1674
P. Gomes et al.
LETTER
e-, CoCl2 0.1 eq
MeCN/pyridine (v/v=9/1)
This process has been extended to various aryl halides
(bromides and chlorides) and several activated olefins
(Table 1).10Aryl halides substituted by an electron-with-
drawing group have been especially studied owing to their
higher reactivity towards electrogenerated Co(I) spe-
cies.11 The reaction has been successfully extended to aryl
chlorides which were found unreactive in the direct elec-
trochemical conjugate addition catalysed by cobalt salts.
Furthermore, the reaction works with a variety of activat-
ed olefins and not only with methylvinylketone. Indeed,
excellent yields are obtained with acrylonitrile (entries 1,
5, 8 and 12) which seemed to polymerise in the one-step
coupling reaction.
ZnX
X
ZnBr2 0.3 eq
Zn anode
FG
FG
RT
Scheme 2
anode and a stainless steel grid as the cathode. Commer-
cial solvents were used without purification. The presence
of a supporting electrolyte was not necessary, since the
ionic conductivity of the medium is ensured by the small
amount of zinc bromide added. Electrolyses were con-
ducted at room temperature, at a constant current intensity Once more, methylvinylketone gives good results (entries
of 0.2 A (0.01 A/cm2) until a charge of 2.1 F per mol of 2, 4, 6, 9 and 13) with all arylzinc compounds studied.
ArX had passed to ensure the complete conversion of the With a substituent such as a methyl on the double bond of
aryl halide. 10 mmol of ArX (0.2 M), 1 mmol of CoCl2 the olefin (entries 11 and 16), conjugate addition products
(0.02 M) and 3 mmol (0.06 M) of ZnBr2 were introduced are isolated in satisfactory yields. These couplings can be
in a solution of acetonitrile and pyridine (v/v = 45/5 mL). extended to the less reactive esters in good yields with
Yields of organozinc compounds obtained were estimated electron-withdrawing group on the arylzinc compounds
as follows: samples of the electrolysis solutions were ex- (entries 7, 10, 14, 15, 17). However, with less reactive
posed to iodine crystals, then to sodium thiosulfate and arylzinc species such as PhZnBr, the yield decreases (en-
extracted with diethyl ether. The amount of the resulting try 3). Nevertheless, there are two olefins which are not
aryl iodide was monitored by GC using an internal refer- suitable for this process at the moment, 2-cyclohexenone
ence. These yields are good to excellent with both aryl and dimethyl maleate. Studies are still in progress to ex-
chlorides and bromides (60–90%/ArX).
tend the process to these alkenes.
In conclusion, we have shown that CoBr2(bpy)2 is very ef-
ficient to perform conjugate addition of electrochemically
prepared arylzinc compounds to several olefins.
2
Coupling Between Organozinc Species and
Activated Olefins
References
Therefore, after completion of the formation of organo-
zinc species, the electrolysis solution was heated at 50 °C.
A mixture of 0.5 equivalents of CoBr2(2,2 -bipyridine)2
and 1.5 equivalents of olefin per equivalent of GC esti-
mated arylzinc reagent were introduced into the medium
(Scheme 3).
(1) Perlmutter, P. Conjugate Addition Reactions in Organic
Synthesis; Pergamon Press: Oxford, 1992.
(2) Gomes, P.; Gosmini, C.; Nédélec, J.-Y.; Périchon, J.
Tetrahedron Lett. 2000, 41, 3385.
(3) (a) Normant, J.-F. Synthesis 1972, 63. (b) Cahiez, G.;
Marquais, S. Pure Appl. Chem. 1996, 68, 53. (c) Klement,
I.; Rottländer, M.; Tucker, C. E.; Majid, T. N.; Knochel, P.;
Venegas, P.; Cahiez, G. Tetrahedron 1996, 52, 7201.
(4) Sibille, S.; Ratovelomanana, V.; Périchon, J. J. Chem. Soc.,
Chem. Commun. 1992, 283.
(5) (a) Gosmini, C.; Rollin, Y.; Périchon, J. Patent application,
99/08480, 1999. (b) Gosmini, C.; Rollin, Y.; Nédélec, J.-Y.;
Périchon, J. J. Org. Chem. 2000, 65, 6024.
W
1.5 eq
1)
ZnX
CH3CN/pyridine (v/v=9/1)
W
CoBr2(bpy)2 0.5 eq
FG
FG
50°C
2) H+
Scheme 3 W = COMe, CN, COOEt
(6) Fillon, H.; Gosmini, C.; Nédélec, J.-Y.; Périchon, J.
Tetrahedron Lett. 2001, 42, 3843.
(7) Avedissian, H.; Bérillon, L.; Cahiez, G.; Knochel, P.
Tetrahedron Lett. 1998, 39, 6163.
(8) Reddy, C. K.; Knochel, P. Angew. Chem., Int. Ed. Engl.
1996, 35, 1700.
The amount of cobalt-complex in this step influences the
reaction rate but not the yields. Indeed, with only 0.3
equivalents of cobalt catalyst, the arylzinc compound is
consumed after 24 hours instead of 15 hours with 0.5
equivalents. With a stoichiometric amount of CoBr2(bpy)2
complex, the reaction is completed in about 10 hours.
(9) Chaussard, J.; Folest, J.-C.; Nédélec, J.-Y.; Sibille, S.;
Troupel, M.; Périchon, J. Synthesis 1990, 5, 369.
(10) Experimental procedure: The electrochemical cell fitted
with a zinc rod as the anode and a stainless steel grid as
cathode was flushed with argon. Acetonitrile (45 mL) and
pyridine (5 mL) were added using a syringe. To the solution
were added anhyd zinc chloride (0.7 g, 0.3 equiv), cobalt
chloride (0.2 g, 0.13 equiv) and tetrabutylammonium tetra-
fluoroborate in order to increase the conductivity of the
medium. The functionalised phenyl halide (10 mmol, 1
equiv) and dodecane (0.2 mL) as internal standard were
An excess of the activated olefin is required to optimise
the yield of conjugate addition. In addition to the coupling
product, the organozinc reagent reacts to give hydrogena-
tion product (ArH). At room temperature, the reaction still
occurs but a higher amount of both ArH and dimer (ArAr)
is formed.
Synlett 2002, No. 10, 1673–1676 ISSN 0936-5214 © Thieme Stuttgart · New York