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M. Shahidzadeh, M. Ghandi / Journal of Organometallic Chemistry 625 (2001) 108–111
clopentane reagent and studied its reaction with benzo-
quinone under the similar conditions (Scheme 4). The
lack of the formation of bisaddition product (5) indi-
cates that only one alkyl groups in cadmium reagent is
active in these reactions. This result is in contrast to the
observed activity of both group in alkylcadmium
reagent which previously has been reported [5]. The
availability of one group of these reagents can be
interpreted on the single electron transfer and the oper-
ation of a radical pathway mechanism in these reac-
tions. The free energy equation of the single electron
transfer reaction is given in Eq. (1):
reagents, the benzyl cadmium reagent leads to 1,4-addi-
tion product even at low temperature.
4. Experimental
4.1. General
1H- and 13C-NMR spectra of CDCl3 solutions were
recorded on an 80 MHz Bruker spectrometer. Chemical
shifts were expressed as ppm with respect to TMS.
Mass spectra were obtained with a GCMS-QP 1000EX,
Shimadzu. Reactions were conducted in oven-dried
glasswares under a nitrogen stream. Diethyl ether and
THF were distilled over Na–benzophenone before use.
The structure of products were identified by compari-
son the spectroscopic data with authentic samples
[3,4c,8].
DG° (kcal mol−1
)
−
=23.06[E°(D+ /D)−E°(A/A )]−DEcoul
(1)
−
where E°(D+ /D) and E°(A/A ) are the standard
reduction potentials of the electron donor and acceptor
redox couples, respectively, and DEcoul is the coulombic
interaction energy for the two singly charged radical
ions formed in the SET process [7]. We suggest that the
involvement of two alkyl groups seems to take place in
4.2. Preparation of organocadmium reagent
two consecutive steps. Since E°(A/A− ) for benzo-
Ethyl, propyl, butyl and benzylcadmium reagents
were prepared according to the standard procedure [9].
Cadmium cyclopentane was prepared as follows.
1,4-Dibromobutane (0.02 mol) dissolved in anhy-
drous THF (150 ml) was added to magnesium turnings
(0.05 mol). A small volume of dihalide solution was
added dropwise at a rate slow enough to avoid exces-
sive heating (40–50°C). After addition was completed,
the clear solution was allowed to stir 1 h at room
temperature (r.t). Dried CdCl2 (0.02 mol) was added
and the solution was refluxed for 2 h to give a negative
test for Grignard reagent.
quinone in both steps is constant, the reduction poten-
tial of the alkyl group, E°(D+ /D), should be the
effective term in Eq. (1). As soon as the organocad-
mium reagent is changed from RꢀCdꢀR to RꢀCdꢀX,
the E°(D+ /D) of the organocadmium reagent is in-
creased. This itself leads to a higher DG° for the second
single electron transfer, which means that a less efficient
reaction would take place for the transferring of the
second alkyl group.
3. Conclusions
From this investigation, it can be concluded that the
reaction of the primary alkylcadmium reagent with
p-benzoquinone proceeds through a SET mechanism.
The products and yields are different and depend on
the temperature, solvent and the nature of the alkyl
group. Reaction at low and high temperature leads to
1,2 and 1,4-addition products, respectively. The yield of
the 1,4-addition product is less than the 1,2-addition
product, since radical ion pairs diffusing out of the
solvent cage is enhanced at higher temperatures. The
formation of high percentage yield of hydroquinone at
high temperature and in the case of all alkyl groups
supports this proposal, as well. Diffusing out of the
solvent cage also depends on the viscosity and basicity
of the solvent. Comparing the yields of hydroquinone
in diethyl ether (DEE) and THF at high temperature
shows the increasing yield in DEE, which has, lower
basicity and viscosity. The nature of the primary alkyl
group is also an important factor in reaction mecha-
nism, since unlike the other primary alkylcadmium
4.3. Procedure for the addition of cadmiumcyclopentane
reagent to benzoquinone at low temperature
The organocadmium reagent solution (0.02 mol) was
cooled to −10°C and a solution of benzoquinone (0.01
mol) in THF was added through dropping funnel to
cadmium reagent. The reaction mixture was stirred for
1 h. After hydrolysis with saturated aqueous ammo-
nium chloride. The mixture was extracted with CH2Cl2
(2×50 ml). The organic phase washed with water (50
ml), and dried over Na2SO4. The solvent was removed
under reduced pressure and the crude product was
purified by PTLC (neutral alumina) eluting with
CHCl3–hexane (9:1). The spectral data of resultant
quinol, which was obtained as pale yellow oil (68%),
1
are as follows: H-NMR 6.8 (2H, d), 6.09 (1H, d), 3.99
(1H, s), 1.73 (2H, m), 1.28 (4H, m), 0.69 (3H, t).
13C-NMR 168.2, 125.5, 127.7, 68.6, 39.6, 25.6, 22.6,
13.6. IR (liquid film, cm−1) 3400, 2956, 1625, 662. MS
(EI, m/z) 166 (M+), 110 (100%), 81, 71, 55.