Determination of the rate constants of the addition of primary alkyl radicals to allylstannanes

Article abstract of DOI:10.1039/b108875k

The rate constants and the activation parameters of the addition of primary alkyl radicals to allyldibutyl-4,7,10-trioxaundecylstannane 1 and (2-ethoxycarbonylprop-2-enyl)dibutyl-4,7,10-trioxaundecylstannane 2 were determined by studying their respective free radical reactions with (2-bromo-1,1-dimethylethyl)benzene and 6-bromohex-1-ene at different temperatures.

Full text of DOI:10.1039/b108875k

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Determination of the rate constants of the addition of primary
alkyl radicals to allylstannanes
Fouad Ferkous,a Marie Degueil-Castaing,b Hervé Deleuzeb and Bernard Maillard*b
2
a Laboratoire de Chimie Organique, Institut de Chimie, Université de Annaba,
BP 12El-Hadjar, Annaba, Algeria
b Laboratoire de Chimie Organique et Organométallique (UMR CNRS 5802), Université
Bordeaux 1, F-33405 Talence-Cedex, France. E-mail: b.maillard@lcoo.u-bordeaux.fr
Received (in Cambridge, UK) 5th October 2001, Accepted 27th November 2001
First published as an Advance Article on the web 14th January 2002
The rate constants and the activation parameters of the addition of primary alkyl radicals to allyldibutyl-4,7,10-
trioxaundecylstannane 1 and (2-ethoxycarbonylprop-2-enyl)dibutyl-4,7,10-trioxaundecylstannane 2 were determined
by studying their respective free radical reactions with (2-bromo-1,1-dimethylethyl)benzene and 6-bromohex-1-ene at
different temperatures.
neophyl [2-methyl(2-phenyl)propyl] rearrangement was selected
as a clock reaction. Then, the rate constant ka could be cal-
culated from the concentrations of the reaction products
(Scheme 1). Indeed, the kinetic analysis of the reaction mechan-
Introduction
Allylstannanes are among the most widely used reagents to
replace bromine atoms by allyl groups.1 However, the difficulty
of separation of organic reaction products from the stannic
compounds is an important limitation. Although several
chemical transformations of a trialkyl- or triaryl-tin halide in a
ism led to the following differential equation:
d[3]/d[4] = (ka/kr)[1]
more easily eliminable compound from the reaction medium
were used, none of them appeared fully satisfactory, as we
reported in a previous article.2 Several substitutes for allyltri-n-
butyltin were proposed to overcome this problem. Thus, the
allyltin moiety has been immobilized on a polymer to readily
eliminate the tin compounds after reaction by precipitation and
simple filtration.3 Other new compounds were designed in our
laboratory, including allylic monoorganotins4 and allyltri-
alkyltins having a polar tail.5 All of them gave good results from
the point of view of separation, however no comparison with
the allyltri-n-butylstannane has been made with regards to the
reactivity. As cascades of free radical reactions are used more
and more frequently in synthesis, it appeared important to
determine the reactivity of the allyltin having a polar tail,
allyldibutyl-4,7,10-trioxaundecylstannane 1, that we designed
to replace the allyltri-n-butyl stannane. This is the primary
objective of this work, part of a larger program devoted to the
study of the free radical reactivity of substitutes of tin reagents
involving free radical reactions by the use of clock radicals.6
The kinetic results will be compared with those obtained by
Curran et al.7 for the allyltri-n-butyltin. The availability of (2-
ethoxycarbonylprop-2-enyl)dibutyl-4,7,10-trioxaundecylstan-
nane 2, an allyltin with an activated double bond and a polar
Scheme 1
tail,5 prompted us to extend this kinetic study.
For measurements of the concentrations of the reaction
products performed at low consumption of 1 ([1] = [1]0), the
integration of the differential equation led to the following
relation:
Results
Kinetic study of the addition of primary alkyl radicals to
allyldibutyl-4,7,10-trioxaundecylstannane 1
[3]/[4] = (ka/kr)[1]0
The absolute rate constants for the rearrangement of certain
radicals are known and can therefore be used to serve as a
reference in competition with the reaction to calibrate. Several
clock radicals were proposed by Griller and Ingold6 for this
purpose. Assuming a value in the range of 104 l molϪ1 sϪ1 for
the rate constant of addition of a primary alkyl radical to the
double bond of 1 (ka), similar to the one of allyltributyltin,7 the
The value of ka/kr can then be easily calculated from the
measured concentrations [3] and [4], determined using gas
chromatography. The reaction of (1,1-dimethyl-2-bromoethyl)-
benzene with 1 was performed with several different concentra-
tions (0.1, 0.2, 0.3 and 0.4 mol lϪ1) of 1 at different temperatures
DOI: 10.1039/b108875k
J. Chem. Soc., Perkin Trans. 2, 2002, 247–250
This journal is © The Royal Society of Chemistry 2002
247

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Table 1 Experimental values of the rate constant ka of addition of
primary alkyl radicals to the double bond of allyldibutyl-4,7,10-tri-
oxaundecylstannane 1 at several temperatures
Table 2 Experimental values of the rate constant kb of addition of
primary alkyl radicals to the double bond of (2-ethoxycarbonylprop-2-
enyl)dibutyl-4,7,10-trioxaundecylstannane 2 at several temperatures
T /K
ka/103 l molϪ1 sϪ1
T /K
kb/105 l molϪ1 sϪ1
393a
63.1
64.3
57.6
65.0
13.1
10.0
11.9
9.1
393a
15.0
16.0
12.3
13.5
9.75
9.55
8.39
10.25
2.98
2.27
3.29
2.89
0.578
0.513
0.509
0.639
353a
313a
273b
353a
313a
273b
1.36
1.6
1.49
1.72
0.229
0.258
0.214
0.266
a Measurements in benzene. b Measurements in decane.
a Measurements in benzene. b Measurements in decane.
in the range 0 to 120 ЊC. The rate constant of addition, ka,
could then be calculated at the various temperatures (Table 1)
taking into account that the rate constant of the neophyl
rearrangement8 (kr) is:
log (kr/sϪ1) = 11.6 Ϫ (49.4/θ) with θ = 2.303RT in kJ molϪ1
The plot of log ka versus 1/T gave a good agreement with an
Arrhenius relationship (r2 = 0.99) between these two variables
leading to the following values for the activation parameters:
log (A/l molϪ1 sϪ1) = 10.2 0.4, Ea = 41.2 2.6 kJ molϪ1
.
For the reactions performed at 0 ЊC, decane was used in place
of benzene which freezes at this temperature. It was verified at
80 ЊC that, as shown previously for the dibutyl-4,7,10-tri-
oxaundecylstannane,2 there was no solvent effect upon the
reaction selectivity.
Kinetic study of the addition of primary alkyl radicals to
(2-ethoxycarbonylprop-2-enyl)dibutyl-4,7,10-trioxaundecyl-
stannane 2
Scheme 2
On the basis that the rate constant of addition of alkyl radicals
to the double bond of 2 (kb) is of the same order of magnitude
as that for addition to methacrylic acid,9 we selected the hex-5-
enyl radical rearrangement as a clock reaction.6 Assuming
a concentration of the allyltin 2 close to its initial value
(less than 10% conversion), the classical kinetic treatment of
the competition, depicted in Scheme 2, led to the following
equation:
Discussion
The rate constant of addition of primary alkyl radicals to
the double bond of 1 at 80 ЊC, about 1.3 × 104 l molϪ1 Ϫ1, is
s
not significantly different from the estimated value for the
homologous reaction involving allyltri-n-butylstannane7 (4 ×
104 l molϪ1
s
Ϫ1). This shows that 1 is a good substitute
for allyltri-n-butylstannane, with the advantage of an easier
separation from the organic reaction products. The influence
of the polar tail on the reactivity of an “acrylic stannane” is
difficult to determine because there is no previous measurement
of the rate constant of addition of primary alkyl radicals to
such unsaturated stannanes.
Taking into account the competition for the primary alkyl
radicals between the addition to the allylstannane (1 or 2) and
to the corresponding allylated derivatives (3, 4 or 5, 6 and 7), it
would be of interest to compare the rate constants determined
in this study with those given for the addition of primary alkyl
radicals to 1-alkenes. The review of Fischer and Radom is a
reference for reaction rates of addition of alkyl radicals to
double bonds.12 To our knowledge, there is no available value
for such reactions of addition relative to primary radicals.
Although it may be questionable to compare the reactivity of
methyl, tert-butyl and primary alkyl radicals with a vinylic
([6] ϩ [7])/[5] = (k6 ϩ k7)/kb[2]0
The ratios of the rate constants (k6 ϩ k7)/kb were determined
at each temperature from the values of the concentration of the
reaction products 5, 6, and 7 measured by GC for the reaction
performed with different initial concentrations (0.1, 0.2, 0.3 and
0.4 mol lϪ1) of 2. The knowledge of k6 10 and k7 11 [log (k6/sϪ1) =
10.4 Ϫ 28.6/θ and log (k7/sϪ1) = 9.9 Ϫ 36.0/θ with θ = 2.303RT in
kJ molϪ1] allowed the calculation of the values of kb (Table 2).
In the case of the reactions performed at 0 ЊC, benzene was
replaced by decane as above.
The plot of log kb versus 1/T showed a good Arrhenius rela-
tionship (r2 = 0.99) between these two variables leading to the
determination of the activation parameters of the addition
of primary alkyl radicals to 2: log (A/l molϪ1 Ϫ1) = 9.2 0.3,
s
Ea = 23.1 1.9 kJ molϪ1
.
248 J. Chem. Soc., Perkin Trans. 2, 2002, 247–250

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250 spectrometer. 13C NMR spectra were recorded at 62.9
MHz. Data are reported as δ-values (ppm) and coupling con-
stants (J/Hz).
Table 3 Rate constants of addition of alkyl radicals to double bonds
of 1, 2, 1-alkenes and methyl methacrylate (MMA)
k/l molϪ1 sϪ1
GC studies were performed with a VARIAN 3400 coupled to
a SPECTRAPHYSIC CHROMJET integrator. The capillary
column used was DB1 type (5% Ph), 30 m in length, 0.25 mm
inner diameter with a film thickness of the stationary phase
of 0.25 µm, while the carrier gas was nitrogen (0.5 bar as flow
rate).
GC-MS were recorded with a Varian Saturn GC/MS 4D
coupled with a Varian 3400 CX gas chromatography apparatus.
Solvents were used without purification when the degree of
purity was high enough or distilled according to the appro-
priate procedure.
ؒ
Rprim ϩ 1
1 × 103
7.6 × 103
1.1 × 103
1.5 × 105
4.9 × 105
6.6 × 105
12
ؒ
Me ϩ 1-alkene
12
ؒ
t-Bu ϩ 1-alkene
ؒ
Rprim ϩ 2
12
ؒ
Me ϩ MMA
12
ؒ
t-Bu ϩ MMA
double bond, one can see in Table 3 that the values of the rate
constants of addition are of the same order of magnitude at, or
near, room temperature. The same conclusion can be drawn
from the comparison involving methacrylic derivatives and
alkyl radicals (Table 3). Similar values are also obtained at
about 70 ЊC for the rate constants of addition of hex-5-enyl
radicals to methacrylic acid9 and 2 (about 7 × 105 l molϪ1 sϪ1).
These results do not agree with the experimental results.5
Indeed, in the course of the reaction of allylstannanes with
bromoalkanes, despite the existence of such a competition, the
allylated derivatives were produced with fair yields.5 According
to Curran et al.,7 allylstannanes must be at least one order of
magnitude more reactive toward alkyl radicals than simple
alkenes to explain the experimental results. The good agree-
ment between the various determinations of the rate constants
of addition of primary alkyl radicals to the different allyl-
stannanes is in favour of the validity of these measurements by
clock reactions.
Syntheses
Allyldi-n-butyl-4,7,10-trioxaundecylstannane 1 and (2-ethoxy-
carbonylprop-2-enyl)di-n-butyl-4,7,10-trioxaundecylstannane 2
were prepared according to a procedure described previously.5
(2-Bromo-1,1-dimethylethyl)benzene was synthesized from
the commercially available chloride via the reaction of its
Grignard reagent with bromine. 6-Bromohex-1-ene was pur-
chased from Aldrich. Bromomethylcyclopentane was prepared
from cyclopentylmethanol with phosphorus tribromide.
5-Methyl-5-phenylhex-1-ene 3 was obtained by condensation
of allylbromide with the Grignard reagent made from 1-chloro-
2-methyl-2-phenylpropane.13
4,4-Dimethyl-5-phenylpent-1-ene 4 was prepared according
to a literature procedure.14
Estimation of the rate constant at different temperatures can
readily be made using the Arrhenius equation. Fischer and
Radom12 have suggested a value for log (A/l molϪ1 sϪ1) of 8.5
0.5 in the addition of primary alkyl radicals to a terminal
The free radical reaction of (2-ethoxycarbonylprop-2-enyl)-
di-n-butyl-4,7,10-trioxaundecylstannane (1.8 g, 3.55 mmol)
and bromomethylcyclopentane (0.5 g, 3.06 mmol), initiated
by BEt3–O2 at room temperature, produced ethyl 2-(2-cyclo-
double bond. The values of log (A/l molϪ1 Ϫ1) obtained in the
s
present work, respectively 10.18 and 9.19, are quite a way from
this recommended value. The determination of the activation
parameters of a reaction by the measurement of rate constants
on a relatively small domain of temperature could lead to
erroneous results, even with relatively accurate values of the
rate constants, which could be the case here. Therefore, we
decided to reconsider the treatment of our results (values of
pentylethyl)prop-2-enoate
6 using a procedure described
previously.5 The product was isolated after chromatography
over silica (petroleum ether–ether 95 : 5) with a yield of
60%. This compound was previously prepared in a different
way.15
Tables 1 and 2) including this value [1/T = 0, log (k/l molϪ1 sϪ1
)
= log (A/l molϪ1 sϪ1) = 8.5] in the linear regression of log ka and
log kb vs. 1/T . On the basis of this analysis, we would like to
recommend the following Arrhenius relationships to estimate
the rate constant of the addition of primary alkyl radicals to 1
and 2:
log (ka/l molϪ1 sϪ1) = 8.6 0.9 Ϫ
1H NMR δ (250 MHz, CDCl3, Me4Si): 6.1 and 5.4 (2s, 2 H,
C9H2), 4.2 (q, J 7.1, 2 H, C11H2), 2.3 (t, J 7.0, 2 H, C7H2), 1.2
(t, J 7.1, 3 H, C12H3), 1.8–0.7 (m, 11 H, cycle H and C6H2);
13C NMR δ (CDCl3): 167.3 (C10), 141.3 (C8), 123.8 (C9), 60.4
(C11), 39.7 (C5), 25.1, 31.0, 32.5 and 34.9 (C1, C2, C3, C4, C6 and
C7), 14.1 (C12).
(32.1 5.9)/θ with θ = 2.303RT in kJ molϪ1
log (kb/l molϪ1 sϪ1) = 8.5 0.3 Ϫ
(19.1 2.6)/θ with θ = 2.303RT in kJ molϪ1
Conclusion
Ethyl 2-(cyclohexylmethyl)prop-2-enoate 7 was prepared
The replacement of a butyl by a 4,7,10-trioxaundecyl group on
the tin atom of an allyltributylstannane does not significantly
change the rate of addition of a primary alkyl radical to a
double bond. Therefore, compounds 1 and 2 should be useful
in the replacement of the halogen atom in alkyl bromide
by prop-2-enyl and 2-ethoxycarbonylprop-2-enyl entities,
affording a facile separation of the pure organic products
from the stannic compounds. The knowledge of the rate of
the addition reactions to the double bonds of these stannanes
will allow integration of these sequences in cascade synthetic
reactions.
earlier by our group.16
Synthesis of ethyl 2-methylenenon-8-enoate 5 was achieved
by the reaction of (2-ethoxycarbonylprop-2-enyl)di-n-butyl-
4,7,10-trioxaundecylstannane 2 used in excess with 6-bromo-
hex-1-ene according to the following procedure.
In a flask containing (2-ethoxycarbonylprop-2-enyl)di-n-
butyl-4,7,10-trioxaundecylstannane 2 (3 g, 5.91 mmol), 6-
bromohex-1-ene (0.19 g, 1.18 mmol) in 2 ml of anhydrous
benzene and 2 ml of a molar solution of triethylborane in
hexane were simultaneously added over 3 hours using two
syringe pumps. Chromatography over a silica column (petrol-
eum ether–ether 98 : 2) did not allow separation of ethyl 2-
methylenenon-8-enoate 5, the major product, from ethyl 2-
(2-cyclopentylethyl)prop-2-enoate 6. The former was identified
by GC-MS: m/z 123 ([CH ᎐CH(CH ) C᎐CH ]ϩ, 90%), 95
Experimental
1H NMR spectra were recorded at 250 MHz in CDCl3 solutions
with Si(CH3)4 as an internal standard using a BRUKER AC
᎐
᎐
2
2
5
2
J. Chem. Soc., Perkin Trans. 2, 2002, 247–250
249

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([CH ᎐CH(CH ) C᎐CH ]ϩ, 34%), 81 ([CH ᎐CH(CH ) CH᎐
᎐
᎐
᎐
᎐
References
2
2
3
2
2
2 2
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Kinetic studies
The kinetic studies were performed according to the following
procedure.
A solution of 0.5 mmol of (2-bromo-1,1-dimethylethyl)-
benzene, 0.05 mmol of AIBN and 0.35 mmol of a suitable
GC reference in anhydrous benzene was made up to 5 ml
in a volumetric flask and a sample of 1 ml was placed in a
Schlenck tube. A 0.1 M solution of 1 in anhydrous benzene
was similarly prepared, and a 1.0 ml sample in a small tube
was carefully placed in the Schlenck tube. The contents of
the Schlenck tube were degassed by three freeze–pump–thaw
cycles, heated to the desired temperature for 5 min and then
mixed. After a suitable time (10–60 min) the mixture was
analysed by GC.
The activation parameters of the addition of primary alkyl
radicals to the double bonds of 1 and 2 were obtained from
the linear regression of log k vs. 1/T using EXCEL 2000 for the
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Acknowledgements
The authors acknowledge Professor H. Fischer for communi-
cation of a paper12 prior to publication and for valuable
advice.
250
J. Chem. Soc., Perkin Trans. 2, 2002, 247–250