Chromous ion

Reaction of free-base porphyrin compounds with iton(II) salts in an appropriate solvent results in loss of the two N—H protons and insertion of iron into the tetradentate porphyrin dianion ligand. Five-coordinate iton(III) porphyrin complexes (hemins), which usually have the anion of the iton(II) salt for the fifth or axial ligand, ate isolated if the reaction is carried out in the presence of air. Iron(II) porphyrin complexes (hemes) can be isolated if the reaction and workup is conducted under rigorously anaerobic conditions. Typically, however, iton(II) complexes are obtained from iton(III) porphyrin complexes by reduction with dithionite, thiolate, borohydtide, chromous ion, or other reducing agents. 

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

TiCl3, H2O, It, 1 h, 85% yield.This is an excellent reagent that works when cleavage of a methoxy oxime with chromous ion fails. 

The ability of chromous ion to efficiently reduce a-halo ketone impurities may be the reason for superior results. 

The stereochemistry of the dibromides does not appear to affect the usefulness for regenerating olefins with zinc or chromous ion however, significant differences in ease of reduction have been observed. Thus, the diequatorial 3j5,4a-dibromocholestane (121) is 1 % reacted under conditions which give a 91 % conversion of the diaxial 3a,4j5-isomer (122). 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Borohydrides reduce a-substituted ketones to the corresponding a-substituted alcohols, and such products can be further reduced to olefins (see section VIII). Other reagents serve, through participation of the carbonyl group, to remove the substituent while leaving the ketone intact. The zinc or chromous ion reduction of a-halo ketones is an example of this second type, which is not normally useful for double bond introduction. However, when the derivative being reduced is an a,jS-epoxy ketone, the primary product is a -hydroxy ketone which readily dehydrates to the a,jS-unsaturated ketone. Since... 

Chromo-hydroxyd, n. chromous hydroxide, chromium(II) hydroxide, -ion, n. chromous ion, chromium(II) ion. 

In compounds, the important oxidation numbers of Cr are +2, +3, and +6. In all of these states the chromium ions are colored and, in fact, the element got its name from this property (ichroma is the Greek word for color). The +2 state is not frequently encountered but it can be made quite easily as the beautiful blue chromous ion in solution by dripping a solution containing CrM over metallic zinc. Air has to be excluded since O rapidly converts Cr1-5 back into Cr. ... 

A general synthesis of pyrrolo[3,4-mediated reductive cyclisation of the pyrrolidinones 16 with amidines. The suggested mechanism is that shown in Scheme 2 < 95JOC7687 >. 

One reagent which has received detailed attention, however, is chromous ion, which possibly acts as a model for other divalent reducing metal ions and is the subject of a recent review . 

In nitrate and sulfate media, Cr(III) is not very stable and the reaction Cr(II) -> Cr tends to be irreversible. The rate constants obtained in these media are tentative because of excessive adsorption of chromous ions in these media. The rate constants have been... 

Robinson et al. [263] have studied both at the laboratory and semi-technical scale the electrochemical generation of chromous ion (Cr(II)) for the reduction of sodium hydroxymandelate (SHM) to 4-hydroxy phenylacetic acid (HPA), a Tenormin intermediate, as a potential replacement for the existing Zn(Hg) reduction process. 

Robinson D, Electrochemical generation of chromous ion for the reduction of hydroxyman-delate, in ref (45a)... 

The main kinetic consideration is the time-scale of the redox reaction if the relevant electron-transfer reaction is slow, then we run the risk that measurements are taken before a true equilibrium has been attained after the addition of each aliquot. In practice, however, most analytes are oxidized or reduced within a very short time-scale - probably within microseconds if mixing is efficient, and provided that the oxidant (e.g. H2O2, Mn04, Ce or Cr20j ) or reductant (e.g. chromous ion, Cr, dithionite, 8204, or thiosulfate, S203 ) is sufficiently powerful. Note that while oxidation of Cr to form is fast, the... 

The principal difference between the two groups lay in the relative ratio of chromous ion to nickel employed [53],... 

This mechanism can be illustrated by the reaction of ferrous ions with hydrogen peroxide (42), the reduction of organic peroxides by cuprous ions (63), as well as by the reduction of perchlorate ions by Ti(III) (35), V(II) (58), Eu(II) (71), The oxidation of chromous ions by bromate and nitrate ions may also be classified in this category. In the latter cases, an oxygen transfer from the ligand to the metal ion has been demonstrated (8), As analogous cases one may cite the oxidation of Cr(H20)6+2 by azide ions (15) (where it has been demonstrated that the Cr—N bond is partially retained after oxidation), and the oxidation of Cr(H20)6+2 by 0-iodo-benzoic acid (6, 8), where an iodine transfer was shown to take place. 

The action of 0-iodobenzoic add as oxidant has also been observed in the oxidation of chromous ions, where an atom transfer has also been demonstrated (6, 8). The double equivalent redox reaction is inferred from the formation of benzoic acid as the product of de-iodination. 

In the case of chromous ions, wtc again looked for the composition of the hydrogen evolved from chromous solutions, again in the presence of isopropyl alcohol. This was the analytical technique used to detect hydrogen atoms. Again we were not able to produce any H—D but H2 only. This suggests a hydride transfer mechanism in the reducing of water by Cr(II). 

Although catalytic hydrogenation is the method most often used, double bonds can be reduced by other reagents, as well. Among these are sodium in ethanol, sodium and f-butyl alcohol in HMPA,243 lithium and aliphatic amines244 (see also 5-10), chromous ion,245 zinc and acids, sodium hypophosphate and Pd-C,246 (EtO)3SiH-Pd(OAc)2,247 trifluoroacetic acid... 

Several gas detector tubes are used in conjunction with common colorimetric reactions to detect butadiene. The reactions include the reduction of chromate or dichromate to chromous ion and the reduction of ammonium molybdate and palladium sulfate to molybdemun blue (Saltzman Harman, 1989). 

Although there are no really stable species with this configuration, there are interesting aspects to its chemistry. The chromous ion, Cr2+, is strongly reducing, but yet may be prepared readily ... 

Chromium metal reacts with aqueous acids in the absence of oxygen to give Cr2+(aq), the beautiful blue chromium(II) (chromous) ion, in which Cr2+ is bound to six water molecules, Cr(H20)62+ (Figure 20.7a) ... 

Ulmer, G. White, W. B. (1966) Existence of chromous ion in the spinel solid solution series FeCr204-MgCr204. J. Amer. Ceram. Soc., 49,50-1. 

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