Chemically induced dynamic nuclear polarization and the mechanism of the reaction of Et3Al with CCl4 in the presence of transition metal complexes

Article abstract of DOI:10.1134/S0023158410040063

Integral effects of chemically induced ¹H and ¹³C nuclear polarization are reported for the reaction of Et₃Al with CCl₄ catalyzed by Pd(acac)₂, Cu(acac)₂, and Cp₂TiCl₂; for the reaction of (n-C₈H ₁₇)₃Al with CCl₄ in the absence of a catalyst and in the presence of Ni(acac)₂; and for the reaction of the cyclic organoaluminum compound 1-ethyl-3-butylaluminacyclopentane with CCl₄ in the presence of Pd(acac)₂. A scheme of the catalytic cycle of this reaction predicting the formation of both radical and nonradical products is derived from the observed chemically induced dynamic nuclear polarization (CIDNP) effects and from data on the products of the reaction between Et ₃Al and CCl₄ in the presence of Pd(acac)₂. According to the results of qualitative analysis of the CIDNP effects, the reactions of the trialkylalanes and the cyclic organoaluminum compound with CCl₄ in the presence of various metal complexes proceeded via similar mechanisms.

Full text of DOI:10.1134/S0023158410040063

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 4, pp. 497–501. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © R.A. Sadykov, G.D. Safina, I.Kh. Teregulov, E.A. Paramonov, 2010, published in Kinetika i Kataliz, 2010, Vol. 51, No. 4, pp. 522–527.
Chemically Induced Dynamic Nuclear Polarization
and the Mechanism of the Reaction of Et3Al with CCl4 in the Presence
of Transition Metal Complexes
R. A. Sadykova, G. D. Safinab, I. Kh. Teregulovc, and E. A. Paramonova
a Institute of Petrochemistry and Catalysis, Ufa, 450075 Bashkortostan, Russia
b Bashkir State Pedagogical University, Ufa, 450000 Bashkortostan, Russia
c Institute of Organic Chemistry, Ufa Scientific Center, Russian Academy of Sciences, Ufa, 450054 Bashkortostan, Russia
eꢀmail: ras6sad@gmail.com
Received June 29, 2009
Abstract—Integral effects of chemically induced 1H and 13C nuclear polarization are reported for the reacꢀ
tion of Et3Al with CCl4 catalyzed by Pd(acac)2, Cu(acac)2, and Cp2TiCl2; for the reaction of (nꢀC8H17)3Al
with CCl4 in the absence of a catalyst and in the presence of Ni(acac)2; and for the reaction of the cyclic orgaꢀ
noaluminum compound 1ꢀethylꢀ3ꢀbutylaluminacyclopentane with CCl4 in the presence of Pd(acac)2. A
scheme of the catalytic cycle of this reaction predicting the formation of both radical and nonradical products
is derived from the observed chemically induced dynamic nuclear polarization (CIDNP) effects and from
data on the products of the reaction between Et3Al and CCl4 in the presence of Pd(acac)2. According to the
results of qualitative analysis of the CIDNP effects, the reactions of the trialkylalanes and the cyclic orgaꢀ
noaluminum compound with CCl4 in the presence of various metal complexes proceeded via similar mechꢀ
anisms.
DOI: 10.1134/S0023158410040063
Organoaluminum compounds find wide use in scheme of radical formation involving transition metal
chemistry. In particular, they are employed as cocataꢀ complexes. Investigation of the second stage of the
lysts in various industrially important processes. Comꢀ reaction between the organoaluminum compounds
plete reaction mechanisms with allowance made for and halomethanes in the presence of transition metal
the possible participation of freeꢀradical intermediates complexes and salts showed that these reaction sysꢀ
should be known for the purposeful formation of these tems activate inert C–C and C–H bonds in saturated
systems. Conventional methods are of little use in the hydrocarbons under mild conditions. Isomerization
elucidation of freeꢀradical routes in reactions involvꢀ and dimerization reactions and skeletal transformaꢀ
ing organoaluminum compounds. Therefore, it seems tions of the starting hydrocarbons with the cleavage of
interesting to study chemically induced dynamic these bonds occur under these conditions [5].
We earlier considered the 1H CIDNP effects in the
reaction between Et3Al and CCl4 catalyzed by
Ni(acac)2 [3]. In the present work, the initial stage of
this reaction in the presence of Pd(acac)2 was studied
by the 1H and 13C CIDNP methods and the influence
of several other transition metal complexes on the
CIDNP effects was considered.
nuclear polarization (CIDNP) effects in homogeꢀ
neous catalytic reactions of organoaluminum comꢀ
pounds with halomethanes in the presence of transiꢀ
tion metal complexes.
Historically, attention to reactions of organoalumiꢀ
num compounds with halomethanes in the presence of
transition metal complexes was due to the ability of
organoaluminum compounds to initiate the freeꢀradiꢀ
cal polymerization of vinyl monomers at the initial
stage [1]. The initiation ceases at the second stage,
which occurs with an induction period [2]. Later it was
found by the 1H CIDNP method that the initial stage
of the reaction of Et3Al with CCl4 catalyzed by
Ni(acac)2 is accompanied by the formation of ethyl
EXPERIMENTAL
A 1 M solution of commercial 98% Et3Al in cycloꢀ
hexaneꢀd12 was used. The starting CCl4 was predried
over P2O5 and distilled.
To observe CIDNP, a 1 M solution of Et3Al in cycloꢀ
and trichloromethyl radicals, whose diffusion pairs hexaneꢀd12 containing 2–4 mg of Pd(acac)2 was placed
show integral polarization effects [3]. A comparison of in a 5ꢀmm NMR tube in an argon atmosphere. Before
the CIDNP effects in this catalytic reaction and in the this, the NMR spectrum of the starting solution was
thermal reaction of Et3Al with CCl4 in the absence of recorded. Next, a certain amount of a CCl4 solution was
a catalyst [4] made it possible to suggest a probable syringed at room temperature into the tube, which was
497

498
SADYKOV et al.
Chemical shifts and signs of polarization of the products of the reaction between (C2H5)3Al (1 M, 0.4 ml) and CCl4 (1.0 : 1.5)
in the presence of Pd(acac)2 (~2 mg) in C6D12 at room temperature
1Н NMR
CIDNP sign**
13С NMR
CIDNP sign**
Product
yields*, %
Products
Group
δ
,
ppm
δ
,
ppm
(CH3CH2)3Al
CH2
CH3
0.37(q)
1.09(t)
–
–
–
–
–
–
–
E
–
A
0.71
8.78
–
–
–
–
–
–
–
A
CCl4
(CH3CH2)2AlCl
96.57
7.80
2.92
CH2
CH3
1.10(t)
0.25(q)
0.84(s)
5.32(s)
7.18(s)
CH3–CH3
CH2=CH2
CHCl3
48
52
7
6.96
122.85
77.45
101.52
49.52
106.1
CH3CH2СCl3
8
СCl3
CH2
СCl3
A
2.67(q)
E
E
–
C2Cl6
СCl2=CCl2
10
3
–
–
* The yields are in percent relative to the initial Et Al.
3
** A is positive polymerization, and E is negative polarization (hyphen means the absence of polarization).
1
mounted directly in the spectrometer sensor. The H
and 13C NMR spectra were recorded during the vigorꢀ
ous reaction on a JEOL FXꢀ90Q spectrometer operatꢀ
ing at 89.5 MHz (1H) and 22.5 MHz (13C).
RESULTS AND DISCUSSION
The exothermic reaction of Et3Al with CCl4 cataꢀ
lyzed by Pd(acac)2 proceeds vigorously and is accomꢀ
panied by the evolution of gaseous products. An
induction period is observed before the second, nonꢀ
radical stage. Its duration depends on the temperature
and on the concentration of the reactants, which
makes it possible to study the initial stage until the secꢀ
ond stage starts [5].
For the analysis of the reaction products, the reacꢀ
tion was carried out in an argon atmosphere in a threeꢀ
necked flask fitted with a magnetic stirrer, a dropping
funnel, and a reflux condenser. A 1.2 M solution (5 ml)
of Et3Al (6 mmol) in hexane was introduced into the
flask, which contained Pd(acac)2 (9 mg, 0.5 mol %
relative to Et3Al) and was connected to a gas meter. To
prevent the onset of the second stage of the reaction, a
solution (1.6 ml) of CCl4 in hexane (1 : 2) was added
dropwise at 13C over 10 min to the resulting dark soluꢀ
tion, and then 0.5 ml of CCl4 (5.1 mmol) was added;
that is, the total amount of CCl4 added was 10.6 mmol.
During the reaction, 121 ml (under normal condiꢀ
tions) of a gas evolved, which is 0.9 mol per mole of
Et3Al. The gas consisted of ethane (48%) and ethylene
(52%). The initial hydrolysis stage was carried out
under cooling, and then the hydrolysis was brought to
the end at room temperature by adding 10% HCl.
During the hydrolysis, 250 ml of a gas (1.85 mole per
mole of triethylamine) was collected, which contained
97.4% ethane and 2.6% ethylene. The organic layer sepꢀ
arated in a separatory funnel from the aqueous phase was
dried with MgSO4 and was analyzed by GC/MS.
The interaction of a 1 M solution of Et3Al in cycloꢀ
hexane with CCl4 (1 : 1.5–3.0) at room temperature
yields Et2AlCl
,
ethane, ethylene, chloroform,
hexachloroethane, and trichloropropane. When the
reaction is carried out directly in the cell of the NMR
spectrometer, the spectra exhibit integral CIDNP
1
effects for H and 13C. The signs of nuclear polarizaꢀ
tion, chemical shifts ( ), and signals assignments are
δ
1
presented in the table. The H and 13C NMR signals
were assigned based on published data and by comparꢀ
isons with the spectra of authentic samples.
1
The H NMR spectra of the starting solution of
Et3Al, the reaction mixture under the reaction condiꢀ
tions, and the reaction products are shown in Fig. 1.
When Et3Al reacts with CCl4 in cyclohexane in the
absence of a catalyst, 1H and 13C CIDNP appear in the
geminal singlet radical pair of the ethyl and trichloꢀ
romethyl radicals [4] formed in one elementary event.
The reaction products were analyzed using a A comparison between the thermal and catalytic proꢀ
Chromꢀ5 chromatograph equipped with a capillary cesses shows that the catalytic reaction is accompaꢀ
column (25 m in length, 5% SEꢀ30 phase) and a nied by the inversion of polarization signs of the chloꢀ
QCMSꢀQP2010 Plus GC/MS system (Shimadzu) roform and trichloropropane nuclei. The inversion
with an SPBꢀ1 column (Supelco, 30 m
0.2 m, 5% phenyl/95% methyl silicone phase). The pair. Therefore, it can be assumed that, in the catalytic
NBS library (United States), containing 75000 mass reaction, integral nuclear polarization appears in the
spectra, was used to identify the compounds. diffusion pair of the ethyl and trichloromethyl radiꢀ
×
0.25 mm
×
can be due to a change in the multiplicity of the radical
µ
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CHEMICALLY INDUCED DYNAMIC NUCLEAR POLARIZATION
499
E
E A
A
A E
CH2=CH2 + CHCl3
Fdisproportionation
•
CH3CH2 CCl3
•
recombination
CH3CH2CCl3
dissociation
•
CCl
3
E
CCl3–CCl3
•
CH3CH2 + CCl3
•
(
F = diffusion radical pair
)
Scheme 1.
cals. The recombination of this pair yields trichloroꢀ
propane (Scheme 1).
Polarized chloroform and ethylene result from the
intracellular disproportionation of this pair. Probably,
polarized hexachloroethane is the product of radical
pair dissociation followed by the recombination of two
trichloromethyl radicals.
Next, PdCl again reacts with excess Et3Al:
PdCl + Et3Al
PdEt + Et2AlCl.
Based on experimental evidence and literature
data, the formation of the major products and radical
intermediates in the reaction of Et3Al with CCl4 in the
presence of Pd2+ can be presented as the following catꢀ
alytic cycle (Scheme 2).
This mechanism of the reaction between Et3Al and
CCl4 in the presence of the Pd complexes coincides
with the scheme proposed previously for the reaction
of Et3Al with CCl4 catalyzed by Ni(acac)2 [9]. Thereꢀ
fore, the mechanisms of the reactions of Pd(acac)2 and
Ni(acac)2 with Et3Al are very similar, and this is in
agreement with the literature [7].
Since the freeꢀradical polymerization of vinyl chloꢀ
ride is efficiently initiated by reaction systems involving
Et3Al and CCl4 in the presence of the titanium and copꢀ
per compounds [1, 10], we verified the existence of
CIDNP effects in this reaction catalyzed by Cu(acac)2
and Cp2TiCl2. A vigorous reaction occurred in both
cases, which was accompanied by emission from the
hydrogen nuclei of chloroform. After the reaction, the
The observed signs of 1H and 13C CIDNP are indiꢀ
cated in Scheme 1. They coincide with those predicted
using the Captain rules for strong magnetic fields [6].
The addition of styrene as an efficient freeꢀradical
scavenger to the reaction mixture suppresses the
polarization effects. This confirms the assumption
about CIDNP generation in diffusion radical pairs.
Thus, all CIDNP effects observed for both protons
and carbon nuclei are well explained in terms of the
diffusion pair of the ethyl and trichloromethyl radicals
and their subsequent transformations.
Let us consider a possible route for the formation of
the ethyl and trichloromethyl radicals in the catalytic
process. According to published data, the interaction
of Et3Al with Pd(acac)2 results in the reduction of the
latter [7]. When the amounts of Et3Al and Pd(acac)2
(Al/Pd < 2) are comparable, the following ligand
exchange occurs:
Et3Al + Pd(acac)2
When Et3Al is in excess, the reaction yields diethꢀ
ylpalladium:
Pd(acac)2 + 2AlEt3
Et2Al(acac) + EtPd(acac).
Products
5
8
7
6
5
4
2 1
0
2Et2Al(acac) + PdEt2.
In the case of a large excess of Et3Al (Al/Pd > 10),
the reaction occurs instantly and the resulting PdEt2 is
unstable and decomposes readily to Pd(0) and gaseous
products (ethane and ethylene). It can be assumed
that part of the PdEt2 decomposes homolytically to
form the ethyl radical:
41 s
4
15 s
3
PdEt2
Pd(0) + C2H6 + C2H4 + CH3CH2.
3 s
2
Pd(0) is aggregated and precipitates as a black colꢀ
loid. In our case, we can assume that Pd(0) atoms react
with CCl4:
1
Initial
.
Pd(0) + CCl4
PdCl + CCl3
8 7 6 5 4 3 2 1 0
This assumption is verified by the existence of the
Kharasch reaction [8] between an organomagnesium
compound RMgX and an organic halide catalyzed by
transition metal salts, which generates intermediate
radicals and yields products of the R–R type:
1
Fig. 1. H CIDNP in the reaction of Et Al with CCl in the
3
4
1
presence of Pd(acac) : (
solution of Et Al (1 mol/l, 0.4 ml) and Pd(acac) (~2 mg) in
cyclohexaneꢀd ; (2–4) H NMR spectra recorded during
the reaction 3, 15, and 41 s after the addition of CCl
1
) H NMR spectrum of the initial
2
3
2
1
12
4
1
M(0) + R'Hal
M H al + R '.
(0.6 mmol); (5) H NMR spectrum of the reaction products.
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500
SADYKOV et al.
Pd(acac)2
Et3Al
Et2Al(acac)
Et2AlCl
PdEt
C2H6, C2H4
Et3Al
•
C2H5
PdCl
Pd(0)
•
CCl3
CCl4
Scheme 2.
2
1
7
6
5
4
3
2
1
0 δ, ppm
1
Fig. 2. H CIDNP observed in the interaction of a solution of 1ꢀethylꢀ3ꢀbutylaluminacyclopentane (0.5 ml, 1 mol/l) in C D
6
12
with (1) CCl (0.2 ml) and (2) CHCl (0.2 ml) at room temperature.
4 3
1H NMR spectra of the reaction mixtures contained under the thermal conditions in the absence of a cataꢀ
lyst in cyclohexaneꢀd12, the protons of chloroform and
the protons at the double bond of octꢀ1ꢀene in the 1H
NMR spectra are positively polarized, the multiplet of
signals from CHCl3, CH2Cl2, CH3CH2CCl3, and
CH3CH2Cl
.
The reactions of other abundant trialkylalanes (
Bu3Al and ꢀBu2AlH) with halomethanes were considꢀ
ered earlier [4, 11, 12]. We discovered integral CIDNP
effects in the reaction of ꢀC8H17)3Al with CCl4,
which occurs under thermal and catalytic (Ni(acac)2ꢀ
catalyzed) conditions. When the process is carried out presence of Ni(acac)2.
iꢀ
protons of the =CH2 group (
polarized, and the doublet of the =CH group (
4.78 ppm) is negatively polarized. In addition, an uniꢀ
dentified positively polarized singlet at = 4.63 ppm is
observed. Chloroform emission was observed in the
δ
= 5.73 ppm) is positively
i
δ
=
(n
δ
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CHEMICALLY INDUCED DYNAMIC NUCLEAR POLARIZATION
501
F
C4H9
Et
. .
CH2
CCl3
Al
Al
Cl
(RP1)
Pd(acac)2
CCl4
EtAl
F
C4H9
Et
Cl
C4H9
.
.
CH2 CCl3
(RP2)
C4H9
Et
Cl
E
disproportionation
Al
RP2
CH2
+ CHCl3
H E
(1)
C4H9
C4H9
Et
Et
Cl
E
E
.
•
CCl3 +
CCl
4
dissociation
RP2
Al
Al
CH2
CH2Cl
Cl
(2)
Scheme 3.
CIDNP effects were also observed in the reaction
of CCl4 with the cyclic organoaluminum compound
1ꢀethylꢀ3ꢀbutylaluminacyclopentane [13] in the presꢀ
ence of Pd(acac)2 in cyclohexaneꢀd12 (Fig. 2). As in
other catalytic reactions studied by us, strong emission
from chloroform protons was observed in the 1H NMR
spectra in this case, but no polarized signals of the
products of the ethyl and trichloromethyl radicals
(1,1,1ꢀtrichloropropane and ethyl chloride) were
observed. Therefore, the ethyl group in the organoaluꢀ
minum compound is not involved in the reaction of
cyclic organoaluminum compounds with CCl4. The
spectra also exhibit broad, negatively polarized signals
with chemical shifts of 5.6 and 3.3 ppm, which are
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likely due to products
1 and 2, respectively
(Scheme 3). This suggests that, in the catalytic reacꢀ
tion of the organoaluminum compounds with CCl4,
the Al–C bond, which is remote from the butyl subꢀ
stituent in the ring, breaks and, accordingly, an RP2
radical pair forms without the formation of an RP1
pair. Only emission from methylene chloride protons
was observed in the reaction of 1ꢀethylꢀ3ꢀbutylalumiꢀ
nacyclopentane with chloroform.
7. Dmitrieva, T.M., Cand. Sci. (Chem.) Dissertation
,
Irkutsk: Irkutsk State Univ., 1983.
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Kashin, A.N., Reaktsii metalloorganicheskikh soedinenii
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Ser. Khim., 1999, p. 1003.
On the whole, our results indicate that the catalytic
reactions of trialkylalanes with CCl4 in the presence of
Cu(acac)2 and Cp2TiCl2, as well as Ni(acac)2 and
Pd(acac)2, in a hydrocarbon solvent occur mainly via
the general mechanism presented in Scheme 2.
10. Breslow, D.S., Christman, D.L., Epsy, H.H., and
Lukach, C.A., J. Appl. Polym. Sci., 1976, vol. 11, p. 73.
11. Sadykov, R.A. and Teregulov, I.Kh., Izv. Akad. Nauk,
Ser. Khim., 1998, p. 2097.
12. Sadykov, R.A. and Dzhemilev, U.M., Izv. Akad. Nauk,
Ser. Khim., 1999, p. 1003.
ACKNOWLEDGMENTS
13. Dzhemilev, U.M., Ibragimov, A.G., Zolotarev, A.P.,
Muslukhov, R.R., and Tolstikov, G.A., Izv. Akad. Nauk
SSSR, Ser. Khim., 1989, p. 201.
This work was supported by the Russian Foundaꢀ
tion for Basic Research, project no. 08ꢀ03ꢀ97016R.
KINETICS AND CATALYSIS Vol. 51
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2010