The 4,4′-(1,2-ethanediyl)bisbenzyl biradical: Its generation, detection, and (photo)chemical behavior in solution

Article abstract of DOI:10.1021/jo001623y

The 4,4′-(1,2-ethanediyl)bisbenzyl biradical (2) is clearly and efficiently generated by photolysis of [3.2]paracyclophane-2-one (8) in cyclohexane solution. This intermediate is also formed via two-photon processes from [2.2]paracyclophane (3) and 1,2-bi

Full text of DOI:10.1021/jo001623y

J . Org. Chem. 2001, 66, 2717-2721
2717
Th e 4,4′-(1,2-Eth a n ed iyl)bisben zyl Bir a d ica l: Its Gen er a tion ,
Detection , a n d (P h oto)ch em ica l Beh a vior in Solu tion
Miguel A. Miranda,*,† Enrique Font-Sanchis,‡ J ulia Pe´rez-Prieto,*,§ and J . C. Scaiano*,‡
Instituto de Tecnologı´a Qu´ımica UPV-CSIC/ Departamento de Quı´mica,
Universidad Polite´cnica de Valencia, Camino de Vera s/ n, Valencia, 46071 Spain,
Departamento de Quı´mica Orga´nica/ Instituto de Ciencia Molecular, Universidad de Valencia,
Vice´nt Andre´s Estelle´s s/ n, Burjassot, 46100 Valencia, Spain, and Department of Chemistry,
University of Ottawa, 10 Marie Curie, Ottawa, Ontario, K1N6N5, Canada
jperez@uv.es
Received November 15, 2000
The 4,4′-(1,2-ethanediyl)bisbenzyl biradical (2) is clearly and efficiently generated by photolysis of
[3.2]paracyclophane-2-one (8) in cyclohexane solution. This intermediate is also formed via two-
photon processes from [2.2]paracyclophane (3) and 1,2-bis(4-chloromethylphenyl)ethane (4). The
products arising thermally from biradical 2 are [2.2]paracyclophane and [2.2.2.2]paracyclophane
(11) (under high-intensity conditions). Furthermore, two-laser two-color flash photolysis shows that
biradical 2 is photostable in solution at room temperature. Thus, formation of p-xylylene (1) from
2 occurs neither thermally nor photochemically.
In tr od u ction
generated n-chloroalkyl radicals have been photolyzed to
the corresponding biradicals.5 On the other hand, these
reactive species have also been generated from acyl alkyl
biradicals, obtained in the photolysis of the readily
accessible 2,n-diphenyl- and 2-phenylcycloalkanones,
through thermal or photochemical loss of carbon mon-
oxide.6,7
Biradicals have often been postulated as reactive
intermediates in thermal and photochemical reactions,1
such as Cope and di-π-methane rearrangements or cy-
clopropane isomerization. In some cases, photoinduced
generation in matrixes has allowed their detection by
increasing the lifetimes to the point that conventional
methods can be used for characterization.2 As chemical
reactivity is inhibited in matrix isolation, its study
usually requires generation of the reactive intermediates
in fluid solution. Sophisticated time-resolved techniques
have allowed the study of the chemical (photo)reactivity
of some biradicals in solution by spectroscopic methods.3
One of the main limitations in the study of reactive
intermediates is the availability of suitable photochemical
precursors, since unambiguous assignment of the reac-
tion products is only possible when they can be clearly
distinguished from the starting substrates.
In this paper, we have focused our attention on the
detection, characterization, and chemical reactivity of
4,4′-(1,2-ethanediyl)bisbenzyl biradical (2) in fluid solu-
tion. This type of reactive species has been postulated
as intermediate in the thermal8 and photochemical
racemization9 of optically active [2.2]paracyclophanes and
in the polymerization of p-xylylene (1)10 (controlled
polymerization of 1 leads to compounds of low molecular
weight, including 3)11 (Scheme 1).
Interestingly, matrix isolation techniques have allowed
the direct detection of biradical 2 in the photolysis of 3
at 77 K.12,13 A two-photon process via the triplet state
(lifetime of a few seconds in such media) has been found
to produce dissociation of paracyclophane to biradical 2.
The formation of p-xylylene when irradiating 3 at 83 K
has been interpreted as an evidence for the thermal
cleavage of 2 to 1.12
In this respect, dihaloalkanes have been used as
precursors of biradicals.4 In particular, we have reported
the versatility of dichlorodiphenylalkanes to provide
linear alkanediyl biradicals through two-photon pro-
cesses; by using two-laser two-color techniques, the first
† Universidad Polite´cnica de Valencia. Tel.: 34-6-3877807. Fax: 34-
6-3877809.
(5) (a) Pe´rez-Prieto, J .; Miranda, M. A.; Garc´ıa, H.; Ko´nya, K.;
Scaiano, J . C. J . Org. Chem. 1996, 61, 3773-3777. (b) Miranda, M.
A.; Font-Sanchis, E.; Pe´rez-Prieto, J .; Scaiano, J . C. J . Org. Chem. 1999,
64, 7842-7845.
(6) Weiss, D. S. In Organic Photochemistry; Padwa, A., Ed.; Marcel
Dekker Inc.: New York, 1981; Vol. 3, pp 347-420.
(7) Miranda, M. A.; Font-Sanchis, E.; Pe´rez-Prieto, J . J . Org. Chem.
1999, 64, 3802-3803.
(8) (a) Cram, D. J .; Hornby, R. B.; Truesdale, E. A.; Reich, H. J .;
Delton, M. H.; Cram, J . M. Tetrahedron 1974, 30, 1757-1768.
(9) Delton, M. H.; Cram, D. J . J . Am. Chem. Soc. 1970, 92, 7623-
7625.
(10) Cram, D. J .; Cram, J . M. Acc. Chem. Res. 1971, 4, 204-213.
(11) Lown, J . W.; Aidoo, A. S. K. Can. J . Chem. 1966, 44, 2507-
2515.
‡ University of Ottawa. Tel: (613) 562-5896. Fax: (613) 562-5170.
§ Universidad de Valencia. Tel: 34-6-3864934. Fax: 34-6-3864939.
(1) (a) Wilson, R. M. In Organic Photochemistry; Padwa, A., Ed.;
Marcel Dekker Inc.: New York, 1985; Vol. 7, pp 339-465. (b) Scaiano,
J . C.; J ohnston, L. J . In Organic Photochemistry; Padwa, A., Ed.;
Marcel Dekker Inc., New York, 1989; Vol. 10, pp 309-357.
(2) (a) Sheridan, R. S. In Organic Photochemistry; Padwa, A., Ed.;
Marcel Dekker Inc., New York, 1987; Vol. 8, pp 159-249. (b) Haider,
K.; Platz, M. S.; Despres, A.; Lejeune, V.; Migirdicyan, E.; Bally, T.;
Haselbach, E. J . Am. Chem. Soc. 1988, 110, 2318-2320.
(3) (a) Scaiano, J . C. J . Am. Chem. Soc. 1980, 102, 5399. (b) Turro,
N. J . Tetrahedron 1982, 38, 809-817. (c) Turro, N. J . Tetrahedron
1982, 38, 809-817. (d) Closs, G. L.; Forbes, M. D. E. J . Phys. Chem.
1991, 95, 1924-1933. (e) J ohnston, L. J . Chem. Rev. 1993, 93, 251-
266.
(12) Kaupp, G.; Teufel, E.; Hopf, H. Angew. Chem., Int. Ed. Engl.
1979, 18, 215-217.
(13) (a) Ishikawa, S.; Nakamura, J .; Nagakura, S. Bull. Chem. Soc.
J pn. 1980, 53, 2476-2480. (b) Ishikawa, S.; Nakamura, J .; Nagakura,
S. Bull. Chem. Soc. J pn. 1981, 54, 685-686.
(4) (a) Biewer, M. C.; Biehn, C. R.; Platz, M. S.; Despre´s, A.;
Migirdicyan, E. J . Am. Chem. Soc. 1991, 113, 616-620. (b) Ouchi, A.;
Koga, Y.; Adam, W. J . Am. Chem. Soc., 1997, 119, 592-599.
10.1021/jo001623y CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/20/2001

2718 J . Org. Chem., Vol. 66, No. 8, 2001
Miranda et al.
Sch em e 1
In solution, the lifetime of the T1 triplet state is very
short. Hegelson and Cram have studied the wavelength-
dependent photochemistry of 3 in alcoholic solutions.
From the analysis of photoproducts they have proposed
that the biradical is produced by exciting 3 with light of
wavelength shorter than 230 nm, which can be able to
populate higher triplet states.14
On the other hand, 3 has been photolyzed with an ArF
laser (193 nm) in the gas phase at 458 K. There has been
no indication of Tn r T1 absorption, but p-xlylene is
generated via a two-photon process.15
F igu r e 1. Top: transient absorption spectra recorded follow-
ing laser excitation of 4 in cyclohexane under nitrogen 2.40
µs (4) and 12.2 µs (0) after the laser (266 nm) pulse. The insert
shows the decay as monitored at 320 nm. Bottom: transient
absorption spectra recorded following laser excitation of 6 in
cyclohexane under nitrogen 2.40 µs (O) and 8.8 µs (0) after
the laser (266 nm) pulse.
Thus, biradical 2 has not been detected in liquid
solution, and it is not clear whether it couples to [2.2]-
paracyclophane or/and dissociates to p-xylylene in this
medium. Trying to obtain direct evidences on these
issues, we have generated this species from different
precursors such as 1,2-bis(4-chloromethylphenyl)ethane
(4) and [3.2]paracyclophane-2-one (8). Photolysis of the
latter precursor has clearly shown formation of biradical
2, which does not cleave to p-xylylene but leads mainly
to [2.2]paracyclophane under the reaction conditions;
traces of the dimerization product, [2.2.2.2]paracyclo-
phane (11), were also observed.
Sch em e 2
and a 308 excimer laser (HCl/Xe/Ne gas mixtures, exci-
mer laser, ∼6 ns, e90 mJ /pulse) was used to photolyze
the generated 5. Though some bleaching of the signal at
320 nm was observed, spectra (not shown) were not clear
enough as to make a safe assignment. GC/MS analysis
of the photolyzate obtained after laser drop photolysis
evidenced the formation of minor amounts of paracyclo-
phane (3) (Scheme 2).18
To rule out the possibility that photocleavage of the
ethane brigde in benzylic radical 5 could occur when
using high-intensity irradiation conditions, we performed
the laser photolysis of the model compound 1-(4-chlo-
romethylphenyl)-2-(4-methylphenyl)ethane (6).19 LFP of
deaerated 10 mM solutions of 6 in cyclohexane at 266
Resu lts
P h otolysis of 1,2-Bis(4-ch lor om eth ylp h en yl)eth -
a n e 4. Laser flash photolysis (LFP) of deaerated 0.9 mM
solutions of 1,2-bis(4-chloromethylphenyl)ethane (4)16 in
cyclohexane at 266 nm (Nd:YAG laser, fourth harmonic,
<10 ns, e16 mJ /pulse) yielded a narrow transient
absorption, with a sharp maximum at 320 nm (see Figure
1). The lifetime of this species was around 9 µs under
our experimental conditions. This signal was quenched
by oxygen at close to the diffusion-controlled limit. Hence,
it was assigned to be the benzylic radical 5 obtained
through homolytic cleavage of one C-Cl bonds17 (Scheme
2).
nm gave a spectrum typical of a benzylic radical (λmax
)
320 nm). Two-laser two-color experiments (266/308 nm)
were performed to study the possibility of photocleavage
of the ethane bridge in benzylic radical 7 generated upon
irradiation of 6 with the synthesis laser at 266 nm; no
bleaching of the signal at 320 nm was observed when
irradiating with the second laser at 308 nm. In agreement
Two-laser two-color experiments were performed in
order to study the photoreactivity of the benzylic radical
5; thus, a 266 nm laser was used as the synthesis laser,
(14) Hegelson, R. C.; Cram, D. J . J . Am. Chem. Soc. 1966, 88, 509-
515.
(18) By contrast, lamp irradiation (medium-pressure mercury,
quartz filter, 1 h) of deaerated 10 mM cyclohexane solutions of 4 led
to complete consumption of the starting material. Analysis of the
photolysate by GC-MS revealed the formation of products arising from
the homolytic cleavage of a carbon-chlorine bond, followed by hydrogen
abstraction from solvent by chlorine atoms, radical-radical recombina-
tion, and some secondary photolysis of monochlorinated products. No
traces of [2.2]paracyclophane were detected in the photolysate.
(19) Amatore, C.; Gaubert, F.; J utand, A.; Utley, J . H. P. J . Chem.
Soc., Perkin Trans. 2 1996, 2447-2452.
(15) Shimizu, S.; Nakashima, N.; Sakata, Y. Chem. Phys. Lett. 1998,
284, 396-400.
(16) (a)Tabushi, I.; Yamada, H.; Kuroda, Y. J . Org. Chem. 1975, 40,
1946-1949. (b) Cram, D. J .; Steinberg, H. J . Am. Chem. Soc. 1951,
73, 5691-5704.
(17) Benzylic radicals normally show absorption in this spectral
region: Chatgilialoglu, C. In Handbook of Organic Photochemistry;
Scaiano, J . C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. II; p 3.

4,4′-(1,2-Ethanediyl)bisbenzyl Biradical
J . Org. Chem., Vol. 66, No. 8, 2001 2719
Sch em e 3
Sch em e 4
F igu r e 2. Transient absorption spectra recorded following
laser excitation of 8 in cyclohexane under nitrogen 0.8 µs (O)
and 16.5 µs (4) after the laser (266 nm) pulse. The insert shows
the decay as monitored at 320 nm.
with maximum at 320 nm (see Figure 2). Its formation
was not instantaneous, and its absorption decayed with
the same kinetics over the entire spectrum (see inserts
in Figure 2), thus supporting that only one species is
responsible for this transient absorption. Its lifetime was
around 8.6 µs under our experimental conditions. The
rounded top of the kinetic trace (see insert in Figure 2)
probably reflects the decarbonylation in Scheme 4, which
is expected to occur in the 100-300 ns time scale.21 The
species was quenched by oxygen at close to the diffusion-
controlled limit. Hence, it was assigned to biradical 2
obtained through thermal loss of carbon monoxide from
the initially generated acyl-alkyl biradical 9 (Scheme 4).
Two-laser two-color flash photolysis of ketone 8 was
also performed. The synthesis laser (266 nm) produced
biradical 2, which is the absorbing chromophore at the
second laser wavelength (308 nm). Neither bleaching of
the biradical by the 308 nm laser nor new signals in the
UV spectra or emission at longer wavelengths were
observed. This is not surprising since for the diphenyl-
methyl radical, where the excited-state lifetime is 260
ns,22 the lifetime in a biradical is 100 times shorter.23 For
the benzyl radical, the room temperature excited state
is only 1 ns.24 If we anticipate a comparable shortening
in the case of biradicals, this emission would not be
detectable. Product studies on the photolysis under high-
intensity irradiation provided further evidences against
photocleavage of 2 to p-xylylene (1). Thus, deaerated 5
mM solutions of ketone 8 in cyclohexane, with the sample
contained in a quartz spectrometer cell, were irradiated
with 4000 laser pulses at 266 nm (Scheme 4). Product
analysis revealed the formation of [2.2]paracyclophane
and minor amounts of [2.2.2.2]paracyclophane (11);25
however, not even traces of [2.2.2]paracyclophane (12)
(which would also be formed if xylylene 1 were an
intermediate)25 were detected.
with these observations, GC/MS analysis of the photo-
lyzate did not show even trace amounts of 3 (Scheme 3).
Thus, 7 is photostable; the same could be expected for
its analogue 5, although it cannot be completely ruled
out that in the case of 5 the presence of the second
chlorine atom could enhance cleavage of the ethane
bridge.
P h otolysis of [3.2]P a r a cyclop h a n -2-on e (8). This
compound can be obtained in low yield upon photolysis
of [3.3]paracyclophane-2,11-dione.20 Although ketone 8
has not previously been used as biradical precursor, it
seemed reasonable to expect photocleavage to the acyl-
alkyl biradical 9, which due to R-phenyl substitution
could (photo)decarbonylate6,7 to provide biradical 2
(Scheme 4). Lamp irradiation (Rayonet photoreactor, 30
min) at 254 nm of deaerated 2 mM benzene solutions of
8 led to [2.2]paracyclophane (3) as the only product; 50%
of the starting ketone was recovered unchanged.
Laser flash photolysis of deaerated 1 mM solutions of
8 in cyclohexane at 266 nm yielded a narrow absorption
These results allow to rule out that biradical 2 ther-
mally or photochemically cleaves to p-xylylene in liquid
(21) Lunazzi, L.; Ingold, K. U.; Scaiano, J . C. J . Phys. Chem. 1983,
87, 529-530. (b) Turro, N. J .; Gould, I. R.; Baretz, B. H. J . Phys. Chem.
1983, 87, 531-532.
(22) Scaiano, J . C.; Tanner, M.; Weir, D. J . Am. Chem. Soc. 1985,
107, 4396-4403.
(23) J ohnston, L. J .; Scaiano, J . C. J . Am. Chem. Soc. 1986, 108,
2349-2353.
(24) Meisel, D.; Das, P. K.; Hug, G. L:; Bhattacharyya, K.; Fes-
senden, R. W. J . Am. Chem. Soc. 1986, 108, 4706-4710.
(25) Otsubo, T.; Mizogami, S.; Otsubo, I.; Tozuka, Z.; Sakagami, A.;
Sakata, Y.; Misumi, S. Bull. Chem. Soc. J pn. 1973, 46, 3519-3530.
(20) Isaji, H.; Sako, K.; Takemura, H.; Tatemitsu, H.; Shinmyozu,
T. Tetrahedron Lett. 1998, 39, 4303-4304.

2720 J . Org. Chem., Vol. 66, No. 8, 2001
Miranda et al.
F igu r e 3. Top: transient absorption spectra recorded follow-
ing laser excitation of 3 in cyclohexane under nitrogen 0.8 µs
(O), 2.96 µs (0) 7.84 µs (9), and 14.5 µs (4) after the laser (266
nm) pulse. Bottom: traces monitored at 380 (left) and 290 nm
(right).
F igu r e 5. Top: (A) transient absorption spectra recorded
following laser excitation of 3 in cyclohexane in the presence
of cyclooctadiene 1.20 µs (4), 3.76 µs (O), and 8.08 µs (0) after
the laser (266 nm) pulse. The insert shows the decay as
monitored at 320 nm. Bottom: Effect of the laser power on
signal intensities monitored at 290 (A) and 320 nm (B). 100%
corresponds to a laser power of 10 mJ .
characteristic features of p-xylylene.26 The oxygen-
quenchable band at 380 nm (with a component at 290
nm) was assigned to the triplet state of 3 (T4 r T1)
absorption (CT band characteristic of the transannular
interaction between the aromatic rings).27
F igu r e 4. Transient absorption spectra recorded following
laser excitation of 3 in cyclohexane under oxygen 2.96 µs (O)
and 7.84 µs (]) after the laser (266 nm) pulse. The insert shows
the decay as monitored at 290.
Deaerated 3 mM solutions of 3 in cyclohexane contain-
ing cyclooctadiene as a triplet quencher were also irradi-
ated at 308 nm. In this case, absorption spectra with two
maxima, one at 290 nm and another one centered at 320
nm, were observed (Figure 5). It seems reasonable to
assign the first absorption band with a longer lifetime
to p-xylylene, while that at 320 nm with a shorter lifetime
(9.6 µs) (nondetected under oxygen) should correspond
to biradical 2. This band appeared as a shoulder in Figure
3.
solution, but supports its coupling to [2.2]paracyclophane.
Irradiation under high-intensity conditions generates
high concentrations of biradical 2, which is then able to
dimerize giving product 11, which is not found under
conditions of lamp irradiation.
H igh -In t en sit y Ir r a d ia t ion of [2.2]P a r a cyclo-
p h a n e (3) in Solu tion . The above results motivated a
reinvestigation of the photolysis of [2.2]paracyclophane
in solution at room temperature.
Moreover, in agreement with the above results on the
photolysis of 8, biradical 2 clearly observed upon laser
flash photolysis of 3 in the presence of the triplet
quencher does not cleave to p-xylylene at room temper-
ature. An investigation of the effects of light intensity
on the size of the signals at 290 and 320 nm was carried
out by attenuating the laser beam with a set of calibrated
neutral density filters. The results (Figure 5) agreed with
previous evidences supporting that both biradical 2 (in
matrix media)13 and p-xylylene (in gas phase)15 originate
through two-photon processes.
Laser flash photolysis of deaerated 1 mM solutions of
3 in cyclohexane at 266 nm yielded a broad spectrum
with maxima at 290 and 380 nm; a similar spectrum was
obtained by irradiation at 308 nm (Figure 3). These
signals decayed with different kinetics, showing the
different nature of the species absorbing at the two
wavelengths (see insets in Figure 3). Thus, while the
band at 380 nm decayed with first-order kinetics, the 290
nm band had two components, a fast one with the same
kinetics as the signal at 380 nm and a slower one. Oxygen
saturated solutions of 3 in cyclohexane led to a narrow
absorption spectrum with maximum at 290 nm (Figure
4). The long lifetime of the species, its absorption in this
spectral region, and its low reactivity toward oxygen are
(26) Kaupp, G. Angew. Chem., Int. Ed. Engl. 1976, 15, 442-443.
(27) Ishikawa, S.; Nakamura, J .; Iwata, S.; Sumitani, M.; Nagakura,
S.; Sakata, Y.; Misumi, S. Bull. Chem. Soc. J pn. 1979, 52, 1346-1350.

4,4′-(1,2-Ethanediyl)bisbenzyl Biradical
J . Org. Chem., Vol. 66, No. 8, 2001 2721
Finally, deaerated 3 mM solutions of paracyclophane
in cyclohexane were irradiated with 4000 laser pulses
either at 248 or 266 nm and with the sample contained
in a quartz spectrometer cell. Product analysis revealed
the formation of [2.2.2]paracyclophane (12)25 and [2.2.2.2]-
paracyclophane (11) (Scheme 4). While the formation of
(11) agrees with the intermediacy of 2, the detection of
12 provided further evidences for the intermediacy of 1,
which can couple either to its trimeric or tetrameric
products.25
quartz tube was irradiated for 1 h with a 125 W medium-
pressure mercury lamp inside a quartz inmersion well,
under continuous magnetic stirring. In the case of
compound 8, a degassed 2 mM benzene solution of the
ketone was irradiated for 30 min with a Rayonet photo-
reactor, seven RPR-253.7 nm lamps. In all cases, after
evaporation of the solvent, the photomixture was ana-
1
lyzed by GC-MS and H NMR.
La ser -Dr op P h otolysis. The beam from a Nd:YAG
laser using the fourth harmonic (266 nm, <10 ns, e16
mJ /pulse) was focused by means of a quartz lens into a
drop of the photolysis solution suspended from a 2 in.
syringe needle (20 gauge). Further details for this experi-
ment have been described previously.28
Con clu sion
4,4′-(1,2-Ethanediyl)bisbenzyl biradical (2) is detectable
at room temperature and does not cleave to p-xylylene
but cyclizes to [2.2]paracyclophane (3) or dimerizes to
[2.2.2.2]paracyclophane (11). Furthermore, biradical 2 is
photostable in liquid solution at room temperature.
La ser F la sh P h otolysis. These experiments were
carried out using either a Nd:YAG laser using the fourth
harmonic (266 nm, <10 ns, e20 mJ /pulse), an excimer
laser operated with HCl/Xe/Ne gas mixtures (308 nm, ca.
6 ns, e90 mJ /pulse), or an excimer laser operated with
F2/Kr/He gas mixtures (248 nm, ca. 6 ns, <100 mJ /pulse).
Transient signals were captured with a Tetronix-2440
digital oscilloscope which was interfaced to a computer
that also controlled the experiment. The system was
operated with software written in the LabVIEW 3.1.1
environment from National Instruments.29 Other aspects
of this instrument are similar to those described earlier.22
The two-laser two-color experiments were performed by
sending a trigger pulse to a delay generator which then
sent TTL pulses which fired the lasers at the desired
sequence. All experiments were carried out using cells
constructed from 7 × 7 mm Suprasil quartz tubing.
Samples were purged with a slow stream of either
nitrogen or oxygen, as required.
Exp er im en ta l Section
Rea gen ts a n d P r od u cts. [2.2]Paracyclophane was
commercially available. The required substrates 619 and
820 were prepared following procedures described in the
literature. Products 11 and 12 were known compounds;25
their identification was based on NMR and MS spectra,
by comparison with literature data.
Syn th esis of 1,2-Bis(4-ch lor om eth ylph en yl)eth an e
(4).16 1,2-Bis(4-hydroxymethylphenyl)ethane16b was added
to concentrated hydrochloric acid (15 mL), and the
solution was allowed to stand at room temperature for 8
h. After this time, the reaction mixture was diluted with
cold water (50 mL) and the organic material extracted
with ether, dried over anhydrous sodium sulfate, and
concentrated under vacuum. The residue was submitted
to semipreparative HPLC using hexane as eluent to
obtain the dichloro compound 4 (90%): 1H NMR (CDCl3,
250 MHz) δ 2.2 (m, 2 H), 2.6 (m, 2 H), 3.5 (m, 2 H), 7.1
(d, J ) 6.5 Hz, 4H), 7.3 (d, J ) 6.5 Hz, 4H); 13C NMR
(CDCl3, 62.5 MHz) δ 215.3 (s), 138.4 (s), 128.6 (d), 128.1
(d), 127.0 (d), 55.8 (d), 29.5 (t); MS m/z 236 (M+, 12), 208
(7), 117 (7), 104 (100). Anal. Calcd for C16H16Cl2: C, 68.83;
H, 5.78. Found: C, 69.16; H, 5.79.
Ack n ow led gm en t. Financial support by the Span-
ish DGES (M.A.M., Project no. PB97-0339) and by the
Natural Sciences and Engineering Research Council of
Canada through an operating grant (J .C.S.) is gratefully
acknowledged.
J O001623Y
(28) Banks, J . T.; Scaiano, J . C. J . Am. Chem. Soc. 1993, 115, 6409-
6413.
(29) Scaiano, J . C. J . Am. Chem. Soc. 1980, 102, 7747-7753.
Con ven tion a l La m p Ir r a d ia tion of Com p ou n d s 4
a n d 8. A degassed 10 mM cyclohexane solution of 4 in a