzene ring is redueed only very slowly in the absence of a proton donor (D. Caine, 1976). 

Selective Catalytic Reduction. Selective catalytic reduction (SCR) is widely used in Japan and Europe to control NO emissions (1). SCR converts the NO in an oxygen-containing exhaust stream to molecular N2 and H2O using ammonia as the reducing agent in the presence of a catalyst. 

WENZEL - IMAMOTO Reduction Selective reductions ol C>C with LaNI alloy... 

Flue gas treatment (FGT) is more effective in reducing NO, emissions than are combustion controls, although at higher cost. FGT is also useful where combustion controls are not applicable. Pollution prevention measures, such as using a high-pressure process in nitric acid plants, is more cost-effective in controlling NO, emissions. FGT technologies have been primarily developed and are most widely used in Japan. The techniques can be classified as selective catalytic reduction, selective noncatalytic reduction, and adsorption. 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

Scheme 22 illustrates a special application of the azide-tetrazole ring closure described by Ponticelli et al. <2004JHC761>. The diazido compound 84 exists as an azide valence bond isomer. When this compound, however, is subjected to reduction by molybdenum hexacarbonyl, one azido group undergoes reduction selectively to an... 

Reduction with LiAlH(OBu )3293 or LAH292 also gives selective hydride addition to the less substituted allyl end (equations 317 and 318). In contrast, formate reductions selectively deliver hydride to the more substituted allyl terminus (equations 319 and 320).302-303 Si—H-mediated reduction, conveniently performed with polymethylhydrosiloxane (PMHS), demonstrates no clear pattern of regioselectivity (equation 321).320 LiHBEt3 delivers hydride regioselectivity to the less substituted allyl terminus (equation 322)289-291... 

Model-based boiler optimization schemes have proved successful in many power plant and industrial boiler applications. Successful NOx reduction through this kind of optimization can avoid or postpone large capital expenditures for low NOx burners, over-fire air modifications, and selective catalytic reduction/selective noncatalytic reduction (SCR/SNCR). 

Virantmycin is a tetrahydroquinoline alkaloid that has inhibitory activity against DNA and RNA viruses. A total synthesis of virantmycin making use of a key type II aziridine has elucidated the absolute stereochemistry at C-2 and C-3 <1996T10609>. An intramolecular photocyclization of an azide onto a Z-alkene produces type II aziridine 351 in excellent yield. A three-step reduction/selective reoxidation procees yields key aziridine alcohol 352 in 76% overall yield (Scheme 71). The alcohol is methylated and the ester hydrolyzed without harm to the azirdine. A TFA-induced ring opening of the aziridine by chloride provides the natural product virantmycin in good yield. This entire process was also carried out with the -alkene to produce /)(-virantmycin, thus proving the stereochemistry at C-2 and C-3. 

WENZEL IMAMOTO Reduction Selective reductnns of C C vwlh LaNI alloy... 

Reduction enanhoseledive 77 Reduction regioselective 375 Reduction selective 42,164 Reduction stereoselective 50 Reduction stereospecilic 60 Reductive ammation 42,233 Reductive coupBng 107 Reductive udmation 200... 

Aromatic nitro compounds (ArN02), are much more electroactive, and specific reduction steps are seen even in acidic aqueous buffers (e.g. at mercury pool cathodes). In general, two successive steps are observed (a four-electron step followed by a two-electron step) with the formation of hydroxylamine and amine, respectively. The reduction selectivity is therefore very sensitive to the potential applied. 

To clarify the difference, the selectivity of NO reduction by CO versus CO oxidation by O2, considered to be the best index of the difference, was measured for fresh, heat-treated and used catalysts. The results are shown in Fig. 2. Here, the closer to the lower right comer the curve is, the lower the selectivity of NO reduction. The A/F s correspond to X = 1.015 at the origin and to X= 1.000 at the upper right comer. The CO conversion was 100% at every point between these two points. At the points near the stoichiometric A/F, at which the selectivity of NO reduction is most important, the curve for the catalyst heated at 1173 K or 1273 K is almost identical to that for the fresh catalyst, but the selectivity of NO reduction on the used catalyst is distinguishably lower. Thus, clearly a decrease in NO reduction selectivity, not explainable by thermal deactivation, occurs on the used catalyst. 

The selectivity of NO reduction was estimated in the same way as in Fig. 2, and the results are shown in Fig. 4. NO reduction selectivity does not seem to decrease by loading Zn or Fe, while the selectivity considerably decreases on the Pb-loaded catalyst. In fact, for the Fe- or Zn-loaded catalyst, the characteristics of the dependence of the catalytic activity on A/F did not change from that of a heat-treated catalyst. However, as shown in Fig. 5a, the CH4 conversion peak was found away from the window on the model Pb-poisoned catalyst at GHSV = 30,000 h-1 and 873 K. Furthermore, at GHSV = 90,000 h-i, all the characteristics in Fig. lb were reproduced qualitatively (Fig. 5b). 

A Pt-Rh three way catalyst used in natural gas-fueled engine systems for 21,000 h showed specific deactivation characteristics, including a decrease in the selectivity of NO reduction, which can neither be reproduced by heat treatment nor explained by physical poisoning such as the blockage of micropores. Through chemieal analyses, EPMA, and activity tests of the used catalyst and model-poisoned catalysts, it was found that the activities of Rh on the used eatalyst were decreased by chemical poisoning due to Pb, causing a decrease in the NO reduction selectivity, and that the absolute rates of NO reduction and other reactions are considerably reduced by a decrease in the effective surface area of the catalyst due to accumulated compounds on the wash coat surface, in addition to thermal effects. 

Fig. 16.2 Solvents in tbe study on enamine reduction. Selected test items Formamide...

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Moreover, the N2 selectivity is higher with olefins than with long chain alkanes. N2O and N2 formation could depend on the covering of metal by hydrocarbon. Hydrocarbon adsorption strength may be an important parameter in the reduction selectivity. 

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