Organometallic reactions in aqueous media. Antimony-mediated allylation of carbonyl compounds with fluoride salts

Article abstract of DOI:10.1016/S0040-4039(00)00795-4

Commercial antimony can be used directly for the allylation of carbonyl compounds in aqueous media in the presence of fluoride salts. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00795-4

Tetrahedron Letters 41 (2000) 5009±5012
Organometallic reactions in aqueous media. Antimony-
mediated allylation of carbonyl compounds with ¯uoride salts
Lian-Hai Li and Tak Hang Chan*
Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada
Received 17 April 2000; accepted 15 May 2000
Abstract
Commercial antimony can be used directly for the allylation of carbonyl compounds in aqueous media in
the presence of ¯uoride salts. # 2000 Elsevier Science Ltd. All rights reserved.
The allylation of carbonyl compounds has proved to be a very important reaction and
numerous reagents and methods have been developed to accomplish this transformation.1 The
discovery2 in the past decade that this transformation could be achieved in aqueous media
through a Barbier-type reaction3 has drawn even more attention to this area of chemistry. Metals
such as Zn,4 In,5 Bi,6 Sn,7 Pb,8 Mn,9 Mg,10 or Sb,11 have been reported to be e€ective in med-
iating the coupling between allyl halides and carbonyl compounds to give the corresponding
homoallylic alcohols in aqueous media (Scheme 1). Among these reported metals, all of them
except Sb were found to e€ect the transformation with commercially available metal powder,
even though in cases such as Sn or Zn, activation was needed. In the case of antimony, however,
commercial antimony metal was apparently not successful in mediating the allylation reaction in
aqueous media. Several publications have appeared in the literature to indicate that the antimony
metal must be formed in situ through the reduction of antimony trichloride with a reducing
reagent such as Al or Fe11a or NaBH4.11b The `active' antimony, thus generated, could then
mediate the allylation reaction in an aqueous/organic solvent system. As part of our general
program12 to study organometallic reactions in aqueous media, we are interested in ®nding con-
ditions which can allow the use of commercial antimony directly without the need for the reductive
generation step. Recently, we found that some ¯uoride salts are quite e€ective in `activating'
aluminum metal in aqueous media to mediate the reduction and/or pinacol coupling of carbonyl
compounds.13 We report here that ¯uoride salts are equally e€ective in `activating' antimony in
aqueous media to mediate the coupling of allyl bromide with aldehydes to give the corresponding
homoallylic alcohols.
* Corresponding author. E-mail: thchan@chemistry.mcgill.ca
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00795-4

5010
Scheme 1.
In agreement with previous reports, the reaction performed in distilled water with the molar
ratio of Sb:benzaldehyde:allyl bromide at 5:1:2.5 gave no trace of the corresponding homoallylic
alcohol even after a week of vigorous stirring of the reaction mixture (Entry 1, Table 1). It was
obvious that activation of the metal was needed for this reaction to proceed. Therefore, we tried a
number of alkaline metal ¯uoride salts as additives in the aqueous media. At 1 M concentration,
RbF and CsF were found to be quite e€ective in activating the antimony to give a good yield of
the corresponding homoallylic alcohol. At 2 M concentration, NaF and KF were found to be equally
e€ective as RbF and CsF. On the other hand, even at 2 M concentration, KCl or KI were found
to be less e€ective (entries 6 and 8, Table 1) and KBr was completely ine€ective (Entry 7, Table 1).
Table 1
The factors that in¯uence the allylation of benzaldehyde
We thus used 2 M KF as the standard reaction conditions to activate commercial antimony
metal and examined its reactions with a number of carbonyl compounds. The results are sum-
marized in Table 2. With these conditions, aldehydes could be allylated to give excellent yields of
the products (Entries 1±10, Table 2). The results in Table 2 showed some interesting features of
the antimony-mediated allylation reaction. First, the reaction was not sensitive to the nature of
the aldehydes used. The reaction proceeded well with either aromatic or aliphatic aldehydes. The
allylation of a,b-unsaturated aldehyde as represented by trans-cinnamaldehyde (entry 10, Table 2)
occurred in a regiospeci®c manner and gave solely the 1,2-addition product. Furthermore, electron
donating or withdrawing groups on the aromatic ring did not seem to a€ect the reaction signi®cantly
either in the yield of the product or the rate of the reaction. Thirdly, the reaction conditions
appeared not to be reductive in nature. Thus, we did not detect alcohols or pinacols as side products
of the reactions. Similarly, nitro function was not reduced under the reaction conditions. Thus,
p-nitrobenzaldehyde was successfully allylated. Usually, the nitro group is sensitive to reduction

5011
by metals and could not be allylated under Barbier conditions.14 In this sense, the use of ¯uoride
salts as activating agent is superior to the use of Al, Fe or NaBH4 reported previously.11 Finally,
the reaction was chemoselective. E€orts to allylate ketones failed (Entries 11±13, Table 2) which
is in agreement with literature reports using `active' antimony.11 Even the reactive carbonyl in
methyl pyruvate (entry 13, Table 2) was left untouched.
Table 2
Allylation of carbonyl compounds mediated by Sb/KF in H2O
Standard procedures for the allylation of aldehydes are as follows: To a mixture of aldehyde
(1 mmol) in 2 M aqueous potassium ¯uoride solution (4 mL) and allyl bromide (2.5 mmol),
antimony powder (2 mmol) was added in one portion and the mixture was vigorously stirred until
the antimony powder was reacted (usually for 16 h). Ethyl ether was added to the reaction mixture
and the organic layer was separated. The aqueous phase was extracted with ethyl ether. The organic
extracts were combined and dried over Na2SO4, and was ®ltered and evaporated. The residue, for

5012
most of the aldehydes, a€orded the corresponding homoallylic alcohols of sucient purity
1
according to H NMR without the need for further puri®cation. If necessary, puri®cation was
performed by ¯ash chromatography over silica gel using EtOAc/hexane (to adjust the Rf value of
alcohol at about 0.15±0.2) as eluent. All the homoallylic alcohols reported in Table 2 are known
compounds: 1-phenylbut-3-en-1-ol (3a),15 1-decene-4-ol (3b),16 1-(4-chlorophenyl)but-3-en-1-ol
(3c),17 a-propenyl-cyclohexanemethanol (3i),18 1-phenyl-1,5-hexadien-3-ol (3j),19 1-(4-nitrophenyl)-
but-3-en-1-ol (3e),20 1-(4-methoxyphenyl)but-3-en-1-ol (3d),21 1-naphthylbut-3-en-1-ol (3h),21 1-(4-
tri¯uoromethylphenyl)but-3-en-1-ol (3g),22 1-(4-methylphenyl)but-3-en-1-ol (3f).22
Acknowledgements
We thank NSERC for ®nancial support of this research.
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