Photochemistry of o-pyrrolylstilbenes and formation of spiro-2H-pyrroles and their rearrangement to dihydroindoles

Article abstract of DOI:10.1021/jo061435t

(Chemical Equation Presented) Excited states of stilbenylpyrroles 1a-1c deactivate by two photochemical processes: cis-trans-isomerization and hydrogen transfer of NH to the stilbene double bond. NH-transfer results in the formation of two quinone dimetha

Full text of DOI:10.1021/jo061435t

Photochemistry of o-Pyrrolylstilbenes and Formation of
Spiro-2H-pyrroles and Their Rearrangement to Dihydroindoles
,†
‡
,§
Nikola Basari c´ ,* Zˇ eljko Marini c´ , and Marija Sˇ indler-Kulyk*
i
DiVision of Organic Chemistry and Biochemistry, and NMR Center, Institute Ruder Bo sˇ koVi c´ , Bijeni cˇ ka
cesta 54, 10 000 Zagreb, Croatia, and Department of Organic Chemistry, Faculty of Chemical
Engineering and Technology, Maruli c´ eV trg 19, 10 000 Zagreb, Croatia
nbasaric@irb.hr; Marija.Sindler@fkit.hr
ReceiVed July 10, 2006
Excited states of stilbenylpyrroles 1a-1c deactivate by two photochemical processes: cis-trans-
isomerization and hydrogen transfer of NH to the stilbene double bond. NH-transfer results in the formation
of two quinone dimethane intermediates, 10 and 11, and biradicals 12. Intramolecular cyclization of
intermediates 10-12 gives rise to polycyclic compounds spiro-2H-pyrroles 7, pyrroloisoindoles 3, and
pyrroloisoquinolines 8. Spiro-2H-pyrroles 7 rearrange on silica gel, giving dihydroindoles 2.
Introduction
quite significant progress has been documented in previous
decades on the synthesis of pyrroloisoindoles, pyrroloisoquino-
2
Synthesis of polycyclic compounds that contain pyrrole or
indole subunits has attracted considerable attention, mainly
because of the interesting biological properties of these com-
3
4
lines, and benzoindoles.
In continuation of our research on the synthesis and photo-
5
6
chemistry of furan and pyrrole derivatives of o-divinylben-
zenes, we discovered a convenient photochemical method for
the transformation of stilbenylpyrroles to indole and isoindole
1
pounds. Besides numerous reports on the synthesis of porfirines,
†
Division of Organic Chemistry and Biochemistry, Institute Ru de r Bo sˇ kovi c´ .
NMR Center, Institute Ruder Bo sˇ kovi c´ .
Faculty of Chemical Engineering and Technology.
7
‡
§
i
derivatives. It was revealed that, after photolysis of stilbe-
nylpyrrole (SP) 1a both dihydroindole (DHI) 2a and pyrrolo-
isoindole (PI) 3a were isolated. We proposed a mechanism for
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G. A., Ed.; The Alkaloids; Academic Press: San Diego, CA, 2002; Vol. 59.
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e) Gupton, J. T. Top. Heterocycl. Chem. 2006, 2, 53-92.
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(
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10.1021/jo061435t CCC: $33.50 © 2006 American Chemical Society
9
382
J. Org. Chem. 2006, 71, 9382-9392
Published on Web 11/16/2006

Photochemistry of o-Pyrrolylstilbenes
In this paper we explore in greater detail the intramolecular
photochemical addition of pyrrole to the stilbene double bond
by changing substituents on the para position of the stilbene
moiety. It was anticipated that the change of substituents may
lead to improved regioselectivity of the reaction and applicability
for the synthesis of DHI or PI. Moreover, an investigation of
the mechanism of intramolecular photochemical addition of
pyrrole to the stilbene double bond by product analysis and
spectroscopic methods is presented.
Results and Discussion
Synthesis. The N-Boc-protected stilbenylpyrroles (6a-c)
were prepared by a Stille reaction following the procedure
the photochemical transformation that included an intramolecular
electron transfer followed by an intramolecular NH hydrogen
transfer. A similar mechanism has also been proposed for the
1
0
7
described in the literature from N-Boc tin derivative 4 and
1
1
the corresponding o-bromostilbenes 5 (eq 1). The removal of
6
8
other examples of intramolecular and intermolecular addition
of pyrrole and amines to double bonds. Whereas the thorough
investigation of the mechanism of the photochemical addition
9
of amines has been carried out, photochemical addition of
pyrrole to double bonds is still under investigation, especially
the intramolecular version.
(4) For example: (a) Katritzky, A. R.; Wang, Z.; Savage, G. P. J.
Heterocycl. Chem. 1991, 28, 321-323. (b) Brown, F. C. R.; Coulston, K.
J.; Eastwood, F. W.; Moffat, M. R. Tetrahedron 1992, 36, 7763-7774. (c)
Suginome, H.; Sakurai, H.; Sasaki, A.; Tekeuchi, H.; Kobayashi, K.
Tetrahedron 1994, 50, 8293-8300. (d) Pinna, G. A.; Prisi, M. A.; Paglietti,
G. Farmaco 1994, 49, 121-126. (e) Macchia, M.; Manera, C.; Nencetti,
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Gonzalez, E.; Pindur, U.; Schollmeyer, D. J. Chem. Soc. Perkin Trans. 1
1
996, 1767-1771. (g) Kobayashi, K.; Taki, T.; Kawakita, M.; Uchida, M.;
Morikawa, O.; Konishi, H. Heterocycles 1999, 51, 349-354.; (h) Barolo,
S. M.; Lukach, A. E.; Rossi, R. A. J. Org. Chem. 2003, 68, 2807-2811.
(5) (a) Sˇ indler-Kulyk, M.; Sˇ poljari c´ , L.; Marini c´ , Zˇ . Heterocycles 1989,
the Boc group and preparation of SPs was accomplished by
treatment of 6 with a solution of NaOMe in MeOH. In that
way mixtures of cis- and trans-SPs (1a-c) were obtained, which
were separated by column chromatography on silica gel.
Preparative Irradiations. The prepared SPs 1a-c were
characterized by absorption bands between 280 and 350 nm,
which allowed for preparative photolyses by irradiating com-
pounds at 300 nm, in Ar-purged benzene or acetonitrile solutions
2
9, 679-682. (b) Sˇ indler-Kulyk, M.; Metelko, B.; Stiplo sˇ ek, Z. Croat.
Chem. Acta 1989, 62, 81-87. (c) Sˇ indler-Kulyk, M.; Stiplo sˇ ek, Z.; Vojnovi c´ ,
D.; Metelko, B.; Marini c´ , Zˇ . Heterocycles 1991, 32, 2357-2363. (d) Sˇ indler-
Kulyk, M.; Kragol, G.; Piantanida, I.; Tom sˇ i c´ , S.; Vujkovi c´ , Cvijin, I.;
Marini c´ , Zˇ .; Metelko, B. Croat. Chem. Acta 1996, 69, 1593-1602. (e)
Vujkovi c´ Cvijin, I.; Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. Spectrosc. Lett. 1998,
3
1, 989-1000. (f) Sˇ indler-Kulyk, M.; Sˇ kori c´ , I.; Tom sˇ i c´ , S.; Marini c´ , Zˇ .;
Mrvo sˇ -Sermek, D. Heterocycles 1999, 51, 1355-1369. (g) Sˇ kori c´ , I.;
Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. Heterocycles 2000, 53, 55-68. (h) Sˇ kori c´ ,
I.; Basari c´ , N.; Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. Heterocycles 2001, 55, 1889-
-
3
(∼1 × 10 M). Besides preparative irradiations, in the case of
1
896. (i) Sˇ kori c´ , I.; Hutinec, A.; Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. ARKIVOC
2
003, 14, 87-97. (j) Sˇ kori c´ , I.; Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. Croat. Chem.
1a, the course of the photochemical reaction was followed by
irradiating a C6D6 solution in a NMR tube and by recording
the NMR spectra after each irradiation. Short irradiations (a few
minutes in a NMR tube) resulted only in cis-trans-isomeriza-
tions. After 10 min of photolysis, the conversion of the cis- to
the trans-isomer and vice versa was the same, at approximately
∼20%. Since the rate of isomerization the cis-isomer to trans
and vice versa is the same, this indicates that the reaction does
not depend on the starting material. On prolonged irradiation,
there is also a competing formation of spiro-2H-pyrroles 7a (SI),
the rate of which also did not depend on the starting material
Acta 2004, 77, 161-166. (k) Sˇ kori c´ , I.; Basari c´ , N.; Marini c´ , Zˇ .; Sˇ indler-
Kulyk, M.; Vi sˇ njevac, A.; Koji c´ -Prodi c´ , B. Chem. Eur. J. 2005, 11, 543-
5
51.
(
6) (a) Sˇ indler-Kulyk, M.; Tom sˇ i c´ , S.; Marini c´ , Zˇ .; Metelko, B. Recl.
TraV. Chim. Pays-Bas 1995, 114, 476-479. (b) Basari c´ , N.; Tom sˇ i c´ , S.;
Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. Tetrahedron 2000, 56, 1587-1593. (c)
Basari c´ , N.; Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. Tetrahedron Lett. 2001, 42,
3
641-3643. (d) Vi sˇ njevac, A.; Basari c´ , N.; Koji c´ -Prodi c´ , B.; Sˇ indler-Kulyk,
M. Acta Crystallogr. E 2001, 57, 01252-01254. (e) Rademacher, P.;
Basari c´ , N.; Kowski, K.; Sˇ indler-Kulyk, M. Eur. J. Org. Chem. 2002, 551-
5
56. (f) Basari c´ , N.; Vi sˇ njevac, A.; Koji c´ -Prodi c´ , B.; Griesbeck, A.; Sˇ indler-
Kulyk, M. Photochem. Photobiol. Sci. 2002, 1, 1017-1023. (g) Basari c´ ,
N.; Ivekovi c´ , D.; Zimmermann, B.; Marini c´ , Zˇ .; Kowski, K.; Rademacher,
P.; Sˇ indler-Kulyk, M. J. Photochem. Photobiol. A 2003, 154, 123-130.
(cis- or trans-1a). Thus, irradiation of 1a does not result in the
(
7
h) Basari c´ , N.; Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. J. Org. Chem. 2003, 68,
formation of a photostationary state. The formation of products
7 is in competition with the observed isomerization.
Prolonged irradiation of all SPs 1a-c, leading to the total
conversion of the starting material, gave similar results, ir-
524-7527.
7) Basari c´ , N.; Marini c´ , Zˇ .; Sˇ indler-Kulyk, M. Tetrahedron Lett. 2003,
4, 7337-7340.
8) (a) Kubota, T.; Sakurai, H. Chem. Lett. 1972, 923-926. (b) Kubota,
(
4
(
T.; Sakurai, H. Chem. Lett. 1972, 1249-1250. (c) Koshima, H.; Ichimura,
H.; Hirotsu, K.; Miyahara, I.; Wang, Y.; Matsuura, T. J. Photochem.
Photobiol. A:Chem. 1995, 85, 225-229. (d) Austin, M.; Covell, C.; Gilbert,
A.; Hendrickx, R. Liebigs Ann./Recueil 1997, 943-946.
(10) Groenendaal, L.; Peerlings, H. W. I.; Havinga, E. E.; Vekemans, J.
A. J. M.; Meijer, E. W. Synth. Met. 1995, 69, 467-470.
(11) (a) Bonini, F.; Tiecco, M. Gazz. Chim. Ital. 1966, 96, 1792-1805.
(b) de Meijere, A.; Song, Z. Z.; Lansky, A.; Hyuda, S.; Rauch, K.;
Noltemeyer, M.; K o¨ nig, B.; Knieriem, B. Eur. J. Org. Chem. 1998, 2289-
2299.
(9) (a) Lewis, F. D.; Crompton, E. M. SET Addition of Amines to
Alkenes. In CRC Handbook of Photochemistry and Photobiology; Horspool,
W., Lenci, F., Eds.; CRC Press: Boca Raton, FL, 2004. (b) Lewis, F. D.
AdV. Electron Transfer Chem. 1996, 5, 1-39.
J. Org. Chem, Vol. 71, No. 25, 2006 9383

Basari c´ et al.
SCHEME 1
TABLE 1. Isolated Yields of the Products after Photolysis of 1a-c
1
respective of the substituent on the stilbene moiety. H NMR
spectra of the crude photolysis mixtures revealed formation of
two major compounds, spiro-isomers 7 (∼50-70%), which were
formed in a ratio of 2:1. Besides 7, some smaller amounts of 3
(in CH3CN or C6H6) and Column Chromatography
2
3
8
9
1
1
1
aa
32.5
27.8
58.4
4.5
9.8
2.0
-d
9.5
-
6.7
b
d
b
3.8
(
∼3-10%), 8 (<5%), and 9 (∼3-10%) (Scheme 1) were also
c
e
c
traces
formed. We were especially interested in spiro-2H-pyrroles 7,
a
Irradiation for 30 min at 300 nm. b After 2 h of irradiation at 300 nm,
hitherto not observed, because of the occurrence of similar
c
1
2
10.9% of 1b was recovered by column chromatography. Irradiation for
compounds in nature and due to their biological activity. For
an example, derivatives of spiro-2H-pyrroles have been found
in Horsfieldia superba, a woody species that grows in South
East Asia, while the derivatives of spirotryprostatins have been
found in the fungus Aspergillus fumigatus. The later showed
inhibitory activity on the cell cycle progression, which is a useful
1
5 min at 300 nm. d Compound was not detected. e The isolated reduction
product did not contain Cl.
13
The protonated forms then undergo rearrangement (1,2-alkyl
shift), resulting in the more stable, tertiary benzylic carbonium
+
+
1
4
ions 2aH -2cH . The loss of a proton yields DHIs 2a-c,
property for the potential application for cancer treatment.
Moreover, spiro-2H pyrroles are intermediates in the biosyn-
which were isolated in moderate to good yields.
thesis of uroporphyrogen III, the important precursor in the
biosynthesis of heme, chlorophyll, and vitamin B12.
Even though we were not able to isolate isomers 7a-c, their
1
2f-h,15
1
13
structure was undoubtedly assigned according to H and
C
Even though spiro compounds 7 were formed as major
products, we were not able to isolate them. Any attempt to
isolate 7 by column chromatography resulted in their rearrange-
ment, giving DHI derivatives 2 in 30-60% yield. The yields
of the isolated products after photolyses and chromatographic
separations are presented in Table 1. Besides isolated compounds
and high molecular weight material that remained on chroma-
tography columns, some smaller amounts (<1%) of unidentified
products were detected, but no attempt was made to isolate and
characterize them. The instability of 7a-c on silica and their
transformation to DHIs can be explained by their basicity, where
NMR spectra of the crude mixtures after photolyses. The
aliphatic regions of the H NMR spectra of the major spiro
1
isomers were characterized by the presence of a doublet of
doublets corresponding to the vicinal H at δ ∼3.6 ppm, and
two doublets of doublets at 3.75 and 2.95 ppm corresponding
to geminal H. On the other hand, protons of the minor isomers
appear in the aliphatic region at δ 4.20 (vicinal H) and 3.10
ppm (corresponding to 2 geminal H). The important character-
1
istic of the aromatic regions of the H NMR of 7a-c (major
isomers) is the presence of three signals corresponding to the
protons of cycloimines: doublets at 5.8 (J ) 4.9 Hz), doublets
of doublets at 6.9 ppm (J ) 4.9 Hz, J ) 0.9 Hz), and broad
singlets at 7.3 ppm. The carbon signals of the cycloimine are
present at δ ∼162, 156, 130 (doublets), and ∼90 (singlet) ppm,
+
+
7
is protonated on the silica, giving 7aH -7cH (Scheme 2).
(
12) (a) James, J. N. G.; Williams, G. J. B. Can. J. Chem. 1972, 50,
2
407-2412. (b) Laurent, A.; Mison, P.; Nafti, A.; Pellissier, N. Tetrahedron
1
3
while the aliphatic region of C spectra shows the presence of
doublets at δ ∼38 and triplets at 54 ppm. The values of the
chemical shifts of the cycloimine carbons and protons are also
in accord with the values found in the literature for similar
Lett. 1978, 46, 4511-4514. (c) Jones, K.; Ho, T. C. T.; Wilkinson, J.
Tetrahedron Lett. 1995, 37, 6743-6744. (d) Escolano, C.; Jones, K.
Tetrahedron Lett. 2000, 41, 8951-8955. (e) Escolano, C.; Jones, K.
Tetrahedron 2002, 58, 1453-1464. (f) Spivey, A. C.; Frampton, C. S.;
Battersby, A. R. J. Chem. Soc. Perkin Trans. 1 1996, 2103-2110. (g)
Hawker, C. J.; Stark, W. M.; Spivey, A. C.; Raithby, P. R.; Leeper, F. J.;
Battersby, A. R. J. Chem. Soc. Perkin Trans. 1 1998, 1493-1508. (h)
Hawker, C. J.; Spivey, A. C.; Leeper, F. J.; Battersby, A. R. J. Chem. Soc.
Perkin Trans. 1 1998, 1509-1517.
12f-h
molecules.
Additional support to the assigned spiro structure
of compounds 7 and their acid-catalyzed rearrangement was
obtained by filtration of the crude photochemical mixture after
photolysis of 1c through a column filled with neutral aluminum
oxide. The filtration furnished a very small quantity of the
separated major stereoisomer of 7c, indicating that spiro
compounds are stable unless exposed to acidic condition or
silica. However, since we were not able to isolate pure isomers
(13) Jossang, A.; Jossang, P.; Hadi, H. A.; S e´ venet, T.; Bodo, B. J. Org.
Chem. 1991, 56, 6527-6530.
(
14) Cui, C.-B.; Kakeya, H.; Osada, H. Tetrahedron 1996, 39, 12651-
1
1
266.
(
37.
15) Mathewson, J. H.; Corwin, A. H. J. Am. Chem. Soc. 1961, 83, 135-
9384 J. Org. Chem., Vol. 71, No. 25, 2006

Photochemistry of o-Pyrrolylstilbenes
SCHEME 2
7
, the stereochemistry on the spiro carbons was not unambigu-
The photolysis of SPs was further investigated by changing
the polarity of the solvent. We have not observed any difference
in the formation of polycyclic compounds (no effort was made
to investigate the influence of solvent polarity on cis-trans-
isomerization) of SPs in solvents of different polarity (aceto-
nitrile or benzene). On the other hand, differently substituted
SPs showed pronounced differences in reactivity when irradiated
under the same conditions. It was found that the chloro-
substituted SPs (1c) were clearly the most reactive, ∼2 times
more reactive than 1a and ∼8 times more than 1b, based on
the conversion of the starting material over the same irradiation
time. This finding indicates that electron-withdrawing substit-
uents on the stilbene moiety facilitates an intramolecular
photochemical reaction giving 3, 7, and 8. Quantum yields for
the product formation were not determined due to the competi-
tive cis-trans-isomerization and the formation of high molecular
weight material.
ously assigned.
The structures of the isolated products 2, 3, 8, and 9 were
1
13
assigned according to H and C NMR and MS. DHIs 2a-c
are characterized by the presence of three signals in the aliphatic
1
region of H NMR which appear as ABX systems at δ ∼4.1,
3
.0, and 2.9-3.0 ppm. On the other hand, in the aromatic region
only two pyrrole C-H multiplets are present at 6.3 and 6.0
13
ppm. In the C NMR spectra, in the aliphatic region the doublets
and the triplets are present (∼40 and 39 ppm), while the
aromatic region clearly indicates the presence of only two
pyrrole doublets at δ ∼118 and 108 ppm. The DHI structures
2
a-c were additionally indicated by MS, where the character-
istic basic signal (m/z ) 168) after the loss of tolyl, methox-
yphenyl, or chlorophenyl group, respectively, was present.
The PIs 3a-c are characterized by the presence of three
1
signals in the aliphatic region of the H NMR spectrum at δ
Absorption and Fluorescence Measurements. In order to
get some further insight into the mechanism of the photochem-
cial transformation of SPs to polycyclic compounds 3, 7, and
∼
4.8, 2.9, and 2.6-2.8 ppm, while three pyrrole protons appear
1
3
as very narrow multiplets at δ ∼6.3-6.5 ppm. However,
C
NMR spectra clearly showed the pyrrole doublets at δ ∼116,
8
, some preliminary photophysical investigations were per-
1
12, and 98 ppm. In the aliphatic region the doublets at δ ∼62
formed. We recorded absorption and emission spectra of SPs
in several solvents. In the absorption spectra of the trans-isomers,
the maximum appeared at ∼280 nm with shoulders at 300 and
and the triplets at 41 ppm indicate the addition to the stilbene
double bond. The final evidence that pointed to the assigned
structure was obtained from MS, where the characteristic basic
signal (m/z ) 154) after cleavage of p-methylbenzyl, p-
methoxybenzyl, and p-chlorobenzyl was present.
The pyrroloisoquinolines (PIQ) 8 were formed only in minor
quantities. The highest yield of PIQ was observed on irradiation
of methoxy-SP (1b) (∼4%). The PIQ derivatives were charac-
terized by the presence of three signals in the aliphatic region
3
40 nm, whereas cis-isomers showed the presence of two
maxima at 280 and 300 nm. Similar spectra have also been
1
6
observed for cis- and trans-o-aminostilbene. The position of
the maxima in the absorption spectra of trans-1a-c and cis-1c
are presented in Table 2.
Whereas solvent polarity did not display any influence on
the absorption spectra of SPs 1a-c, solvatochromic effects were
observed in their emission spectra. For example, the emission
spectra of trans-1a obtained by recording fluorescence in four
aprotic solvents of different polarity are presented in Figure 1.
All the fluorescence spectra were broad and lacked any vibronic
structure, regardless of solvent polarity. The same behavior was
also observed for trans-SPs 1b and 1c. The biggest solvato-
chromic effects were observed for the chloro-substituted SPs
cis- and trans-1c and the smallest for methoxy-SP 1b. Stokes
shifts were not determined due to the difficulties caused by
1
of H NMR at δ ∼4.6, 2.8, and 2.6 ppm. The most important
evidence that points to the presence of the PIQ structure is the
characteristic fragmentation in MS, namely the cleavage of the
chlorophenyl or methoxyphenyl group.
The reduction products 9a and 9c were characterized by the
1
presence of multiplets in H NMR at δ ∼2.9 and 2.7 ppm and
1
3
the triplets in C NMR at ∼37 and 35 ppm. The structure of
the reduction products was additionaly indicated by their MS.
It should be mentioned that photolysis of SP 1c furnished
reduction product without a chlorine atom. The cleavage of
chlorine in the formation of the reduction product is not
surprising when taking into account the radical mechanism of
the reaction (vide infra) and the relatively low dissociation
energy of the C-Cl bond.
(
16) (a) Lewis, F. D.; Kalgutkar, R. S.; Yang, J.-S. J. Am. Chem. Soc.
1
999, 121, 12045-12053. (b) Lewis, F. D.; Kalgutkar, R. S. J. Phys. Chem.
A 2001, 105, 285-291.
(17) Olmsted, J., III. J. Phys. Chem. 1979, 83, 2581-2584.
J. Org. Chem, Vol. 71, No. 25, 2006 9385

Basari c´ et al.
TABLE 2. Photophysical Properties of SPs 1a-c
λabs
λem
(max/nm)
(max/nm)a
b
φf
c
compd
solvent
trans-1a
cyclohexane
CH3CN
CH3OH
cyclohexane
CH3CN
CH3OH
cyclohexane
CH3CN
CH3OH
cyclohexane
CH3CN
CH3OH
277
277
277
280
281
281
278
277
277
404
453
451
402
444
445
408
474
468
411
469
475
0.27
0.42
0.54
0.22
0.37
0.48
0.30
0.38
0.50
0.07
0.14
0.16
trans-1b
trans-1c
cis-1c
277
277
278, 298
a
Maxima in the absorption spectra. b Maxima in the emission spectra.
c
Quantum yield of fluorescence is measured by using quinine sulfate as a
reference (Φ ) 0.55 in 1.0 N H2SO4).17 Estimated error is (0.05.
FIGURE 1. Normalized fluorescence spectra of SP 1a recorded in
four solvents: cyclohexane, toluene, THF, and acetonitrile (λex ) 300
nm).
TABLE 3. Decay Times of SPs Measured by Using the Single
Photon Timing Technique
overlapping of the lowest energy absorption transition with those
of higher energy.
compd
solvent
τ1/ns
τ2/ns
1a
cyclohexane
CH3CN
cyclohexane
CH3CN
0.5 ( 0.2
3.3 ( 0.2
0.5 ( 0.1
3.0 ( 0.2
0.3 ( 0.1
0.3 ( 0.1
and 5 ( 1
4.3 ( 0.1
7.9 ( 0.1
1.7 ( 0.1
6.6 ( 0.1
4.4 ( 0.1
8.9 ( 0.2
SPs 1a-c are characterized by a mezomeric electron-donating
pyrrole unit and an electron-withdrawing stilbene moiety,
revealing that this excitation probably possesses pyrrolefstilbene
charge-transfer character. Therefore substitution of the stilbene
moiety by an electron-withdrawing (chloro) substituent would
further enhance the charge-transfer character of the excited state,
while an electron-donating substituent (OMe) would decrease
this effect. It is interesting to note that the higher polarity of
the excited state (as it is the case with chloro substituent)
coincides with the higher reactivity of SP toward intramolecular
reaction, giving polycyclic compounds 7, 3, and 8.
1b
1c
cyclohexane
CH3CNa
a
The fluorescence decay was fitted using a three-exponential function.
lower quantum yield of SPs fluorescence in cyclohexane might
be attributed to a lower solubility of SPs in that solvent.
Although we have not observed the influence concentration on
the quantum yield of fluorescence, fluorescence quenching due
to the aggregation in nonpolar solvent cannot be totally
disregarded. The position of the maxima in the emission spectra
of SPs, as well as the associated quantum yields of fluorescence,
are presented in Table 2.
Contrary to the usual emission properties of the stilbene
derivatives, which can undergo cis-trans-isomerization, SPs
were characterized by quite strong emissions of trans-isomers
with quantum yields of fluorescence of 0.20-0.60 and some-
what lower quantum yields of fluorescence for the cis-isomers.
Moreover, to our surprise, quantum yields of fluorescence
increased in solvents of higher polarity, which appears to be in
contradiction with the observed solvatochromic effects in the
emission spectra. However, such a behavior was seen in some
To further investigate the photophysics of SPs, we measured
singlet lifetimes in nonpolar (cyclohexane) and polar solvent
(CH CN). Fluorescence decays were measured by a single
3
1
8
systems such as push-pull 2,6-disubstituted anthracenes or
photon counting technique and analyzed by global analysis
fitting for the decay times and associated preexponential
3
,5-dimethoxystilbenes.19 Higher fluorescence quantum yields
(
in more polar solvents might be explained by relatively small
solvent reorganization due to charge-transfer stabilization. In
that way some other nonradiative processes (such as cis-trans-
isomerization or intersystem crossing that may take place with
lower quantum yields in solvents of higher polarity) might
compensate for the loss in energy due to solvent reorganization,
resulting in an overall increase of the quantum yield of
fluorescence. Since shifting of the maxima in the emission
spectra of SPs (by increasing polarity) is not that big, there is
no extra stabilization of the emissive state via large amplitude
nuclear motion. The similar solvatochromic behavior was seen
for trans-o-aminostilbene, where the position of the maximum
in the fluorescence spectrum shifted from 407 nm in cyclohex-
ane to 445 nm in CH3CN. However, the quantum yield of
fluorescence of o-aminiostilbene decreased from 0.88 to 0.69
factors). Since we were able to perform measurements on a setup
with nanosecond time resolution, we could not get information
about the solvation effects taking place on picosecond and
subpicosecond time scales. However, measured lifetimes in
combination with quantum yields of fluorescence could provide
the values of the rate constants of radiative and nonradiative
deactivation pathway. Analysis of the SPs fluorescence decays
revealed that none could be fitted by a one-exponential function
(
giving one lifetime), so a second exponent was needed to be
2
introduced (as judged by øg and plots of the weighted residuals
and autocorrelation functions). For 1c in CH3CN, a third decay
time was necessary. The decay times of SPs obtained by global
analysis are presented in Table 3.
As can be seen, SPs 1a-c are characterized by relatively
long decay times, which is not usual for the stilbenes character-
ized by double bonds that can undergo cis-trans-isomerization.
However, it coincides with high quantum yields of fluorescence
and is in accord with similarly long lifetimes that were measured
for ortho- or meta-substituted amino- and methoxystilbenes1
16
by going from cyclohexane to acetonitrile. In addition, the
(
18) Bohne, C.; Ihmels, H.; Weidelich, M.; Yihwa, C. J. Am. Chem.
Soc. 2005, 127, 17158-17159.
19) Roberts, J. C.; Pincock, J. A. J. Org. Chem. 2006, 71, 1480-1492.
6,19
(
9386 J. Org. Chem., Vol. 71, No. 25, 2006

Photochemistry of o-Pyrrolylstilbenes
FIGURE 2. (a) Transient absorption spectra of SPs 1a-c recorded in O
2
-purged CH
and O2.
3
CN, immediately after the laser excitation. (b) Decay of
transient absorbance at 520 nm recorded for SP 1b in the presence of N
2
and sterically hindered stilbenes.20 The existence of more than
one decay time in the fluorescence decay may be explained by
the existence of more than one emissive tautomer. Such
conformational equilibria and the existence of more than one
emissive tautomer are quite common in unsymmetrical stil-
benes. Hence, the thorough investigation of the dependence
of decay times and quantum yields of fluorescence on temper-
ature, viscosity, and excitation wavelengths was not in the scope
of this manuscript.
Laser Flash Photolysis (LFP). Nanosecond LFP was used
to probe for intermediates formed on photolyses of SPs 1a-c,
with the transient absorption spectra recorded in N2 and O2
purged CH3CN solutions. The obtained spectra, recorded
immediately after the laser pulses, were characterized by the
presence of strong transient absorptions in the whole visible
range with the broad maxima between 450 and 550 nm (Figure
reported for the radical cation, and the decay could not be fitted
to a one-exponential function. Therefore, it was evident that
the observed transient absorption corresponded to several
species. Since we have not observed shortening of the decay in
the presence of oxygen, we may tentatively assign the transient
absorption to the presence of quinone dimethanes 10a and 11a
(Scheme 3) and not to the presence of biradicals or triplets.
Moreover, the similarity of the observed transient absorptions
to those corresponding to cis- and trans-enoles obtained by
H-abstraction reaction in the derivatives of 2-methylbenzophe-
2
1
2
4
nones encouraged us to tentatively assign the shorter lifetime
(1.4 µs) to 10a and the longer one (20-30 µs) to 11a.
Transients obtained by LFP (in N purged CH CN) of 1b
2
3
were more complex than those of 1a. They were characterized
by the decays that were fitted to the sum of minimally four
exponential functions characterized by the rate constants k )
1
6
-1
5
-1
2) and additional bands at wavelengths shorter than 375 nm.
(2.0 ( 0.3) × 10 s (τ ∼ 510 ns), k ) (7.5 ( 0.4) × 10 s
2
-1
4
The intensity of the observed bands at shorter wavelengths was
reduced, due to the reversible bleaching caused by the absorption
of the laser flash by the substrate. In order to decrease the
probability of two-photon excitation resulting in the formation
(τ ∼ 1.3 µs), k ) (9 ( 1) × 10 s (τ ∼ 11 µs), and k ∼ 3
3 -1
3
4
× 10 s (τ ∼ 330 µs). Contrary to the other SPs, transients
obtained from 1b were affected by O . Namely, the shortest-
2
living transient species was not detected when the solution was
22
of stilbene radical cations, laser pulses were kept at low power
<30 mJ per pulse). However, two-photon excitation could not
purged with O (Figure 2b). The following rate constants were
2
(
obtained for the decay of the transient absorbance in O -purged
2
5
-1
be totally suppressed and to some extent contributed to the
transient absorption intensity.
CH CN solution: k ) (8.0 ( 0.3) × 10 s (τ ∼ 1.2 µs), k
3
1
4
2
-1
3
-1
) (7.8 ( 0.4) × 10 s (τ ∼ 13 µs), and k ∼ 5 × 10 s (τ
3
Transient absorption spectra of SP 1a and decay of the
transient absorbance were not affected by the presence of O2.
The observed transient absorption was fitted to the sum of three
exponential functions giving the following rate constants k1 )
∼ 200 µs). The disappearance of the short-lived transient in
the presence of O encouraged us to tentatively assign it to the
2
presence of biradical 12b, while the other three transients were
tentatively assigned to the presence of intermediates 10b (1.2
5
-1
5
-1
22
(
(
7.3 ( 0.2) × 10 s (τ ∼ 1.4 µs), k2 ) (1.5 ( 0.1) × 10 s
τ ∼ 6.6 µs), and k3 ∼ 4 × 10 s (τ ∼ 20-30 µs). The position
µs), stilbene radical-cation (13 µs), and 11b (∼200 µs).
4
-1
Substitution of the stilbene moiety by an electron-donating
substituent partly stabilizes the formed biradical 12b, and
therefore, this biradical has a longer lifetime than 12a and 12c.
Transient absorption spectra of chloro SP 1c were also
characterized by the presence of decays that were fitted to a
sum of minimally three exponential functions, giving three rate
constants (or lifetimes). The following rate constants were
of the maximum at 483 nm and the associated lifetime of ∼6-7
2
2,23
µs is in accord with the formation of stilbene radical-cation.
However, transient absorption spectra were much broader than
(20) (a) Saltiel, J.; Charlton, J. L. Rearrangements in ground and excited
states; de Mayo, P., Ed.; Academic Press: New York, 1980. (b) Dutt, G.
B.; Konitsky, W.; Waldeck, D. H. Chem. Phys. Lett. 1995, 245, 437-440.
6
-1
revealed: k1 ) (1.0 ( 0.1) × 10 s (τ ∼ 1.0 µs), k2 ) (8.4
(
c) Penn, J. H.; Gan, L.-X.; Eaton, T. A.; Chan, E. Y.; Lin, Z. J. Org.
4
-1
3
-1
Chem. 1988, 53, 1519-1523. (d) Lewis, F. D.; Hoyle, C. E. J. Am. Chem.
Soc. 1977, 11, 3779-3786.
( 0.5) × 10 s (τ ∼ 12 µs), and k3 ∼ 5 × 10 s (τ ∼ 200
µs). In the presence of O2, neither of the transients showed
(
21) (a) Mazzucato, U. Pure Appl. Chem. 1982, 54, 1705-1721. (b)
Saltiel, J.; Sun, Y.-P. Photocromism Molecules and Systems; Durr, H.,
Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990.
(23) Samori, S.; Hara, H.; Tojo, S.; Fujitsuka, M.; Majima, T. J.
Photochem. Photobiol. A: Chem. 2006, 179, 115-124.
(22) (a) Hara, M.; Samori, S.; Xichen, C.; Fujitsuka, M.; Majima, T. J.
Org. Chem. 2005, 70, 4270-4374. (b) Miranda, M. A.; Angeles, Izquierdo,
M.; Galindo, F. J. Org. Chem. 2002, 67, 4138-4142. (c) Majima, T.; Tojo,
S.; Ishida, A.; Takamuku, S. J. Org. Chem. 1996, 61, 7793-7800. (d)
Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401, 449.
(24) (a) Konosonokos, A.; Wright, P. J.; Tsao, M.-L.; Pika, J.; Novak,
K.; Mandel, S. M.; Krause, Bauer, J. A.; Bohne, C.; Gudmundsd o´ ttir, A.
D. J. Org. Chem. 2005, 70, 2763-2770. (b) Johnston, L. J.; Scaiano, J. C.
Chem. ReV. 1989, 89, 521-547.
J. Org. Chem, Vol. 71, No. 25, 2006 9387

Basari c´ et al.
SCHEME 3
significant shortening of their lifetime. Thus, we tentatively
assigned the transient absorptions to the presence of 10c (1 µs),
stilbene radical cation (12 µs), and 11c (200 µs). However, with
the data at hand, we can only tentatively assign the observed
transients to 10a-c, 11a-c, and 12b, with the data from LFP
consistent with the proposed mechanism (vide infra). In order
to verify assignments of transients, quenching experiments
should be performed, which is difficult due to the concomitant
formation of stilbene radical cations.
conjugated neutral intermediate molecules, 10a-c and 11a-c.
However, formation of intermediates 10 and 11 may also be
explained by a [1,9]-hydrogen shift, especially in the case of
11a-c, where the H-atom transfer is less probable due to the
longer distance. The intermediates 10a-c decay by two
processes: by intramolecular cyclization, giving 3a-c, and
reverse [1,9]-hydrogen shift (giving starting material). Due to
the steric hindrance caused by the two hydrogens, intramolecular
cyclization from 11 to pyrroloindenes is less favored and
probably does not take place. Thus, 11a-c probably decay only
by the [1,9]-hydrogen shift, thereby yielding the longer lifetime.
Besides isolation of products 3a-c, formation of 10 and 11
was also indicated by LFP experiments, where we saw the
presence of the short-lived transients tentatively assigned to 10
and longer-living transients tentatively assigned to 11 (vide
supra).
Reaction Mechanism. From the present product study,
fluorescence, and LFP measurements we may propose a reaction
mechanism for the photochemical transformation of SPs 1a-
c, which is presented in Scheme 3. Electronic excitation of SPs
results in the population of singlet states that have substantial
charge-transfer character. Polar singlet excited states of 1a-c
can deactivate by photophysical processes and by two competing
photochemical pathways: cis-trans-isomerization and NH-
hydrogen transfer. Due to the existence of at least two
conformers in the ground state, excitation leads to the formation
of more than one excited-state conformer, the presence of which
was indicated by the biexponential decay of fluorescence. One
of the conformers with the pyrrole NH oriented toward the
stilbene double bond has a shorter singlet lifetime, possibly
attributed to the higher efficiency of intramolecular hydrogen
transfer of NH to the stilbene double bond. Hydrogen transfer
is not specific and results in formation of the three intermediates
Proton transfer to the R-position of the stilbene double bond
and formation of biradicals 12 was highly indicated by the
isolation of PIQ compounds 8. On the other hand, formation of
spiro products 7 seems somewhat surprising at first, but it is
well-supported by the resonant stabilization of 12. Thus, benzylic
radical form 12′ probably has higher contribution in the
resonance than its N-radical form 12. Therefore, recombination
of 12 and 12′ gives a higher yield of spiro comound 7 than PIQ
8. Moreover, radical cyclization in the similar system described
by Jones et al. also resulted primarily in the formation of five-
membered rings and spiro compounds, rather than six-membered
1
0-12.
1
2c
In the case of H-transfer to the â-C-atom of the stilbene
double bond, reaction from the singlet state gives rise to two
rings. Additional indication for the presence of biradical 12
was obtained from the LFP data of 1b. Namely, we saw the
9388 J. Org. Chem., Vol. 71, No. 25, 2006

Photochemistry of o-Pyrrolylstilbenes
presence of a very short-lived transient in N2-purged CH3CN,
which was tentatively assigned to 12b and which disappeared
upon exposure to O2. Since 12a-c are true biradicals (unlike
the solvent was evaporated, and the brown residue was chromato-
graphed on a column with silica gel using CH Cl /petroleum ether
1:4) as eluent. In the first fraction, unreacted stilbene derivatives
were isolated, followed by the isolation of the mixture of cis- and
trans-N-tert-butoxycarbonyl-2-{2-[2-(4-substituted phenyl)ethenyl]-
phenyl}pyrrole.
2
2
(
1
0 and 11) in a singlet state, they should react with O2 at a rate
25
that is diffusion controlled. The very short lifetime of these
biradicals (12a-c) is not surprising, since they can probably
react very efficiently in an intramolecular process, giving cis-
and trans-7. Only stabilization with an electron-donating group
A mixture of 2.47 g (98%) of cis- and trans-N-tert-butoxycar-
bonyl-2-{2-[2-(4-methylphenyl)ethenyl]phenyl}pyrrole (6a) was
1
obtained. cis-6a: colorless oil; H NMR (C
7
6 6
D
) δ/ppm (300 MHz)
(as in the case of 12b) is expected to induce a longer lifetime
.55 (m, 1H), 7.54 (d, 1H, J ) 7.8 Hz), 7.39 (d, 2H, J ) 8.1 Hz),
of these species. Besides formation of cyclic products 7 and 8,
biradicals 12 probably also undergo intermolecular hydrogen
abstraction or C-Cl bond cleavage (in the case of 12c), giving
reduction products 9a and 9c, respectively.
Substitution of the stilbene moiety has significant influence
on the degree of SP photochemical reactivity. Even though
photolyses furnish the same type of products, the photochemical
reaction is more efficient (vide supra) and selective with
electron-withdrawing substituents on the stilbene moiety. Thus,
photolysis of 1c is the most selective giving products 7c and
7.33 (d, 1H, J ) 7.5 Hz), 7.05 (dd, 1H, J ) 7.5 Hz, J ) 7.8 Hz),
6.98 (dd, 1H, J ) 7.5 Hz, J ) 7.8 Hz), 6.92 (d, 2H, J ) 8.1 Hz),
6.53 (d, 1H, J ) 12.6 Hz), 6.45 (d, 1H, J ) 12.6 Hz), 6.25 (m,
1
9
H), 6.22 (dd, 1H, J ) 3.3 Hz, J ) 3.6 Hz), 2.09 (s, 3H), 1.19 (s,
H); 13C NMR (C
D
6
) δ/ppm (75 MHz) 149.6 (s), 138.5 (s), 137.3
6
(s), 135.5 (s), 135.0 (s), 134.2 (s), 131.2 (d), 130.9 (d), 129.9 (d),
1
29.6 (d), 129.5 (d), 129.3 (d), 127.8 (d), 127.3 (d), 122.5 (d), 115.4
-
1
(
(
(
d), 111.4 (d), 83.1 (s), 27.9 (q), 21.6 (q); IR (oil) νmax/cm 1752
1
CO), 1739 (CO). trans-6a: colorless oil; H NMR (C
300 MHz) 7.71 (dd, 1H, J ) 2.4 Hz, J ) 2.7 Hz), 7.62 (d, 1H, J
7.8 Hz), 7.32 (d, 2H, J ) 8.1 Hz), 7.31 (d, 1H, J ) 6.9 Hz),
6 6
D ) δ/ppm
)
3
c in a ratio of ∼30:1. On the other hand, methoxy-substituted
7.28 (d, 1H, J ) 16.5 Hz), 7.20 (dd, 1H, J ) 7.8 Hz, J ) 7.5 Hz),
7.10 (dd, 1H, J ) 6.9 Hz, J ) 7.5 Hz), 7.05 (d, 1H, J ) 16.5 Hz),
6.93 (d, 2H, J ) 8.1 Hz), 6.23 (d, 2H, J ) 2.7 Hz), 2.09 (s, 3H),
SP 1b gave photoproducts 7b and 3b in a ratio of ∼3:1. The
reason for the higher efficiency and selectivity with electron-
withdrawing groups probably lies with the formation of a more
polar excited state (which was indicated by fluorescence
measurements). The driving force for the NH hydrogen transfer
is probably more in such a polar excited state, explaining the
higher efficiency of the reaction. On the other hand, selectivity
of the H-transfer to the R or â position is explained by the
competing substituent effects of the pyrrole unit and the stilbene
moiety. Thus, the more polar excited-state of 1c (as compared
to 1a or 1b) is probably characterized by a bigger difference in
electron density on the R- and â-position of the stilbene double
bond, resulting in a more selective hydrogen transfer to the
R-position.
1
.10 (s, 9H); 13C NMR (C
6
D
6
) δ/ppm (75 MHz) 149.8 (s), 138.4
(s), 137.9 (s), 135.5 (s), 135.1 (s), 133.5 (s), 131.5 (d), 130.5 (d),
1
1
1
30.1 (d), 127.23 (d), 127.18 (d), 126.6 (d), 125.2 (d), 122.6 (d),
-
1
15.8 (d), 111.5 (d), 83.3 (s), 27.6 (q), 21.5 (q); IR (oil) νmax/cm
+
738 (CO); MS m/z (%) 359 (30, M ), 303 (35), 258 (45), 167
: C 80.19, H 7.01, N
(50), 57 (100). Anal. Calcd for C24
H25NO
2
3.90. Found: C 80.36, H 6.76, N 4.15.
A mixture of 3.23 g (86.0%) of cis- and trans-N-tert-butoxy-
carbonyl-2-{2-[2-(4-methoxyphenyl)ethenyl]phenyl}pyrrole (6b)
1
was obtained. cis-6b: yellowish oil; H NMR (C D ) δ/ppm (300
6
6
MHz) 7.48 (dd, 1H, J ) 1.5 Hz, J ) 7.5 Hz), 7.47 (dd, 1H, J )
3.3 Hz, J ) 3.3 Hz), 7.33 (d, 2H, J ) 8.7 Hz), 7.29 (dd, 1H, J )
1
7
.5 Hz, J ) 7.5 Hz), 7.02 (ddd, 1H, J ) 1.5 Hz, J ) 7.5 Hz, J )
.5 Hz), 6.95 (ddd, 1H, J ) 1.5 Hz, J ) 7.5 Hz, J ) 7.5 Hz), 6.62
(
d, 2H, J ) 8.7 Hz), 6.41 (d, 1H, J ) 12.3 Hz), 6.35 (d, 1H, J )
Conclusion
1
2.3 Hz), 6.19 (dd, 1H, J ) 3.3 Hz, J ) 3.3 Hz), 6.16 (dd, 1H, J
3.3 Hz, J ) 3.3 Hz), 3.24 (s, 3H), 1.14 (s, 9H); 13C NMR (C
δ/ppm (75 MHz) 159.1 (s), 148.9 (s), 138.1 (s), 134.8 (s), 133.7
s), 130.6 (d), 129.9 (d), 129.6 (s), 128.9 (d), 127.6 (d), 127.2 (d),
26.6 (d), 121.8 (d), 114.7 (d), 113.6 (d), 110.7 (d), 81.2 (s), 55.0
)
6 6
D )
Excitation of SPs 1a-c to the singlet state gives rise to more
polar species, which can deactivate by photophysical processes
and two competing photochemical pathways: cis-trans-isomer-
ization and intramolecular NH hydrogen transfer. The NH
hydrogen transfer results in formation of biradicals and quinone
dimethane intermediates, which cyclize to spiro-2H-pyrroles 7,
PIs 3, and PIQ 8. The reaction of NH transfer is more efficient
and more selective (toward formation of 7) if the stilbene moiety
is substituted with an electron-withdrawing group. The major
photochemical products, spiro-2H-pyrroles 7, easily rearrange
on silica, giving DHI derivatives 2.
(
1
-
1
(q), 27.9 (q); IR (oil) νmax/cm 1751 (CO); MS m/z (%) 375 (55,
+
M ), 319 (55), 274 (68), 168 (45), 84 (50), 57 (100). Anal. Calcd
for C24 : C 76.77, H 6.71, N 3.73. Found: C 76.71, H 7.04,
H25NO
3
1
N 3.94%. trans-6b: yellowish crystals; mp 88-89 °C; H NMR
(C D ) δ/ppm (300 MHz) 7.67 (dd, 1H, J ) 3.0 Hz, J ) 3.0 Hz),
6
6
7.58 (d, 1H, J ) 7.8 Hz), 7.26 (d, 2H, J ) 7.8 Hz), 7.22-7.29 (m,
1H), 7.12-7.20 (m, 1H), 7.12 (d, 1H, J ) 16.2 Hz), 7.05 (dd, 1H,
J ) 7.2 Hz, J ) 7.5 Hz), 6.97 (d, 1H, J ) 16.2 Hz), 6.63 (d, 2H,
1
3
J ) 7.8 Hz), 6.19-6.22 (m, 2H), 3.25 (s, 3H), 1.05 (s, 9H);
NMR (C
(
C
D
6 6
) δ/ppm (75 MHz) 159.9 (s), 149.5 (s), 138.2 (s), 134.6
s), 133.2 (s), 131.2 (d), 130.6 (s), 129.7 (d), 128.1 (d), 128.0 (d),
Experimental Section
1
26.6 (d), 125.0 (d), 124.6 (d), 122.1 (d), 115.3 (d), 114.5 (d), 111.1
Synthesis of 4-Substituted 2-{2-[2-Phenylethenyl]phenyl}-
-
1
10
(d), 82.9 (s), 54.7 (q), 27.2 (q); IR (KBr) νmax/cm 1733 (CO).
pyrroles. Stille Reaction, General Procedure. N-Boc-pyrrolyl-
7
A mixture of 3.21 g (84.6%) of cis- and trans-N-tert-butoxy-
stannane (4) (0.01 mol) and 4′-substituted 2-bromostilbene (5a-
11
carbonyl-2-{2-[2-(4-chlorophenyl)ethenyl]phenyl}pyrrole (6c) was
obtained. cis-6c: yellowish oil, H NMR (C
c) (0.01 mol) were dissolved in toluene (15 mL). To the mixture
was added a 1 M aqueous solution of Na CO (15 mL) and the
solution was purged with argon. A spatula-point of Pd(PPh was
1
6 6
D ) δ/ppm (600 MHz)
2
3
3
)
4
7.43 (dd, 1H, J ) 1.8 Hz, J ) 3.0 Hz), 7.27 (d, 1H, J ) 7.5 Hz),
7.26 (d, 1H, J ) 7.5 Hz), 7.12 (d, 2H, J ) 8.4 Hz), 6.99 (ddd, 1H,
J ) 7.5 Hz, J ) 7.5 Hz, J ) 1.5 Hz), 6.95 (d, 2H, J ) 8.4 Hz),
6.89 (ddd, 1H, J ) 7.5 Hz, J ) 7.5 Hz, J ) 1.5 Hz), 6.42 (d, 1H,
J ) 12.0 Hz), 6.16 (d, 1H, J ) 12.0 Hz), 6.12-6.15 (m, 2H), 1.10
added and the reaction mixture heated under argon at reflux
temperature for 1 day. The layers were separated, and the water
layer was extracted three times with CH
layers were dried over anhydrous MgSO
2
Cl
2
. The combined organic
4
. The solution was filtered,
(
1
(
s, 9H); 13C NMR (C
D
6 6
) δ/ppm (150 MHz) 149.5 (s), 137.8 (s),
36.2 (s), 135.4 (s), 134.1 (s), 133.5 (s), 131.3 (d), 131.2 (d), 130.7
d), 129.4 (d), 129.3 (d), 128.9 (d), 127.9 (d), 127.6 (d), 122.6 (d),
(25) Miranda, M. A.; Font-Sanchis, E.; P e´ rez-Prieto, J.; Scaiano, J. C.
J. Org. Chem. 2001, 66, 2717-2721. (b) Font-Sanchis, E.; Miranda, M.
A.; P e´ rez-Prieto, J.; Scaiano, J. C. J. Org. Chem. 2002, 67, 6131-6135.
-
1
115.4 (d), 111.4 (d), 83.2 (s), 27.9 (q); IR (oil) νmax/cm 1750
J. Org. Chem, Vol. 71, No. 25, 2006 9389

Basari c´ et al.
1
(
CO). Anal. Calcd for C23
Found: C 72.32, H 5.99, N 4.03. trans-6c: yellowish oil; H NMR
) δ/ppm (600 MHz) 7.62 (dd, 1H, J ) 1.8 Hz, J ) 3.0 Hz),
H
22ClNO
2
: C 72.72, H 5.84, N 3.69.
orless crystals, mp 92-93 °C; H NMR (C
6
D
6
) δ/ppm (300 MHz)
1
7.47 (d, 1H, J ) 7.3 Hz), 7.36 (d, 1H, J ) 16.2 Hz), 7.30 (br s,
1H), 7.06-7.20 (m, 3H), 7.04 (d, 2H, J ) 8.4 Hz), 6.93 (d, 2H, J
) 8.4 Hz), 6.72 (d, 1H, J ) 16.2 Hz), 6.45 (ddd, 1H, J ) 1.5 Hz,
J ) 2.7 Hz, J ) 3.3 Hz), 6.39 (ddd, 1H, J ) 1.5 Hz, J ) 1.5 Hz,
J ) 2.7 Hz), 6.35 (ddd, 1H, J ) 2.7 Hz, J ) 2.7 Hz, J ) 3.3 Hz);
6 6
(C D
7
.48 (d, 1H, J ) 7.8 Hz), 7.22 (d, 1H, J ) 7.8 Hz), 7.13 (dd, 1H,
J ) 7.2 Hz, J ) 7.8 Hz), 7.08 (d, 1H, J ) 16.2 Hz), 7.06 (dd, 1H,
J ) 7.2 Hz, J ) 7.8 Hz), 6.95 (d, 2H, J ) 9.0 Hz), 6.94 (d, 2H, J
13
)
9.0 Hz), 6.75 (d, 1H, J ) 16.2 Hz), 6.17 (dd, 1H, J ) 3.0 Hz,
6 6
C NMR (C D ) δ/ppm (75 MHz) 136.8 (s), 135.7 (s), 133.7 (s),
J ) 3.6 Hz), 6.15 (dd, 1H, J ) 1.8 Hz, J ) 3.6 Hz), 1.02 (s, 9H);
133.5 (s), 131.0 (s), 129.6 (d), 129.5 (d), 129.30 (d), 129.26 (d),
128.42 (d), 128.37 (d), 127.51 (d), 127.46 (d), 119.4 (d), 110.9
(d), 110.4 (d); UV (EtOH) λmax/nm (ꢀ) 280.0 (27956), 302.0
1
3
C NMR (C
35.2 (s), 133.8 (s), 133.3 (s), 131.6 (d), 129.5 (d), 129.0 (d), 128.5
d), 128.3 (d), 127.6 (d), 125.4 (d), 122.6 (d), 115.8 (d), 111.5 (d),
6 6
D ) δ/ppm (75 MHz) 149.7 (s), 137.8 (s), 136.6 (s),
1
(
8
3
-
1
14
(20298); IR (KBr) νmax/cm 3348 (NH). Anal. Calcd for C18H -
-
1
3.6 (s), 27.6 (q); IR (KBr) νmax/cm 1739 (CO); MS m/z (%)
81 (5, M ), 379 (15, M ), 323 (25), 278 (20), 168 (30), 57 (100).
ClN: C 77.28, H 5.04, N 5.01. Found: C 77.15, H 5.43, N 5.13.
Irradiation of 2-{2-[2-(4-Methylphenyl)ethenyl]phenyl}-
pyrrole. In a quartz vessel, 200 mg of the SP (0.77 mmol; c ) 1.5
+
+
Removal of the Boc Group. N-Boc-2-{2-[2-(4-substituted
-
3
phenyl)ethenyl]phenyl}pyrrole (6a-c) (6.88 mmol) was dissolved
in 50 mL of CH OH. A solution of freshly prepared sodium
methoxide [prepared by reacting 950 mg of Na (41 mmol) in 20
mL of CH OH] was added and the reaction mixture was heated
under reflux over 1 h. The solvent was evaporated and the residue
treated with water (100 mL) and extracted with CH Cl
(4 × 50
mL). The organic extracts were dried over anhydrous MgSO , the
solvent was evaporated, and the residue was chromatographed on
a column with silica gel using CH Cl /petroleum ether (1:4) as
× 10 M) was dissolved in 500 mL of benzene (or acetonitrile).
The solution was purged with argon for 30 min, sealed, and
irradiated in a Rayonet reactor at 300 nm over 30 min. The solvent
was removed on the rotary evaporator and the residue chromato-
3
3
2 2
graphed on a column filled with silica gel using CH Cl /petroleum
ether (3:7) as eluent. In the first fractions, 9 mg (4.5%) of 4H-4-
(4-methylbenzyl)-pyrrolo[2,1-a]isoindole was isolated, followed by
19 mg (9.5%) of 2-{2-[2-(4-methylphenyl)ethyl]phenyl}pyrrole and
65 mg (32.5%) of 4,5-dihydro-4-(4-methylphenyl)benzo[g]indole.
High molecular weight material remained on the column.
Following the Reaction Course of Cis-Trans Isomerization
of 2-{2-[2-(4-Methylphenyl)ethenyl]phenyl}pyrrole (1a) by H
2
2
4
2
2
7
eluent. In the first fractions cis-2-{2-[2-(4-substituted phenyl)-
ethenyl]phenyl}pyrroles (1a-c) were isolated, followed by the
isolation of trans-isomers. Reaction furnished the following
1
7
yields: 81.5% (p-Me), 99% (p-OMe), and 86% (p-Cl).
NMR. In a NMR tube was placed 10 mg of cis-2-{2-[2-(4-
methylphenyl)ethenyl]phenyl}pyrrole (1a) and it was dissolved in
cis-2-{2-[2-(4-Methoxyphenyl)ethenyl]phenyl}pyrrole (1b): yel-
1
lowish crystals; mp 50-51 °C; H NMR (C
6 6
D ) δ/ppm (300 MHz)
1 mL of C D
6 6
. In the other NMR tube was placed 10 mg of the
. To both NMR
8
7
1
.18 (br s, 1H), 7.35 (d, 2H, J ) 8.7 Hz), 7.17 (d, 2H, J ) 8.7 Hz),
trans-isomer and it was dissolved in 1 mL of C D
6 6
.05 (ddd, 1H, J ) 7.5 Hz, J ) 7.5 Hz, J ) 1.2 Hz), 6.89 (ddd,
H, J ) 7.5 Hz, J ) 7.5 Hz, J ) 1.2 Hz), 6.75 (ddd, 1H, J ) 3.6
tubes was added 1 µL of cyclohexane. NMR tubes were irradiated
in a Rayonet reactor (equipped with 2 lamps) at 300 nm for short
period of time, after which the H NMR spectra were recorded.
From the ratio of integrals of signals in H NMR spectra, the ratio
1
Hz, J ) 2.1 Hz, J ) 1.5 Hz), 6.56 (d, 2H, J ) 8.7 Hz), 6.55 (d,
1
1
1
3
(
1
H, J ) 12.0 Hz), 6.47 (dd, 1H, J ) 2.7 Hz, J ) 1.5 Hz), 6.41 (d,
H, J ) 12.0 Hz), 6.38 (dd, 1H, J ) 3.6 Hz, J ) 2.7 Hz), 3.20 (s,
of the compounds was calculated.
H); 13C NMR (C
D ) δ/ppm (75 MHz) 159.8 (s), 135.1 (s), 132.9
6 6
Irradiation of 2-{2-[2-(4-Methoxyphenyl)ethenyl]phenyl}-
pyrrole (1b). In a quartz vessel, 184 mg (0.67 mmol; c ) 1.3 ×
s), 131.7 (s), 131.2 (d), 131.0 (d), 129.9 (s), 129.7 (d), 128.1 (d),
-
3
28.0 (d), 126.7 (d), 119.4 (d), 114.4 (d), 110.4 (d), 110.1 (d), 55.06
10 M) of the SP was dissolved in 500 mL of benzene (or
acetonitrile). The solution was purged with argon for 30 min, sealed,
and irradiated in a Rayonet reactor at 300 nm over 2 h. The solvent
was removed by rotary evaporation and the residue chromato-
-1
(
(
q); IR (KBr) νmax/cm 3436 (NH); UV (EtOH) λmax/nm (ꢀ) 282.0
+
17 185), 301.0 (18 190); MS m/z (%) 275 (100, M ), 168 (40,
+
M
3
- CH OPh).
trans-2-{2-[2-(4-methoxyphenyl)ethenyl]phenyl}pyrrole (1b): col-
2 2
graphed on a column filled with silica gel using CH Cl /petroleum
1
orless crystals; mp 112-113 °C; H NMR (C
.57 (d, 1H, J ) 7.2 Hz), 7.42 (br s, 1H), 7.42 (d, 1H, J ) 16.2
Hz), 7.23 (d, 2H, J ) 8.4 Hz), 7.10-7.26 (m, 3H), 6.95 (d, 1H, J
6 6
D ) δ/ppm (300 MHz)
ether (3:7) as eluent. In the first fractions, 7 mg (3.8%) of 4,5-
dihydro-4-(4-methoxyphenyl)pyrrolo[2,1-b]isoquinoline (8b) was
isolated, followed by 18 mg (9.8%) of 4H-4-(4-methoxybenzyl)-
pyrrolo[2,1-a]isoindole (3b), 20 mg (10,9%) of the mixture of cis-
and trans-isomers of the starting material (1b), and 51 mg (27.8%)
of 4,5-dihydro-4-(4-methoxyphenyl)benzo[g]indole (2b). High mo-
lecular weight material remained on the column.
7
)
16.2 Hz), 6.72 (d, 2H, J ) 8.4 Hz), 6.54 (ddd, 1H, J ) 1.8 Hz,
J ) 2.7 Hz, J ) 3.3 Hz), 6.40 (ddd, 1H, J ) 1.2 Hz, J ) 1.5 Hz,
J ) 2.7 Hz), 6.37 (ddd, 1H J ) 0.9 Hz, J ) 2.7 Hz, J ) 3.3 Hz),
13
3
1
3 6 6
.28 (s, 3H, OCH ); C NMR (C D ) δ/ppm (75 MHz) 160.2 (s),
36.4 (s), 133.2 (s), 131.4 (s), 131.2 (s), 130.4 (d), 129.4 (d), 128.6
4,5-Dihydro-4-(4-methoxyphenyl)pyrrolo[2,1-b]isoquinoline
(8b): colorless crystals; H NMR (C
1
(
1
(
d), 127.8 (d), 127.5 (d), 127.4 (d), 126.7 (d), 119.2 (d), 114.9 (d),
6
6
D ) δ/ppm (600 MHz) 7.65
-
1
10.6 (d), 110.2 (d), 55.1 (q); IR (KBr) νmax/cm 3363 (NH); UV
EtOH) λmax/nm (ꢀ) 283.0 (24940), 301.0 (20987). Anal. Calcd for
19
C H
17NO: C 82.88, H 6.22, N 5.09. Found: C 82.54, H 6.51, N
5.12.
cis-2-{2-[2-(4-Chlorophenyl)ethenyl]phenyl}pyrrole (1c): yel-
1
lowish crystals; mp 61-62 °C; H NMR (C
6
D
6
) δ/ppm (300 MHz)
7
7
6
6
.76 (br s, 1H), 7.23 (d, 1H, J ) 7.8 Hz), 7.16 (d, 1H, J ) 7.5 Hz),
.03 (dd, 1H, J ) 7.5 Hz, J ) 7.8 Hz), 6.93 (d, 2H, J ) 8.7 Hz),
.89 (d, 2H, J ) 8.7 Hz), 6.82 (dd, 1H, J ) 7.5 Hz, J ) 7.5 Hz),
.69 (ddd, 1H, J ) 3.6 Hz, J ) 2.7 Hz, J ) 1.5 Hz), 6.59 (d, 1H,
(d, 1H, J ) 7.2 Hz, H-2), 7.17 (dd, 1H, J ) 7.2 Hz, J ) 7.2 Hz,
H-3), 6.99 (dd, 1H, J ) 7.2 Hz, J ) 7.2 Hz, H-4), 6.92 (d, 1H, J
) 7.2 Hz, H-5), 6.86 (dd, 1H, J ) 3.6 Hz, J ) 1.2 Hz, H-3′), 6.80
(d, 2H, J ) 9.0 Hz, H-10), 6.66 (d, 2H, J ) 9.0 Hz, H-11), 6.43-
6.47 (m, 2H, H-4′ and H-5′), 4.69 (t, 1H, J ) 6.6 Hz, H-A), 3.25
J ) 12.0 Hz), 6.46 (ddd, 1H, J ) 1.2 Hz, J ) 1.5 Hz, J ) 2.7 Hz),
6
.38 (dd, 1H, J ) 2.7 Hz, J ) 3.6 Hz), 6.20 (d, 1H, J ) 12.0 Hz);
13
C NMR (C
6 6
D ) δ/ppm (75 MHz) 135.9 (s), 134.3 (s), 133.5 (s),
1
3
1
(
33.0 (s), 132.3 (d), 131.2 (s), 131.0 (d), 129.8 (d), 129.1 (d), 127.9
(s, 3H, OCH ), 2.90 (d, 2H, J ) 6.6 Hz, H-B and H-C);
C
3
-
1
d), 126.7 (d), 119.6 (d), 110.6 (d), 110.4 (d); IR (KBr) νmax/cm
6 6
NMR (C D ) δ/ppm (150 MHz) 160.1 (s), 130.6 (s), 130.3 (s), 128.8
(d, 2C-10), 128.7 (d), 127.8 (d), 126.0 (d), 122.9 (d, C-2), 121.3
3
428 (NH); UV (EtOH) λ+max/nm (ꢀ) 281.0 (18 509), 298.0 (18 190);
+
+
MS m/z (%) 281 (30, M ), 279 (100, M ), 168 (60, M - ClPh).
(d, C-5′), 114.7 (d, 2C-11), 109.9 (d, C-4′), 105.2 (d, C-3′), 58.6
(d, C-8), 55.1 (q, OCH ), 38.6 (t, C-7), two singlets were not seen;
3
trans-2-{2-[2-(4-Chlorophenyl)ethenyl]phenyl}pyrrole (1c): col-
9390 J. Org. Chem., Vol. 71, No. 25, 2006

Photochemistry of o-Pyrrolylstilbenes
MS m/z (%) 275 (75, M ), 168 (60, M+ - p-methoxyphenyl);
+
HRMS calcd for C19H18NO 276.1388, found 276.1391.
4
H-4-(4-Methoxybenzyl)pyrrolo[2,1-a]isoindole (3b): yellowish
1
6 6
crystals; mp 102-104 °C; H NMR (C D ) δ/ppm (600 MHz) 7.31
(
d, 1H, J ) 7.2 Hz, H-2), 7.08 (dd, 1H, J ) 7.2 Hz, J ) 7.2 Hz,
H-3), 6.91 (dd, 1H, J ) 7.2 Hz, J ) 7.2 Hz, H-4), 6.82 (d, 1H, J
)
9
1
1
7.2 Hz, H-5), 6.74 (d, 2H, J ) 9.0 Hz, H-10), 6.68 (d, 2H, J )
.0 Hz, H-11), 6.43-6.48 (m, 3H, H-3’, H-4’ and H-5’), 4.73 (dd,
H, J ) 6.0 Hz, J ) 7.2 Hz, H-A), 3.25 (s, 3H, OCH ), 2.84 (dd,
H, J ) 6.0 Hz, J ) 14.1 Hz, H-B), 2.66 (dd, 1H, J ) 7.2 Hz, J
(d, 1H, J ) 7.2 Hz, H-2), 7.07 (dd, 1H, J ) 7.2 Hz, J ) 7.8 Hz,
H-3 or H-4), 6.97 (d, 2H, J ) 8.1 Hz, H-10), 6.88 (dd, 1H, J ) 7.2
Hz, J ) 7.8 Hz, H-3 or H-4), 6.71 (d, 1H, J ) 7.2 Hz, H-5), 6.45
(d, 2H, J ) 8.1 Hz, H-11), 6.42 (dd, 1H, J ) 3.3 Hz, J ) 2.7 Hz,
H-4’), 6.40 (d, 1H, J ) 3.3 Hz, H-3’), 6.34 (d, 1H, J ) 2.7 Hz,
H-5’), 4.56 (dd, 1H, J ) 6.0 Hz, J ) 7.2 Hz, H-A), 2.64 (dd, 1H,
J ) 6.0 Hz, J ) 13.8 Hz, H-B), 2.47 (dd, 1H, J ) 7.2 Hz, J )
3
1
3
)
14.1 Hz, H-C); C NMR (C
), 144.8 (s), 137.8 (s), 134.6 (s), 131.2 (d, 2C-10), 128.9 (s),
28.7 (d, C-3), 125.0 (d, C-4), 123.6 (d, C-5), 119.1 (d, C-2), 116.7
d, C-5’), 114.5 (d, 2C-11), 113.4 (d, C-3’), 99.2 (d, C-4’), 62.8
6 6
D ) δ/ppm (150 MHz) 159.5 (s,
OCH
3
1
13.8 Hz, H-C); 13C NMR (C
(
(
6 6
D ) δ/ppm (150 MHz) 131.5 (d, 2C-
+
d, C-7), 55.0 (q, OCH
3
), 41.5 (t, C-8); MS m/z (%) 275 (75, M ),
54 (100, M - methoxybenzyl); HRMS calcd for C19
76.1388, found 276.1395.
,5-Dihydro-4-(4-methoxyphenyl)benzo[g]indole (2b): colorless
11), 129.0 (d, 2C-10), 128.9 (d, C-3 or C-4), 125.0 (d, C-3 or C-4),
123.4 (d, C-5), 119.1 (d, C-2), 116.5 (d, C-5’), 113.6 (d, C-4’),
99.4 (d, C-3’), 62.2 (d, C-7), 41.4 (t, C-8), singlets were not seen
due to the small quantitie of the sample; MS m/z (%) 281 (30,
+
1
2
H18NO
4
1
+
+
+
crystals; mp 122-123 °C; H NMR (C
D
6 6
) δ/ppm (300 MHz) 7.26
M ), 279 (100, M ), 154 (75, M - chlorobenzyl); HRMS calcd
for C18 15NCl 280.0893, found 280.0895.
-[2-(2-Phenylethyl)phenyl]pyrrole (9c): colorless oil; H NMR
H
1
2
(
1
br s, 1H, NH), 7.10-7.20 (m, 4H), 6.95-7.05 (m, 2H), 6.87 (d,
H, J ) 7.2 Hz, H-2 or H-5), 6.76 (d, 2H, J ) 8.7 Hz, H-11), 6.31
(
dd, 1H, J ) 2.7 Hz, J ) 2.7 Hz, H-5’), 6.24 (dd, 1H, J ) 2.4 Hz,
J ) 2.7 Hz, H-4’), 4.09 (dd, 1H, J ) 8.4 Hz, J ) 8.7 Hz, H-A),
13
3
.29 (s, 3H, OCH
NMR (C ) δ/ppm (75 MHz) 158.6 (s, C-12), 137.4 (s), 134.4
s), 129.5 (s), 129.0 (d, 2C-10), 128.6 (d), 126.5 (d), 125.1 (d),
3
), 3.03 (d, 2H, J ) 8.4 Hz, H-B and H-C);
C
6 6
(C D ) δ/ppm (600 MHz) 7.13-7.17 (m, 2H), 7.05-7.12 (m, 5H),
D
6 6
7.00-7.05 (m, 1H), 6.96 (dd, 2H, J ) 7.8 Hz, J ) 1.2 Hz), 6.41
(m, 1H), 6.37 (m, 1H), 6.35 (m, 1H), 2.95 (t, 2H, J ) 7.8 Hz),
(
2.72 (t, 2H, J ) 7.8 Hz); 13C NMR (C
D ) δ/ppm (150 MHz) 141.4
1
1
24.1 (s), 118.5 (d, C-5’), 118.3 (d, C-2 or C-5), 113.9 (d, 2C-11),
08.2 (d, C-4’), 54.5 (q, OCH ), 40.1 (t, C-7), 39.8 (d, C-8), one
6 6
3
(s), 139.3 (s), 133.2 (s), 130.3 (s), 129.4 (d), 129.2 (d), 128.1 (d),
127.9 (d), 126.7 (d), 125.6 (d), 125.5 (d), 117.2 (d), 108.8 (d), 108.1
+
singlet was covered by solvent; MS m/z (%) 275 (75, M ), 168
+
+
(
70, M - methoxyphenyl); HRMS calcd for C19
H18NO 276.1388,
(d), 37.2 (t), 35.1 (t); MS m/z (%, fragment) 247 (100, M ), 156
+
found 276.1389.
(25, M - benzyl); HRMS calcd for C18
H18N 248.1439, found
Irradiation of 2-{2-[2-(4-Chlorophenyl)ethenyl]phenyl}-
pyrrole (1c). In a quartz vessel, 250 mg (0.90 mmol; c ) 1.8 ×
248.1437.
4,5-Dihydro-4-(4-chlorophenyl)benzo[g]indole (2c): colorless
-
3
1
1
0
M) of the SP was dissolved in 500 mL of benzene (or
6 6
crystals; mp 170-171 °C; H NMR (C D ) δ/ppm (600 MHz) 7.24
acetonitrile). The solution was purged with argon for 30 min, sealed,
and irradiated in a Rayonet reactor at 300 nm over 30 min. The
solvent was removed by rotary evaporation and the residue
2 2
chromatographed on a column filled with silica gel using CH Cl /
petroleum ether (3:7) as eluent. In the first fractions, traces of 4,5-
dihydro-4-(4-chlorophenyl)pyrrolo[2,1-b]isoquinoline (8c) were
isolated, followed by 5 mg (2.0%) of 4H-4-(4-chlorobenzyl)pyrrolo-
[2,1-a]isoindole (3c), 15 mg (6.7%) of 2-[2-(2-phenylethyl)phenyl]-
pyrrole (9c), and 146 mg (58.4%) of 4,5-dihydro-4-(4-chlorophenyl)-
benzo[g]indole (2c). High molecular weight material remained on
the column.
(br s, 1H, NH), 7.14-7.24 (m covered by solvent, 1H H-2 or H-5),
7.06 (d, 2H, J ) 8.4 Hz, H-10), 6.95-7.02 (m, 2H, H-2 and H-5),
6.92 (d, 2H, J ) 8.4 Hz, H-11), 6.84 (d, 1H, J ) 7.8 Hz, H-2 or
H-5), 6.27 (dd, 1H, J ) 2.4 Hz, J ) 3.0 Hz, H-5’), 5.90 (dd, 1H,
J ) 2.4 Hz, J ) 3.0 Hz, H-4’), 3.92 (dd, 1H, J ) 6.6 Hz, J ) 10.5
Hz, H-A), 2.89 (dd, 1H, J ) 6.6 Hz, J ) 15.0 Hz, H-B), 2.83
4
,5-Dihydro-4-(4-chlorophenyl)pyrrolo[2,1-b]isoquinoline
1
(
(
8c): aliphatic part of the H NMR (C
6 6
D ) δ/ppm (600 MHz) 4.61
dd, 1H, J ) 5.4 Hz, J ) 6.6 Hz, H-A), 2.81 (dd, 1H, J ) 5.4 Hz,
J ) 15.0 Hz, H-B), 2.68 (dd, 1H, J ) 6.6 Hz, J ) 15.0 Hz, H-C);
(dd, 1H, J ) 10.5 Hz, J ) 15.0 Hz); 13C NMR (C
6 6
D ) δ/ppm (150
+
+
+
MS m/z (%) 281 (30, M ), 279 (100, M ), 168 (12, M
-
MHz) 144.5 (s), 134.4 (s), 132.6 (s), 130.1 (d, 2C-11), 129.9 (s),
129.2 (d), 129.1 (d, 2C-10), 127.3 (d), 125.9 (d), 123.6 (s), 119.2
(d, C-5’), 118.9 (d, C-2 or C-5), 108.5 (d, C-4’), 40.4 (d, C-8),
chlorophenyl).
4
H-4-(4-Chlorobenzyl)pyrrolo[2,1-a]isoindole (3c): colorless
1
+
+
crystals; mp 101-103 °C; H NMR (C
6 6
D ) δ/ppm (600 MHz) 7.27
40.2 (t, C-7); MS m/z (%) 281 (30, M ), 279 (100, M ), 168 (70,
J. Org. Chem, Vol. 71, No. 25, 2006 9391

Basari c´ et al.
M+ - p-chlorophenyl); HRMS calcd for C18
H15NCl 280.0893,
Fluorescence lifetimes were measured on an instrument equipped
with a hydrogen flash lamp, using a time-correlated single photon
counting technique in 1023 channels. Histograms of the instrument
response functions (using LUDOX scatterer) and sample decays
found 280.0884.
Comparing Relative Photochemical Reactivity of 2-{2-[2-(4-
Substituted phenyl)ethenyl]phenyl}pyrrole (1a-c). In three
quartz cuvettes was placed 20 mg of a mixture of cis- and trans-
isomer of SP 1a, 1b, or 1c, respectively. The compounds were
dissolved in 17 mL of benzene, purged with a stream of Ar, sealed,
and irradiated in a Rayonet reactor over 15 min. After irradiation,
3
were recorded until they typically reached 3 × 10 counts in the
peak channel. The half-width of the instrument response function
was typically ∼1.5 ns. The time increment per channel was 0.020
or 0.049 ns. Three obtained fluorescence decays were fitted by
global analysis as sums of exponentials using Gaussian-weighted
nonlinear least-squares fitting based on Marquardt-Levenberg
minimization implemented in the software package of the instru-
1
solvent was removed and H NMR spectra of the crude photomix-
tures were recorded. From the integrations of the signals, the relative
amounts of the starting material and the photoproducts were
obtained.
ment. The fitting parameters (decay times and pre-exponential
2
Laser Flash Photolysis (LFP). All LFP studies were conducted
at the University of Victoria LFP facility using a XeCl excimer
laser with 308-nm excitation. Flow cells (0.7 cm) were used and
solutions were purged with nitrogen or oxygen for 30 min prior to
measurements. Optical densities at 308 nm were ∼0.4. The
factors) were determined by minimizing the global reduced ø
g
.
An additional graphical method was used to judge the quality of
the fit that included plots of surfaces (“carpets”) of the weighted
residuals vs channel number.
3
measurements were performed in CH CN (HPLC grade).
Acknowledgment. Support of this work was provided by
the grants from the Ministry of Science, Education and Sports
of the Republic of Croatia (project no. 0125004, 0098052, and
Steady State and Time-Resolved Fluorescence Measurements.
For the fluorescence measurements, solvents were of spectroscopic
grade purity. All the compounds were additionally purified by three
recrystallization using cyclohexane/chloroform. The steady state
measurements were performed in three solvents: acetonitrile,
methanol, and cyclohexane. Concentrations were adjusted to have
absorbance at the excitation wavelength (280 nm) < 0.1. Solutions
were purged with argon for 15 min prior to analysis. Corrected
excitation and emission spectra were obtained on a fluorometer.
Quantum yields were determined by comparison of the integral of
emission bands with the one of quinine bisulfate (Φ ) 0.55 in 1.0
0
098059). The authors would like to thank University of Victoria
(BC, Canada) and Profs. Cornelia Bohne and Peter Wan for
using their LFP and SPT equipment. The authors would also
like to thank Ms. E. Arctander for English corrections.
Supporting Information Available: Detailed experimental
procedures, UV and fluorescence spectra of SPs 1a-1c, transient
absorption spectra of 1a-1c recorded on the nanosecond LFP, and
1
13
H and C NMR spectra of all the prepared compounds. This
17
2 4
N H SO ). Typically, three absorption traces were recorded (and
material is available free of charge via the Internet at http://
pubs.acs.org.
averaged) and three fluorescence emission traces, exciting at three
different wavelengths (340, 300, and 280 nm). Three quantum yields
were calculated, and the mean value was reported.
JO061435T
9392 J. Org. Chem., Vol. 71, No. 25, 2006