C O M M U N I C A T I O N S
Scheme 1
Figure 1. UV-visible spectra showing the conversion of 3 to 5 with
photolysis time. Traces were recorded after 0, 0.25, 0.5, 1, 2, 3, 5, and 7
min of photolysis time in a Rayonet photoreactor fitted with 300 nm lamps
(9:1 CH3CN-H2O, deaerated).
position. This finding rules out 8 as being a common intermediate
to both 3-2′D and 5-7′D. A preferred mechanism involves water-
mediated ESIPT to the 7′-carbon atom, forming quinone methide
9 (Scheme 1). Ring closure gives 5-7′D directly. This mechanism
is consistent with exclusive isolation (from D2O runs) of 5-7′D in
which the 7′-position is exactly 50% exchanged with deuterium.
While it is apparent from the deuterium labeling experiments
that 3-2′D and 5-7′D do not result from the same quinone methide
intermediate, the similarity in their water dependence does suggest
a common intermediate prior to quinone methide formation. One
such possible intermediate could result from initial water-mediated
ESIPT from the phenol to the naphthyl π-system, forming the
zwitterionic “protonated π-complex” 10. Once formed, intra-
molecular rearrangement can lead to formation of the two possible
quinone methides. While further study is required to address such
mechanistic details, it is apparent from this work that the photo-
cyclization of o-(1-naphthyl)phenols is a new reaction pathway
initiated by ESIPT. It is a clean, efficient, and high-yielding route
to benzoxanthenes and dihydrobenzoxanthenes, which are not
readily accessible by thermal methods. Moreover, the proliferation
of Pd-based aryl-aryl coupling reactions enables the synthesis of
an abundant number of starting phenols and hence the ability to
make a wide variety of benzoxanthene derivatives.
Figure 2. Plot of yield of 5-7′D (9) and 3-2′D (O) vs D2O content (in
CH3CN). Samples (∼10-3 M in 50 mL of solvent) were irradiated in a
Rayonet reactor with 16 lamps (300 nm) for 20 min. Estimated errors are
(10%.
for 3. Recovered was starting material with deuterium incorporation
at the 2′-position of the naphthalene ring (Φ ) 0.11 ( 0.02)8 and
photoproduct 7 (Φ ) 0.11 ( 0.02)8 with 50% deuterium incorpora-
tion at the 7′-position (Ha).
In the ESIPT reaction reported for 1, it was observed that
deuterium exchange efficiency (at the 2′-position) reached a
maximum at less than 1% D2O content. It was concluded that the
proton transfer was not mediated by water. Studying the effect of
water content on reactivity of 3 (and 4) should provide additional
information on the mechanisms of cyclization versus deuterium
exchange. Therefore, preparatory photolyses of 3 in varying
concentrations of D2O (in CH3CN) were carried out. The efficiency
for formation of 3-2′D and 5-7′D showed a similar strong
dependence on water content (Figure 2). With no reaction observed
in neat CH3CN, the efficiency rose to a maximum at g10% water.
The formation of 3-2′D and 5-7′D required much higher water
content to reach maximum efficiency than 1, suggesting that the
photoproducts do not arise via an explicit ESIPT as proposed for
1, but instead from a water-mediated ESIPT. We propose provi-
sional mechanisms for the deuterium incorporation and the cy-
clization reactions observed for 3 and 4 (Scheme 1). A water-
mediated ESIPT in 3-OD from the phenol to the 2′-carbon atom
on the appended naphthyl ring gives rise to the quinone methide
intermediate 8. Reverse proton transfer in 8 furnishes 3-2′D. If
formation of 5-7′D were to proceed via electrocyclic ring closure
of 8 (after isomerization about the exocyclic alkene), followed by
a [1,7]-hydrogen shift, the 2′-position of 5-7′D produced from 3-OD
should show a minimum of 50% deuterium exchange, regardless
of the extent of conversion. Isolation of 5-7′D generated in a low
conversion run showed 50% deuterium exchange of the methylene
protons (Ha), but with only residual exchange (5%) at the 2′-
Acknowledgment. We acknowledge the continued support of
the Natural Sciences and Engineering Research Council of Canada
(NSERC). M.L. thanks NSERC for a postgraduate scholarship.
References
(1) (a) Ormson, S. M.; Brown, R. G. Prog. React. Kinet. 1994, 19, 45. (b)
LeGourrierec, D.; Ormson, S. M.; Brown, R. G. Prog. React. Kinet. 1994,
19, 211. (c) Formosinho, S. J.; Arnaut, L. G. J. Photochem. Photobiol., A
1993, 75, 21. (d) Chou, P.-T.; Liao, J.-H.; Wei, C.-Y.; Yang, C.-Y.; Yu,
W.-S.; Chou, Y.-H. J. Am. Chem. Soc. 2000, 122, 986. (e) Fischer, M.;
Wan, P. J. Am. Chem. Soc. 1999, 121, 4555. (f) Chou, P.-T.; Chen, Y.-
C.; Yu, W.-S.; Chou, Y.-H.; Wei, C.-Y.; Cheng, Y.-M. J. Phys. Chem. A
2001, 105, 1731.
(2) (a) Chou, P.-T.; McMorrow, D.; Aartsma, T. J.; Kasha, M. J. Phys. Chem.
1984, 88, 4596. (b) Kasha, M.; McMorrow, D.; Parthenopoulos, D. A. J.
Phys. Chem. 1991, 95, 2668. (c) Kasha, M. J. Chem. Soc., Faraday Trans.
2 1986, 82, 2379. (d) Kasha, M. Acta Phys. Pol. 1987, 71A, 717.
(3) (a) Isaks, M.; Yates, K.; Kalanderopoulos, P. J. Am. Chem. Soc. 1984,
106, 2728. (b) Kalanderopoulos, P.; Yates, K. J. Am. Chem. Soc. 1986,
108, 6290.
(4) (a) Lukeman, M.; Wan, P. J. Chem. Soc., Chem. Commun. 2001, 1004.
(b) Lukeman, M.; Wan, P. J. Am. Chem. Soc. 2002, 124, 9458.
(5) Oxygen was removed to eliminate possible residual photooxidation.
(6) The corresponding methyl ether derivative of 3 was not reactive under
similar conditions, indicating that the phenol group is required for reaction.
(7) A small amount (less than 5% of product yield) of other as yet unidentified
products were formed in addition to the primary photoproduct under these
conditions.
(8) Reaction quantum yields were estimated by using the deuterium exchange
reaction of 2-phenylphenol (ref 4b) as a secondary reference standard.
The reported values are the average of three runs.
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