Bi(III) carboxylates as efficient reagents for preparation of esters under mild conditions. A new method for the protection of carboxylic acids

Article abstract of DOI:10.1016/S0040-4039(00)02125-0

Bi(III) carboxylates have been obtained by reaction of triphenylbismuth with various monocarboxylic acids. By a ligand-exchange reaction, these Bi(III) carboxylates react with tertiary or benzylic-type bromides and afford esters. This reaction sequence constitutes a new efficient way for the temporary protection of carboxylic acids under mild conditions.

Full text of DOI:10.1016/S0040-4039(00)02125-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 855–857
Bi(III) carboxylates as efficient reagents for preparation of
esters under mild conditions. A new method for the protection of
carboxylic acids
El-Mehdi Keramane, Bernard Boyer* and Jean-Pierre Roque
Laboratoire de Chimie Organique Physique, De´partement de Chimie Organique Fine, Universite´ Montpellier II,
Place E. Bataillon, 34095 Montpellier Cedex 5, France
Received 12 October 2000; accepted 14 November 2000
Abstract—Bi(III) carboxylates have been obtained by reaction of triphenylbismuth with various monocarboxylic acids. By a
ligand-exchange reaction, these Bi(III) carboxylates react with tertiary or benzylic-type bromides and afford esters. This reaction
sequence constitutes a new efficient way for the temporary protection of carboxylic acids under mild conditions. © 2001 Elsevier
Science Ltd. All rights reserved.
In a series of preliminary studies, we have established
the general usefulness of bismuth(III) reagents for the
halogenation of alcohols,1 the halogen exchange
reactions2 and the protection of aliphatic alcohols as
benzyl ethers.3 More recently, we showed that the
behaviour of bismuth(III) halides (BiX3; X=Cl, Br and
I) is totally different according to the nature of the
halide bonded to the bismuth atom4 and proposed a
mechanism to explain such behaviour. We noted that
bismuth(III) derivatives acting as reagent, react accord-
ing to a general ligand-exchange mechanism. Their
reactivities agree with the Hard and Soft Acids and
Bases (HSAB) Pearson’s principle. Referring to the
bismuth atom, ligand exchange occurs if the entering
group is a softer base than the leaving one (Scheme 1).
To check this new way to access ester function, we first
prepared bismuth(III) acetate and benzoate by reacting
the commercially available triphenylbismuth with the
corresponding carboxylic acid (Scheme 3). This
exchange reaction took advantage of the very weak
phenyl/bismuth bond which has one of the weakest
aryl/element bond energies6 and was achieved in good
yield.7 This reaction is all the easier as carboxylates are
softer bases than phenyl anion. Only a stoichiometric
amount of the carboxylic acid was required. Moreover,
the by-product of the reaction was the volatile benzene.
Both bismuth acetate and benzoate were in turn reacted
with a series of bromides in CCl4 to promote exchange
reaction under stoichiometric conditions (Scheme 2).
This reaction was easy to monitor because the colour of
the solid changed from white [Bi(OCOCH3)3 or
Bi(OCOC6H5)3] to yellow [like BiBr3]. That colour
On this basis we thought that it would be possible to
achieve ligand exchange between a bismuth carboxylate
and an alkyl bromide, as the bromide ion is a softer
base than carboxylate,5 the result being an ester
(Scheme 2).
Scheme 2.
Scheme 3.
Scheme 1.
Keywords: bismuth; esters; protection; carboxylic acids.
* Corresponding author. Fax: (33) 04 67 14 38 88; e-mail: bboyer@univ-montp2.fr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02125-0

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E.-M. Keramane et al. / Tetrahedron Letters 42 (2001) 855–857
change proved that the ligand exchange reaction
occurred. Table 1 summarises the results obtained.
above described procedure.7 Because of its high reactiv-
ity, tert-butyl bromide was then added.
The data reported in Table 1 call for the following
comments:
The white solid [Bi(OTs)3] became yellow [like BiBr3]
and the bromide disappeared within a few minutes
from the liquid phase. Both observations agree with an
exchange reaction between the bromide and tosylate
groups. However, we did not detect any product in the
organic phase. Moreover, during the reaction, the vol-
ume of the solid increased.
(i) The primary bromides, 1-bromohexane and 1-bro-
mooctane, did not react whatever the bismuth
carboxylate. This agrees with results previously
reported2 dealing with the reaction involving bis-
muth(III) halides as a reagent. Therefore, this
method is not applicable for the synthesis of esters
of primary alcohols.
(ii) Secondary bromides afforded alkene formation
simultaneously with the expected acetate or ben-
zoate. The aliphatic 2-bromopentane led to ester as
major product (75%). On the contrary, with cyclo-
hexyl bromide, the alkene was the major product
(70%).
(iii) Tertiary and benzylic bromides appeared to be the
most reactive substrates, yielding exclusively the cor-
responding esters. Except for benzene, no other by-
product was detected. Thus, bismuth(III) carboxy-
lates acted as a reagent. It is worthwhile to remark
that these two groups (tert-butyl and benzyl) are also
common protecting groups for carboxylic acids.
Recently,4 we have shown that Bi(III) (a soft Lewis
acid) can form intermediate complexes possessing sev-
eral coordination-type around the bismuth atom.
Besides, an example of intramolecular coordination of
a sulfonyl group with a bismuth atom has previously
been described.8 So we suspected that when the tosylate
(TsOtBu) was formed, it would give a complex with the
Lewis acid BiBr3 produced during the reaction. That
could explain the swelling of the solid and the absence
of the expected tosylate in the organic phase. To verify
that point, we ran the following experiment: 3-tosylpen-
tane and BiBr3 in a 3:1 molar ratio were refluxed in
CCl4 for 1 h. We observed that the volume of the solid
rapidly increased. At the same time, 3-tosylpentane
disappeared from the solution while no other product
was detected. This observation would seem to agree
with our former assumption.
Thus, it appears that carboxylic acids can be easily
converted to their tert-butyl or benzyl esters via their
reaction with triphenylbismuth, followed by tert-butyl
or benzyl bromides.
In conclusion, this study confirms that bismuth, acting
as a reagent, reacts according to a general ligand-
exchange mechanism in agreement with the HSAB
principle.
The fact that primary bromides are unreactive affords
an interesting functional group selectivity. Indeed, a
specific exchange could be expected with primary/ter-
tiary dibromides as previously observed for halogen
exchange of alkyl halides in the presence of BiX3.2
Our procedure based on the use of bismuth(III) car-
boxylates in the presence of bromides provides a new
simple, efficient and useful alternative for the prepara-
tion of tertiary and benzyl-type esters from bromides.
Our results offer the possibility of using this method to
protect carboxylic acids as tert-butyl, benzyl or 1-
phenylethyl esters, which are largely used since they can
We have applied the same approach to prepare tosy-
lates from bromides according to Scheme 4. Bis-
muth(III) tosylate was first obtained following the
a
Table 1. Reaction of Bi(III) acetate and benzoate with various bromide derivatives in CCl4
b
RBr
T°C
Reaction times Products with Bi(OAc)3
Products with Bi(OBz)3
1-Bromohexane
1-Bromooctane
2-Bromopentane
Cyclohexyl bromide Reflux
tert-Butyl bromide 25°C
Benzyl bromide
1-Phenylethyl
bromide
Reflux
Reflux
Reflux
10 days
12 days
45 h
48 h
1 h
No reaction
No reaction
75% 2-Pentyl acetate+25% 2-pentene
30% Cyclohexyl acetate+70% cyclohexene
tert-Butyl acetate (100%)
Benzyl acetate (100%)
No reaction
No reaction
75% 2-Pentyl benzoate+25% 2-pentene
30% Cyclohexyl benzoate+70% cyclohexene
tert-Butyl benzoate (100%)
Benzyl benzoate (100%)
25°C
25°C
2 h
1 h
1-Phenylethyl acetate (100%)
1-Phenylethyl benzoate (100%)
a Bi(OCOC6H5)3 or Bi(OCOCH3)3 (1.25 mmol) in CCl4 (5 ml) at room temperature or at reflux was stirred vigorously (1200 rpm). To this stirred
suspension, bromide derivative (R%Br, 3.75 mmol) was added and the stirring was continued. The reaction was followed by GC analysis of the
liquid phase until complete bromide consumption. The mixture was filtered and the solvent evaporated under reduced pressure to provide the
product (yield range 90–96%). The products were analysed by GC, 1H NMR and their spectral data compared with those of authentic samples.
b Identical results were obtained with commercially available Bi(OAc)3.
Scheme 4.

E.-M. Keramane et al. / Tetrahedron Letters 42 (2001) 855–857
857
be easily removed.9 The advantages of this procedure
are mainly the mild conditions, easy work-up, good
yields and short reaction times.
5. (a) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533; (b)
Pearson, R. G. Science 1966, 151, 172; (c) Parr, R. G.;
Pearson, R. G. J. Am. Chem. Soc. 1983, 105, 7512; (d)
Pearson, R. G. J. Chem. Ed. 1987, 64, 561.
We are presently exploring the scope of this new
reaction.
6. Finet, J. P. Chem. Rev. 1989, 89, 1487.
7. Preparation of bismuth(III) carboxylate: In a 10 ml flask,
triphenylbismuth (1.25 mmol) was dissolved in anhydrous
CCl4 (5 ml) under vigorous stirring (1200 rpm) at reflux.
Then, benzoic or acetic acid (3.75 mmol) was introduced.
The reaction was monitored by TLC. After 16 hours, the
white solid formed was filtered and dried under reduced
pressure (89% yield). The products were analysed by 1H
NMR and the data are consistent with its assigned struc-
ture. Furthermore, mass spectroscopy (FAB+) showed the
characteristic signals of such products (e.g. m/z for
Bi(OCOC6H5)3: 473 [M+H]+; 495 [M+Na]+).
References
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3. Boyer, B.; Keramane, E. M.; Roque, J. P.; Pavia, A. A.
Tetrahedron Lett. 2000, 41, 2891.
8. Suzuki, H.; Murafugi, T.; Azuma, N. J. Chem. Soc.,
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