Rearrangement of the carbon skeleton in the intramolecular photoadduct of anthracene and benzene rings

Article abstract of DOI:10.1016/S0040-4020(03)00787-7

The effectivity of optical switching between anthracene derivatives 3a,b and their intramolecular photocycloadducts 4a,b is impaired by traces of acid. The systematic treatment of 4a,b with an increasing excess of formic acid revealed that - apart from the normal enolether cleavage 4a,b→6a,b→7a,b - a cleavage with rearrangement of the carbon skeleton can occur: 5b→6b′. The driving force is a stability enhancement of the involved carbenium ions 5b→5b′. A further increased excess of formic acid leads finally to a competitive ether cleavage in the tetrahydrofuran ring 5b→8.

Full text of DOI:10.1016/S0040-4020(03)00787-7

Tetrahedron 59 (2003) 5323–5327
Rearrangement of the carbon skeleton in the intramolecular
photoadduct of anthracene and benzene rings
Derong Cao,a Silvia Dobis,b Dieter Schollmeyerb and Herbert Meierb
,
*
aLCLC, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, 510650 Guangzhou, People’s Republic of China
bInstitute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany
Received 20 March 2003; revised 12 May 2003; accepted 12 May 2003
Abstract—The effectivity of optical switching between anthracene derivatives 3a,b and their intramolecular photocycloadducts 4a,b is
impaired by traces of acid. The systematic treatment of 4a,b with an increasing excess of formic acid revealed that—apart from the normal
enolether cleavage 4a,b!6a,b!7a,b—a cleavage with rearrangement of the carbon skeleton can occur: 5b!6b0. The driving force is a
stability enhancement of the involved carbenium ions 5b!5b0. A further increased excess of formic acid leads finally to a competitive ether
cleavage in the tetrahydrofuran ring 5b!8. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Results and discussion
The anthracene chromophore attracts great attention in
photochemistry because of its high capability of cycloaddi-
tion reactions.1 Applications in materials science or
molecular recognition seem to be particularly promising
in the field of optical switching processes.1–3
The reaction of 9-chloromethylanthracene (1) and di- or
trimethoxybenzylalcohols (2a,b) leads under phase-transfer
conditions in good yields to the anthracene derivatives 3a,b.
Irradiation (l$300 nm) of 3a,b in a ,1022 M solution in
benzene furnishes the polycyclic compounds 4a,b in
quantitative yields (Scheme 1). The compounds 3a,b exhibit
the typical, ‘finger-like’ anthracene absorption with maxima
at 333, 349, 367 and 387 nm (3.730#log 1#4.178). The
cycloreversion can be quantitatively accomplished either by
heating to 1208C or by irradiation with l¼254 nm.
Recently we prepared dendrimers with an anthracene core
´
and dendrons of the Frechet type and some model
compounds for these light-harvesting systems.4,5 Common
to all these compounds is a benzene ring, which is linked by
a CH2–O–CH2 chain to the 9-position of anthracene. A
new type of [4pþ4p] photocycloaddition was found, in
which an electron-rich benzene ring adds in 1,4-position to
the 9,10-position of the anthracene moiety.
The intramolecular variant5 seems to be particularly suitable
for optical switching processes since the photocycloaddition
by irradiation into the anthracene chromophore as well as
the reverse photoreaction with light of shorter wavelengths
are quantitative processes. However, it turned out that the
retrocycloaddition is impaired by the presence of traces of
acid—on the surface of silica or in chloroform solution.
Therefore we studied the influence of formic acid on two
model systems that we now report.
Keywords: rearrangements; carbenium ions; ether cleavage; ketones.
*
Corresponding author. Tel.: þ49-6131-3922605; fax: þ49-6131-
Scheme 1. Preparation and reversible intramolecular cycloaddition of the
anthracene derivatives 3a,b.
3925396; e-mail: hmeier@mail.uni-mainz.de
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00787-7

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D. Cao et al. / Tetrahedron 59 (2003) 5323–5327
Figure 1. 1H and 13C NMR data of 4b (Cs symmetry) in CDCl3, related to
TMS as internal standard. The arrows indicate the most important
homonuclear NOEs.
The structure elucidation of 4a,b is based on NMR and mass
spectroscopic measurements. Figure 1 demonstrates the 1H
and 13C chemical shifts of 4b. The correlations with certain
nuclei were obtained by nuclear Overhauser effect (NOE)
measurements, homonuclear and heteronuclear shift corre-
lations (HMQC and HMBC).
The treatment of 4a with formic acid leads to a stepwise
cleavage of the two enolether substructures. The transform-
ation of 4a to the monoketone 6a and the diketone 7a
prevents a cycloreversion of the polycyclic system. The
thermal activation barrier is probably too high because the
retrocycloaddition would not restore the aromatic benzene
rings; the reverse photoreaction fails presumably because
of the formation of a first electronically excited singlet state
S1, which has a npp character at the carbonyl groups
(Scheme 2).
Scheme 2. Enolether cleavage of 4a,b with formic acid.
force for the 1,2-C shift is rationalized by the formation of a
more stable cation. The positive charge in 5b and 5b0 is
stabilized by the adjacent heteroatom O, but 5b0 represents
additionally an allylic system.7 The subsequent hydrolysis
5b0!6b0 yields an a,b-unsaturated monoketone 6b0.
Although 6b0 has still an enolether substructure, it is stable
towards formic acid—even at 808C. Diketone 7b can be
When 4b is subjected to the same procedure, a surprising
result is obtained. It turns out that the reaction route depends
strongly on the molar ratio HCOOH–4b (Table 1). The
formation of the monoketone 6b, proved by 1H NMR
measurements, and of the isolated diketone 7b is at room
temperature a slow process, provided that a not much higher
than fourfold excess of formic acid is used. When the ratio
HCOOH–4b amounts to 10:1, the reaction rate of the
enolether cleavage becomes high, but another product is
generated. The normal A–SE2 mechanism6 for the enol-
ether cleavage is almost completely suppressed, because the
primary protonation generates a carbenium ion 5b, which
shows a rearrangement of the carbon skeleton. The driving
1
merely observed as a trace in the H NMR spectrum of the
raw product 6b0. The latter measurement reveals another
trace of a byproduct, namely the formate 8. When the molar
ratio HCOOH–4b is further changed to 13.7:1 and finally to
58.8:1, the amount of formed 6b0 is reduced and the
percentage of 8 enhanced (Table 1). An extremely high
excess of formic acid leads obviously to a competitive ether
cleavage in the perhydrofuran ring (Scheme 2).8
Table 1. Product distribution in the acid-catalyzed cleavage of the bisenolether 4b at room temperature
Entry
Molar ratio HCOOH–4b
Reaction time
Conversion (%)
Product distribution (%)
6b0
7b
8
1
2
3
4
5
4.1:1
8.2:1
10.3:1
13.7:1
58.8:1
10 d
12 h
1 h
2 h
1 h
100
60
100
100
100
–
100
80
Trace
–
–
–
20
100
95
74
Trace
5
26
–

D. Cao et al. / Tetrahedron 59 (2003) 5323–5327
5325
elemental analyses were determined in the Microanalytical
Laboratory of the Chemistry Department of the University
of Mainz.
4.2. General procedure for the preparation of the 9-
(alkoxymethyl)antracenes 3a,b
A mixture of 9-chloromethylanthracene 110 (454 mg,
2.0 mmol), 3,5-dimethoxybenzylalcohol 2a11 (370 mg,
2.2 mmol) or 3,4,5-trimethoxybenzylalcohol 2b12
(440 mg, 2.4 mmol), KOH (161 mg, 2.4 mmol), (n-
Bu)4NBr (90 mg, 0.28 mmol), 1 mL water and 20 mL
chlorobenzene was stirred for 3–4 d at 658C. The volatile
parts were removed under reduced pressure (1 hPa) and the
residue treated with 20 mL CH2Cl2 and 15 mL water. The
separated water layer was three times extracted with 10 mL
CH2Cl2 each. The combined organic phases were dried with
Na2SO4 and the products purified by column chromatog-
raphy (50£3 cm SiO2, cyclohexane–ethylacetate 93:7).
4.2.1. 9-[(3,5-Dimethoxyphenyl)methoxymethyl]anthra-
cene (3a). Yield 588 mg (82%), mp 1008C (ethanol–
1
dichloromethane 1:1). H NMR (CDCl3): d¼3.76 (s, 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, 10-H, anthracene); 13C NMR
(CDCl3): d¼55.3 (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, anthracene), 140.9 (i-C, benzene), 160.9 (m-C,
benzene); EI MS: m/z (%)¼358 (17) [Mþ], 152 (100); Anal.
calcd for C24H22O3 (358.4): C, 80.42; H, 6.19. Found: C,
80.47; H, 6.21.
Figure 2. ORTEP plot of 6b0. The displacement ellipsoids are shown at the
50% probability level. The numbering does not correspond to the
nomenclature.
Apart0from spectroscopic methods, the structure elucidation
of 6b is based on an X-ray crystal structure analysis.9
Figure 2 shows an ORTEP plot of 6b0.
4.2.2. 9-[(3,4,5-Trimethoxyphenyl)methoxymethyl]an-
1
thracene (3b). Yield 488 mg (63%), viscous oil. H NMR
3. Conclusion
(CDCl3): d¼3.79 (s, 6H, OCH3), 3.85 (s, 3H, OCH3), 4.62
(s, 2H, b-CH2), 5.49 (s, 2H, a-CH2), 6.58 (s, 2H, o-H,
benzene), 7.48 (m, 4H, anthracene), 8.00 (m, 2H, anthra-
cene), 8.32 (m, 2H, anthracene), 8.45 (s, 1H, 10-H,
anthracene); 13C NMR (CDCl3): d¼56.0 (OCH3), 60.8
(OCH3), 63.9 (a-CH2), 72.3 (b-CH2), 104.7 (o-CH,
benzene), 124.4, 125.0, 126.1, 128.6 (CH, anthracene),
128.5, 131.1, 131.5 (Cq, anthracene), 134.2 (i-C, benzene),
137.4 (p-C, benzene), 153.2 (m-C, benzene; FD MS: m/z
(%)¼389 (100) [MþHþ]. Anal. calcd for C25H24O4 (388.5):
C, 77.30; H, 6.23. Found: C, 77.20; H, 6.30.
A quantitative optical switching between the anthracene
derivatives 3a,b and their polycyclic isomers 4a,b requires
an acid-free medium (or surface). In the presence of formic
acid, 4a and 4b exhibit a normal enolether cleavage to the
monoketones 6a,b and the diketones 7a,b. If however, the
molar ratio of formic acid: 4b is increased above a certain
limit a competitive process 4b!6b0 can take place, which
becomes then the major route. A 1,2-C-shift, based on the
relative stability of the involved carbenium ions 5b and 5b0,
is the characteristic step in the alternative ether cleavage. A
further enhancement of the excess of HCOOH reduces
finally the yield of 6b0, because a new byproduct, namely the
ester 8 is generated.
4.3. Preparation of the photocycloadducts
4.3.1. 7,22-Dimethoxy-3-oxahexacyclo[7.6.6.25,8.01,5.010,15
16,21] tricosa-6,10,12,14,16,18,20,22-octaene (4a). A sol-
.
0
ution of 30 mg (8.4£1022 mmol) of 3a in 165 mL of benzene
was purged with a slow stream of argon. After 30 min
irradiation was started with a middle pressure mercury lamp
(Hanovia 450 W) equipped with a Duran glass filter. The
quantitative photocycloaddition was finished after 15 min, the
solvent was removed and the product 4a isolated as light
yellow solid, which started to isomerize to 3a at mp 988C. 1H
NMR (C6D6): d¼2.82 (s, 6H, OCH3), 3.42 (dd, 3J¼10.7 Hz,
4J¼2.4 Hz, 1H, 8-H), 3.90 (s, 2H, 4-H), 4.20 (d, 3J¼10.7 Hz,
4. Experimental
4.1. General remarks
Melting points were measured on a Bu¨chi melting point
1
apparatus and are uncorrected. H and 13C NMR spectra
were recorded with a Bruker AM 400 spectrometer. The
mass spectra were obtained on a Finnigan MAT 95
spectrometer with the field desorption technique (FD) or
with the electron impact method (EI) at 70 eV. The
3
1H, 9-H), 4.34 (d, J¼2.4 Hz, 2H, 6-H, 23-H), 4.70 (s, 2H,

5326
D. Cao et al. / Tetrahedron 59 (2003) 5323–5327
2-H), 7.09 (m, 6H, aromat. H), 7.30 (m, 2H, aromat. H); 13
C
4.4.3. 3-Oxahexacyclo[7.6.6.25,8.01,5.010,15.016,21]tricosa-
10,12,14,16,18,20-hexaen-7,22-dione (7a). A solution of
20 mg (5.6£1022 mmol) of 4a in 0.5 mL of chloroform was
treated with 0.05 mL (61.2 mg, 1.33 mmol) of formic acid.
After 1 h stirring at room temperature, the volatile parts
were removed under reduced pressure (1 hPa) and the raw
product 7a (18.4 mg, 100%) obtained as a colorless,
thermally stable solid; mp 2078C (dichloromethane–
cyclohexane 1:9). 1H NMR (CDCl3): d¼1.92/2.14 (AB,
2J¼218.5 Hz, 4H, 6-H), 23-H), 3.80 (s, 2H, 4-H), 3.96 (d,
3J¼12.2 Hz, 1H, 8-H), 4.61 (d, 3J¼12.2 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): d¼47.7 (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 (C-7, C-22); FD MS: m/z (%)¼331
(100) [MþHþ]. Anal. calcd for C22H18O3 (330.4): C, 79.98;
H, 5.49. Found: C, 79.96; H, 5.64.
NMR (C6D6): d¼51.3 (C-9), 53.3 (C-8), 54.8 (OCH3), 56.9
(C-5), 65.7 (C-1), 71.2 (C-2), 81.1 (C-4), 105.8 (C-6, C-23),
122.8,125.2,125.8, 127.7(aromat. CH), 145.0, 146.8(aromat.
Cq), 163.7 (C-7, C-22); FD MS: m/z (%)¼359 (100) [MþHþ].
Anal. calcdfor C24H22O3 (358.4):C, 80.42;H, 6.19. Found:C,
80.44; H, 6.17.
4.3.2. 7,8,22-Trimethoxy-3-oxahexacyclo[7.6.6.25,8
1,5.010,15.016,21]tricosa-6,10,12,14,16,18,20,22-octaene
.
0
(4b). 39 mg (0.10 mmol) 3b yielded in the irradiation process,
described above, 39 mg (100%) of 4b, a yellow solid, which
isomerizes to 3a at mp 1208C. The 1H and 13C NMR data are
shown in Figure 1. FD MS: m/z (%)¼389 (100) [MþHþ].
Anal. calcd for C25H24O4 (388.5). C, 77.30; H, 6.23. Found: C,
76.96; H, 6.42.
4.4. Enolether cleavage
4.4.1. (5R p,8R p)-22-Methoxy-3-oxahexacyclo[7.6.6.25,8
1,5.010,15.016,21]tricosa-10,12,14,16,18,20,22-heptaen-7-
.
4.4.4. 8-Methoxy-3-oxahexacyclo[7.6.6.25,8.01,5
10,15.016,21]tricosa-10,12,14,16,18,20-hexaen-7,22.dione
.
0
0
one (6a). To a solution of 20 mg (5.58£1022 mmol) of 4a in
3 mL toluene 0.02 mL (24.4 mg, 0.53 mmol) formic acid was
added. After 25 min stirring at room temperature, the volatile
parts were removed under reduced pressure. The raw product
(19.2 mg, 100%), a yellowish solid was recrystallized from
dichloromethane–cyclohexane (1:9). The thermally stable
compound melted at 1058C. 1H NMR (CDCl3): d¼1.90/2.04
(AB, 2J¼218.4 Hz, 2H, 6-H), 2.93 (s, 3H, OCH3), 3.44 (dd,
3J¼11.7 Hz, 4J¼1.6 Hz, 1H, 8-H), 3.70/3.95 (AB,
2J¼29.0 Hz, 2H, 4-H), 4.37 (s, 1H, 6-H), 4.38 (d,
(7b). To a solution of 25 mg (6.4£1022 mmol) of 4b in 3 mL
toluene 0.01 mL (12.2 mg, 0.265 mmol) formic acid was
added. After stirring for 10 d at room temperature, the volatile
parts were removed under reduced pressure and the raw
product 7b (23.1 mg, 100%) obtained as a thermally stable,
colorless solid; mp 2418C (dichloromethane–cyclohexane
1:9). 1H NMR (CDCl3): d¼2.03/2.13 (AB, 2J¼218.8 Hz, 4H,
6-H, 23-H), 3.35 (s, 3H, OCH3), 3.74 (s, 2H, 4-H), 4.35 (s, 1H,
9-H), 4.87 (s, 2H, 2-H), 7.28 (m, 6H, aromat. H), 7.35 (m, 2H,
aromat. H); 13C NMR (CDCl3): d¼46.8, 56.0 (C-1, C-5), 48.6
(OCH3), 56.5, 58.9 (C-6, C-9), 73.0 (C-2), 81.5 (C-4), 100.9
(C-9), 124.0, 127.8, 128.0, 129.3 (aromat. CH), 137.1, 141.9
(aromat. Cq), 203.4 (C-7, C-22); FD MS: m/z (%)¼361 (100)
[MþHþ]. Anal. calcd for C23H20O4 (360.1):C, 76.65;H, 5.59.
Found: C, 76.56; H, 5.54.
2
3J¼11.7 Hz, 1H, 9-H), 4.68/4.88 (AB, J¼210.6 Hz, 2H,
2-H); 13C NMR (CDCl3): d¼45.5 (C-6), 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-23), 122.9, 123.9, 125.9, 126.4, 127.0, 127.1, 127.5,
128.8 (aromat. CH), 140.2, 141.0, 141.6, 144.9 (aromat Cq),
158.8 (C-22), 209.7 (C-7); FD MS: m/z (%)¼345 (100)
[MþHþ]. Anal. calcd for C23H22O3 (344.4):C, 80.21;H, 5.85.
Found: C, 80.13; H, 5.90.
4.4.5. 9-Anthrylmethyl formate (8). A solution of 35 mg
(9.0£1022 mmol) of 4b in 0.5 mL of chloroform was
treated with 0.20 mL (244 mg, 5.30 mmol) formic acid.
After 1 h stirring at room temperature, the volatile parts
were removed under reduced pressure. The residue was
separated by column chromatography (20£2 cm SiO2,
cyclohexane–ethyl acetate gradient 95:5 to 80: 20 to yield
5 mg (25%) of 8 as yellowish needles; mp 1308C. 1H NMR
(CDCl3): d¼6.24 (s, 2H, CH2), 7.53 (m, 4H, aromat. H),
8.03 (m, 2H, aromat. H), 8.19 (s, 1H, CHO), 8.32 (m, 2H,
aromat. H), 8.52 (s, 1H, anthracene 10-H); 13C NMR
(CDCl3): d¼58.2 (CH2), 123.7, 125.2, 126.9, 129.2, 129.6
(aromat. CH), 128.0, 131.4, 134.1 (aromat. Cq), 161.1
(CHO); EI MS: m/z (%)¼237 (61) [MþHþ], 192
(100); identification by comparison with an authentic
sample.13
4.4.2. (5R p,9R p)-7,9-Dimethoxy-3-oxahexacyclo
[8.6.6.15,9.01,5.011,16.017,22]tricosa-6,11,13,15,17,19,21-
heptaen-8-one (6b0). To a solution of 39 mg (0.1 mmol of
4b in 0.5 mL chloroform, 0.03 mL (36.6 mg, 0.795 mmol)
of formic acid was added. After stirring at room
temperature for 1 h, the volatile parts were removed
under reduced pressure and the product 6b0 isolated in a
quantitative yield. Colorless crystals, mp 189–1908C
(dichloromethane–cyclohexane 1:9). 1H NMR (CDCl3):
2
4
d¼1.32 (dd, J¼212.6 Hz, l Jl¼1.9 Hz, 1H, endo-23-H),
2.45 (d, 2J¼212.6 Hz, 1H, exo-23-H), 3.15 (s, 3H,
2
OCH3), 3.17 (s, 3H, OCH3), 3.77/4.00 (AB, J¼29.0 Hz,
2H, 4-H), 4.17 (s, 1H, 10-H), 4.74/4.88 (AB,
2J¼29.9 Hz, 2-H), 5.35 (d, 4J¼1.9 Hz, 6-H), 6.94 (m,
1H, aromat. H), 7.05 (m, 2H, aromat. H), 7.19 (m, 4H,
aromat. H), 7.52 (m, 1H, aromat. H); 13C NMR (CDCl3):
d¼40.9 (C-23), 48.4, 59.7 (C-1, C-5), 51.9 (C-10), 55.0,
58.3 (OCH3), 69.6, 80.5 (C-2, C-4), 78.7 (C-9), 121.6
(C-6), 122.0, 123.0, 126.9, 127.0, 127.0, 127.0, 127.8,
129.2 (aromat. CH), 138.9, 139.6, 141.7, 141.9 (aromat.
Cq), 158.8 (C-7), 193.7 (C-8); FD MS: m/z (%)¼375
(100) [MþHþ]. Anal. calcd for C24H22O4 (374.2): C,
76.99; H, 5.92. Found: C, 76.93; H, 6.03.
Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft,
the Fonds der Chemischen Industrie, the National
Natural Science Foundation of China and the Guangdong
Natural Science Foundation of China for financial
support.

D. Cao et al. / Tetrahedron 59 (2003) 5323–5327
5327
6. See for example Smith, M. B.; March, J. Advanced Organic
Chemistry. 5th ed., 2001; p 467.
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7. Since 5e does not show such a 1,2-C shift, both stabilizations
of the cation seem to be a precondition for the rearrangement.
8. The remainder part of 4b in the cleavage 4b!8 did not give a
uniform compound, the 1H NMR spectrum of the raw product
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a benzene ring.
9. The crystallographic data of 6b0 have been deposited at
Cambridge Crystallographic Data Centre as supplementary
publication No. 6b0 CCDC 206152. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union
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