Intramolecular photocycloaddition of anthracene and benzene ring systems

Article abstract of DOI:10.1016/S0040-4039(02)01476-4

The anthracenes 3a-c, substituted in the 9-position with 3,5-dialkoxybenzyloxymethyl groups exhibit, in diluted solutions, intramolecular [4π+4π] photocycloaddition reactions to the polycyclic compounds 4a-c. The quantitative processes are completely reversible by heating, unless acid-catalyzed cleavages lead to thermally stable mono- and diketones: 4a-c→5a-c→6.

Full text of DOI:10.1016/S0040-4039(02)01476-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6853–6855
Intramolecular photocycloaddition of anthracene and benzene
ring systems
Derong Cao,a Silvia Dobisb and Herbert Meierb,*
aLCLC, Guangzhou Institute of Chemistry, Chinese Academy of Science, China
bInstitute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz, Germany
Received 1 July 2002; accepted 16 July 2002
Abstract—The anthracenes 3a–c, substituted in the 9-position with 3,5-dialkoxybenzyloxymethyl groups exhibit, in diluted
solutions, intramolecular [4p+4p] photocycloaddition reactions to the polycyclic compounds 4a–c. The quantitative processes are
completely reversible by heating, unless acid-catalyzed cleavages lead to thermally stable mono- and diketones: 4a–c“5a–c“6.
© 2002 Elsevier Science Ltd. All rights reserved.
Compounds which are suitable for photochemical addi-
tion or cycloaddition reactions of the AB type lead to
competitive intra- and intermolecular processes, which
depend on the concentration c and particularly on the
average lifetime ~ of the excited species. The prediction
of the major route seems to be possible on the basis of
the diffusion equation, provided that the values of c
and ~ are known.1 However, the situation becomes
more difficult, if ground-state aggregates are involved,
which render the diffusion needless. Scheme 1 summa-
rizes the mono-, bi- and polymolecular reaction routes
for AB photo(cyclo)addition reactions. Both reactive
sites A and B represent separate chromophores in the
same molecule. Due to the energy transfer ET from the
energy-higher to the energy-lower excited state, the
excitation can take place by irradiation into the absorp-
tion of the A or B part.
Recently we published in the dendrimer series2,3 some
antenna systems of anthracenes A substituted in 9,10-
position with Fre´chet dendrons B.4 These compounds
show in aggregates reversible photocycloaddition reac-
tions to dimers (Scheme 1). We modified the systems
and the reaction conditions for the photochemistry, so
that a monomolecular switching process became easily
feasible. The starting compounds 3a–c were prepared
by the reaction of 9-chloromethylanthracene 1 and the
3,5-dialkoxybenzylalcohols 2a–c (Scheme 2).
Irradiation (u>350 nm) of 3a–c in diluted solutions
(c<3.3×10−3 mol L−1) in benzene does not lead to
dimers or oligomers;4 we observed a quantitative
intramolecular photocycloaddition to 4a,8 4b9 and 4c,10
respectively, which is completely reversed by heating to
100°C. Consequently, a 1:1 mixture of 3b and 3c gave
Scheme 1. Competition between intra- and intermolecular photo(cyclo)addition reactions.
* Corresponding author. Fax: +49 (0) 6131 3925396; e-mail: hmeier@mail.uni-mainz.de
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01476-4

6854
D. Cao et al. / Tetrahedron Letters 43 (2002) 6853–6855
Scheme 2. Preparation of 9-alkoxymethylanthracenes: model compound 3a,5 dendrimers 3b6 and 3c.7 (Phase transfer catalysis:
Bu4N+Br−, chlorobenzene/water 20:1.)
Scheme 3. Formation of the cycloadducts 4a–c and enol ether cleavage to 5a–c and 6.
only 4b and 4c under these conditions. (Photodimeriza-
tions would also yield a mixed product.) The observed
[4p+4p] photocycloaddition between an anthracene and
a benzene ring system is so far unprecedented11,12 and
corresponds principally to the intermolecular variant
that we found recently.4
Acknowledgements
We are grateful to the Deutsche Forschungsgemein-
schaft, the Fonds der Chemischen Industrie and the
Guangdong Natural Science Foundation of China for
financial support.
Treatment of 4a–c with formic acid led to a stepwise
cleavage of the enol ether moieties yielding the monoke-
tones 5a,13 5b,14 5c15 and the diketone 6.16 The 1,3-
cyclohexanedione system 6 exists in the di-keto form,
because the enolization would lead to a double bond on
the bridgehead C-8. Moreover, compounds 5 and 6
proved to be stable on heating to 100°C. The thermal
retrocycloadditions would not restore the aromatic ben-
zene rings; 5a–c, for example, would lead to energeti-
cally high-lying cyclohexa-2,4-dien-1-one substructures
(Scheme 3).
References
1. Meier, H.; Mu¨ller, K.; Fetten, M. J. Inform. Recording
1996, 22, 421–426.
2. Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Den-
drimers and Dendrons; Wiley-VCH: Weinheim, 2001.
3. Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and other
Dendritic Polymers; Wiley: New York, 2001.
4. Cao, D.; Meier, H. Angew. Chem. 2001, 113, 193–195;
Angew. Chem., Int. Ed. 2001, 40, 186–188.

D. Cao et al. / Tetrahedron Letters 43 (2002) 6853–6855
6855
1
5. Compound 3a, mp 100°C; H NMR (CDCl3): l=3.76 (s,
11. Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.;
Lapouyade, R. Chem. Soc. Rev. 2000, 29, 43–55.
12. Bouas-Laurent, H.; Desvergne, J.-P. In Photochromism,
Molecules and Systems; Du¨rr, H.; Bouas-Laurent, H., Eds.;
Elsevier: Amsterdam, 1990; p. 561.
6H, OCH3), 4.65 (s, 2H, b-CH2), 5.48 (s, 2H, a-CH2), 6.43
(t, 1H, p-H, benzene), 6.57 (d, 2H, o-H, benzene), 7.48 (m,
4H, anthracene), 8.00 (m, 2H, anthracene), 8.32 (m, 2H,
anthracene), 8.45 (s, 1H, anthracene); 13C NMR (CDCl3):
l=55.3 (OCH3), 64.0 (a-CH2), 72.3 (b-CH2), 100.1 (p-CH,
benzene), 105.5 (o-CH, benzene), 124.3, 124.9, 126.1, 128.5,
129.0 (CH, anthracene), 128.7, 131.1, 131.5 (Cq, anthra-
cene), 140.9, 160.9 (Cq, benzene).
13. Compound 5a, mp 105°C; 1H NMR (CDCl3): l=1.90/2.04
2
(AB, J=−18.4 Hz, 2H, 23-H), 2.93 (s, 3H, OCH3), 3.44
(dd, 3J=11.7 Hz, 4J=1.6 Hz, 1H, 8-H), 3.70/3.95 (AB,
2J=−9.0 Hz, 2H, 4-H), 4.37 (‘s’, 1H, 6-H), 4.38 (d, 3J=11.7
Hz, 1H, 9-H), 4.68/4.88 (AB, 2J=−10.6 Hz, 2H, 2-H), 7.20
(m, 7H, aromat. H), 7.48 (m, 1H, aromat. H); 13C NMR
(CDCl3): l=45.5 (C-23), 49.4 (C-9), 51.6, 60.4 (C-1, C-5),
54.6 (OCH3), 60.6 (C-8), 72.0 (C-2), 81.0 (C-4), 105.9 (C-6),
122.9, 123.9, 125.9, 126.4, 127.0, 127.1, 127.5, 128.8 (aro-
mat. CH), 140.2, 141.0, 141.6, 144.9 (aromat. Cq), 158.8
(CqO), 209.7 (CO).
1
6. Compound 3b, mp 108°C; H NMR (CDCl3): l=4.64 (s,
2H, b-CH2), 4.97 (s, 4H, g-CH2), 5.48 (s, 2H, a-CH2), 6.57
(t, 1H, p-H, benzene), 6.65 (d, 2H, o-H, benzene), 7.38 (m,
10H, benzene), 7.48, m, 4H/8.01, m, 2H/8.33, m, 2H/8.46,
s, 1H (anthracene).
1
7. Compound 3c, mp 132°C; H NMR (CDCl3): l=4.62 (s,
2H, b-CH2), 4.90 (s, 4H, g-CH2), 4.99 (s, 8H, d-CH2), 5.47
(s, 2H, a-CH2), 6.54, t, 3H/6.61, d, 2H/6.65, d, 4H/7.36,
m, 20H (benzene), 7.38, m, 4H/7.99, m, 2H/8.30, m,
2H/8.44, s, 1H (anthracene).
14. Compound 5b, mp 72°C; 1H NMR (CDCl3): l=1.91/2.09
(AB, 2J=−18.5 Hz, 2H, 23-H), 3.52 (dd, 3J=11.7 Hz,
2
4J=1.8 Hz, 1H, 8-H), 3.67/4.21 (AB, J=−10.7 Hz, 2H,
8. Compound 4a, mp 98°C; 1H NMR (C6D6): l=2.82 (s, 6H,
4-H), 3.71/3.96 (AB, 2J=−8.8 Hz, 2H, g-CH2), 4.41 (d,
3J=11.7 Hz, 1H, 9-H), 4.51 (d, 4J=1.8 Hz, 1H, 6-H),
4.68/4.90 (AB, 2J=−10.2 Hz, 2-H), 7.23 (m, 12H, aromat.
H), 7.52 (m, 1H, aromat. H).
3
4
OCH3), 3.42 (dt, J=10.7 Hz, J=2.4 Hz, 1H, 8-H), 3.90
3
4
(s, 2H, 4-H), 4.20 (d, J=10.7 Hz, 9-H), 4.34 (d, J=2.4
Hz, 2H, 6-H, 23-H), 4.70 (s, 2H, 2-H), 7.09, m, 6H/7.30,
m, 2H (benzene rings); 13C NMR (C6D6): l=51.3 (C-9),
53.3 (C-8), 54.8 (OCH3), 56.9, 65.7 (C-1, C-5), 71.2 (C-2),
81.1 (C-4), 105.8 (C-6, C-23), 122.8, 125.2, 125.8, 127.7
(C-11, 12, 13, 14, 17, 18, 19, 20), 145.0, 146.8 (C-10, 15,
16, 21), 163.7 (C-7, 22).
15. Compound 5c, mp 68°C; 1H NMR (CDCl3): l=1.91/2.08
(AB, 2J=−19.0 Hz, 2H, 23-H), 3.52 (m, 1H, 8-H), 3.57/4.11
2
2
(AB, J=−10.7 Hz, 2H, g-CH2), 3.70/3.95 (AB, J=−9.3
Hz, 2H, 4-H), 4.40 (d, 3J=11.2 Hz, 1H, 9-H), 4.48 (‘s’, 1H,
6-H), 4.68/4.89 (AB, 2J=−10.3 Hz, 2H, 2-H), 5.01 (s, 4H,
d-CH2), 6.39 (d, 2H, aromat. H), 6.54 (t, 1H, aromat. H),
7.31 (m, 18H, aromat. H).
1
9. Compound 4b, mp 70°C; H NMR (CDCl3): l=3.24 (m,
1H, 8-H), 3.59/4.25 (AB, 2J=−10.6 Hz, g-CH2), 3.90 (s, 2H,
3
4
4-H), 4.28 (d, J=11.4 Hz, 1H, 9-H), 4.48 (d, J=2.0 Hz,
6-H, 23-H), 4.75 (s, 2H, 2-H), 7.07–7.36 (m, 18H, aromat.
H).
16. Compound 6, mp 207°C; 1H NMR (CDCl3): l=1.92/2.14
(AB, 2J=−18.5 Hz, 4H, 6-H, 23-H), 3.80 (s, 2H, 4-H), 3.96
(d, 3J=12.2 Hz, 1H, 8-H), 4.61 (d, 3J=12.2 Hz, 1H, 9-H),
4.89 (s, 2H, 2-H), 7.28 (m, 6H, aromat. H), 7.50 (m, 2H,
aromat. H); 13C NMR (CDCl3): l=47.7 (C-6, C-23), 47.7
(C-9), 48.3, 56.7(C-1, C-5), 72.5(C-8), 73.5(C-2), 81.2(C-4),
124.5, 127.7, 127.9, 128.9 (aromat. CH), 138.4, 141.8
(aromat. Cq), 205.6 (CO).
1
10. Compound 4c, mp 74°C; H NMR (CDCl3): l=3.22 (dt,
3J=10.6 Hz, J=1.9 Hz, 1H, 8-H), 3.48/4.14 (AB, J=
4
2
−11.0 Hz, 4H, g-CH2), 3.85 (s, 2H, 4-H), 4.23 (d, 3J=10.6
4
Hz, 1H, 9-H), 4.41 (d, J=1.9 Hz, 2H, 6-H, 23-H), 4.71
(s, 2H, 2-H), 5.01 (s, 8H, d-CH2), 6.44 (d, 4H, aromat. H),
6.53 (t, 2H, aromat. H), 6.97–7.42 (m, 28H, aromat. H).