Benzopentalenonaphthalenones from the intramolecular capture of a merocyanine derived from a naphthopyran

Article abstract of DOI:10.1039/c0cc02986f

Novel, highly coloured benzopentalenonaphthalenones result from a cascade process initiated by the thermally-induced ring-opening of diarylmethanol substituted 2H-naphtho[1,2-b]pyrans in the presence of acid.

Full text of DOI:10.1039/c0cc02986f

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Benzopentalenonaphthalenones from the intramolecular capture of a
merocyanine derived from a naphthopyranw
Christopher D. Gabbutt, B. Mark Heron,* Colin Kilner and Suresh B. Kolla
Received 2nd August 2010, Accepted 23rd September 2010
DOI: 10.1039/c0cc02986f
Novel, highly coloured benzopentalenonaphthalenones result
from a cascade process initiated by the thermally-induced ring-
opening of diarylmethanol substituted 2H-naphtho[1,2-b]pyrans
in the presence of acid.
Interest in the design and properties of new functional dye
systems1 has increased at a steady rate over the last few years
as a consequence of emerging technologies including dye
sensitised solar cells2 and organic conducting and emitting
systems.3 More established functional dye systems, such as
those used for photochromism,4,5 photodynamic therapy6 and
assorted sensor systems,7 continue to attract attention. In
particular, the control of various phenomena through the
cycling sequence of a photochromic unit has recently been
reviewed.8
Our interest of late has focussed upon cationic dye systems
Scheme 2 Synthesis of benzopentalenonaphthalenones 5.
whose colour can be modulated by the switching of a photo-
chromic naphthopyran unit.9 In this study the cationic centre
the cationic centre is generated adjacent to the naphthalene
unit in close proximity to the photomerocyanine moiety.
Alkyl 2,2-diaryl-2H-naphtho[1,2-b]pyran-5-carboxylates 2a,
b were readily prepared using the established acid-catalysed
was generated at the terminus of a thienyl group on the
3H-naphtho[2,1-b]pyran 1 and through variation of pH and
irradiation switching between one colourless and three
coloured states was accomplished (Scheme 1). We now report
our observations on the naphtho[1,2-b]pyran system 3, wherein
route from
a
1,1-diarylprop-2-yn-1-ol and an alkyl
4-hydroxynaphthalene-2-carboxylate.10 The addition of excess
4-methoxyphenyllithium to 2 at low temperature gave the
diarylmethanols 3a, b in good yield (Scheme 2).11 The
1H NMR spectra of these compounds displayed a readily
identifiable doublet at d 5.8 (J = 10.1 Hz) for the pyran ring
proton, 3-H.5
Examination of the photochromic response of the new
diarylmethanol substituted naphthopyran 3a in CH2Cl2
solution (ca. 1 Â 10À5 mol dmÀ3) under irradiation with
ultraviolet light (275–375 nm) resulted in the generation of a
red solution with lmax 514 nm which faded gradually to
colourless upon cessation of irradiation. A solution of 3b
developed a weak blue colour with lmax 615 nm which faded
very rapidly, a feature that was not wholly unexpected since it
has been established that a combination of strong electron
donation groups at 2-C of a naphtho[1,2-b]pyran combined
with a bulky substituent at the 5-position results in only
transient photocolouration.5 Treatment of the colourless
solutions of 3a, b in CH2Cl2 with MeSO3H (98%, 2 mL
injected directly into 10 mm path length cuvette) resulted in
the development of an intense violet shade derived from the
Scheme 1 Colour modulation of naphthopyran 1 through changes in
pH and irradiation.
two new strong bands in the absorption spectrum [3a lmax
=
498 nm, emax = 7.87 Â 104 molÀ1 dm3 cmÀ1, lmax = 648 nm,
School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK.
E-mail: b.m.heron@leeds.ac.uk; Fax: +44 (0)113 3436565;
Tel: +44 (0)113 3432925
w Electronic supplementary information (ESI) available: Selected
spectroscopic data for all new compounds and X-ray crystallographic
data for compound 5a are provided. CCDC 787182. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc02986f
e
max = 1.26 Â 104 molÀ1 dm3 cmÀ1, 3b lmax = 476 nm, emax
=
5.11 Â 104 molÀ1 dm3 cmÀ1, lmax = 568 nm, emax = 1.72 Â
104 molÀ1 dm3 cmÀ1, after dilution to ca. 1 Â 10À6 mol dmÀ3].
These bands are considered to arise from cation generation
and compare favourably with unsymmetrically substituted
c
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Chem. Commun., 2010, 46, 8481–8483 8481

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triarylmethine dyes.12 In marked contrast to the behaviour of
1, irradiation of these acidic solutions of 3a, b resulted in no
appreciable change in their absorption spectra.w
In order to further explore this unexpected phenomenon,
toluene solutions of 3 containing a catalytic amount of
4-TsOH were examined. Upon warming, the initial colour
gradually intensified and after stirring at 60 1C for 20 h 3a had
been irreversibly converted into a new, intense coloured,
turquoise species 5a (76%) (Scheme 2).13 The 1H NMR
spectrum of 5a displayed signals at ca. d 3.8 for four non-
equivalent methoxy groups and a distinct singlet at d 6.42.
Additionally the 13C NMR spectrum displayed a signal at d
184 which is attributed to an a,b-unsaturated CQO group.
Scheme 3 Proposed mechanism for the formation of 5a.
1
The absence of any H NMR signals that could be attributed
to the pyran ring combined with a hydrogen atom count that
was two hydrogen atoms too few for the predicted species 4
resulting from intramolecular trapping of the cation by
the proximal extended p-system was also of note. Mass
spectrometry confirmed the molecular formula of the product
as C43H34O6 and the pentacyclic structure 5a was established
by X-ray crystallography (Fig. 1).14 Naphthopyran 3b
underwent a similar cyclisation process to afford 5b (58%)
though more forcing reaction conditions were required (48 h,
100 1C). Pentalenones 5a, b exhibited two absorption bands
with the longer wavelength band bathochromically shifted
relative to that of their respective precursors with 5a (turquoise)
Scheme 4 Photochemical rearrangement of the benzo-2,3-naptho-
barrelene sysetm.
The formation of 5 may be rationalised by a cascade process
involving initial intramolecular trapping of the cation to
presumably afford 4. The latter subsequently undergoes a
second intramolecular cyclisation to the naphthol 6, tauto-
merisation of which followed by aerial oxidation results in the
generation of the stabilising, extended conjugated p system
(Scheme 3). In order to establish the proposed mechanism, a
solution of 3a in degassed toluene under N2 containing
4-TsOH was stirred for 5 days at rt; work-up and column
chromatography afforded a pale blue solid whose initial
1H NMR spectrum indicated the presence of a small amount
of 5a [ca. 3%, dH 7.80 (d, 6-H), 8.51 (1H, d, 4-H)] together
with the major component which was identified as the
naphthol 6 [1H NMR d 5.23 (s) 4b-H, d 5.55 (s) OH (D2O
exchange) and d 7.12 (s) 10-H]. An OH stretch at 3372 cmÀ1
and the absence of a significant CQO signal in the 13C NMR
spectrum supported this assignment. Interestingly, re-recording
lmax = 586 nm, emax = 1.06 Â 104 molÀ1 dm3 cmÀ1, lmax
=
376 nm, emax = 1.59 Â 104 molÀ1 dm3 cmÀ1 and 5b (violet)
lmax = 545 nm, emax = 1.61 Â 104 molÀ1 dm3 cmÀ1, lmax
=
370 nm, emax = 1.04 Â 104 molÀ1 dm3 cmÀ1 (CH2Cl2),
respectively, which is presumably a consequence of the
extended conjugated pathway coupled with the fixed planar
arrangement of the fused rings. There was no evidence of any
aggregation of these dyes in CH2Cl2 solution from dilution
experiments, a feature attributed to the geminal diaryl groups
(Ar2) which are directed above and below the main plane of
the molecule.
1
the H NMR spectrum of the initial solution after 16 hours
indicated an increased proportion (ca. 15%) of 5a, presumably
generated by the facile air oxidation process.
The thermally induced ring-opening of a naphthopyran unit
and subsequent trapping of the merocyanine with a proximal
cationic centre provides a reasonably efficient route to this
unusual unsaturated pentacyclic system and interesting new
dyes. To our knowledge the only other report of the
benzo[5,6]pentaleno[1,2-b]naphthalene ring system appears in
an exploratory mechanistic study concerned with the photo-
chemical rearrangement of the benzo-2,3-naphthobarrelene
system (Scheme 4).15 The conversion of pyrans and their
benzologues into carbocycles is particularly rare although
examples are known.16 Further examination of the synthesis
of these systems and their application are ongoing.
Notes and references
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Fig. 1 X-Ray crystal structure of 5a with thermal ellipsoids scaled at
the 50% probability level.
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¨
c
8482 Chem. Commun., 2010, 46, 8481–8483
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¨
1171.9, 1032.3, 944.4, 824.9, 585.9 cmÀ1, lmax = 514, 429 nm
(CH2Cl2) then lmax = 498 nm, emax = 7.87 Â 104 molÀ1 dm3 cmÀ1
,
+
lmax = 648 nm, emax = 1.26 Â 104 molÀ1 dm3 cmÀ1 (CH2Cl2
2 mL MeSO3H), dH 2.96 (1 H, s, OH), 3.76 (6 H, s, OMe), 3.82
(6 H, s, OMe), 3.92 (3 H, s, OMe), 5.78 (1H, d, J = 10.1 Hz, 3-H),
6.68 (1H, s, 6-H), 6.82 (8H, m, Ar-H), 6.89 (1H, d, J = 10.1 Hz,
4-H), 7.03 (1H, dd, J = 9.2, 2.4 Hz, 8-H), 7.14 (4H, m, Ar-H), 7.32
(4H, m, Ar-H), 7.39 (1H, d, J = 9.2 Hz, 7-H), 7.56 (1H, d,
J = 2.4 Hz, 10-H), dC 55.47, 55.51, 55.76, 81.80, 82.36, 100.86,
113.44, 113.55, 116.52, 119.02, 121.86, 124.10, 126.20, 126.84,
128.28, 128.57, 129.24, 130.04, 137.53, 138.29, 139.52, 148.58,
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Kim, Elsevier, Amsterdam, 2006, pp. 85–135.
158.35, 158.80, 159.07. Found [M
C43H38O7 requires [M + Na]+ = 689.2510.
+ = 689.2503
Na]+
12 H. Zollinger, Color Chemistry, VCH, Weinheim, 2nd edn, 1991,
pp. 71–80; M. Oda, T. Kawase and C. Wei, Pure Appl. Chem.,
1996, 68, 267–274.
13 3,7-Dimethoxy-11,11,12-tris(4-methoxyphenyl)-11H-benzo[5,6]-
pentaleno[1,2-b]naphthalene-5-one 5a as deep purple microcrystals
0.37 g (76%) after elution from silica with 5% ethyl acetate in
toluene and recrystallisation from acetone/methanol, mp
252–255 1C, nmax 3002.7, 2949.2, 2831.4, 1583.4, 1505.3, 1491.4,
1244.1, 1222.9, 1175.1, 1030.5, 824.8, 787.2, 551.9 cmÀ1, lmax
=
=
6 M. Wainwright, Anti-Cancer Agents Med. Chem., 2008, 8,
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Rev., 2010, 110, 2839–2857.
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586 nm, emax = 1.06 Â 104 molÀ1 dm3 cmÀ1, lmax = 376 nm,
emax = 1.59 Â 104 molÀ1 dm3 cmÀ1 (CH2Cl2), dH 3.75 (6H, s,
OMe), 3.78 (3H, s, OMe), 3.92 (3H, s, OMe), 3.95 (3H, s, OMe),
6.42 (1H, s, 10-H), 6.71 (6H, m, Ar-H), 6.75 (1H, dd, J = 8.2, 2.6
Hz, 2-H), 6.95 (3H, m, Ar-H), 7.11 (1H, d, J = 8.4, 2.8 Hz, 8-H),
7.19 (5H, m, Ar-H), 7.80 (1H, d, J = 2.8 Hz, 6-H), 8.51 (1H, d,
J = 2.6 Hz, 4-H), dC 55.66, 56.13, 56.35, 58.66, 110.15, 113.72,
113.94, 115.54, 116.33, 120.66, 122.28, 122.80, 126.58, 130.17,
130.21, 130.56, 131.93, 131.99, 132.16, 133.21, 136.52, 140.85,
144.10, 150.53, 156.95, 158.65, 159.50, 159.91, 160.34, 160.46,
184.16. Found [M
[M + H]+ = 647.2428.
+ = 647.2419 C43H34O6 requires
H]+
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11 9-Methoxy-5-[1,1-bis(4-methoxyphenyl)-1-hydroxymethyl]-2,2-bis-
(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran 3a as colourless
microcrystals (2.13 g) 79% after elution from silica with 40%
EtOAc in hexane and recrystallisation from acetone/methanol,
mp = 181–182 1C, nmax 3446.8, 1607.1, 1504.9, 1297.7, 1246.7,
14 Crystal data for 5a crystallised from CH2Cl2/hexane by vapour
diffusion: C43H34O6, M = 646.7, monoclinic, space group C2/c,
a = 23.5830(17) A, a = 901, b = 16.0599(13) A, b = 96.983(3)1,
c = 17.8227(15) A, g = 901, volume = 6700.1(9) A3, rcalcd
=
1.282 Mg mÀ3, T = 150(2) K, Z = 8, 51 816 reflections measured,
independent reflections 7460 R(int) 0.0785, R1 0.0611
=
=
(I 4 2s(I)), wR2 (all data) = 0.1930. Full data are provided in
the ESIw.
15 H. E. Zimmerman and M.-L. Viriot-Villaume, J. Am. Chem. Soc.,
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c
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Chem. Commun., 2010, 46, 8481–8483 8483