New approaches to syntheses of diethers and haloethers from cyclopentadiene

Article abstract of DOI:10.1023/A:1015803815051

Cyclopentadiene (CPD) is oxidized by copper(II) bromide and chloride in alcohol solutions to form dialkoxy- and haloethers and exhibits a higher reactivity than butadiene. Dialkoxy-, chloroalkoxy-, and dichlorocyclopentenes are formed from CPD and CuCl2 w

Full text of DOI:10.1023/A:1015803815051

616
Russian Chemical Bulletin, International Edition, Vol. 51, No. 4, pp. 616—624, April, 2002
New approaches to syntheses of diethers and haloethers
from cyclopentadiene
M. Ya. Skumov,a S. M. Brailovskii,b and O. N. Temkinb
ꢀ
аAsen Zlatarov University,
1 ul. Prof. Yakimova, 8010 Burgas, Bulgaria.
Eꢀmail: skumov@btu.bg
bM. V. Lomonosov Moscow Academy of Fine Chemical Technology,
86 prosp. Vernadskogo, 117571 Moscow, Russian Federation.
Fax: +7 (095) 434 8711. Eꢀmail: lbruk@cityline.ru
Cyclopentadiene (CPD) is oxidized by copper(II) bromide and chloride in alcohol soluꢀ
tions to form dialkoxyꢀ and haloethers and exhibits a higher reactivity than butadiene.
Dialkoxyꢀ, chloroalkoxyꢀ, and dichlorocyclopentenes are formed from CPD and CuCl2
without catalysts. The reaction selectivity (yield of ethers calculated per consumed CuBr2)
reaches 98%. The characteristic feature of the oxidation mechanism is the parallel formation
of all reaction products through a common intermediate (presumably, the bromonium catꢀ
ion) and bromineꢀcontaining carbocationic intermediates.
Key words: cyclopentadiene, copper(II) halides, oxidation, dialkoxycyclopentenes, mechaꢀ
nism, kinetics.
Cyclopentadiene (CPD) is a byꢀproduct in the pyꢀ
However, we can expect that they will be oxidants in the
rolysis of gasoline fractions in ethylene production and
exhibits a high reactivity. CPD can be used as a raw
material for the preparation of valuable products: synꢀ
thetic fragrants, medicines, chlorineꢀcontaining pestiꢀ
cides, special polymers, plasticizers, thermoꢀ and photoꢀ
stabilizers, etc.
case of CPD. Rather simple technology of oxidation
processes make them very promising for the production
of the CPDꢀbased oxygenꢀcontaining substances.
This work was aimed at studying the products of CPD
oxidative alkoxylation in the presence of metal complex
oxidants and catalysts and the kinetics of CPD oxidation.
One of the promising ways for CPD processing is the
preparation of oxygenꢀcontaining substances (viz.,
alcohols, esters, and carbonyl compounds) used in the
pharmaceutical1—24 and perfumery25,26 industries.
The fiveꢀmembered carbon cycle in the CPD derivaꢀ
tives is a convenient initial structure for the syntheꢀ
sis of cyclopentaneꢀderived naturally occurring subꢀ
stances.3,5,17,24 The operation sequence includes, as a
rule, the preparation of cyclopenteneꢀderived ethers and
esters followed by their partial hydrolysis to hydroxyethers
and ꢀesters.3—20,27 Enzymes are often used as catalysts at
the hydrolysis stage.3—5,8,12—20 Synthons thus obtained
possess a high optical purity necessary for the creation of
molecules with a specific biological activity.
The known methods for the synthesis of the oxygenꢀ
containing CPD derivatives are multistage and produce
the target products in low yields. The interaction of acyꢀ
clic conjugated dienes with solutions of CuBr2 in alcohols
and acids results in the formation of diethers and diesters
or bromoethers and bromoesters.28—30 The oxidation of
conjugated and nonconjugated cyclic dienes by copper(II)
halides in alcoholic solutions was not studied earlier.
Experimental
The concentration of copper(II) in reaction mixtures was
determined titrimetrically. GLC analyses were carried out on a
Sigma 2000 instrument (Perkin Elmer) with a flameꢀionization
detector using nitrogen as the carrier gas and capilalry columns
with lengths of 100 m (ОVꢀ101) and 50 m (Carbowaxꢀ20000).
The internal standard method (1,2,3ꢀtriethylꢀ and 1,2ꢀdichloꢀ
robenzene) was used for the quantitative analysis of products.
IR spectra were recorded using a URꢀ20 spectrometer in soluꢀ
tions of CCl4. The spectra of all analyzed compounds contain
intense bands of C—O stretching vibration in a region of
1060—1150 cm–1. Н NMR spectra were recorded on a Bruker
1
АМꢀ250 instrument (250 MHz) in CDCl3 solutions using Me4Si
as the internal standard. The 1Н NMR spectra of the syntheꢀ
sized compounds are presented in Table 1. Cyclopentadiene
(content of the main substance is 95—97%) was prepared by
the monomerization of dicyclopentadiene 13 (180—200 °С)
prior to syntheses.
CPD oxidation with alcoholic solutions of copper(II) halides
(general procedure). A solution of CPD in the corresponding
alcohol was added with stirring to a solution of copper(II) haꢀ
lide in the same alcohol (in particular cases, together with a
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 572—579, April, 2002.
1066ꢀ5285/02/5104ꢀ616 $27.00 © 2002 Plenum Publishing Corporation

Syntheses of ethers and haloethers
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 4, April, 2002
617
catalyst) heated to a specified temperature. Alcohol excess with
respect to CPD was 23—35 vol.%. The moment of CPD addiꢀ
tion was taken as the beginning of reaction. After the cessation
of reaction, the mixture was diluted with equal volume of water
and triply extracted with equal volume of the solvent (usually
cyclohexane, after double extraction, the degree of extraction
of the reaction products was 93%), at the solution to extragent
ratio equal to 10 : 1. The copper(I) halide that precipitated was
filtered off, and the extract was washed with water and dried
with MgSO4. Then the extragent was distilled off at an atmoꢀ
spheric pressure, and the residue was distilled in vacuo. In some
cases, a DMF —alcohol mixture (1 : 1) was used as a solvent. A
suspension of copper salts in alcohols was also used.
Table 1. Chemical shifts in the 1Н NMR spectra of ethers 2, 3,
and 5—9 obtained by CPD oxidation with copper(II) bromide
in MeOH and EtOH (Me4Si, CDCl3)
The catalytic oxidation of CPD with dioxygen in the presꢀ
ence of copper(II) halides was conducted in a flask with a stirrer
equipped with a bubbler for O2 supply. The oxidation rate of
CuBr was preliminarily measured at P0 = 96 kPa, [HBr]0
=
O
2
0.6 mol L–1, [CuBr]0 = 0.4 mol L–1, and 25 °C in a shaken
"catalytic necked flask." Under these conditions, the oxidation
rate of CuBr was proportional to PO2 and [CuBr]. Experiments
on the oxidation of CPD with copper bromide and regeneration
of the oxidant with dioxygen were conducted in a reactor with a
volume of a solution of 250 mL at [CuBr2]0 = 1.18 mol L–1 and
[HBr]0 = 1.1 mol L–1. To the reaction mixture CPD (6.1 mL,
the amount corresponding to the 50% conversion of the oxiꢀ
dant) was added at 35 °C. After 1.5 h, the reaction ceased when
a 48% degree of conversion with respect to CuBr2 was reached.
Then air was passed through the reactor with a flow rate of
3 L h–1. The concentration of CuBr2, close to the initial conꢀ
centration, was reached in 1.5 h at the same temperature. Six
oxidation—regeneration cycles were carried out.
Comꢀ
pound
δ
Н(1) Н(2) Н(3) Н(4) Н(4´) Н(5) Н(5´)
2
3
5
6
7
8
9
6.10 6.10 4.55 2.00 2.00 4.55
5.95 5.95 4.15 2.50 1.50
5.87 5.78 4.41 3.96
5.77 5.88 4.41 3.98
5.80 5.91 4.70 4.21
—
4.15
2.25 2.76
2.49 2.23
2.68 3.16
—
—
—
—
6.09 6.09 4.87 2.04 2.04 4.87
6.02 6.02 4.32 2.62 1.58
—
4.32
—
Results and Discussion
CPD oxidation in alcoholic solutions of CuBr2. By
Table 2. Properties of ethers 2, 3, and 5—9 obtained by the
analogy to butaꢀ1,3ꢀdiene, the formation of diethers
oxidation of CPD with copper(II) bromide in MeOH and EtOH
[CuBr2]/mol L–1
2.0
Comꢀ
pound
B.p.a/°С
nD
Comꢀ
pound
B.p.a/°С
nD
2
3
5
6
45.2
48.7
55.8
59.3
1.4442b
1.4508b
1.4405c
1.4405c
7
8
9
62.8
67.8
69.9
1.4510c
1.4440c
1.4452c
1.5
1
1.0
2
a The b.p. values were determined at 11 hPa.
b The nD index was determined at 22 °C;
c nD was determined at 20 °C.
3
0.5
4
0
5
10
15
t/min
(С5H6(OR)2), bromoethers (C6H5(Br)OR), and diꢀ
bromides (C5H6Br2) is possible in the reactions of CPD
with alcohols.
Fig. 1. Kinetic curves for the reaction of CuBr2 with CPD in
methanol.
[CuBr2]0 [CPD]0 T/°C
It turned out that copper(II) bromide in methanol in
the reaction with CPD is reduced already at ∼20 °C
(Fig. 1). Four organic products of CPD oxidation with
95—98% purity were isolated on a preparative chromatoꢀ
mol L–1
1
2
3
4
2.010
1.141
0.519
0.512
0.500
0.571
0.200
0.200
30
37
30
50
1
graph. According to the Н NMR spectra (Table 1) and
characteristics of the CPD oxidation products (Table 2),

618
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 4, April, 2002
Skumov et al.
Table 3. Conditions for synthesis of methyl ethers of cycloꢀ
oxidation in the CuBr2—MeOH system can be described
by the following equations:
pentene, yields of ethers, and conversion of CuBr2 (XCuBr
)
2
Entry [CuBr2]0 [CPD]0 T/°C
t
Yield of
XCuBr
2
/min ethers* (%)
mol L–1
1
2
3
4
5
6
0.443
0.443
0.519
1.436
1.337
0.512
0.405
0.405
0.200
0.358
0.632
0.211
30
30
30
30
37
50
20
60
20
20
70
20
87
90
94
92
98
95
25
28
26
56
95
65
(1)
* Calculated per reacted CuII.
used the reaction time of 70 min at 50 °С, which proꢀ
vided the 95% copper(II) conversion.
(2)
The temperature plots of the degree of conversion of
CuBr2 were studied for the reactions in PrOH, BuOH,
and methyl cellosolve (Fig. 2).
An increase in the concentration of Br– when LiBr is
added to the reaction mixture somewhat decreases the
yield of cyclopentene diethers. For example, in a methaꢀ
nol solution containing 5 mol L–1 LiBr, the yields of the
(3)
target compounds decrease by 10—15% ([CuBr2]0 =
0.221 mol L–1, 30 °С) due to the selectivity decrease.
Kinetics of CPD oxidation in alcohol solutions of CuBr2.
Typical plots for the concentrations of diethers and
bromoethers formed by CPD oxidation in methanol and
time changes in the copper(II) concentration at different
initial concentrations of CPD and CuBr2 are presented
in Fig. 3. The error of measuring the concentrations of
the products and reactants found by repeated experiꢀ
ments in the middle of the studied concentration region
for CuBr2 is 5—7%.
Bromomethoxycyclopentene is formed along with
dimethoxycyclopentenes
(4)
An important feature of CPD oxidation in alcoholic
solutions of CuBr2 is the absence of the Sꢀlike character
of the curves for diether accumulation and a maximum
Cyclic ethers are the main (87—98%) products of
CPD oxidation with copper(II) bromide in methanol.
The experimental conditions and results are presented in
Table 3.
XCuBr2 (%)
3
1
The initial CuBr2 to CPD molar ratio correspond to
stoichiometry of reactions (1)—(3). The complete conꢀ
90
2
version of the oxidant XCuBr in a methanolic solution is
2
80
70
reached at 37 °С and a reaction time of 87 min. The
degree of conversion of CuBr2 remained almost unꢀ
changed when CaCO3 was added to neutralize HBr.
The oxidation of CPD in ethanol was conducted usꢀ
ing a general procedure (see Experimental). Five fracꢀ
tions with a content of the main substance of 95—99%
(see Tables 1 and 2, compounds 5—9) were obtained
using the preparative chromatography. The degree of
copper(II) conversion is 90% already at 20 °С and inꢀ
creases to 97% at 80 °С. In further preparation runs, we
40
60
80
100
120
T/°С
Fig. 2. Temperature effect on the degree of CuBr2 conversion
(X ) (t = 180 min) in PrOH (1), BuOH (2), and methyl
cellosolve (3).

Syntheses of ethers and haloethers
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 4, April, 2002
619
[CuBr2]/mol L–1
0.5
[P]/mol L–1
0.025
Scheme 1
1
5
0.4
0.3
0.2
0.1
0.020
0.015
0.010
0.005
2
3
4
0
5
10
15
t/min
Fig. 3. Concentrations of CuBr2 ([CuBr2]) and the products
of CPD oxidation ([P]) as functions of time ([CuBr2]0
0.505 mol L–1, [CPD]0 = 0.250 mol L–1, 30 °С): 1, CuBr2;
2, transꢀ3,4ꢀdimethoxyꢀ1ꢀcyclopentene (1); 3, transꢀ3,5ꢀdiꢀ
methoxyꢀ1ꢀcyclopentene (2); 4, cisꢀ3,5ꢀdimethoxyꢀ1ꢀcycloꢀ
pentene (3); and 5, 3ꢀmethoxyꢀ4ꢀbromoꢀ1ꢀcyclopentene (4).
=
in the curve for bromoether accumulation (see Fig. 3).
This indicates a low concentration of the products preꢀ
ceding diether formation and a low rate of further transꢀ
formations of the bromoether. The latter fact can be
explained under the assumption that the bromine atom
in the bromoether is not allylic and the observed
bromoether has structure 4 or 5.
The ratios of the concentrations of isomeric diethers
and bromoether remain unchanged during the reaction
(beginning from the third minute of the reaction). This
can be treated as a proof for the formation of all detected
ethers through one common intermediate (Scheme 1).
Good convergence of the material balance of the
process (92—98%) suggests that in the whole studied
temperature and concentration interval the following
equality is fulfilled:
concentrations [Br]– and [CuBr], the reaction rate was
considered as a function of the overall concentration of
CuBr2. The following variants were analyzed.
1. Power equation with a constant order with respect
to [CuBr2]
r = –d[CuBr2]/2dt = k[CPD][CuBr2]n.
(7)
2. Power equation with a variable order with respect
to [CuBr2] depending on [CuBr2],
–0.5d[CuBr2]/dt =
r = –d[CuBr2]/2dt = k[CPD][CuBr2]n,
= d[1]/dt + d[2]/dt + d[3]/dt + d[4]/dt,
(5)
n = f ([CuBr2]).
(8)
where [1]—[4] are the concentrations of compounds
shown in Fig. 3 and in Eqs. (1)—(4). The constancy of
the ratios of product concentrations when the initial conꢀ
centrations of the reactants and time are varied
3. Fractionꢀrational equation with constant orders n1
and n2 with respect to [CuBr2]
r = –d[CuBr2]/2dt =
n
= k1[CPD][CuBr2]n /(1 + k2[CuBr2] ).
(9)
1
2
[1] : [2] : [3] : [4] = 0.38 : 0.26 : 0.33 : 1
(6)
allows us to restrict the kinetic model by the equation
describing the change in the overall rate of copper(II)
bromide consumption during CPD oxidation because
the rates of formation of all oxidation products are proꢀ
portional to the overall reaction rate.
Taking into account a complicated dependence of
the concentration of free CuBr2 on the overall concenꢀ
tration of copper bromide in solutions with the changing
The reaction order with respect to CPD was taken
equal to 1 by analogy to the data obtained for the kinetꢀ
ics of butadiene oxidation in solutions of copper(II)
halides.30
The constants in the kinetic equation were estimated
using the nonlinear leastꢀsquares method by the minimiꢀ
zation of the adequacy criterion Ф,31 which is the sum of
squares of relative deviations of the calculated CuBr2

620
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 4, April, 2002
Skumov et al.
Table 4. Results of calculation of the kinetic parameters for CPD oxidation by copper(II) bromide in methanol at 30 °С in
different kinetic models
a
b
c
Number
of model
Form of equation
Ф
(%)2
∆
k1
k2
k3
(%)
r = k1[CPD][CuBr2]k
1833
1605
1580
5.6
5.2
5.2
0.29 ( 0.03)
0.29 ( 0.03)
0.47 ( 0.05)
3.4 ( 0.3)
3.8 ( 0.4)
0.59 ( 0.15)
—
–0.84 ( 0.42)
—
2
1
2
3
r = k1[CPD][CuBr2]k +k [CuBr ]
2
3
2
r = k1[CPD][CuBr2]4/(1 + k2[CuBr2]3)
a The dimensionality of k1 in model 1 is L3.4 mol–3.4 min–1, and that in model 3 is L4 mol–4 min–1; the dimensionality of
k1 in model 2 depends on the concentration of CuBr2 in the exponent.
b In models 1 and 2, k2 is dimensionless; in model 3, the dimensionality is L3 mol–3
.
c The dimensionality of k3 is L mol–1
.
concentrations (С calc) from those measured experimenꢀ
tally (С exp
Based on the values of the k2 constants in Eqs. (1)
and (2) and the k2 constant in Eq. (3) (see Table 4), we
can conclude that the reaction order with respect to
[CuBr2] ranges from 3 to 4. The high orders with respect
to [CuBr2] (exceeding the stoichiometric coefficient with
respect to copper(II) bromide in Eqs. (1)—(3)) are assoꢀ
ciated with a possible participation of the (CuBr2)n polyꢀ
nuclear neutral associates or the CumBrn(n–m–2)– anionic
complexes in the reaction and with the influence of CuBr
on the concentration of free Br–, which slows down the
reaction (see above). Note that according to the thermoꢀ
dynamic concepts, at least two CuX2 molecules must
participate in the limiting step of the twoꢀelectron subꢀ
strate oxidation by copper(II) chloride or bromide.32,33
Two CuBr2 molecules can be involved in the formation
of the cyclopentenebromonium cation and bromineꢀconꢀ
taining carbocations (see Scheme 1). The formation
of the polynuclear CuII chloride and bromide comꢀ
plexes has been repeatedly described in the literature.
We present only some examples: (MeCN)2Cu3Cl6,34
)
(10)
where i is the run number, and j is the number of the
measurement time.
The rootꢀmeanꢀsquare deviation (∆) is the following:
∆ = (Ф/n)0.5
,
(11)
where n is the number of measurements.
The results of estimation of the constants are preꢀ
sented in Table 4. Analysis of the found values of the
constants and orders showed that all three kinetic modꢀ
els are adequate to the experimental data. The distincꢀ
tions in accuracy of description between the models are
much lower than the experimental error. Therefore, the
discrimination of the models using objective criteria is
impossible.
The calculated curves and experimental values for
the time plots of the CuBr2 concentration obtained usꢀ
ing model 1 are presented in Fig. 4.
35
36
(MeCH2CH2OH)2Cu5Cl10
,
(Me4N)2Cu4Cl10,
(Et4N)4Cu4Cl12,37 and (Me3NH)2Cu4Br10.38
For each model (Table 5), we determined the rate
constants for the formation of all ethers taking into acꢀ
count that correlation (6) between the concentrations of
the ethers depends only on the temperature and is valid
in the whole concentration interval studied.
[CuBr2]/mol L–1
1
2
3
4
5
2.0
1.5
1.0
0.5
Table 5. Rate constants for CuBr2 consumption
(k1) and formation of ethers 1—4 (k1(1)—k1(4))
at 30 °С
Number
of model
k1
k1(1) k1(2) k1(3) k1(4)
1
2
3
0.29 0.056 0.038 0.049 0.15
0.29 0.056 0.038 0.049 0.15
0.47 0.091 0.062 0.079 0.24
5
10
15
t/min
Fig. 4. Concentrations of CuBr2 ([CuBr2]) as functions of time
calculated using model 1. [CPD] = 0.5 mol L–1, 30 °C.
Points are experiment, and solid lines are calculation;
[CuBr2]0/mol L–1: 1, 2.0; 2, 1.5; 3, 1.0; 4, 0.75; and 5, 0.5.
Note. For dimensionalities of the k1 constants,
see Table 4.

Syntheses of ethers and haloethers
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 4, April, 2002
621
Table 6. Rate constants for CPD oxidation by
copper(II) bromide (k1) in a DMF solution in
the presence of various alcohols at 30 °С
Based on the results of analysis of the reaction prodꢀ
ucts of CPD oxidation by copper(II) chloride in methaꢀ
nol, we can describe oxidation by the following reacꢀ
tions:
Alcohol
k1
Alcohol
k1
MeOH
EtOH
PrOH
PriOH
0.32
0.34
0.32
0.35
BuOH
BuiOH
BusOH
ButOH
0.37
0.32
0.33
0.36
(12)
(13)
(14)
(15)
Note. The k1 constants were calculated using
model 1 (L3.4 mol–3.4 min–1). Volume raꢀ
tio alcohol : DMF = 1 : 1; molar ratio
[CuBr2]0 : [CPD]0 = 2.
The influence of the alcohol nature on the formation
of cyclopentene ethers was quantitatively estimated by
the comparison of the rate constants of CPD oxidation
by copper(II) bromide (at the stoichiometric molar ratio)
in a DMF—alcohol solution (2 : 1 v/v). The experimenꢀ
tal data were processed using model 1 (see Table 4)
assuming that the presence of the solvent (DMF) and
variation of the alcohol nature have no effect on the
form of the kinetic equation. The found values of the
constants (Table 6) confirmed these assumptions in agreeꢀ
ment with the scheme of product formation through comꢀ
mon carbocationic intermediates.
CPD oxidation in a methanolic solution of CuCl2. In
alcoholic solutions of CuCl2, the oxidation of conjuꢀ
gated dienes in the presence of diiodine or iodineꢀconꢀ
taining compounds, which act as catalysts,39,40 produces
mainly diethers. In this connection, it was of interest to
study the interaction of CPD with alcoholic solutions of
CuCl2 and the influence of additives of KI or I2 on
oxidation. It was found that, unlike acyclic dienes, CPD
is oxidized by CuCl2 even when a catalyst is absent.
Changes in the copper(II) concentration depending on
the conditions are shown in Fig. 5.
(16)
[CuCl2]/mol L–1
The reaction of CPD with copper(II) chloride always
affords dicyclopentadiene (13) and the products of its
chlorination along with the products of CPD oxidation.14
2.0
1.8
1.6
(17)
1
1.4
1.2
2
1.0
3
4
0.8
20
40
60
t/min
(18)
Fig. 5. Curves for CuCl2 consumption in MeOH (T = 24—26 °С,
molar ratio CuCl2 : CPD = 2 : 1), [CuCl2]0/mol L–1: 1, 2.109;
2, 1.360; 3, 1.067; and 4, 1.027.

622
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 4, April, 2002
Skumov et al.
Table 7. Results of studying CPD oxidation by copper(II) chloride in methanol
Entry [CuCl2]0 [KI]0
mol L–1
T
t
Conversion
XCuCl2 (%)
Overall
Yield of b (%))
/°C /min
yielda (%)
ethers
chlorides oxidation
products
I
II
1
2
3
4
5
6
7
8
1.027
1.027
2.109
2.109
3.313
1.067
1.067
0.982
1.098
1.075
1.059
0
0
0
0
25
25
24
24
26
23
23
26
50
23
25
30
60
30
60
60
60
140
30
60
60
60
19
22
33
34
49
24
26
20
36
23
22
22
24
31
39
43
26
30
—
14
15
22
24
27
11
15
20
14
13
21
9
9
81
83
61
60
39
35
14
76
58
39
83
90
92
82
93
84
97
74
—
21
33
45
62
60
—
32
59
14
0
0.0213
0.0213
0.0107
0.0211
0.0108
0.0210
9
10 c
11 d
36
27
27
90
98
97
a The yield calculated per taken CPD using Eqs. (19) and (20) for the oxidation products (I) and dimerization (II),
respectively.
b The yield calculated per change in the concentration of CuCl2.
c The experiment was carried out in the presence of I2.
d The experiment was carried out in the presence of KBr.
The main quantitative findings for the reaction under
different conditions are presented in Table 7.
netic regularities of oxidation in the CuCl2—MeOH and
CuCl2—KI—MeOH systems are presented in Figs. 6
and 7. The characteristic feature of the curves in these
figures (similarly to the reactions with CuBr2) is the abꢀ
sence of the Sꢀlike shape, which is pronounced for butaꢀ
diene oxidation under similar conditions.41 It is most
likely that in the chloride system chloroether 11 is not a
precursor of diethers and does not contain the chlorine
atom in the allyl position. Chloroethers with such a chloꢀ
rine atom are much more reactive and rapidly transform
into diethers (Scheme 2).
The yield of ethers calculated per converted copper(II)
is at most 62% (see Table 7, entry 6). Unlike butadiene
oxidation,41 the KI additives to a solution of CuCl2 do
not virtually increase the oxidation rate but substantially
change the ratio of the products: the yield of diethers
increases and the yield of dichlorides decreases. The
chloroether : dichloride ratio decreases simultaneously.
In the presence of KI, one more compound, which
was preliminarily identified as iodoether, appeared
in the products. The KBr additive to a solution of
CuCl2 has no effect on the products composition and
copper(II) conversion (see Table 7, entry 11). The kiꢀ
Synthesis of cyclopentene diethers using CPD oxidaꢀ
tion by dioxygen in the presence of CuBr2 as a catalyst.
[P]/mol L–1
[P]/mol L–1
0.04
1
0.04
1
0.03
0.03
0.02
2
2
0.02
3
0.01
0
4
5
0.01
3
6
4
5
5
10
15
20
25 t/min
0
5
10
15
20
25 t/min
Fig. 7. Change in the concentration of the products ([P]) of
Fig. 6. Change in the concentration of the products ([P]) of
CPD oxidation by copper(II) chloride in the presence of KI
CPD oxidation by copper(II) chloride ([CuCl2]0 = 1.027 mol L–1
,
([CuCl2]0 = 1.067 mol L–1, [CPD]0 = 0.505 mol L–1, [KI]0
=
[CPD]0 = 0.493 mol L–1, 25 °С): 1, 3,5ꢀdichlorocyclopentene
(10); 2, 3, 3,4ꢀdichlorocyclopentene (10); 4, 13; and 5, 11.
0.0213 mol L–1, T = 23 °С): 1, 12; 2, 10; 3, 14; 4, curves for 2
and 3 coincide; 5, 13; and 6, 11.

Syntheses of ethers and haloethers
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 4, April, 2002
623
Scheme 2
It is inconvenient to isolate diethers by distillation
from the reaction solution because of a great difference
in the boiling temperatures of diethers and methanol. In
addition, the complete removal of methanol would be
accompanied by the formation of a precipitate containꢀ
ing inorganic salts and the products of CPD oxidation.
Therefore, we developed the variant of isolation of the
target products by extraction with saturated hydrocarꢀ
bons. When this method was used to separate dimethoxyꢀ
cyclopentenes, the concentration of the extragent (cycloꢀ
hexane) in the catalytic solution containing 1.5 mol L–1
CuBr2 in methanol was at most 2.7%. The practice
showed that this amount of cyclohexane has no effect on
the solubility of CuBr2.
Thus, CPD oxidation by copper(II) chloride and broꢀ
mide in alcoholic solutions, similarly to acyclic conjuꢀ
gated dienes, produces isomeric dialkoxyethers and haloꢀ
gen ethers containing no halogen in the allyl position.
Cyclopentadiene exhibits a higher reactivity in these reꢀ
actions than butadiene and, in the case of CuCl2, does
not require catalysts. The selectivity of ether formation
reaches 98%. In the case of CuCl2, oxidation begins only
To check the possibility of the formation of cyclopentene
diethers according to the stoichiometric equation
at high concentrations of the oxidant, at least 2.5 mol L–1
.
The oxidation is characterized by the parallel formation
of all products through the common intermediate, preꢀ
sumably the bromonium cation, and bromineꢀcontainꢀ
ing carbocationic intermediates.
(19)
and the estimations of stability of the products under the
conditions of copper(I) oxidation, we carried out experiꢀ
ments in which the stages of formation of diethers acꢀ
cording to the equation
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the same. In each cycle (CPD oxidation for 1.5 h and
CuBr2 regeneration for 1.5 h), 50% conversion of CuBr2
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Received May 29, 2001;
in revised form October 24, 2001