Facile synthesis, fluorescence, and photochromism of novel helical pyrones and chromenes

Article abstract of DOI:10.1021/ol0619260

(Chemical Equation Presented) Aryl coumaryl ethylenes undergo oxidative photocyclization readily to yield helical pyrone-annulated condensed aromatics. The pyrones are conveniently converted to the corresponding photochromic diphenylpyrans/chromenes. Both

Full text of DOI:10.1021/ol0619260

ORGANIC
LETTERS
2006
Vol. 8, No. 21
4891-4894
Facile Synthesis, Fluorescence, and
Photochromism of Novel Helical
Pyrones and Chromenes
Jarugu Narasimha Moorthy,* Parthasarathy Venkatakrishnan,
Sanchita Sengupta, and Mahiuddin Baidya
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
moorthy@iitk.ac.in
Received August 3, 2006
ABSTRACT
Aryl coumaryl ethylenes undergo oxidative photocyclization readily to yield helical pyrone-annulated condensed aromatics. The pyrones are
conveniently converted to the corresponding photochromic diphenylpyrans/chromenes. Both pyrones and chromenes exhibit helicity-dependent
fluorescence efficiency and persistence, respectively.
Helicity is ubiquitous in nature and pervades every sphere
of day-to-day life. Historically, helical structures have elicited
tremendous interest as aesthetic marvels. However, remark-
able applications of helicenes in the development of nonlinear
optical materials,1 liquid crystals,2 chiroptical materials,3 etc.4
have unfolded only in recent years. Consequently, there is a
sudden surge of interest in helices with unique properties
and in the development of novel synthetic protocols to access
them.5 Among various routes for the synthesis of organic
helical structures,5,6 oxidative photocyclization of diaryleth-
ylenes continues to be most favored.6c,7
In our recent investigations aimed at exploring modifica-
tion of the reactivity of stilbenes via 2-pyranone-annulation,
we uncovered novel cis f trans isomerization of some
derivatives in the solid state.8 In a continuation of these
studies, we reasoned that it should be possible to convert
2-pyranone-annulated phenyl aryl ethylenes into novel helical
structures containing the basic benzopyrone, i.e., coumarin,
moiety; coumarins are fantastic fluorophores and are wide-
spread as dyes, DNA intercalators, biologically active
(1) (a) Verbiest, T.; Elshocht, S. V.; Kauranen, M.; Hellemans, L.;
Snauwaert, J.; Nuckolls, C.; Katz, T. J.; Persoons, A. Science 1998, 282,
913. (b) Verbiest, T.; Sioncke, S.; Persoons, A.; Vyklicky´, L.; Katz, T. J.
Angew. Chem., Int. Ed. 2002, 41, 3882.
(2) Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998, 120, 9541.
(3) (a) Feringa, B. L.; van Delden, R. A.; ter Wiel, M. K. J. In Molecular
Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim, 2001; Chapter 5, p
123. (b) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E.
M. Chem. ReV. 2000, 100, 1789.
(4) (a) Meurer, K. P.; Vogtle, F. Top. Curr. Chem. 1985, 127, 1. (b)
Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63. (c)
Katz, T. J. Angew. Chem., Int. Ed. 2000, 39, 1921.
(6) (a) Liu, L.; Katz, T. J. Tetrahedron Lett. 1990, 31, 3983. (b) Carreno,
M. C.; Garc´ıa-Cerrada, S.; Urbano, A. J. Am. Chem. Soc. 2001, 123, 7929.
(c) Norsten, T. B.; Peters, A.; McDonald, R.; Wang, M.; Branda, N. R. J.
Am. Chem. Soc. 2001, 123, 7447.
(7) For example, see: (a) Nakamura, Y.; Yamazaki, T.; Nishimura, J.
Org. Lett. 2005, 7, 3259. (b) Wigglesworth, T. J.; Sud, D.; Norsten, T. B.;
Lekhi, V. S.; Branda, N. R. J. Am. Chem. Soc. 2005, 127, 7272 (c) Kelly,
T. R.; Sestelo, J. P.; Tellitu, I. J. Org. Chem. 1998, 63, 3655.
(8) (a) Moorthy, J. N.; Venkatakrishnan, P.; Savitha, G.; Weiss, R. G.
Photochem. Photobiol. Sci. 2006, DOI: 10.1039/b606027g.
(5) Urbano, A. Angew. Chem., Int. Ed. 2003, 42, 3986.
10.1021/ol0619260 CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/21/2006

ingredients, etc. Further, we envisioned that the helical
pyrones can be most conveniently transformed into chromenes,
which are helical in nature; chromenes are well-known in
the realm of photochromism and readily exhibit photoinduced
color changes via heterolysis of the C-O bond in the singlet-
excited state, which results in the formation of colored
quinonoid intermediates.9 Herein, we report facile and regio-
selective cyclization of coumarins 1 and 2 to 2-pyranone-
annulated helical systems, which are subsequently trans-
formed easily to the corresponding chromenes. It is shown
that helical pyrones are novel fluorescent molecules and that
the chromenes derived therefrom exhibit readily observable
photochromism. Further, a remarkable influence of helicity
on the fluorescence and photochromic properties is unraveled.
Scheme 1
The cis and trans mixtures of coumarins 1 and 2 were
synthesized in 64-90% isolated yields from the Wittig
reaction between the triphenylphosphonium salts of ap-
propriately substituted 6-bromomethylcoumarins and ben-
zaldehyde (for 1) or 2-naphthaldehyde (for 2). Photolysis
(Luzchem photoreactor, λ = 350 nm) of the solutions of cis
isomers of 1 and 2 in benzene under N2 atmosphere led to
a readily observable color change, and the color in each case
was found to disappear rapidly when kept in the dark; the
color in the case of coumarins 1 varied from orange to violet,
whereas it was red in coumarins 2. The attendant color
changes were monitored by electronic absorption spectros-
copy; see Supporting Information. The transients responsible
for the absorptions are attributed to the formation of dihydro
(DH) intermediates 5 and 6,10 Scheme 1. Indeed, the
formation of oxidation products in the presence of O2 or
added I2 (vide infra) lends credence to this assignment.
(73%) and 2c (65%). Remarkably, the cyclizations were
found to occur with a noticeable regioselectivity, as revealed
1
by 400 MHz H NMR spectroscopy of the mixtures of
regioisomeric cyclization products. Whereas coumarins 1
exhibited preference for cyclization at position 5 of the
coumarin leading to 2-pyranone-annulated phenanthrenes
3-[c], coumarins 2, with the exception of 2a, underwent
cyclization selectively at position 7 yielding pyran-2-one-
annulated tetrahelicenes 4-[b]; cf. Scheme 1. In coumarins
1, the selectivity for cyclization ranged from 69% to 89%,
whereas it was 53-90% in coumarins 2 (Table 1). The
observed regioselectivities in oxidative cyclizations can, in
principle, be reconciled from a composite of steric and
electronic interactions.11,12 Although energies of transition
states that lead to the two regioisomers, as calculated from
The photolyses of solutions of 1 and 2 as mixtures of cis
and trans isomers (λ = 350 nm) in the presence of I2 led to
the formation of electrocyclized aromatic products. The
photolysis in each case was continued until the disappearance
1
Table 1. Results of Oxidative Photocyclizations of 1 and 2a
of the coumarin, as monitored by H NMR spectroscopy
(Table 1), and the photolysates were subjected to chromato-
graphic techniques to isolate mixtures of regioisomeric
pyrone-annulated phenanthrenes 3 and tetrahelicenes 4
(Scheme 1). The pure regioisomers were thoroughly char-
duration of
irradiation
(h)
relative ratios of isolated
concn
(M)
cyclized productsb
[c]:[b]
yield
(%)
substrate
1a
1b
1c
1d
2a
2b
2c
2d
15
36
36
36
24
24
36
36
2 × 10-4
8 × 10-5
5 × 10-4
2 × 10-4
2 × 10-4
4 × 10-4
1 × 10-4
4 × 10-4
89:11
69:31
72:28
78:22
47:53
25:75
10:90
29:71
92
95
73
92
94
92
65
93
1
acterized by IR, H and 13C NMR, and mass spectroscopic
techniques (Supporting Information). As given in Table 1,
the isolated yields of 2-pyrone-annulated aromatic products
were >90% except for the chloro-substituted coumarins 1c
(9) (a) Gemert B. V. In Organic Photochromic and Thermochromic
Compounds; Crano, J. C., Guglielmetti, R., Eds.; Plenum Press: New York,
1999; Vol. 1, Chapter 3. For a recent example, see: (b) Zhao, W.; Carreira,
E. M. Org. Lett. 2006, 8, 99.
(10) (a) Muszkat, K. A. Top. Curr. Chem. 1980, 88, 89. (b) Laarhoven,
W. H. In Photochromism, Molecules and Systems; Durr, H., Bouas-Laurent,
H., Eds.; Elsevier: Amsterdam, 1990; p 270. (c) Irie, M. Chem. ReV. 2000,
100, 1685.
a Photolyses were carried out in Pyrex flasks containing solutions of 1
or 2 in benzene (ca. 10-4 to 10-5 M) in the presence of iodine (0.5 equiv);
λ = 350 nm. b From 1H NMR analyses; [b] and [c] refer to the faces of
annulation (see text).
4892
Org. Lett., Vol. 8, No. 21, 2006

semiempirical calculations, may shed light on the regiose-
lectivity, it has been pointed out recently that the regiose-
lectivities are determined by relative rates of ring opening,
sigmatropic rearrangements of the DH-intermediates as well
as their rates of oxidation.13 All of these processes are
susceptible to variation in structure, solvent, temperature,
concentration, reactivity of the oxidant, etc. to cause any
generalizations as to the regioselectivities less meaningful.
As the helicity in the cyclization products of the naph-
thalene derivatives 2 was expected to be substantial, we
specifically examined the X-ray crystal structures of the
photocyclization products of 2a and 2b. That these molecular
systems are indeed helical is evident from their X-ray
determined structures shown in Figure 1. From the angle
annulation imparts fluorescence property to the aromatic
systems. As expected, the pyrones 3 and 4 were found to
exhibit high emission intensities at as low concentrations as
10-5 to 10-6 M. While the absorption in all cases tapers off
at ca. 400 nm, the pyrones were found to exhibit blue
emission in the range of 400-550 nm. In Figure 2 are shown
Figure 2. Normalized emission spectra for 4a-[c] and 4a-[b]. The
spectra were recorded for λexc ) 366 nm.
the fluorescence spectra recorded typically for 4a-[c] and
4a-[b]. It is noteworthy that the [b]-face-annulated phenan-
threne and tetrahelicene, i.e., 3a-[b] and 4a-[b], exhibit 20
nm shift in their structured emission maxima relative to those
of the [c]-face annulated analogues, i.e., 3a-[c] and 4a-[c].
Their fluorescence efficiencies were determined in a nonpolar
cyclohexane and a polar protic methanol solvents using 9,-
10-diphenylanthracene as a standard.
Figure 1. X-ray determined structures of 2-pyrone-annulated
tetrahelicenes 4a-[c], 4a-[b], 4b-[c], and 4b-[b].
between the mean planes constructed for the terminal
aromatic rings of these pyrones, it can be readily discerned
that the helicity in the case of [c]-face-annulated pyrones,
i.e., 4a-[c] and 4b-[c], is higher (ca. 44°-45°) than those of
the [b]-face annulated analogues (ca. 26°), i.e., 4a-[b] and
4b-[b]. It is noteworthy that the substituent at the fourth
position of the pyrone ring as in 4b-[c] does not result in
any perceptible difference in the helicity as compared to 4a-
[c]. In the absence of structural studies on pyrone-annulated
phenathrenes 3a-[b] and 3a-[c], it is logical to believe that
the helicity in the case of 3a-[c], which may be considered
equivalent to a tetrahelicene, should be comparable or more
than that in the case of [b]-face-annulated tetrahelicenes,
namely, 4a-[b]. Of course, the [b]-face-annulated phenan-
threnes 3a-[b] must be planar.
Table 2. Photophysical Properties of Helical Pyrones 3 and 4
emissiona
λmax (nm)
quantum
yield,b Φf
substrate abs λmax (nm) C6H12 MeOH C6H12 MeOH
3a-[c]
3a-[b]
4a-[c]
4a-[b]
380
400
360
390
410
423
421
438
430
452
450
475
0.08
0.04
0.14
0.03
0.06
0.03
0.10
0.03
a λexc ) 366 nm. b Error ( 10%
As given in Table 2, the quantum yields were found to be
as high as 0.14.14 Quite remarkably, they are higher for
molecules with higher helicity. In both polar methanol and
nonpolar cyclohexane solvents, one observes a ca. 2- to
4-fold increase in the quantum yields for 3a-[c] and 4a-[c]
as compared to their regioisomers 3a-[b] and 4a-[b]. The
In light of these structural insights, it was interesting to
inquire into the dependence of fluorescence efficiencies on
the helicity. As mentioned at the outset, the pyrone-
(11) Zimmerman, T. J.; Muller, T. J. Eur. J. Org. Chem. 2002, 2269.
(12) (a) Laarhoven, W. H.; Cuppen, Th. J. H. M.; Nivard, R. J. F.
Tetrahedron 1970, 26, 4865. (b) Muzkat, K. A.; Seger, G.; Sharafi-Ozeri,
S. J. Chem. Soc., Faraday Trans. 2 1975, 1529.
(13) Lewis, F. D.; Kalgutkar, R. S.; Yang, J.-S. J. Am. Chem. Soc. 2001,
123, 3878.
(14) For fluorescence studies of azahelicenes, see: Bazzini, C.; Brovelli,
S.; Caronna, T.; Gambarotti, C.; Giannone, M.; Macchi, P.; Meinardi, F.;
Mele, A.; Panzeri, W.; Recupero, F.; Sironi, A.; Tubino, R. Eur. J. Org.
Chem. 2005, 1247.
Org. Lett., Vol. 8, No. 21, 2006
4893

reason as to why the molecules with higher helicity exhibit
higher efficiency is intriguing. Such a remarkable dependence
of fluorescence efficiency on helicity is, to the best of our
knowledge, heretofore unknown.
Scheme 3
The pyrones can be readily converted to chromenes that
are photochromic. The reaction of pyrones 3a-[b], 3a-[c],
4a-[c], and 4a-[b], representative cases, with excess PhMgBr
followed by dehydration led to diphenylpyrans (chromenes)
in 55-92% isolated yields as shown in Scheme 2.15 A brief
Scheme 2
chromenes 7a-[c], 8a-[c], and 8a-[b] were found to be longer
lived as monitored from UV-vis absorption spectroscopy,
the transient in the case of 7a-[b] was found to be very short-
lived. Following a brief irradiation (30 s) of the solutions in
toluene, the decay rate constants for the intermediates derived
from 7a-[c], 8a-[c], and 8a-[b] were determined at 295 K to
be 0.080, 0.066, and 0.129 s-1, respectively. The intermediate
in the case of 7a-[b] was too short-lived; see Supporting
Information for details. The relatively longer lifetimes of the
intermediates derived from chromenes 8a-[b] and 8a-[c], as
compared to those derived from 7a-[b] and 7a-[c], attest to
the unique role of helicity in influencing the lifetimes.
In conclusion, we have shown that oxidative photocy-
clization of aryl coumaryl ethylenes is a facile protocol to
access regioisomeric helical pyrones. The helicity of the
pyrones is unequivocally established by X-ray crystal
structure determinations. The pyran-2-one-annulated phenan-
threnes and tetrahelicenes are found to exhibit fluorescence
efficiencies that are remarkably dependent on the magnitude
of helicity. We have shown that helical pyrones can be
conveniently transformed into photochromic chromenes. In
these cases, the persistence of color upon photolysis is found
to depend on the helicity. The helicity-dependent fluores-
cence efficiency and persistence of quinonoid intermediates
generated via heterolysis of chromenes bring out the
importance of structural attributes on physical properties.
exposure (20-30 s) of the solutions of helical chromenes 7
and 8 in dry benzene (ca. 10-3 M) to UV radiation (λ = 350
nm) under a N2 gas atmosphere led to instantaneous color
change. The UV-vis absorption spectral changes for the
photolyzed solutions of chromenes 7 and 8 before and after
irradiation at -20 °C are shown in Figure 3. The species
Figure 3. UV-vis absorption spectra for 7a-[c] and 8a-[c] before
(s) and after (- - -) photoirradiation.
Acknowledgment. J.N.M. is thankful to DST, India for
funding and P.V. to CSIR for a research fellowship.
Supporting Information Available: Experimental pro-
cedures, spectral data (UV, fluorescence, and NMR), X-ray
structure determination details and CIF files. This material
is available free of charge via the Internet at http://pubs.acs.org.
responsible for color are attributed to the o-quinonoid
intermediates as shown in Scheme 3. Whereas the absorp-
tions due to the quinonoid intermediates in the case of
(15) Cottam, J.; Livingstone, R. J. Chem. Soc. 1965, 6646.
OL0619260
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