Bimolecular concerted elimination as an elementary stage in free-radical reactions. Reaction of organocobaloximes with bromotrichloromethane

Article abstract of DOI:10.1023/A:1013427306364

In free-radical reaction of cis-β-phenylvinylcobaloxime with bromotrichloromethane the primary product, phenylacetylene, arises as a result of elimination, and further it adds the bromotrichloromethane. Similarly proceeds the reaction between β-phenylethylcobaloxime with bromotrichloromethane. The values of absolute rate constants and their relation to the nature of the axial ligand completely agree with the assumption that in the reactions in question similarly to the E2 mechanism of nucleophilic elimination the elimination occurs as one-stage bimolecular process with the simultaneous rupture of C-Co and C-H bonds.

Full text of DOI:10.1023/A:1013427306364

Russian Journal of Organic Chemistry, Vol. 37, No. 10, 2001, pp. 1444 1450. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 10, 2001,
pp. 1513 1519.
Original Russian Text Copyright
2001 by Dneprovskii, Kasatochkin.
Bimolecular Concerted Elimination as an Elementary Stage
in Free-radical Reactions. Reaction of Organocobaloximes
*
with Bromotrichloromethane
A. S. Dneprovskii and A. N. Kasatochkin
St. Petersburg State University, St. Petersburg, 198904 Russia
Received December 6, 2000
Abstract In free-radical reaction of cis- -phenylvinylcobaloxime with bromotrichloromethane the primary
product, phenylacetylene, arises as a result of elimination, and further it adds the bromotrichloromethane.
Similarly proceeds the reaction between -phenylethylcobaloxime with bromotrichloromethane. The values
of absolute rate constants and their relation to the nature of the axial ligand completely agree with the
assumption that in the reactions in question similarly to the E2 mechanism of nucleophilic elimination the
elimination occurs as one-stage bimolecular process with the simultaneous rupture of C Co and C H
bonds.
It is undoubtedly of interest to investigate the
reactivity in free-radical reactions of organocobal-
In preceding publications we reported on extensive
studies of reaction kinetics of organocobaloximes and
bromotrichloromethane at broad spectrum of ligand
character and organic fragment structures [10 13].
The reaction of bimolecular homolytic substitution
was shown to extend to alkylcobaloximes [13]. We
developed a procedure for evaluation of relative [10]
oximes RCo(dmgH) L where R is hydrocarbon
2
radical, dmgH is a dimethylglyoxime monoanion, L
is a neutral organic ligand. These compounds are
synthetic analogs of cobalamines, in particular, of
vitamin B , and many reactions of these compounds
1
2
simulate biochemical processes. The chemical
behavior of cobalt complexes to a significant degree
depends on the low strength of the -bond Co C
and absolute [12] rate constants of S 2 reaction, and
H
all the kinetic data were consistent with SH2
mechanism. The comparison of the results obtained
with the data on homolysis rate of the substrates
studied led to conclusion that the main factors affect-
(
80 100 kJ mol 1 [1]). As a result the Co C bond
easily undergoes homolysis yielding a cobalt-centered
radical Co(dmgH) L. On the one hand this radical
shows low activity in the most free-radical reactions
ing the rate of S 2 reactions was the dissociation
2
H
energy of C Co bond and the steric accessibility of
the reaction site [13]. The relations between structure
and kinetics were totally like those established for
bimolecular nucleophilic substitution.
(
save halogen abstraction). On the other hand the
Co(dmgH) L species is a good leaving group in
2
radical reactions. Therefore were successfully carried
out relatively rare for organic chemistry reactions,
e.g., allyl substitutions [2 4] and cyclization afford-
ing cyclopropane structures [5].
Taking into account the established similarity of
free-radical and nucleophilic substitution reactions we
investigated the reaction of bromotrichloromethane
and cis- -phenylvinylcobaloxime (I) expecting that
the reaction would proceed as free-radical vinyl sub-
stitution. However the expected reaction product was
not obtained neither at thermal nor at photochemical
initiation. At the same time we found that the main
reaction product of reaction between compound I
The reaction of benzylglyoximate cobalt complexes
PhCN Co(dmgH) L with some free-radical agents
2
2
affords substitution products at carbon atom (S 2)
H
[6 9].
PhCH Co[dmgH) L + X
PhCH X + Co(dmgH) L
2 2
2
2
(
L = Py) and fivefold excess of bromotrichloro-
X = CCl , ArSO .
methane in chloroform at 100 C was 1-phenyl-3,3,3-
trichloroprop-1-ene (III). Besides in the reaction
mixture was found a small amount (about 2%) of
phenylacetylene. It is presumable that phenylacetylene
is a primary reaction product that further adds bromo-
3
2
*
The study was carried out under support of Ministry of
Education of Russian Federation, program Russian Univer-
sities. Fundamental Research (grant no. 1216¶12).
1
070-4280/01/3710-1444$25.00 2001 MAIK Nauka/Interperiodica

BIMOLECULAR CONCERTED ELIMINATION
1145
CCl Br
3
.
PhCH CHCo(dmgH) L + CCl3
PhCBr= CHCCl3
III
2
I
(1)
Co(dmgH) L
2
PhCH CH
CCl3
PhCH= CHCCl3
II
trichloromethane along free-radical mechanism to
further reaction with bromotrichloromethane affords
afford compound III.
product III.
The reaction is initiated by thermolysis of the
original cobaloxime I.
The GLC analysis of the reaction mixtures
revealed that their composition changes both in the
course of reaction and at workup. At the early reac-
tion stages the peak of compound III is lacking, but
accumulates a substance with the retention time very
close to that of compound III. As the reaction
proceeds the relative amount of the unknown com-
pound decreases, and occurs accumulation of product
III. At further keeping of the reaction mixture the
unknown substance completely disappears transform-
ed into product III. We reckon that this is due to
stereoisomerization of the reaction product in the
course of the process. We have shown before [14]
that the bromotrichloromethane first adds to methyl
propiolate stereospecifically in trans-position, and
then the primary product rearranges into a more
stable stereoisomer. It is presumable that here the
reaction also proceeds as trans-addition and then
stereoisomerization of the primary product affords
more stable stereoisomer III.
Further investigation of reaction (1) revealed that
the direction of the process is notably affected both by
temperature and by the presence in the system of
alkylcobaloximes. At 50 C at the lack of free radical
initiators compound I is stable, and no compound III
or phenylacetylene form in the system. It is known
[
13] at the same time that the thermolysis rate of
alkylcobaloximes under these conditions is sufficient
for initiation of a free-radical process. The stability of
compound I obviously is due to increase in the C Co
bond strength on going to vinylcobaloximes because
2
of sp -hybridization of carbon atom. In this connec-
tion further we introduced into the reaction two sub-
strates: phenylvinylcobaloxime (I) and benzyl-
glyoximate complex PhCH Co(dmgH) Py (IV). Since
2
2
under conditions of concurrent reactions the
stationary concentration of trichloromethyl radicals
arising from reactions RCo(dmgH)2L
Co(dmgH) L and CCl Br + Co(dmgH) L
R +
CCl3
2
3
2
was determined by the rate of decomposition of the
more reactive complex, we could expect that applying
compound IV that decomposed with appreciable rate
already at 40 C [10, 13] we would be able to perform
reaction (1). Actually we found that at 47.7 C under
conditions of concurrent reactions formed both
phenylacetylene and product III. The study of
dependence of reaction products composition on con-
version showed that at the early stage of the reaction
the phenylacetylene was the only reaction product,
and kinetics of its formation fit the equation of the
pseudofirst order. Later on formed product III, and
the variation in time of phenylacetylene concentration
is consistent with a kinetic scheme regarding it as an
intermediate in the series of consecutive processes
Table 1. Composition of products obtained in reaction
of cis- -phenylvinylcobaloxime (I) with bromotrichloro-
methane at 47.7 C
3
4
[
PhC CH] 10 ,
[III] 10 ,
mol l
Time, h
1
1
mol l
0.33
0.58
0.83
1.08
1.5
2.0
3.5
4.5
9.5
0.08
0.13
0.16
0.16
0.19
0.30
0.44
0.48
0.53
0
0
0
0
0
0
0
0.5
2.0
(Table 1). Thus all the data obtained evidence that the
primary reaction product is phenylacetylene, and its
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

1
446
DNEPROVSKII, KASATOCHKIN
till now is a subject to discussions we have thorough-
Ph
Br
Ph
Br
H
CCl3
ly studied the kinetics of reactions (1) and (2) paying
special attention to the dependence of the reaction rate
on the character of the axial ligand L. Using the
method of concurrent reaction we evaluated the krel at
C= C
C= C
H
CCl3
III
4
7.7 C for the pair of compounds I/IV (Table 2).
-
Phenylethylcobaloxime (V) reacts with bromotri-
chloromethane in a similar way. First styrene is
formed that further transforms into addition product
VI.
CCl Br
3
PhCH=CHCo(dmgH) Py
PhC CH
2
CCl Br
3
PhCHBr=CHCCl3
PhCH=CHCo(dmgH) Py
PhC=CH2
2
CCl Br
3
CCl Br
3
PhCHBrCH CCl
(2)
2
3
PhCH=CHCo(dmgH) Py
PhCH CCl
3
(3)
2
2
Taking into account that free-radical elimination is
a relatively rare phenomenon and its mechanism up
The value of krel was calculated by the following
formula:
k(I)
k(IV)
[PhCCH] + [PhCBr= CHCCl3 ]
[PhCH Co(dmgH) Py ]
2 2
.
=
[PhCH CCl ]
[PhCH= CHCoCo(dmgH) Py ]
2
2
3
We revealed that the value krel is considerably
affected by the concentration of free pyridine in the
reaction system (Table 2). We already observed
analogous dependence in the kinetic study of reaction
between alkylcobaloximes and bromotrichloro-
methane. It was demonstrated that the effect of the
free axial ligand on the reaction rate is due to the
possible participation in the reaction of a pentaco-
ordinate alkylglyoximate complex [13]. Therefore
further we used only the values of the relative rate
constants measured at large excess of pyridine. Since
we already determined previously the value of the
absolute rate constant for reaction of trichloromethyl
radical with compound IV (L = Py) [13] we were
able to calculate the absolute rate constant of the
reaction between this radical and compound I (L =
3
Py): kabs = 3.8 10 .
Similarly were measured the relative rate constants
of reaction between bromotrichloromethane and
-phenylethylglyoximate complexes V [reaction (2)]
and complex IV [reaction (3)].
k(V)
k(IV)
[PhC= CH ]+ [PhCHBrCH CCl ]
[PhCH Co(dmgH) Py ]
2 2
2
2
3
.
=
[PhCH CCl ]
[PhCH CH CoCo(dmgH) Py ]
2 2 2
2
3
The measured values of the relative rate constants
for all ligands are listed in Table 3, and the calculated
absolute rate constants are given in Table 4.
25 C is for complex IV 500 times smaller than that
for vinyl complex (R = CH CH=CH).
3
The kinetic data obtained are sufficient for con-
sidering the mechanism of elimination reaction that
we had revealed. Analogous to ionic reactions three
possible mechanisms should be taken into account.
As seen from Table 3, variation of axial ligand
significantly affects the rate of elimination. As we
have shown before [13], the main cause of the change
in reactivity at variation of the axial ligand is the
alteration of the dissociation energy of the C Co
bond. Unlike reaction (1) the rate of reaction (2) is
virtually insensitive to the addition of free axial
ligand L. This may be connected with unlike stability
of the hexacoordinate complexes I and V. For
instance, as measured in [15], the association constant
(1) Unimolecular decomposition with initial homo-
lysis of the C Co bond followed by disproportiona-
tion in the radical pair (analogous to E1 mechanism
in ionic reactions).
PhCH=CHCo[dmgH) L
[PhCH=CH
2
RCo(dmgH) Py
RCo(dmgH) + Py in water at
+
Co(dmgH) L]
2
PhC CH + HCo(dmgH) L
2
2
2
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

BIMOLECULAR CONCERTED ELIMINATION
1147
Table 2. Effect of pyridine addition on the relative rate
constants k(I)/k(IV) in reaction of organocobaloximes with
bromotrichloromethane
Table 3. Relative rate constants k(V)/k(IV) in reaction
of -phenylethylcobaloximes with bromotrichloromethane
L
Time, h
[L]/[V]
k(V)/k(IV)
Time, h
[Py]/[I]
k(I)/k(IV)
Py
Py
Py
Py
Py
Py
Py
Py
3-MePy
3-MePy
3-MePy
4-MePy
4-MePy
4-MePy
3-BrPy
3-BrPy
3-BrPy
3-BrPy
Im
0.17
0.67
1.33
0.67
1.0
0
0
0
8
8
12
12
12
0
0
0
0
0
0
0
0
0
0
0
8.6
9.2
8.6
10.0
9.4
9.0
8.8
8.5
7.72
7.9
8.4
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
1
0
0
0
0
.17
.33
.58
.75
.22
.3
.48
.12
.3
.48
.75
.33
.67
.67
.0
0
0
0
156
156
156
104
121
107
127
18.5
26.7
21.5
20.4
9.2
0
0.3
0.3
0.3
1.0
1.0
1.0
1.0
4.0
4.0
8.0
8.0
12
12
12
15
15
15
15
0.5
0.75
1.33
0.5
0.75
1.33
0.5
0.75
1.33
0.5
0.75
1.33
1.66
0.5
8.15
9.1
9.2
9.4
7.1
7.3
7.1
7.5
7.1
7.3
7.1
5.35
4.64
5.14
5.35
11.75
11.5
11.0
6.16
6.66
6.62
.5
.75
.33
.17
.32
.48
.72
Im
Im
PPh3
PPh3
0.75
1.33
0.5
0.75
1.33
0
0
0
0
6.7
7.3
PPh3
0
It is assumed that olefins arise in thermolysis of
primary and secondary cobalamines along this
mechanism [16, 17]. But for reaction we studied this
mechanism should be rejected: Firstly, it does not
explain why complex I is stable at 47 C in the
absence of trichloromethyl radicals but elimination
reaction easily starts with it in the presence of benzyl
complex IV. Secondly, absolute rate constants of
Table 4. Absolute rate constants of reaction between
-phenylethylcobaloximes (V) and bromotrichloromethane
(kelim), of bimolecular homolytic substitution for complexes
IV (ksubst), and constants of homolysis rate of complexes
IV (khom) at 47.7 C
L
Py 3-MePy 4-MePy 3-BrPy Im PPh
3
homolysis are in the range 10 5 10
7
s
1
[13],
3
therefore they are by several orders of magnitude
smaller than those of the reactions under study.
k
k
elim 10
subst 10
4.6
5.2
4.0
4.6
5.7
4.5
5.7
6.5
6.8
7.5 4.6 32.4
14 4.0 50
9.0 4.8 47
2
6
khom 10
(
2) Two-stage mechanism with hydrogen abstrac-
tion by the trichloromethyl radical with subsequent
fragmentation of the radical yielding the elimination
-
Two-stage mechanism was taken as the main one
in the majority of publications on radical elimination
reactions.
product (analogous to E1sB mechanism).
PhCH=CHCo(dmgH) L + CCl3
2
Therewith as the principal proof of stage-like
process were regarded the data on the stereoselec-
tivity of the reaction. It was found that nonstereo-
selective elimination occurred in reactions of
-bromosulfones [19] and -bromosulfides [20] with
trialkylstannyl radicals. In the cases where the elimin-
ation occurred stereoselectively Boothe et al.
PhC=CHCo(dmgH) L
PhC CH
2
Co(dmgH) L
2
In this scheme the limiting stage is the first one,
since the hydrogen abstraction by trichloromethyl
radical is irreversible [18].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

1
448
DNEPROVSKII, KASATOCHKIN
suggested that the process was two-stage, and the
stereoselectivity was due to intermediate formation of
bridge-like radicals [21 23]. Unfortunately the lack of
kinetic data does not permit considering the multi-
stage mechanism as rigorously proved in the majority
of the above publications.
considerably smaller than the figures we obtained for
elimination reactions (Table 4). Secondly, if the
elimination occurs in two stages, and the first one is
limiting, then the variation of the leaving group
should not affect the reaction rate as is observed in
nucleophilic eliminations along E1cB mechanism.
This also is in disagreement with our results (Table 4).
Our data are not consistent with the two-stage
mechanism. Firstly, in a reaction proceeding by this
mechanism the overall rate of the process should be
equal to that of free-radical hydrogen abstraction by
trichloromethyl radical. For cyclohexane, toluene,
and ethylbenzene these rates are respectively 79 [24],
All results we obtained are logically understood in
assumption that the reaction is a one-stage process
going via transition state VII with concerted rupture
of C H and C Co bonds analogously to the
mechanism of bimolecular nucleophilic elimination
E2.
1
1
2
.9 [25] and 15 mol s [26]. These values are
If the reaction follows this mechanism, a re-
EXPERIMENTAL
semblance should be expected to the other processes
where the limiting stage consists in the rupture of
C Co bond. This statement is quite consistent with
the data of Table 4. An obvious similarity is seen
between the rate of the reaction under study and both
the rates of bimolecular substitution at carbon atom
and organocobaloximes homolysis. The greater
reactivity of the C H bond as compared to reactions
of hydrogen transfer is also quite understandable.
Same as in E2 reactions were the process rate is
notably higher than calculated from the kinetic acidity
of the compound, the reaction is facilitated due to
partial formation of the C=C bond in the transition
state. Thus it can be considered as established that the
reaction we studied proceeded along one-stage con-
1H NMR spectra were recorded on spectrometer
Tesla BS-567A from solutions in deuterochloroform.
The GLC analyses were carried out on gas chromato-
graph Chrom-5 equipped with a flame-ionization
1
detector, carrier gas argon, flow rate 30 ml min ,
glass column 3000 2 mm, stationary phase 10% OV
on Chromaton-N-Super, oven temperature 165
170 C. The composition of reaction mixtures was
analyzed by GLC with the use of authentic reference
compounds. The bromotrichloromethane was distilled
before use through a Vigreux column (40 cm)
collecting the fraction of bp 143 145 C. Phenyl-
acetylene and (2-bromoethyl) benzene were distilled
collecting fractions 143 145 C and 93 95 C
(15 mm Hg) respectively. Chloroform of pure for
chromatography grade was used without further
purification.
certed mechanism of free-radical elimination E 2.
R
There are published examples suggesting that the
concerted mechanism of free-radical elimination is
not an exception. For instance, it was shown in [27]
that in reaction of dibutylmercury with trimethyl
radicals formed in high yield 1-butene evidencing the
enhanced reactivity of C H bond in 2 position. The
prevailing anti-elimination in the reaction of
cis-(2-Phenylvinyl)cobaloxime (I), 2-phenylethyl-
cobaloximes (V), and benzylcobaloximes (IV).
Complex I was prepared along procedure [32] by
reaction of phenylacetylene with a highly nucleophilic
I
complex Na[Co (dmgH) Py]. The crystals of the
2
3
-deutero-3-trimethylstannylethane with trichloro-
complex were dried in a vacuum-desiccator in the
presence of P O and purified by column chromato-
methyl radical [28] and stereoselective course of some
other elimination reactions [29 31] also may be inter-
preted within the framework of the one-stage con-
certed mechanism.
2
5
graphy on silica gel, eluent acetone ethyl acetate,
1
1: 1. Yield 20%. H NMR spectrum (CDCl3,
,
ppm): 1.9 s (12H), 5.5 d (1H, J 8.9 Hz), 6.4 d (1H,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

BIMOLECULAR CONCERTED ELIMINATION
1149
J 8.9 Hz), 7.1 m (5H), 7.3 m (2H), 7.7 m (1H). 8.5 m methane. The solvent was preliminary freed of oxy-
2H).
gen traces. The ampule was flushed with argon at
C, sealed, and placed into a thermostat at 47.7 C.
(
0
Complex V (L = Py) was prepared by proce-
Intermittently the content of ampules was passed
through short (3 4 cm) column packed with silica gel
to remove cobalt complexes (eluent dichloromethane).
The solution obtained was cautiously evaporated to a
volume of 0.2 0.5 ml, and the products were sub-
jected to GLC.
dure [33] proceeding from (2-bromoethyl)benzene.
The crystals of the complex were dried in a vacuum-
desiccator in the presence of P O and purified by
2
5
column chromatography under the same conditions
1
as above. Yield 60%. H NMR spectrum (CDCl , ,
3
ppm): 1.7 1.9 m (4H), 2.1 s (12H), 7.1 m (5H), 7.3 m
(
2H), 7.6 m (1H), 8.6 m (2H).
Study of material balance in reaction of
complexes I and V with bromotrichloromethane.
The reaction mixture of complexes I and IV with
bromotrichloromethane in deuterochloroform after
heating to 47.7 C for 20 h was subjected to GLC and
Complexes V with various axial ligands (L =
-MePy, 4-MePy, 3-BrPy, Im, PPh ) were prepared
3
3
1
and purified in the similar way. The H NMR spectra
are similar to that of complex V (L = Py) save the
signals from the axial ligands. The synthesis of
benzylglyoximate complexes (IV) (L = Py, 3-MePy,
4
[
1
1
H NMR. In the H NMR spectrum the signals of
vinyl protons of the original complex I were lacking
evidencing virtually total conversion thereof. Accord-
ing to GLC from 0.226 mmol of the original complex
formed 0.017 mmol of phenylacetylene and
-MePy, 3-BrPy, Im, PPh ) we described elsewhere
3
11]. The purity of all complexes was checked by
TLC on Silufol-UV plates, eluent acetone ethyl
acetate, 1: 1.
0
.199 mmol of compound III corresponding to over-
all yield of reaction products 95%. Similarly were
analyzed the products of reaction between complexes
V and bromotrichloromethane. Chromatographic
yield of the reaction products amounted to 98 100%.
1
-Bromo-1-phenyl-3,3,3-trichloropropene (III)
and 1-bromo-1-phenyl-3,3,3-trichloropropane (VI).
Compound III was obtained by addition of bromo-
trichloromethane to phenylacetylene at photochemical
initiation (UV lamp of moderate pressure) in a quartz
tube at 30 35 C within 20 h. The conversion of
phenylacetylene according to GLC data is nearly
quantitative. On completing the reaction the excess
bromotrichloromethane was removed in a vacuum at
room temperature, the residue was distilled in a
vacuum (0.2 mm Hg) at 70 C. According to GLC the
reaction product contained 92% of compound III and
8
spectrum (CCl , , ppm): 6.91 s and 6.95 s (overall
1
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1
% of impurity (presumably C H CCCl ). H NMR
6
5
3
4
H), 7.18 7.35 m (5H). Found, %: C 36.60; H 2.03.
4
5
C H BrCl . Calculated, %: C 35.98; H 2.01.
9
6
3
Compound VI was prepared under similar condi-
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distilled styrene. The conversion of the initial alkene
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6
7
. Funabiki, T., Gupta, B.D., and Johnson, M.D.,
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1
00 C. The obtained adducts mixture contained 85%
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chloropropane [34]. Yield 60%, mp 49 51 C.
1
8
9
. Gupta, B.D. and Roy, S., J. Chem. Soc., Perkin
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H NMR spectrum (CDCl3, , ppm): 3.66 d (2H,
J 6.5 Hz), 5.25 t (1H, J 6.5 Hz), 7.05 7.35 m (5H).
Kinetic measurements along procedure of con-
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