Chromium reagent

The topic of chromium oxdiations has been the subject of an excellent review by Wiberg. Discussions of the chemistry of chromium reagents have also been given by Stewart, Fieser, House, ° Neustaedter and Lee. ... 

Further oxidation of an aldehyde product to the corresponding carboxylic acid does not take place. Moreover, the SM>ern oxidation reaction does not require the use of toxic and pollutant chromium reagents. The activated DMSO species, however, are stable only at low temperature, which might in some cases be a drawback of this method. 

The chromium reagent, prepared from 3-bromopropene, exhibits a good ability to discriminate between axial and equatorial attack in 4-fcrf-butylcyclohexanone1. 

The stereoselectivity reverts in favor of the. rpn-isomer when bulky aldehydes such as 2,2-di-methylpropanal are employed8. This unusual feature is attributed to a higher reactivity of the (Z)-isomer in the equilibrating reagents15 or to the competition of the twist-boat transition states8. The diastereoselectivity decreases, when DMF is used as a solvent, or pyridine is added to the THF solution. Presumably, the complexation ability of the chromium reagent towards the aldehyde is decreased by these additives8. 

The stereoselectivities of reactions when both components are chiral, were also studied12. In matched pairs and, as well, in several mismatched combinations very high selcctivities of the same sense are observed. This indicates that the stereodirecting influence of the chromium reagents usually dominates (Table 3). 

The synthesis of the C(17)-C(24) segment also began with a diastereoselective boron enolate aldol addition. The adduct was protected and converted to an aldehyde in sequence H. The terminal diene unit was installed using a y-silylallyl chromium reagent, which generates a (3-hydroxysilane. Peterson elimination using KH then gave the Z-diene. 

Oxidations are often carried out using stoichiometric amounts of oxidants, such as permanganate, manganese dioxide or chromium reagents. Catalytic alternatives, therefore, hold great promise to substitute such processes. 

Each substrate was oxidized to the corresponding carbonyl compounds in good yields. Moreover, the coexisting olefin linkage remained intact upon treatment with the oxodiperoxochromium complex and no epoxy compounds were observed in the reaction mixture. Hexavalent chromium reagents such as anhydrous chromium trioxide and pyridinium... 

Matsubara, S. Oshima, K. Olefination of Carbonyl Compounds by Zinc and Chromium Reagents. In Modem Carbonyl Olefination-, Takeda, T., Ed. Wiley-VCH Weinheim, 2004 pp 200—222. 

For a review of this oxidation with chromium reagents, sec Cainclli Cardillo, Ref. 10. pp. 92-117. 

A selection of cases in which 2 has been found to be particularly efficacious is given in the Table. Additional examples are cited in references 1 and 5. Particularly noteworthy examples include the oxidation of acid- and base-sensitive systems, systems containing sulfur and selenium, and 1,3-diols to 1,3-dicarbonyl compounds. Use of chromium reagents in these latter cases often leads to fragmentation products. 

A number of new methods for the preparation of cyclopropanols from carbonyl derivatives via 1,3-bond formation between the carbonyl and carbons have been developed. 7>ons-2-alkylcyclopropanols are stereoselectively produced from 2- or 3-substituted acrolein upon exposure to chromium(II) chloride in the presence of a catalytic amount of nickel chloride in DMF (equation 56)73. 2,3-Disubstituted acroleins are, in contrast, inert to the chromium reagent. Treatment of / -stannyl carbonyls with titanium(IV) chloride affords cyclopropanols in good yields when the substrates are ketones not bearing j -alkyl... 

See also page 1128 for intramolecular examples using chromium reagents. 

In the second step the alcohol is transformed to an aldehyde by means of the Swem oxidation. Other reagents to oxidize alcohols to aldehydes are e. g. Dess-Martin-periodinane and chromium reagents like PCC or PDC. 

The mechanism also explains why an aldehyde product is resistant to further oxidation when methylene chloride is the solvent (i.e. no OH present to react with the chromium reagent). When aqueous conditions are used the aldehyde is hydrated and this generates two OH groups that are available to bond to the chromium reagent and result in further oxidation. 

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