Study of the time-resolved IR spectrum of α-arylvinyl cations generated by laser flash photolysis

Article abstract of DOI:10.1246/cl.2003.294

The characteristic stretching band of the C(sp²)=C(sp)⁺ moiety in vinyl cations was observed for the first time around 1870 cm^-1 by a time-resolved IR spectrometry with 266 nm or 308 nm laser flash irradiation of (4-methoxyphenyl)alkynes in TFE and α-arylvinyl bromide in acetonitrile. The ultraviolet-visible and the IR absorption maxima were affected by the β-substituents.

Full text of DOI:10.1246/cl.2003.294

294
Chemistry Letters Vol.32, No.3 (2003)
Study of the Time-resolved IR Spectrum of ꢀ-Arylvinyl Cations Generated by
Laser Flash Photolysis
Masashi Kotani, Shinjiro Kobayashi,ꢀ Masaaki Mishima, and Yuji Horiy
Institute for Fundamental Research of Organic Chemistry, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
yDepartment of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo, Saga 840-8502
(Received November 25, 2002; CL-021003)
The characteristic stretching band of the C(sp2)=C(sp)þ
moiety in vinyl cations was observed for the first time around
12
1870 cmꢁ1 by a time-resolved IR spectrometry with 266 nm or
308 nm laser flash irradiation of (4-methoxyphenyl)alkynes in
10
8
TFE and ꢀ-arylvinyl bromide in acetonitrile. The ultraviolet-
visible and the IR absorption maxima were affected by the ꢁ-
substituents.
6
4
2
Laser flash photolysis (LFP) have been shown to be one of the
most useful methods for the observation of reactive transient
species and for the direct measurement of the rate constants of
0
their decay under the reaction conditions. Most detection methods
involve the measurement of the electronic spectrum of the
transient species. Vinyl cations generated by photoprotonation of
arylalkynes or by photolysis of haloalkenes have been detected by
the UV-vis spectrometry1{9 but time-resolved IR spectrometry,
which was recently developed, had not been used yet for
measuring the IR spectra of vinyl cations.
We report here the first example of measurement of time-
resolved IR spectra of vinyl cations. The transient species were
generated by applying the nanosecond laser flash of Nd:Yag
(266 nm) or Excimer (308 nm) to a solution of arylalkynes 1 in
2,2,2-trifluoroethanol (TFE) or to a solution of bromoalkenes 3 in
acetonitrile (Scheme 1). The samples had flown into a calcium
fluoride cell at room temperature and the generated transient
species were detected by using the time-resolved IR spectro-
meter.10
-2
1896
1904
wavenumber / cm
1912
1920
-1
Figure 1. Time-resolved IR spectra of 1-(4-
methoxyphenyl)vinyl cation (2a) generated
by laser flash photoprotonation of 1a in TFE.
The spectra are the averages of data collected
every 400 ns.
methyl-substituted vinyl cation 2b have an absorption maximum
at 1903 cmꢁ1, a closely similar position to that of the ꢁ-
unsubstituted vinyl cation 2a. In the case of the laser flash
photoprotonation of 1-(4-methoxyphenyl)-2-phenylethyne (1c),
the transient species has an absorption maximum at 1885 cmꢁ1
which could be assigned to 1-(4-methoxyphenyl)-2-phenylvinyl
cation 2c which has a UV-vis absorption maximum at 342 nm and
is more stable than its 2-(4-methoxyphenyl)-1-phenyl cation
,
R
C
H
TFE
1.2
1.0
An
C
C
C
R
An
Ar
C
LFP
1
2
Ph
Ph
Ar
Br
Ph
MeCN
LFP
C
C
C
0.8
0.6
0.4
0.2
0
Ph
3
4
Scheme 1.
In the case of 4-methoxyphenylethyne (1a) in TFE, a
transient species was detected at 1914 cmꢁ1 (Figure 1). Since it
is known that the 1-(4-methoxyphenyl)vinyl cation (2a) was
generated and detected at 328 nm by 266 nm laser flash irradiation
of 1a in TFE4;9 the transient species which has an absorption
maximum at 1914 cmꢁ1 could be assigned to the vinyl cation 2a.
Similarly, a 266 nm laser flash irradiation of 1-(4-methoxyphe-
nyl)prop-1-yne (1b) in TFE gave the 1-(4-methoxyphenyl)-2-
methylvinyl cation (2b) as the only transient species with an
absorption maximum at 333 nm in the UV-vis spectrum.11 The ꢁ-
1840
1860
wavenumber / cm
1880
1900
-1
Figure 2. Time-resolved IR spectra 1-(4-methox-
yphenyl)-2,2-diphenyl vinyl cation (2a) generated
by laser flash photolysis of triarylbromoethene 3a
in acetonitrile. The spectra are the averages of data
collected every 400 ns.
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.3 (2003)
295
Table 1. IR absorption maxima of the vinyl cations
R2
R1
C C
R3
Precursor
Cation
R1
R2
R3
H
H H
H
Detected/cmꢁ1
Calculated/cmꢁ1 a
1a
1b
1c
3
2a
2b
2c
a
b
c
CH3O
CH3O
CH3O
C4Ha3O
C4Hb3
H
C
Ph
Ph
Ph
Ph
1914b
1903b
1885b
1868c
1862c
1867c
1945
1937
1901
1871
1857
1868
3
Ph
Ph
Ph
3
3
C4Hc3S
a
Calculated at the B3LYP/6-31Gꢀ level.
Detected in TFE.
Detected in acetonitrile.
b
c
isomer.
References and Notes
The laser flash photolysis of triarylbromoethenes 3 gave
transient species which were clearly detected in the IR spectrum.
It was reported that the laser flash photolysis of 3a in acetonitrile
gave the 1-(4-methoxyphenyl)-2,2-diphenyl cation with an
absorption maximum at 350 nm as the only detectable transient
species.8 Therefore, the generated transient species which display
an absorption maximum at 1868 cmꢁ1 could be assigned to the
vinyl cation 4a (Figure 2). Similarly, the laser flash photolysis of
1-bromo-1-(4-methylphenyl)-2,2-diphenylethene (3b) gave the
1-(4-methylphenyl)-2,2-diphenylvinyl cation (4d) with an IR
maximum absorption at 1862 cmꢁ1 and 1-(4-methylsulfanylphe-
nyl)-2,2-diphenylvinyl cation (4c) was generated from 1-bromo-
1-(4-methylsulfanylphenyl)-2,2-diphenylethene (3c) which was
1
2
3
4
5
6
7
8
9
W. Schnabel, I. Naito, T. Kitamura, S. Kobayashi, and H.
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detected at 1867 cmꢁ1
.
The decay rate constant of 4c measured by following the
change in its IR absorption with time was 7:1 ꢂ 105 Mꢁ1sꢁ1. This
value is reasonably close to the value measured by the UV-vis
spectrophotometry (9:3 ꢂ 105 Mꢁ1sꢁ1).12 The stretching bands
of the C(sp2)=C(sp)þ bond of vinyl cations detected and
calculated at the B3LYP/6-31Gꢀ(scale factor 0.9613) level are
summarized in Table 1. The calculated values are in relatively
good agreement with the observed values.13
We conclude that the vinyl cations 2 and 4 generated by the
laser flash photolysis of 4-methoxyphenylalkynes 1 or of
triarylbromoethenes 3 could be detected by their time-resolved
IR spectra. The absorption maxima of vinyl cations are affected
by the conjugation with the ꢁ-substituents.
M. Kotani, S. Kobayashi, and J.-A. Chang, J. Phys. Org.
Chem., 15, 863 (2002).
10 TRIR1000 in JASCO Co. Cf. T. Yuzawa, C. Kato, M. W.
George, and H. Hamaguchi, Appl. Spectrosc., 48, 684 (1994).
11 The same vinyl cation was detected at 333 nm by 266 nm
irradiation of 1-chloro-1-(4-methoxyphenyl)prop-1-ene in
TFE.
12 The decay rate constants were measured in the presence of
1 M of ethanol as the nucleophile in TFE.
13 The stretching bands were calculated with Gaussian 98,
Revision A.5.