Chromium-based reagents

Third, chromium-based reagents prepared by reduction of 1,1-dihalo compounds with CrCl2 are suitable for the transformation of aldehydes to (i4)-iodoalkenes, (it)-alkenylsilanes, " and ( )-alkenylboronic esters with one-carbon homologation (Equation (8)). The amount of CrCl2 can be reduced to a catalytic amount by using manganese (or zinc) and MesSiCl. " ... 

Other chromium-based reagents are also found to oxidize alcohols, following a mechanism like the one depicted above for oxidation with chromic acid.4... 

Tertiary allylic alcohols form a chromate ester that, as it lacks a hydrogen on a to the alcohol, instead of suffering a normal oxidation to ketone rearranges to an enone. This transformation, which can be brought about by other chromium-based reagents, is normally carried out with PCC when it is purposefully sought at (see page 55). 

PDC has a lesser tendency to effect oxidative transposition of allylic alcohols than other chromium-based reagents.163... 

Similar to other chromium-based reagents, kinetic evidence shows that oxidation of alcohols by PCC operates via a chromate ester intermediate that evolves to an aldehyde or ketone in the rate-determining step.194... 

This oxidative transposition of tertiary allylic alcohols into enones or enals is carried out under mild conditions and has ample application in organic synthesis. Although, it can be carried out with other chromium-based reagents (see pages 16 and 35), PCC is the reagent of choice.272... 

Normally, nitrocompounds resist307 the action of PCC although, on rare occasions, PCC can promote the attack of nucleophiles on nitro groups, in a similar way to the other chromium-based reagents (see pages 10 and 35). 

When the oxidation of a primary alcohol with PCC results in the formation of an aldehyde, activated with an electron withdrawing group at the a-position sometimes, a stable dimeric hemiacetal is formed that is further oxidized to a dimeric ester.331 This reaction, that can also happen with other chromium-based reagents (see page 42), can be minimized by adjusting the reaction conditions. 

Chromium-based oxidants tend to react quicker with unsaturated alcohols, although the difference of oxidation speed with saturated alcohols is normally not sufficient for synthetic purposes. Nevertheless, the chromium-based reagent pyridinium dichromate (PDC) possesses a mildness and, therefore, a relative greater selectivity that allows its occasional employment for selective oxidations of allylic and benzylic alcohols.134... 

Chromium-based reagent systems, for pinacol coupling, 11, 63 Chromium carbenes, in ene-yne metathesis, 11, 272 Chromium carbonyl compounds with bridging hydrides, 5, 206 computational studies and spectroscopy, 5, 203 experimentally determined structures, 5, 204 nitro and nitroso compounds, 5, 205 silatropic migrations, 5, 249 with very weakly bonded ligands, 5, 205 Chromium carbonyl hydrides, preparation and characteristics,... 

A number of other chromium-based reagents have been developed for allylic oxidation for example that of stnoids by t-butyl hydroperoxide in the presence of a catalytic amount (0.0S-0.S mol equiv.) of chromium trioxide in dichloromethane solution at room temperature (equation 39). Yields vary from 32 to 69%. This modification is useful in terms of cost, operational simplicity and yields. 

Because of the toxicity of chromium-based reagents, other methods for the oxidation of alcohols have been developed. One of the most widely employed methods, called the Swem oxidation, uses dimethyl sulfoxide [(CH3)2SO], oxalyl chloride [(COCl)2], and triethylamine. Since the reaction is not carried out in an... 

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