Reductants homogeneous

Castro CE, WC Kray (1963) The cleavage of bonds by low-valent transition metal ions. The homogeneous reduction of alkyl halides by chromous sulfate. J Am Chem Soc 85 2768-2773. 

In cases of both the ionic and non-ionic homogeneous reduction processes, a reducing agent figures which oxidizes itself and, consequently, results in the precipitation of a metal. This description may wholesomely be conveyed by the following scheme ... 

Zinc/copper couple (Zn/Cu), produced by the treatment of zinc with HC1, followed by the addition of copper(n) sulfate, has been known since the seminal work of Simmons and Smith.67 A less reactive form of activated zinc, but one that is sufficiently active for most applications, is produced by the treatment of the metal with 1,2-dibro-moethane, followed by Me3SiCl.68 The most reactive form of activated zinc, the so-called Rieke zinc (Zn ), is finely divided metallic zinc produced by the homogenous reduction of zinc halides in THF.69... 

The first examples of a homogeneous reduction of this type were reported in 1971. Cobalt carbonyl was found to reduce anhydrides such as acetic anhydride, succinic anhydride and propionic anhydride to mixtures of aldehydes and acids. However, scant experimental details were recorded [94]. In 1975, Lyons reported that [Ru(PPh3)3Cl2] catalyzes the reduction of succinic and phthalic anhydrides to the lactones y-bulyrolaclone and phthalide, respectively [95], The proposed reaction sequence for phthalic anhydride is shown in Scheme 15.15. Conversion of phthalic anhydride was complete in 21 h at 90 °C, but yielded an equal mixture of the lactone, phthalide (TON = 100 TOF 5) and o-phthalic acid, which is presumably formed by hydrolysis of the anhydride by water during lactoniza-tion. Neither acid or lactone were further hydrogenated to any extent using this catalyst system, under these conditions. 

Tsai, K.-L. and Dye, J.L., Synthesis, properties, and characterization of nanometer-size metal particles by homogeneous reduction with alkalides and electrides in aprotic solvents, Chem. Mater., 5, 540,1993. 

The preceding sections dealt with thermal heterogeneous and homogeneous reductive and oxidative cleavages. These reactions may also be triggered photochemically by means of a sentitizer as pictured in Scheme 3.7 in the... 

Aniline has also been obtained (93%) from a homogenous reduction with preformed tetraethylammonium hydridotri-iron undecacarbonyl [10], In this reduction, the yield is comparable, or superior, to that obtained when the more reactive iron pentacarbonyl or di-iron nonacarbonyl complex is used in the absence of a phase-transfer catalyst. 

The catalytic effect of tetra-n-butylammonium fluoride in the homogeneous reduction of heterocyclic A-oxides and nitroarenes by hexamethyldisilane in tetra-hydrofuran can occur with EXPLOSIVE violence, but can be controlled by the slow addition of the disilane to the A-oxide (or nitroarene) and tetra-n-butylammonium fluoride to yield the parent heterocycle (>70%) (or azobenzene 84%). In a similar manner, azoxybenzene is converted into azobenzene (95%), and 4-nitropyridine-l-oxide, is reduced to azoxypyridine-l,l -dioxide (78%), with minor amounts of azopyridine-1, l -dioxide and azopyridine-1-oxide [5,6]. 

The treatment of the bromoacetal (255) with catalytic amounts ( 10%) of a Co(I) species, generated by the electroreduction of cobaloxime (232) in an Me0H-LiCl04 system at —1.8 V, produces the cis-fused adduct (256) in 60 70% yield (Scheme 96) [392]. Cathodic reduction is used for the synthesis of a [Co(CO)3PBu3] complex in a methanol-methyl formate medium, which catalyzes the alkoxycarbonylation of dichloromethane to dimethyl mal-onate in up to 75% yield [393]. The Co(II) complexes are found to be effective for the homogeneous reduction of gem-dichlorocyclopropanes in the presence of anthracene [394]. The formation of the C—C double bond of (258) may be ascribed to the a-elimination of the Co-H species. Thus, benzalchloride (257) can be converted to a mixture consisting primarily of ds- and trans-stilbenes (258) by the action of electrogenerated Co(I)(salen) (Scheme 97) [395-398]. 

The mechanism of the thermal 5 rnI reaction, using 4-nitrocumyl chloride and 2-nitropropanate ion as a model has been investigated. The results provided unambiguous evidence that a decrease in driving force is able to change the mechanism of homogeneous reductive cleavage reactions from stepwise to concerted. ... 

Again, the potassinm cation acts as a coordination center, which ensures the template organization of anionic fragments during homogeneous reduction. The cavity diameter of the obtained mnltithiaheterocycle matches the doubled radius of potassium cation. 

Homogeneous reductive cleavage of the sulphur-sulphur bond in some diphenyl disulphides by electron transfer from added radical-anions in the solution illustrate points from the two previous paragraphs [32], Rate data for two diphenyl disulphides are given in Figure 4.6. The variation in electron transfer rate with electron... 

Table 13 Electrochemical, kinetic and thermodynamic parameters for the homogeneous reduction of para-substituted diaryl disulfides (XPhS)2.

More recent data were collected on the reduction of triphenylmethyl sulfides having the formula PhjC-SPhX. The data pertains to X = CN, " Cl, OMe, H, and COMe. " The voltammetric pattern is similar to that of the unsubstituted compound. " The homogeneous reduction of each compound provided curved log/Chom vs. E d/d - plots in the activation-controlled zone. The relevant data are collected in Table 15. 

Table 15 Dissociative ET data for the homogeneous reduction of triphenylmethyl aryl sulfides."...

When an aldehyde or a ketone is treated with ammonia or a primary or secondary amine in the presence of hydrogen and a hydrogenation catalyst (heterogeneous or homogeneous), reductive alkylation of ammonia or the amine (or reductive amination of the carbonyl compound) takes place.165 The reaction can formally be regarded as occurring in the following manner (shown for a primary amine), which probably does correspond to the actual sequence of steps 166... 

Useful for homogeneous reduction of alkenes. As a consequence of the reagent bulk, it is understandable that the reactivity of alkene reduction is dependent on substitution the less-substituted alkenes react faster. Also, reduction occurs from the less-hindered face in a cis-stereochemistry. Many other functional groups are tolerated by conditions. 

Homogeneous reduction of carbon dioxide to ligated carbon monoxide has been achieved from the reaction of [Na][(t/5-C5H5)Fe(CO)2] with C02 and two equivalents of acid [Eq. (40)] (108). Evans (23) has previously presented infrared spectral evidence for the existence of 28. This reduction... 

Many of the comments related to C=0 reduction in this section also apply to the homogeneous reductions of C=N bonds, a process for which heterogeneous methods predominate192. This section will therefore concentrate on recent developments and, in particular, on asymmetric transformations. Transfer hydrogenations of C=N bonds by homogeneous catalysts have been reported264. 

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