achieve the arylation of allylic acetates from aryl halides by
introducing all the allylic substrate in the cell at the beginning
of the reaction (unlike the electrochemical process using a
nickel complex as a catalyst) and to use a less toxic catalyst
(Scheme 1).
Scheme 3. Aryl Chloride Allylation by Allyl Acetate with
Co-Fe-Mn
Scheme 1. Arylation of Allylic Acetate Catalyzed by CoBr2
by the Electrochemical Method
(0.16 g, 1.5 mmol). The reaction medium was activated by
adding acetic acid (0.024 g, 4 × 10-4 mmol) and stirred for
30 min at room temperature until bromobenzene was totally
consumed. Allyl acetate (3.03 g, 30 mmol) and ethyl
4-bromobenzoate (2.427 g, 15 mmol) were then introduced
into the solution. After being stirred for 3 h at room
temperature, the reaction mixture was poured into a solution
of 2 M HCl (40 mL) and extracted with diethyl ether (3 ×
40 mL). The combined extracts were dried over MgSO4.
Evaporation of ether and purification by column chroma-
tography on silica gel (pentane/ether, 99/1) afforded 1.852
g (65%) of 4-(2-propenyl)ethyl benzoate as a colorless oil.
Acetic acid was necessary to activate the zinc dust. Iodine
or trifluoroacetic acid could also play this part. Nevertheless,
with iodine as an activating agent, ethyl 4-bromobenzoate
is totally consumed only after 48 h and the amount of zinc
dust must be increased to 100 mmol. The presence of the
zinc bromide is not necessary but enhances the yield of the
reaction. To decrease the amount of the reduction product,
a catalytic amount of bromobenzene is added to the solution
before the introduction of the functionalized aryl bromide.
Bromobenzene is readily converted into the reduced product.
This procedure is the same as described before for the
formation of the aryl zinc species11 except that allylic acetate
is present in the medium and reacts directly by substitution
with this compound without a supplementary catalyst. The
byproducts of this reaction are the reduction product and the
dimer of the aryl bromide. The results of the allylation of
various aryl bromides by allyl acetate are reported in Table
1.
These electrochemical methods favorably compare with
known chemical processes. However, all the electrochemical
reactions are generally considered more difficult to handle
than conventional chemical methods. As a result, electro-
chemical syntheses are poorly applied by organic chemists
and are not used in a larger scale than in the laboratory.
Recently, we have demonstrated that in some cases a
purely chemical reaction could be extended from our initial
electrochemical process. Indeed, we have established that
the low-valent cobalt generated from the chemical reduction
of cobalt halide can unprecedently activate aryl bromide in
acetonitrile to form arylzinc species.11
Herein, we describe that the cobalt catalyst is suitable for
the efficient arylation of allylic acetate not only with aryl
bromides but also with aryl chlorides, using a different
reducing agent in the appropriate medium.
Aryl bromides react with allylic acetate in the presence
of zinc dust in pure acetonitrile (Scheme 2). However, aryl
Scheme 2. Aryl Bromide Allylation by Allyl Acetate with
Co-Zn
Good yields are obtained with several aryl bromides
substituted either by an electron-withdrawing group in the
para, meta, and ortho positions (Table 1, entries 1-3 and
5-8) or by an electron-donating group (Table 1, entry 4). It
can be pointed out that the position of the substituent has a
slight influence on the yield. For all aryl bromides, the
reaction time was ca. 3 h. However, the yield decreases with
a substituted allyl acetate, and the cross-coupling of crotyl
acetate or cinnamyl acetate with p-BrPhCOOEt leads to 21
or 30%, respectively, of the corresponding coupling products.
Undoubtedly, these yields can be improved through optimi-
zation of the process toward substituted allylic acetates. When
the halogen on ArX is chlorine instead bromine, the aryl
chloride is not consumed even if the aromatic nucleus is
substituted by an electron-withdrawing group as we have
already observed in the formation of the aryl zinc species in
this medium.
chlorides require stronger reducing metal (manganese pow-
der) in a mixture of acetonitrile-pyridine with a stoichio-
metric amount of iron salt to carry out the same reaction
(Scheme 3).
We first investigated the reaction between an aryl bromide
and allyl acetate. To an acetonitrile solution (20 mL) was
successively added zinc dust (3.25 g, 50 mmol), CoBr2 (0.657
g, 3 mmol), ZnBr2 (0.338 g, 1.5 mmol), and bromobenzene
(11) (a) Fillon, H.; Gosmini, C.; Pe´richon, J. Fr. Pat. Appl. 01/08880,
July 4, 2001. (b) Fillon, H.; Gosmini, C.; Pe´richon, J. J. Am. Chem. Soc.
2003, in press.
However, this process has been successfully extended to
aryl chlorides by changing several experimental conditions.12
1044
Org. Lett., Vol. 5, No. 7, 2003