Nicotinium dichromate oxidation

A compound prepared and first described as nicotinium dichromate (NDC) by Palomo et al.,379 was later shown by X-ray-crystal analysis380 to be a betainic mixed anhydride of nicotinic and chromic acid (NACAA). Because of its unique structure, it deserves a close scrutiny of its oxidative properties.381 Replacement of the chloride anion in the quaternary ammonium resin, Dowex 1-X8, for the dichromate anion, leads to a polymer supported dichromate, which is able to make selective benzylic oxidations.382 Finally, poly[vinyl(pyridinium dichromate)] (PVPDC), a polymeric analogue of PDC, must be mentioned whose use in the oxidation of alcohols allows for a very easy work-up.383... 

The oxidation of meta- and para-substituted anilines with imidazolium fluorochro-mate (IFC)18 and nicotinium dichromate (NDC),19 in several organic solvents, in the presence of p-toluenesulfonic acid (TsOH) is first order in the oxidant and TsOH and is zero order with respect to substrate. A correlation of rate data in different solvents with Kamlet-Taft solvatochromic parameters suggests that the specific solute-solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute-solvent complexation. The oxidation rates with NDC exhibited negative reaction constants, while the oxidation with IFC did not correlate well with any linear free energy relationships. 

F. Roldan, A. Gonzalez, and C. Palomo, Nicotinium dichromate a new cheap reagent for high-yielding large-scale oxidation of carbohydrates, Carbohydr. Res., 149 (1986) C1-C4. 

Nicotinium dichromate (16) is a stable, nonhygfx copic, nonphotosensitive, mild oxidant.Nicotinium dichromate alone in dichloromethane gives moderate selectivity for benzylic and allylic over saturated alcohols. This selectivity is further increased by the use of benzene as solvent... 

Nicotinium dichromate has been reported to be particularly useful for large scale carbohydrate oxidations where other modified chromium oxidants have failed. ... 

In the presence of pyridine, nicotinium dichromate is also a sufficiently strong oxidant for the preparation of quinones from hydroquinones,... 

Carboxypyridinium dichromate (isonicotinium dichromate) has also been prepared by Palomo and coworkers. It appears to have very similar properties to nicotinium dichromate, and offers no further advantage for the oxidation of alcohols. 

The rate of oxidation of chalcones in ACOH-H2O has first order in imidazolium dichromate (INDC) and H+ ion and fractional order in the chalcones decreased with decreasing dielectric constant." The oxidation of phenoxyacetic acids in ACOH-H2O-HCIO4 is first order in nicotinium dichromate (NDC) and substrates, and second order in H+ ion. The electron-releasing substituents and increased percentage of AcOH increased the rate, which was decreased by electron-attracting substituents. The rate correlated with the Hammett relation, and Mn + ions exhibited noticeable catalytic effect." ... 

For direct aerobic a, -dehydrogenation of aldehydes and ketones with a new Pd(TFA)2/4,5-diazafluorenone catalyst, the turnover-limiting step is a-C-H cleavage. Means of oxidative cleavage of C-C bonds in aldehydes by O2, cyclic ketones " by nicotinium dichromate, and a-hydroxy ketones by O2 have been discussed. 

Suitable mechanisms have been proposed following determination of the kinetic and activation parameters for oxidation of 2-naphthol and cyclic ketones by nicotinium dichromate some a-amino acids by tripropylammonium fluorochromate " distyryl ketone by quinaldinium fluorochromate methanol by benzyltriethylammonium chlorochromate catalysed by 1,10-phenanthroline substituted benzyl alcohols by tetraethylammonium bromochromate L-cysteine by pyridinium bromochromate lactic acid and 3,5-dimethyl-2,6-diaryl piperidin-4-one oximes by pyridinium chlorochromate allyl alcohol by IDC benzophenoxime by bispyridine silver(I) dichromate and alkyl phenyl sulfides by cetyltrimethylammonium dichromate. A non-linear Hammett plot obtained for the oxidation of substituted benzyl alcohols by IDC has been attributed to the operation of substituent effect on two steps of the proposed mechanism. " Kinetic and activation parameters for oxidation of o-toluidine and of A-methyl-2,6-diphenyl piperidin-4-one oxime and its 3-alkyl derivatives by sodium dichromate have been determined and suitable mechanisms have been suggested. Micellar catalysis in the 1,10-phenanthroline-promoted chromic acid oxidation of propanol... 

---