3,5-dimethylpyrazolium fluorochromate(VI)-catalysed oxidation of organic substrates by hydrogen peroxide under solvent-free conditions

Article abstract of DOI:10.1002/adsc.200505116

3,5-Dimethylpyrazolium fluorochromate (VI), C₅H ₈N₂H[CrO₃F], DmpzHFC, serves as an efficient catalyst for oxidation of primary, secondary and allylic alcohols to the corresponding carbonyl compounds using H₂O₂ at room temperature under solvent-free conditions. Oxidation of methyl phenyl sulfides and triphenylphosphine were also carried out successfully.

Full text of DOI:10.1002/adsc.200505116

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3,5-Dimethylpyrazolium Fluorochromate(VI)-Catalysed
Oxidation of Organic Substrates by Hydrogen Peroxide under
Solvent-Free Conditions
Mihir K. Chaudhuri,a,* Sanjay K. Dehury,a Sahid Hussain,a Ankur Duarah,a
Nayanmoni Gogoi,a M. Lakshmi Kantamb,*
a
Indian Institute ofTechnology Guwahati, Guwahati – 781039, India
Fax: (þ91)-361-269-0762, e-mail: mkc@iitg.ernet.in
b
Indian Institute ofChemical Technology, Hyderabad – 500 007, India
Fax: (þ91)-40-272-60921, e-mail: mlakshmi@iict.res.in
Received: March 27, 2005; Accepted: May 11, 2005
Abstract: 3,5-Dimethylpyrazolium fluorochromate
(VI), C5H8N2H[CrO3F], DmpzHFC, serves as an ef-
ficient catalyst for oxidation of primary, secondary
and allylic alcohols to the corresponding carbonyl
compounds using H2O2 at room temperature under
solvent-free conditions. Oxidation of methyl phenyl
sulfides and triphenylphosphine were also carried
out successfully.
augmented by the significant contributions of Muzart
et al.[12,13] in the domain ofCr(VI)-catalysed oxidations
by TBHP, UHP (urea hydrogen peroxide), SPC (sodium
percarbonate) and H2O2. The prevalence ofcontrolled
acidity ofa Cr(VI)(cat)-H O2 system appeared to be
2
one ofthe most important prerequisites for the success.
Based on these factors and also considering the pka val-
ues of PFC, QFC and DmpzHFC being 2.7, 4.7 and
7.8,[7–9] respectively, it was perceived that DmpzHFC
should be the preferred catalyst for this purpose.
Since DmpzHFC has an inherently controlled acidity
(pka, 7.8),[9] and also since Cr(VI) is capable ofactivating
peroxide through the formation ofa variety ofperoxo-
Cr(VI) species, it was thought to be reasonable to try
out the DmpzHFC(cat.)-H2O2 oxidation oforganic sub-
strates, especially alcohols. There have been a number of
reasons as to why H2O2 was selected as the oxidant of
Keywords: alcohols; catalytic oxidation; chromium;
3,5-dimethylpyrazolium fluorochromate(VI); hydro-
gen peroxide; solvent-free organic reactions
Over the years, partial oxidation has been one ofthe fun-
damental procedures in synthetic organic chemistry. It choice. Among TBHP, UHP, SPC and H2O2, H2O2 is
not only finds application in basic research and pharma- the least costly, most readily available and more impor-
ceutical industries but also is regarded as a core technol- tantly a non-waste-producing oxidant. However, H2O2
ogy for converting petroleum-based materials or bio- alone does not do the job efficiently, it needs to be acti-
mass-based feedstocks to useful chemicals.[1] Although vated to utilize its full potentiality.
a wide variety ofoxidants, catalysts and reaction systems
The present communication reports the results ofox-
have been developed, yet, Cr(VI)-based oxidants are idation ofa variety oforganic substrates by H O2 under
2
extensively used owing to their commendable perform- solvent-free conditions using DmpzHFC as the catalyst
ance under mild conditions with high efficiency in (Scheme 1).
amounts ranging from stoichiometric to a large ex-
cess.[2–10] Since the chromium residues are environmen-
tally harmful and at times pose difficulties in carrying
out the work-up, it would be advantageous to develop
Cr(VI)-catalysed oxidation protocols.[11–13] A large
Scheme 1.
number ofcatalytic systems has been described using
metal complexes associated to peroxo species for alco-
hol oxidations. Although Cr(VI)-catalysed H2O2 oxida-
tions often do not seem to work very efficiently,[12] our
Higher valent transition metals react with H2O2 quite
long association with developing Cr(VI) reagents[7–9] readily with the formation of a peroxo-metal species al-
and peroxo-metal chemistries[14} led us to believe that though their chemistry is rather complicated. What we
under appropriately worked out conditions hexavalent learnt from our experience in the field of peroxo-metal
chromium would be an effective catalyst for H2O2 oxida- chemistry[14] was that the formation of peroxo inter-
tion oforganic substrates. Our perception was uf rther
mediates is quite pH dependent and a higher pH gener-
Adv. Synth. Catal. 2005, 347, 1349–1352
DOI: 10.1002/adsc.200505116
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Mihir K. Chaudhuri et al.
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Table 1. DmpzHFC-catalysed oxidation ofselected organic substrates using H O2 as oxidant.
2
Entry
Substrate
Substrate/DmpzHFC/H2O2 molar ratio
Time [min]
5
Product
Yield [%]
1
1: 0.01: 2
81, 18,[a} 41[b]
2
1: 0.01: 2
5
82
3
4
1: 0.01: 2
1: 0.01: 2
3
2
82
80
5
1: 0.01: 2
1: 0.01: 2
2
5
84
87
6
7
1: 0.01: 2
1: 0.01: 2
30
5
75
85
¼
8
PPh3
Reaction with PFC(cat.)-H2O2.
O PPh3
a
b
Reactions with QFC(cat.)-H2O2.
ally favours the formation of complexes with a metal: tion did not pose any difficulty. We believe that it is the
O22À ratio of1: <1, and that the loss ofperoxide is rather
controlled with the raise in pH value ofthe reaction sol-
ution.
pH ofthe reaction solution ( cf. higher pH), rather than
the quantity ofwater, which is more important.
Similar reactions when conducted with either PFC or
Accordingly, the oxidations ofa selection ofprimary,
QFC as the catalyst gave very poor yield. For instance, n-
secondary and allylic alcohols were carried out with decanol (entry 1) was reacted with H2O2 (2 mmol,
DmpzHFC(cat.)-H2O2 and the results are summarized 23 mL) and either PFC (0.01 mmol, 0.0019 g) or QFC
in Table 1. The reactions went on quite fast affording (0.01 mmol, 0.0024 g) as catalyst for 5 min to provide
the products in very high yield. In fact the conversions the product only in 18% or 41% yield, respectively.
appeared to be complete in a few minutes. It is notable Thus the trend is just in the line with the controlled acid-
À
that secondary OH group is chemoselectively oxidized ity ofthe chromium catalyst. This supports the view that
to ketone in the presence ofa primary OH group (entry the lesser the acidity ofa Cr(VI)(cat)-H 2O2 system the
À
4). Allylic alcohols such as cinnamyl alcohol and gera- better it is for the oxidation of alcohols, at least within
niol (entries
5
and 6) upon treatment with the limits ofour experimental conditions. Itmay be men-
DmpzHFC(cat.)-H2O2 was converted to the corre- tioned that on addition ofthe catalyst, a violet colour
sponding aldehydes. It may be mentioned that acid-sen- was first formed which turned brown as the reactions
sitive substrates like geraniol need, in many instances, a proceeded. This clearly indicates that H2O2 interacts
buffer in solution-phase oxidations.
with DmpzHFC to form chromium-peroxo complexes
As representative examples ofoxo-transfer reactions,
in situ, which oxidise alcohols to the corresponding car-
DmpzHFC(cat.)-H2O2 oxidizes methyl phenyl sulfide bonyls. Relevantly, the involvement ofa peroxo-chromi-
and triphenylphosphine to methyl phenyl sulfoxide um intermediate was also suggested by Muzart et al.[12]
and triphenylphosphine oxide, respectively, (entries 7 in their report on alcohol oxidation with TBHP as the
and 8).
oxidant and CrO3 as the catalyst. The oxidation reaction
Importantly, the reactions are very facile, unlike many is likely to proceed via the nucleophilic attack by oxygen
other cases ofCr(VI)-catalysed aqueous H O2 oxida- ofalcohols to the metal as depicted in Scheme 2.
2
tions.[12] We did not face any problem with the commer-
The oxo-transfer reaction is expected to progress via a
cial H2O2. The amount ofH 2O present in the H2O2 solu- metal-oxygen shift mechanism. The ease of formation of
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3,5-Dimethylpyrazolium Fluorochromate(VI)-Catalysed Oxidation
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Scheme 2.
Scheme 3.
purification by column chromatography afforded the product
in good yields (Table 1).
the sulfoxide or phosphine oxide is likely to happen
through the nucleophilic attack by the hetero-atoms
(S, P) to the electrophilic O O bond ofa peroxo-metal
species thus also facilitating regeneration of the catalyst
(Scheme 3).
It may be stated in conclusion that DmpzHFC appears
to emerge as an appropriate catalyst, for the oxidation of
organic substrates, especially alcohols, by H2O2, which is
capable ofworking well with high selectivity. Its inher-
ently controlled acidity seems to be at the hub ofits suc-
cessful performance. This is well supported by the poor
performance of its companion catalysts PFC and QFC
having relative higher inherent acidity.
À
Acknowledgements
S. K. D. is an IITG research fellow. S. H. thanks the Council of
Scientific and Industrial Research (CSIR), India, for providing
a research fellowship.
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