Oxidation of organic compounds with pentavalent vanadium

Vanadium (V) only possesses oxidising properties in acid solutions. The main species present in these conditions is that cation VO2+, but this is protonated to a small extent to give the more powerfully oxidising species V(OH)23+.
Previous work in this laboratory has shown that vanadium (V) is a one-electron oxidising agent and is reduced by organic compounds to vanadium (IV) only. No vanadium (III) has yet been detected in any of these oxidation reactions while the presence of free radicals, which are usually associated with one-electron oxidations, has been demonstrated in many vanadium (V) oxidations of organic compounds.
To extend the knowledge of these oxidations a number of alcohols, α-hydroxy acids, acyloins and aldehydes, and also pinacol and oxalic acid have been oxidised by acid solutions of vanadium (V). The products of these oxidations have been investigated and in many cases a detailed investigation of the kinetics of the reactions has been made.
1. α-t-Butyl=benzyl alcohol PhCHOH.C(CH3)3 was shown to be oxidised initially to benzaldehyde with direct carbon-carbon bond fission. The yield of pivalophenone Ph CO.C(CH3)3, which is not oxidised under the conditions used, was only about 10-15%. This direct carbon-carbon bond fission has been noted previously in the oxidation of various secondary alcohols with chromium (VI) and manganese (VII), but in general the yield of fission products is much smaller with these reagents. The following mechanism is proposed for the fission reaction in the vanadium (V) oxidation.
[For the diagram omitted here, please consult the PDF.]
2. 2-Phenyl=ethyl alcohol was also found to be oxidised by vanadium (V) with carbon-carbon bond fission. Benzaldehyde was the main product of the reaction. Phenyl=acetic acid, which is not oxidised by vanadium (V) was not found in the reaction products.
3. The oxidations of the α-hydroxy acids lactic acid, malic acid, mandelic acid and α-hydroxy iso-butyric acid, and also of pinacol, were found to follow the rate equation
-d[VV]⁄dt = k[VV][Substrate](1 + a h0)
where a is a constant and h0 is the Hammett acidity function. The acid independent reaction must be due to VO2+ while the acid dependent reaction must be due to V(OH)23+. The above expression contrasts with that obtained for the oxidation of pinacol monomethyl ether:-
-d[VV]⁄dt = k[VV][ether](H+)
This has no acid independent term and is the same as the expression obtained for the oxidation of cyclohexanol. Apparently pinacol monomethyl ether is oxidised as an alcohol:-
[For the diagram omitted here, please consult the PDF.]
The mechanism of oxidation of the hydroxy acids is postulated to be
[For the diagram omitted here, please consult the PDF.]
and for pinacol
[For the diagram omitted here, please consult the PDF.]
Complex I contains a molecule of water which is not present in II or III and this fact has been used to explain the difference in the observed acidity dependences of the two reactions. The products of the oxidations were shown to be
lactic acid → acetaldehyde + carbon dioxide
malic acid → acetaldehyde + carbon dioxide
mandolic acid → benzaldehyde + carbon dioxide
α-hydroxy iso-butyric acid → acetone + carbon dioxide
pinacol → Acetone    
These products are consistent with the above mechanisms.
4. In two respects the oxidation of oxalic acid is unique among vanadium (V) oxidations which have been investigated so far; firstly oxidation can occur in the complete absence of mineral acid; secondly at acid concentrations above 3.5 M, the addition of mineral acid decreases the rate of oxidation. The kinetics of the oxidation were found to be somewhat complex and have been interpreted by postulating that a series of complexes is formed between the reactants. It was shown that a complex of formula (H2C2O4)2. VV is formed in the absence of mineral acid. The main reaction at low acidities is probably assisted by the formation of a cyclic chelate structure
[For the diagram omitted here, please consult the PDF.]
5. Acetoin was found to be oxidised by vanadium according to the rate equation
-d[VV]⁄dt = k[Acetoin][VV].h0
but the oxidation of 3-hydroxy-3-methyl 2-butanone (methyl acetoin) was found to have the rate equation
-d[VV]⁄dt = k[Methyl acetoin][VV].(1 + ah0)
Methyl acetoin must therefore be oxidised in a similar manner to the hydroxy acids giving acetone and acetic acid as products.
[For the diagram omitted here, please consult the PDF.]
Acetoin was found to be oxidised to diacetyl initially. In order to account accurately for the observed oxidation kinetics it has been necessary to postulate a vanadium (V) complex containing acetoin as a tridentate ligand. The following mechanism is suggested for the oxidation,
[For the diagram omitted here, please consult the PDF.]
and in the discussion it is explained that this mechanism is equivalent to an oxidation of acetoin both as a ketone and as an alcohol.
6. Three aldehydes, propionaldehyde, n-butyraldehye and iso-butyraldehyde were oxidised by vanadium (V) and were found to undergo carbon-carbon bond fission rather than to be oxidised to the corresponding carboxylic acids.
CH3 CH2 CHO →4 VV CH3 CHO + HCOOH
CH3 CH3 CH2 CHO →3 VV CH3 CHO + 2 HCOOH
(CH3)2 CH CHO →4 VV (CH3)2 CO + HCOOH
The kinetics of the iso-butyraldehyde oxidation followed the rate equation
-d[VV]⁄dt = k[aldehyde][H+]
The rate of oxidation was independent of the concentration of oxidising agent and was equal to the rate of enolisation of the aldehyde. The aldehyde is therefore oxidised in its enolic form.
For propionaldehyde and n-butyraldehyde the rate equation was found to be
-d[VV]⁄dt = k[VV][aldehyde][H+]
at low acid concentrations, but the rate increased more rapidly at high acid concentrations than this equation would indicate. The rate of enolisation of these two aldehydes was found to be faster than the rate of oxidation and although the reaction kinetics are consistent with a reaction of the aldehyde in its enolic form, they cannot confirm this definitely. Various possible mechanisms for the reaction have been discussed.