Solid-state photochemistry of o-aroylbenzothioates: Absolute asymmetric phthalide formation involving 1,4-aryl migration

Article abstract of DOI:10.1021/ja982696q

Solid-state photoreactions of S-(o-tolyl), S-phenyl, and S-(m-tolyl) 2- benzoylbenzothioates, which formed chiral crystals by spontaneous resolution, underwent an unprecedented intramolecular cyclization involving phenyl migration to afford optically acti

Full text of DOI:10.1021/ja982696q

12770
J. Am. Chem. Soc. 1998, 120, 12770-12776
Solid-State Photochemistry of o-Aroylbenzothioates: Absolute
Asymmetric Phthalide Formation Involving 1,4-Aryl Migration
Masaki Takahashi,† Norio Sekine,† Tsutomu Fujita,† Shoji Watanabe,†
Kentaro Yamaguchi,‡ and Masami Sakamoto*,†
Contribution from the Department of Materials Technology, Faculty of Engineering and Chemical Analysis
Center, Chiba UniVersity, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
ReceiVed July 29, 1998
Abstract: Solid-state photoreactions of S-(o-tolyl), S-phenyl, and S-(m-tolyl) 2-benzoylbenzothioates, which
formed chiral crystals by spontaneous resolution, underwent an unprecedented intramolecular cyclization
involving phenyl migration to afford optically active corresponding 3-phenyl-3-(arylthio)phthalide in good
chemo- and enantioselectivities. Stereochemical assignment and substituent probe experiment of this
photoreaction sequence led to a reliable conclusion that this reaction was rationalized on the basis of the aryl
migration rather than the well-recognized radicalic mechanism. Furthermore, the chirality preservation in the
crystals was demonstrated by means of the bisignated CD spectra, which allowed assignment of the origin of
the specific CD band and the helicity of the chiral molecules.
Introduction
standing of reactions in the anisotropic constrained media is of
great advantage when solution reactions perturb the reaction
outcome and preclude any formulation of the mechanism. In
cases where molecules in crystal lattices are allowed to react
under topochemical control due to well-defined atomic arrange-
ment, solid-state reactions proceed through the most favored
pathway leading to clear results. Then, reaction courses can be
rationalized from practices of the geometrical preferences.15,16
Since Scheffer and co-workers described the first example
of an intramolecular version of the “absolute” asymmetric
syntheses by means of the photochemical di-π-methane rear-
rangement and Norrish/Yang cyclization systems,4 further
investigations concerning intramolecular absolute asymmetric
transformations have been reported.5-9,17-23 Although there has
been considerable attention attracted to this field, variations of
the methodology still remain underdeveloped. Recently, we
described the solid-state photoreaction of S-(o-tolyl) 2-benzoyl-
benzothioate (1a), which formed chiral crystals, led to optically
active 3-phenyl-3-phenylthiophthalide (2a) in good enantiose-
lectivity (Scheme 1).20 According to the correlation that was
found by X-ray crystal structure determination, between the
absolute configurations of the starting compound and photo-
product, the product formation can be rationalized by an unusual
pathway involving phenyl migration. To establish the validity
The asymmetric generation influenced by the chiral crystalline
environment can be considered as an attractive phenomenon to
obtain optically active compounds from achiral compounds.1-3
Of particular interest in this context is the “absolute” asymmetric
transformations which are influenced only by the chiral crystal-
line environment without any external chiral source in the
starting compounds.4-9 This situation arises when compounds
crystallized by spontaneous resolution undergo reactions in the
crystalline lattices yielding new stereocenters in the products.
Since the solid-state reaction proceeds with least atomic or
molecular movement, the X-ray crystallographic approach is
very useful for investigation of the mechanistic features and
reactivities.10-14 From a mechanistic point of view, an under-
† Department of Materials Technology.
‡ Chemical Analysis Center.
(1) Addadi, L.; Lahav, M. Origin of Optical ActiVity in Nature; Walker,
D. C., Ed.; Elsevier: New York, 1979; Chapter 14.
(2) Green, B. S.; Lahav, M.; Rabinovich, D. Acc. Chem. Res. 1979, 12,
191-197.
(3) Sakamoto, M. Chem. Eur. J. 1997, 3, 684-689.
(4) Evans, S. V.; Garcia-Garibay, M.; Omkaram, N.; Scheffer, J. R.;
Trotter, J.; Wireko, F. J. Am. Chem. Soc. 1986, 108, 5648-5650.
(5) Gudmundsdottir, A. D.; Lewis, T. J.; Randall, L. H.; Scheffer, J. R.;
Rettig, S. J.; Trotter, J.; Wu, C.-H. J. Am. Chem. Soc. 1996, 118, 6167-
6184.
(6) Toda, F.; Yagi, M.; Soda, S. J. Chem. Soc., Chem. Commun. 1987,
1413-1414.
(15) (a) Zimmerman, H. E.; Zuraw, M. J. J. Am. Chem. Soc. 1989, 111,
2358-2361. (b) Zimmerman, H. E.; Zuraw, M. J. J. Am. Chem. Soc. 1989,
111, 7974-7989. (c) Zimmerman, H. E.; Zhu, Z. J. Am. Chem. Soc. 1995,
117, 5245-5262.
(7) Sekine, A.; Hori, K.; Ohashi, Y.; Yagi, M.; Toda, F. J. Am. Chem.
Soc. 1989, 111, 697-699.
(8) Toda, F.; Miyamoto, H.; Koshima, H.; Urbanczyk-Lipkowska, Z. J.
Org. Chem. 1997, 62, 9261-9266.
(16) Choi, T.; Peterfy, K.; Khan, S. I.; Garcia-Garibay, M. A. J. Am.
Chem. Soc. 1996, 118, 12477-12478.
(9) Roughton, A. L.; Muneer, M.; Demuth, M. J. Am. Chem. Soc. 1993,
115, 2085-2087.
(17) Sakamoto, M.; Hokari, N.; Takahashi, M.; Fujita, T.; Watanabe,
S.; Iida, I.; Nishio, T. J. Am. Chem. Soc. 1993, 115, 818.
(18) Sakamoto, M.; Takahashi, M.; Fujita, T.; Watanabe, S.; Iida, I.;
Nishio, T. J. Org. Chem. 1995, 60, 3476-3477.
(19) Sakamoto, M.; Takahashi, M.; Shimizu, M.; Fujita, T.; Nishio, T.;
Iida, I.; Yamaguchi, K.; Watanabe, S. J. Org. Chem. 1995, 60, 7088-7089.
(20) Sakamoto, M.; Takahashi, M.; Moriizumi, S.; Yamaguchi, K.; Fujita,
T.; Watanabe, S. J. Am. Chem. Soc. 1996, 118, 8138-8139.
(21) Sakamoto, M.; Takahashi, M.; Kamiya, K.; Yamaguchi, K.; Fujita,
T.; Watanabe, S. Am. Chem. Soc. 1996, 118, 10664-10665.
(22) Takahashi, M.; Fujita, T.; Watanabe, S.; Nishio, T.; Iida, I.; Aoyama,
H. J. Org. Chem. 1997, 62, 6298-6308.
(10) Scheffer, J. R.; Garcia-Garibay, M.; Nalamasu, O. Organic Pho-
tochemistry; Padwa, A., Ed.; Marcel Dekker: New York and Basel, 1987;
Vol. 8, pp 249-338.
(11) Gamlin, J. N.; Jones, R.; Leibovitch, M.; Patrick, B.; Scheffer, J.
R.; Trotter, J. Acc. Chem. Res. 1996, 29, 203-209.
(12) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433-481.
(13) Venkatesan, K.; Ramamurthy, V. Photochemistry in Organized and
Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; pp
133-184.
(14) Ito, Y. Synthesis 1998, 1-32.
10.1021/ja982696q CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/01/1998

Solid-State Photochemistry of o-Aroylbenzothioates
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12771
Scheme 1
Figure 1.
and generality of this reaction, we have investigated the solid-
state photoreactions of various thioester analogues. In the present
publication, complete details on the mechanistic events are
described. Furthermore, we disclose the correlation between the
absolute configuration and the nominally achiral molecules with
chiral conformations shown by circular dichroism (CD) spec-
trum.
Results and Discussion
The starting thioesters 1 were readily prepared from com-
mercially available thiols and the corresponding 2-aroylbenzoic
acids by condensation reactions. For methyl-substituted thioester
1g, the requisite 4-methyl-2-benzoylbenzoic acid was prepared
by the Friedel-Crafts condensation of 3-methylphthalic anhy-
dride with benzene. Recrystallization of these thioesters from
chloroform-hexane solution afforded colorless prisms for all
cases. All crystals were subjected to X-ray crystallographic
analyses to obtain details on the molecular architecture in the
crystals.
In regard to thioesters 1a-c, the constituent molecules
adopted orthorhombic chiral space group P212121 and were
frozen in helical conformations. Obviously, these molecules
formed chiral crystals and spontaneous optical resolution
occurred during their crystallization process, in which the
molecules were associated and immobilized in chiral fashion.
These interesting features were demonstrable through a mea-
surement of the circular dichroism (CD) spectra of crystalline
samples in KBr pellets.24,25 Under such conditions conforma-
tional interconversion owing to carbon-carbon bond rotations
was severely restricted and their optical activities evolving in
the crystalline environment were maintained. Generally, there
is equal possibility in the selection of enantiomorphic P- and
M-forms of molecular configurations (Figure 1). Once crystals
formed, a large amount of chiral crystals with the same optical
rotatory can be selectively prepared through recrystallization
by seeding the desired crystals.
Figure 2. CD and UV spectra of enantiomeric crystals of both
antipodes of 1a-1c in KBr. A mixture of 10 mg of 1 and 100 mg of
KBr was well ground and formed into a disk with a radius of 5 mm.
It is of great significance to correlate the absolute configura-
tions of helical molecules frozen in the chiral crystalline
environment with the sign of specific rotations. An attempt to
determine the absolute configuration of (+)-1a giving rise to a
positive Cotton effect on its CD spectrum was first successfully
achieved by X-ray crystallography taking account of anomalous
dispersion due to the relatively heavy sulfur atom. Consequently,
the crystals (+)-1a were determined to contain P-configuration
in the molecules with regard to the helix of two carbonyl groups,
while its antipode was confidently assigned to possess left-
handed M-helicity (Figure 3). Furthermore, absolute configura-
tions of both (-)-1b and 1c were determined to be M-config-
urations by X-ray crystallography. While the latter one utilized
the same method as the case of (+)-1a for the determination,
the former was established through refinement of Roger’s η
parameter (final value was 0.581).26 Compatible with 1a, both
antipodes of 1b and 1c gave symmetrical CD absorption curves
to their antipodes with almost the same intensities in all regions
Figure 2 shows CD spectra of two enantiomorphic crystals
of 1a-c in KBr pellets, which were independently obtained by
spontaneous resolution. These crystals gave specific curves in
the region between 200 and 400 nm, which were mirror images
designated as (+) and (-) at the wavelength of 346 nm within
inevitable error.
(23) Fu, T. Y.; Liu, Z.; Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc.
1993, 115, 12202-12203.
(24) Hashizume, D.; Kogo, H.; Sekine, A.; Ohashi, Y.; Miyamoto, H.;
Toda, F. J. Chem. Soc., Perkin Trans. 2 1996, 61-66.
(25) For the observation of CD spectra in the crystalline state, see:
Azumaya, I.; Yamaguchi, K.; Okamoto, I.; Kagechika, H.; Shudo, K. J.
Am. Chem. Soc. 1995, 117, 9083-9084. Toda and co-workers also studied
this topic, cited in ref 8.

12772 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Takahashi et al.
Figure 3. ORTEP drawings of the absolute configurations of (+)-1a, (-)-1b, and (-)-1c.
observed. Interestingly, a similar tendency in relationships
between helicities and absolute configurations for the chiral
crystals 1a-c was observed, in which minus activity of the
crystals was attributed to M-helicity while P-helicity was
responsible for plus activity.
reaction courses in chemical yields and enantiomeric excesses
of 2a as a function of reaction proceeding at 0 °C. While the
chemoselectivity decreased proportionately with the extent of
irradiation, the enantioselectivity fell off seriously at earlier
stages. These results indicate that nontopochemically controlled
processes occurred in the crystal lattices at later reaction stages,
which promoted nonselective reactions leading to complicated
products including phthalide dimer as well as reaction in the
solution phase.27 Indeed, the change in enantioselectivity was
more sensitive than that in chemoselectivity. Pronounced
changes in the product profiles were recognized when the
reaction was carried out at low (5%) conversion or at low
temperature (-78 °C). In the former case, a good ee value (77%
ee) was obtained with exclusive product formation (>95% as
observed by 1H NMR of the crude mixture). Under these
conditions, the molecules were supposed to be still under
topochemical control leading to the better stereoselective
reaction in the crystals. Additionally, the reaction at -78 °C
gave similar results (50% yield, 65% ee) for the same reason,
where the molecules were strongly frozen in the crystal lattices.
Obviously, the topochemical control was much more effective
at lower temperature since 65% ee was obtained even at 50%
conversion, which value corresponded to about 39% ee for 0
°C on estimation of the curve. Apart from 1a, S-phenyl and
S-(m-tolyl) derivatives 1b and 1c formed chiral crystals (space
groups P212121) and their solid-state photoreactions also led to
optically active products with respective ee values depending
on the reaction conditions (Table 1). Under well-optimized
conditions, good enantiomeric excesses were eventually obtained
in the products as high as 87% ee for 2b (10% conversion at
-78 °C) and 74% ee for 2c (67% conversion at -78 °C).
Figure 2 includes UV spectra of the crystals of thioesters 1a-
c, indicating the molecules absorbed photons sufficiently in the
solid state beyond 290 nm (Pyrex filtered light).27 Accordingly,
crystals of 1d-g bearing almost the same structures were
expected to behave similarly on irradiation. For the practical
experiment, a light source from a 500-W high-pressure mercury
lamp was employed for the solid-state photoreactions of 1a-g.
These crystals weighing 100 mg were well ground, sandwiched
between two Pyrex slides, and covered with polyethylene bags
to prevent the sample from being flattened by cooling solvents
during irradiation. For instance, 6 h of irradiation of powdered
(+)-1a at 0 °C led to production of 3-phenyl-3-(o-tolylthio)-
phthalide (2a) as a major product (65% yield after purification
by silica gel chromatography) with complete consumption of
the starting material. This compound was well-characterized by
spectroscopic methods,28 in which a lactone carbonyl appeared
at 1772 cm-1 in its infrared spectrum and the benzoyl carbon
of 1a at δc 189.8 ppm disappeared after the reaction. Finally,
this structure was unequivocally established by X-ray crystal-
lography as shown in Figure 4. This solid-state photoreaction
establishes the ability of the crystal lattice to immobilize the
molecules in chiral conformations, and the reaction seemed to
proceed under the influence of the crystalline environment. As
expected, the asymmetric generation in 2a was realized by an
observation of its specific rotation as +37° which corresponded
to 30% ee.29 It should be emphasized that this solid-state
photoreaction proceeded heterogeneously, giving the product
in low chemo- and enantioselectivities owing to undesirable
reactions in incidental broken lattice sites. Figure 5 shows the
These intramolecular cyclization reactions in the solid-state
were also examined for other thioesters 1d-g, giving the
corresponding phthalides 2d-g, but as racemates. The failure
of generation of optical activity in the products was well in
agreement with their centrosymmetric crystal systems such as
P21/c and P1h (Table 2). In such crystals, both P- and M-helical
molecules were regularly arranged and complementarily coupled
each other. Eventually, the enantioselective transformation of
each species led to the formation of a 1:1 mixture of (R)- and
(S)-enantiomers (racemates) throughout the reaction stages.
(26) Although the η value of (-)-1b is relatively small, the parameter
test favored only one chirality for M-1b with the final R and RW being
0.0448 and 0.0499 for 1609 reflections, which may guarantee the absolute
helicity of the frozen molecules.
(27) Photoreactions of the thioesters in solution phase were investigated.
Takahashi, M.; Fujita, T.; Watanabe, S.; Sakamoto, M. J. Chem. Soc., Perkin
Trans. 2 1998, 487-491.
(28) The spectroscopic data for all compounds have been reported in
ref 27 and cited therein.
In regard to the mechanism for these solid-state photoreac-
tions, there are two possible pathways from the starting thioesters
(29) The ee value was determined by HPLC by using a chiral cell OJ
(Daicel Chemical Industries).

Solid-State Photochemistry of o-Aroylbenzothioates
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12773
Figure 4. ORTEP drawings of the absolute configurations of (R)-(+)-2a and (S)-(-)-2b.
Table 1. Solid-State Photoreactions of Thioesters 1
temp
(°C)
conversion
(%)
yield of 2
(%)
thioester 1
[R]20D of 2a-c
a
0
0
100
5
65
>95d
92
+37° (30% ee)
+94° (77% ee)
+80° (65% ee)
-37° (35% ee)
-75° (71% ee)
-92° (87% ee)
-27° (23% ee)
-83° (71% ee)
-87° (74% ee)
0° (0% ee)
-78
50
20
10
10
56
10
67
100
67
63
71
b
c
0
0
97
>95d
>95d
96
-78
0
0
95
-78
>95d
89
d
e
f
0
0
0
0
78
91
85
0° (0% ee)
0° (0% ee)
0° (0% ee)
Figure 5. A plot for the conversion of 1a vs chemical and enantiomeric
yields of 2a. The chemical yields were determined on the basis of
consumed thioester 1a.
g
a Measured in CHCl3 at c 1.0. b Each enantiomorph could be
prepared selectively in bulk by seeding methods. c Determined by HPLC
by using chiral cell OJ (Daicel Chemical Industry). d The chemical
to phthalides (Scheme 2). In the first model (Path A), the
reaction is initiated by homolytic dissociation between C-S
bonding to form a radical pair intermediate. Such a pathway is
well-recognized as the excitation reaction of thioester com-
pounds. The other model (Path B) consists of direct cyclization
and subsequent phenyl migration sequences, where a zwitteri-
onic intermediate would be involved. To solve this mechanistic
problem, stereochemical correlation studies before and after the
solid-state photoreactions were employed. The optically pure
(+)-2a was obtained by optical resolution of (+)-enantiomer
rich 2a with HPLC using a chiral cell OJ, whose absolute
structure was determined by X-ray analysis with the Bijvoet
difference method in the correction process. Finally, the X-ray
results indicated that the molecules were crystallized in triclinic
chiral space group P1 and the unit cell included two discrete
1
yields were determined on the basis of H NMR spectra.
rotamers (2a-A and 2a-B) owing to carbon-carbon bond
rotation around the o-tolyl groups. Furthermore, statistic evalu-
ation taking into account the anomalous dispersion determined
the single stereocenter in (+)-2a to be (R)-configuration (Figure
4). Consequently, this reaction elucidated the stereochemical
relationship from P-1a to (R)-2a.
Assuming Path A was involved in the reaction, this sequence
would lead to (S)-2a as a major product. This stereochemical
outcome is incompatible with the aforementioned results, and
this mechanistic interpretation is, therefore, ruled out. On the
other hand, Path B may offer a complementary demonstration
to satisfy the stereochemical relationship. The course of the

12774 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Takahashi et al.
Table 2. Space Group, Optical Rotatory, and Absolute
Configuration of the Starting Crystal and the Photoproduct
Scheme 3
space
group
optical
abs config
abs config
thioester 1
rotatorya
of 1
of 2
a
b
c
d
e
f
P212121
P212121
P212121
P21/c
P-1
P-1
P-1
(+)b
(-)b
(-)b
racc
racc
racc
racc
P
M
M
PM
PM
PM
PM
R
S
Sd
RS
RS
RS
RS
g
a Determined on the basis of their CD spectra at 346 nm. b Enan-
tiomorphic crystals were also obtained by spontaneous crystallization.
c
Racemic crystals. d Predicted configuration on the basis of the Path
B mechanism.
Scheme 2
Scheme 4
discussion, the stereochemical relationship between M-1c and
(S)-2c can be readily predicted.
An alternative approach to determine the reaction pathway
was elaboration of the regiochemical correlation employing a
methyl probe on the central aryl ring in the starting material.
The demonstration based on the methyl probe between the
starting material and final product provides straightforward proof
for the determination of the pathway since the position of this
substituent in the product may represent the reaction course.
When the powdered sample 1e was irradiated at 0 °C for 6 h,
a single regioisomer 2e was obtained in 78% yield (67%
conversion). This photochemical event was evaluated by an
observation of the 1H NMR spectrum of the crude photolyzate.
Remarkably, only one strong methyl signal, except for that of
1e, evolved around the region of 2.0-2.5 ppm, with weak
signals due to phthalide dimer. The demonstration is incompat-
ible with reaction in the solution phase.27 The characterization
of the photoproduct was unequivocally established by the X-ray
crystallographic analysis, which demonstrated the methyl probe
was located at the 6-position of the phthalide system.30 As is
apparent from the regiochemical correlation, the reaction
pathway B was indispensable for the rationalization of the
product formation since Path A would lead to another regio-
isomer (Scheme 3). Therefore, we could draw the same
conclusion as was obtained by the correlation of the absolute
configurations described above for stereochemical demonstra-
tions.
initial cyclization step may govern the absolute configuration
of the final product, and the phenyl migration step proceeded
with steric restrictions resulting in stereoselective formation of
a single enantiomer. Thus, the enantioselective transformation
from P-(+)-1a to (R)-2a can be rationalized by Path B involving
a convergent sequence of nucleophilic cyclization and phenyl
migration. It is noteworthy that this reaction provides new
mechanistic insights into the photochemistry of thioester and
aromatic compounds since both nucleophilic cyclization of the
thioester group and the photoinduced 1,4-phenyl migration
process have never been recognized, to the best of our
knowledge.
To clarify the feasibility of stereochemical assignment during
the reaction, another correlation study of the absolute configura-
tions of the starting compound and the product was carried out
for the reaction of M-1b to (-)-2b. The absolute structure of
enantiomerically pure (-)-2b was determined as (S)-enantiomer
by X-ray diffraction solving through refinement of Roger’s η
parameter, the final value of which was 0.865 (Figure 4). This
indicated the aforementioned stereochemical relationship was
applicable to this reaction system (M-1b to (S)-2b), supporting
the mechanistic proposal for Path B. Unfortunately, additional
assignment on the transformation of M-1c to (-)-2c could not
be achieved since optically pure (-)-2c isolated by HPLC using
a chiral cell gave only thin plates unsuitable for X-ray
crystallographic studies. On consideration of the aforementioned
To gain further insight into the mechanistic features of the
phenyl migration, solid-state photoreactions of p-methyl- and
p-chlorophenylcarbonyl systems were explored (Scheme 4). The
starting thioesters 1f and 1g were prepared from commercially
available o-arylbenzoic acids and thiophenol by condensation
with the DCC method. Along with the aforementioned cases,
solid-state photolyses of crystalline samples 1f and 1g led to
formation of cyclized products 2f and 2g in good yields (91
and 85%). Observation of clear methyl and aromatic signals in
(30) The X-ray crystallographic structures of both 1e and 2e have been
reported in ref 27, and their crystal data were cited therein.

Solid-State Photochemistry of o-Aroylbenzothioates
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12775
Figure 7.
Table 3. Geometrical Parameters in the Path B Mechanisma
thioester 1
R (deg)
â (deg)
dc (Å)
dm (Å)
a
b
c
d
e
f
g
13
34
23
19
8
7
9
81
65
75
80
93
92
77
2.68
2.77
2.73
2.71
2.65
2.66
2.69
3.72
3.93
3.91
3.80
3.68
3.65
3.54
ideal
<3.22
<3.40
a R: torsional angle formed by the aroyl carbonyl against central
aryl ring. â: tortional angle formed by the thioester carbonyl against
the central aryl ring. dc: distance between the thioester oxygen and
the aroyl carbonyl carbon. dm: distance between the carbonyl carbon
of the thioester and the aryl carbon of the aroyl group.
by R and â (Figure 7, Table 3). Remarkably, the thioester groups
were inclined to exist in almost the same plane to the aryl rings
(R values: 7-34°), and the benzoyl groups were orthogonally
twisted with their â values of 65-93°. It is noteworthy that the
sulfur atoms were located in the external spaces away from the
benzoyl carbons probably due to the repulsive CdO‚‚‚S
interactions. With these atomic arrangements, it is difficult for
straightforward radical cyclization by acyl radicals generated
in Path A to occur in the crystal lattices. In such cases, C-C
bond rotations around the acyl radicals should be required.
However, this assumption was incompatible with the results in
the improvement of enantioselectivities in lower reaction
conversion and temperature since the drastic atomic movements
may destroy the chiral crystalline environment, resulting in loss
of enantioselective transformation. Table 3 also lists interatomic
distances responsible for the photochemical events via Path B.
The dc values were much less than the sum of van der Waals
radii (3.22 Å), ranging from 2.65 to 2.77 Å. Although the dm
values in this mechanism were slightly longer (3.54-3.93 Å)
than the sum of van der Waals radii (3.40 Å), the subsequent
aryl migration might occur since some conformational changes
enhanced by initial cyclization would allow the reaction centers
to be close. These atomic arrangements in the crystal lattices
satisfy the reactive conformations, in which the electrophilic
aroyl carbons are sufficiently exposed to the nucleophilic
n-electrons of excited thioesters with favorable angles. Conse-
quently, these geometrical studies also led to the same conclu-
sion that these reactions were rationalized on the basis of the
unprecedented reaction sequences consisting of the nucleophilic
cyclization and subsequent aryl migration giving the cyclized
products.
Figure 6. ORTEP drawings of 1f and 2f.
1
the H NMR spectra of the crude photolyzate indicated these
reactions occurred with perfect regioselectivities during aryl
migrations. The reaction course was also demonstrated by X-ray
structural analysis of 2f, which unequivocally ensured the aryl
migration occurred in parallel fashion giving phthalide with the
tolyl group at the 3-position (Figure 6).
It has been generally argued that solid-state reactions
proceeded with minimum atomic or molecular movement.
Therefore, the reactivities are mainly governed by atomic
arrangement represented by distances and angles between
reaction centers. Ultimately, geometrical assessment may offer
abundant mechanistic information including crucial criteria for
the feasible reactions. Twisting around the C-C bonds defined
by benzoyl and thioester bending against the centered phenyl
ring may be the most important factor for the formation of the
helical and chiral structures. From the X-ray structural studies
of the starting compounds, several trends were found in the
twisting angles formed by the two functional groups, represented
Conclusion
Investigations of the solid-state photoreactions of S-aryl
2-aroylbenzothioates 1 led to the remarkable conclusion that
the major process involved was an unprecedented reaction
sequence, starting with intramolecular cyclization, followed by
aryl migration to the photoproducts 2. The “absolute” asym-
metric generations into the achiral starting molecules were

12776 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Takahashi et al.
observed with good enantioselectivities, which seriously de-
pended on the conversion and reaction temperature. The
mechanistic studies based on stereochemical correlation before
and after the reaction gave strong evidence for the putative
reaction pathway as well as the geometrical analyses of the
starting molecules in the crystal lattices. Thus, the absolute-to-
absolute correlation study may offer a powerful tool for
manifesting obscure reaction pathways during solid-state reac-
tions. In conclusion, continuing elaboration of the photochem-
istry of such aromatic systems will provide further insights into
mechanistic details of the unprecedented photochemical behav-
ior.
Supporting Information Available: Experimental section
and X-ray crystallographic data of 1b, 1c, 1d, 1f, 1g, 2b, and
2f (36 pages, print/PDF). See any current masthead page for
ordering information and Web access instructions.
JA982696Q