Mechanistic studies on the photogeneration of o-and p-xylylenes from aαα′-dichloroxylenes

Article abstract of DOI:10.1039/a802016g

Two-colour two-laser techniques have unambiguously proved that photolysis of the o-/p-(chloromethyl)benzyl radical leads to the sequential two-photon generation of o-/p-xylylene from α,α′-dichloro-o-/p-xylene.

Full text of DOI:10.1039/a802016g

View Article Online / Journal Homepage / Table of Contents for this issue
Mechanistic studies on the photogeneration of o- and p-xylylenes from
a,aA-dichloroxylenes
a
b
a
c
Miguel A. Miranda,* † Julia P e´ rez-Prieto,* Enrique Font-Sanchis, and J. C. Scaiano*
a
Instituto de Tecnolog ´ı a Qu ´ı mica/Departamento de Qu ´ı mica, Universidad Polit e´ cnica de Valencia, Camino de Vera s/n, 46071
Valencia, Spain
b
Departamento de Qu ´ı mica Org a´ nica, Facultad de Farmacia, Universidad de Valencia, Vic e´ nt Andr e´ s Estell e´ s s/n, Burjasot,
4
6100 Valencia, Spain
Department of Chemistry, University of Ottawa, Ottawa, Canada K1N 6N5
c
3
Two-colour two-laser techniques have unambiguously
proved that photolysis of the o-/p-(chloromethyl)benzyl
radical leads to the sequential two-photon generation of o-/p-
xylylene from a,aA-dichloro-o-/p-xylene.
obtained. According to the literature, they were assigned to 2a
(with a maximum at 330 nm) and o-xylylene (3a) (maximum at
3
3
21
21 4
360 nm, emax = 3 3 10 dm mol cm ), generated through
one- and two-photon processes, respectively.
When irradiating with a 266 nm laser it is useful to place a
beam diffuser (which eliminates ‘hot’ spots in the laser beam)
close to the sample in order to get a better observation of
monophotonic transients. Actually, a new spectrum obtained
under these conditions showed a higher ratio of radical 2a to the
two photon intermediate, 3a (Fig. 1).
To confirm the radical nature of the transient at 330 nm,
oxygenated samples were examined, showing that, while the
band at 330 nm was quenched at close to the diffusion
controlled limit (see insert Fig. 1), the lifetime of the transient at
360 nm appeared insensitive to the presence or absence of
oxygen (data not shown).
Xylylenes are molecules of considerable theoretical and
synthetic interest, which have been detected and characterized
by different spectroscopic methods.1 Although benzylic
dichlorides have been used as very simple and readily
accessible precursors, the involved mechanisms are not com-
5
2
3
pletely understood. It has been recently reported that 266 nm
laser irradiation of a,aA-dichloro-o-xylene (1a) produces the
dissociation of both C–X bonds via a two-photon process;
however, the nature of the intermediate species which absorbs
the second photon is uncertain. Three possibilities have been
considered: the
S
1
and
T
1
states of 1a and the
o-(chloromethyl)benzyl radical (2a). Based on the short lifetime
of the singlet state of the dichloride and the apparent relative
Furthermore, to study the photobehaviour of 2a, two-colour
6
two-laser flash photolysis experiments were carried out using
2
1
yields of the observed transients, the undetected T
1
state of the
266 nm laser pulses (20 mJ pulse ) to photolyze 1a and a 308
2
1
dichloride has been suggested as the photochemical precursor of
o-xylylene.
nm excimer laser (90 mJ pulse ) to irradiate 2a. The two
pulses were typically separated by ca. 2.5 ms, a time sufficiently
long that the irradiation of excited states of 1a by the 308 nm
laser is impossible. We note that 1a is transparent at 308 nm.
That the benzyl radical 2a can be the precursor of o-xylylene is
clearly demonstrated by comparing the spectra obtained upon
irradiation with the first and the second laser (Fig. 2). The
permanent and irreversible bleaching of the transient with a
maximum at 330 nm was concurrent with the jump of the band
with a maximum at 360 nm.
Our aim was to explore the photochemistry of the benzyl
radical 2a produced after dissociation of one C–X bond of a,aA-
dichloro-o-xylene, using two-laser two-colour techniques, in
order to obtain new data to support or reject the intermediacy of
2
a in the formation of o-xylylene. We report here our findings
and compare them with those obtained in the photolysis of a,aA-
dichloro-p-xylene. Based on the obtained results, a mechanism
for the formation of o- and p-xylylenes from their correspond-
ing dichloro precursors is presented.
Thus, it seems clear that 2a photolyzes to o-xylylene, and a
mechanism for the photogeneration of o-xylylene from a,aA-
dichloro-o-xylene is shown in Scheme 1; while triplet 1a may or
may not produce o-xylylene, it is clear that there is no need to
invoke this undetected process.
Laser flash photolysis of deaerated 1.5 mm solutions of 1a in
cyclohexane at 266 nm (Nd: YAG laser, fourth harmonic, < 10
2
1
ns, @20 mJ pulse ) yielded the transient absorption spectra
shown in Fig. 1. Two transients with different lifetimes were
On the other hand, laser flash photolysis of deaerated 1.5 mm
solutions of a,aA-dichloro-p-xylene (1b) in cyclohexane at 266
nm (with a beam diffuser) yielded the transient absorption
spectrum shown in Fig. 3. Irradiation of oxygenated samples
showed that the band at 320 nm was quenched at close to the
diffusion controlled limit, but the lifetime of the transient at 290
nm appeared to be insensitive to the presence of oxygen. Thus,
the band at 320 nm can be assigned to the p-(chloro-
methyl)benzyl radical (2b) by comparison with the band of the
(
a)
(
b)
(
b)
(
a)
7
p-methylbenzyl radical in solution. On the other hand, the band
l / nm
at 290 nm is assigned to p-xylylene in accordance with literature
8
data. At first glimpse, the formation of p-xylylene from 1b
appeared to be more efficient than the generation of o-xylylene
from 1a, even if both dihalides were photolyzed under the same
conditions [compare curve (b) in Fig. 1 and the transient in
Fig. 3]. However, this could also reflect a different molar
extinction coefficient of both xylylenes. Although emax of 3b
has not been reported, electronic spectra calculated for xyly-
l / nm
Fig. 1 Transient absorption spectra recorded following laser excitation (266
nm) of 1a under nitrogen 2 ms after laser pulse (a) with diffuser and (b)
without diffuser. Insert: (a) spectrum of o-xylylene obtained 2.32 ms after
irradiation in the presence of oxygen without diffuser; (b) spectrum of 2a
obtained by normalizing and subtracting the spectrum of o-xylylene from
the spectrum obtained in the absence of oxygen.
9
lenes indicate that a much higher intensity should be expected
for the absorption of the para than for the ortho derivative.
Chem. Commun., 1998
1541

View Article Online
(
a)
(b)
l / nm
t / µs
Fig. 4 Two laser-two colour photolysis of 1b. Top: kinetic trace at 320 nm;
the bleaching corresponds to the disappearance of 2b. Bottom: kinetic trace
at 290 nm; the jump corresponds to the formatio of 3b.
t / µs
t / µs
Fig. 2 Top: (a) Transient absorption spectrum recorded following laser
excitation (266 nm) of 1a under nitrogen 1.12 ms after laser pulse. (b)
Transient absorption spectrum obtained upon two laser-two colour
excitation of 1a. The intermediate generated by means of a 266 nm laser
pulse is photolyzed after 2 ms by a second laser at 308 nm. Bottom: Kinetic
trace at 330 (left) and 360 nm (right); the bleaching at 330 nm corresponds
to the disappearance of 2a and the jump at 360 nm corresponds to the
formation of 3b.
The photobehaviour of radical 2b was studied by using two-
colour two-laser experiments; the 266 nm laser was used to
generate this radical, which was further irradiated with pulses
from the 308 nm laser. That 2b can be the precursor of 3b is
clearly demonstrated by comparing traces obtained for both
transients (Fig. 4). Thus, the bleaching at 320 nm was
coincident with the jump at 290 nm. Based on this evidence the
mechanism for the photogeneration of p-xylylene from a,aA-
dichloro-p-xylene is also that of Scheme 1.
In summary, this work has demonstrated that benzyl radicals
are the key light absorbing transients in the two-photon
generation of o- and p-xylylenes from the corresponding
dichloroxylenes.
CH2Cl
CH2•
3
a
hn1, –Cl•
•
2
hn , –Cl
CH2Cl
CH2Cl
ortho: 1a
para: 1b
2a,b
J. C. S. thanks the National Sciences and Engineering
Research Council of Canada for support. Spanish DGICYT
3b
(
MAM, Project no. PB94-0539) and Spanish Ministry of
Scheme 1
Education (EFS, Grant) are gratefully acknowledged.
(
a)
Notes and References
†
E-mail: mmiranda@qim.upv.es
(b)
1
L. A. Errede, J. Am. Chem. Soc., 1961, 83, 949; E. Migirdicyan and
J. Baudet, J. Am. Chem. Soc., 1975, 97, 7400; J. J. McCullough, Acc.
Chem. Res., 1980, 13, 270; Y. Ito, M. Nakatsuka and T. Saegusa, J. Am.
Chem. Soc., 1982, 104, 7609; V. Wintgens, J. C. Netto-Ferreira,
H. L. Casal and J. C. Scaiano, J. Am. Chem. Soc., 1990, 112, 2363.
K. L. Tseng and J. Michl, J. Am. Chem. Soc., 1977, 99, 4840; K. Haider,
M. S. Platz, A. Despres, V. Lejeune, E. Migirdicyan, T. Bally and
E. Haselbach, J. Am. Chem. Soc., 1988, 110, 2318.
2
l / nm
3
4
5
6
M. Fujiwara, K. Mishima, K. Tamai, Y. Tanimoto, K. Mizuno and
Y. Ishii, J. Phys. Chem., 1997, 101, 4912.
W. S. Trahanovsky and J. R. Macias, J. Am. Chem. Soc., 1986, 108,
Fig. 3 Transient absorption spectrum recorded following laser excitation
266 nm) of 1b under nitrogen 0.8 ms after laser pulse. Inserts (a) and (b)
show the effect of the laser power on signal intensities monitored at 320 and
(
6
820.
M. A. Miranda, J. P e´ rez-Prieto, E. Font-Sanchis, K. K o´ nya and
J. C. Scaiano, J. Org. Chem., 1997, 62, 5713.
J. C. Scaiano, L. J. Johnston, W. G. McGimpsey and D. Weir, Acc. Chem.
Res., 1988, 21, 22.
2
90 nm, respectively.
To establish the nature of the process leading to radical 2b
and p-xylylene, an investigation of the effects of light intensity
on the relative intensity of their signals was carried out by
attenuating the laser beam with a set of calibrated neutral
density filters. Inserts (a) and (b) in Fig. 3 show that there is a
linear relationship between the formation of 2b and the laser
power, while a parabola is obtained in the case of 3b; thus, the
evidence supports a one-photon process for the benzylic radical
and a two-photon process for p-xylylene.
7 R. F. C. Claridge and H. Fisher, J. Phys. Chem., 1983, 87, 1960.
8
G. Kaupp, Angew. Chem., Int. Ed. Engl., 1976, 15, 442; J. M. Pearson,
M. A. Six, D. J. Williams and M. Levy, J. Am. Chem. Soc., 1971, 93,
5
1
034; R. Marquardt, W. Sander, T. Laue and H. Hopf, Leibigs Ann., 1995,
643.
9
C. R. Flynn and J. Michl, J. Am. Chem. Soc., 1974, 96, 3280.
Received in Liverpool, UK, 12th March 1998; 8/02016G
1542
Chem. Commun., 1998