Allylic oxidation with Collins reagent

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... 

The mode of interaction of the oxidant with the acetoxypalladation adduct is not certain. The oxidant could be removing electrons from Pd as the Pd(II)—C bond is broken and Pd(0) is never formed, or the Pd(II) could be oxidized to Pd(IV) which would leave much more easily than Pd(II). Another possibility is that the organic radical is transferred to the oxidant followed by decomposition. It would be difficult to distinguish between the various possibilities. Related reactions are the cleavage of a-bonded palladium complexes with Collins reagent (280), decomposition of rr-allyls with oxidants (164), and the decomposition of oxypalladation adducts of diolefins with oxidants (Section IV, B). 

Oxidative decomplexation of the above described palladium alkyl or allyl complexes with Collin s reagent gave norbornenones 20 and nortricyclenones 21 in varying ratios depending on the starting material. ... 

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

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). 

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. 

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). ... 

---