Reductions with Metals

It has been reported that gem-dihalocyclopropanes, usually undergo electrochemical or metallic reduction with predominant retention of configuration as shown in Scheme 34. 124,126) The stereoselectivity of these reactions may result from the intermediacy of a cyclopropyl carbanion which is unusually stable to racemization compared to other alkyl carbanions. 125>... 

More versatile reactions are being discovered in metal-templated reactions involving C-N bond formation.21-23 Co-condensation of formaldehyde, nitroethane and the aminothioether 26 in the presence of copper(II) yields an intermediate di-imino nitro macrocycle. Subsequent dissolving metal reduction with zinc in HC1 yields the saturated aza-thia macrocycle 27 in which the hydroxy group and amino groups are /rani-related (Scheme 3.10).22,23... 

The MoS2 in ores is concentrated by the foam flotation process the concentrate is then converted by roasting into Mo03 which, after purification, is reduced with hydrogen to the metal. Reduction with carbon must be avoided because this yields carbides rather than the metal. 

Sinclair, S., Jorgensen, W. L. Computer Assisted Mechanistic Evaluation of Organic Reactions. 23. Dissolving Metal Reductions with Lithium in Liquid Ammonia Including the Birch Reduction. J. Org. Chem. 1994, 59, 762-772. 

Reduction (electrochemical or metals ) of dibromodifluoromethane produces difluorocarbene (carbenoid) which reacts with alkenes to form 1,1-difluorocyclopropanes. Electrochemical reduction was carried out on a platinum electrode at 20 C, while metal reduction with lead in the presence of tetrabutylammonium bromide (TBAB) in various solvents was carried out at 40-60°C. In both cases 1,1-difluorocyclopropanes were formed in moderate yield (Houben-Weyl, Vol. E19b, p 1475). 

C, b.p. 907"C, d 713. Transition element occurring as zinc blende, sphalerite (Zn,Fe)S calamine or smithsonite (ZnCO j), willemite (Zo2Si04), franklinite (ZnFe204). Extracted by roasting to ZnO and reduction with carbon. The metal is bluish-white (deformed hep) fairly hard and brittle. Burns... 

Reference to Figure 3.4 shows that the reduction is not feasible at 800 K. but is feasible at 1300 K. However, we must remember that energetic feasibility does not necessarily mean a reaction will go kinetic stability must also be considered. Several metals are indeed extracted by reduction with carbon, but in some cases the reduction is brought about by carbon monoxide formed when air, or air-oxygen mixtures, are blown into the furnace. Carbon monoxide is the most effective reducing agent below about 980 K, and carbon is most effective above this temperature. 

The less hindered f/ans-olefins may be obtained by reduction with lithium or sodium metal in liquid ammonia or amine solvents (Birch reduction). This reagent, however, attacks most polar functional groups (except for carboxylic acids R.E.A. Dear, 1963 J. Fried, 1968), and their protection is necessary (see section 2.6). 

Gaylord, N. G, 1956, Reduction with Complex Metal Hydrides, Interscience New York... 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Both halogens of the dihalogenothiazoles can be replaced by nucleophiles. At any rate, the halogen in position 2 is always more reactive than those in positions 4 or 5, as previously discussed. Analogously,the halogen can be selectively removed only from position 2 by reduction with metals (Table V-5). 

Reduction to alcohols (Section 15 2) Aide hydes are reduced to primary alcohols and ketones are reduced to secondary alcohols by a variety of reducing agents Catalytic hydrogenation over a metal catalyst and reduction with sodium borohydride or lithium aluminum hydride are general methods... 

The mote electropositive metals react with cryohte, Hberating aluminum or aluminum monofluotide (22,23). The reduction of cryohte by magnesium is a current method for removal of magnesium in the refining of aluminum. Upon contact with strong acids cryohte Hberates hydrogen fluoride. 

Gravimetric methods more suitable for general use involve the precipitation of metallic gold from tetrachloraurate solutions by reduction with oxaUc acid, SO2, or hydroquinone. Formaldehyde, hydrazine, ferrous sulfate, and hypophosphorous acid also have been used but are considered less efficient (40). 

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