SCHEME 1
Ca ta lytic Hu n sd ieck er Rea ction of
r,â-Un sa tu r a ted Ca r boxylic Acid s: How
Efficien t Is th e Ca ta lyst?
J aya Prakash Das and Sujit Roy*
Organometallics & Catalysis Laboratory, Chemistry
Department, Indian Institute of Technology,
Kharagpur 721302, India
sroy@chem.iitkgp.ernet.in
Received April 28, 2002
the above protocols, within their limitations, is potential
means to synthesize unsaturated organic halides in
general and 1-halo-1-alkenes and 1-halo-1-alkynes in
particular. Tokuda and co-workers8 have recently re-
ported a microwave-induced version of CHR with lithium
acetate as catalyst in acetonitrile-water, originally
developed by us.5 Longer reaction time has been cited as
one of the limitations of microwave-free CHR.3,8 Since in
a bimolecular reaction reaction time is concentration
dependent, efficiency judgment based on total reaction
time becomes difficult at times unless one compares
experiments performed under identical conditions. In
view of this, we felt it appropriate to evaluate the
comparative efficiency of the catalysts in the CHR of R,â-
unsaturated acids and found UV-vis spectrophotometric
methods to be the ideal probe. A delightful outcome of
this study is the finding that triethylamine in dichloro-
methane or acetonitrile-water can also trigger CHR
giving rise to high yields of products in short reaction
time. The results of the above studies are presented in
this paper.
To evaluate the effective rate of the reaction, initially
1H NMR monitoring was attempted for the bromodecar-
boxylation of substituted cinnamic acids with N-bromo-
succinimide in the presence of catalysts (Scheme 1).
However, the method was found unsuitable due to
difficulties associated with integrating overlapping peaks
and very fast reaction in NMR-approachable concentra-
tion. For example, for a reaction with acid (10-2 M),
N-bromosuccinimide (10-2 M), and catalyst (10-5 M), the
total reaction time was less than 5 min. Henceforth, UV-
vis spectrophotometry is attempted for reaction monitor-
ing, for which 4-methoxycinnamic acid 1 is found to be
the ideal substrate. Compound 1 and the corresponding
â-bromostyrene 1a show well-separated absorption
maxima (Table 1). N-Bromosuccinimide and all catalysts
are also transparent within the spectral region scanned
(230-350 nm).
Abstr a ct: UV-vis spectrophotometry is utilized to measure
the relative efficiency of lithium acetate, tetrabutylammo-
nium trifluoroacetate, and triethylamine as catalysts for the
conversion of 4-methoxycinnamic acid to 4-methoxy-â-bromo-
styrene. In acetonitrile-water as solvent, the efficiency order
is lithium acetate > triethylamine > tetrabutylammonium
trifluoroacetate. For triethylamine as catalyst, solvent-
dependent order is acetonitrile-water > dichloromethane
> acetonitrile. Using triethylamine as catalyst (5-20 mol
%), cinnamic acids, and propiolic acids are converted to
corresponding â-bromostyrenes and 1-halo-1-alkynes in 60-
98% isolated yields within 1-5 min.
The decarboxylative halogenation of carboxylic acids
is a synthetically useful route to organic halides. Various
modifications of the classical Hunsdiecker reaction are
testimony to the unabated interest of organic chemists
in this reaction.1 However, until recently, it remained a
difficult task to adopt a synthetically meaningful Huns-
diecker strategy in acids bearing unsaturation directly
at the C-terminus of the carboxyl group. Toward this,
recent strategies developed are (a) oxidative halodecar-
boxylation including the green variation,2,3 (b) biscol-
lidinehalogenium ion as a new Br+ or I+ source in
additive-free halodecarboxylation,4 and (c) electrophilic
halodecarboxylation catalyzed by either a metal salt
such as lithium acetate,5 and manganese acetate,6 or a
tetraalkylammonium salt.7 The latter is termed as the
catalytic Hunsdiecker Reaction (hereafter CHR). Each of
(1) (a) Borodin, B. Liebigs Ann. 1861, 119, 121. (b) Hunsdiecker, H.
C. Chem. Ber. 1942, 75, 291. (c) Chrich, D. In Comprehensive Organic
Synthesis; Trost, B. M., Steven, V. L., Eds.; Pergamon: Oxford, 1991;
Vol. 7, pp 723-734. (d) Sheldon, R. A.; Kochi, J . K. Org. React. (NY)
1972, 19, 326. (e) Mckillop, A.; Bromly, D.; Tayler, E. C. J . Org. Chem.
1969, 34, 1172. (f) Kochi, J . K. J . Am. Chem. Soc. 1965, 87, 2500. (g)
Cristol, S. J .; Firth, W. C., J r. J . Org. Chem. 1961, 26, 280. (h) Hassner,
A.; Stumer, C. Organic Synthesis based on Name Reactions and
Unnamed Reactions; Pergamon: Oxford, 1994; p 183. (i) Amouri, H.
E.; Gruselle, M.; Vaissermann, J .; McGlinchey, M. J .; J aouen, G. J .
Organomet. Chem. 1995, 485, 79. (j) Barton, D. H. R.; Chrich, D.;
Motherwell, W. B. Tetrahedron Lett. 1983, 24, 4979. (k) Bunce, N. J .;
Murray, N. G. Tetrahedron 1971, 27, 1369. (l) Camps, P.; Lukach, A.
E.; Pujol, X.; Vazquez, S. Tetrahedron 2000, 56, 2703. (m) J acques, J .
Compt. Rend. IIC 1999, 2, 181.
The spectral profile during the bromodecarboxylation
of 1 to 1a in the presence of excess N-bromosuccinimide
(2-6.5 equiv) and catalytic lithium acetate and triethyl-
(2) (a) Concepcion, J . I.; Francisco, C. G.; Freire, R.; Hernandez, R.;
Salazar, J . A.; Suarez, E. J . Org. Chem. 1986, 51, 402. (b) Graven, A.;
J orgensen, K. A.; Dahl, S.; Stanczak, A. J . Org. Chem. 1994, 59, 3543.
(c) Sinha, J .; Layek, S.; Mandal, G. C.; Bhattacharjee, M. J . Chem.
Soc., Chem. Commun. 2001, 1916.
(3) Roy, S. C.; Guin, C.; Maiti, G. Tetrahedron Lett. 2001, 42, 9253.
(4) (a) Homsi, F.; Rousseau, G. Tetrahedron Lett. 1999, 40, 1495.
(b) Homsi, F.; Rousseau, G. J . Org. Chem. 1999, 64, 81.
(5) (a) Chowdhury, S.; Roy, S. J . Org. Chem. 1997, 62, 199; Chem.
Eng. News 1997, J anuary 27, 24. (b) Naskar, D.; Roy, S. J . Chem. Soc.,
Perkin Trans. 1 1999, 2435.
(6) Chowdhury, S.; Roy, S. Tetrahedron Lett. 1996, 37, 2623.
(7) (a) Naskar, D.; Roy, S. J . Org. Chem. 1999, 64, 6896. (b) Naskar,
D.; Roy, S Tetrahedron 2000, 56, 1369. (c) Naskar, D.; Das, S. K.;
Giribabu, L.; Maiya, B. G.; Roy, S. Organometallics 2000, 19, 1464.
(8) Kuang, C.; Senboku, H.; Tokuda, M. Synlett 2000, 10, 1439.
10.1021/jo025868h CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/03/2002
J . Org. Chem. 2002, 67, 7861-7864
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