Allyl alcohols Collins reagent

Collins reagent is used for the introduction of carbonyl groups at allylic positions." This transformation of alkenes into enones is much slower than the oxidation of alcohols, requiring a great excess of Cr03 2Py and prolonged reaction times. Consequently, alcohols can be oxidized to aldehydes and ketones by Collins reagent without interference from alkenes. 

Collins reagent can transform tertiary allylic alcohols into rearranged enones,101 similar to PCC, which is routinely used for this purpose (see page 55). As this reaction is normally slower than the oxidation of primary and secondary alcohols, these can be oxidized with Collins reagent with no interference from tertiary allylic alcohols present in the same molecule.102... 

Similar to Jones reagent, Collins reagent can produce a hydroxy directed epoxidation of allylic alcohols. This side-reaction only occurs in a limited number of allylic alcohols, most of them being oxidized uneventfully to the corresponding enones.117... 

Alternatively it is possible to oxidize a primary alcohol no further than to give the aldehyde. This is the domain of the Collins reagent, PCC, PDC, or activated dimethyl sulfoxide. The oxidation of primary alcohols with K2Cr207 in aqueous solution to nothing but the aldehyde, (i.e., without further oxidation to the carboxylic acid) is possible only if a volatile aldehyde results and is distilled off as it is formed. This is the only way to prevent the further oxidation of the aldehyde in the (aqueous) reaction mixture. Selective oxidations of primary alcohols to aldehydes with the Jones reagent succeed only for allylic and benzylic alcohols. Otherwise, the Jones reagent directly converts alcohols into carboxylic acids (see above). 

Table 3 Rearrangement of Tertiaiy Allylic Alcohols with the Collins Reagent...

In a similar fiashion to the Collins reagent, PCC will also induce oxidative rearrangement of tertiary allylic alcohols (Table S). PCC, and several other chromium oxidants, will also cause tertiary cyclopropyl alcohols to rearrange to give 3,y-unsaturated carbonyl compounds (equation 8). ... 

Pyridinium chlorochromate (PCC) Corey and Suggs prepared PCC by mixing CrOs with pyridine in HCl. PCC is used for the oxidation of primary and secondary alcohols in CH2CI2. This reagent is less efficient than Collins reagent for the oxidation of allyl alcohols. 

Chromium, trioxobisfpyridine)- Collins reagent, CrO)Py2, CrOy 2Py) allylic oxidation of alkenes, 116,120 oxn. of prim, alcohols, 125 oxn. of sec. alcohols (selective), 135 Chugaev pyrolysis of thioesters, 138,140-141 Cinchona alkaloids pr., 290... 

This chromyl chloride based reagent is inferior to Collins reagent for oxidation of allylic alcohols since cis-trans isomerization occurs. It has also been found to be unsatisfactory for oxidation of m-hydroxybenzyl alcohol (38%... 

Pyridinium Chlorochromate. The need for improved oxidation of primary alcohols and greater ease for isolation of products prompted further research into the nature of Cr(VI) reagents. Corey found that addition of pyridine to a solution of chromium trioxide in aqueous HCl allowed crystallization of a solid reagent characterized as 31, pyridinium chlorochromate (PCC). This reagent was superior for the conversion of primary alcohols to aldehydes in dichloromethane but less efficient than the Collins oxidation when applied to allylic alcohols. Oxidation of 1-heptanol with PCC in dichloromethane gave 78% of heptanal, for example. As stated by Corey, PCC is an effective oxidant in dichloromethane although aqueous chlorochromate species are not very effective oxidants. Oxidation of secondary alcohols to ketones is straightforward, as in Banwell s synthesis of y-lycorane, in which 32 was oxidized by PCC to the ketone (33). ... 

Eremophilane sesquiterpenes are not widely distributed in Eremophila species. In fact, the only other example so far known is the aldehyde (70) which occurs in E. rotundifolia (66). The structure of 70 was deduced largely from H- and C-nmr spectral data and, the absolute, stereochemistry by correlation with eremophilone. The keto aldehyde was reduced with sodium borohydride-cerium trichloride to give a mixture of diastereomeric diols. The major compound was assigned the 9a-configuration. The derived diacetate, on treatment with lithium-diethylamine, gave mainly the 9-allylic alcohol which was oxidised to eremophilone with Collin s reagent. 

Write out the product formed when allyl alcohol is oxidized with Collins s reagent and when benzyl alcohol is oxidized with Collins s reagent. 

I se of chromyl chloride in the Collins oxidation does not require large excesses of the reagent however, it is less selective than is Cr03, and tends to isomerize double bonds of allylic alcohols. 

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