Ytterbium(III) triflate as a recyclable catalyst for the selective atom economic oxidation of benzyl alcohols to benzaldehydes

Article abstract of DOI:10.1055/s-1999-2857

Ytterbium(III) triflate (10mol%) was found to catalyse the selective oxidation of a range of simple benzyl alcohols to benzaldehydes in good to excellent yield using a stoichiometric quantity of 69% nitric acid as the oxidant, and where the only by-products were water and oxides of nitrogen. The catalyst can be readily recovered and recycled by a simple evaporative process.

Full text of DOI:10.1055/s-1999-2857

LETTER
Ytterbium(III) Triflate as a Recyclable Catalyst for the Selective Atom Economic Oxidation
1489
Ytterbium(III) Triflate as a Recyclable Catalyst for the Selective Atom
Economic Oxidation of Benzyl Alcohols to Benzaldehydes
Anthony G. M. Barrett,*a D. Christopher Braddock,a R. Murray McKinnella and Francis J. Wallerb
aDepartment of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, UK SW7 2AY
E-mail: agmb@ic.ac.uk
bAir Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501
Received 27 July 1999
Abstract: Ytterbium(III) triflate (10mol%) was found to catalyse
the selective oxidation of a range of simple benzyl alcohols to ben-
zaldehydes in good to excellent yield using a stoichiometric quanti-
ty of 69% nitric acid as the oxidant, and where the only by-products
were water and oxides of nitrogen. The catalyst can be readily re-
covered and recycled by a simple evaporative process.
Keywords: alcohols, aldehydes, catalysis, lanthanides, oxidations
The selective oxidation of benzylic alcohols to benzalde-
hydes is a transformation of considerable importance in
organic synthesis. Whilst numerous reagents have been
developed to effect this process in excellent yield, many
of them employ greater than stoichiometric quantities of
toxic heavy metals or co-oxidants which severely handi-
cap their applicability to large scale industrial process-
es.1,2 We have instigated a research program in the area of
clean technology whereby we seek to develop atom-eco-
nomic transformations in which little or no waste is gen-
erated, and have recently reported the use of
lanthanide(III) salts as recyclable catalysts for the nitra-
tion of simple arenes, affording water as the only side-
product.3,4 During our attempts to apply this nitration
methodology to benzyl alcohol, we noticed a rapid evolu-
tion of brown gas as the reaction mixture approached re-
flux temperature. Within a very short space of time the
starting alcohol had undergone almost complete conver-
sion to a less polar product, which upon isolation and
analysis proved to benzaldehyde. Whilst oxidation of ben-
zyl alcohols to benzaldehydes using nitric acid has been
described previously, the procedure demanded the use of
opposed to the uncatalyzed case, which failed to give any
conversion whatsoever. The reaction was unsuccessful for
electron rich arenes, which presumably suffered compli-
cations due to competing nitration of the aromatic ring.
at least 3 equivalents of nitric acid and a resultant neutral-
ization step at the end of the reaction.5
Optimization of the reaction conditions revealed that sim-
ply refluxing a solution of benzyl alcohol (1 equivalent),
69% w/w nitric acid (1 equivalent) and ytterbium triflate
(10 mole%) in 1,2-dichloroethane effected the formation
of benzaldehyde in 91% isolated yield within 0.5h. In the
absence of ytterbium triflate, the reaction proceeded to
only 17% conversion in 6h. Further studies showed that
this oxidation protocol can be applied to a wide range of
benzyl alcohols (Table 1).
A distinct advantage of the present method is the ease with
which the ytterbium triflate can be recycled without detri-
ment to the yields of aldehyde. Simple evaporation of the
aqueous phase allows almost quantitative recovery of the
catalyst. In the case of 4-bromobenzyl alcohol, 3 consec-
utive oxidations with the same batch of catalyst effected
conversion to the aldehyde in similar timescales and
yields (Table 2).
Although the reaction becomes slower as electron with-
drawing substituents are added to the ring, the advantages
of the catalyst are still clear, with 3,5-dinitrobenzyl alco-
hol giving a 70% conversion to the aldehyde over 24h, as
In conclusion, we have shown that ytterbium triflate is a
suitable catalyst in a novel procedure employing 69% ni-
tric acid for the highly selective and fast conversion of
Synlett 1999, No. 9, 1489–1490 ISSN 0936-5214 © Thieme Stuttgart · New York

1490
A. G. M. Barrett et al.
LETTER
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benzyl alcohols to benzaldehydes. Further applications of
ytterbium triflate and related catalysts in organic chemis-
try are being investigated.6
(4) For reviews on the use of lanthanide and scandium triflates in
organic synthesis see: Kobayashi, S. Eur. J. Org. Chem. 1999,
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(6) Barrett, A. G. M.; Braddock, D. C.; Henschke, J. P.; Walker,
E. R. J. Chem. Soc., Perkin Trans. 1 1999, 873; Waller, F. J.;
Barrett, A. G. M.; Braddock, D. C.; Ramprasad, D.;
McKinnell, R. M.; White, A. J. P.; Williams, D. J.; Ducray, R.
J. Org. Chem. 1999, 64, 2910.
Acknowledgement
We thank Air Products and Chemicals, Inc., for support of our rese-
arch under the auspices of the Joint Strategic Alliance, the EPSRC
for a studentship (to R. M. M.), Glaxo Group Research Ltd. for the
most generous endowment (to A. G. M. B.), and the Wolfson Foun-
dation for establishing the Wolfson Centre for Organic Chemistry
in Medical Science at Imperial College.
References
Article Identifier:
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Synlett 1999, No. 9, 1489–1490 ISSN 0936-5214 © Thieme Stuttgart · New York