Competitive enantiodifferentiating anti-Markovnikov photoaddition of water and methanol to 1,1-diphenylpropene using a sensitizing cyclodextrin host

Article abstract of DOI:10.1021/jo9012628

(Chemical Equation Presented) UV-vis, circular dichroism (CD), fluorescence, and NMR spectral studies on the self-inclusion behavior of a newly synthesized sensitizing host, 6-(5-cyanonaphthyl-1-carboamido)-6-deoxy-β- cyclodextrin (1), showed that the app

Full text of DOI:10.1021/jo9012628

pubs.acs.org/joc
Competitive Enantiodifferentiating Anti-Markovnikov Photoaddition of
Water and Methanol to 1,1-Diphenylpropene Using A Sensitizing
Cyclodextrin Host
Gaku Fukuhara, Tadashi Mori, and Yoshihisa Inoue*
Department of Applied Chemistry, Osaka University, 2-1Yamada-oka, Suita 565-0871, Japan
inoue@chem.eng.osaka-u.ac.jp
Received June 16, 2009
UV-vis, circular dichroism (CD), fluorescence, and NMR spectral studies on the self-inclusion
behavior of a newly synthesized sensitizing host, 6-(5-cyanonaphthyl-1-carboamido)-6-deoxy-β-cyclo-
dextrin (1), showed that the appended naphthalene moiety of 1 perches laterally on the cyclodextrin rim
in aqueous methanol but is shallowly included and somewhat tilted in its own cavity in water. UV-vis
and CD spectral examinations of the complexation of guest substrate 1,1-diphenylpropene (DPP) with
host 1 revealed the formation of a stoichiometric 1:1 complex of DPP with 1. The naphthyl fluorescence
of 1 was efficiently quenched by the addition of DPP in aqueous solutions of low methanol contents
(e25%) but was less efficiently quenched in more hydrophobic solvents (g50% methanol), where the
fluorophore is not included in the cavity and allows the external attack of DPP to form an exciplex in the
bulk solution. Upon irradiation in aqueous solutions of different methanol contents, competitive
photoaddition of water and methanol to DPP occurred to give chiral water adduct 3 and methanol
adduct 4, favoring the latter product by a factor of 2.5 due to the higher nucleophilicity of methanol. The
enantiomeric excess (ee) values of the photoadducts were generally low in highly methanolic solutions,
but was greatly improved by increasing the water content to reach 18% ee for 3 and 13% ee for 4 in 10%
methanol solution at -10 ꢀC. Interestingly, the ee of methanol adduct 4 was consistently lower than that
of water adduct 3 particularly in water-rich solvents, revealing that the product’s ee is not a simple
thermodynamic function of the enantioface-selectivity upon complexation of DPP by chiral host 1 but
also kinetically controlled by the subsequent photoinduced enantioface-differentiating nucleophilic
attack of water and methanol to radical cationic DPP generated photochemically. Compatible with this
mechanism, the compensation plot of the differential activation enthalpy versus entropy, which were
obtained from the van’t Hoff analysis of the temperature-dependent ee’s obtained in aqueous solutions
of varying methanol contents, gave an excellent straight line for water adduct 3 but an unprecedented
bent plot for methanol adduct 4, indicating a switching of the mechanism in between 35 and 50%
methanol solution. By using high pressure, low temperature, and/or added salt, the ee of water adduct 3
was further enhanced to 24-26%.
Introduction
alternative to the conventional asymmetric syntheses using
chiral catalysts or enzymes.2-4 One of the most intriguing
features of photochirogenesis is the critical control of en-
antioselectivity, including the switching of chiral sense, of
Photochirogenesis,1 which enables us to access the ther-
mally forbidden routes to chiral compounds, is an attractive
6714 J. Org. Chem. 2009, 74, 6714–6727
Published on Web 08/11/2009
DOI: 10.1021/jo9012628
r
2009 American Chemical Society

Fukuhara et al.
JOCArticle
photoproduct through the manipulation of a variety of en-
tropy-related environmental variants such as temperature,5
pressure,6 and solvation,7 which provides us with a unique,
versatile tool for multidimensionally optimizing a photochiro-
genic reaction without using a harsh condition.8 More impor-
tantly, these observations unambiguously revealed the essential
roles of the entropy played in the enantiodifferentiating step
of photochirogenic reaction.
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by β-cyclodextrin derivatives with a tethered sensitizing
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CHART 1. Modified β-Cyclodextrin As a Photosensitizing Host 1 and Reference Sensitizer 2
expand the scope of the entropy control concept to supra-
molecular photochirogenesis.
particular emphasis on water adduct 3 which was examined
for the first time, and (3) the temperature and solvent effects
on the yield and ee of both adducts and the roles of entropy
upon supramolecular complexation and photochirogenesis.
Supramolecular photochirogenesis with a built-in sensiti-
zer is not restricted to the unimolecular reactions but is also
applicable to bimolecular reactions. Thus, the enantiodiffer-
entiating anti-Markovnikov photoaddition of methanol
to 1,1-diphenylpropene (DPP) (Scheme 1), which was ori-
ginally studied in organic solvents18 and more recently in
supercritical carbon dioxide,19 was effected by using 6-(5-
cyanonaphthyl-1-carboamido)-6-deoxy-β-cyclodextrin (1)
as a sensitizing host (Chart 1).12i For such a reaction
performed in aqueous methanol, it is likely that radical
cationic DPP formed upon photoelectron transfer to the
sensitizing host is attacked not only by methanol but also by
water, but we could not definitely identify the water adduct
under the gas chromatographic conditions established for
analyzing the methanol adduct, probably due to the dehy-
dration of the water adduct.
Results and Discussion
Self-Inclusion Behavior of Host 1. Circular Dichroism
Spectral Examinations. Modified cyclodextrins possessing a
hydrophobic substituent are known to intramolecularly
include the appended substituent moiety to give self-inclu-
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SCHEME 1. Enantiodifferentiating Anti-Markovnikov Photo-
addition of Water and Methanol to 1,1-Diphenylpropene (DPP) in
the Presence of Sensitizing Host 6-(5-Cyanonaphthyl-1-
carboamido)-6-deoxy-β-cyclodextrin (1)
In the present study, we wanted to fully elucidate the
factors and mechanisms operating in the supramolecular
complexation and the competitive photochirogenic nucleo-
philic attack of water and methanol to DPP sensitized
by cyanonaphthalene-modified β-cyclodextrin 1, as well as
the role of entropy in this bimolecular supramolecular
photochirogenic reaction. Thus, the following aspects will
be discussed in detail: (1) the self-inclusion behavior of the
sensitizing moiety in host 1 and the complexation of DPP by
host 1 in the ground state, (2) the competitive enantiodiffer-
entiating anti-Markovnikov photoaddition of water and
methanol to DPP included and sensitized by 1, with a
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2007, 36, 1488. (d) Nishiyama, Y.; Wada, T.; Asaoka, S.; Mori, T.; McCarty,
T. A.; Kraut, N. D.; Bright, F. V.; Inoue, Y. J. Am. Chem. Soc. 2008, 130,
7526.
6716 J. Org. Chem. Vol. 74, No. 17, 2009

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CHART 2. Directions of the 1Bb and 1La Transition Moments of
2 Calculated by TD-DFT Calculations
The TD-DFT calculations were performed with the
B3LYP functional24 and the aug-cc-pVDZ basis set25 to give
a major band at 223 nm with an oscillator strength of 0.53,
which is assigned to the 1Bb transition, and a minor band at
318 nm with an oscillator strength of 0.13 assignable to the
1La transition, both of which nicely agree in energy and
relative intensity with the experimental spectra,26 as shown in
Figure 1a. This allowed us to determine the directions of the
1Bb and 1La transition moments of 2, as illustrated in Chart 2,
and also the orientation of the appended naphthalene moiety
in the cyclodextrin cavity by using the sector rules.23 In pure
FIGURE 1. (a) UV-vis and (b) CD spectra of a 1.45 ꢀ 10-5
M
solution in pure methanol (black line), a 1.52 ꢀ 10-5 M solution in
50% aqueous methanol (red line) and a 1.37 ꢀ 10-5 M solution in
pure water (blue line) of 1 at 25 ꢀC. The red bars indicate the
oscillator strengths calculated by the TD-DFT calculations, and the
inset shows the magnified CD spectra in the 1La band region.
1
1
or aqueous methanol, both of the Bb and La bands show
negative Cotton effects, indicating that these two transition
moments lay in the negative region outside of the sector cone
and hence the naphthalene moiety of 1 is deduced to rest
laterally on the cyclodextrin rim, as schematically drawn in
in aqueous solutions and in the solid state. Hence, the self-
inclusion behavior of host 1 was first investigated before
examining the complexation of DPP added as a guest sub-
strate.
Circular dichroism (CD) spectroscopy is a powerful and
indispensable tool22 for elucidating the position and orienta-
tion of an achiral chromophoric guest included in the
cyclodextrin cavity from the signs of the induced circular
dichroism (ICD) signals at the chromophore’s absorption
bands, as proposed by Kajtar et al.,23a Harata et al.,23b and
Kodaka et al.23c
1
Figure 2a. In water, the La band switches the ICD sign to
positive, while the 1Bb band signal is significantly enhanced
in intensity keeping the ICD sign negative, which indicates
the tilted conformation shown in Figure 2b. This conforma-
tional change is likely to be caused by the increased solvent
UV-vis and CD spectra of 1 were measured in aqueous
solutions of varying methanol contents. As shown in
1
Figure 1, the Bb band at ca. 225 nm displayed a negative
ICD signal, intensity of which was dramatically decreased
with increasing methanol content from 0 to 100%, indicating
less efficient self-inclusion of the naphthalene moiety in
highly methanolic solutions. Interestingly, the 1La band
centered at ca. 300 nm switched the sign of ICD from positive
to negative by increasing the methanol content, while keep-
ing the UV original spectral shape and intensity. To apply the
sector rules23 to the observed ICD signals, the directions of
the transition moments of the naphthalene choromophore
of 1 have to be determined, and hence we carried out the
TD-DFT calculations on reference compound 2, which con-
tains the essential chromophore of 1.
FIGURE 2. Schematic drawing of the sector rule23a applied to host
1 in (a) aqueous methanol and (b) water.
(24) (a) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.;
Shirley, W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193. (b) Stephens,
P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98,
11623. (c) Stephens, P. J.; Devlin, F. J.; Ashwar, C. S.; Chabalowski, C. F.;
Frisch, M. J. Faraday Discuss., Chem. Soc. 1994, 99, 103.
(22) (a) Harada, N.; Nakanishi, K. In Circular Dichronic Spectroscpy-
Exciton Coupling in Organic Sterochemistry; University Science Books: Mill
Valley, CA, 1983. (b) Berova, N.; Nakanishi, K. In Circular Dichroism:
Principles and Applications, 2nd ed.; Berova, N., Nakanishi, K., Woody,
R. W., Eds.; Wiley: New York, 2000; pp 337-382. (c) Berova, N.; Bari, L. D.;
Pescitelli, G. Chem. Soc. Rev. 2007, 36, 914.
(23) (a) Kajtar, M.; Horvath-Toro, C.; Kuthi, E.; Szejtli, J. Acta Chim.
Acad. Sci. Hung. 1982, 110, 327. (b) Harata, K.; Uedaira, H. Bull. Chem. Soc.
Jpn. 1975, 48, 375. (c) Kodaka, M. J. Am. Chem. Soc. 1993, 115, 3702.
(25) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007.
(26) Judging from the consistent CD spectra of 1 at 10 μM to 1 mM
concentrations in 25% aqueous methanol at 25 ꢀC (Figure S2 in Supporting
Information), we can rule out the formation of supramolecular aggregates at
least under the conditions employed.
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FIGURE 3. (a) Fluorescence spectra of 1 excited at 300 nm in water, 50% methanol, and methanol at room temperature; [1] = 1.0 ꢀ 10-5
M
and (b) the fluorescence maxima of 1 (O) and 2 (0), both at 1.0 ꢀ 10-5 M concentration, as a function of methanol content.
polarity to more efficiently accommodate the hydrophobic
part of naphthalene in the cavity and simultaneously expose
the polar cyano group to bulk water. This result shows that
the naphthalene chromophore suffers a dynamic conforma-
tional change from the capping to slant orientation by simply
increasing the solvent polarity.
CHART 3. Notation of Pyranose Units and Numbering of
Protons of 1
Fluorescence Spectral Examinations. Naphthalene deriva-
tives are highly fluorescent in general, and can be used as a
probe for sensing the environmental changes upon inclusion
complexation with cyclodextrin. In the present case, the
appended cyanonaphthalene moiety of 1 functions as a
built-in fluorescence probe. As shown in Figure 3a, the
fluorescent intensity of 1 was significantly enhanced by
increasing the water content in methanol. However, refer-
ence compound 2 also exhibited a similar fluorescence
intensity enhancement to a slightly larger extent (Figure S3
in the Supporting Information), and therefore the fluores-
cence intensity does not appear to be suitable for examining
the self-inclusion behavior of the naphthalene moiety. In
contrast to the fluorescent intensity which is very sensitive to
the solvent polarity, the fluorescence maximum of 2, that is,
the 0-1 band, stayed constant at 343.0-343.5 nm over the
entire solvent composition from pure methanol to pure
water, as shown in Figure 3b. Interestingly, the fluorescence
maximum of 1 showed appreciable bathochromic shifts only
in the aqueous solutions containing <25% methanol, sug-
gesting the inclusion of the naphthalene chromophore in the
hydrophobic cavity of 1 with accompanying conformational
changes, which is consistent with the conclusion derived
from the CD spectral examinations.
NMR Spectral Examination. A series of sophisticated
2D-NMR techniques have been developed for the use in
detailed conformational analysis of modified cyclodex-
trins.27 In the present study, we assigned all of the protons
in 1 with the aid of 2D-COSY, TOCSY, ROESY, HMBC
and HMQC techniques.20g The assignments are summarized
in Figure 4; see Chart 3 for the notation and numbering.
The chemical shifts of H1-H6 protons of each pyranose
unit (A-G) in 1 are deviated to different extent from the
original positions observed in native β-cyclodextrin. The
deviation in chemical shift is a direct measure of the anisot-
ropy of the naphthalene’s ring current, and therefore
the degree of deviation, summarized for H1-H6 of each
pyranose in Figure 5, is indispensable in assigning the
location and orientation of the appended naphthalene in
the cyclodextrin cavity. The H3 protons, located inside the
cavity near the secondary rim, do not show any appreciable
deviation and only the H5 and H6 protons, located near the
primary rim, suffer substantial changes in chemical shift,
indicating that the naphthalene is shallowly included in the
cavity and no supramolecular aggregation occurs even at this
concentration. Thus, only the H5 and H6a protons of
pyranose A and the H5 and H6a,b protons of pyranose
G deviate to the down fields, while the H5 and H6 protons of
pyranoses B-E and the H6b of pyranose A show moderate
to large upfield shifts. These chemical shift changes are fully
compatible with the shallow inclusion of tilted naphthalene
deduced from the CD spectral examinations, and further
indicate that the naphthalene moiety inserts into the cavity in
between the pyranose A and G, causing the downfield shifts
of their H5 protons, and leans over the pyranose E-G,
causing the up-fields shifts of their H5 and H6 protons.
The contrasting behavior of two H6 protons of pyranose A
and the large upfield shift of the H6b of pyranose B support
€
(27) Schneider, H.-J.; Hacket, F.; Rudiger, V.; Ikeda, H. Chem. Rev.
1998, 98, 1755 and references therein .
6718 J. Org. Chem. Vol. 74, No. 17, 2009

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1
FIGURE 4. Aromatic (top) and sugar (middle) region of the 600 MHz H NMR spectrum of a 19.2 mM solution of 1 in D2O at room
temperature; see Chart 3 for the notation of the pyranose units and the numbering of the protons.
the above assignment that the naphthalene moiety is tilted to
the pyranose G side.
SCHEME 2. Complexation of DPP by Host 1
ROESY spectral study more clearly revealed the location
and orientation of the appended naphthalene moiety in the
cyclodextrin cavity. As shown in Figure 6, the naphthalene’s
a, b, and d protons are correlated with the H5 protons of
pyranose E, D and B, respectively. These results indicate that
the naphthalene moiety is shallowly included and tilts to
some extent to the E-G rings. The strong NOE cross peaks
of the naphthalene’s c proton with the H6 of pyranose G and
of the d proton with the H6 of pyranose C further support the
above assignment.
isodichroic point. As illustrated in Figure 7d, the CD in-
tensity at 260 nm increased almost linearly with increasing
DPP concentration, but suddenly leveled off at the stoichio-
metric concentration, most probably due to the precipitation
of uncomplexed DPP at the higher concentrations, as in-
dicated by the UV absorbance plot (Figure 7c).28 This
observation indirectly indicates the formation of 1:1 complex
of DPP with 1 as illustrated in Scheme 2.
Complexation of DPP with Host 1. The complexation
behavior of DPP with 1 was first examined by UV-vis and
CD spectral titrations. As shown in Figure 7a, gradual
addition of DPP to a 0.128 mM solution of 1 in 3:1 (v/v)
water-methanol did not cause any appreciable change in the
naphthalene absorption but simply increased the absorbance
at shorter wavelengths, which is assignable to the absorption
of DPP. However, the addition of DPP induced significant
changes in CD spectrum (Figure 7b), that is, the reduction of
intensity in the naphthalene’s 1La region and the significant
(28) Upon addition of DPP into D2O solution of 1, the white solid that
would be supramolecular complex consisting of 1 and DPP precipitated
under NMR condition.
1
enhancement in the DPP’s Lb region, with accompanying
J. Org. Chem. Vol. 74, No. 17, 2009 6719

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FIGURE 5. Deviation in chemical shift (Δδ) of the H1-H6 protons
of pyranose units A-G in 1 from those of native β-cyclodextrin.
The complexation behavior of host 1 with DPP was
further investigated in aqueous methanol of different com-
positions by examining the CD spectral changes upon addi-
tion of DPP. As shown in Figure 8a, the original CD
spectrum of 1 (1 mM) was not appreciably affected by the
addition of an equimolar amount of DPP in pure methanol,
indicating no interaction with 1 in the organic solvent.
However, the addition of an equimolar amount of DPP to
a less-concentrated 0.1 mM solution of 1 caused noticeable
changes in CD spectrum in 50% aqueous methanol and
particularly in pure water. The negative Cotton effect in-
duced in the DPP-absorbing region, clearly shown in the
difference CD spectra (Figure 8b), indicates the inclusion
complexation of DPP by host 1 in the aqueous solutions. It is
interesting that, despite the difference in the original orienta-
tion of the naphthalene moiety, the ICD signals observed in
50% methanol and pure water, share the same sign, indicat-
ing similar penetration mode upon inclusion complexation
of DPP by 1. Unfortunately, the absolute CD spectral
FIGURE 6. Partial ROESY spectrum of 1 in D2O obtained with a
mixing time of 1.5 s and the assignment of the cross peaks.
different from the previous observations that a fluorophore
appended to cyclodextrin generally gives two lifetimes attri-
butable to free and included species.20j-l,o The present ob-
servation may be ascribed to the total inclusion of the
naphthyl moiety as a result of its highly hydrophobic nature
or to the comparable lifetimes for both species as a result of
the shallow inclusion. The inherent lifetime of 1 gradually
increased with increasing water content, as was the case with
the fluoresce intensity (Figure 9). In the presence of DPP, the
lifetime was not apparently changed (within the instrumental
error of ( 0.1 ns) at higher methanol contents, but was
appreciably shortened due to the quenching by DPP in
solutions of high water content. These results indicate that
in high water-content solutions the naphthyl moiety is totally
self-included in the cyclodextrin cavity and its fluorescence is
quenched even by 10 μM DPP without accompanying
appreciable exciplex fluorescence.
Competitive Photoaddition of Water and Methanol to DPP
Included and Sensitized by Host 1. The enantiodifferentiating
anti-Markovnikov photoaddition of water and methanol to
DPP (Scheme 3) was mediated by sensitizing host 1 in
aqueous methanol at different temperatures to give chiral
water adduct 3 and methanol adduct 4 in varying yields and
ee’s. As shown in Table 2,29 no adduct was produced in pure
methanol upon irradiation of 0.1 mM DPP with 1 mM 1.
This seems reasonable since the affinity of DPP to host 1 is
extremely low in methanol in particular at the concentrations
employed, as demonstrated by the lack of ICD (Figure 8b)
and the negligible fluorescence spectral change (Figure 9)
upon addition of DPP to a methanol solution of 1. By
increasing the DPP concentration up to 20 mM and extend-
ing the irradiation period to 16 h, a trace amount of methanol
adduct 4 was detected by GC but the ee was only 1.7% at
changes were not sufficiently large (ΔΔε < 1 M-1 cm-1
)
for accurately determining the binding constant even in pure
water.
Steady-state fluorescence spectral and fluorescence life-
time studies were performed to elucidate the excited-state
supramolecular interactions of DPP with host 1. Figure 9
shows the fluorescence spectra of 1 (1.0 ꢀ 10-5 M) in the
presence and absence of an equimolar amount of DPP in
pure methanol, 50% aqueous methanol, and pure water. In
pure methanol, the fluorescence spectrum of 1 was not
affected at all by the addition of DPP, which is reasonable
as DPP does not form any complex with 1 in methanol (see
the CD Spectral Examination) and is too dilute to dynami-
cally quench the fluorescence of 1. In 50% methanol, the
shape of the fluorescence spectrum in the presence of DPP
was slightly different in particular at longer wavelengths.
However, in pure water, where the complexation of DPP
with 1 is highly encouraged, the fluorescence of 1 was clearly
quenched by DPP included in the cavity. More detailed
analysis of the fluorescence quenching behavior in 50%
methanol and in water will be discussed later.
The fluorescence lifetimes of 1 in the presence and absence
of DPP were measured in water-methanol mixed solvents
of varying compositions by means of the time-correlated
single-photon-counting technique. The results obtained are
summarized in Table 1. The naphthyl fluorophore in 1
showed an apparently single-exponential decay in aqueous
solutions containing 0-50% methanol, which is somewhat
(29) The method of supramolecular photosensitizations of DPP included
by 1 was shown in the Experimental Section.
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FIGURE 7. (a) UV-vis and (b) CD spectra of 3:1 (v/v) H2O-MeOH solutions of 1 (0.128 mM; fixed) and DPP (0-0.178 mM) at 25 ꢀC and
(c) the absorbance and (d) circular dichroism changes at 260 nm as functions of the DPP concentration.
FIGURE 8. (a) CD spectra of 1 in the absence (solid line) and presence (dashed line) of DPP at 25 ꢀC; Black, [1] = [DPP] = 1.0 mM in pure
methanol (1 mm cell); Red, [1] = [DPP] = 0.1 mM in 50% aqueous methanol (1 cm cell); Blue, [1] = [DPP] = 0.1 mM in pure water (1 cm cell).
(b) Corresponding CD spectral changes (ΔΔε: difference in circular dichroism) caused by the addition of DPP to 1.
25 ꢀC and -2.8% at -40 ꢀC.30 This indicates that DPP is not
included in the cavity and hence the photoreaction occurs in
bulk methanol, where the cyclodextrin simply functions as a
chiral auxiliary attached to the naphthalene sensitizer, as was
the case in the conventional asymmetric photosensitization
in organic solutions.5-8,18 However, the addition of g25%
water to methanol greatly accelerated the photoreaction
even in a dilute solution containing 0.1 mM each of DPP
and 1, as shown in Table 2. Thus, 71-99% of DPP was
consumed after 2 h irradiation in aqueous methanol to give
water adduct 3 and methanol adduct 4 in 25-74% combined
yields, along with a varying amount of benzophenone as an
oxidation product despite the argon bubbling prior to the
irradiation. Judging from the CD spectral changes of 1 upon
addition of DPP in aqueous methanol shown in Figure 8, we
may conclude that this remarkable acceleration was accom-
plished through the enhanced binding of DPP by 1 and the
subsequent photoinduced electron transfer from DPP to the
excited naphthalene moiety closely located in the cavity.
Consequently, the photosensitization mechanism is switched
from the conventional molecular level in pure methanol to
the supramolecular regime in aqueous methanol solutions.
Effect of Conversion on Product ee. This photoaddition
reaction is irreversible and the product’s ee is not expected to
depend on the conversion of substrate as long as the host-
guest complexation is maintained until the end of the reac-
tion. To confirm this, we performed the photoreaction of
(30) GC chromatographic condition of 4 was shown in the Supporting
Information (Instruments in General Experimental Methods).
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SCHEME 3. Enantiodifferentiating Anti-Markovnikov Photo-
addition of Water and Methanol to DPP Included and Sensitized
by 1
FIGURE 9. Fluorescence spectra of 1 in the absence (solid lines)
and presence (dashed lines) of DPP at room temperature; [1] =
[DPP] = 1.0 ꢀ 10-5 M, excitation wavelength was 300 nm.
TABLE 1. Fluorescence Lifetimes (τ/ns) of 1 and 2 in the Presence/
Absence of DPP in Aqueous Methanola
in view of the nucleophilic nature of the water/methanol
attack to DPP and the higher acidity, or lower basicity, of
water (pKa = 14.0 at 25 ꢀC)31 than that of methanol (pKa =
15.5 at 25 ꢀC).31
additive
compd
% methanol
none
DPPb
1
50
35
25
10
0
0.6
0.8
1.0
1.0
1.4
0.7
0.6
0.8
0.8
0.8
Enantiodifferentiation Mechanisms. There are two poten-
tially enantiodifferentiating steps in this supramolecular
photochirogenic reaction, which are the complexation of
DPP by host 1 in the ground state and the subsequent
photoaddition of water/methanol in the excited state. In
principle, both of the thermodynamic and kinetic processes
contribute to the discrimination of the enantiofaces of DPP.
As illustrated in Scheme 4, the two enantiofaces of the
olefinic part of DPP are differentiated upon complexation
with chiral host 1 to give a pair of diastereomeric complexes
(with equilibrium constants Kr and Ks). In these complexes,
either re- or si-enantioface of DPP is more exposed to the
bulk solution and hence attacked by water/methanol upon
photoinduced electron transfer to yield (R)- or (S)-3 or 4 at
rate constant krR or ksS, although the hindered face may be
attacked less efficiently to give antipodal (S)- or (R)-3 or 4 at
krS or ksR. Thus, the enantiomer ratio of 3 or 4 can be defined
by eq 1.
1.0
c
2
25
aMeasured with 1.0 ꢀ 10-5 M solutions of 1 and 2 at room
temperature under aerated (nondegassed) conditions by the time-corre-
lated single-photon-counting technique; error ( 0.1 ns. b[DPP] = 1.0 ꢀ
10-5 M. cNot determined.
DPP mediated by 1 for varying periods of irradiation (from 5
min to 2 h) in 25% methanol solution at -10 ꢀC (a condition
that guarantees strong complexation), and the conversion
and the ee of methanol adduct 4 were determined. As shown
in Table 2, the ee of 4 obtained for different irradiation
periods nicely agreed with each other to give an average value
of 10.6 ( 0.7%, indicating that we can safely discuss the
product’s ee obtained at any irradiation period or conversion
at least for those obtained in high water-content solutions at
low temperatures.
Effects of Solvent Composition and Temperature on Pro-
duct Ratio 3/4. In this competitive photoaddition of water
and methanol to DPP, the product ratio 3/4 is a critical
function of the solvent composition, as can be seen from
Table 2. However, by taking into account of the molar ratio
of the attacking agents, i.e. water and methanol, the relative
reactivity for water versus methanol was calculated as shown
in Table 3. The relative reactivity thus obtained at 45, 25, or
-10 ꢀC is constant, irrespective of the water/methanol ratio.
Interestingly, the temperature effect on the relative reactivity
is modest and somewhat puzzling, displaying an appreciable
increase from 0.35 to 0.45 by decreasing the temperature
from 45 to -20 ꢀC but a subsequent decrease to 0.34 at
-45 ꢀC. By assuming that the temperature does not greatly
affect the relative reactivity, we obtain an overall average of
0.40 ( 0.06 for the competitive photoaddition of water and
methanol to DPP. This value indicates that the attack of
water to radical cationic DPP, which is produced upon
photoinduced electron transfer to the naphthyl moiety of
1 within the cyclodextrin cavity, is slower or less efficient by
a factor of 2.5 than that of methanol. This seems reasonable
½Rꢁ=½Sꢁ ¼ ðKr=KsÞðkrR þ ksRÞ=ðksS þ krSÞ
ð1Þ
By neglecting the minor paths (the attack from the hindered
side), eq 1 is simplified to eq 2.
½Rꢁ=½Sꢁ ¼ ðKr=KsÞðkrR=ksSÞ
ð2Þ
This equation clearly indicates that the product’s ee is
controlled thermodynamically by the stability difference
between the diastereomeric re- and si-complexes in the
ground state and also kinetically by the difference in rate
constant of the subsequent water or methanol attack to the
radical cationic DPP.
Enantioselectivity and the Eyring Analysis. As discussed
above, the enantioface of DPP can be discriminated upon
(1) complexation with chiral host 1 and/or (2) the subsequent
nucleophilic attack of water/methanol to radical cationic DPP
within the chiral host cavity. If only the first step is enantioface
selective and the second is racemic (nonenantioface-selective),
the water and methanol adducts should give exactly the same ee.
(31) David. R. L. CRC Handbook of Chemistry and Physics; CRC Press:
Boca Raton, FL, 2003-2004.
6722 J. Org. Chem. Vol. 74, No. 17, 2009

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TABLE 2. Enantiodifferentiating Anti-Markovnikov Photoaddition of Water and Methanol to DPP Sensitized by Host 1 in Aqueous Methanola
yield/%d
ee/%
% MeOH ([H2O]/[MeOH]b) [DPP]/mM [1]/mM temperature/ꢀC irradiation time/h conversion/%c
3e 4 f
3/4
3g
4h
i
100 (0)
0.1
20
1.0
1.0
25
25
-10
-40
45
2
16 j
16 j
16 j
2
2
2
2
2
k
<1
<1
1.7
-0.9
-2.8
k
k
l
<1
l
l
l
75 (0.75)
50 (2.24)
0.1
0.1
0.1
0.1
0.28 -0.9
0.32 -0.3
0.34
0.27
0.77
0.79
25
71
74
77
97
6
10
12
23
19
19
29
45
30
24
32
1.4
2.0
2.8
-10
-40
45
1.2
2.7
1.9 -2.1
3.9 -0.8
6.5
25
2
2
2
2
2
2
2
97
>99
96
-10
-40
-45
45
25
0
26
0.81
2.1
5.8
l
l
l
61
l
l
l
l
0.76
1.4
1.5
1.7
1.9
2.4
2.9
3.1
12.0
3.5
6.0
9.7
14.9
4.6
6.9
8.9
35 (4.16)
25 (6.73)
0.1
0.1
0.1
0.1
>99
>99
91
24
28
32
41
36
53
49
17
19
19
22
15
18
16
18
9
13
15
15
6
4.3
4.8
6.8
9.6
6.5
7.8
8.0
10.1
9.7
11.4
10.5
10.9
8.3
-20
45
25
15
0
-10
2
2
2
2
89
>99
95
>99
>99
68
2
0.083
0.5
1
56
3.1
12.3
l
l
l
l
l
l
l
l
l
88
93
2
94
46
42
63
3.1
7.0
14.9
5.7
9.4
13.0
10 (20.2)
0.1
0.1
0.1
0.1
45
25
5
-5
-10
45
25
10
0
2
2
2
2
2
2
2
2
>99
93
91
6
10.5
9.3
8.7
56
6
4
3
10.9
11.6
13.4
l
l
l
85
71
l
l
l
l
l
l
18.1
6.4
9.4
13.9
15.6
l
0 (¥)
l
l
l
2
aIrradiated under an argon atmosphere in aqueous methanol with an ultrahigh-pressure Hg lamp (500 W) fitted with a UV-29 glass filter, unless
stated otherwise. bMolar ratio. cLoss of starting material determined by GC. dYield based on the conversion; error in yield <1%; despite the argon
purge, benzophenone was produced as a side product in varying yields particularly at higher temperatures. eChemical yield of water adduct 3 determined
by HPLC. fChemical yield of methanol adduct 4 determined by GC. gEnantiomeric excess of 3 determined by chiral HPLC; error in ee <1% (see the
Supporting Information in Figure S4). hEnantiomeric excess of 4 determined by chiral GC; error in ee <0.5%. iNo reaction. jIrradiated with a high-
pressure Hg lamp (300 W) fitted with a uranium glass filter. kValue not determined due to low conversion. lNot determined.
TABLE 3. Relative Reactivity of Water versus Methanol upon Photoaddition to DPP Included and Sensitized by Host 1 in Aqueous Methanol
temperature/ꢀC
[H2O]/[MeOH]
product ratio 3/4
relative reactivity H2O/MeOH
45
0.75
2.24
4.16
6.73
20.2
0.28
0.77
1.4
2.4
7.0
0.37
0.34
0.34
0.36
0.35
average
0.32
0.79
1.5
0.35 ( 0.01
0.43
0.35
25
0.75
2.24
4.16
6.73
20.2
0.36
0.43
0.52
2.9
10.5
average
3.1
1.7
3.1
0.42 ( 0.07
0.46
0.41
15
0
6.73
4.16
6.73
0.46
average
9.3
0.34
0.81
3.1
0.44 ( 0.03
0.46
0.45
5
-10
20.2
0.75
2.24
6.73
0.36
0.46
average
1.9
0.27
0.76
0.43 ( 0.05
0.45
0.36
0.34
0.40 ( 0.06
-20
-40
-45
65:35 [4.16]
25:75 [0.75]
50:50 [2.24]
overall average
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Fukuhara et al.
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FIGURE 10. Eyring plots: Temperature dependence of the ee values of (a) water adduct 3 and (b) methanol adduct 4 obtained in the
enantiodifferentiating anti-Markovnikov photoaddition of water and methanol to DPP included and sensitized by host 1 in methanol ( ꢀ ),
75% methanol (2), 50% methanol (9), 35% methanol (1), 25% methanol (b), 10% methanol ((), and water (þ).
SCHEME 4. Thermodynamic Enantioface Differentiation
(Kr/Ks) upon Complexation of DPP with Host 1 in the Ground
State and Kinetic Enantioface Differentiation (krR/ksS) upon
Subsequent Nucleophilic Attack of ROH (Water or Methanol) to
Radical Cationic DPP Generated by Photoinduced Electron
Transfer to the Cyanonaphthalene Moiety of Sensitizing Host 1a
It is also interesting that the product chirality is inverted
indeed or expected to be inverted within the temperature
range employed. Such a remarkable phenomenon has
already been reported for several conventional asymmetric
photosensitizations in isotropic media5-8,18 and also for the
supramolecular enantiodifferentiating photoisomerization
of (Z)-cyclooctene mediated by sensitizer-appended per-
methylcyclodextrins.12g,k This apparently unusual inversion
of product chirality by temperature can be rationalized by
using the differential Gibbs-Helmholtz equation: ΔΔGq =
ΔΔHq-TΔΔSq. At low temperatures, the differential activa-
tion free energy ΔΔGq, which determines the product’s ee, is
dominated by the enthalpy term ΔΔH‡. However, when the
signs of ΔΔHq and ΔΔSq are the same as in the present cases,
the entropy term TΔΔSq overwhelms the enthalpy term
ΔΔHq at a critical (equipodal) temperature (T0) to invert
the sign of ΔΔGq and give the antipodal product at tempera-
tures higher than T0.5-8 In the present case, the differential
activation parameters ΔΔHq and ΔΔSq obtained in the
Eyring analysis are considered to be a weighted sum of the
differential thermodynamic parameter for the enantioface-
differentiating complexation of DPP with host 1 and the
differential activation parameter for the enantioface-differ-
entiating attack of water/methanol to radical cationic DPP
within the host 1 cavity.
To quantitatively evaluate the enthalpic and entropic
contributions to the enantiodifferentiation upon com-
plexation and photoaddition, the ee data obtained at
different temperatures in each aqueous methanol solution
were subjected to the Eyring analysis.5-8,18 Thus, the
natural logarithm of the apparent relative rate constants
for the formation of (R)- and (S)-form of 3 and 4, that is
kR/kS = (100 þ % ee)/(100 - % ee), were plotted against
the reciprocal temperature.
As illustrated in Figure 10, each set of the data points
obtained for 3 in aqueous methanol of different composi-
tions falls on a single straight line, indicating that the
enantiodifferentiation mechanism does not change in the
temperature range employed in each solvent. However,
the data for 4 do not appear to converge at a single point
as is the case for 3 in particular in solvents of high methanol
contents, suggesting that the operating mechanism may not
be the same upon attack of water and methanol to DPP in
a“re” and “si” indicate the open enantioface (front side) in the pair of
diastereomeric 1•DPP complexes, while the dotted arrow indicate the minor
path involving the attack from the hindered side.
However, the experimental ee’s obtained for 3 and 4 (Table 2)
significantly differ from each other (by 3-5%, which exceeds
the experimental error of (1%) in several runs particularly
those performed at low temperatures. This reveals that both of
the complexation and the subsequent photoaddition contribute
to the enantiodifferentiation. Thus, the higher ee’s of 15-18%
were obtained for water adduct 3 in the photoreaction per-
formed in more hydrophilic solvents at 0 to -20 ꢀC, while the
best ee’s for methanol adduct 4 (10-13% ee) were appreciably
lower under the comparable conditions, probably because the
faster attack of methanol is less enantioselective. The higher ee’s
obtained generally in more water-rich solvents at lower tem-
peratures may be ascribed to the enhanced and tighter binding
of DPP in host 1 cavity.
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Fukuhara et al.
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TABLE 4. Differential Activation Parameters for Competitive Enantiodifferentiating Photoaddition of Water and Methanol to DPP Mediated by Host 1
in Aqueous Methanol Solutions at 298 Ka
water adduct 3
ΔΔSqR-S/J mol-1 K-1
methanol adduct 4
Solvent
ΔΔHqR-S/kJ mol-1
ΔΔHqR-S/kJ mol-1
ΔΔSqR-S/J mol-1 K-1
100% MeOH
75% MeOH
50% MeOH
35% MeOH
25% MeOH
10% MeOH
H2O
0.8
-0.3
-1.2
-1.1
-1.2
-1.0
2.9
-0.6
-4.0
-2.9
-2.6
-1.9
-0.5
-1.3
-2.3
-2.7
-3.1
-3.1
-1.8
-3.9
-6.8
-7.7
-8.8
-8.8
a Error: <0.3 kJ mol-1 for ΔΔHq and <1.0 J mol-1 K-1 for ΔΔSq.
FIGURE 11. Fluorescence spectra of a 0.01 mM solution of 1 excited at 300 nm upon addition of various concentrations of DPP in (a) 50%
MeOH and (b) 25% MeOH at room temperature. In 50% MeOH, exciplex emission obtained by spectral subtraction is shown in the inset.
highly methanolic solutions. Nevertheless, the differential
activation enthalpy (ΔΔH‡R-S) and entropy (ΔΔS‡R-S) can
be calculated from the slope and intercept of the Eyring plots
in Figure 10; the results are listed in Table 4.
Fluorescence Quenching of 1 by DPP. To further elucidate
the excited state involved in the enantiodifferentiation process
and also to evaluate the rate constants of the relevant processes,
we performed the fluorescence quenching experiments in aqu-
eous solutions containing 25-50% methanol. In 50% methanol,
the fluorescence of 1 was only slightly quenched by 0.0425 mM
DPP to show a new weak emission at ca. 380 nm attributable to
an exciplex intermediate (Figure 11, inset), with accompanying
isoemissive point at 345 nm. In contrast, the fluorescence
quenching was much more efficient in 25% methanol solution,
FIGURE 12. Stern-Volmer plot for fluorescence quenching of
0.01 mM 1 by DPP in 25% MeOH at room temperature.
displaying appreciable decrease of fluorescence intensity upon
addition of DPP of up to 0.029 mM, which however did not
accompany any new emission at longer wavelengths. The effi-
cient quenching in the latter solvent may be ascribed to the
enhanced binding of DPP by host 1, which facilitates the
subsequent static quenching within the cyclodextrin cavity.
The fluorescence quenching data obtained in 25% metha-
nol were analyzed by using the conventional Stern-Volmer
equation (eq 3) to evaluate the apparent quenching rate
constant in this supramolecular complex. The relative fluor-
escence intensity (IF0/IF) was plotted against the concen-
tration of added DPP to give a good straight line up to
0.0174 mM but then level off, as shown in Figure 12. The
saturation behavior may be ascribed to the full occupation of
host 1 cavity by DPP, although the binding constant could
not be determined for this host-guest system for the solu-
bility reason.32 From the slope of the plot, the Stern-Volmer
constant (kQτ0) was determined to be 5600 M-1. The appar-
ent quenching constant (kQ) calculated by using the kQτ0
value thus obtained and the fluorescence lifetime (τ0
=
1.0 ns, Table 1) amounts to 5.6 ꢀ 1012 M-1 s-1. This value
far exceeds the diffusion-controlled rate constants (kdiff
)
)
in both water (6.5 ꢀ 109 M-1 s-1 33
and methanol (1.1 ꢀ
1010 M-1 s-1 33
and is therefore assignable to the static
)
quenching of the naphthyl moiety of 1 by DPP tightly
packed in the cyclodextrin cavity, leading to the immediate
electron transfer affording radical cationic DPP. In more
(32) As shown in the Supporting Information (Figure S5), under the
diluted concentration, the binding constant was not obtained even by the
fluorescence titration experiments due to the low solubility of DPP as
revealed by UV titration.
(33) Murov. S. L. Handbook of Photochemistry; Marcel Dekker: New
York, 1973.
J. Org. Chem. Vol. 74, No. 17, 2009 6725

Fukuhara et al.
JOCArticle
which is not available upon the addition of water, is operat-
ing for the methanol addition in the solutions of low metha-
nol contents. As one of such mechanisms, we may suggest the
static intracavity attack of coincluded methanol34 to DPP
tightly bound to cyclodextrin in particular in highly hydro-
phobic, less methanolic solutions, which may be related
to the pronounced bathochromic shift of fluorescence at
low methanol contents discussed above (Figure 3). The
contribution of such a mechanism, and therefore the devia-
tion from the original compensation plot, are negligible in
50-100% methanol, but become clearer as the methanol
content in aqueous solution is further reduced to 35% or less,
as shown in Figure 13.
Optimization of Product ee. As shown in Table 2, the ee’s of
3 and 4 are critical functions of the methanol content and the
temperature, and the highest values of 18% ee for 3 and 13%
for 4 were obtained upon irradiation of an equimolar mixture
of DPP (0.1 mM) and host 1 (0.1 mM) in 10% methanol at
-10 ꢀC. This is reasonable as the major enantiodifferentiation
occurs only within the chiral cavity of host 1. In this context,
it is sensible to examine the photosensitization at a much
higher host concentration. Indeed, the photoreaction of DPP
(0.1 mM) sensitized by host 1 of 2.0 mM in 10% methanol
solution at -10 ꢀC afforded water adduct 3 in an enhanced
ee of 24.3% and methanol adduct 4 in a decreased ee of 8.5%.
The contrasting behavior of ee observed for the water and
methanol adducts is consistent with the above-mentioned
mechanism. Thus, the use of a 20-fold excess amount of host
enhances the complexation of DPP, which simply leads to a
higher ee for water adduct 3 but allows the static, and therefore
less enantiodifferentiating, intracavity attack of coincluded
methanol to produce adduct 4 in lower ee.
We also examined the pressure effect on this enantiodif-
ferentiating supramolecular photoaddition. However, the
photosensitization of DPP (0.1 mM) by host 1 (2.0 mM) in
10% methanol at -19 ꢀC under a pressure of 210 MPa gave
water adduct 3 in 24.4% ee, which is comparable to that
obtained above in the same solvent at -10 ꢀC and atmo-
spheric pressure (0.1 MPa). This means that the effects of
increasing pressure and decreasing temperature are canceled
to each other in this supramolecular photosensitization.
Since the temperature turned out to be an important factor
to critically control the ee of 3, we tried to decrease the freezing
point of 10% methanol not by applying high pressure but by
adding a salt. A 10% methanol solution of DPP (0.1 mM), 1
(2.0 mM), and aluminum perchlorate (1.5 M) was photolyzed
at -17 ꢀC to afford 3 in an appreciably better ee of 26.4%.
FIGURE 13. Enthalpy-entropy compensation plot of the differ-
ential activation parameters obtained for water adduct 3 (red) and
methanol adduct 4 (black) in aqueous solutions of varying methanol
contents from 0 to 100%.
lipophilic 50% methanol solution, it is likely that the com-
plexation of DPP is much weaker and most of the DPP
is populated in the bulk solution even at the concentration
of 0.0425 mM. Furthermore, the naphthyl moiety of 1 is
less tightly bound and more exposed to the bulk solution
(as judged from the CD spectral changes in Figure 8),
allowing the dynamic quenching by DPP in the bulk solution
and the subsequent exciplex formation as was the case in
homogeneous solution.18
IF0=IF ¼ 1þkQτ0½Qꢁ
ð3Þ
Enthalpy-Entropy Compensation. Compensatory enthalpy-
entropy relationship has widely been observed not only for the
inclusion complexation of various organic guests with cyclodex-
trin and other supramolecular hosts10 but also for the enantio-
differentiating photoisomerization and photoaddition reactions
sensitized by chiral aromatic compounds,5-8 and used as a tool
for globally analyzing the ground- and excited-state processes.
Hence, we performed the enthalpy-entropy compensation
analysis of the differential activation parameters obtained in
this study for a better understanding of the enantiodifferentiat-
ing mechanism and factors operating in the photosensitized
addition of water and methanol in the mixed solvents.
In Figure 13, all of the ΔΔHqR-S values listed in Table 4 are
plotted against the corresponding ΔΔSqR-S values to reveal
distinctly different profiles for 3 and 4. Thus, the parameters
for water adduct 3 obtained in pure water to 75% methanol
gives an excellent straight line that passes through the origin,
confirming that the enantiodifferentiating mechanism oper-
ating does not change throughout the methanol contents
from 0 to 75%; the isokinetic temperature (Tiso), which is
equivalent to the slope, is 365 K. In contrast, methanol
adduct 4 gives a very different compensation plot with a
kink (Figure 13, black line), which is related to the less
regular behavior of the ee of 4 in the differential Eyring plot
(Figure 10). The plot of the parameters for 4 obtained at high
methanol contents (50-100%) looks normal, affording a
good straight line that crosses the origin; the Tiso value of
283 K obtained from the slope is significantly lower than that
for 3. However, the parameters obtained for 4 in 10-35%
methanol are clearly deviated from the straight line
(Figure 13), probably because some different mechanism,
Conclusions
In the present study, we first investigated the self-inclusion
behavior of host 1 and the complexation behavior of DPP
with 1 in the ground state, and then the competitive photo-
addition of water and methanol to DPP mediated by 1. The
supramolecular photochirogenesis mechanism turned out to
involve both of the thermodynamically controlled enantio-
face-differentiating complexation of DPP with 1 in the
ground-state equilibrium and the kinetically controlled en-
antioface-differentiating attack of water/methanol to radical
cationic DPP within the host 1 cavity in the excited-state
(34) Huang, J.; Catena, G. C.; Bright, F. V. Appl. Spectrosc. 1992, 46, 606.
6726 J. Org. Chem. Vol. 74, No. 17, 2009

Fukuhara et al.
JOCArticle
rate. However, the enthalpy-entropy compensation analy-
sis of the differential activation parameters obtained for the
supramolecular asymmetric photoaddition further revealed
a difference in the mechanisms that determine the ee’s of 3
and 4, for which the static intracavity attack of coincluded
methanol to DPP tightly included in the cyclodextrin cavity
is likely to be responsible. Utilizing these mechanistic under-
standings, we were able to enhance the ee of water adduct 3
up to 26.4%, which is the highest value ever obtained for a
bimolecular photochirogenic reaction using a chiral photo-
sensitizing supramolecular host.
pure methanol were directly injected into the GC for the
analyses of 4.
Synthesis of 2. Aqueous methylamine (40%) solution (1.0 mL,
11.6 mmol) and 5-cyanonaphthalene-1-carboxylic acid35 (1.15 g,
5.83 mmol) were dissolved in dehydrated N,N-dimethylfor-
mamide (10 mL) in a 100 mL round-bottomed flask under an
argon atmosphere. To the mixture was added 1-ethyl-3-(30-di-
methylaminopropyl)-carbodiimide hydrochloride (EDC) and
the reaction mixture was stirred for 52 h at room temperature.
The resulting solution was extracted with ethyl acetate, and the
extract was washed with water and then with a saturated
aqueous sodium hydrogen carbonate solution. The organic
layer was dried over magnesium sulfate and concentrated, and
the residue was dried under high vacuum. This crude product
was purified by normal phase column chromatography to give 2
(170 mg, 0.81 mmol) as white solid in 14% yield; mp 185 ꢀC; EI-
MS: m/z 210[M]þ; 1H NMR (DMSO-d6, 18 ꢀC): δ 8.65 (q, 1H,
-NH-), 8.55 (d, J 8.64 Hz, 1H, H-8), 8.23 (dd, 2H, H-6, H-4),
7.87-7.71 (m, 3H, H-3, H-2, H-7); 13C NMR (DMSO-d6,
26 ꢀC): δ 168.1 (CdO), 135.9 (C-1), 133.6 (C-6), 131.6 (C-10),
131.3 (C-8), 129.4 (C-9), 128.3 (C-3), 126.7 (C-2), 126.3 (C-7),
125.9 (C-4), 117.5 (CN), 109.2 (C-5), 26.2 (-Me); Anal. Calcd for
C13H10N2O: C, 74.27; H, 4.79; N, 13.33%; Found: C, 74.25; H,
4.83; N, 13.04%.
Experimental Section
Methods. The self-inclusion behavior of host 1 was investi-
gated by UV-vis, CD, and fluorescence spectrometry in aqu-
eous methanol solutions of different compositions at various
temperatures. The self-inclusion was examined also by 2D-
NMR technique. The complexation of DPP by host 1 was
examined by the CD and fluorescence spectroscopy as well as
the fluorescence lifetime measurement. Aqueous methanol solu-
tions (2 mL) of various compositions, containing DPP (0.1 mM)
and 1 (0.1 mM), were irradiated for 2 h under an Ar atmosphere
at temperatures varying from 45 to -45 ꢀC by using a 500-W
ultrahigh-pressure mercury lamp fitted with a UV-29 glass filter,
while methanol solutions (5 mL) in Pyrex tubes (1 cm i.d.),
containing DPP (20 mM) and 1 (1.0 mM), were irradiated at
>320 nm for 16 h under Ar in the same temperature range by
using a 300 W high-pressure mercury lamp fitted with a uranium
glass filter. The photolyzed sample, excepting for that in pure
methanol, was poured onto a 10% aqueous potassium hydro-
xide solution (1 mL) for decomplexation. After further addition
of a saturated sodium chloride solution (1 mL), the mixture was
extracted with diethyl ether (1 mL). A known amount of cyclo-
pentadecane was added to the separated organic layer as an
internal standard for GC, and the ether solution was con-
centrated by Ar bubbling and subjected to the chiral GC
analysis for determining the consumption of DPP and the
chemical yield and ee of 4, and also to the chiral HPLC analysis
for the yield and ee of 3. Representative HPLC chromatograms
of 3 were shown in Figure S4. The photolyzed samples in
Acknowledgment. This work was supported by
a
Research Fellowship for Young Scientists (No. 08918) and
a Grant-in-Aid for Young Scientists (start-up) (No. 20850023)
to G.F., and a Grant-in-Aid for Scientific Research (A) to Y.I.
(No. 21245011) from Japan Society for the Promotion of
Science, which are gratefully acknowledged. We are grateful
to Dr. Toshiko Muneishi for the measurement of 2D-NMR
spectra, Prof. Takehiko Wada at Tohoku University for useful
discussion, and Dr. Cheng Yang and Ms. Yumi Origane for the
assistance in pressure experiments and in the HPLC analyses,
respectively.
Supporting Information Available: Instruments and materials
used in the present study, reaction scheme and NMR spectra for
reference sensitizer 2, CD spectra of host 1, fluorescence spectra of
2, HPLC analyses of 3, UV-vis spectra of host 1 in the presence of
DPP, and DFT optimized geometry of 1. This material is available
free of charge via the Internet at http://pubs.acs.org.
(35) (a) Short, W. F.; Wang, H. J. Chem. Soc. 1950, 991. (b) Dewar, M. J.
S.; Grisdale, P. J. J. Chem. Soc. 1962, 84, 35.
J. Org. Chem. Vol. 74, No. 17, 2009 6727