Lamp versus laser photolysis of a bichromophoric dichloroalkane: Chemical evidence for the two-photon generation of the 1,5-diphenylpentanediyl biradical

Article abstract of DOI:10.1021/jo960033q

Low intensity (lamp) photolysis of 1,5-dichloro-1,5-diphenylpentane (1) leads to the formation of the 1-chloro-1,5-diphenylpentyl radical (7) through C-Cl bond cleavage. Radical 7 leads to the final products through typical free radical reactions. No cyclopentanes are formed under low intensity conditions. In contrast, high intensity laser irradiation leads to C-Cl photocleavage of radical 7 to yield the 1,5-diphenylpentanediyl biradical (11), which results in the formation of isomeric cis- and trans-1,2-diphenylcyclopentanes; the behavior of these biradicals agrees well with that observed when their precursor is 2,6-diphenylcyclohexanone. Two-color two-laser experiments suggest that both singlet and triplet biradicals are formed, even if only the latter are detectable with nanosecond techniques.

Full text of DOI:10.1021/jo960033q

J . Org. Chem. 1996, 61, 3773-3777
3773
La m p ver su s La ser P h otolysis of a Bich r om op h or ic
Dich lor oa lk a n e: Ch em ica l Evid en ce for th e Tw o-P h oton
Gen er a tion of th e 1,5-Dip h en ylp en ta n ed iyl Bir a d ica l
J ulia Pe´rez-Prieto,*,† Miguel Angel Miranda,*,‡ Hermenegildo Garc´ıa,‡ Kla´ra Ko´nya,§ and
J . C. Scaiano*,§
Departamento de Qu´ımica Orga´nica, Facultad de Farmacia, Universidad de Valencia,
Vicent Andre´s Estelles s/ n, Burjasot, 46100 Valencia, Spain, Instituto de Tecnologı´a Quı´mica,
Universidad Polite´cnica de Valencia, Camino de Vera s/ n, Valencia, 46071 Spain, and
Department of Chemistry, University of Ottawa, Ottawa, Canada K1N 6N5
Received J anuary 3, 1996X
Low intensity (lamp) photolysis of 1,5-dichloro-1,5-diphenylpentane (1) leads to the formation of
the 1-chloro-1,5-diphenylpentyl radical (7) through C-Cl bond cleavage. Radical 7 leads to the
final products through typical free radical reactions. No cyclopentanes are formed under low
intensity conditions. In contrast, high intensity laser irradiation leads to C-Cl photocleavage of
radical 7 to yield the 1,5-diphenylpentanediyl biradical (11), which results in the formation of
isomeric cis- and trans-1,2-diphenylcyclopentanes; the behavior of these biradicals agrees well with
that observed when their precursor is 2,6-diphenylcyclohexanone. Two-color two-laser experiments
suggest that both singlet and triplet biradicals are formed, even if only the latter are detectable
with nanosecond techniques.
In tr od u ction
with high-intensity light may lead to a difunctional
reactive intermediate from which completely different set
of products may be formed. For example, our earlier
work with 1,5-diiodo-1,5-diphenylpentane revealed the
formation of new products by photoexcitation of the
intermediate hypervalent iodine intermediate.7
Photolysis of monohalogenated organic compounds is
known to induce carbon-halogen (C-X) bond cleavage,
to give products from either heterolysis or homolysis,
depending on the structure of the compound, the nature
of the leaving group, and the polarity and nucleophilicity
of the solvent.1 In the case of homolytic cleavage, the
final products result from radical reactions.
In the case of the photolysis of 1,8-bis(chloromethyl)-
naphthalene and 1,8-bis(bromomethyl)naphthalene it is
possible to achieve the double-activation of dihalides.8-10
Irradiation of these compounds with a high-intensity KrF
excimer laser (248 nm) gives acenaphthene via biphotonic
generation of a biradical intermediate and successive
radical coupling. However, the yields reported for
acenaphthene are very low and decrease upon further
irradiation. This has been attributed to secondary reac-
tions of the primary photoproduct.8-10 In these sub-
strates the C-X bonds are connected through a single
naphthyl group and therefore are not fully independent
chromophores.
To our knowledge, no photochemical study of this type
has been reported on dichloro derivatives where the C-Cl
bonds are separated by a fully saturated carbon skeleton,
which would ensure the true bichromophoric nature of
the substrates. This has prompted us to undertake a
systematic study on 1,5-dichloro-1,5-diphenylpentane (1),
in order to compare the results of conventional lamp
photolysis with those obtained with KrF (248 nm) exci-
mer laser excitation, with two-photon two-color tech-
niques, and with the laser-drop method. Our results
clearly show that 1,2-diphenylcyclopentanes are exclu-
sively formed using the high intensity laser source and
can be safely assigned as two-photon products.
In many respects the photobehavior of geminal diha-
lides resembles that previously observed for monohalides.
Both reduction and elimination products are formed,
which is consistent with initial homolytic cleavage of one
of the carbon-halogen bonds, to afford a radical pair.
Hydrogen abstraction from the solvent is common and
in polar media can be followed by deprotonation or
electron transfer reactions.2
Vicinal dibromides undergo photodecomposition to
yield bromine atoms. The cleavage is frequently followed
by hydrogen abstraction from the solvent to yield HBr.3-5
Comparatively little is known about the photobehavior
of organic dihalides in which the C-X bonds are sepa-
rated by two or more carbon atoms. The few examples
found in the literature show that photolysis frequently
gives rise to isomerization products.6 In principle, si-
multaneous or sequential excitation of both chromophores
† Universidad de Valencia.
‡ Universidad Polite´cnica de Valencia.
§ University of Ottawa.
X Abstract published in Advance ACS Abstracts, May 1, 1996.
(1) (a) Kropp, P. J .; Adkins, R. L. J . Am. Chem. Soc. 1991, 113, 2709.
(b) Bartl, J .; Steenken, S.: Mayr, H.; McClelland, R. A. J . Am. Chem.
Soc. 1990, 112, 6918. (c) Slocum, G. H.; Schuster, G. B. J . Org. Chem.
1984, 49, 2177. (d) Kropp, P. J . Acc. Chem. Res. 1984, 17, 131. (e) For
a review of the photobehavior of alkyl halides see: Sammes, J . P.;
Chemistry of the Carbon-Halogen Bond; Patai, S.; Ed.; Wiley: New
York, 1973; Chapter 11.
Resu lts
In this work we have taken advantage of a range of
techniques that allowed us to establish the nature and
(2) Kropp, P. J .; Pienta, N. J . J . Org. Chem. 1983, 48, 2084.
(3) Scaiano, J . C.; Barra, M.; Krzywinski, M.; Sinta, R.; Calabrese,
G. J . Am. Chem. Soc. 1993, 115, 8340.
(4) Gannon, T.; McGimpsey, W. G. J . Org. Chem. 1993, 58, 913.
(5) Weldon, D.; Barra, M.; Sinta, R.; Scaiano, J . C. J . Org. Chem.
1995, 60, 3921.
(7) Banks, J . T.; Garc´ıa, H.; Miranda, M. A.; Pe´rez-Prieto, J .;
Scaiano, J . C. J . Am. Chem. Soc. 1995, 117, 5049.
(8) Ouchi, A.; Yabe, A. Tetrahedron Lett. 1992, 33, 5359.
(9) Adam, W.; Ouchi, A. Tetrahedron Lett. 1992, 33, 1875.
(10) Ouchi, A.; Yabe, A.; Adam, W. Tetrahedron Lett. 1994, 35, 6309.
(6) Golub, M. A. J . Am. Chem. Soc. 1972, 92, 2615.
S0022-3263(96)00033-3 CCC: $12.00 © 1996 American Chemical Society

3774 J . Org. Chem., Vol. 61, No. 11, 1996
Pe´rez-Prieto et al.
Sch em e 1
Sch em e 2
under conditions similar to those used for compound 1.
Thus, 1-chloro-1,5-diphenylpentane (2) was converted
into a mixture of 1,5-diphenylpentane, 1-cyclohexyl-1,5-
diphenylpentane, and two other isomeric cyclohexane-
derived products (m/ z 306 for the molecular ion). Since
intermediate 8 has a delocalized unpaired electron, the
later photoproducts could in principle result from the
attack of a cyclohexyl radical to different positions of the
aryl ring (Scheme 2).
Unambiguous synthesis of 1-(p-cyclohexylphenyl)-5-
phenylpentane (10a ) showed that this compound was
identical to one of the minor photoproducts from 1 with
a molecular weight of 306. Minor amounts of 1-(o-
cyclohexylphenyl)-5-phenylpentane (10b) were also ob-
tained during the synthesis of 10a . This allowed us to
compare its retention time and fragmentation pattern
with those of the other minor photoproduct of 1, which
was clearly different. Thus, the latter may be the third
possible isomer: 1-(m-cyclohexylphenyl)-5-phenylpentane
(10c). The formation of products of meta substitution is
surprising and suggests that these photoproducts may
be formed through radical alkylation15 of the aryl ring of
4.
mechanism of a range of mono- and two-photon processes
involved in the photodecomposition of 1,5-dichloro-1,5-
diphenylpentane (1). The results are grouped according
to the type of technique employed.
Likewise, irradiation of 1-chloro-5-cyclohexyl-1,5-diphen-
ylpentane (3) led to 1-cyclohexyl-1,5-diphenylpentane (5)
and 1,5-dicyclohexyl-1,5-diphenylpentane (6). In this
case, no alkylation products were observed.
La m p Ir r a d ia tion . Photolysis of 1,5-dichloro-1,5-
diphenylpentane (1) was performed in deaerated cyclo-
hexane solution (ca. 10-2 M) for 1 h. Analysis of the
photolysate by GC/MS revealed the presence of six
products, whose structures were assigned as the follow-
ing: 1-chloro-1,5-diphenylpentane (2), 1-chloro-5-cyclo-
hexyl-1,5-diphenylpentane (3), 1,5-diphenylpentane (4),11
1-cyclohexyl-1,5-diphenylpentane (5), 1,5-dicyclohexyl-
1,5-diphenylpentane (6) and cyclohexylcyclohexane.12
This structural assignment was confirmed by unam-
biguous synthesis of the products, using well-established
procedures (see Experimental Section), or by comparison
with available authentic samples. Minor amounts of two
other compounds with m/ z 306 for the molecular ion
were also detected.
The product distribution can be explained through
homolytic cleavage of C-Cl bond(s) to give benzylic
radicals (7-9) and a chlorine atom (Scheme 1). No
disproportionation products were detected, due to ef-
ficient hydrogen abstraction from the solvent by chlo-
rine,13,14 which yields cyclohexyl radicals. The formation
of compounds 3, 6, and cyclohexylcyclohexane must occur
through recombination of the intermediate benzylic and
cyclohexyl radicals, while compounds 2 and 4 can be
accounted for through hydrogen abstraction from the
medium. Compound 5 could arise via a combination of
both routes.
In summary, C-Cl homolysis of compound 1 takes place
as a monophotonic process upon conventional lamp
photolysis. No dimerization products (i.e., containing two
1 moieties) were obtained, probably as a consequence of
the low substrate concentration, low light intensity, and
relatively reactive solvent.
High In ten sity P r od u ct Stu d ies. Two different
conditions of high intensity laser irradiation were em-
ployed. The laser drop technique7,16 provides a way of
performing high intensity photolysis, while minimizing
the amounts of secondary products. The products of 266
nm excitation revealed the formation of cis and trans
isomers of 1,2-diphenylcyclopentane in approximately an
equimolar ratio. Interestingly, no products of radical
recombination or products containing the cyclohexyl
moiety were detected. While conversions were small
(<10%), the results suggest that radical 7 was quanti-
tatively photodecomposed to biradical 11, which in turn
undergoes predominant cyclization (Scheme 3).
Laser irradiations were also conducted with deaerated
10-2 M solutions of 1 in cyclohexane, using a KrF excimer
laser (248 nm) and with the sample contained in a quartz
spectrometer cell. Compounds 2-6 were again obtained,
but in this case significant amounts of cis- and trans-
1,2-diphenylcyclopentane (12a /12b), ca. 1/1 ratio), were
also found in the photolysis mixtures (Scheme 3). It is
noticeable that these compounds are also formed during
conventional lamp irradiation of 2,6-diphenylcyclohex-
anone (13) which is known to yield the 1,5-diphenylpen-
tanediyl biradical (11).17 Formation of the diphenylcy-
In order to check the feasibility of the proposed
mechanistic scheme, compounds 2 and 3 were photolyzed
(11) Borsche, W.; Wollemann, J . Chem. Ber. 1912, 45, 3713.
(12) Beilstein Handbuch der Organische Chemie H, 5, 71.
(13) Reported bond energies (kcal/ mol): H-Cl, 103.2; H-C6H11
,
(15) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemistry; Harper and Row: New York, 1981; pp 730.
(16) Scaiano, J . C.; Banks, J . T. J . Brazil Chem. Soc. 1995, 6, 211.
(17) Zimmt, M. B.; Doubleday, C., J r.; Turro, N. J . Chem. Phys. Lett.
1987, 134, 549.
95.5; Weast, R. C. Handbook of Chemistry and Physics; CRC Press:
Boca Raton, FL; 1976; pp F-221, F-231.
(14) Bunce, N. J .; Ingold, K. U.; Landers, J . P.; Lusztyk, J .; Scaiano,
J . C. J . Am. Chem. Soc. 1985, 107, 5464.

Lamp vs Laser Photolysis of a Bichromophoric Dichloroalkane
J . Org. Chem., Vol. 61, No. 11, 1996 3775
Sch em e 3
clopentane derivatives was therefore a fingerprint for the
two-photon chemistry of 1,5-dichloro-1,5-diphenylpentane
(1) by means of laser photolysis. Clearly, a mixture of
mono- and multiphoton processes occur under these
conditions, in contrast with the clear dominance of two-
photon processes in the laser drop experiments.
La ser F la sh P h otolysis. Laser flash photolysis of
deaerated 1 mM solutions of 1 in cyclohexane at 266 nm
were carried out using two different excitation geom-
etries. Front face irradiation of the sample with a highly
concentrated beam, in a geometry that tends to favor
multiphoton processes, and 90° excitation using a quartz
beam diffuser in front of the sample; this avoids hot spots
in the beam and tends to promote monophotonic pro-
cesses.
In the case of front face excitation we observed the
formation of an intermediate with λmax 320 nm which
decays with a first-order kinetics and a lifetime of ∼600
ns. This is in agreement with the known properties of
biradical 11,17 a result that is further confirmed by our
two-color two-laser experiments (vide infra). In contrast,
this short lived transient is not detected under the low
intensity (monophotonic) conditions, where a much longer
lived intermediate (g2 µs) showing typical monoradical
behavior was detected. In the case of cyclohexanone 13,
we monitored a biradical lifetime of ca. 800-900 ns in
cyclohexane; it is possible that its somewhat shorter
biradical lifetime when 1 is the precursor is related to
the need to carry out these experiments under conditions
(high laser intensity) that favor two-photon processes. It
is known that under these conditions even very short
lived biradicals can undergo biradical-biradical reactions
that will lead to a shortening of the experimental
lifetimes.18
These data suggest the formation of the biradical in
the first case, since the front-face irradiation favors
biphotonic processes. Laser-drop experiments agree well
with the conclusions of front-face laser flash experiments.
The long lived transient in the 90° irradiation is assigned
to the benzylic radical 7. An investigation of the effects
of light intensity on the lifetime of this transient and the
size of the signal was carried out by attenuating the laser
beam with a set of calibrated neutral density filters. The
power dependence of the signal with a diffuse laser beam
at 90° showed a linear relationship between transient
absorption and laser power, consistent with the behavior
anticipated for monophotonic processes.
F igu r e 1. Two-laser two-color experiment showing excitation
of 1 in cyclohexane at 266 nm (see A), followed by excitation
at 308 nm (B) (monitoring wavelength at 320 nm). The insert
shows an enlargement of the signal following excitation by the
second laser.
laser excites the reaction intermediate.19 The wavelength
of the photolysis laser is usually selected so as to match
the optical absorption of the intermediate of interest
Thus, a 1 mM solution of 1,5-dichloro-1,5-diphenylpen-
tane in cyclohexane was excited using 266 nm pulses (Nd:
YAG laser, fourth harmonic) to produce the transient,
while a second laser with 308 nm pulses to photolyze it;19
both lasers were incident from the front face. Photolysis
of the transient derived from 1,5-dichloro-1,5-diphenyl-
pentane at 308 nm led to permanent and irreversible
bleaching of the transient signal as monitored at 320 nm
(Figure 1). The low point near the B marker are due to
scattered light from the second laser. Interestingly, the
small component of fast decay observed following the 308
nm pulse decays with the same lifetime as the initial fast
component (∼600 ns). We believe that the same biradical
(11) is produced by two-photon processes at 266 nm or
by 308 nm excitation of the monoradical 7.
We have also analyzed the reaction mixture (3 mL, 1
mM deaerated solution of 1) from a two-laser experiment
in which the sample was irradiated with a 500 pulse
series using both lasers, 266 and 308 nm, with the pulses
separated by 3 µs. GC/MS analysis showed that more
than 50% of the starting material had been photolyzed
and mainly 1,2-diphenylcyclopentane products were
formed (0.58/0.42, cis/ trans ratio). Minor amounts of
monoradical products were also identified.
Discu ssion
It is clear from our experiments that the low intensity
photolysis of 1,5-dichloro-1,5-diphenylpentane (1) can be
understood using the same conceptual framework as in
the case of simple monochloro compounds. Under these
conditions the second chlorine atom is not involved in
the primary cleavage step, and the intermediate radical
appears to have no tendency for the formation of hyper-
valent intermediates of the type recently detected7 for
the 1,5-diphenyl-5-iodo-1-pentyl radical. This is probably
not surprising given the reduced tendency toward bridg-
ing in the case of chlorine. Thus, the reaction intermedi-
ates observed under attenuated laser excitation, and the
Tw o-La ser Tw o-Color La ser F la sh P h otolysis.
This technique allows the direct examination of the effect
of laser excitation on the behavior of reaction intermedi-
ates. A first laser pulse (referred to as synthesis laser)
produces the intermediate of interest (e.g., the mono-
radical), while the second laser pulse from the photolysis
(19) Scaiano, J . C.; J ohnston, L. J .; McGimpsey, W. G.; Weir, D. Acc.
Chem. Res. 1988, 21, 22.
(18) Scaiano, J . C. Tetrahedron 1982, 38, 819.

3776 J . Org. Chem., Vol. 61, No. 11, 1996
Pe´rez-Prieto et al.
La ser -Dr op P h otolysis.7,16 The beam from a Continuum
Surelite Nd-YAG laser using the fourth harmonic (266 nm,
<10ns, 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.7 Drops of deaerated 2 mM
solution of 1,5-dichloro-1,5-diphenylpentane were irradiated
by the focused output from the 266 nm laser. GC/MS analysis
showed that cis- and trans-1,2-diphenylcyclopentane were
formed in nearly equimolar amounts. The efficiency of trans-
formation was very low (<10%), but interestingly no mono-
radical products were detected.
La ser F la sh P h otolysis. These experiments were carried
out using either a Continuum Surelite Nd-YAG laser using
the fourth harmonic (266 nm, <10ns, e16 mJ /pulse) or a
Lumonics EX-530 excimer laser operated with HCl/Xe/Ne gas
mixtures (308 nm, ca. 6 ns, e50 mJ /pulse). Transient signals
were captured with a Tektronix-2440 digital oscilloscope which
was interfaced to a PowerMacintosh computer which also
controlled the experiment. The system was operated with
software written in the LabVIEW 3.1.1 environment from
National Instruments. Other aspects of this instrument are
similar to those described earlier.23,24 The two-laser two-color
experiments were performed by sending a trigger pulse to a
Stanford Research Systems Model DG 535 delay generator
which then sent TTL pulses which fired the lasers at the
desired sequence. All experiments were 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.
products characteristic of lamp irradiation, can be readily
explained on the basis of Scheme 1. This form of irradia-
tion does not lead to the formation of any 1,2-diphenyl-
cyclopentanes.
In contrast, high intensity laser irradiation leads to
the formation of cis- and trans- 1,2-diphenylcyclopentanes
with approximately equimolar yields. It is interesting
that laser excitation in bulk (i.e. in a spectrometer cell)
leads to good conversions to diphenylcyclopentanes, but
the presence of monophotonic products (from Scheme 1)
is unavoidable under these conditions. Thus, it is clear
that many radicals 7 do not undergo photodecomposition
under these conditions. While the products under laser-
drop conditions also reveal the importance of two-photon
processes, the complete absence of monophotonic prod-
ucts indicates that while conversion on the starting
material 1 is low, the radical 7 undergoes quantitative
photodehalogenation.
It is interesting to examine the structure of radical 7.
Clearly, the dominant chromophore will be the radical
center, rather than the intact chlorobenzyl moiety; this
will be particularly true at the 308 nm laser wavelength,
which is quite close to the absorption maximum for 7
(∼320 nm). Thus, the efficient photodecomposition of the
radical must require energy transfer from the excited
radical (which is expected to have a short excited state
lifetime)20 to the C-Cl bond.
Another interesting question relates to the multiplicity
with which 11 is formed by photoinduced C-Cl bond
cleavage of radical 7. In principle, we expect the statisti-
cal ratio of 3:1 triplets-to-singlets. Our results can only
provide a qualitative indication on this question. Note
that in Figure 1 308 nm laser excitation of the radical
leads to bleaching. Yet, one could have anticipated an
enhanced absorption, since excitation transforms a mono-
chromophoric radical (7) into a bichromophoric biradical
(11), both chromophores being expected to maintain their
spectroscopic characteristics.21 We believe that the rea-
son for this apparent discrepancy may reflect that bi-
radicals produced in the singlet state undergo rapid
decay, in time scales too short for detection with nano-
second techniques. On the other hand triplet biradicals
survive long enough for easy detection. Interestingly, the
ratio of cis- and trans-1,2-diphenylcyclopentanes (12) is
essentially the same reported for the photolysis of 13.22
Syn th esis of 1,5-d ich lor o-1,5-d ip h en ylp en ta n e (1). 1,5-
Diphenylpentane-1,5-diol25 (0.66 g, 2.6 mmol) was added to
concentrated hydrochloric acid (15 mL), and the mixture was
stirred for 12 h at room temperature. Afterwards, the solution
was neutralized with NaOH, extracted with ether, and dried
with anhydrous sodium sulfate. Solvent was removed under
reduced pressure to give an oil (0.70 g, 92%). The product was
purified by HPLC chromatography (hexane). 1H NMR
(CDCl3): 1.1-1.9 (m, 2H), 2.1 (m, 4H), 4.8 (dd, J ) 8, 6Hz,
2H), 7.2-7.4 (m, 10H). 13C NMR (CDCl3): 141.4 (s), 128.2 (d),
128.1 (d), 126.8 (d), 63.2 (d), 39.1 (t), 24.6 (t). MS m/ z M+
(292, 0), 221 (3), 220 (13), 142 (14), 129 (100), 128 (42), 117
(30), 115 (46), 104 (22), 92 (38), 91 (65). Anal. Calcd for C17H18
-
Cl2: C, 69.63; H, 6.19; Cl, 24.18. Found: C, 69.69; H, 6.10;
Cl, 24.09.
Syn th esis of 1-Ch lor o-1,5-d ip h en ylp en ta n e (2). 1,5-
Diphenyl-1-pentanol26 (0.40 g, 1.8 mmol) was added to con-
centrated hydrochloric acid (10 mL), and the mixture was
stirred for 12 h at room temperature. The workup procedure
was analogous to that previously described. An oil (0.40 g,
82%) was obtained and purified by HPLC (hexane). 1H NMR
(CDCl3): 1.4-1.8 (m, 4H), 2.0-2.3 (m, 2H), 2.5 (t, J ) 7 Hz,
2H), 3.9 (t, J ) 7 Hz, 1H), 7.2-7.6 (m, 10H). 13C NMR
(CDCl3): 142.3 (s), 141.9 (s) 128.6 (d), 128.3 (d), 128.2 (d), 126.9
(d), 125.7 (d), 63.7 (d), 39.9 (t), 35.7 (t), 30.9 (t), 26.7 (t). MS
m/ z M+ (258, 0), 222 (11), 131 (70), 118 (22), 117 (73), 115
(49), 105 (30), 104 (66), 92 (41), 91 (100). Anal. Calcd for
C17H19Cl: C, 78.90; H, 7.40; Cl, 13.70. Found: C, 79.07; H,
7.43; Cl, 13.32.
Syn th esis of 1-Ch lor o-5-cycloh exyl-1,5-d ip h en ylp en -
ta n e (3). 5-Oxo-5-phenylpentanoic acid (1.92 g, 10 mmol) was
added dropwise to a 2 M degassed solution of cyclohexylmag-
nesium chloride (10 mL, 20 mmol) in cyclohexane. After the
addition was complete, the mixture was stirred for 12 h at
room temperature and quenched by adding ice-water. The
reaction mixture was acidified with HCl, extracted with ether,
and dried over magnesium sulfate. Solvent was removed to
give an oil which was dissolved in acetic acid (67 mL) and
perchloric acid (3.4 mL). After addition of 10% palladium on
charcoal (0.50 g) the reaction mixture was stirred under
Exp er im en ta l Section
Gen er a l P r oced u r e. IR spectra were run in a Perkin-
Elmer 843 spectrometer. 1H and 13C NMR spectra were
recorded in a Bruker AC 250 spectrometer; chemical shifts (δ)
are reported in ppm relative to TMS. GC/MS measurements
were made on a Trio 1000 quadrupole mass selective detector
connected to a Fisons Series 8000 HRGC gas chromatograph
equipped with DB-1 capillary column (15 m, film thickness 1
mm i.d. 0.25 mm). Combustion analysis were performed at the
Instituto de Quimica Bio-Organica of the CSIC in Barcelona.
High-resolution mass spectra were conducted on a VG AU-
TOSPEC instrument.
Con ven tion a l La m p Ir r a d ia tion . A degassed 10-2
M
cyclohexane solution of the corresponding chloro compound in
a quartz tube 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 irradiated mixture was analyzed by GC/MS.
(23) Scaiano, J . C. J . Am. Chem. Soc. 1980, 102, 7747.
(24) Scaiano, J . C.; Tanner, M.; Weir, D. J . Am. Chem. Soc. 1985,
107, 4396.
(25) Beilstein Handbuch der Organische Chemie H, 6, 1015.
(26) Henbest, H. B.; Trocha-Grimshow, J . J . Chem. Soc., Perkin
Trans. 1 1974, 601.
(20) Meisel, D.; Das, P. K.; Hug, G. L.; Bhattacharyya, K.; Fes-
senden, R. W. J . Am. Chem. Soc. 1986, 108, 4706.
(21) Small, R. D., J r.; Scaiano, J . C. Chem. Phys. Lett. 1977, 50, 431.
(22) Peyman, A.; Beckhaus, H. D.; Ru¨chardt, C. Chem. Ber. 1988,
121, 1027.

Lamp vs Laser Photolysis of a Bichromophoric Dichloroalkane
J . Org. Chem., Vol. 61, No. 11, 1996 3777
pressure (12 kg/cm2) during 2 h. The mixture was extracted
with ether and dried over magnesium sulfate. Solvent was
removed to give 5-cyclohexyl-5-phenylpentanoic acid (15) (1.27
g, 49%) which was used without further purification. IR
(CCl4): 1710 (CdO). 1H NMR (CDCl3): 0.6-2.0 (m, 15H), 2.1-
2.4 (m, 3H), 7.0-7.4 (m, 5H), 10.5 (bb, 1H). 13C NMR
(CDCl3): 180.0 (s), 143.8 (s), 128.3 (d), 127.9 (d), 125.8 (d), 51.8
(d), 43.1 (d), 34.0 (t), 31.8 (t), 31.3 (t), 30.9 (t), 26.8 (t), 22.9 (t).
HRMS Calcd for C17H24O2: 260.1776. Found: 260.1780.
To a solution of acid 15 (1.27 g, 4.9 mmol) in anhydrous
diethyl ether (30 mL), a 2 M solution of phenyllithium (5 mL,
9.8 mmol) was added dropwise over ca. 20 min. After the
addition was complete the mixture was stirred for 1 h at room
temperature and quenched by adding ice-water. The organic
phase was separated and the alkaline aqueous phase was
further extracted with two 10 mL portions of ether. The
combined ethereal extracts were washed with brine and dried
over magnesium sulfate. Solvent was removed to give an oil
(1.07 g, 68%). Purification of the residue by column chroma-
tography (hexane/ethyl acetate, 10/1) left 5-cyclohexyl-1,5-
diphenyl-1-pentanone (16). White solid (mp 90 °C). IR
(CCl4): 2924, 1690 (CdO), 1214. 1H NMR (CDCl3): 0.6-2.0
(m, 15H), 2.3 (m, 1H), 2.9 (m, 2H), 7.1-7.6 (m, 10H), 7.9 (d, J
) 6 Hz, 5 H). 13C NMR (CDCl3): 200.3 (s), 144.1 (s), 136.9
(s), 132.8 (d), 128.5 (d), 128.4 (d), 127.9 (d), 125.8 (d), 52.0 (d),
43.2 (d), 38.6 (t), 32.1 (t), 31.3 (t), 30.9 (t), 26.5 (t), 22.7 (t).
HRMS Calcd for C23H28O : 320.2140. Found: 320.2129.
To a solution of 2.14 g (6.7 mmol) of ketone 16 in 20 mL of
ethanol was added sodium borohydride (0.28 g, 7.4 mmol) in
small portions over 5 min. The mixture was stirred at room
temperature for 20 min and then cooled by means of an ice
bath. Subsequently, 40 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 an oil to which concentrated
hydrochloric acid (20 mL) was added. The mixture was stirred
for 12 h at room temperature. Then, the solution was
neutralized with NaOH, extracted with ether, and dried with
anhydrous sodium sulfate. Solvent was removed to give an
oil (1.50 g, 66%). The product was purified by column
chromatography (hexane/ethyl acetate, 10/1) to leave 1-chloro-
5-cyclohexyl-1,5-diphenylpentane (3). 1H NMR (CDCl3): 0.6-
1.0 (m, 2H), 1.0-2.1 (m, 14 H), 2.3 (m, 2H), 4.7 (m, 1H), 7.0-
7.5 (m, 10H). 13C NMR (CDCl3): 142.1 (s), 141.8 (s), 128.5
(d), 128.1 (d), 128.0 (s), 126.9 (d), 126.8 (d), 125.8 (d), 63.7(d),
63.5 (d), 51.9 (d), 51.8 (d), 43.3 (d), 43.2 (d), 40.0 (t), 39.9 (t),
31.8 (t), 31.8 (t), 31.4 (t), 31.0 (t), 29.7 (t), 26.9 (t), 26.6 (t),
26.5 (t), 25.3 (t), 25.2 (t). MS m/ z M+ (340, 0), 304 (2), 221
(12), 171 (28), 131 (15), 129 (18), 118 (19), 117 (100), 115 (36),
105 (18), 104 (31), 92 (16), 91 (87), 83 (17). Anal. Calcd for
C23H29Cl: C, 81.03; H, 8.57. Found: C, 81.21; H, 8.68.
Syn th esis of 1-Cycloh exyl-1,5-d ip h en ylp en ta n e (5).
1,5-Diphenyl-1-pentanone (2.38 g, 10 mmol) in anhydrous
diethyl ether (40 mL) was added dropwise to a 2 M solution of
cyclohexylmagnesium chloride (5 mL, 10 mmol) in cyclohexane.
After the addition was complete, the mixture was stirred for
30 min 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 a yellow oil to which acetic acid (40 mL), perchloric acid
(2 mL), and 10% palladium on charcoal (0.30 g) were added.
The reaction mixture was stirred under pressure (12 kg/cm2)
during 2 h. Then, the mixture was extracted with ether and
dried over magnesium sulfate. Solvent was removed to give
1-cyclohexyl-1,5-diphenylpentane (0.75 g, 50%). The product
was purified by HPLC chromatography (hexane). 1H NMR
(CDCl3): 0.6-1.0 (m, 3H), 1.0-2.0 (m, 14H), 2.3 (m, 1H), 2.5
(m, 2H), 7.0-7.3 (m, 10 H). 13C NMR (CDCl3):144.2 (s), 142.4
(s), 128.5 (d), 128.3(d), 128.1 (d), 127.9 (d), 125.6 (d), 125.5 (d),
52.1 (d), 43.2(d), 35.8 (t), 32.3 (t), 31.6 (t), 31.4 (t), 31.1 (t),
27.5 (t), 26.9 (t), 26.5 (t). MS m/ z M+ (306, 3), 224 (8), 145
(41), 131 (26), 117 (9), 105 (9), 104 (14), 92 (18), 91 (100), 83
(6). Anal. Calcd for C23H30: C, 90.13; H, 9.87. Found: C,
89.90; H, 10.01.
Syn th esis of 1,5-Dicycloh exyl-1,5-d ip h en ylp en ta n e (6).
1,5-Diphenyl-1,5-pentanedione (1.26 g, 5 mmol) in anhydrous
diethyl ether (20 mL) was added dropwise to a 2 M solution of
cyclohexylmagnesium chloride (5 mL, 10 mmol) in cyclohexane.
After the addition was complete, the mixture was stirred for
30 min 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 an oil which was dissolved in 40 mL of acetic acid.
Perchloric acid (2 mL) and 0.30 g of 10% palladium on charcoal
were added. Hydrogenation was performed under pressure
(12 kg/cm2) during 2 h. The workup procedure was analogous
to that previously described. An oil (0.65 g, 34%) was obtained
and purified by HPLC (hexane). 1H NMR (CDCl3): 0.6-1.0
(m, 6H), 1.0-1.3 (m, 6H), 1.3-1.5 (m, 4H), 1.5-2.0 (m, 12H),
2.2 (m, 2H), 7.0-7.4 (m, 10H). 13C NMR (CDCl3): 144.7 (s),
144.6 (s), 128.5 (d), 127.7 (d), 125.5 (s), 125.4 (d), 125.8 (d),
51.7 (d), 51.6 (d), 43.4 (d), 42.9 (t), 32.3 (t), 32.2 (t), 31.4 (t),
31.3 (t), 31.0 (t), 30.7 (t), 26.6 (t), 26.5 (t), 25.6 (t), 25.4 (t). MS
m/ z M+ (388, 1), 306 (4), 224 (6), 145 (35), 131 (50), 117 (23),
105 (14), 104 (14), 92 (12), 91 (100). Anal. Calcd for C29H40
C, 89.62; H, 10.37. Found: C, 89.66; H, 10.47.
:
Syn th esis of 1-(p-Cycloh exylp h en yl)-5-p h en ylp en ta n e
(10a ). Thionyl chloride (1.2 mL, 16.8 mmol) was added to
5-phenylpentanoic (1.99 g, 11.2 mmol), and the mixture was
heated under reflux for 1 h. The excess of thionyl chloride
was removed in a rotary evaporator to leave the acid chloride,
which was used for the following step. To a mixture of
cyclohexylbenzene (1.14 g, 7.1 mmol) and the acid chloride was
added aluminum chloride (1.50 g, 11.2 mmol) in three portions.
After heating on a boiling water bath for 20 min, the reaction
mixture was poured onto ice-water, made alkaline by the
addition of a 10% NaOH solution, extracted with ether and
dried over magnesium sulfate. Solvent was removed to give
a reddish oil. Purification of the residue by column chroma-
tography (hexane/ ethyl acetate, 10/1) left 1-(p-cyclohexylphen-
yl)-5-phenyl-1-pentanone (17) (0.29 g, 13%). IR (CCl4): 1686
(CdO). 1H NMR (CDCl3): 1.2-1.5 (m, 5H), 1.6-2.0 (m, 9H),
2.5-2.8 (m, 1H), 2.7 (t, J ) 7 Hz, 2H), 3.1 (t, J ) 7 Hz, 2H),
7.1-7.4 (m, 7H), 7.9 (d, 2H). 13C NMR (CDCl3): 200.1 (s),
153.4 (s), 142.2 (s), 134.8 (s), 128.3 (d), 128.2 (d), 126.9 (d),
125.6 (d), 44.6 (d), 38.2 (t), 35.7 (t), 34.0 (t), 31.1 (t), 26.7 (t),
26.0 (t), 24.0 (t). HRMS Calcd for C23H28O: 320.2140.
Found: 320.2133.
To a solution of ketone 17 in 20 mL of ethyl acetate, several
drops of perchloric acid and 0.30 g of palladium-carbon
catalyst were added. Hydrogenation was performed under
pressure (12 kg/cm2) during 2 h. The mixture was extracted
with ether and dried over magnesium sulfate. Solvent was
removed to give 1-(p-cyclohexylphenyl)-5-phenylpentane as a
colorless oil (0.25 g, 89%), which was further purified by HPLC
(hexane). 1H NMR (CDCl3): 1.1-1.5 (m, 7H), 1.5-1.9 (m, 9H),
2.5-2.7 (m, 5H), 7.0-7.2 (m,7H), 7.2-7.3 (m, 2H). 13C NMR
(CDCl3): 145.4 (s), 143.0 (s), 140.1 (s), 128.4 (d), 128.2 (d), 126.6
(d), 125.6 (d), 44.1 (d), 35.9 (t), 35.5 (t), 34.5 (t), 31.4 (t), 29.1
(t), 27.0 (t), 26.2 (t). MS m/ z M+ (306, 26), 173 (21), 159 (48),
145 (22), 131 (16), 117 (25), 105 (10), 92 (15), 91 (100). Anal.
Calcd for C23H30: C, 90.13; H, 9.87. Found: C, 89.98; H, 10.04.
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; HG, Project no. PB93-
0380 and J PP Project no. PR95-289) are gratefully
acknowledged. J .C.S. is the recipient of a Killam
Fellowship awarded by the Canada Council. Thanks
are also due to Dr. J . T. Banks for his contribution to
some of the early experiments.
J O960033Q