Laser Flash, Laser-Drop, and Lamp Photolysis of 1,3-Dichloro-1,3-diphenylpropane. One- versus Two-Photon Reaction Pathways

Article abstract of DOI:10.1021/jo9704772

Low intensity irradiation of 1,3-dichloro-1,3-diphenylpropane (1) in cyclohexane leads to the formation of 3-chloro-1,3-diphenylpropyl radical (2) through homolytic C-Cl bond cleavage. Radical 2 gives rise to final products typical of free radical reactions. Neither 1,2-diphenylcyclopropanes (6) nor 3-cyclohexyl-1,3-diphenylpropene (20) are obtained under these conditions. Nevertheless, high intensity laser irradiation of the initially formed monoradical 2 leads to the 1,3-diphenylpropenyl radical (3) detected with nanosecond techniques. This intermediate is completely trapped by the cyclohexyl radicals generated in the medium to yield 20. Photolysis of 1 with the laser-drop technique leads to the cyclopropanes 6 as major photoproducts.

Full text of DOI:10.1021/jo9704772

J . Org. Chem. 1997, 62, 5713-5719
5713
La ser F la sh , La ser -Dr op , a n d La m p P h otolysis of
1,3-Dich lor o-1,3-d ip h en ylp r op a n e. On e- ver su s Tw o-P h oton
Rea ction P a th w a ys
Miguel Angel Miranda,*,† J ulia Pe´rez-Prieto,*,‡ Enrique Font-Sanchis,† Kla´ra Ko´nya§ and
J . C. Scaiano*,§
Instituto de Tecnologı´a Quı´mica/ Departamento de Quı´mica, Universidad Polite´cnica de Valencia,
Camino de Vera s/ n, Valencia, 46071 Spain, Departamento de Qu´ımica Orga´nica, Facultad de Farmacia,
Universidad de Valencia, Vice´nt Andre´s Estelles s/ n, Burjasot, 46100 Valencia, Spain, and Department
of Chemistry, University of Ottawa, Ottawa, Canada K1N 6N5
Received March 14, 1997X
Low intensity irradiation of 1,3-dichloro-1,3-diphenylpropane (1) in cyclohexane leads to the
formation of 3-chloro-1,3-diphenylpropyl radical (2) through homolytic C-Cl bond cleavage. Radical
2 gives rise to final products typical of free radical reactions. Neither 1,2-diphenylcyclopropanes
(6) nor 3-cyclohexyl-1,3-diphenylpropene (20) are obtained under these conditions. Nevertheless,
high intensity laser irradiation of the initially formed monoradical 2 leads to the 1,3-diphenylpro-
penyl radical (3) detected with nanosecond techniques. This intermediate is completely trapped
by the cyclohexyl radicals generated in the medium to yield 20. Photolysis of 1 with the laser-drop
technique leads to the cyclopropanes 6 as major photoproducts.
Sch em e 1
In tr od u ction
The photochemistry of dihalides in which the C-X
bonds are separated by two or more carbon atoms is a
subject of recent interest.1-4 We have demonstrated the
intermediacy of a hypervalent iodine radical in the
photolysis of 1,5-diodo-1,5-diphenylpentane in cyclohex-
ane,1 while in the case of the dichloro analogue the typical
benzyl radical is formed.4 By contrast, we and others
have demonstrated that excitation of dihalides with high-
intensity light can lead to difunctional intermediates,
biradicals, from which a completely different set of
products can be formed. For instance, high intensity
irradiation of 1,5-dichloro- and 1,5-diiodo-1,5-diphenyl-
pentane leads to cis and trans-1,2-diphenylcyclopentanes
by a two-photon route. The 1,5-biradical intermediate
has been detected with nanosecond techniques.
Futhermore, we have already communicated5 that
different products are obtained when 1,3-dichloro-1,3-
diphenylpropane (1) is submitted to low or high intensity
irradiation. Thus, cis- and trans-1,2-diphenylcyclopro-
panes (6) are only formed when using the latter condi-
tions. However, in this case the difunctional biradical
precursor 5 has not been observed probably because of
its very short lifetime;6,7 instead a longer lived mono-
functional 1,3-diphenylpropenyl radical intermediate (3)
arising from a two-photon procces has been detected.
In this paper we report in full our results on the
photochemistry of 1,3-dichloro-1,3-diphenylpropane in
cyclohexane. A systematic study on this dihalide was
undertaken in order to compare the results of conven-
tional lamp photolysis with those obtained upon laser
excitation [Nd: YAG (266 nm), or KrF (248 nm)] and with
the laser-drop method. Furthermore, since 3 can be
generated from irradiation of 3-chloro-1,3-diphenylpro-
pene (4), product studies on the lamp irradiation of this
compound were performed to clarify which products arise
from the propenyl radical 3 in the high-intensity irradia-
tion of the 1,3-dichloride.
Our results clearly show that the propenyl radical 3 is
not involved in the formation of the 1,2-diphenylcyclo-
propanes. Instead, it is trapped by the cyclohexyl
radicals formed after reaction of chlorine atoms with the
solvent. Moreover a mechanism involving intramolecular
electron transfer from the excited benzylic radical to the
benzylic chloride is proposed for the formation of radical
3 in the high intensity irradiation of the 1,3-dichloride.
† Instituto de Tecnolog´ıa Qu´ımica/Departamento de Qu´ımica, Uni-
versidad Polite´cnica de Valencia.
‡ Departamento de Qu´ımica Orga´nica, Facultad de Farmacia, Uni-
versidad de Valencia.
§ University of Ottawa.
X Abstract published in Advance ACS Abstracts, J uly 15, 1997.
(1) Banks, J . T.; Garc´ıa, H.; Miranda, M. A.; Pe´rez-Prieto, J .;
Scaiano, J . C. J . Am. Chem. Soc. 1995, 117, 5049.
(2) Ouchi, A.; Koga, Y.; Adam, W. J . Am. Chem. Soc. 1997, 119, 592.
(3) Ouchi, A.; Yabe, A.; Adam, W. Tetrahedron Lett. 1994, 35, 6309.
(4) Pe´rez-Prieto, J .; Miranda, M. A.; Garc´ıa, H.; Ko´nya, K.; Scaiano,
J . C. J . Org. Chem. 1996, 61, 3773.
Resu lts a n d Discu ssion
(5) Pe´rez-Prieto, J .; Miranda, M. A.; Font-Sanchis, E.; Ko´nya, K.;
Scaiano, J . C. Tetrahedron Lett. 1996, 37, 4923.
(6) Mizuno, K.; Ichinose, M.; Otsuji, Y.; Caldwell, R. A. J . Am. Chem.
Soc. 1985, 107, 5797.
(7) Zimmt, M. B.; Doubleday, C., J r.; Turro, N. J . Chem. Phys. Lett.
1987, 134, 549.
Low In ten sity Ir r a d ia tion s. 1,3-Dichloro-1,3-diphe-
nylpropane (1) was prepared from 1,3-diphenyl-1,3-
propanedione by sodium borohydride reduction and
subsequent exchange of the hydroxyl group by chlorine,
S0022-3263(97)00477-5 CCC: $14.00 © 1997 American Chemical Society

5714 J . Org. Chem., Vol. 62, No. 17, 1997
Miranda et al.
Sch em e 2
Sch em e 3
The formation of other products with m/z 278 in the
irradiation of 1,3-dichloro-1,3-diphenylpropane (1) is in
agreement with radical alkylation of the aryl ring by the
cyclohexyl radicals.12 This type of compound was also
found in the photolysis of 1,5-dichloro-1,5-diphenylpen-
tane. Unambiguous synthesis of 1-(p-cyclohexylphenyl)-
3-phenylpropane (15) showed that this compound was
identical to one of these minor products.
The structural assignment of photoproducts was con-
firmed by unambigous synthesis of the products, using
well-established methods, or by comparison with avail-
able authentic samples. The chloro compounds 7 and 8
were prepared from the corresponding ketones, 14a and
14b, respectively, by sodium borohydride reduction and
subsequent exchange of the hydroxyl group by chlorine,
using HCl. Compounds 12 and 15 were obtained by
hydrogenation of the corresponding carbonyl compound
14b and 14c in presence of palladium on charcoal.
Addition of cyclohexylmagnesium chloride to the carbonyl
group of 14b followed by hydrogenation afforded com-
pound 13. The above synthetic routes are outlined in
Scheme 3. Details and appropriate references are pro-
vided in the Experimental Section.
La ser F la sh P h otolysis. LFP of deaerated 1 mM
solutions of 1 in cyclohexane at 266 nm yielded a narrow
spectrum, with maxima at 320 (weak), 340 (weak), and
360 nm (strong), see Figure 1. The lifetime of this species
was around 10 µs under our experimental conditions.
Oxygen saturated solutions of 1 in cyclohexane led to a
reduced lifetime of 0.6 µs at 360 nm.
This intermediate does not appear to be a benzylic
radical, which would be expected to absorb at ca. 320
nm;13 thus, the possibility of an interaction between the
benzylic radical center and the distant â-phenyl group
was first considered, in order to interpret the spectrum.
To check this hypothesis, LFP of deaerated 1 mM
solutions of 7 in cyclohexane at 266 nm were obtained.
However, a spectrum typical of a benzylic radical (λ )
320 nm) was obtained in this case, see Figure 2.
using HCl. Lamp irradiation (medium pressure mercury,
quartz filter, 1 h) of a deaerated 10 mM cyclohexane
solution of the dichloride led to complete consumption of
the starting material. Analysis of the photolysate by GC/
MS revealed the formation of 1-chloro-1,3-diphenylpro-
pane (7, 5%), 1-chloro-3-cyclohexyl-1,3-diphenylpropane
(8, 26%), 1,3-diphenylpropane (11, 5%),8 1-cyclohexyl-1,3-
diphenylpropane (12, 29%), 1,3-dicyclohexyl-1,3-diphe-
nylpropane (13, 17%), and two minor products with m/z
278 for the molecular ion (7%). The product of coupling
of cyclohexyl radicals, cyclohexylcyclohexane,9 was also
detected.
The product distribution can be explained through
homolytic cleavage of C-Cl bond(s) from the excited
singlet state(s) to give benzylic radicals (2, 9, and 10) and
a chlorine atom. No disproportionation products were
detected, due to efficient hydrogen abstraction from the
solvent by chlorine,10,11 which yields cyclohexyl radicals.
Similar behavior has been earlier observed in the pho-
tochemistry of 1,5-dichloro-1,5-diphenylpentane.4 Pho-
toproduct formation can be explained from the interme-
diate benzylic radicals by either hydrogen abstraction
from the medium (7, 11), or coupling with cyclohexyl
radicals (8, 13), or a combination of both processes (12).
Chloro compounds 1-chloro-1,3-diphenylpropane (7)
and 1-chloro-3-cyclohexyl-1,3-diphenylpropane (8) were
irradiated under similar conditions. Thus, 1-chloro-1,3-
diphenylpropane was partially converted into 11 (13%)
and 12 (32%) along with minor amounts of products with
m/z 278 for the molecular ion (7%). Similarly, GC/MS
analysis of the photolysate from 1-chloro-3-cyclohexyl-
1,3-diphenylpropane showed the formation of 12 (39%)
and 13 (32%).
In view of the absorption at relatively long wavelengths
and the long lifetime, the intermediate formed upon LFP
of 1 was thought to be the stabilized 1,3-diphenylpropenyl
(8) Gilman, H.; Tomasi, R. A.; Wittenberg, D. J . Org. Chem. 1959,
24, 821.
(9) Beilstein Handbuch der Organischen Chemie H, 5, 71.
(10) Reported bond energies (kcal/mol): H-Cl, 103.2; HC6H11, 95.5;
Weast, R. C. Handbook of Chemistry and Physics; CRC Press: Boca
Raton, FL; 1976; pp F-221, F-231.
(12) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemistry; Harper and Row: New York, 1981; p 730.
(13) Chatgilialoglu, C. Handbook of Organic Photochemistry; Sca-
iano, J . C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. II, p 3.
(11) Bunce, N. J .; Ingold, K. U.; Landers, J . P.; Lusztyk, J .; Scaiano,
J . C. J . Am. Chem. Soc. 1985, 107, 5464.

Photolysis of 1,3-Dichloro-1,3-diphenylpropane
J . Org. Chem., Vol. 62, No. 17, 1997 5715
F igu r e 3. Transient absorption spectra recorded following
laser excitation (266 nm) of 1 under nitrogen 0.64 µs (4), 5.76
µs (0), and 34.1 µs (O) after the laser pulse.
F igu r e 1. Transient absorption spectra recorded following
laser excitation (266 nm) of 1 under nitrogen 0.72 µs (4), 8.48
µs (0), and 15 µs (O) after the laser pulse.
F igu r e 4. Effect of laser power on signal intensity following
266 nm laser flash photolysis of 1 (monitored at 360 nm). 100%
laser power corresponds to ca. 20 mJ /pulse.
F igu r e 2. Transient absorption spectra recorded following
laser excitation (266 nm) of 7 under nitrogen 4.16 µs (4), 13.4
µs (0), and 31.4 µs (O) after the laser pulse.
band at 320 nm, together with a less prominent 360 nm
band (see Figure 3). In this case the decay of both bands
did not follow the same kinetics. Time-resolved absorp-
tion spectra at 320 nm showed a short-lived part (τ )
2-3 µs) and a long-lived one (τ ) 10 µs). Moreover the
fast component was readily quenched by oxygen at close
to the diffusion-controlled limit. These data allowed us
to assign confidently this transient to the benzylic radical
2.13 An investigation of the effects of light intensity on
the size of the signal at 360 nm was carried out by
attenuating the laser beam with a set of calibrated
neutral density filters. Figure 4 has been fitted with a
parabola, consistent with all the evidence pointing to a
two-photon process.
Two-laser two-color laser flash photolysis of 1,5-dichloro-
1,5-diphenylpentane4 has already been performed. The
synthesis laser (266 nm) produces the benzylic mono-
radical which is the only absorbing chromophore at the
photolysis laser wavelength (308 nm). Bleaching of the
monoradical by the second laser was accompanied by
radical 3. To confirm this assignment, the laser flash
photolysis of 0.5 mM solutions of 3-chloro-1,3-diphenyl-
propene (4) was carried out. This gave rise to transient
3, whose absorption matched that described above. The
same spectrum was obtained when a 50/50 di-tert-butyl
peroxide/benzene mixture was irradiated (355 nm, Nd:
YAG laser) in presence of 1,3-diphenylpropene. Here
allylic hydrogen abstraction14 by tert-butoxy radicals
should provide a clean source of 3.
The formation of propenyl radical 3 was not consistent
with the products obtained by lamp irradiation, thus
suggesting that a two-photon process was taking place
under laser excitation. Accordingly, a different transient
spectrum was obtained by placing a beam diffuser made
with a frosted quartz plate in front of the sample, in order
to minimize multiphoton processes in the laser flash
experiments. The main spectral features were a stronger
(14) Paul, H.; Small, R. D., J r.; Scaiano, J . C. J . Am. Chem. Soc.
1978, 100, 4520.

5716 J . Org. Chem., Vol. 62, No. 17, 1997
Miranda et al.
Sch em e 4
minimizing the amounts of products arising from irradia-
tion of (thermally) nonreactive photoproducts.25 When
drops of deaerated solutions of compound 1 in cyclohex-
ane were irradiated by the focused output from a 266 nm
laser (1 cycle) only a 10% conversion was observed. The
major products were cis- and trans-diphenylcyclopro-
panes (6) (cis/trans ratio: 1/2) and significant amounts
of 1,3-diphenylpropene, 1-phenylindane, and 3-cyclo-
hexyl-trans-1,3-diphenylpropene (20) were also obtained.
As in the lamp photolysis experiment, minor amounts of
1-chloro-1,3-diphenylpropane and products containing
the cyclohexyl moiety were also detected.
photodecomposition of the remaning chlorobenzyl moiety
and formation of 1,2-diphenylcyclopentanes. In the case
of the 1,3-dichloro analogue, we assume that photode-
composition of radical 2 might also involve intramolecular
quenching of its excited state by the benzylic chloride.15-19
This process could give rise to the radical ion pair 19,20
which would undergo deprotonation to the propenyl
radical 3. Alternatively, geminate electron transfer from
the chloride anion to the 1,3-radical cation within 19
would ultimately result in the formation of 1,2-diphe-
nylcyclopropane (Scheme 4). Related precedents on
quenching of cyclopropyl radical cations by chloride can
be found in the recent literature.21
In principle, the possibility of an alternative intramo-
lecular energy transfer in 2* was also envisaged; how-
ever, it was ruled out in view of the endothermicity of
the process. Thus, the excitation energy of the radical
(around 62 kcal mol-1) was insufficient to populate the
S1 (107 kcal mol-1 ) or T1 (73 kcal mol-1 ) states of the
excited benzylic chloride.22,23
Though the laser-drop technique might be more ef-
ficient in the conversion of the dihalide than the laser
irradiation of a flowing sample, the former technique is
not suitable to perform the irradiations under oxygen
atmosphere.24 Thus, in order to compare results under
both conditions, deaerated and oxygenated samples run-
ning through the cell were irradiated using a 266 nm
laser excitation. The dihalide conversion was even lower,
but GC/MS analysis of the photolyzed deaerated solutions
revealed the formation of the same products as the case
of the laser-drop experiment. By contrast, irradiations
of oxygenated samples gave rise to lower amounts of
cyclohexyl derivatives, without affecting the formation
of 1,2-diphenylcyclopropanes.
These results ruled out any role of the propenyl radical
in the formation of the 1,2-diphenylcyclopropanes, since
the formation of these compounds is not affected by
oxygen, while the propenyl radical is scavenged under
these conditions. However, the intermediacy of biradical
5 in their formation is easily understood in view of the
short lifetime of this intermediate (τ ) 15 ns)6,7 that could
preclude its quenching by oxygen. Futhermore, the
formation of 1-phenylindane is also rationalized through
the involvement of 5,26 while 1,3-diphenylpropene could
arise either from 5 (H shift) or from 3 (hydrogen abstrac-
tion). By contrast, 3-cyclohexyl-trans-1,3-diphenylpro-
pene is clearly a product arising from coupling of cyclo-
hexyl and 1,3-diphenylpropenyl radicals. As the later
intermediate is formed via a two-photon process, the
formation of 20 in the laser-drop experiment and its
quenching by oxygen can be explained.
High In ten sity P r od u ct Stu d ies. 1,3-Dichloro-1,3-
diphenylpropane was irradiated under different high-
intensity conditions. The laser-drop technique1,24 pro-
vides a way of performing high intensity photolysis while
(15) Application of the Rehm-Weller equation to the (PhCHCH3)*/
PhCH2Cl pair, with excited benzylic radical acting as donor, showed
that an electron-transfer reaction is thermodynamically possible for
this system.
∆G° ) -23.06(E°A/A•- - E° /D) - eo2/Ea - ∆Eo,o
+•
D
The value of ∆Eo,o for benzyl radicals has been estimated16 to be around
62 kcal mol-1
. The oxidation potential of 1-phenylethyl radical has
been found to be 0.37 V (vs SCE).17 The reduction potential of benzyl
chloride is -2.2 V (vs SCE).18 These data lead to a ∆G° value of ca.
-3 kcal mol-1 without taking into account the favorable Coulombic
term. However, it must be taken into account that the redox measure-
ments were performed in acetonitrile or dimethylformamide, which
are more polar than cyclohexane. Hence, in this solvent the electron
transfer step could be less favored. Anyway, the above calculations
are to be taken only as a rough indication on the feasibility of the
process. Furthermore, in addition to the negative free energy change,
other factors such as the proximity of the two centers or proper orbital
overlap could enhance the intramolecular electron transfer process.
As a matter of fact, analogous intermolecular quenching of excited
diphenylmethyl radicals by carbon tetrachloride involving charge
transfer have been reported.19
(16) Meisel, D.; Das, P. K.; Hug, G. L.; Bhattacharyya, K.; Fes-
senden, R. W. J . Am. Chem. Soc. 1986, 108, 4706.
(17) Wayner, D. D. M.; McPhee, D. J .; Griller, D. J . Am. Chem. Soc.
1988, 110, 132.
(18) Andrieux, C. P.; Gorande, A. L.; Save´ant, J .-M. J . Am. Chem.
Soc. 1992, 114, 6892.
(19) Arnold, B. R.; Scaiano, J . C.; McGimpsey, W. G. J . Am. Chem.
Soc. 1992, 114, 9978.
Laser irradiations were also performed with deaerated
10 mM solutions of 1 in cyclohexane, using a KrF excimer
laser (248 nm) and with the sample contained in a quartz
spectrometer cell. Compounds 7, 8, and 11-13 were
obtained, but significant amounts of cyclopropanes were
also formed. Clearly, in contrast with the clear domi-
nance of two-photon products arising from irradiation of
(thermally) reactive intermediates in the laser-drop
experiments other products arising from irradiation of
(thermally) nonreactive photoproducts were also formed
under these conditions.
La m p P h ot olysis of 3-Ch lor o-1,3-d ip h en ylp r o-
p en e. At this point it was neccesary to determine which
products originate from the propenyl radical generated
in the high intensity irradiation experiments. Homolysis
of the C-Cl bond of 3-chloro-1,3-diphenylpropene (4)
generates the propenyl radical as shown in the laser flash
photolysis of this compound. Low intensity irradiation
(Pyrex filter) of deaerated 10 mM solutions of 4 in
(20) Direct detection of this species was not possible, probably due
to its short lifetime, (<100 ps, according to Karki, S. B.; Dinnocenzo,
J . P.; Farid, S.; Gooman, J . L.; Gould, I. R.; Zona, T. A. J . Am. Chem.
Soc. 1997, 119, 431).
(21) Mizuno, K.; Ichinose, N.; Otsuji, Y. J . Org. Chem. 1992, 57,
1855.
(22) Ichimura, T.; Hikida, T.; Mori, Y. J . Phys. Chem. 1975, 79, 291.
(23) Takemura, T.; Fujita, M.; Ohta, N. Chem. Phys. Lett. 1988, 145,
215.
(25) High intensity photolysis studies have also been carried out
by means of the laser-jet technique: Wilson, R. M. ; Adam, W.; Schulte-
Oestrich, R. The Spectrum 1991, 4, 8.
(26) Griffin, G. W.; Covell, J .; Petterson, R. C.; Dodson, R. M.; Klose,
G. J . Am. Chem. Soc. 1965, 87, 1410.
(24) Scaiano, J . C.; Banks, J . T. J . Brazil Chem. Soc. 1995, 6, 211.

Photolysis of 1,3-Dichloro-1,3-diphenylpropane
J . Org. Chem., Vol. 62, No. 17, 1997 5717
Sch em e 5
1,3-diphenylpropene (trans-20). The 1,3-diphenylpropyl
biradical 5 must be the precursor of 1,2-diphenylcyclo-
propanes, though this intermediate was not observed due
to its short lifetime. Formation of 1,3-diphenylpropene
and 1-phenylindane can be also explained through the
intermediacy of the 1,3-biradical, since they are also
obtained during the photoisomerization of 1,2-diphenyl-
cyclopropane.26 However, 3-cyclohexyl-trans-1,3-diphe-
nylpropene must arise from the 1,3-diphenylpropenyl
radical 3 detected in the laser flash photolysis experi-
ments. The possibility that the propenyl radical is an
intermediate in the formation of 1,2-diphenylcyclopro-
pane can be ruled out because lamp irradiation of
3-chloro-1,3-diphenylpropene (4) leads only to trans-3-
cyclohexyl-1,3-diphenylpropene, which under our irradia-
tion conditions is rapidly transformed into 1-cyclohexyl-
2,3-diphenylcyclopropane (21). No formation of 1,2-
diphenylcyclopropane is observed in this case.
cyclohexane resulted in the formation of a product with
m/z 276 for the molecular ion. In principle it could be
expected that the intermediate propenyl radical 3 could
be trapped by the cyclohexyl radical from the medium to
give rise to 3-cyclohexyl-1,3-diphenylpropene (20), a
product observed in the laser-drop experiment of dichlo-
ride 1. However, NMR data showed that the structure
of the photoproduct did not correspond to this compound,
but instead to 1-cyclohexyl-cis,trans-2,3-diphenylcyclo-
propane (21). The stereochemistry assignment was
based on its 1H-NMR spectrum, since 21 is the only
geometric isomer with two nonequivalent benzylic hy-
drogens. These hydrogens gave rise to signals at 2.3 ppm
Exp er im en ta l Section
(t, J ) 5.6 Hz, 1H) and 2.5 ppm (dd, J a ) 5.6 Hz, J b
)
Gen er a l P r oced u r es. 1H and 13C NMR spectra were
recorded in a 250 MHz spectrometer; chemical shifts (δ) are
reported in ppm relative to TMS. GC/MS measurements were
made on a quadrupole mass selective detector connected to a
gas chromatography equipped with DB-1 capillary column (15
m, film thickness 1 mm i.d. 0.25 mm). Combustion analyses
were performed at the Instituto de Qu´ımica Bio-Orga´nica of
the CSIC in Barcelona. Compound 14a is commercially
available (Aldrich). Compound 4 was synthesized from 1,3-
diphenyl-2-propenone following a literature method.29 High
intensity irradiations at 248 nm were realized using a excimer
laser (KrF, 248 nm).
Con ven tion a l La m p Ir r a d ia tion of Ch lor id es 1, 4, 7,
a n d 8. A degassed 10-2 M cyclohexane solution of the
corresponding chloro compound either in a quartz tube (1, 7,
and 8) or Pyrex tube (4) was irradiated for 1 h with a 125-W
medium-pressure mercury lamp inside a quartz immersion
well, under continuous magnetic stirring. After evaporation
of the solvent, the photomixture was analyzed by GC/MS.
Dir ect Ir r a d ia tion of 3-cyloh exyl-tr a n s-1,3-d ip h en yl-
p r op en e (tr a n s-20). A degassed solution of trans-20 (10-2
M) in cyclohexane was irradiated in a Pyrex tube for 2.5 h
with a 125-W medium-pressure mercury lamp inside a quartz
immersion well, under continuous magnetic stirring. After
evaporation of the solvent, NMR data showed the complete
transformation of the alkene in 1-cyclohexyl-cis,trans-2,3-
diphenylcyclopropane (21). 1H NMR (CDCl3): 0.8-1.9 (m,
12H), 2.3 (t, J ) 5.6 Hz, 1H), 2.5 (dd, J a ) 5.6 Hz, J b ) 9.3 Hz,
1H), 7.1-7.4 (m, 10H). 13C NMR (CDCl3): 143.1 (s), 138.7 (s),
128.6 (d), 128.4 (d), 128.3 (d), 127.9 (d), 126.35 (d), 125.7 (d),
125.5 (d), 37.15 (d), 36.5 (d), 33.3 (t), 32.4 (t), 32.1 (d), 27.1 (t),
26.4 (t), 26.1 (t), 25.8 (t). MS m/z 276 (M+, 8), 193 (100), 180
(22), 179 (20), 178 (23), 165 (15), 115 (72), 91 (35). Anal. Calcd
for C21H24: C, 91,25; H 8,75. Found: C, 91,17; H, 8,83.
La ser -Dr op P h otolysis.1,24 The beam from a Nd-YAG
laser using the fourth harmonic (266 nm, <10 ns, e20 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 experiment have been
described earlier.1,24
9.3 Hz, 1H). As expected for vicinal cyclopropyl hydro-
gens, cis coupling was larger than trans coupling.27 The
benzylic hydrogen cis to the cyclohexyl group was shielded
(0.22 ppm) with respect to the benzylic hydrogen with
trans arrangement.
As compound 21 could arise from a secondary di-π-
methane reaction of the trans-alkene 20, the latter
compound was irradiated under similar conditions. This
led to a clean transformation of trans-20 into the cyclo-
propane derivative 21 (Scheme 5). A common photore-
action of phenylolefins is trans-cis isomerization.28 How-
ever, we were not able to detect cis-20 in the photolysate
obtained from trans-20. A reasonable possibility would
be that the expected cis-20 also undergoes efficient di-
π-methane rearrangement under the irradiation condi-
tions. This was confirmed by an independent photolysis
experiment.
Overall, the above results on the photolysis of 4 confirm
that the formation of 20 in the laser-drop photolysis of 1
occurs via the intermediacy of radical 3. The lack of di-
π-methane rearrangement under the employed conditions
is in agreement with the fact that this high intensity
irradiation technique produces almost exclusively pri-
mary photoproducts.
Con clu sion s
It is clear from the above results that monophotonic
processes take place in low-intensity irradiations of
dichloride 1. Homolytic cleavage of the first C-Cl bond
gives rise to a typical benzylic radical, and no hypervalent
intermediate is detected due to the reduced tendency
toward bridging in the case of chlorine. Thus, the
reaction intermediate observed under attenuated laser
excitation and the products characteristic of lamp ir-
radiation can be readily explained on the basis of Scheme
2. No cyclopropanes were obtained under these condi-
tions.
La ser F la sh P h otolysis. These experiments were carried
out using a Nd-YAG laser using the fourth harmonic (266 nm,
<10 ns, e20 mJ /pulse). Transient signals were captured with
a digital oscilloscope which was interfaced to a computer which
also controlled the experiment. The system was operated with
software written in the LabVIEW 3.1.1 enviromment from
National Instruments. Other aspects of this instrument are
similar to those described earlier.30,31 All experiments were
In contrast, high intensity irradiations led mainly to
the formation of 1,2-diphenylcyclopropanes (6) as major
products, together with significant amounts of 1,3-
diphenylpropene, 1-phenylindane, and 3-cyclohexyl-trans-
(27) Williams, D. H.; Fleming, Y. Spectroscopic Methods in Organic
Chemistry; McGraw Hill Book Co. (U.K.) Ltd.: London, 1973; p 107.
(28) Kropp, P. J . Organic Photochemistry; Padwa, A., Ed.; Marcel
Dekker: New York, 1979; Vol 4, p 1.
(29) Hayashi, T.; Yamamoto, A.; Ito, Y.; Nishioka, E.; Miura, H.;
Yanagi, K. J . Am. Chem. Soc. 1989, 111, 6301.
(30) Scaiano, J . C. J . Am. Chem. Soc. 1980, 102, 7747.

5718 J . Org. Chem., Vol. 62, No. 17, 1997
Miranda et al.
carried out using flow cells constructed from 7 × 7 mm
Suprasil quartz tubing. Samples were contained in a 100 mL
reservoir tank which was purged with a slow stream of either
nitrogen or oxygen, as required.
0), 276 (10), 193 (100), 115 (63), 91 (20). Anal. Calcd for
C21H25Cl: C, 80.66; H, 7.99. Found: C, 81.01; H, 8.16.
Syn th esis of 1-Cycloh exyl-1,3-d ip h en ylp r op a n e (12).
To a solution of 3-cyclohexyl-1,3-diphenyl-1-propanone34 (0.10
g, 0.34 mmol) in 20 mL of ethyl acetate were added perchloric
acid (1 mL) and 0.10 g of palladium-carbon catalyst. Hydro-
genation was performed under pressure (12 kg/cm2) during 1
h. The mixture was filtered, extracted with ether, and dried
over magnesium sulfate. Solvent was removed to give 1-cy-
clohexyl-1,3-diphenylpropane as a colorless oil (0.06 g, 64%),
which was further purified by HPLC (hexane/ethyl acetate,
10/1). 1H NMR (CDCl3): 0.7-1.8 (m, 11 H), 1.8-2.0 (m, 2 H),
2.0-2.2 (m, 1 H), 2.2-2.5 (m, 2 H), 7.1-7.4 (m, 10 H). 13C NMR
(CDCl3): 144.2 (s), 142.8 (s), 128.6 (d), 128.3 (d), 128.2 (d),
128.0 (d), 125.8 (d), 125.5 (d), 51.7 (d), 43.3 (d), 34.5 (t), 34.0
(t), 31.3 (t), 31.0 (t), 26.5 (t). MS m/z 278 (M+, 13), 196 (13),
117 (50), 91 (100). Anal. Calcd for C21H26: C, 90.65; H, 9.34.
Found: C, 90.67; H, 9.38.
Syn th esis of 1,3-Dicycloh exyl-1,3-diph en ylpr opan e (13).
3-Cyclohexyl-1,3-diphenylpropanone34 (0.58 g, 2.00 mmol) in
anhydrous diethyl ether (20 mL) was added dropwise to a 2
M solution of cyclohexylmagnesium chloride (1 mL, 2.00 mmol)
in cyclohexane. After the addition was complete, the mixture
was stirred for 1 h at room temperature and quenched by
adding 20 mL of ice-water. The reaction mixture was
extracted with ether and dried over magnesium sulfate.
Solvent was removed to give 1,3-dicyclohexyl-1,3-diphenylpro-
panol (17) as an oil (0.62 g, 83%). IR (oil): 3574 (OH). 1H
NMR (CDCl3): 0.5-2.6 (m, 26H), 6.7-6.8 (m, 1H), 6.85-6.95
(m, 1H), 7.0-7.35 (m, 8H). 13C NMR (CDCl3): 145.3 (s), 144.8
(s), 144.0 (s), 128.7 (d), 128.5 (d), 127.5 (d), 127.0 (d), 126.4
(d), 126.0 (d), 125.9 (d), 125.8 (d), 125.6 (d), 125.1 (d), 79.7 (s),
79.0 (s), 49.7 (d), 48.0 (d), 47.4 (d), 47.0 (d), 44.1 (d), 43.7 (d),
43.0 (t), 41.6 (t), 31.7 (t), 31.1 (t), 30.4 (t), 30.2 (t), 27.4 (t),
26.9 (t), 26.6 (t), 26.5 (t), 26.4 (t). MS m/z 376 (M+, 8), 375
(21), 356 (19), 294 (78), 209 (100), 190 (81), 120 (84), 92 (79),
83 (48). HRMS Calcd for C27H36O: 376.2766. Found 376.2737.
1,3-Dicyclohexyl-1,3-diphenylpropanol (0.62 g, 1.64 mmol)
was disolved in 20 mL of acetic acid. Perchloric acid (1 mL)
and 0.10 g of 10% palladium on charcoal were added. Hydro-
genation was performed under pressure (12 kg/cm2) during 2
h. Then, the mixture was filtered, extracted with ether, and
dried over magnesium sulfate. Solvent was removed to give
1,3-dicyclohexyl-1,3-diphenylpropane (0.46 g, 78%). The prod-
uct was purified by HPLC (hexane/ethyl acetate, 10:1) and
obtained as a diastereoisomeric mixture. 1H NMR (CDCl3):
0.6-2.1 (m, 24H), 2.3-2.5 (m, 2H), 6.9-7.0 (m, 4H), 7.1-7.4
(m, 6H). 13C NMR (CDCl3): 144.8 (s), 144.3 (s), 128.8 (d), 128.7
(d), 127.8 (d), 127.6 (d), 125.6 (d), 125.5 (d), 49.5 (d), 49.3 (d),
43.7 (d), 42.5 (d), 35.9 (t),35.0 (t), 31.9 (t), 31.2 (t), 29.9 (t),
26.8 (t), 26.6 (t), 26.5 (t). MS m/z 360 (M+, 9), 278 (8), 173
Syn th esis of 1,3-Dich lor o-1,3-diph en ylpr opan e (1). 1,3-
Diphenylpropane-1,3-diol (0.37g, 1.61 mmol), prepared accord-
ing with the literature,32 was added to concentrated hydro-
chloric acid (20 mL). The mixture was stirred for 12 h at room
temperature. Afterwards, the solution was neutralized with
NaOH, extracted with ether, and dried over anhydrous sodium
sulfate. Solvent was removed under reduced pressure to give
an oil (0.41 g, 96%). The product was purified by HPLC
chromatography (hexane). 1,3-Dichloro-1,3-diphenylpropane
was obtained as
a
diastereoisomeric mixture. 1H NMR
(CDCl3): (1R,3S), 2.65 (dt, J a ) 15 Hz, J b ) 8 Hz, 1H), 2.95
(dt, J a ) 15 Hz, J b ) 8 Hz, 1H), 4.8 (t, J ) 8 Hz, 2H), 7.2-7.3
(m, 10H); (1R, 3R)/ (1S, 3S), 2.6 (t, J ) 8 Hz, 2H), 5.2 (t, J )
8 Hz, 2H), 7.2-7.3 (m, 10H). 13C NMR (CDCl3): 140.6 (s),
140.0 (s), 128.8 (d), 128.7 (d), 128.6 (d), 128.5 (d), 127.0 (d),
126.9 (d), 60.7 (d), 60.0 (d), 49.5 (t), 49.4 (t). MS m/z 264 (M+,
5), 228 (15), 193 (35), 125 (100). Anal. Calcd for C15H14Cl2:
C, 67.94; H, 5.32. Found: C, 67.56; H, 5.34.
Syn th esis of 1-Ch lor o-1,3-d ip h en ylp r op a n e (7). 1,3-
Diphenylpropanol (0.40 g, 1.80 mmol), prepared as in the
literature,33 was added to concentrated hydrochloric acid (20
mL), and the mixture was stirred for 12 h at room tempera-
ture. The workup procedure was analogous to that previously
described. An oil (0.41 g, 95%) was obtained and purified by
HPLC (hexane). 1H NMR (CDCl3): 2.2-2.5 (m, 2H), 2.6-2.9
(m,2 H), 4.8 (dd, J a ) 8 Hz, J b ) 6 Hz, 1H), 7.2-7.4 (m, 10 H).
13C NMR (CDCl3): 141.6 (s), 140.6 (s), 128.7 (d), 128.5 (d), 128.3
(d), 127.0 (d), 126.2 (d), 62.8 (d), 41.4 (t), 33.1 (t). MS m/z 230
(M+, 17), 194 (25), 91 (100). Anal. Calcd for C15H15Cl: C,
78.12; H, 6.50. Found: C, 78.16; H, 6.58.
Syn th esis of 1-Ch lor o-3-cycloh exyl-1,3-d ip h en ylp r o-
p a n e (8). To a solution of 3-cyclohexyl-1,3-diphenyl-1-pro-
panone34 (0.18 g, 0.62 mmol) in 20 mL of ethanol was added
sodium borohydride (0.03 g, 0.80 mmol) was added in small
portions over 5 min. The mixture was stirred at room
temperature for 24 h and then cooled by means of an ice bath.
Subsequently, 20 mL of water was added and then 1 mL of 6
M hydrochloric acid. The reaction mixture was extracted with
ether and dried with anhydrous sodium sulfate. Solvent was
removed to give 3-cyclohexyl-1,3-diphenylpropanol (16) as a
mixture of stereoisomers (0.15 g, 82%). IR (oil): 3354 (OH).
1H NMR (CDCl3): 0.6-2.2 (m, 14H), 2.6-2.75 (m, 1H), 4.1-
4.25 (m, 1H), 6.9-7.4 (m, 10H). 13C NMR (CDCl3): 145.7 (s),
143.9 (s), 143.7 (s), 143.6 (s), 128.6 (d), 128.4 (d), 128.3 (d),
128.1 (d), 127.7 (d), 127.10 (d), 126.6 (d), 126.1 (d), 125.4 (d),
73.5 (d), 71.75 (d), 48.4 (d), 43.4 (t), 43.3 (d), 42.7 (t), 41.3 (t),
31.2 (t), 31.1 (t), 30.5 (t), 26.4 (t). MS m/z 294 (M+, 85), 276
(47), 193 (100), 174 (97), 107 (81), 92 (31). HRMS Calcd for
C21H26O: 294.1984. Found 294.1984.
(70), 91 (100). Anal. Calcd for C27H36
Found: C, 90.03; H, 10.08.
: C, 90.00; H, 9.99.
3-Cyclohexyl-1,3-diphenylpropanol (0.15 g, 0.51 mmol) was
added to concentrated hydrochloric acid (20 mL). The mixture
was stirred for 12 h at room temperature. The solution was
then neutralized with NaOH, extracted with ether, and dried
over sodium sulfate. The solvent was removed to give an oil
(0.14 g, 88%). The product was purified by column chroma-
tography (hexane/ethyl acetate, 10/1) to give 1-chloro-3-cyclo-
hexyl-1,3-diphenylpropane as a diastereoisomeric mixture. 1H
NMR (CDCl3): 0.6-2.2 (m, 24 H), 2.5-2.9 (m, 4 H), 4.4 (dd,
J a ) 11 Hz, J b ) 2 Hz, 1 H), 4.5 (dd, J a ) 11 Hz, J b ) 5 Hz, 1
H), 7.0 (m, 2 H), 7.2-7.3 (m, 18 H). 13C NMR (CDCl3): 142.8
(s), 142.6 (s), 142.4 (s), 140.6 (s), 128.6 (d), 128.5 (d), 128.4 (d),
128.3 (d), 128.2 (d), 128.0 (d), 127.4 (d), 126.6 (d), 126.3 (d),
62.5 (d), 61.5 (d), 49.3 (d), 43.5 (d), 43.1 (d), 43.0 (t), 42.7 (t),
31.25 (t), 31.0 (t), 30.7 (t), 26.5 (t), 26.4 (t). MS m/z 312 (M+,
Syn th esis of 1-(p-Cycloh exylp h en yl)-3-p h en ylp r op a n e
(15). To a mixture of cyclohexylbenzene (1.20 g, 7.50 mmol)
and 3-phenylpropanoyl chloride (2.00 g, 11.86 mmol) was
added aluminum chloride (1.59 g, 11.86 mmol) in three
portions. After stirring 1 h at room temperature, the reaction
mixture was poured onto ice-water, extracted with ether, and
dried over anhydrous sodium sulfate, and then solvent was
removed. Purification of the residue by column chromatog-
raphy (hexane/ethyl acetate, 10/1) left 1-(p-cyclohexylphenyl)-
3-phenyl-1-propanone (14c) (0.33 g, 15%). IR (oil): 1680 (CdO).
1H NMR (CDCl3): 1.2-1.5 (m, 5H), 1.7-1.9 (m, 5H), 2.5 (m,
1H), 3.1 (t, J ) 8 Hz, 2H), 3.3 (t, J ) 8 Hz, 2H), 7.2-7.3 (m,
7H), 7.9 (d, J ) 8 Hz, 2H). 13C NMR (CDCl3): 198.8 (s), 153.6
(s), 141.3 (s), 134.6 (s), 128.4 (d), 128.3 (d), 128.2 (d), 127.0
(d), 126.0 (d), 44.6 (d), 40.3 (t), 34.0 (t), 30.1 (t), 26.6 (t), 25.9
(t). MS m/z 292 (M+, 15), 209 (55), 187 (100), 91 (22). HRMS
Calcd for C21H24O: 292.1827. Found 292.1831.
(31) Scaiano, J . C.; Tanner, H.; Weir, D. J . Am. Chem. Soc. 1985,
107, 4396.
To a solution of 1-(p-cyclohexylphenyl)-3-phenylpropanone
(0.10 g, 0.34 mmol) in 20 mL of ethyl acetate were added
several drops of perchloric acid and 0.10 g of palladium-carbon
catalyst. Hydrogenation was performed under pressure (12
kg/cm2) during 1 h. The mixture was extracted with ether and
dried over magnesium sulfate. Solvent was removed to give
(32) Zimmerman, H. E.; English, J ., J r. J . Am. Chem. Soc. 1954,
76, 2285.
(33) Huisgen, R.; Grashey, R.; Hans, H.; Seidl, H. Chem. Ber. 1968,
101, 2548.
(34) Totleben, M. J .; Curran, D. P.; Wipf, P. J . Org. Chem. 1992,
57, 1740.

Photolysis of 1,3-Dichloro-1,3-diphenylpropane
J . Org. Chem., Vol. 62, No. 17, 1997 5719
1-(p-cyclohexylphenyl)-3-phenylpropane as a colorless oil (0.08
g, 85%), which was further purified by HPLC (hexane:ethyl
acetate, 10:1). 1H NMR (CDCl3): 1.1-1.4 (m, 5H), 1.6-1.9
(m, 7H), 2.3-2.5 (m, 1H), 2.5-2.6 (m, 4H), 7.0-7.3 (m, 9H).
13C NMR (CDCl3): 145.5 (s), 142.4 (s), 139.6 (s), 128.4 (d), 128.3
(d), 126.7 (d), 125.7 (d), 44.1 (d), 35.5 (t), 35.0 (t), 34.5 (t), 33.0
(t), 26.9 (t), 26.2 (t). MS m/z 278 (M+, 50), 187 (17), 173 (20),
1,3-diphenylpropene in a Pyrex tube was irradiated during 2.5
h with a 125-W medium-pressure mercury lamp inside a
quartz immersion well, under continuous stirring. After
evaporation of the solvent, the 1H NMR spectrum showed a
mixture of isomeric olefins (cis-20/trans-20, ratio 72/28). Both
isomers were separated by preparative chromatography. Data
for cis-20: 1H NMR (CDCl3): 0.7-2.0 (m, 11H), 3.5 (dd, J a
)
159 (25), 131 (20), 117 (42), 91 (100). Anal. Calcd for C21H26
C, 90.65; H, 9.34. Found: C, 90.63; H, 9.50.
:
9 Hz, J b ) 11 Hz, 1H), 5.9 (t, J ) 11 Hz, 1 H), 6.5 (d, J ) 11
Hz, 1H). 13C NMR (CDCl3): 144.3 (s), 137.6 (s), 135.1 (d), 129.1
(d), 128.7 (d), 128.4 (d), 128.0 (d), 126.5 (d), 125.9 (d), 50.2 (d),
43.9 (d), 31.0 (t), 30.9 (t), 26.4 (t), 26.35 (t). MS m/z 276 (M+,
10), 193 (100), 178 (24), 165 (9), 115 (100), 91 (37). Anal. Calcd
for C21H24: C, 91,25 ; H, 8,75. Found: C, 91,30; H, 8,71.
Syn th esis of 3-Cycloh exyl-tr a n s-1,3-d ip h en ylp r op en e
(tr a n s-20). 3-Cyclohexyl-1,3-diphenylpropanol was dehy-
drated following a literature procedure35 to give 3-cyclohexyl-
trans-1,3-diphenylpropene, (solid). 1H NMR (CDCl3): 0.8-2.0
(m, 11H), 3.1 (m, 1H), 6.4 (m, 2H), 7.2-7.5 (m, 10H). 13C NMR
(CDCl3): 144.0 (s), 137.6 (s), 133.2 (d), 130.2 (d), 130.0 (d),
128.4 (d), 128.1 (d), 128.0 (d), 126.9 (d), 126.1 (d), 126.0 (d),
56.7 (d), 43.7 (d), 31.5 (t), 26.5 (t), 26.4 (t). MS m/z 276 (M+,
17), 193 (100), 178 (33), 165 (12), 115 (95), 91 (46). Anal. Calcd
for C21H24: C, 91.25; H, 8.75. Found: C, 90.91; H, 9.07.
Syn th esis of 3-Cycloh exyl-cis-1,3-diph en ylpr open e (cis-
20). A degassed 10-2 M acetone solution of 3-cyclohexyl-trans-
Ack n ow led gm en t. Financial support by the Natu-
ral Science and Engineering Council of Canada through
an operating grant (J .C.S.) and Spanish DGICYT
(M.A.M., Project no. PB94-0539; J .P.P., Project no.
PR95-289) are gratefully acknowledged. E.F.S. also
thanks the Spanish Ministry of Education for a grant.
J .C.S. is the recipient of a Killam Fellowship awarded
by the Canada Council.
(35) Hoffman, R. V.; Bishop, R. D.; Fitch, P. M.; Hardenstein, R. J .
Org. Chem. 1980, 45, 917.
J O9704772