Evidence for a transition state model compound of in-plane vinylic S N2 reaction

Article abstract of DOI:10.1021/ol050961o

(Chemical Equation Presented) To isolate a transition state model compound of an in-plane vinylic S_N2 reaction, vinyl bromide 6 bearing a newly synthesized tridentate ligand derived from 1,8-dimethoxythioxanthen-9-one (5) was prepared as a prec

Full text of DOI:10.1021/ol050961o

ORGANIC
LETTERS
2005
Vol. 7, No. 13
2739-2742
Evidence for a Transition State Model
Compound of In-Plane Vinylic SN2
Reaction
Torahiko Yamaguchi,† Yohsuke Yamamoto,*,† Yoshihisa Fujiwara,‡ and
Yoshifumi Tanimoto‡
Department of Chemistry, Graduate School of Science, Hiroshima UniVersity,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan, and Department of
Mathematical and Life Science, Graduate School of Science, Hiroshima UniVersity,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
yyama@sci.hiroshima-u.ac.jp
Received April 29, 2005
ABSTRACT
To isolate a transition state model compound of an in-plane vinylic SN2 reaction, vinyl bromide 6 bearing a newly synthesized tridentate ligand
derived from 1,8-dimethoxythioxanthen-9-one (5) was prepared as a precursor. Although irradiation of 6 gave demethylated benzofuran 12, a
transient broad peak which indicates formation of the desired transition state model compound was observed in the laser flash photolytic
study.
A bimolecular nucleophilic substitution (SN2) reaction at an
sp3 carbon atom proceeds via a five-coordinate carbon
compound as a transition state.1 Recently, we reported the
synthesis and crystal structure of hypervalent pentacoodinate
carbon compounds bearing a sterically rigid anthracene
ligand 12 or a flexible van Koten type ligand 23 as a transition
state model compound of the SN2 reaction.
reaction center, has been reported by Ochiai et al.4 Recently,
Okuyama et al. concluded that the transition state of this
reaction should be a four-coordinate carbon compound 3
based on detailed analysis of the kinetics.5 However, the four-
coordinate carbon compound as the transition state model
of the in-plane vinylic SN2 reaction has not been detected
or isolated. Herein, we report the observation of the transition
state model compound 4 using laser flash photolysis although
the lifetime is very short (several hundreds microseconds).
An example of an in-plane SN2 reaction at an sp2 vinylic
carbon atom, which proceeds with complete inversion of the
† Department of Chemistry.
To isolate the transition state model compound, we
synthesized 1,8-dimethoxythioxanthen-9-one (5) first as
shown in Scheme 1.
‡ Department of Mathematical and Life Science.
(1) Morrison, R. T.; Boyde, R. N. Organic Chemistry, 6th ed.; Prentice
Hall, Inc.: New Jersey, 1992; Chapter 5.
10.1021/ol050961o CCC: $30.25
© 2005 American Chemical Society
Published on Web 05/26/2005

However p-methoxybenzylpotassium could not be generated
by similar procedures, so p-methoxy derivative 6c was
synthesized by another route (Scheme 3). Thioxanthone 5
Scheme 1
Scheme 3
Bis(3-methoxyphenyl) sulfide (7) was synthesized by a
copper-catalyzed coupling reaction6 of commercially avail-
able 3-iodoanisole and 3-methoxybenzenthiol. The sulfide
7 was selectively dilithiated by refluxing in n-hexane, and
bis(2-bromo-3-methoxyphenyl) sulfide (8) was obtained after
treatment with 1,2-dibromotetrafluoroethane. The sulfide 8
was then converted to 5 via dilithiation followed by treatment
with methyl chloroformate.
was converted to 10 via treatment with MeMgBr and
p-toluenesulfonic acid, followed by bromination with pyri-
dinium hydrobromide perbromide. A Pd-catalyzed cross-
coupling reaction of 11 with p-methoxyphenylboronic acid
and subsequent bromination with pyridinium hydrobromide
perbromide afforded 6c. Colorless crystals suitable for X-ray
analysis of 6c were obtained by recrystallization from
n-hexane/CH2Cl2, and the X-ray structure is shown in Figure
1. Daylight irradiation of vinyl bromide 6a was carried out
Because it was known that a vinyl halide affords a vinyl
cation by photolysis,7 we synthesized vinyl bromide 6 as a
precursor as shown in Scheme 2.
Scheme 2
The reaction of thioxanthone 5 with benzylpotassium8
afforded an alcohol which was treated with p-toluenesulfonic
acid to give olefin 9. Subsequent bromination of 9a
proceeded using Br2, and the desired vinyl bromide 6a was
obtained (Scheme 2). In the case of 9b, bromination using
Br2 afforded a byproduct, but it proceeded cleanly using
pyridinium hydrobromide perbromide, and 6b was obtained.
Figure 1. Crystal structure (30% thermal ellipsolids) of 6c.
(2) (a) Akiba, K.-y.; Yamashita, M.; Yamamoto, Y.; Nagase, S. J. Am.
Chem. Soc. 1999, 121, 10644-10645. (b) Yamashita, M.; Yamamoto, Y.;
Akiba, K.-y.; Hashizume, D.; Iwasaki, F.; Takagi, N.; Nagase, S. J. Am.
Chem. Soc. 2005, 127, 4354-4371.
(3) Akiba, K.-y.; Moriyama, Y.; Mizozoe, M.; Inohara, H.; Nishii, T.;
Yamamoto, Y.; Minoura, M.; Hashizume, D.; Iwasaki, F.; Takagi, N.;
Ishimura, K.; Nagase, S. J. Am. Chem. Soc. 2005, 127, 5893-5901.
(4) Ochiai, M.; Ohshima, K.; Masaki, Y. J. Am. Chem. Soc. 1991, 113,
7059-7061.
in chloroform-d. Analysis of the 1H NMR spectroscopic data
of the reaction mixture indicated the formation of benzofuran
(5) (a) Okuyama, T.; Takino, T.; Sato, K.; Ochiai, M. J. Am. Chem. Soc.
1998, 120, 2275-2282. Okuyama, T.; Takino, T.; Sato, K.; Oshima, K.;
Imamura, S.; Yamataka, H.; Asano, T.; Ochiai, M. Bull. Chem. Soc. Jpn.
1998, 71, 243-257. (b) Okuyama, T.; Yamataka, H. Can. J. Chem. 1999,
77, 577-583.
Scheme 4
(6) Bates, C. G.; Gujadhur, R. K.; Venkataraman, D. Org. Lett. 2002, 4,
2803-2806.
(7) Suzuki, T.; Kitamura, T.; Sonoda, T.; Kobayashi, S.; Taniguchi, H.
J. Org. Chem. 1981, 46, 2275-2282.
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508-509.
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Org. Lett., Vol. 7, No. 13, 2005

12a and MeBr (Scheme 4). In the photolysis by 266 nm laser
light in CD2Cl2, the formation of 12a was also observed.
To exchange the counteranion, we attempted reaction of
6 with AgPF6,9 but the reaction gave the mixture shown in
Scheme 5.
methoxy derivative 13 recorded after 266 nm pulse laser
irradiation (8 ns fwhm). The spectrum recorded in dichlo-
romethane solution (Figure 2a) at a 0.4 µs delay time has a
broad peak at ca. 400-650 nm which includes two compo-
nents. One component appears at 400-500 nm and has a
lifetime of less than 10 µs (A). Another has a spectrum
widely spread over 400-650 nm and a lifetime of several
hundreds µs (B). In the presence of oxygen, B is still present
but A is absent (Figure 2b). Because the lifetime of B is
hardly affected by O2 bubbling, this peak should not be
derived from either a radical species or a triplet state which
should be quenched in the presence of O2. The results
strongly suggest that the spectrum of 400-650 nm observed
after a 10 µs delay time should be assigned to a cation
species. In our study on the hypervalent pentacoodinate
carbon compound of 2, a similar broad peak at 500-800
nm, which was considered to be derived from a charge
transfer (CT) between apical aryloxy groups and the central
carbocation, was observed.3 Based on the formation of 12a
after photolysis of 6, the intermediate should be the desired
cation 4, and the broad peak at ca. 400-650 nm should be
derived from CT between ambilateral methoxy groups to the
vinyl cation center. Consistent with the discussion, non-
methoxy derivative 13 did not show a similar broad peak
during the laser photolytic study (Figure 2c). In addition,
the optimized structure of 4a with the hybrid density
functional theory (DFT) at the B3PW91/6-31G(d) level11
using the Gaussian 98 program12 showed that the sym-
metrically coordinated structure is the energy minimum
(Figure 3). The two C-O distance are almost identical (2.292
Scheme 5
Therefore, to detect the transient species in the photolysis
of 6, a laser flash photolytic study10 was carried out. Figure
2 shows the transient absorption spectra of 6a and non-
Figure 2. Transient absorption spectra of 6a in CH2Cl2 in the (a)
absence and (b) presence of O2. (c) Transient absorption spectra
of 13 in CH2Cl2.
Figure 3. Optimized structure (upper) and orbitals (lower) of 4a
calculated at the B3PW91/6-31G(d) level.
Org. Lett., Vol. 7, No. 13, 2005
2741

and 2.313 Å). It should be noted that the calculated λmax
(529.1 nm: HOMO-2 to LUMO) of the optimized structure
at the TD-B3PW91/6-311+G(2d, p) level is quite consistent
with the observed spectra of B.
the Ministry of Education, Culture, Sports, Science and
Technology of the Japanese Government. We thank Professor
Tsugio Kitamura (University of Saga) for helpful discussions.
Supporting Information Available: Experimental pro-
cedures and CIF files of 6c. This material is available free
of charge via the Internet at http://pubs.acs.org.
In conclusion, the transient species B should be the
transition state model compound of the in-plane vinylic SN2
reaction.
To prohibit the demethylation reaction, attempts at prepa-
ration of the compounds bearing aryloxy groups instead of
the methoxy groups are in progress.
OL050961O
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Acknowledgment. This work was supported by a Grant-
in-Aid (No. 14078220) for Scientific Research provided by
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