m-Quinone methides from m-hydroxy-1,1-diaryl alkenes via excited-state (formal) intramolecular proton transfer mediated by a water trimer [16]

Full text of DOI:10.1021/ja974144y

2680
J. Am. Chem. Soc. 1998, 120, 2680-2681
m-Quinone Methides from m-Hydroxy-1,1-Diaryl
Alkenes via Excited-State (Formal) Intramolecular
Proton Transfer Mediated by a Water Trimer
Maike Fischer and Peter Wan*
The photohydration of hydroxy-substituted styryl compounds
1-3 were initially studied, chosen such that if the corresponding
QM were photogenerated, its UV-vis absorption spectra would
be readily detected by nanosecond laser flash photolysis (LFP),
based on results of earlier work.3,6 Photolysis of 1 in 1:1 H2O/
CH3CN (∼10-3 M; Rayonet RPR-100 photochemical reactor; 254
nm lamps; <15 °C; argon-purged solutions; 2-10 min) gave
diarylmethanol 7 cleanly with high quantum yield (> 40%; Φ )
0.22) (eq 2), whereas photolysis of m-methoxy-substituted deriva-
tive 4 gave a residual amount of the corresponding photohydration
product only after extended photolysis (Φ e 0.02). Photolysis
Department of Chemistry, UniVersity of Victoria,
P.O. Box 3065, Victoria, British Columbia, Canada V8W 3V6
ReceiVed December 8, 1997
Excited-state intramolecular proton transfer (ESIPT) reactions
are of fundamental interest.1 ESIPT generally occurs when both
basic and acidic groups are present on the same molecule and
when excitation leads to enhancement of their basicity and acidity,
respectively. The most commonly observed examples are when
the basic and acid groups share a hydrogen bond,1 which facilitates
the proton transfer, although in many of these cases one may
view these reactions as hydrogen transfers or the tunneling of a
proton.1 Although much less common and restricted to only a
few molecular systems, examples in which one or more water
(or alcohol) molecules or substrate mediates the ESIPT (via a
“proton relay” mechanism resulting in formal intramolecular
proton transfer) are also known.1,2 In continuing work3 on the
use of hydroxybenzyl alcohols and related compounds as precur-
sors for the photogeneration of quinone methides (QMs), we came
upon the opportunity of studying the photochemistry of m- and
p-hydroxy-substituted 1,1-diaryl alkenes 1-3 in aqueous solution.
of 1,1-diphenylethylene even under extended photolysis times
resulted in no reaction. Photolysis of the p-isomer 2 also yielded
the corresponding photohydration product cleanly (Φ ≈ 0.1).
Similar photohydration chemistry was observed for indene 3 (Φ
) 0.24). In this case, the photohydration product was found to
be thermally labile and readily dehydrated back to 3 on workup.
However, photolysis in an NMR tube in D2O/CD3CN showed
clean formation of the expected alcohol. All of these results
indicate that the hydroxyl group is necessary for efficient
photohydration in these compounds suggesting the possibility of
ESIPT as the primary photochemical step.7
Kalanderopoulos and Yates4 reported the efficient photohydration
of o-hydroxystyrenes via ESIPT from the phenol to the â-carbon
of the alkene moiety, to generate proposed o-QMs,5 resulting in
â-arylethanol products, as shown in eq 1 for the parent o-
hydroxystyrene (6). We decided to test whether ESIPT can
operate for m- and p-hydroxy-substituted styryl systems 1-3
(which would give rise to m- and p-QMs, respectively) where
the acid/base moieties are distal from each other. We find that
all of 1-3 react efficiently in aqueous solution, to give the
corresponding photohydration product, and show that the mech-
anism involves proton transfer (formally intramolecular) requiring
a “proton relay” mechanism assisted by solvent water.
Steady-state fluorescence studies provided additional details
with respect to the photohydration mechanism. As shown in
Figure 1, addition of small amounts of H2O to a CH3CN solution
of 1 (or 3 but not 4, 5, or 1,1′-diphenylethylene) efficiently
quenched the fluorescence emission. CH3OH quenches to a much
lesser extent, and THF quenches not at all. However, the Stern-
Volmer plots are not linear and are curved upward. This curved
dependence on H2O concentration in Stern-Volmer analysis has
been previously observed for several other excited-state proton
transfer systems where a modified plot using higher order de-
pendence on water concentration was used to linearize such plots
thus providing information with regard to the molecularity of the
quencher.8 The fluorescence quenching data for 1 and 3 fit the
modified Stern-Volmer plot with a cubed quencher concentration
dependence (Figure 2). This is consistent with an ESIPT
mechanism in which water in the form of a trimer is involved in
the deactivation of the singlet state and responsible for the reac-
tion. The rate constants for quenching of 1 and 3 by H2O were
3.2 × 108 M-3 s-1 and 1.5 × 108 M-3 s-1, respectively. Use of
D2O resulted in ∼50% less efficient quenching for both 1 and 3,
again consistent with proton transfer as the primary photochemical
(1) (a) Formosinho, S. J.; Arnaut, L. G. J. Photochem. Photobiol. A 1993,
75, 21. (b) Kasha, M. J. Chem. Soc., Faraday Trans. 2 1986, 82, 2379. (c)
Heldt, J.; Gormin, D.; Kasha, M. Chem. Phys. 1989, 136, 321. (d) Le
Gourrierec, D.; Ormson, S. M.; Brown, R. G. Prog. React. Kinet. 1994, 19,
211. (e) Hibbert, F. AdV. Phys. Org. Chem. 1986, 22, 113.
(2) Selected references: (a) Chou, P.-T.; Wei, C.-Y.; Chang, C.-P.; Chiu,
C.-H. J. Am. Chem. Soc. 1995, 117, 7259. (b) Chen, Y.; Gai, F.; Petrich, J.
W. J. Am. Chem. Soc. 1993, 115, 10158. (c) Suzuki, T.; Okuyama, U.;
Ichimura, T. J. Phys. Chem. A 1997, 101, 7047. (d) Herbich, J.; Hung, C.-Y.;
Thummel, R. P.; Waluk, J. J. Am. Chem. Soc. 1996, 118, 3508. (e) Itoh, M.;
Adachi, T.; Tokumura, K. J. Am. Chem. Soc. 1984, 106, 850. (f) Konijnenberg,
J.; Ekelmans, G. B.; Huizer, A. H.; Varma, C. A. G. O. J. Chem. Soc., Faraday
Trans. 2 1989, 85, 39. (g) Itoh, M.; Yoshida, N.; Takashima, M. J. Am. Chem.
Soc. 1985, 107, 4819. (h) Itoh, M.; Adachi, T. J. Am. Chem. Soc. 1984, 106,
4320. (i) Nimbos, M. R.; Kelly, D. F.; Bernstein, E. R. J. Phys. Chem. 1989,
93, 643. (j) Choi, J. D. C.; Fugate, R. D.; Song, P.-S. J. Am. Chem. Soc.
1980, 102, 5293.
(3) (a) Diao, L.; Yang, C.; Wan, P. J. Am. Chem. Soc. 1995, 117, 5369.
(b) Wan, P.; Barker, B.; Diao, L.; Fischer, M.; Shi, Y.; Yang, C. Can. J.
Chem. 1996, 74, 465. (c) Shi, Y.; Wan, P. J. Chem. Soc., Chem. Commun.
1995, 1217. (d) Shi, Y.; Wan, P. J. Chem. Soc., Chem. Commun. 1997, 273.
(4) Kalanderopoulos, P.; Yates, K. J. Am. Chem. Soc. 1986, 108, 6290.
(5) The authors did not explicitly state that an o-QM intermediate is formed
in their photohydration reactions, although the structure drawn for their
zwitterionic intermediate is the commonly accepted important resonance form
for such a species.
(6) The lifetime for the m-QM from m-hydroxystyrene is expected to be
much less than 20 ns in aqueous solution and hence undetectable by
nanosecond LFP.3 For this reason, we have studied only m-hydroxystyryl
system possessing an R-phenyl group (1 and 3).
(7) Photolysis of 1 or 3 at pH 13 (excitation of the phenolate) gave
essentially no reaction under similar photolysis times consistent with the
requirement of the phenol proton in the reaction mechanism. It also rules out
a stepwise mechanism in which the phenol dissociates adiabatically on
excitation, to generate an excited phenolate, which then reacts by protonation
(by water or hydronium ion) at the â-carbon.
(8) (a) Tolbert, L. M.; Haubrich, J. E. J. Am. Chem. Soc. 1994, 116, 10593.
(b) Moore, R. A.; Lee. J.; Robinson, G. W. J. Chem. Phys. 1985, 89, 3648.
(c) Lee, J.; Robinson, G. W.; Webb, S. P.; Philips, L. A.; Clark, J. H. J. Am.
Chem. Soc. 1986, 108, 6538.
S0002-7863(97)04144-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/07/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 11, 1998 2681
Figure 1. Fluorescence quenching of 1 by added H2O in CH3CN (λexcit
) 290 nm; Φf (100% CH3CN) ) 0.20; τ (100% CH3CN) ) 5.5 ns). The
fluorescence observed is that of the phenol (ArOH); emission from ArO-
is not observed.
Figure 3. LFP transients (taken 50-60 ns after the pulse) assigned to
the corresponding m-QMs (λexcit ) 266 nm) of 1 and 3 in aqueous solution.
All transient spectra are observed within the laser pulse (∼10 ns). b
m-QM 8 from 1 (in 100% H2O), τ ) 47 ns; 9 m-QM 9 from 3 (in 9:1
H2O/CH3CN), τ ) 5.0 µs.
Scheme 1
to baseline. No transients were observed on photolysis in 100%
CH3CN but addition of a small amount of H2O (<3 M) to the
CH3CN solution produced increasing amounts of the above
transients which corroborates the interpretation of the fluorescence
quenching results. In addition, the observed lifetimes decreased
with increasing water concentration consistent with nucleophilic
quenching (probably concerted with proton transfer to the
phenolate) by water as the main mode of reaction. LFP of 4 and
5 gave weaker signals probably assignable to radical cations (and
to a minor amount of the corresponding diarylmethyl cation for
5). These results indicate that the transients shown in Figure 3
are m-QMs 8 and 9, formed from water-trimer-mediated ESIPT
of 1 and 3 (in S1), respectively (Scheme 1).12,13
Figure 2. Modified Stern-Volmer plots for fluorescence quenching by
H2O and D2O (in CH3CN) for 1 (b D2O; 9 H2O). The points represent
experimental data and the lines are fits to the modified Stern-Volmer
equation, I°/I ) 1 + kq[Q]3.
step responsible for the quenching of fluorescence.9 Fluorescence
emission from the p-isomer 2 was too weak for reliable quenching
studies.10 These H2O-mediated ESIPTs are irreversible since
photolysis of 3 in D2O/CD3CN did not result in formation of any
deuterium label at the â-carbon in substrate, which would be
expected if back proton transfer was significant. Instead, these
photogenerated m-QMs are completely trapped by H2O.
In summary, ESIPT from the phenol moiety to the â-carbon
of a conjugated alkene is a general reaction for all o-, m-, and
p-isomers, giving rise to the corresponding QM intermediates.
For the m-isomers, we have shown that the ESIPT is unique in
that a very small amount of water acting in the form of a water
trimer is able to catalyze the reaction in a pool of solvent
CH3CN. We envision that these systems may provide a new and
valuable probe of the dynamics of solvent-mediated reactions.
Coupled with the high quantum yields observed for these isomers,
this is another example of Zimmerman’s “meta-ortho effect”14
manifesting in the hydroxy substituent.
Nanosecond laser flash photolysis (LFP) studies provided
evidence that the primary photochemical step responsible for
fluorescence quenching of 1 and 3 results in the formation of the
corresponding m-QMs. Thus, LFP of 1-3 (Nd:YAG laser; 266
nm; <30 mJ/pulse; oxygen-purged and flowing solutions) in
aqueous solution gave strongly absorbing long wavelength
transients centered at 425, 350, and 415 nm, respectively (Figure
3), similar to those observed and assigned to m- and p-QMs
reported recently.3 The transients are produced within the laser
pulse (∼10 ns) and undergo clean single-exponential decay back
(9) Fluorescence quenching data from lifetime measurements also show a
cubed dependence on water concentration (in the same concentration range
used for the steady-state measurements) in the modified Stern-Volmer plot
but the quenching effect is less than observed from steady-state measurements
suggestive of both static and dynamic components in the quenching mecha-
nism, the details of which will be reported in the full paper. Preliminary data
indicate the formation of a weak substrate-water-trimer complex in the ground
state which is responsible for the static component.
Acknowledgment. Support of this research by NSERC (Canada) and
the University of Victoria is gratefully acknowledged. M.F. thanks
NSERC for a post-graduate scholarship.
JA974144Y
(11) Lewis, F. D.; Yang, J.-S. J. Am. Chem. Soc. 1997, 119, 3834.
(12) Only singlet state reactivity (for ESIPT) of these compounds is studied
in this work. The corresponding triplet states are not expected to undergo
ESIPT due to their expected diradicaloid character. In addition, it is well-
known that triplet excited phenols have similar pKa values as in the ground
state and would not provide a driving force for the proton transfer.
(13) There is no evidence to indicate that the ESIPT of these compounds
is adiabatic. Indeed, even if the m-QMs were photogenerated adiabatically,
they would be very short-lived due to their high intrinsic reactivity (carboca-
tionic species), resulting in negligible fluorescence emission, if any. In addi-
tion, fast radiationless deactivation to the ground state would be expected due
to the inherent conformational flexibility of these carbocation-like structures.
(14) (a) Zimmerman, H. E. J. Am. Chem. Soc. 1995, 117, 8988. (b)
Zimmerman, H. E.; Sandel, V. R. J. Am. Chem. Soc. 1963, 85, 915.
(10) Lewis and Yang11 have observed that p-substituted amino styryl
systems are only weakly fluorescent, in contrast to the much more emissive
m-isomers. They suggested that differences in the ability to twist about the
Ar-CH bond between the m- and p-isomers (the m-isomers being more
twisted) might offer a possible explanation for the difference in fluorescence
quantum yields (and lifetimes) for these isomers. In our systems, essentially
no difference in fluorescence characteristics was observed between alkenes 1
and 3 although 3 is an indene where twisting about the Ar-CH bond would
be prohibitive. This clearly suggests that twisting about this bond is not required
for charge-transfer character in our system. Our results suggest that these
reactions proceed via planar, highly polarized (charge transfer) singlet states.
A study of the corresponding p-hydroxy-substituted indene derivative might
provide more information as to why the p-isomer is much less fluorescent.