Determination of the rate constant for ring opening of an a-cyclopropylvinyl radical

Article abstract of DOI:10.1039/b412676a

The rate constant for ring opening of the l-(trans-2-phenylcyclopropyl)ethen-l-yl radical, 4, generated by photolysis of the corresponding vinyl iodide 2, is reported. The value of the rate constant was determined by the tin hydride method and was found t

Full text of DOI:10.1039/b412676a

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A R T I C L E
Determination of the rate constant for ring opening of an
a-cyclopropylvinyl radical
Kaarina K. Milnes, Stephen E. Gottschling and Kim M. Baines*
Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7.
E-mail: kbaines2@uwo.ca
Received 16th August 2004, Accepted 29th September 2004
First published as an Advance Article on the web 5th November 2004
The rate constant for ring opening of the 1-(trans-2-phenylcyclopropyl)ethen-1-yl radical, 4, generated by photolysis
of the corresponding vinyl iodide 2, is reported. The value of the rate constant was determined by the tin hydride
method and was found to be (1.6 0.2) × 1010 s−1, one order of magnitude smaller than the rate constant for
rearrangement of the trans-2-phenylcyclopropylcarbinyl radical.
Introduction
Cyclopropylcarbinyl radical rearrangements have been utilized
as both mechanistic probes and radical clocks for over two
decades.1 The success of these radical probes can be attributed,
for the most part, to the large and precisely determined rate
constants for rearrangement.2 A particularly useful derivative of
these probes contains a phenyl substituent at the 2-position on
the cyclopropyl ring. The introduction of the phenyl substituent
results in an increase in the rate of rearrangement by three
orders of magnitude.3 These hypersensitive radical probes have
been effectively utilized in a variety of mechanistic investiga-
Scheme 1
tions including studies of the mechanisms of enzyme-catalyzed
hydroxylation reactions,4 of Co(III)-catalyzed autoxidation of
alkenes with Et3SiH,5 and of the addition of aldehydes to Group
14 dimetallenes.6
yield as determined by 1H NMR spectroscopy (Scheme 2). The
1H and 13C NMR chemical shifts and the IR spectral data
of the allene agreed well with the literature values.9,10 trans-2-
Phenylcyclopropylethene, 5, was also detected in the reaction
mixture by gas chromatography (ratio of 3/5 = 43 : 1). The
alkene was identified by co-injection with an authentic sample
prepared by a Wittig reaction of the corresponding aldehyde.11
No evidence for the formation of the regioisomeric allene, 4-
phenylpenta-1,2-diene, was found.
In contrast to the well known cyclopropylcarbinyl radical
system, there are only three reports describing the reactivity of
the analogous a-cyclopropylvinyl radicals.7 The results suggest,
at least qualitatively, that the rate constant of ring opening of
the vinyl radical is slower than the carbinyl analog. Given the
importance of cyclopropylcarbinyl radical rearrangements as
well as the widespread occurrence of vinyl radicals in organic
chemistry, we believe that the corresponding cyclopropylvinyl
radical rearrangement may also be of general interest as a
mechanistic probe. To act effectively as a probe, the rearrange-
ment must compete with potentially very fast pseudo-first-order
processes, and thus, a large rate constant for rearrangement is
necessary. With the aim of developing a hypersensitive probe for
vinyl radicals and to evaluate its potential, we have determined
the rate constant for the rearrangement of the 1-(trans-2-
phenylcyclopropyl)ethen-1-yl radical, 4, using an indirect kinetic
method. Our results are reported herein.
Scheme 2
We employed the tin hydride competition method, outlined
in Scheme 3, to determine the rate constant for rearrangement,
kR, of the a-cyclopropylvinyl radical 4. The rate constant for the
competition reaction, kH, abstraction of a hydrogen atom from
Bu3SnH by radical 4, is not known, and thus it was necessary to
use a comparable value from the literature. The rate constant for
reaction of the 2,2-dimethylvinyl radical, formed by photolysis
of the corresponding diacyl peroxide, with tributyltin hydride
has been reported12 and appears to be an appropriate choice.
However, it was later demonstrated that decarboxylation of the
related aroyloxy radicals is a relatively slow process,13 and thus
the actual rate constant reported for the reaction of the 2,2-
dimethylvinyl radical with Bu3SnH is likely the rate constant
for reaction of the vinyl-substituted carboxyl radical instead.14
There are no other reports concerning the rate constants for the
reaction of a vinyl radical with a tin hydride; however, the rate
constant for the reaction of aryl radicals with Bu3SnH has been
determined using two independent methods;13 the recommended
value is 7.8 × 108 M−1s−1. Given that the vinyl and the phenyl
radicals are both r-radicals located at an sp2-hybridized carbon,
Results and discussion
The most appropriate precursor for generation of the radical 4
was determined to be vinyl iodide 2. Vinyl iodide 2 was easily
synthesized by iodinolysis of 1-(trans-2-phenylcyclopropyl)-1-
tributylstannylethene, 1, which was prepared by hydrostan-
nylation of trans-2-phenylcyclopropylethyne8 (Scheme 1). The
hydrostannylation reaction yields
a mixture of the de-
sired stannylethene 1 and trans-2-(trans-2-phenylcyclopropyl)-1-
tributylstannylethene. Upon chromatography, the latter product
undergoes a hydrodestannylation reaction forming trans-2-
phenylcyclopropylethene, 5. It was difficult to remove all traces
of 5 from the final product.
Irradiation of 1-iodo-(trans-2-phenylcyclopropyl)ethene, 2, at
350 nm in the presence of one equivalent of Bu3SnH resulted
in the formation of 5-phenylpenta-1,2-diene, 3, in quantitative
3 5 3 0
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 5 3 0 – 3 5 3 4
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
©

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we, and others,14 believe that the rate constant for the reaction
of a vinyl radical with Bu3SnH should be very similar to that
of an aryl radical. To conclude that radical 4 is a reasonable
model for the phenyl radical, it is necessary to assume that the a-
phenylcyclopropyl substituent will not greatly influence the rate
constant for the abstraction of a hydrogen atom by the r-radical.
The available evidence suggests that the analogous assumption
is valid in the related cyclopropylcarbinyl system: that is, that
the cyclopropylcarbinyl radical is a reasonable model for the
primary alkyl radical in fast trapping reactions.15 Finally, we
have also assumed that solvent effects on these radical reactions
will be minimal. With these assumptions, we have utilized 7.8 ×
108 M−1s−1 as the rate constant for the competition reaction, kH,
in this study. The error in the basis reaction was incorporated
into the precision calculations of this study.
order of magnitude of that reported for the rearrangement of the
trans-2-phenylcyclopropylcarbinyl radical (3 × 1011 s−1).3 The
plotted data have a non-zero intercept indicating that the ring
opening rearrangement of vinyl radical 4 to benzyl radical 6 is
reversible.2a As such, one would expect to observe the formation
of cis-2-phenylcyclopropylethene due to non-stereoselective ring
closure of benzyl radical 6. No evidence for the formation of cis-
2-phenylcyclopropylethene was observed during the course of
these studies as noted by the comparison of GC retention times
with an authentic sample.11 The reversibility of the ring opening
rearrangement of 4 to 6 does not affect the rate constant of the
forward reaction. The rate constant of the reverse reaction was
estimated to be 1.6 × 104 s−1.2a
Incorporation of an alkoxy group at the unsubstituted
carbon of phenyl-substituted cyclopropylcarbinyl probes allows
for the discrimination between the formation of radical and
cationic intermediates.17 The alkoxy group does not signifi-
cantly influence the rate constant for rearrangement of the
radical.3b To investigate the influence of a 3-alkoxy substituent
on the rate constant for rearrangement for ring-opening of
radical 4, the preparation of 1-iodo-1-(trans,trans-2-methoxy-
3-phenylcyclopropyl)ethene, 8, was attempted. Stannylethene 7
was prepared as described above for stannylethene 1; however,
iodinolysis of 7 only gave a low yield of impure vinyl iodide
8 (Scheme 4). The major product formed is a diastereomeric
mixture of allene 9. Iodide 8 is likely the result of tributylio-
dostannane elimination from the putative b-stannyl cation.
Presumably, the methoxy substituent provides anchimeric as-
sistance which results in concurrent rearrangement of the
intermediate cation to give an oxonium ion which is trapped by
pyridine. Subsequent elimination of tributyliodostannane would
result in the formation of allene 9. Thus, we were unable to
synthesize the iodide precursor in the required amounts and
purity to complete the kinetic study on this molecule.
Photolyses of 1-iodo-(trans-2-phenylcyclopropyl)ethene, 2,
were performed at 350 nm in cyclohexane in the presence of
Bu3SnH (1 M to 2 M) at 20 ◦C. All runs proceeded to 100%
conversion; the reactions were extremely clean; only allene 3 and
alkene 5 were detected as products. The identities of the products
were confirmed by comparison of GC retention times or by
coinjection of authentic samples. During the course of these
studies, it was found that the injector temperature of the GC
◦
must be kept well below 200 C since at elevated temperatures
trans-2-phenylcyclopropylethene, 5, is in equilibrium with its
cis-isomer.16 Furthermore, the trans-isomer also rearranges at
elevated temperatures to give 4-phenylcyclopentene.16 To avoid
the formation of these compounds, the GC injector temperature
was maintained at 100 ◦C.
Scheme 3
According to the kinetic expression, ([5]/[3]) = (kH/kR)
[Bu3SnH]avg , the ratio of the unrearranged to the rearranged
products, [5]/[3], and the concentration of Bu3SnH provide
the ratio of the rate constants for the direct trapping, kH,
and the ring opening, kR, reactions; the results are presented
graphically in Fig. 1. Given that kH is 7.8 × 108 M−1s−1 (vide
supra), the rate constant for the rearrangement of the 1-(trans-
2-phenylcyclopropyl)ethen-1-yl radical, 4, kR, was found to be
(1.6 0.2) × 1010 s−1. The error includes the competition reaction
rate constant error13 as well as the random error of the method.
The value of the rate constant for ring-opening is within an
Scheme 4
Conclusions
We have determined the rate constant for rearrangement of
the 1-(trans-2-phenylcyclopropyl)ethen-1-yl radical, 4. Based on
the qualitative behavior of a-cyclopropylvinyl radicals,7 it was
expected that the value of the rate constant for the vinyl radical
rearrangement would be less than that of the analogous carbinyl
system; however, it is only one order of magnitude smaller. We
believe this very fast radical rearrangement will find general use
by organic, bioorganic, organometallic and surface chemists as
a mechanistic probe.
Experimental
Reactions were performed under ambient conditions un-
less otherwise noted. THF and CH2Cl2 were purged with
N2 and passed through an alumina column prior to use.
Cyclohexane was distilled from LiAlH4, and then purged
with Ar prior to use. Tributyltin hydride was purchased
Fig. 1 Plot of the ratio of [5]/[3] vs. [Bu3SnH]avg at 20 ◦C.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 5 3 0 – 3 5 3 4
3 5 3 1

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from Aldrich Chemical Co. trans-2-Phenylcyclopropylethyne8
and trans-2-phenylcyclopropylethene11 were prepared accord-
ing to the literature procedures. A mixture of cis/trans-2-
phenylcyclopropylethene was prepared by initial formation of
the isomeric ethyl 2-phenylcyclopropylcarboxylates by CuSO4-
catalyzed cyclopropanation of styrene with ethyl diazoacetate,
reduction of the esters to the alcohols using LiAlH4 and
oxidation to the aldehydes using Swern oxidation conditions.
The mixture of aldehydes was then converted to the alkenes
Synthesis of 1-(trans,trans-2-methoxy-3-phenylcyclopropyl)-
1-tributylstannylethene (7)
A solution of tributyltin hydride (2.8 mL, 10.5 mmol) dis-
solved in THF (20 mL) was added dropwise to a solution
of trans,trans-2-methoxy-1-methyl-3-phenylcyclopropylethyne18
(1.80 g, 10.5 mmol) and Cl2Pd(PPh3)2 (152 mg, 0.2 mmol) in
THF (35 mL). After stirring at RT for 15 min, the solution was
dark brown in colour. The solvent was removed to yield a dark
brown oil. A mixture of stannylethene 7 and trans-2-(trans,trans-
2-methoxy-3-phenylcyclopropyl)-1-tributylstannylethene (70 :
30) was obtained as determined by 1H NMR spectroscopic
analysis. Chromatographic separation (silica gel, 1 : 1 hexanes–
CH2Cl2) of the crude reaction mixture yielded stannylethene 7 as
a clear, colourless oil (3.1 g, 63%). mmax(film)/cm−1 2955 s, 2925 s,
1603 m, 1496 m, 1460 m, 1219 m, 697 s; dH (400 MHz, C6D6) 7.35
1
by a Wittig reaction. H and 13C NMR spectral data of cis-2-
phenylcyclopropylethene agreed well with the literature values.11
NMR spectra were recorded on Varian Mercury 400 or
Varian Inova 400 spectrometers. The standards used were as
follows: residual C6D5H (7.15 ppm), residual CHCl3 (7.24 ppm),
residual CDHCl2 (5.32 ppm) for 1H NMR spectra; C6D6 central
transition (128.00 ppm), CDCl3 central transition (77.00 ppm)
for 13C NMR spectra. J values are given in Hz. Mass spectra
were obtained on a Finnegan MAT model 8400 mass spec-
trometer with an ionizing voltage of 70 eV (reported in mass-
to-charge units, m/z, with ion identity and intensities relative
to the base peak in parentheses). IR spectra were recorded
(cm−1) on a Perkin-Elmer System 2000 FT IR spectrometer.
Ultraviolet spectra were recorded on a Varian Cary 100 UV-
Visible Spectrophotometer. Gas chromatographic analyses were
performed on a dimethylpolysiloxane column on an Agilent
6850 Series GC. Photolyses were performed in a Rayonet
Photochemical Reactor. The temperature within the reactor
was maintained at 20 ◦C using an Endocal model ULT-70 low
temperature external bath circulator to force cooled (20 ◦C)
methanol through a vacuum-jacketed Pyrex immersion well.
(2H, d, o-PhH, J 7.4), 7.18 (2H, t, m-PhH, J 7.4), 7.06 (1H, t,
3
=
119
p-PhH, J 7.2), 5.71 (1H, br t, C C(H)Htrans, J 1.8, J Sn–H 137,
J
3
3
3
=
117
119
117
Sn–H 131), 5.26 (1H, d, C C(H)Hcis, J 2.0, J Sn–H 64, J
60), 3.42 (1H, dd, CHOMe, J 6.6, 3.5), 2.98 (3H, s, OMe), 2.39
(1H, dd, C(H)C(Sn) C, J 6.6, 3.5), 2.16 (1H, t, C(H)Ph, J 6.6),
1.56-1.61 (2H, m, SnCH2CH2), 1.34 (2H, tq, SnCH2CH2CH2,
J 7.6, 7.3), 0.99-1.02 (2H, m, SnCH2), 0.90 (3H, t, CH2CH3, J
7.2); dC (100 MHz, C6D6) 152.8 (SnC C) 138.2 (i-PhC), 128.3,
128.2 (o,m-PhC), 125.9 (p-PhC), 123.0 (SnC C), 67.7 (COMe),
Sn–H
=
=
=
3
=
119/117
57.8 (OMe), 36.2 (CC C), 32.9 (C(H)Ph, J
15), 29.5
119
Sn–C
3
2
119/117
(SnCH2CH2, J
21), 27.8 (SnCH2CH2CH2, J
57,
Sn–C
Sn–C
Sn–C
3
1
117
119
J
54), 13.9 (SnCH2CH2CH2CH3), 9.9 (SnCH2, J
Sn–C
1
330, J Sn–C 315); m/z (CI) 464 (M+, 1.2%), 432 (M+ − MeOH,
3.5), 407 (M+− Bu, 17), 291 (Bu3Sn+, 91), 265 (50), 235 (40),
179 (45), 142 (M+− Bu3Sn-OMe, 100); high resolution MS (CI)
for C24H40O120Sn (M+) calcd 464.2101, found 464.2104.
117
Synthesis of 1-(trans-2-phenylcyclopropyl)-1-
tributylstannylethene (1)
Synthesis of 1-iodo-1-(trans-2-phenylcyclopropyl)ethene (2)
A solution of tributyltin hydride (18.1 mL, 67 mmol) dis-
solved in THF (50 mL) was added dropwise to a solu-
tion of trans-2-phenylcyclopropylethyne (9.6 g, 67 mmol)
and Cl2Pd(PPh3)2 (0.94 g, 1.34 mmol) dissolved in THF
(150 mL). Upon complete addition, the reaction mixture
appeared opaque and brown. The solvent was removed by
rotary evaporation yielding a brown oil consisting of a mixture
of stannylethene 1, and trans-2-(trans-2-phenylcyclopropyl)-1-
tributylstannylethene (54 : 46) as determined by 1H NMR
spectroscopic analysis. Upon column chromatography (silica
gel, 9 : 1 hexanes–CH2Cl2) trans-2-(trans-2-phenylcyclopropyl)-
1-tributylstannylethene undergoes hydrodestannylation yielding
1-(trans-2-phenylcyclopropyl)ethene, 5. Minor amounts of 5
were always present in the final product. Stannylethene 1
was obtained as a clear colourless oil (5.89 g, 20% yield).
mmax(film)/cm−1 2956 s, 2926 s, 2870 m, 2852 m, 1603 m,
1501 m, 1460 m, 782 m, 695 m; dH (400 MHz, C6D6) 7.09–
7.14 (2H, m, m-PhH), 6.99–7.02 (1H, m, p-PhH), 6.97-6.99
A solution of I2 (1.41 g, 5.55 mmol) in CH2Cl2 (200 mL)
was added dropwise to a solution of stannylethene 1 (2.41 g,
5.55 mmol) and pyridine (2 mL, 25 mmol) in CH2Cl2 (50 mL).
After addition, the reaction mixture was allowed to stir for
0.5 h and was then quenched with a saturated solution of
Na2S2O3. The layers were separated and the aqueous layer was
extracted with CH2Cl2 (3 × 20 mL). The combined organic layers
were dried over MgSO4, the solids were removed by gravity
filtration and the solvent was removed by rotary evaporation
yielding a yellow oil. Hexanes were added to the crude product.
The salts were removed by gravity filtration and the hexanes
removed from the filtrate by rotary evaporation. The product
was dissolved in EtOAc (20 mL) and an aqueous solution of
excess KF was added. The biphasic solution was allowed to stir
vigorously for 18 h. The layers were separated and the aqueous
layer was extracted with EtOAc (3 × 20 mL). The combined
organic layers were dried over MgSO4, the solids were removed
by gravity filtration and the solvent was removed by rotary
evaporation. The product was triturated with CH3CN to remove
any remaining KF. The product was further purified by column
chromatography (silica gel, 8 : 2 hexanes–CH2Cl2) yielding 1-
iodo-1-(trans-2-phenylcyclopropyl)ethene, 2, as a clear, colorless
oil (0.78 g, 52%, 96% by GC). The amount of 1-(trans-2-
phenylcyclopropyl)ethene, 5, present was determined to be 1%
by GC. kmax (cyclohexane)/nm 219 (e/dm3mol−1cm−1 8520);
mmax(film)/cm−1 3084 w, 3064 w, 3025 w, 3002 w, 2924 w, 2847
w, 1604 s, 1495 s, 1456 m, 1196 m, 1169 m, 1095 s, 1076 m, 885
m, 750 s, 695 s; dH (400 MHz, C6D6) 7.03-7.08 (2H, m, m-PhH),
6.97-7.02 (1H, m, p-PhH), 6.81-6.85 (2H, m, o-PhH), 5.71 (1H,
=
(2H, m, o-PhH), 5.78 (1H, dd, C C(H)Htrans, J 1.8, 1.2,
3
3
3
=
119
117
J
J
138,
64,
J
132), 5.25 (1H, d, C C(H)Hcis, J 2.4,
60), 1.92 (1H, t, C(H)C(Sn) C, J 7.2),
Sn–H
Sn–H
3
=
119
117
J
Sn–H
Sn–H
1.92 (1H, t, C(H)Ph, J 7.2), 1.52-1.60 (6H, m, SnCH2CH2),
1.33 (6H, tq, SnCH2CH2CH2, J 7.2, 7.2), 1.26-1.30 (1H, m,
CH2cycloprop), 1.04-1.10 (1H, m, CH2cycloprop), 0.96-1.00 (6H, m,
SnCH2, J 7.2), 0.89 (9H, t, SnCH2CH2CH2CH3, J 7.4); dC
(100MHz, C6D6) 155.0(SnC C), 143.2(i-PhC), 128.6 (m-PhC),
125.9 (o-PhC), 125.8 (p-PhC), 122.6 (SnC C, J
=
2
=
119/117
29),
20),
Sn–C
2
2
=
119/117
119/117
3
33.2 (CC C, J
27.8 (SnCH2CH2CH2, J
11), 16.9 (CH2,
10.0 (SnCH2, J
50), 29.5 (SnCH2CH2, J
Sn–C
3
Sn–C
119/117
119/117
55), 26.9 (C(H)Ph, J
17), 13.9 (SnCH2CH2CH2CH3),
Sn–C
Sn–C
3
=
=
119/117
J
br t, C C(H)Htrans, J 1.2), 5.54 (1H, dd, C C(H)Hcis, J 1.6,
0.4), 1.99 (1H, ddd, C(H)Ph, J 4.4, 6.0, 9.2), 1.50-1.56 (1H,
Sn–C
1
1
119
117
329, J
314); m/z (CI) 435 (M +
Sn–C
Sn–C
H+, 1.3%), 377 (C19H29120Sn (M+− Bu), 94), 291 (C12H27120Sn
(Bu3Sn), 100), 235 (C8H18120Sn (Bu2Sn), 33), 177 (C4H9120Sn
(BuSn), 22), 128 (C10H8 (M+− Bu3Sn-CH3), 17), 91 (C7H7 (M+−
Bu3Sn-C4H4), 7); high resolution MS (CI) for C23H39120Sn (M +
H+) calcd 435.2073, found: 435.2070.
m, C(H)C(I) C), 1.06 (1H, ddd, CH2cycloprop, J 5.2, 5.6, 9.2),
=
0.86 (1H, ddd, CH2cycloprop, J 4.8, 6.0, 8.4); dC (100 MHz, C6D6)
141.0 (i-PhC), 128.6 (m-PhC), 126.5 (o-PhC), 126.3 (p-PhC),
=
=
=
123.9 (IC C), 113.4 (IC C), 35.6 (CC C), 27.2 (C(H)Ph),
18.3 (CH2); m/z (EI) 270 (M+, 19%), 192 (M+− C6H6, 6), 143
3 5 3 2
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 5 3 0 – 3 5 3 4

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(M+− 127I, 100), 128 (C10H8, 89), 115 (27). High resolution MS
Table 1 Ratio of 5/3 obtained at varying tributyltin hydride concen-
trations.
(EI) Calc. for C11H11127I (M+): 269.9865, found 269.9913.
Run
2/mmol
Vcy/mL
V
tot/mL
[Bu3SnH]avg /M
5/3
Iodinolysis of 1-(trans,trans-2-methoxy-3-phenylcyclopropyl)-
1-tributylstannylethene (7)
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
0.164
0.157
0.141
0.180
0.168
0.150
0.195
0.176
0.150
0.237
0.180
0.155
0.151
0.143
1.2
1.2
1.2
0.9
0.9
0.9
0.7
0.7
0.7
0.5
0.5
0.5
0.4
0.4
1.7
1.7
1.7
1.4
1.4
1.4
1.2
1.2
1.2
1.0
1.0
1.0
0.9
0.9
1.013
1.015
1.020
1.224
1.229
1.235
1.422
1.430
1.441
1.685
1.714
1.727
1.921
1.925
0.069
0.071
0.072
0.082
0.083
0.079
0.090
0.092
0.088
0.099
0.102
0.108
0.113
0.118
A solution of iodine (37 mg, 0.14 mmol) dissolved in CH2Cl2
(5 mL) was added to a solution of stannylethene 7 (54 mg,
0.12 mmol) and pyridine (49 lL, 0.60 mmol) dissolved in CH2Cl2
(5 mL). After 15 min, the solvent was removed by rotary evap-
oration to give an orange-brown oil. The oil was triturated with
benzene to yield a light brown solid identified as 1-(1-methoxy-
2-phenylpenta-3,4-dienyl)pyridinium iodide, 9, (38 mg, 87%).
The product contained minor amounts of tributyltin impurities
(∼9% as determined by 1H NMR spectroscopy).
The benzene extracts were concentrated by rotary evaporation
to yield a brown oil containing 1-iodo-1-(trans,trans-2-methoxy-
3-phenylcyclopropyl)ethene, 8, as well as significant amounts of
tributyliodostannane and other impurities. Attempts were made
to purify crude iodo compound 8 by chromatography (silica
gel, 2 : 1 hexanes–CH2Cl2); however, the desired compound
was never obtained cleanly or in any significant amount. 8: dH
(400 MHz, CD2Cl2): 7.2-7.3 (5H, m, o,m,p-PhH), 6.12 (1H, t,
irradiation the reaction had gone to completion. The reaction
mixture was analyzed by GC to determine the ratio of the
area of trans-2-phenylcyclopropylethene, 5, to the area of 5-
phenylpenta-1,2-diene, 3 (see Table 1). The precursor, 1-iodo-
1-(trans-2-phenylcyclopropyl)ethene, 2, was contaminated with
alkene 5, as determined by GC analysis; the ratio of alkene
5 to allene 3 was corrected appropriately. Three independent
experiments at each concentration were performed (with the
exception of run 5).
Allene 3 formed in the kinetic runs was identified by
comparison of its GC retention time. The identity of trans-2-
phenylcyclopropylethene, 5, was confirmed by co-injection of a
sample prepared by independent synthesis.11
The concentration of Bu3SnH was the average of the initial
and final concentrations of Bu3SnH (where the final concentra-
tion was calculated as [the initial amount of Bu3SnH (mmol)
minus the initial amount of 1-iodo-1-(trans-2-phenylcyclo-
propyl)ethene, 2, (mmol)] divided by the total volume used).
The GC response factors for 5-phenylpenta-1,2-diene, 3,
and trans-2-phenylcyclopropylethene, 5, were determined to be
equal.
=
=
C C(H)Htrans, J 1.8), 5.77 (1H, dd, C C(H)Hcis, J 1.8, 0.6),
3.48 (1H, dd, C(H)OMe, J 7.2, 3.0), 3.29 (3H, s, OMe), 2.41
=
(1H, t, C(H)C(I) C, J 6.6), 2.30-2.35 (1H, m, C(H)Ph). 9:
mmax(film)/cm−1 diastereomeric mixture: 3034 m, 2956 s, 2924 s,
2854 m, 1953 m, 1623 s, 1476 s, 1456 m, 1211 m, 1150 m,
1097 s, 1024 m, 865 m, 681 s. dH (400 MHz, CDCl3), major
diastereomer: 8.99 (2H, d, o-PyH, J 5.6), 8.63 (1H, t, p-PyH,
J 7.6), 8.00 (2H, t, m-PyH, J 6.7) 7.2-7.4 (5H, m, o,m,p-PhH),
= =
6.58 (1H, d, C(H)OMe, J 7.3), 5.56 (1H, q, HC C CH2, J
= =
6.7), 4.79 (1H, ddd, C C CH2, J 11.4, 6.7, 2.1), 4.60 (1H,
ddd, C C CH2, J 11.4, 6.7, 2.1), 3.79-3.87 (1H, m, C(H)Ph),
= =
3.65 (3H, s, OMe); minor diastereomer: 9.12 (2H, d, o-PyH,
J 5.9), 8.50 (1H, t, p-PyH, J 7.6), 8.11 (2H, t, m-PyH, J 7.0),
7.2–7.4 (5H, m, o,m,p-PhH), 6.77 (1H, d, C(H)OMe, J 6.7),
5.54 (1H, q, HC C CH2, J 6.7), 4.86 (1H, ddd, C C CH2,
J 11.4, 6.7, 2.3), 4.82 (1H, ddd, C C CH2, J 11.4, 6.7, 2.3),
= =
= =
= =
3.79–3.87 (1H, m, C(H)Ph), 3.54 (3H, s, OMe). dC (100 MHz,
CDCl3), diastereomeric mixture: 208.7, 208.2 (C C C), 147.2,
146.9 (p-PyC), 141.3, 141.2 (o-PyC), 135.5, 135.5 (i-PhC), 128.9,
128.8, 128.7, 128.3, 128.2, 128.1 (o,m,p-PhC), 127.9, 127.8 (m-
= =
Acknowledgements
We thank the University of Western Ontario, the NSERC
(Canada) and the Canadian Foundation of Innovation for
financial support.
= =
PyC), 101.2, 100.4 (C(H)OMe), 88.3, 87.6 (HC C CH2), 77.7,
= =
77.2 (HC C CH2), 59.4, 59.3 (OMe), 52.9, 52.6 (C(H)Ph). m/z
(ES) diastereomeric mixture: 631 (2M − I−, 6%), 252 (M − I−,
100), 173 (M − I−-C5H5N, 4). High resolution MS (ES): Calc.
for C17H18ON (M − I−): 252.1388, found 252.1394.
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