Molecular Structure and Photochemistry of (E)- and (Z)-Ethyl 3-(2-Indolyl)propenoate. Ground State Conformational Control of Photochemical Behavior and One-Way E -> Z Photoisomerization

Article abstract of DOI:10.1021/jp960907c

The molecular structure, electronic spectra, and photoisomerization of (E)- and (Z)-ethyl 3-(2-indolyl)propenoate, two methylated indole derivatives, and their N,N-dimethylamide analog have been investigated.The E ester exists in the ground state as a mixture of anti and syn rotational isomers.The spectroscopic and photochemical behaviors of the individual anti and syn conformers were characterized with the assistance of comparisons with the behavior of the methylated indole derivatives.The major anti conformer of the E ester absorbs and emits at shorter wavelength than the minor syn conformer.The rate constant for singlet state isomerization of the anti conformer is substantially larger than that of the syn conformer, resulting in a shorter singlet lifetime and smaller fluorescence quantum yield for the anti conformer.The behavior of the E amide in both the ground and excited states is similar to that of the ester.The Z isomers of the ester and amide possess a relatively strong intramolecular hydrogen bond.Their singlet states are weakly fluoroscent and photoisomerize ineffeciently in nonpolar solvents.Thus photostationary states highly enriched in the Z isomers are obtained in nonpolar solvents.The red-shifted, structureless emission observed upon irradiating the Z amide in an EPA or methylcyclohexane glass at 77 K is attributed to an excited state tautomer formed via intramolecular hydrogen transfer.

Full text of DOI:10.1021/jp960907c

14560
J. Phys. Chem. 1996, 100, 14560-14568
Molecular Structure and Photochemistry of (E)- and (Z)-Ethyl 3-(2-Indolyl)propenoate.
Ground State Conformational Control of Photochemical Behavior and One-Way E f Z
Photoisomerization
Frederick D. Lewis* and Jye-Shane Yang
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: March 26, 1996; In Final Form: June 4, 1996X
The molecular structure, electronic spectra, and photoisomerization of (E)- and (Z)-ethyl 3-(2-indolyl)-
propenoate, two methylated indole derivatives, and their N,N-dimethylamide analog have been investigated.
The E ester exists in the ground state as a mixture of anti and syn rotational isomers. The spectroscopic and
photochemical behaviors of the individual anti and syn conformers were characterized with the assistance of
comparisons with the behavior of the methylated indole derivatives. The major anti conformer of the E ester
absorbs and emits at shorter wavelength than the minor syn conformer. The rate constant for singlet state
isomerization of the anti conformer is substantially larger than that of the syn conformer, resulting in a shorter
singlet lifetime and smaller fluorescence quantum yield for the anti conformer. The behavior of the E amide
in both the ground and excited states is similar to that of the ester. The Z isomers of the ester and amide
possess a relatively strong intramolecular hydrogen bond. Their singlet states are weakly fluorescent and
photoisomerize inefficiently in nonpolar solvents. Thus photostationary states highly enriched in the Z isomers
are obtained in nonpolar solvents. The red-shifted, structureless emission observed upon irradiating the Z
amide in an EPA or methylcyclohexane glass at 77 K is attributed to an excited state tautomer formed via
intramolecular hydrogen transfer.
Introduction
CHART 1
A large number of arylethylenes are known to exist as
equilibrium mixtures of nearly isoenergetic conformers.1-4 In
some cases these conformers display sufficiently different
spectroscopic properties to permit spectral characterization of
the individual conformers. The excited conformers can also
differ in their rates of unimolecular isomerization and bimo-
lecular reactions. We recently investigated the molecular
structure, spectroscopy, and photochemistry of the E and Z
isomers of 2-(2-(2-pyridyl)ethenyl)indole, 1 (Chart 1), and found
that E-1 exists as a mixture of anti and syn conformers which
have distinct fluorescence excitation and emission spectra and
very different rate constants for photoisomerization.4 Further-
more, Z-1 exists as a single conformer due to the presence of
an intramolecular hydrogen bond. The singlet state of Z-1 is
nonfluorescent and stable with respect to photoisomerization
in both nonpolar and polar solvents. While the intramolecular
hydrogen bond is clearly implicated in the nonradiative decay
of singlet Z-1, the nonradiative decay pathway has not been
elucidated. Photoisomerization of the E but not the Z isomer
results in one-way E f Z photoisomerization, a relatively rare
phenomenon.3-7
The pronounced differences in the spectroscopic properties
and photochemistry of the anti and syn conformers of E-1 and
the ability of the indole nitrogen to serve as a good hydrogen
bond donor make the 2-vinylindoles attractive targets for further
investigation. We report here the results of our investigation
of the molecular structure, spectra, and photoisomerization of
the E and Z ethyl 3-(2-indolyl)propenoates 2-4, and the N,N-
dimethylamide 5 (Chart 2). The methylated indoles 3 and 4
serve as single-conformer models for the anti and syn conform-
ers of 2, respectively. Whereas the photochemistry of â-aryl-
propenoate derivatives has been extensively investigated, little
is known about the effects of the carboxylate group on the
spectroscopy and photochemistry of the conformers of singlet
arylethylenes. The anti and syn conformers of the ester and
amide derivatives of 3-(2-naphthyl)propenoic acid are fluores-
cent; however, strong overlap of the fluorescence spectra
prevented dissection of the properties of the anti and syn
conformers.8, 9 The carbonyl groups of esters and amides have
previously been found to serve as hydrogen bond acceptors in
the Z isomers of urocanic acid (3-(4-imidazolyl)propenoic acid)
derivatives; however, the absence of fluorescence from either
the E or Z isomers of these and other mononuclear â-arylpro-
penoic esters and amides prevented a detailed investigation of
the photophysics of these molecules.6b We find that the E
isomers of 2 and 5 exist as mixtures of ground state anti and
syn conformers with distinct spectral properties, whereas the Z
isomers exist predominantly as the anti conformers due to the
presence of an intramolecular hydrogen bond. Ground state
conformation has a profound effect upon both spectroscopic
properties and photochemical behavior.
Results and Discussion
Molecular Structure. The 2-indolylpropenoic acid deriva-
tives 2-5 were prepared from commercial indole starting
materials using standard synthetic procedures. The E and Z
isomers are separable by column chromatography and are
X Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)00907-0 CCC: $12.00 © 1996 American Chemical Society

(E)- and (Z)-Ethyl 3-(2-Indolyl)propenoate
J. Phys. Chem., Vol. 100, No. 34, 1996 14561
TABLE 1: 1H NMR and IR Dataa and the Difference in
Relative Ground State Energiesb between Z and E Isomers
δ N-H
(ppm)
νN-H
νCdO
νCdC
∆G°
(kcal/mol)
compd
(cm-1
)
(cm-1
)
(cm-1
)
E-2
Z-2
E-3
Z-3
E-4
Z-4
E-5
Z-5
8.49
11.87
8.58
3471
3300
3478
3300
1695
1694
1689
1688
1701
1709
1648
1640
1634
1614
1626
1599
1631
1622
1596
1591
-0.7 ( 0.2
0.7 ( 0.1
>1.5
11.80
8.76
12.15
3467
3246
-0.9 ( 0.2
1
a CDCl3 solvent for H NMR and CHCl3 for IR. b ∆G° ) G°(Z) -
G°(E).
CHART 2
Figure 1. NOE data (%) for (E)-ethyl 3-(2-indolyl)propenoate (E-2)
in CDCl3 (A) and DMSO-d6 (B), (E)-ethyl 3-(3-methyl-2-indolyl)-
propenoate (E-3) in CDCl3 (C), and (E)-ethyl 3-(N-methyl-2-indolyl)-
propenoate (E-4) in CDCl3 (D).
(or N1-CH3) and the other by indole C3-H (or C3-CH3). NOE
data indicate that the lowest energy vinyl-indole conformations
are those shown in Figure 1. The observed vinyl-indole
conformations for E-3 and E-4 are consistent with the anticipated
effect of nonbonded repulsion between the methyl substituent
and the vinyl group.4,12 The NOE data for E-2 more closely
resemble that for E-3 than for E-4. On this basis we conclude
that the major and minor conformers of E-2 are anti and syn,
respectively. The larger NOE values observed for E-2 in
DMSO-d6 vs CDCl3 may reflect a difference in solvent viscosity,
and the larger values for E-3 and E-4 vs E-2 in CDCl3 may
reflect a reduction in rotational rates upon methylation.13
The gas phase conformational energies of the four possible
conformers of E-2-E-5 were calculated using the MM2 force
field.14 In accord with IR assignment, the two s-trans enone
conformations have lower energy by 2.4 ( 0.2 kcal/mol than
the s-cis conformations in the cases of E-2-E-4, whereas the
two s-cis enone conformations of E-5 are more stable by 2.3 (
0.2 kcal/mol than the s-trans conformations. In addition, the
two lower energy conformers in E-2 and E-5 are nearly
isoenergetic (∆G ) 0.2 ( 0.2 kcal/mol), but only one lowest
energy conformer exists in the cases of E-3 and E-4 (∆G > 1.2
( 0.2 kcal/mol). These calculations do not rule out the possible
presence of higher energy conformations of E-2-E-5 in
solution.15
The difference in relative ground state energies between E
and Z isomers of 2-5 in benzene solution was determined by
free-radical-initiated isomerization using irradiation of either
iodine or diphenyl diselenide as the radical source (Table 1).16
Z-2 and Z-5 are more stable than their corresponding E isomers
by ∼0.8 kcal/mol, as is the case for Z-1 (∆G ) 0.2 kcal/mol in
benzene solution4). In contrast, Z-4, which is incapable of
forming an intramolecular hydrogen bond, is less stable than
its E isomer by >1.5 kcal/mol. Z-3 is less stable than its E
isomer by 0.7 kcal/mol, in spite of the existence of an
intramolecular hydrogen bond. The nonbonded interaction
between the 3-methyl substituent and the vinyl hydrogen (A(1,3)
strain12) may be larger for the more planar Z isomer vs the E
isomer. Clearly there is no simple relationship between the
relative thermodynamic stability of the Z vs E isomer and the
N-H chemical shift or vibrational frequency.
crystalline solids (except in the case of Z-4 which is an oil)
which were purified by recrystallization and fully characterized
(see Experimental Section). The existence of intramolecular
1
hydrogen bonds in Z-2, Z-3, and Z-5 was established by H
NMR and IR spectroscopies. Downfield chemical shifts of
N-H (>3 ppm) and lower N-H stretching frequencies (>170
cm-1) are observed in comparison to the values for the
corresponding E isomers (Table 1). The larger downfield
chemical shift and the smaller νN-H observed for Z-5 vs Z-2
indicate that the amide group behaves as a slightly stronger
hydrogen bond acceptor than the ester group.
The ground state conformations of 2-5 have been investi-
gated by both experimental and computational methods. The
E isomers of 2-5 can adopt any of four planar conformations
as a consequence of rotation about the vinyl-indole and enone
single bonds. The relative intensities of infrared νCdC vs νCdO
bands can be employed to assign the enone conformations.10
By analogy to our previous studies of cinnamate ester and amide
conformation,11 the IR data are consistent with s-trans enone
conformations for esters E-2-E-4 and s-cis for amide E-5 (Chart
2). The hydrogen-bonded conformers of Z-2, Z-3, and Z-5 have,
of necessity, s-cis enone conformations.
NOE difference experiments for E-2-E-4 in CDCl3 solution
and for E-2 in DMSO-d6 solution were carried out to provide
information about their vinyl-indole conformations. The results
are depicted in Figure 1. Two irradiation frequencies were used
in each case: one at the frequency absorbed by indole N1-H
Absorption Spectra. The electronic absorption spectra of
E and Z isomers of 2-4 in hexane solution are shown in Figure
2. The appearance of absorption spectra of E-5 and Z-5 (not

14562 J. Phys. Chem., Vol. 100, No. 34, 1996
Lewis and Yang
Figure 3. ZINDO-calculated highest occupied and lowest unoccupied
molecular orbitals for (E)-ethyl 3-(2-indolyl)propenoate (E-2).
polarity also results in loss of vibrational structure and a red-
shift of the absorption maximum (Table 2).
The electronic structure and spectra of syn-E-2, anti-E-2, and
the Z-2 have been investigated by means of semiempirical
INDO/S-SCF-CI (ZINDO) calculations using the algorithm
developed by Zerner and co-workers.18 The conformations of
molecules for calculation are adopted from the results of MM2
optimization. The ZINDO-derived highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) for anti-E-2 are shown in Figure 3. The HOMO is
localized on the vinylindole portion of the molecule and the
LUMO on the enone portion of the molecule. Thus the ester
and amide are expected to have similar electronic structures, in
accord with their similar absorption spectra. The energy of the
HOMO is well separated energetically from that of the two
lower energy π-bonding orbitals and the nonbonding orbital.
The first allowed transitions for syn-E-2, anti-E-2, and Z-2 are
all calculated to be of relatively pure and allowed HOMO f
LUMO, π f π* character. The ZINDO calculated frontier
orbitals and singlet states of syn-E-2, anti-E-2, and the Z-2 are
provided as supporting information.
Figure 2. UV spectra of (E)- (s) and (Z)- (- - -) ethyl 3-(2-indolyl)-
propenoate (2) (A), ethyl 3-(3-methyl-2-indolyl)propenoate (3) (B), and
ethyl 3-(N-methyl-2-indolyl)propenoate (4) (C) in hexane solution.
The calculated transition energies and oscillator strengths are
in the order anti-E-2 (314 nm, f ) 0.88) > syn-E-2 (321 nm, f
) 0.76) > Z-2 (337 nm, f ) 0.50). The longer wavelength
and lower oscillator strength for Z-2 vs E-2 are in agreement
with the observed spectra (Figure 2). In addition to the allowed
π f π* transition, ZINDO calculations indicate the presence
of n f π* transitions of very low oscillator strength (0.001-
0.005) at energies below their lowest π-π* transitions by ∼2700
cm-1. The n,π* states have extensive configuration interaction
involving all of the low-energy π* orbitals. Similar results have
been obtained from ZINDO calculations for 2-naphthylpropenoic
acid derivatives.9 It has been noted that ZINDO may not
provide reliable energies for n,π* states in such cases.19 On
the basis of the fluorescence properties described in the
following section, it seems likely that the E isomers of 2-5 all
possess lowest energy π,π* singlet states.
TABLE 2: Absorption and Fluorescence Spectral Data
compd
solvent
λmax,abs (nm)
ꢀmax
λmax,fl (nm)
E-2
C6H14
EtOH
C6H14
EtOH
C6H14
EtOH
C6H14
EtOH
C6H14
EtOH
C6H14
EtOH
C6H14
EtOH
C6H14
EtOH
330, 346
340
352
348
340
350
356
356
340
344
342
342
330, 346
340
348
342
32 200, 27 200
31 500
379, 400
449
Z-2
E-3
Z-3
E-4
Z-4
E-5
Z-5
22 300
26 400
28 700
24 600
26 700
26 600
23 600
19 700
18 700
380, 397
438
397, 417
458
378, 398
433
34 100
22 100
Excited State Behavior of the E Isomers. The fluorescence
emission and excitation spectra of E-2 in hexane solution
bubbled with nitrogen or oxygen are shown in Figure 4a.
Oxygen has little effect on the appearance of the emission
spectrum but reduces the intensity of the long wavelength band
of the excitation spectrum. This selective quenching is indica-
tive of the presence of two fluorescent species with different
shown) are very similar to those of E-2 and Z-2, respectively.
The absorption maxima of hydrogen-bonded Z isomers (Z-2,
Z-3, and Z-5) are at longer wavelengths than those of their E
isomers in both nonpolar and polar solvents (Table 2). In
contrast, the absorption maximum of Z-4, which does not
possess an intermolecular hydrogen bond, is similar to that of
E-4. The effect of conformation upon the absorption spectra
of â-substituted styrenes has been discussed by Fueno et al.17
They observed that planar Z isomers absorb at lower energy
than the corresponding E isomers, whereas nonplanar Z isomers
absorb at higher energy. Intramolecular hydrogen bonding may
favor a more nearly coplanar geometry for the vinylindole
chromophores in Z-2, Z-3, and Z-5 than in their E isomers or
in Z-4. Methylation on the indole C3 or N1 positions results in
a small red-shift in the absorption maximum (5 -10 nm) and
a loss of vibrational structure (Figure 2). Increasing solvent
excitation spectra and lifetimes. A synchronous scan (λem
-
λex ) 10 nm) of the fluorescence spectrum for a highly dilute
hexane solution of E-2 is shown in Figure 5. The appearance
of two distinct bands at 346 and 366 nm provides further
evidence for the presence of two fluorescing species.20 The
ratio of band intensities (366/346 nm ∼6) does not change at
lower concentrations, indicating that reabsorption of emitted
light is not influencing this ratio.
Fluorescence quantum yield (ΦF) and lifetime data for E-2
obtained in several solvents at room temperature are reported

(E)- and (Z)-Ethyl 3-(2-Indolyl)propenoate
J. Phys. Chem., Vol. 100, No. 34, 1996 14563
TABLE 3: Fluorescence Quantum Yields (ΦF) and Lifetime
Data (τ) with Preexponential Values (A) at Room
Temperature
a
b
c
b
compd solvent λex
ΦF
τ1,ns A1
%
τ2,ns
A2
%
E-2
C6H14
322
362
1.4 0.09 52 0.13 0.91 48
1.3 0.54 90 0.17 0.46 10
PhH
338 0.030
CH3CN 334 0.048 1.1 0.45 76 0.29 0.55 24
DMSO 342 0.037
EtOH
EPAd
PhH
EtOH
PhH
EtOH
PhH
340 0.028 0.85 0.59 87 0.19 0.41 13
336 4.0 0.30 40 2.6 0.70 60
348 0.013 0.52 1.00 100
348 0.0040
E-3
E-4
E-5
346 0.26
2.47 1.00 100
346 0.086 1.35 1.00 100
336 0.051
CH3CN 330 0.033 1.0 0.53 80 0.28 0.47 20
DMSO 338 0.036
a Excitation wavelength corresponding to the maximum in the
fluorescence excitation spectrum used for both fluorescence quantum
yield and lifetime measurements. b Contribution to total fluorescence
decay [A1τ1 or A2τ2/(A1τ1 + A2τ2)]. c Decay times <0.3 ns are near the
time resoltuion of our instrumentation and are considered approximate
((0.1 ns). d Data obtained at 77 K.
Figure 4. Fluorescence emission and excitation spectra of (A) (E)-
ethyl 3-(2-indolyl)propenoate (E-2) in hexane solution bubbled with
nitrogen (a) and oxygen (b) (λex ) 330 nm and λem ) 400 nm) and (B)
(E)-ethyl 3-(3-methyl-2-indolyl)propenoate (E-3) (λex ) 325 nm and
λem ) 420 nm) (c) and (E)-ethyl 3-(N-methyl-2-indolyl)propenoate (E-
4) (λex ) 325 nm and λem ) 397 nm) (d) in hexane solution bubbled
with nitrogen.
precedents, E-3 and E-4 were expected to serve as single-
conformer models for anti-E-2 and syn-E-2, respectively. The
fluorescence emission and excitation spectra of E-3 and E-4 in
hexane solution are shown in Figure 4B. Both display a
vibrational structure similar to that for E-2. Except for the
absence of the shoulder at the onset of the emission spectrum,
the vibrational bands of E-3 overlap with those of E-2. The
vibrational bands of E-4 are shifted by ∼20 nm to the red, and
thus its 0-0 band lies at the same position as the 0-1 band of
E-3. Single-exponential decays are observed for E-3 and E-4
in both benzene and ethanol. E-3 exhibits lower values of ΦF
and shorter lifetimes than those for E-4 (Table 3).
Correlation of the behavior of E-3 and E-4 with that of E-2
suggests that syn-E-2 absorbs and emits at longer wavelength
with larger fluorescence yield and longer lifetime than anti-E-
2. This assignment is consistent with the selective quenching
of the long-wavelength fluorescence excitation band by oxygen
(Figure 4A) and with the effect of excitation and emission
wavelength on the preexponentials for fluorescence decay (Table
3). The low total fluorescence quantum yields observed for
E-2 (Table 3) indicate that anti-E-2 is the major conformer, in
accord with the NOE experiments for E-2 (Figure 1). Increasing
solvent polarity results in red-shifted (Table 2) and broadened
fluorescence spectra for E-2-4. The fluorescence quantum
yields for both E-3 and E-4 are lower in ethanol than in benzene
solution. The fluorescence quantum yields and lifetimes of E-2
display only minor variation with solvent (Table 3).
Figure 5. Fluorescence synchronous scan (λem - λex ) 10 nm) of
(E)-ethyl 3-(2-indolyl)propenoate (E-2) in hexane solution.
in Table 3. Dual exponential fluorescence decays are observed
in all solvents. The decay times for the shorter-lived component
of E-2 are near the time resolution of our instrument and thus
should be viewed as approximate (0.1 < τ2 < 0.3 ns). The
dependence of preexponentials of the fluorescence decay of E-2
in hexane on excitation and emission wavelengths was also
investigated. When the solution is excited at 322 nm and the
emission decay is detected at 358 nm, emission from the short-
lived component has the larger preexponential. In contrast,
emission from the long-lived component has the larger preex-
ponential when the excitation and emission wavelengths are 362
and 419 nm, respectively (Table 3). On the basis of the above
observations, the short-lived and long-lived components can be
assigned to species with shorter and longer wavelength 0,0
bands, respectively. Both the appearance of the fluorescence
spectrum and the lifetimes for E-2 are similar to those previously
reported for E-1,4 in accord with the assignment of the lowest
singlets to vinyl-indole π,π* states.
Quantum yields for photoisomerization of E-2-E-4 (ΦE,Z
)
are reported in Table 4. The significantly higher value for E-3
vs E-4 indicates that the shorter lifetime of E-3 is due, at least
in part, to more rapid isomerization. The value of ΦE,Z for E-2
is even higher than that for E-3, in accord with our assignment
of the anti conformation to the short-lived major conformer.
The values of ΦE,Z for E-2-E-4 decrease with increasing solvent
polarity. The decrease in ΦE,Z for E-2 might be attributed to a
decrease in the population of the reactive anti conformer in more
polar solvents; however, NOE data indicate that the conformer
populations are similar in CDCl3 and DMSO solution. Fur-
thermore, increasing solvent polarity also results in a decrease
in the isomerization quantum yields of E-3 and E-4 which exist
as single conformers. The solvent dependence of ΦE,Z for E-2-
E-4 is similar to that previously observed for the photoisomer-
ization of 3-(2-pyridyl)propenamides.5 Solvation of the polar
The use of arene methylation to “lock” a vinylarene into a
single conformer has been successfully employed in previous
investigations of rotational isomers.2,4,21 On the basis of these

14564 J. Phys. Chem., Vol. 100, No. 34, 1996
Lewis and Yang
TABLE 4: Quantum Yields for Isomerization and Isomer
Content for the Photostationary Statesa
compd
solvent
ΦE,Z
ΦZ,E
%Zb
2
PhH
0.65
0.61
0.37
0.22
0.47
0.06
0.034
0.012
0.77
0.60
0.25
0.30
0.047
0.10
0.19
0.073
0.02
0.005
0.34
0.12
0.0049
0.056
95
72
51
67
98
CH3CN
DMSO
EtOH
PhH
EtOH
PhH
EtOH
PhH
CH3CN
DMSO
EtOH
3
4
5
68
<22c
<20c
98
96
82
85
a Excitation wavelength is 313 nm. b Same results obtained starting
with either pure E or pure Z isomers. c Compound not very stable upon
prolonged irradiation.
TABLE 5: Summary of Ground and Excited State
Properties of Conformers anti-E-2 and syn-E-2 and Model
Compounds E-3 and E-4 in Nonpolar Solvent at Room
Temperature
anti-E-2
syn-E-2
E-3
E-4
Figure 6. Fluorescence emission and excitation spectra of (A) (E)-
ethyl 3-(2-indolyl)propenoate (E-2) (λex ) 342 nm and λem ) 392 nm)
and (B) (E)-ethyl 3-(3-methyl-2-indolyl)propenoate (E-3) (λex ) 320
nm and λem ) 400 nm) (a) and (E)-ethyl 3-(N-methyl-2-indolyl)-
propenoate (E-4) (λex ) 342 nm and λem ) 434 nm) (b) in EPA at 77
K.
τ, ns
∼ 0.2
1.0
0.52
0.013
0.25
2.5
0.26
1.0
ΦF
0.006 ( 0.003
0.3 ( 0.2
0
0.11 ( 0.03
1.1 ( 0.2
0.8 ( 0.2
76
0.06 ( 0.02
0.6 ( 0.2
kf, ×10-8 s-1 a
∆Go, kcal/molb
Es, kcal/molc
ΦE,Z
80
75
0.47
9.0
72
0.034
0.13
0.83 ( 0.2
42 ( 20
ki, ×10-8 s-1 a
vs E-3 and syn-E-2 vs E-4 may reflect the lower singlet energies
of the methylated indoles. It is interesting to note that the values
of kf for anti-E-2 and E-3 and for syn-E-2 and E-4 are essentially
identical. Thus methylation of the indole nucleus results in a
decrease in the singlet energy and isomerization rate constant
but has little effect upon either the fluorescence rate constant
or the vibronic structure.
a Estimated errors for anti-E-2 are larger than those for syn-E-2b
due to its short lifetime (Table 3, footnote c). b Free energies relative
to anti-E-2 estimated from experimental data in hexane or benzene
solutions. c Singlet energies estimated from emission spectra in hexane
solution.
amide or ester group might result in a solvent-induced barrier
for photoisomerization.
The spectroscopy, photophysical, and photochemical behavior
of E-5 are all similar to those of E-2. Thus the difference in
enone conformation (s-trans for E-2 and s-cis for E-5) has no
noticeable consequences. This result is in accord with the results
of ZINDO calculations for E-2 (Figure 2 and supporting
information), which indicate that the HOMO and LUMO are
largely localized on the vinylindole portion of the molecule.
The fluorescence spectra of E-2-4 in EPA glasses formed
by slow cooling to 77 K are shown in Figure 6. The spectra of
E-3 and E-4 display enhanced vibrational resolution and are
red-shifted by ∼12 nm (Figure 6B) compared to their room
temperature spectra in hexane solution (Figure 4A). The 77 K
spectrum of E-2 (Figure 6A) is assigned to the anti conformer
on the basis of the simple mirror image relationship between
the excitation and emission spectrum and the 14 nm red-shift
of the 0,0 band compared to the room temperature spectrum in
hexane solution. Increases in the population and singlet lifetime
of the anti conformer upon cooling evidently result in its
domination of the total emission spectrum. The fluorescence
decay of E-2 at 77 K is, however, biexponential with decay
times of 4.0 and 2.6 ns. The observation of dual exponential
decay from a single conformer can be attributed to the formation
of different solvent domains upon cooling the mixed solvent
EPA.22 Different solvent domains could also account for minor
variations in the fluorescence excitation and emission spectra
of E-2 in EPA at 77 K that are observed when the emission or
excitation wavelength is changed.
The ground and excited state properties of conformers syn-
E-2 and anti-E-2 and of their single conformer models E-3 and
E-4 in nonpolar solvent are summarized in Table 5. The ground
state conformer populations E-2 are estimated to be 77% anti
and 23% syn in nonpolar solvents, on the basis of the 1:6 ratio
of peak intensities in the synchronous scan spectrum (Figure
5) and the assumption that the fluorescence quantum yield ratio
for the conformers is the same as that for the single-conformer
models E-3 and E-4 (Table 3). These estimated populations
are consistent with the NOE data (Figure 1) and can be used
with the measured value of Φf for E-2 to obtain approximate
values of ΦF ) 0.006 for anti-E-2 and 0.11 for syn-E-2.
Similarly, these populations can be used with the measured
quantum yields for isomerization of E-2-E-4 to obtain ap-
proximate values of ΦE,Z ) 0.83 for anti-E-2 and 0.06 for syn-
E-2.
Values of kf and ki for the anti and syn conformers of E-2
and for E-3 and E-4 can be calculated from the estimated or
measured quantum yields and singlet lifetimes (Table 5). Values
of ki are not corrected for possible partitioning of a twisted
intermediate between E and Z isomers. The estimated errors
are largest in the case of E-2 because of possible errors in the
measurement of its short singlet lifetime as well as in estimation
of the quantum yields. The smaller value of kf and larger value
of ki for the anti vs syn conformer of E-2 parallel the differences
observed for E-3 vs E-4. The larger values of ki for anti vs
syn-E-2 and for E-3 vs E-4 plausibly reflect the higher singlet
energies of the more reactive conformational or geometric
isomer, which could result in a lower barrier for twisting about
the double bond. Similarly, the larger values of ki for anti-E-2
Excited State Behavior of the Z Isomers. The Z isomers
of 2-5 are all very weakly fluorescent at room temperature in
solution. In the case of Z-4 the room temperature fluorescence
is dominated by a 2-3% residual impurity of E-4. Quantum

(E)- and (Z)-Ethyl 3-(2-Indolyl)propenoate
J. Phys. Chem., Vol. 100, No. 34, 1996 14565
Figure 8. Fluorescence emission and excitation spectra of (Z)-N,N-
dimethyl-3-(2-indolyl)propenamide (Z-5) in EPA at 77 K irradiated at
λex ) 363 nm and monitered at 546 nm.
Figure 7. Fluorescence emission and excitation spectra of (Z)-ethyl
3-(2-indolyl)propenoate (Z-2) in EPA at 77 K (λex ) 394 nm and λem
) 458 nm). Spectra of (E)-ethyl 3-(2-indolyl)propenoate (E-2) (- - -)
are included for purposes of comparison.
yields for photoisomerization of Z-2-Z-5 (ΦZ,E) are reported
in Table 4. The values of ΦZ,E for Z-2, Z-3, and Z-5 in nonpolar
solvents are all very small (<0.05). Since the quantum yields
for isomerization of the corresponding E isomers are relatively
large, the photostationary states are enriched in the Z isomers
(Table 4). In the case of 4 the value of ΦZ,E is significantly
larger than that for ΦE,Z in nonpolar solvents, and the photo-
stationary state is enriched in the E isomer. The much larger
values of ΦZ,E for Z-4 compared to those for Z-2, Z-3, and Z-5
can be attributed to the absence of an intramolecular hydrogen
bond for the former molecule. Highly inefficient photoisomer-
ization has previously been observed for Z-14 and several other
molecules possessing intramolecular hydrogen bonds.5,6 The
intramolecular hydrogen bond may either inhibit twisting about
the double bond or introduce competing nonradiative decay
pathways, including intramolecular hydrogen transfer.
The values of ΦZ,E for Z-2 and Z-5 are larger in polar, aprotic
solvents than in benzene, resulting in a decrease in the Z isomer
content of the photostationary state. Polar solvents might disrupt
the intramolecular hydrogen bond as previously observed for
the 3-(2-pyridyl)propenamides,5 but not for Z-1.4 The low
values of ΦZ,E for Z-2-Z-4 in ethanol solution may reflect a
solvent-induced barrier for isomerization, as proposed for the
E isomers.
Figure 9. Schematic potential energy surfaces for the singlet state
photoisomerization and hydrogen transfer of (E) and (Z)-ethyl 3-(2-
indolyl)propenoate (2) and N,N-dimethyl-3-(2-indolyl)propenamide (5).
CHART 3
The fluorescence excitation and emission spectra of Z-2 at
77 K in an EPA glass are shown in Figure 7, along with the
spectra of E-2. Excitation at 394 nm, a wavelength absorbed
by Z-2 but not anti-E-2, results in a broad spectrum with poorly
resolved vibrational structure and a 0,0 band at 431 nm. The
red-edge of the excitation spectrum is in the same spectral region
as the long-wavelength tail of the room temperature absorption
spectrum of Z-2 (Figure 2). Excitation at 342 nm, a wavelength
absorbed by both E-2 and Z-2, results in overlapping emission
from both isomers. Since E-2 is more strongly fluorescent than
Z-2 in the glass as well as in solution, trace amounts of E-2
which are either present initially or formed via photoisomer-
ization in the glass can account for the observation of emission
from E-2. Repeated scanning results in a growth of E-2
fluorescence, indicating that isomerization of Z-2 does occur
in the glass.
The fluorescence excitation and emission spectra of Z-5 at
77 K in an EPA glass are shown in Figure 8. Similar spectra
were obtained in a nonpolar methylcyclohexane glass. The
maximum in the excitation spectrum of Z-5 is at shorter
wavelength than that for Z-2 (376 vs 394 nm); however, the
emission spectrum of Z-5 is broadened and substantially red-
shifted compared to that of Z-2 (Figure 7). The strongly red-
shifted fluorescence of Z-5 may result from adiabatic intramo-
lecular hydrogen transfer resulting in the formation of the
fluorescent tautomer Z-5′ (Chart 3). Adiabatic formation of a
fluorescent tautomer has been observed both at room temper-
ature and at 77 K for other arylethylenes possessing intramo-
lecular hydrogen bonds.3,6 Tautomerization may account for
the low value of ΦZ,E as well as the absence of fluorescence
from Z-5 in solution or the glass. The observation of structured
fluorescence from Z-2 but tautomer fluorescence from Z-5′
suggests that the tautomerization process may be more favorable
for singlet Z-5. One possible explanation for this difference is
the higher singlet energy for Z-5 vs Z-2 estimated from the 0,0
bands of their fluorescence excitation spectra.
Concluding Remarks. The results of our investigation of
the structure and photochemical behavior of the E and Z isomers
of 2 and 5 can be summarized using the potential energy
diagram in Figure 9. Both E-2 and E-5 exist as mixtures of
two ground state conformers, which are anti (major) or syn
(minor) with respect to rotation about the vinyl-indole bond
and have the same enone conformation (s-trans for E-2 and s-cis
for E-5). The methylated indoles E-3 and E-4 provide single-
conformer models for anti-E-2 and syn-E-2, respectively. The
excited state behaviors of the anti conformers of E-2 and E-5
are similar to that of E-3, but quite different from that of the
syn conformers or E-4. The lower singlet energies of the syn
conformers (Figure 9) are consistent with the results of ZINDO

14566 J. Phys. Chem., Vol. 100, No. 34, 1996
Lewis and Yang
calculations. Photoisomerization of the anti conformers and E-3
is ∼70 times faster than for the syn conformers or E-4, resulting
in shorter lifetimes and lower fluorescence quantum yields for
the anti conformers. Faster isomerization may be attributed to
the higher singlet energies of the anti conformers.
The Z isomers of 2, 3, and 5 exist as single ground state
conformers which possess a strong intramolecular hydrogen
bond. The Z isomers are very weakly fluorescent and have low
photoisomerization quantum yields in nonpolar solution. Their
singlet energies (Figure 9) can be estimated from fluorescence
excitation spectra in low-temperature glasses. The lower singlet
energy of Z-2 vs E-2 is consistent with the results of ZINDO
calculations (supporting information). The low-temperature
fluorescence of Z-2 and Z-5 are assigned to the singlet of Z-2
and the tautomer of Z-5, respectively. The occurrence of
tautomerization for Z-5 but not Z-2 may reflect the higher singlet
energy of Z-5 (Figure 9).
window or Oxford Instruments variable temperature liquid
nitrogen cryostat model DN1704 equipped with a model ITC4
temperature controller. Fluorescence quantum yields were
determined relative to equiabsorbing solutions of pyrene (ΦF
) 0.60 in nonpolar and 0.53 in polar solvent23a), anthracene
(ΦF ) 0.27 in nonpolar and polar solvent23a), or 2,5-bis(5-tert-
butyl-2-benzoxazolyl)thiophene (BBOT, ΦF ) 0.6723b,c) in
methylcyclohexane. Fluorescence decays were obtained on a
Photon Technology International LS-1 single-photon-counting
apparatus with a gated hydrogen arc lamp (time resolution ∼0.2
ns) using a scatter solution to approximate the lamp decay.
Decays were analyzed using deconvolution and single- or
multiexponential least-squares fitting as described by James et
al.24 The goodness of the fit was judged by the reduced ø2
value (<1.2 in all cases), the randomness of the residuals, and
the autocorrelation function.
Photoisomerization quantum yields and E,Z photoequilibria
were determined for 2 mL aliquots of 0.005 M solutions of esters
or amides contained in 1 cm path length quartz cuvettes sealed
with white rubber septa and purged for 5-10 min with dry
nitrogen. Samples were irradiated in triplicate with light from
the Oriel Optical bench equipped with a 200 W high-pressure
mercury-xenon lamp and a Bausch and Lomb high-intensity
monochromator set at 313 nm. Quantum yields were analyzed
for isomer formation at less than 10% conversion with trans-
stilbene as the actinometer.25 Product ratios were analyzed with
Hewlett-Packard 5890A Chromatograph equipped with 10 m
× 0.53 mm capillary columns coated with poly(dimethylsilox-
ane) or poly(methylphenylsiloxane). Ground state thermal
equilibria were determined by irradiating benzene solutions of
2, 3, and 4 containing a few iodine crystals with an incandescent
lamp or by irradiation of benzene solutions of 5 and diphenyl
diselenide (both 5 × 10-3 M) with 450 nm lamps.16
These results provide the first detailed information about the
effects of â-carboxylate substituents on the spectroscopy and
photochemistry of the conformational isomers of an arylethyl-
ene. The photophysical properties of the anti and syn conform-
ers of E-2 and E-5 in nonpolar solvents are quite similar to
those of the diarylethylene E-1.4 A â-carboxylate substituent
significantly shortens the singlet lifetimes of vinylnaphthalene9
and styrene11 due to the introduction of a low-energy n,π*
singlet state. This is not the case for the vinylindoles E-2 and
E-5 which have lowest energy π,π* excited states. Since the
lowest singlet states of E-1, E-2, and E-5 are localized on the
vinylindole portion of the molecule, it is not surprising that the
behavior of the excited state is more dependent upon vinylindole
conformation (anti or syn) than the enone conformation (s-trans
for E-2 and s-cis for E-5). Thus the presence of alternate ground
state enone conformers (s-cis for E-2 and s-trans for E-5) may
have gone undetected in our fluorescence decay measurements.
The â-carboxylate substituent has a more pronounced effect
on the structure and behavior of the Z-2 and Z-5 than on their
E isomers due to the presence of an intramolecular hydrogen
bond. The observation of red-shifted fluorescence from Z-5 in
a low-temperature glass indicates that excited state hydrogen
transfer followed by return transfer in the ground state provides
a pathway for nonradiative decay which may be responsible
for the inefficient photoisomerization of the Z isomers in
solution. In view of the polar nature of the ester and amide
groups, it is not surprising that the behavior of both the E and
Z isomers of 2 and 5 is strongly solvent dependent.
Minimum energy conformations were calculated using a
Macintosh IIsi computer with a MM2 type force field as
supplied in the Chem 3D software package.14 The calculations
of electronic structure and optical spectra were performed using
the semiempirical INDO/S-SCF-CI (ZINDO) algorithm devel-
oped by Zerner and co-workers.18 The ZINDO calculations
were performed on a Stellar mini supercomputer and required
approximately 5-10 min of CPU time.
Materials. All solvents were spectral grade (Aldrich or
Fisher) unless otherwise noted. Dichloromethane was distilled
over calcium hydride, and benzene was distilled over sodium
metal prior to use. Cyclohexane, hexane, acetonitrile, ethyl
alcohol (anhydrous, Aldrich), and dimethyl sulfoxide (HPLC
grade, Aldrich) were used as received. trans-Stilbene (Aldrich)
was recrystallized two times from benzene and once from ethyl
alcohol. All other compounds were used as received unless
otherwise indicated.
Experimental Section
General Methods. 1H NMR and 13C NMR (proton-de-
coupled) spectra were recorded using a Varian Gemini 300
spectrometer. NOE experiments were performed with a Varian
Unity Plus 400 spectrometer using samples degassed by four
freeze-pump-thaw cycles. Elemental analyses were performed
by Oneida Research Services Inc., Whitesboro, NY. UV-vis
absorption spectra were recorded using a Hewlett-Packard
8542A diode array spectrophotometer in 1 cm path length quartz
cuvettes. Infrared spectra of ca. 0.01 M solutions were recorded
on a Mattson FT-IR spectrometer. Mass spectra were deter-
mined with a Hewlett-Packard 5985 GC/VG Analytical 70-
250SE MS system using an ionizing voltage of 70 V. Melting
points were determined on a Fisher-Johns melting point ap-
paratus.
trans- and cis-Ethyl 3-(2-Indolyl)propenoate (E-2 and Z-2).
Reduction of indole-2-carboxylic acid (Aldrich) with LiAlH4
afforded 2-hydroxymethylindole [89%, mp 73-73.5 °C (lit26
mp 75-76 °C)]. Oxidation of the alcohol using sodium
dichromate dihydrate and concentrated sulfuric acid in DMSO27
yielded 2-indolecarboxaldehyde [34%, mp 141.5-142.5 °C (lit28
mp 139-141 °C)]. A salt-free Wittig reaction28 of the aldehyde
with (carbethoxymethylene)triphenylphosphorane (Aldrich) af-
forded a mixture of E-2 and Z-2 which was separated by column
chromatography using benzene as eluent to provide E-2 and
Z-2 (73% and 7%, respectively) as pale yellow solids of high
Fluorescence spectra of degassed solutions were recorded with
a Spex Fluoromax spectrometer. Low-temperature emission
spectra and fluorescence spectra were obtained using a liquid
nitrogen cooled fluorescence Dewar with a hanging finger
1
purity (>99% by GC analysis). E-2: mp ) 121-122 °C; H
NMR (CDCl3) δ 1.36 (t, J ) 7.1 Hz, 3H), 4.30 (q, J ) 7.1 Hz,
2H), 6.26 (d, J ) 16.1 Hz, 1H), 6.83 (s, 1H), 7.15 (t, J ) 8.0

(E)- and (Z)-Ethyl 3-(2-Indolyl)propenoate
J. Phys. Chem., Vol. 100, No. 34, 1996 14567
Hz, 1H), 7.27 (t, J ) 8.2 Hz, 1H), 7.39 (d, J ) 8.2 Hz, 1H),
7.63 (d, J ) 8.0 Hz, 1H), 7.69 (d, J ) 16.2 Hz, 1H), 8.49 (bs,
1H); 13C NMR (CDCl3) δ 14.3, 60.6, 109.0, 111.1, 115.4, 120.6,
121.5, 124.6, 128.4, 133.3, 134.3, 137.7, 167.0; IR (CHCl3)
3471, 1695, 1634, 1268, 1180, 1128 cm-1; MS (m/e) 215 (M+,
89), 169 (100), 141 (33), 115 (24), 89 (9); HRMS 215.0958
(obsd) and 215.0947 (calcd). Anal. Calcd for C13H13N2O: C,
72.53; H, 6.09; N, 6.51. Found: C, 72.29; H, 6.12; N, 6.40.
123.5, 121.9, 119.9, 118.2, 109.4, 108.2, 60.3, 29.7, 14.2; IR
(CHCl3) 1709, 1622, 1446, 1181 cm-1.
trans-N,N-Dimethyl-3-(2-indolyl)propenamide (E-5). The
ester E-2 was converted to the amide E-5 (72%) on standing
for 3 days in a saturated methanolic solution of dimethylamine
in the presence of sodium methoxide.31 mp ) 234.5-235 °C;
1H NMR (CDCl3) δ 3.11 (s, 3H), 3.21 (s, 3H), 6.78 (d, J )
15.6 Hz, 1H), 6.81 (s, 1H), 7.11 (t, J ) 7.1 Hz, 1H), 7.23 (t, J
) 7.1 Hz, 1H), 7.37 (d, J ) 8.0 Hz, 1H), 7.61 (d, J ) 8.0 Hz,
1H), 7.76 (d, J ) 15.4 Hz, 1H), 8.76 (bs, 1H); 13C NMR (CD3-
OD) δ 36.3, 37.8, 108.9, 112.1, 115.4, 120.8, 122.1, 124.9,
129.8, 134.2, 135.8, 139.7, 169.1. IR (CHCl3) 3467, 1648,
1596, 1126 cm-1; MS (m/e) 214 (M+, 70), 170 (100), 142 (31),
115 (34), 89 (11); HRMS 214.1117 (obsd) and 214.1106 (calcd).
cis-N,N-Dimethyl-3-(2-indonyl)propenamide (Z-5). A 0.01
M solution of E-5 in dichloromethane was irradiated in a
Rayonet reactor fitted with RPR 3500 lamps. The photolysis
was stopped after reaching the photostationary state consisting
of ca. 96% cis isomer. The solvent was removed and the residue
chromatographed on silica gel using hexane/ethyl acetate (1:1)
eluent to provide the Z-5 in >99% purity on the basis of GC
1
Z-2: mp ) 84.5 -85.5 °C; H NMR (CDCl3) δ 1.35 (t, J )
7.1 Hz, 3H), 4.27 (q, J ) 7.1 Hz, 2H), 5.78 (d, J ) 12.6 Hz,
1H), 6.76 (s, 1H), 6.93 (d, J ) 12.6 Hz, 1H), 7.09 (t, J ) 7.1
Hz, 1H), 7.26(t, J ) 7.1Hz, 1H), 7.44 (d, J ) 8.1 Hz, 1H),
7.62 (d, J ) 8.0 Hz, 1H), 11.87 (bs, 1H); 13C NMR (CDCl3) δ
14.2, 60.8, 111.7, 112.0, 112.9, 120.1, 121.4, 124.7, 127.5,
133.7, 135.1, 137.6, 168.5; IR (CHCl3) 3300, 1694, 1614, 1201,
1125 cm-1; MS (m/e): 215 (M+, 87), 169 (100), 141 (36), 115
(26), 89 (15); HRMS 215.0962 (obsd) and 215.0947 (calcd).
trans- and cis-Ethyl 3-(3-Methyl-2-indolyl)propenoate (E-3
and Z-3). 2,3-Dimethylindole (TCI) was converted to 3-methyl-
2-indolecarboxaldehyde by the method of Sakai et al.29 [21%,
mp 137-139 °C (lit29 mp 139 °C)]. Wittig reaction of aldehyde
as described above followed by column chromatography
provided E-3 and Z-3 (96% and 4%, respectively) as pale yellow
solids of high purity (>99% by GC analysis). Irradiation of
E-3 in dichloromethane solution with 350 nm light afforded a
20:1 mixture of Z-3:E-3 from which pure Z-3 could be obtained
by column chromatography followed by recrystallization.
E-3: mp ) 157-158 °C; 1H NMR (CDCl3) δ 1.34 (t, J ) 7.1
Hz, 3H), 2.39 (s, 3H), 4.30 (q, J ) 7.1 Hz, 2H), 6.20 (d, J )
15.9 Hz, 1H), 7.09 (t, J ) 7.1 Hz, 1H), 7.22-7.36 (m, 2H),
7.56 (d, J ) 7.8 Hz, 1H), 7.82 (d, J ) 15.9 Hz, 1H), 8.58 (bs,
1H); 13C NMR (CDCl3) δ 8.9, 14.3, 60.0, 111.0, 113.7 (2C),
118.7, 119.8, 125.0, 129.0, 129.9, 132.3, 137.4, 167.4; IR
(CHCl3) 3478, 1689, 1626, 1284, 1179 cm-1. Z-3: mp )
1
analysis. Mp ) 79-80 °C; H-NMR (CDCl3) δ 3.02 (s, 3H),
3.07 (s, 3H), 5.97 (d, J ) 12.7 Hz, 1H), 6.61(s, 1H), 6.75(d, J
) 12.7 Hz, 1H), 6.99 (t, J ) 7.9 Hz, 1H), 7.14 (t, J ) 8.0 Hz,
1H), 7.36 (d, J ) 8.3 Hz, 1H), 7.52 (d, J ) 8.1 Hz, 1H), 12.15
(bs, 1H); 13C NMR (CDCl3) δ 36.1, 38.4, 109.8, 112.0, 113.4,
119.7, 121.0, 123.9, 127.8, 131.5, 134.1, 137.4, 167.7; IR
(CHCl3) 3246, 1640, 1591, 1121 cm-1; MS (m/e) 214 (M+,
93), 170 (100), 142 (29), 115 (33), 89 (10); HRMS 214.1098
(obsd) and 214.1106 (calcd).
Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
for support of this research.
1
56.5-57 °C; H NMR (CDCl3) δ 1.37 (t, J ) 7.1 Hz, 3H),
Supporting Information Available: Figures S1-S3 show-
ing symmetries and energies of frontier orbitals and Table S1
reporting calculated absorption maxima, oscillator strengths, and
configuration interactions of the singlet states of anti-E-2, syn-
E-2, and Z-2 (4 pages). Ordering information is given on any
current masthead page.
2.45 (s, 3H), 4.28 (q, J ) 7.1 Hz, 2H), 5.77 (d, J ) 12.7 Hz,
1H), 7.04-7.12 (m, 2H), 7.27 (t, J ) 7.0 Hz, 1H), 7.41(d, J )
8.3 Hz, 1H), 7.59 (d, J ) 8.1 Hz, 1H), 11.80 (bs, 1H); 13C
NMR (CDCl3) δ 9.1, 14.2, 60.7, 111.4 (2C), 111.8, 119.3, 119.8,
125.0, 127.8, 130.2, 132.1, 136.6, 168.8; IR (CHCl3) 3300, 1688,
1599, 1336, 1191 cm-1. Anal. Calcd for C14H15N2O: C, 73.33;
H, 6.60; N, 6.11. Found: C, 72.99; H, 6.58; N, 5.99.
References and Notes
trans- and cis-Ethyl 3-(N-Methyl-2-indolyl)propenoate (E-4
and Z-4). Methylation of E-2 and Z-2 by the method of
Eenkhoom et al.30 using sodium hydride as the base and methyl
iodide as the methylating reagent in N,N-dimethylformamide
afforded E-4 and Z-4 (55-60%). E-4 was purified (>99% pure
on the basis of GC analysis) by recrystallization from benzene/
hexane and then by column chromatography using benzene as
the eluent. Z-4 was purified by column chromatography and
then by preparative TLC using benzene as the eluent and the
mobile phase. The resulting pale yellow oil contained ca. 2-3%
impurity of E-4. E-4: mp ) 87-87.5 °C; 1H NMR (CDCl3) δ
1.36 (t, J ) 7.1 Hz, 3H), 3.83 (s, 3H), 4.29 (q, J ) 7.1 Hz,
2H), 6.49 (d, J ) 15.7 Hz, 1H), 6.96 (s, 1H), 7.12 (t, J ) 8.0
Hz, 1H), 7.21-7.37 (m, 2H), 7.62 (d, J ) 8.0 Hz, 1H), 7.80
(d, J ) 15.7 Hz, 1H). 13C NMR (CDCl3) δ 14.4, 30.0, 60.6,
103.7, 109.6, 118.1, 120.4, 121.3, 123.6, 127.4, 132.6, 134.9,
139.0, 167.0. IR (CHCl3) 1701, 1631, 1282, 1184 cm-1. Anal.
Calcd for C14H15N2O: C, 73.33; H, 6.60; N, 6.11. Found: C,
73.30; H, 6.56; N, 6.15. Z-4: oil; 1H NMR (CDCl3) δ 7.66 (t,
J ) 7.6 Hz, 1H), 7.67 (s, 1H), 7.32-7.23 (m, 2H), 7.10 (t, J )
7.9 Hz, 1H), 6.97 (d, J ) 12.9 Hz, 1H), 5.99 (d, J ) 12.8 Hz,
1H), 4.26 (q, J ) 7.1, 2H), 3.78 (s, 3H), 1.32 (t, J ) 7.1, 3 Hz,
3H); 13C NMR (CDCl3) δ 165.9, 138.0, 133.2, 130.3, 127.4,
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