Photochemical reactions of thiobenzamides bearing an allylic substituent on the nitrogen atom: Double-bond migration via tandem 1,4- And 1,6-hydrogen transfer

Article abstract of DOI:10.1039/a700247e

N-(2-Phenylprop-2-enyl)thiobenzamides 1a-d underwent double-bond migration on irradiation to give N-(2-phenylprop-1-enyl)thiobenzamides 2a-d via consecutive 1,4- and 1,6-hydrogen transfer. Photoreaction of an N-(prop-2-enyl)thiobenzamide 1e and an N-(3-phenylprop-2-enyl)thiobenzamide if did not give migration products, but afforded pyrroles 3e and 3f and dealkylation products 4a in low yields.

Full text of DOI:10.1039/a700247e

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Photochemical reactions of thiobenzamides bearing an allylic
substituent on the nitrogen atom: double-bond migration via tandem
1,4- and 1,6-hydrogen transfer
Hiromu Aoyama
Department of Material Creation Chemistry, Faculty of Textile Science and Technology,
Shinshu University, Ueda, Nagano 386, Japan
N-(2-Phenylprop-2-enyl)thiobenzamides 1a–d underwent double-bond migration on irradiation to give
N-(2-phenylprop-1-enyl)thiobenzamides 2a–d via consecutive 1,4- and 1,6-hydrogen transfer.
Photoreaction of an N-(prop-2-enyl)thiobenzamide 1e and an N-(3-phenylprop-2-enyl)thiobenzamide 1f
did not give migration products, but afforded pyrroles 3e and 3f and dealkylation products 4a in low
yields.
Table 1 Photoreactions of compounds 1 in benzene
Introduction
Thiocarbonyl compounds such as thioketones1 and thioimides2
Yield (%)
undergo photochemical reactions similar to those of carbonyl
Compound
Conversion (%)
2
3
4
5
compounds. Thioamides are, however, photochemically much
less reactive than these compounds and few reports have been
published on their photochemical reactions. Recently, Nishio
and Okuda reported [2 ϩ 2] cycloaddition of cyclic thio-
anilides.3 Similar reactions of thiobenzamides have also been
reported by Oda et al.4 In relation to our previous study on
the photochemistry of nitrogen-containing thiocarbonyl com-
pounds,5 I report here photochemical reactions of thio-
benzamides having an allylic substituent on the nitrogen atom.
These reactions involve a novel double-bond migration via con-
secutive 1,4- and 1,6-hydrogen transfer and provide the first
example of hydrogen abstraction by excited thioamides.
1a
1b
1c
1d
1e
1f
28
26
20
31
32
36
77
69
60
22
b
5
9
a
a
a
a
a
a
a
b
b
b
12
14
12
3
10
6
b
Trace. b Not detected.
a
was found to be a secondary product from thioenamide 2a since
irradiation of this substrate 2a gave compound 5a. The photo-
reaction of compound 1a was clean at low conversion (Table 1),
but became sluggish at higher conversion and a considerable
amount of the cyclization product 5a was produced along with
unidentified by-products. This is apparently due to the second-
ary reactions of intermediate 2a; the extinction coefficient of
compound 2a is larger than that of substrate 1a in the range
300–440 nm. Reactions similar to the photocyclization of com-
pound 2a to the isoquinolinethione 5a have been reported.6
Photoreaction of 4-chloro- and 4-methoxy-thiobenzamide 1b
and 1c gave similar results (Table 1). The structures of the
thioenamides 2a–c were confirmed by independent syntheses
via thionation of the corresponding enamides,6 and the E
geometry of the enamides was determined on the basis of
nuclear Overhauser effect (NOE) difference spectra. The struc-
tures of the pyrroles 3a and 3b were also established by
unequivocal syntheses (see Experimental section).7
Results and discussion
Photoreactions in solution
When N-isopropyl-N-(2-phenylprop-2-enyl)thiobenzamide 1a
in benzene was irradiated with a high-pressure mercury lamp, a
thioenamide 2a and a pyrrole 3a were obtained as main prod-
ucts together with a dealkylation product 4a and a cyclization
product 5a (Scheme 1). Among these products, compound 5a
Irradiation of an N-methyl derivative 1d also gave the cor-
responding thioenamide 2d and pyrrole 3d as with the reactions
of analogues 1a–c, although the yield of compound 2d was low.
In this case product 2d was a mixture of E and Z isomers.
Meanwhile, photochemical reaction of N-allyl-N-isopropyl-
thiobenzamide 1e and N-cinnamyl-N-isopropylthiobenzamide
1f did not yield thioenamides but instead afforded pyrroles 3e
and 3f and N-isopropylthiobenzamide 4a as main products,
although these reactions were not clean. The structure of com-
pound 3d was determined by direct comparison with an authen-
tic sample,7 and that of compound 3e was confirmed by its
independent synthesis via photocyclization of an enamino
ketone,8 whereas that of compound 3f was elucidated on the
basis of spectral data and elemental analysis.
Mechanism
The formation of the thioenamides 2 and the pyrroles 3 can be
Scheme 1 Condition: i, hν
J. Chem. Soc., Perkin Trans. 1, 1997
1851

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bond; some amides and esters are known to undergo similar
photoreactions.10 The cyclization of compounds 2 to isoquino-
linethiones 5 is explicable by 6π electrocyclization and sub-
sequent 1,5-hydrogen shift as in the case of photocyclization of
enamides.6
The present reaction provides the first example of photo-
chemical hydrogen abstraction of thioamides and the double-
bond shift via consecutive 1,4- and 1,6-hydrogen transfer is
without precedent to our best knowledge. The efficient intra-
molecular hydrogen abstraction by these excited thioamides is
presumed to be due to the high reactivity of the allylic hydro-
gens toward abstraction; the hydrogens are further activated by
the adjacent nitrogen atom. Moreover, it is known that excited
thioketones undergo 1,4- or 1,6-hydrogen transfer efficiently11
in contrast to the selective 1,5-hydrogen transfer (γ-hydrogen
abstraction) of excited carbonyl compounds.
Scheme 2 Condition: i, hν
rationalized by the mechanism shown in Scheme 2. β-Hydrogen
abstraction (1,4-hydrogen transfer) by the excited thiocarbonyl
group gives a diradical 6. Hydrogen transfer in diradicals 6 via a
seven-membered cyclic transition state (1,6-hydrogen transfer)
yields the thioenamide 2, whereas cyclization of diradical 6
followed by elimination of hydrogen sulfide produces the
pyrrole 3.
In confirmation of this mechanism, photoreaction of a deu-
teriated thiobenzamide 1a-d2 was carried out. The resulting
thioenamide 2a-d2 possessed one of the deuterium atoms in the
allylic methyl group and the other one on the olefinic carbon
adjacent to the nitrogen atom, whereas the pyrrole 3a-d carried
a deuterium atom at the 5-position: the positions of the deuter-
ium atoms were determined on the basis of NMR spectroscopy.
This observation is consistent with the above mechanism. Fur-
ther, the comparison of the quantum yield for the formation of
2a-d2 (Φ = 0.0050) with that for the parent 2a (Φ = 0.026) sup-
Experimental
Mps were measured on a Yanagimoto micromelting point
apparatus and are not corrected. Distillation was effected with
a cold-finger-type apparatus for molecular distillation (only
oven temperature is given). IR Spectra were measured in CHCl3
and NMR spectra were obtained in CDCl3. J-Values are in Hz.
General procedure for preparation of thiobenzamides 1
N-Isopropyl-N-(2-phenylprop-2-enyl)amine,
N-isopropyl-N-
(prop-2-enyl)amine, N-isopropyl-N-(3-phenylprop-2-enyl)-
amine and N-methyl-N-(2-phenylprop-2-enyl)amine were syn-
thesized from the corresponding alkenyl bromide and an
alkylamine. After benzoylation of the amines with benzoyl
chloride or other aroyl chlorides, the resulting amides were
treated with the Lawesson’s reagent in refluxing toluene for 1 h.
After evaporation off of the solvent, the residue was chromato-
graphed on silica gel to give the thioamides 1. The thioamides
obtained were mixtures of two rotational isomers in solution.
N-Isopropyl-N-(2-phenylprop-2-enyl)thiobenzamide 1a. Mp
121–122 ЊC; νmax/cmϪ1 1460; λmax(hexane)/nm 245 (ε 17 300),
280sh (10 100), 379 (220) and 400 (210); λmax(MeOH)/nm 250
(15 700), 282 (10 900) and 369 (135); δH 1.20 and 1.36 (6 H in
total, ~3:1, both d, J 7), 4.34 and 5.01 (2 H in total, ~1:3, both
s), 4.42 and 6.03 (1 H in total, ~3:1, both sept, J 7), 5.19, 5.23,
5.42 and 5.47 (2 H in total, ~3:1:1:3, each s) and 6.9–7.55 (10
H, m); δC 19.8 and 20.8 (CH3), 49.2 and 50.7 (CH2), 52.8 and
ports the mechanism since the large isotope effect (ΦH /
ΦD = 5.2) indicates that the cleavage of the allylic C᎐H bond is
involved in the transition state of the photoreaction of com-
pounds 1.
It may be conceivable that compounds 2 are formed by direct
1,3-hydrogen transfer of the styrene moiety of compounds 1
since concerted suprafacial 1,3-sigmatropic reactions of alkenes
are photochemically allowed. This mechanism is, however,
improbable as detailed below. Photochemical 1,3-hydrogen
shifts of styrenes have not been reported in spite of extensive
studies on their photochemistry.9 The thioenamide 2a was
formed on selective excitation of the thioamide group of com-
pound 1a with visible radiation (436 nm). The concomitant
formation of the pyrroles is consistent with the mechanism
involving the corresponding diradical intermediate 6. Further,
the amide 7, an oxygen analogue of compound 1a, did not
55.6 (CH), 111.9 and 114.6 (᎐CH ), 124.9–129.1, 139.0 (C),
᎐
2
139.7 (C), 140.8 (C), 144.1 (C), 144.8 (C), 202.4 and 203.3
(C᎐S) (Found: C, 76.85; H, 7.12; N, 4.76. C H NS requires C,
᎐
19 21
77.24; H, 7.16; N, 4.74%).
4-Chloro-N-isopropyl-N-(2Ј-phenylprop-2Ј-enyl)thiobenz-
amide 1b. Mp 133–136 ЊC; νmax/cmϪ1 1465; δH 1.19 and 1.35 (6
H in total, ~3:1, both d, J 7), 4.35 and 4.99 (2 H in total, ~1:3,
both s), 4.38 and 5.99 (1 H in total, ~3:1, both sept, J 7), 5.14,
5.20 and 5.46 (2 H in total, ~3:1:4, each s) and 6.9–7.6 (9 H,
m); δC 19.77 and 20.77 (CH3), 49.2 and 50.7 (CH2), 53.0 and
55.8 (CH), 111.9, 114.6 (᎐CH ), 125.8–128.8, 134.1 (C), 138.6
᎐
2
(C), 139.5 (C), 140.8 (C), 142.4 (C), 144.6 (C), 200.9 and 201.7
(C᎐S) (Found: C, 68.72; H, 6.12; N, 4.10. C H ClNS requires
᎐
19 20
C, 69.18; H, 6.11; N, 4.24%).
N-Isopropyl-4-methoxy-N-(2Ј-phenylprop-2Ј-enyl)thiobenz-
amide 1c. Mp 155–156 ЊC; νmax/cmϪ1 1455; δH 19.9 and 20.8
(CH3), 49.3 and 50.8 (CH2), 53.2 and 55.4 (CH), 1.19 and 1.35
(6 H in total, ~5:1, both d, J 7), 3.77 and 3.82 (3 H in total,
~1:5, both s), 4.39 and 5.01 (2 H in total, ~1:5, both s), 4.51
and 6.00 (1 H in total, both sept, J 7), 5.16, 5.22 and 5.45 (2 H
in total, each s) and 6.85–7.6 (9 H, m); δC 55.7 (CH3), 111.8 and
undergo double-bond migration on direct excitation of the
styryl group or on xanthone-sensitized irradiation. These find-
ings clearly show that the present photoreactions are reactions
of the excited thioamides rather than those of the excited
styrenes.
The fact that the N-cinnamyl thioamide 1f does not undergo
the double-bond shift is not unreasonable since the reaction
requires destruction of the conjugation in the cinnamyl group.
The formation of the N-alkylthiobenzamides 4 (cleavage
products) can be rationalized in terms of homolysis of the C᎐N
114.5 (᎐CH ), 113.2, 113.9, 125.9–128.5, 136.7 (C), 136.9 (C),
᎐
2
139.7 (C), 144.0 (C), 144.7 (C), 159.6 (C), 202.7 (C᎐S) (Found:
᎐
C, 73.68; H, 7.19; N, 4.20. C20H23NOS requires C, 73.81; H,
7.12; N, 4.30%).
N-Methyl-N-(2-phenylprop-2-enyl)thiobenzamide 1d. Bp
1852
J. Chem. Soc., Perkin Trans. 1, 1997

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150 ЊC/2 mmHg; νmax/cmϪ1 1495; δH 2.91 and 3.55 (3 H in total,
both s, ~1:0.9), 4.46 and 5.30 (2 H in total, both s), 5.19, 5.30,
5.52 and 5.60 (2 H in total, each s) and 7.0–7.6 (10 H, m); δC
40.7 and 41.4 (CH3), 57.4 and 59.6 (CH2), 114.8 and 115.4
cmϪ1 1605; δH 1.41 (6 H, d, J 7), 4.48 (1 H, sept, J 7), 6.44 (1 H,
d, J 1.7) and 7.1–7.7 (11 H, m); δC 24.1 (CH3), 47.4 (CH), 106.3
(3-C), 114.0 (5-C), 124.5–129.2, 133.5 (C), 135.0 (C) and 135.8
(C) (Found: C, 87.28; H, 7.36; N, 5.36. C19H19N requires C,
87.30; H, 7.33; N, 5.36%).
(᎐CH ), 125.4–128.7, 137.6 (C), 138.1 (C), 142.6 (C), 143.05 (C),
᎐
2
143.11 (C), 143.4 (C), 202.4 and 202.6 (C᎐S) (Found: C, 76.08;
H, 6.39; N, 5.24. C17H17NS requires C, 76.36; H, 6.41; N,
5.24%).
2-Isopropyl-4-methyl-4-phenyl-3,4-dihydroisoquinoline-1(2H)-
thione 5a. Mp 144–145 ЊC; νmax/cmϪ1 1485; δH 0.78 (3 H, d, J 7),
1.22 (3 H, d, J 7), 1.74 (3 H, s), 3.42 and 3.70 (2 H, ABq, J 13),
6.02 (1 H, sept, J 7), 7.0–7.5 (8 H, m) and 8.7 (1 H, m); δC 18.2
(CH3), 18.5 (CH3), 24.5 (CH3), 41.7 (C), 51.9 (CH), 54.5 (CH2),
᎐
N-Isopropyl-N-(prop-2-enyl)thiobenzamide 1e. Mp 55–57 ЊC;
νmax/cmϪ1 1455; δH 1.18 and 1.35 (6 H in total, ~3:1, both d, J 7),
4.03 and 4.66 (2 H in total, ~1:3, both m), 4.29 and 5.93 (1 H in
total, ~3:1, both sept, J 7), 4.90–5.07, 5.23–5.35, 5.56–5.70 and
6.05–6.19 (3 H in total, each m) and 7.1–7.4 (5 H, m); δC 19.9
and 21.0 (CH3), 48.6 and 50.2 (CH2), 52.7 and 55.5 (CH),
117.3 and 117.8 (᎐CH ), 114.7–134.2, 114.0 (C), 114.1 (C),
125.0–133.1, 134.6 (C), 140.4 (C), 143.7 (C) and 190.5 (C᎐S)
᎐
(Found: C, 77.01; H, 7.20; N, 4.73. C19H21NS requires C, 77.24;
H, 7.16; N, 4.74%).
(E)-4-Chloro-N-isopropyl-N-(2Ј-phenylprop-1Ј-enyl)thiobenz-
amide 2b. Mp 62–64 ЊC; νmax/cmϪ1 1420; δH 1.34 (6 H, d, J 7), 1.68
(3 H, d, J 1), 5.78 (1 H, sept, J 7), 6.28 (1 H, q, J 1) and 7.0–7.5
(9 H, m); δC 16.9 (CH3), 19.1 (CH3), 54.5 (CH), 124.8–128.6,
᎐
2
201.0 and 202.2 (C᎐S) (Found: C, 71.26; H, 7.70; N, 6.30. Calc.
᎐
for C13H17NS: C, 71.18; H, 7.81; N, 6.39%).
N-Isopropyl-N-(3-phenylprop-2-enyl)thiobenzamide 1f. Mp
108–109 ЊC; νmax/cmϪ1 1460; δH 1.21 and 1.38 (6 H in total,
~3:1, both d, J 7), 4.17 and 4.81 (2 H in total, ~1:3, both d, J 5),
4.31 and 5.97 (1 H in total, both sept, J 7), 5.83–6.09 and 6.51–
6.68 (2 H in total, both m) and 7.2–7.5 (10 H, m); δC 19.9 and
21.1 (CH3), 48.4 and 49.7 (CH2), 52.5 and 55.5 (CH), 124.5–
133.1, 136.0 (C), 136.7 (C), 144.0 (C), 144.1 (C), 200.9 and
134.7 (C), 139.3 (C), 139.5 (C), 143.1 (C) and 200.3 (C᎐S)
(Found: C, 69.20; H, 6.12; N, 4.32. C19H20ClNS requires C,
69.18; H, 6.11; N, 4.25%).
2-(4Ј-Chlorophenyl)-1-isopropyl-4-phenylpyrrole 3b. Mp 131–
132 ЊC; δH 1.43 (6 H, d, J 7), 4.42 (sept, J 7), 6.43 (1 H, s) and
7.1–7.6 (10 H, m); δC 24.1 (CH3), 47.5 (CH), 106.6 (3-C), 114.4
(5-C), 124.7–130.4, 131.9 (C), 133.2 (C), 133.7 (C) and 135.6 (C)
(Found: C, 77.14; H, 6.12; N, 4.71. C19H18ClN requires C, 77.15;
H, 6.13; N, 4.74%).
᎐
202.2 (C᎐S) (Found: C, 77.16; H, 7.16; N, 4.57. C H NS
᎐
19 21
requires C, 77.24; H, 7.16; N, 4.74%).
6-Chloro-2-isopropyl-4-methyl-4-phenyl-3,4-dihydroisoquino-
line-1(2H)-thione 5b. Mp 146–149 ЊC; νmax/cmϪ1 1475; δH 0.78 (3
H, d, J 7), 1.21 (3 H, d, J 7), 1.73 (3 H, s), 3.41 and 3.69 (2 H,
ABq, J 13), 5.99 (1 H, sept, J 7), 6.95–7.5 (7 H, m) and 8.67 (1
H, d, J 9); δC 18.2 (CH3), 18.5 (CH3), 41.8 (quat), 52.0 (CH),
54.4 (CH2), 125.1–128.9, 135.1, 133.0 (C), 138.3 (C), 142.1 (C),
Synthesis of the deuterium-labelled compound 1a-d2
N-Isopropyl-N-phenacylamine was prepared by the reaction of
phenacyl bromide (α-bromoacetophenone) with an excess of
isopropylamine and was converted to the corresponding benz-
amide by treatment with benzoyl chloride in the presence of
triethylamine. To a mixture of D2O (3 g) and dry tetrahydro-
furan (THF) (30 cm3) was added sodium (100 mg). N-
Isopropyl-N-phenacylbenzamide(500mg)wasadded to themix-
ture and the resulting solution was refluxed for 12 h. After
removal of most of the THF, benzene (30 cm3) was added and
the organic layer was separated and dried over anhydrous
MgSO4. The benzene solution was dried by azeotropic distil-
lation and concentrated to 15 cm3. The dry benzene solution
was added to a cold THF solution (30 cm3; 0 ЊC) of the phos-
phorus ylide prepared from methyltriphenylphosphonium
bromide (3 g) and butyllithium. After the solution had been
stirred for 1 h at room temperature, methanol (5 cm3) and water
(100 cm3) were added. The aqueous layer was extracted with
benzene and the combined organic layer was washed with sat-
urated aq. NaCl, dried over anhydrous MgSO4 and evaporated.
The deuteriated N-isopropyl-N-(2-phenylprop-2-enyl)benz-
amide was isolated by flash chromatography on silica gel and
thionated as in the case of compound 1a.
142.9 (C) and 189.3 (C᎐S) (Found: C, 69.13; H, 6.10; N, 4.14.
᎐
C19H20ClNS requires C, 69.18; H, 6.11; N, 4.24%).
(E)-N-Isopropyl-4-methoxy-N-(2Ј-phenylprop-1Ј-enyl)thio-
benzamide 2c. Bp 200 ЊC/1 mmHg; νmax/cmϪ1 1420; δH 1.34 (6 H,
d, J 7), 1.64 (3 H, d, J 1), 3.75 (3 H, s), 5.79 (1 H, sept, J 7), 6.32
(1 H, q, J 1), 6.74 (2 H, d, J 9) and 7.1–7.5 (7 H, m); δC 16.7
(CH3), 19.1 (CH3), 54.6 (CH), 55.3 (OCH3), 112.5 (CH), 125.2–
129.5, 137.3 (C), 138.5 (C), 139.9 (C), 160.1 (C) and 201.7 (C᎐S)
᎐
(Found: C, 73.77; H, 7.04; N, 4.27. C20H23NOS requires C,
73.81; H, 7.12; N, 4.30%).
2-Isopropyl-6-methoxy-4-methyl-4-phenyl-3,5-dihydroiso-
quinoline-1(2H)-thione 5c. Mp 130–131 ЊC; νmax/cmϪ1 1480; δH
0.77 (3 H, d, J 7), 1.20 (3 H, d, J 7), 1.72 (3 H, s), 3.39 and 3.67
(2 H, ABq, J 13), 3.78 (3 H, s), 6.03 (1 H, sept, J 7), 6.49 (1 H, d,
J 3), 6.87 (1 H, dd, J 3 and 9), 7.1–7.35 (5 H, m) and 8.71 (1 H,
d, J 9); δC 18.3 (CH3), 18.6 (CH3), 24.5 (CH3), 41.8 (C), 51.6
(CH), 54.4 (CH2), 55.4 (OCH3), 110.4, 112.1, 127.1, 127.2,
128.4 and 136.0 (CH), 128.0, 142.6, 143.6 and 162.6 (C) and
189.8 (C᎐S) (Found: C, 73.74; H, 7.20; N, 4.32. C H NOS
᎐
20 23
General procedure for preparative photolyses
requires C, 73.80; H, 7.12; N, 4.30%).
A benzene solution (100 cm3) of a thioamide (500 mg) in a
Pyrex tube was de-aerated by argon bubbling and irradiated
with a high-pressure mercury lamp (1000 W) for 4–10 h. After
removal of the solvent, products were isolated by flash chrom-
atography on silica gel or HPLC. The thioenamide 2 and the
cyclization product 5 were separated by fractional recrystalliz-
ation. N-Isopropylthiobenzamide 4a, N-isopropyl-4-chlorothio-
benzamide 4b, N-isopropyl-4-methoxythiobenzamide 4c and
N-methylthiobenzamide 4d were identified by direct com-
parison with authentic samples.13
(E)-N-Isopropyl-(2-phenylprop-1-enyl)thiobenzamide 2a. Mp
161–164 ЊC; νmax/cmϪ1 1425; λmax(hexane)/nm 265 (ε 10 700)
and 420sh (290); δH 1.34 (6 H, d, J 7), 1.67 (3 H, d, J 1.6), 5.82 (1
H, sept, J 7), 6.25 (1 H, br s) and 7.0–7.5 (5 H, m); δC 17.0
(CH3), 19.1 (CH3), 54.3 (CH), 125.1–128.8, 139.1 (C), 139.8 (C),
N-Methyl-N-(2-phenylprop-1-enyl)thiobenzamides 2d. This
was a mixture of E and Z isomers and only the Z isomer was
isolated by fractional recrystallization: mp 86–88 ЊC; νmax/cmϪ1
1460; δH 1.89 and 2.18 (3 H in total, ~2:1, both d, J 2), 2.78 and
3.38 (3 H in total, ~1:2, both s), 6.23 and 6.88 (1 H in total,
~2:1, both q, J 2) and 7.1–7.5 (10 H, m); δC 21.58 and 21.64
(CH3), 43.4 and 44.0 (NCH3), 125.6–133.2, 138.2 (C), 139.1 (C),
143.1 (C), 143.5 (C) and 201.5 and 201.9 (C᎐S) (Found: C,
᎐
76.48; H, 6.36; N, 5.26. C17H17NS requires C, 76.36; H, 6.41; N,
5.24%). The E isomer was not completely purified: character-
istic signals δH 1.90 (d, J 1), 3.70 (s) and 6.37 (q, J 1).
1-Methyl-2,4-diphenylpyrrole 3d. This was identified by direct
comparison (IR, 1H NMR, 13C NMR and HPLC) with an
authentic sample.7
2,4-Dimethyl-4-phenyl-3,4-dihydroisoquinoline-1(2H)-thione
5d. Mp 126–128 ЊC; νmax/cmϪ1 1500; δH 1.71 (3 H, s), 3.56 (3 H,
s), 3.70 and 4.00 (2 H, ABq, J 13), 7.1–7.5 (8 H, m) and 8.66 (1
H, dd, J 1 and 8); δC 24.9 (CH3), 42.4 (C), 44.9 (CH2), 63.3
144.8 (C) and 202.0 (C᎐S) (Found: C, 77.19; H, 7.20; N, 4.70.
᎐
C19H21NS requires C, 77.24; H, 7.16; N, 4.74%).
1-Isopropyl-2,4-diphenylpyrrole 3a. Bp 200 ЊC/1 mmHg; νmax
/
J. Chem. Soc., Perkin Trans. 1, 1997
1853

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(NCH3), 125.0–132.7, 133.8 (C), 140.5 (C), 143.7 (C) and 191.4
filter solution containing 0.002 M K2CrO4 and 1.0 M NiSO4 in
(C᎐S) (Found: C, 75.93; H, 6.38; N, 5.00. C H NS requires C,
5% aq. K2CO3.14 The sample, in a Pyrex tube, was de-aerated
by argon bubbling. After irradiation, the degree of reaction
(formation of 2a or 2a-d2) was determined by HPLC (ODS, aq.
MeOH) using xanthone as internal standard.
᎐
17 17
76.36; H, 6.41; N, 5.24%).
1-Isopropyl-2-phenylpyrrole 3e. Bp 100 ЊC/2 mmHg; νmax
/
cmϪ1 1600; δH 1.39 (6 H, d, J 7), 4.48 (1 H, sept, J 7), 6.13 (1 H,
dd, J 2 and 3), 6.25 (1 H, t, J 3), 6.88 (1 H, dd, J 2 and 3) and
7.3–7.5 (5 H, m); δC 24.1 (CH3), 47.1 (CH), 108.0 and 108.2
(3- and 4-C), 126.9, 128.3 and 129.3 (CH), 133.86 (C) and
133.94 (C) (Found: C, 84.38; H, 8.09; N, 7.38. C13H15N requires
C, 84.28; H, 8.16; N, 7.56%).
Acknowledgements
I thank the Ministry of Education, Science and Culture, Japan,
for Grant-in-Aid for Scientific Research on Priority Areas, No.
06242201.
1-Isopropyl-2,3-diphenylpyrrole 3f. Mp 131–132 ЊC; νmax/cmϪ1
1600; δH 4.36 (6 H, d, J 7), 4.23 (1 H, sept, J 7), 6.47 (1 H, d, J
3), 6.88 (1 H, d, J 3) and 7.0–7.5 (10 H, m); δC 23.9 (CH3), 47.1
(CH), 108.1 (4-C), 116.2 (5-C), 124.8–131.3, 122.0, 133.3 (C)
and 136.6 (C) (Found: C, 86.99; H, 7.42; N, 5.31. C19H19N
requires C, 87.30; H, 7.33; N, 5.36%).
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1-Isopropyl-2,4-diphenylpyrrole 3a
To an ethereal solution (30 cm3) of (E)-β-bromomethyl-
chalcone13 (500 mg) was added isopropylamine (400 mg) and
the resulting mixture was stirred overnight at room tempera-
ture. After filtration of isopropylamine hydrochloride, the solu-
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silica gel. The pyrrole obtained in this reaction (72%) was iden-
tical with the photoproduct 3a above (IR, NMR and HPLC).
1-Isopropyl-2-phenylpyrrole 3e
A benzene solution (250 cm3) of β-[methyl(isopropyl)amino]-
vinyl phenyl ketone (600 mg), which was prepared by the reac-
tion 8 of β-chlorovinyl phenyl ketone with methylisopro-
pylamine, was irradiated with a 1000 W high-pressure lamp
through a Pyrex filter for 5 h. After removal of the solvent, the
residue was chromatographed on silica gel. The pyrrole 3e (7%)
obtained in this reaction was identical with the product in the
photoreaction of compound 1e (IR, NMR and HPLC) (see
above).
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Determination of quantum yield
Benzophenone–benzhydrol actinometry14 was used. Irradiation
was performed with a 500 W high-pressure mercury lamp in a
merry-go-round apparatus. The 313-nm line was isolated with a
Paper 7/00247E
Received 10th January 1997
Accepted 6th March 1997
1854
J. Chem. Soc., Perkin Trans. 1, 1997