Novel photorearrangement of a 4-chloromethylazulene

Article abstract of DOI:10.1016/S0040-4039(01)00256-8

1-Chloro-4,6,8-trimethylazulene is formed almost exclusively upon UV irradiation of a deaereted benzene solution of 4-chloromethyl-6,8-dimethylazulene. This photoproduct was identified by ¹H and ¹³C NMR spectroscopy of both the isolated photoproduct and the one prepared by a thermal route. The mechanism proposed involves the recombination of the intimate radical pair initially formed by the photohomolysis of the C-Cl bond, followed by a sigmatropic [1,9] hydrogen shift.

Full text of DOI:10.1016/S0040-4039(01)00256-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2613–2615
Novel photorearrangement of a 4-chloromethylazulene
Carlos A. A. Ruiz, Vicente G. Toscano, Daisy de Brito Rezende and Wilhelm J. Baader*
Instituto de Qu´ımica, Universidade de Sa˜o Paulo, Cx. Postal 26077, 05599-970 Sao Paulo, SP, Brazil
Received 7 February 2001; accepted 13 February 2001
Abstract—1-Chloro-4,6,8-trimethylazulene is formed almost exclusively upon UV irradiation of a deaereted benzene solution of
4-chloromethyl-6,8-dimethylazulene. This photoproduct was identified by 1H and 13C NMR spectroscopy of both the isolated
photoproduct and the one prepared by a thermal route. The mechanism proposed involves the recombination of the intimate
radical pair initially formed by the photohomolysis of the CꢀCl bond, followed by a sigmatropic [1,9] hydrogen shift. © 2001
Published by Elsevier Science Ltd.
Although various studies on the photocleavage of sub-
stituted arylmethyl compounds have been reported
since the 1960s,1 there is a continued interest in the
subject2 because the mechanisms operative in this pro-
cess remain not entirely understood.
When photolysis of 1 was performed in the presence of
the well established hydrogen donor 1,4-cyclohexadiene
(CHD), the rate for the consumption of 1 decreased ca.
25% (see Figs. 1 and 2) and the relative amount of 3
was greater than that formed when the irradiations
were performed in pure benzene (see Table 1, where the
yields reported for product formation have been calcu-
lated as percentages of the total amount of 1, in moles,
consumed after irradiating for 1 h). The quantum yields
determined3 for the consumption of 1 are greater in
benzene than in benzene/CHD due to the competitive
absorption by CHD at 254 nm.
In the course of our investigation on the photolysis of
halomethylazulenes in apolar solvents, we have
observed for the first time, a rearrangement of 4-
chloromethyl-6,8-dimethylazulene (1) to 1-chloro-4,6,8-
trimethylazulene (2), which is interesting from both
preparative and mechanistic points-of-view, because the
only low-molecular-weight by-products formed occur in
trace amounts (Scheme 1). This is an unexpected result,
as the products coming from biradicalar recombination
are present only in negligible amounts, as opposed to
what we have observed in a previous study concerning
the related a-bromomethylxylenes.3
1
The H and 13C NMR spectra of 2 prepared either by
thermal or by photochemical pathways are identical,
indicating that the only possible structure for 2 is the
1-chloro-isomer. They contain three signals due to the
methyl substituents, two doublets for the five-mem-
bered ring protons and two (slightly broadened) singlets
for the remaining aromatic protons. These features
indicate that product 2 does not present a C2 symmetry
axis and so it cannot be the 2-chloro isomer. Further-
more, if 2 were the 5-chloro-isomer, then the H-2
Photolysis of an oxygen-free benzene solution of 1
yields 2 and the minor products 4,6,8-trimethylazulene
(3) and 1,2-bis(4,6-dimethylazulen-8-yl)ethane (4), which
were characterized by GC/MS and GC (coinjection of
authentic standards, prepared by thermal routes).
Scheme 1.
* Corresponding author. Fax: +55-11 3815-5579; e-mail: wjbaader@iq.usp.br
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)00256-8

2614
C. A. A. Ruiz et al. / Tetrahedron Letters 42 (2001) 2613–2615
natural isotopic abundance ratios of C, H and Cl (see
Experimental).
Our results can be explained by assuming the initial
photoevent to be the homolytic cleavage of the car-
bonꢀhalogen bond, forming an intimate radical pair
that deactivates preferably by the formation of the
isomer 2, a process that can be rationalized by the
mechanism outlined in Scheme 2: initial chlorine atom
attachment to the five-membered ring in position 3
leads to compound 5, which should rearomatize rapidly
through a thermally allowed, suprafacial, sigmatropic
[1,9] hydrogen shift. In the presence of CHD, those
radicals that diffuse away from the initial intimate
radical-pair are able to abstract hydrogen from it,
forming 3 in appreciable amounts. The occurrence of
trace amounts of 3 in neat benzene can be assigned to
H-abstraction from another molecule of 1 in the
ground state.
Figure 1. Photolysis kinetic profile (uexc=254 nm) for 1, in
benzene. 1: in moles consumed, at time t; C0=1.1×10−2 mol
L−1; b=(2.60 0.07)×10−5 mol L−1 min−1; r=0.995. 2: b=
(1.19 0.03)×10−5 mol L−1 min−1; r=0.997.
Experimental. Instruments and methods—All qualitative
and quantitative chromatographic analyses were per-
formed on a Shimadzu CG-14-A gas chromatograph
using a 50 m×0.25 mm×0.25 mm Quadrex methylsili-
cone column. Quantitative determinations were per-
formed after correcting for the experimentally
determined FID response factors for each compound.
GC–MS analyses were done on a Finnigan-MAT ITD-
800 ion trap mass spectrometer. 1H and 13C NMR
spectra were recorded at 20°C on a Varian unity Inova
300 spectrometer, at 300 and 75 MHz, respectively. The
acquisition and processing parameters employed were
analogous to those already described.4 Irradiations—A
solution of 2.04 g (10.00 mmol) of compound 1 in 25.00
cm3 of spectroscopic grade benzene was irradiated
inside the cavity of an Ultraviolet Products PCQ-X1
Irradiation System, consisting of four 18 W doughnut-
type low-pressure mercury-vapor lamps contained in a
cylindrical mirror. The solution was flushed with ben-
zene-saturated argon for 30 minutes before irradiation,
the argon flux being maintained throughout the irradia-
tion. Quantum yields3—Calculated from the angular
coefficients (b) of the linear regression fits presented in
Figs. 1 and 2, using the equation b=b·V/I0, where
V=3.0×10−3 L and the photon flux, I0=1.2×10−8 ein-
stein min−1, determined with a potassium ferrioxalate
actinometer.5 Materials—4-Chloromethyl-6,8-dimethyl-
Figure 2. Photolysis kinetic profile (uexc=254 nm) for 1, in
benzene/CHD 1.0 mol L−1. 1: in moles consumed, at time t;
C0=1.0×10−2 mol L−1; b=(1.96 0.06)×10−5 mol L−1 min−1
;
r=0.993. 2: b=(4.0 0.2)×10−6 mol L−1 min−1; r=0.987. 3:
b=(1.42 0.06)×10−6 mol L−1 min−1; r=0.990.
should be present as a triplet (due to coupling with
both H-1 and H-3), which is not the case. The presence
of a chloro substituent in the molecule is confirmed by
the consistence between the observed and the calculated
+
’
relative intensities of the molecular-ion peak (M ,
which is also the base peak) and those due to the
Table 1. Photolysis of 1 in benzene at 254 nm
Yielda (%)
b (10−2
)
Recovereda,b (%)
1
2
3
4
1
2
3
4
Neat
19c
40c
53d
23d
Traces
11d
Traces
Traces
6.6e
4.9e
3.0f
1.1f
–
–
–
53
34
CHD (1.0 mol L−1
)
0.4f
a After 1 h of UV-exposition.
b Recovered refers to the total amount of photoproducts detected and quantified by GC analysis relative to the moles of 1 consumed, as a
percentage; the remainder are non-volatile high-molecular-weight materials for which characterization was not pursued.
c Moles of 1 consumed, as a percentage.
d Relative to the moles of 1 consumed, as a percentage.
e Quantum yield (b) for the consumption of 1.
f Quantum yield (b) for product formation calculated from b, in Figs. 1 and 2 (see Experimental).

C. A. A. Ruiz et al. / Tetrahedron Letters 42 (2001) 2613–2615
2615
Scheme 2.
Acknowledgements
azulene (1) was prepared from 4,6,8-trimethylazulene as
previously described.6 1,2-Bis(4,6-dimethylazulen-8-
yl)ethane (4) was prepared as reported by Hafner et al.7
1-Chloro-4,6,8-trimethylazulene (2): (i) by preparative
photolysis—50.0 cm3 solution of 1, irradiated as
described above, the product being purified by elution
with hexane/ether (20:1) through a column of basic
alumina (100–230 mesh) of activity II–III (80 g of
alumina+2 g of H2O). The product, a deep-blue oily
liquid, was isolated in 56% yield. (ii) By thermal route—
a solution of N-chlorosuccinimide (0.3 g; 2.2 mmol) in
anhydrous ether was added to 2 mmol of sodium
4,6-dimethyl-8-methylene-azuleneate,7 dissolved in the
same solvent. The reaction mixture was stirred for
2 h at room temperature, while being filtered; the
filtrate was washed until neutrality with 2 mol L−1
hydrochloric acid and water and dried over anhydrous
magnesium sulphate. After total removal of the solvent,
the product was redissolved in hexane and purified by
column chromatography on neutral alumina (activity
II) eluted with hexane/ether 95:5. The product was
This work was supported by grants from the FAPESP,
CNPq and FINEP/PADCT. C.A.A. Ruiz thanks
CAPES for a fellowship. The authors are indebted to
Dr. Ivan P. de Arruda Campos (Universidade Paulista)
for the stimulating discussions, fruitful suggestions and
careful revision of the manuscript. The gift of 4,6,8-
trimethylazulene by Professor K. Hafner (Technische
Universita¨t Darmstadt) is also gratefully acknowledged.
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+
’
153 (79.1%, m/z=169-CH4); 169 (71.1%; M −Cl); 189
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35
+
.
37
+
.
’
’
(24.4%; M −CH3); 191 (22.73%; M −CH3); 204
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35
+
35
+
’
’
(100%; M ); 205 (17.8%;
M
+1); 206 (33.3%;
35
+
37
+
37 +
’
’
’
M +1); requires:
[ M +2 and M ]; 207 (4.44%;
M =100%;
=33.54%;
35
+
35
+
35
+
37
+
’
’
’
’
M
M
+1=14.26%;
M
+2 and
37
+
’
M
+1=4.65.
.