Zinc Complex Reducing Agents

Rusling et al. performed electrochemically and light mediated radical additions of alkyl iodides to cyclohexenone in conductive microemulsions catalyzed by 20 mol% of 247 in 14—81% yield [303]. Radical allylations of alkyl bromides 249 with allyl sulfides, sulfones, or phosphates catalyzed by 5 mol% of cobalt (iminate) complex 250 in the presence of zinc as reducing agent proceeded in 52-85% yield [304],... 

Further studies of complex reducing agents based on sodium hydride have shown that a mixture of sodium hydride, sodium t-amylate, and zinc chloride (ZnCRA) gives regioselective 1,2-reduction of a, 8-enones to allylic alcohols, in contrast to the 1,4-reduction preference shown by the earlier developed NiCRA (3,135). The activity is enhanced by the addition of MgBr2. 

There has been an increasing number of reports of the utilization of zinc hydride complexes as precursors in organic transformations and as reducing agents. Zinc hydrides have been less well investigated than the other group 13 hydrides due to the formation of polymeric and oligomeric species that are difficult to characterize. 

Chromium(III) chloride has very low solubility in pure water. However, it readily dissolves in the presence of Cr2+ ion. Reducing agents such as SnCla can solubilize CrCls in water. It forms adducts with many donor ligands. For example, with tetrahydrofuran (THF) in the presence of zinc, it forms the violet crystals of the complex CrCF 3THF. 

These have been known for many years.1052-1054 Chromium(III) is approximately octahedral ( ie(f = 3.69-4.1 BM) the compounds have a layer structure. In the chloride, r(Cr—Cl) is 5.76 A between layers and 3.46 A within layers. The iodide is isomorphous with the chloride and the bromide has a similar but distinct structure. All may be prepared by the direct halogenation of the metal. Other methods are available, e.g. CrCl3 may be prepared by heating Cr203-xH20 in CCU vapour at 650 °C.1055 The anhydrous halides are insoluble in water, however reducing agents such as zinc catalyze dissolution. The trichloride reacts with liquid ammonia to form ammine complexes. 

The complexes are insensitive to mild reduction, but are decomposed by strong reducing agents such as zinc. The redox properties of these anions will be discussed later. 

Ni(0) catalyst. A radical 5-exo cyclization to the potentially zinc or nickel-complexed ketone provides an alkoxyl radical that combines with the co-produced Ni(I) species. A transmetalation to a zinc alkoxide regenerates the catalyst and forms the zinc cyclopentoxide, from which products 79 are liberated on hydrolysis. A bimetallic Cu(I)-Mn(II) system provided similar results (see Sect. 8.4). Analogous samarium diiodide-mediated reactions require in contrast stoichiometric amounts of the reducing agent and are less diastereoselective [26, 27],... 

The catalytic effect of several alcohols in the preparation of dichlorotetrakis(pyridine)rhodium(III) cation has long been known.1 In recent years, a variety of reducing agents, present in catalytic quantities, have been used in the preparation, of several rhodium(III) complexes.2 In the absence of catalysts, these reactions are often laborious, and/or incomplete, by comparison with the catalyzed reaction, for example, the preparation of pentaamminechlororhodium(III) chloride (Claus salt) by the method of Lebedinsky.3 Conversion of [Rh(NII3)[,CI]Cl2 to the pentaamminehydridorhodium(III) salt [Rh(NII3) I 1JW()4 by treatment with zinc and ammonia is rapid, and the reaction is relatively clean.4 The formation of hydrido species by tetrahydroborate treatment6 is not a satisfactory preparative procedure. 

Zinc-modified cyanoborohydride, prepared from anhydrous zinc chloride and sodium cyanoborohy-dride in the ratio 1 2 in ether, selectively reduced aldehydes and ketones but not acids, anhydrides, esters and tertiary amides. In methanol the reactivity paralleled the unmodified reagent. Zinc and cadmium borohydrides form solid complexes with DMF, which may prove to be convenient sources of the reducing agents.Aromatic and a,p-unsaturated ketones were reduced much more slowly than saturated ketones, so chemoselective reduction should be possible. 

A variety of reducing agents have been used to reduce nitroarenes to azo compounds. However, a mixture of zinc and sodium hydroxide is used most frequently. - Reduction under these conditions produces hydrazo compounds, which are then oxidized to azo compounds by dissolved atmospheric oxygen alternatively, air can be drawn through the product solution to achieve the conversion. - Occasionally, activation of the zinc dust is required prior to its use, and, since metal ions form chelated complexes with azo compounds, a vigorous post-treatment with acids is recommended. The zinc-sodium hydroxide reduction conditions are sufficiently mild that p-nitrostyrene (2 equation 2), can be reduced without reduction of the vinyl groups. ... 

Replacement of aromatic halogens by hydrogen can be accomplished by catalytic hydrogenation and by reduction with complex hydrides, with sodium, with zinc, and with inorganic and organic reducing agents. 

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