Rhodium(II)-mediated reactions of thiobenzoylketene S,N-acetals with α-diazo carbonyl compounds: Synthesis of 2-substituted 3-alkylamino-5-phenylthiophenes

Article abstract of DOI:10.1039/b203931a

Treatment of 3-methylamino-3-methylsulfanyl-1-phenylpropenethione 1 with excess (2.5 equiv.) α-diazo carbonyl compounds such as α-diazoketones and α-diazoesters in the presence of a catalytic amount of Rh(II) acetate in CH₂Cl₂ at rt gave 2-acyl- or 2-aroyl-3-methylamino-5-phenylthiophenes and alkyl 3-methylamino-5-phenylthiophene-2-carboxylates, respectively, as major products along with 1-phenyl-2-methylsulfanylethanones. The formation of the major products indicates that the carbenes or carbenoids generated interact initially with the thione sulfur of 1.

Full text of DOI:10.1039/b203931a

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Rhodium(II)-mediated reactions of thiobenzoylketene S,N-acetals
with ꢀ-diazo carbonyl compounds: synthesis of 2-substituted
3-alkylamino-5-phenylthiophenes†
1
Hyun Min Song and Kyongtae Kim*
School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151-742,
Korea. E-mail: kkim@plaza.snu.ac.kr; Fax: 82 2874 8858; Tel: 82 2886 6636
Received (in Cambridge, UK) 24th April 2002, Accepted 16th August 2002
First published as an Advance Article on the web 14th October 2002
Treatment of 3-methylamino-3-methylsulfanyl-1-phenylpropenethione 1 with excess (2.5 equiv.) α-diazo
carbonyl compounds such as α-diazoketones and α-diazoesters in the presence of a catalytic amount of Rh()
acetate in CH2Cl2 at rt gave 2-acyl- or 2-aroyl-3-methylamino-5-phenylthiophenes and alkyl 3-methylamino-
5-phenylthiophene-2-carboxylates, respectively, as major products along with 1-phenyl-2-methylsulfanylethanones.
The formation of the major products indicates that the carbenes or carbenoids generated interact initially with the
thione sulfur of 1.
ation of the reactivity of the C᎐S bond of 1 toward carbenes
Introduction
᎐
because compound 1 has two other heteroatoms, i.e., a sulfur
atom of the R2S group and a nitrogen atom of the R1NH
group, which might act as electron donors to the electron-
deficient carbenes.11 Therefore, we have studied the reactions of
1 with α-diazo carbonyl compounds. The results are described
herein.
The reaction of keto carbenoids with thiocarbonyl compounds
is rapidly gaining prominence as an efficient method for
synthesizing sulfur-containing compounds, especially three-
and five-membered heterocycles. A survey of the literature
shows that diverse classes of thiocarbonyl compounds such
as thioketones,1 thioketenes,2 alkyl and aryl isothiocyanates,3
O-alkyl thioesters,4 dithioesters,5 thioamides,6 thioureas,7 and
1,3-thiazole-5(4H)-thiones,8 have been employed in the absence
or presence of a catalyst for synthetic and mechanistic studies.
It has been shown that the reactions of carbenes or carb-
enoids generated from α-diazo carbonyl compounds with the
Results and discussion
Treatment of a mixture of 1a (Ar = Ph, R1 = R2 = Me) and a
catalytic amount of Rh2(OAc)4ؒ2H2O (8 mg) in CH2Cl2 with
ethyl diazoacetate (R = OEt, 1.5 equiv.) in CH2Cl2 for 72 h at
rt gave 3-methylamino-5-phenyl-2-thiophenecarboxylate (4a,
R = OEt) and 1-methylpyrrole derivative 5 in 53% and 16%
yields, respectively (Scheme 2). Similar reactions with other
C᎐S bonds of thiocarbonyl compounds initially proceeds with
᎐
the formation of thiocarbonyl ylides, which subsequently
undergo 1,3-dipolar cycloaddition or electrocyclic ring closure
to give thiiranes followed by extrusion of sulfur leading to
α,β-unsaturated carbonyl compounds. Nevertheless, it may be
difficult to predict the chemo- and regioselectivities from the
same reactions of thiocarbonyl compounds with multifunc-
tionalities. Very recently, we reported the synthesis of 5,6-
dihydro-4H-thieno[3,2-b]pyrrolid-5-ones 3 by treatment of
alkyl 3-(thien-2-yl)-3-oxo-2-diazopropanoates 2, prepared from
thioaroylketene S,N-acetals 1, Hg(OAc)2, and 2-diazo-3-tri-
methylsilyloxybut-3-enoic acid alkyl esters,9 with Rh2(OAc)4ؒ
2H2O10 (Scheme 1). The result spurred us towards an examin-
Scheme 2
α-diazoesters and α-diazoketones under the same conditions
gave the corresponding 2-thiophenecarboxylates 4b,c and acyl-
4d,e and aroylthiophene derivatives 4f,j in moderate yields.
Interestingly, the reactions with the foregoing α-diazo carbonyl
compounds we tried (entries 2–10) did not give pyrrole deriv-
atives analogous to 5 other than for ethyl diazoacetate. Instead,
1-phenyl-2-methylsulfanylethanone 6f and 1-(4-tolyl)-2-methyl-
sulfanylethanone 6g were isolated in the reactions with benzoyl-
and 4-methylbenzoyldiazomethanes, respectively. Reaction time
and yields of compounds 4, 5, and 6 are summarized in Table 1.
When excess (2.5 equiv.) diazo compounds were used under
the same conditions, yields of 4 increased significantly except
for that of 4h (entry 8). Nevertheless unreacted 1a albeit in
Scheme 1
† Electronic supplementary information (ESI) available: spectral and
analytical data. See http://www.rsc.org/suppdata/p1/b2/b203931a/
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This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b203931a

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Table 1 Reactions of 1 with α-diazo carbonyl compounds in the presence of Rh2(OAc)4ؒ2H2O
Timea(t/h)
Yieldb(%)
Entry
RN2CHCOR
A
B
4
5
6
1
1
2
3
4
5
6
7
8
9
EtO
t-BuO
NCCH2O
Cl2CH
ClCH2CH2
Ph
p-MeC6H4
p-MeOC6H4
m-BrC6H4
p-ClC6H4
72
72
72
72
72
72
72
72
72
72
72
72
72
20
25
35
45
40
43
41
a
b
c
d
e
f
53 (61)
58c (64)d
55 (74)e
54 (69)
48 (74)
63 (85)
64 (86)
67 (67)
64 (76)
61 (79)f
16 (21)
16 (87)
15 (90)
(0)
12 (7)
g
h
i
17 (17)
19 (10)
14 (14)
19 (10)
(80)
(56)
10
j
a Reaction times A and B represent stirred time when 1.5 and 2.5 equivalents of diazo compounds were employed, respectively. b Isolated yields.
Numbers in parentheses represent yields when 2.5 equiv. of diazo compounds were used. c In addition, t-butyl fumarate 7 was isolated in 9% and 12%
yields, respectively. d In addition, t-butyl fumarate 7 was isolated in 9% and 12% yields, respectively. e In addition, compound 8 (16%) was isolated.
f In addition, 1,4-bis(4-chlorophenyl)-2-methylsulfanylbutane-1,4-dione 9 was isolated in 7% yield.
lower percentage (7–17%) compared with those involving
1.5 equiv. of diazo carbonyl compounds (12–19%), was
recovered. Compounds 4 are all new except for 4a,9b,12 and
4f.9a,12 Synthesis of alkyl 3-amino-5-phenylthiophene-2-
carboxylates has been mainly achieved by treatment of either
β-chlorocinnamonitriles with thioglycolic acid esters in the
presence of
a base or a base-catalyzed cyclization of
β-alkylsulfanyl-α-cyanocinnamonitriles.14 However, synthesis
of 2-acyl- and 2-aroyl-3-aminothiophenes has received little
attention. Recently, compound 4a was prepared in high yield
by treating a mixture of 1a and Hg(OAc)2 in CH2Cl2 at rt
with active methylene compounds such as diethyl 1,3-
acetonedicarboxylate (83%),9b ethyl 3-nitrobenzoylacetate
(89%),9b ethyl methylsulfonylacetate (74%),9b methylphenyl-
sulfinylacetate (89%),9b and triethylphosphonoacetate (82%).9b
Similarly 4f was prepared using pentane-2,4-dione (91%),9a
1-phenylbutane-1,3-dione (90%)9a or 1-phenyl-4,4,4-trifluro-
butane-1,3-dione (47%)9a by the same methodology. The
reported method involving Hg(OAc)2 appears to be superior to
the present method involving α-diazo carbonyl compounds
from various standpoints such as yield, reaction time, and
reaction temperature, providing the corresponding active
methylene compounds are readily available.
12 (Scheme 3). Loss of a methanethiolate ion, followed by
aromatization would give 4. On the other hand, either 1,3-
dipolar cycloaddition of 10 or addition reaction of carbene into
the C᎐S bond of 1 would give thiirane derivative 13, whose C–S
᎐
bond is cleaved by an intramolecular nucleophilic attack of the
amino group to give a pyrroline intermediate 14. A proton-
transfer, followed by loss of a H2S molecule would give 5.
Alternatively, one may envisage the formation of the inter-
mediate 14 via an intramolecular cyclization of a nitrogen ylide
15. However, the involvement of the intermediate 13 rather
than 15 may be more plausible since no insertion product 16
was isolated.11d
Interestingly, the methanethiol liberated from 1 in the course
of the reaction was trapped only by aroylcarbenes (entries
6–10) to give 1-aryl-2-methylsulfanylethanones 6 whose yields
increased significantly in the presence of excess (2.5 equiv.)
diazo compounds as expected.15,16 However, no compounds
analogous to
6 were isolated from the reactions with
α-diazoesters and α-diazoketones. From the reaction with tert-
butyl diazoacetate (entry 2) was isolated di-tert-butyl fumarate
7, presumably formed by the reaction of tert-butoxycarbonyl-
carbene with its carbene precursor. The stereochemistry of
7 was assigned to be trans based on the chemical shift of
the olefinic protons (6.66 ppm), which is in accord with the
reported values (6.65 ppm).17 The reaction with excess
cyanomethyl diazoacetate (entry 3) afforded compound 8
(16%) as a minor product, which was envisaged to be formed
through the insertion reaction of excess carbene, generated
from cyanomethyl diazoacetate into the methylamino group of
4c.15 The reaction with excess α-diazo-p-chloroacetophenone
(entry 10) gave 1,4-bis(4-chlorophenyl)-2-methylsulfanyl-
butane-1,4-dione 9 in 7% yield. Compound 9 may be formed
by dimerization of p-chlorobenzoylcarbene to give 1,4-bis-
(4-chlorophenyl)but-2-ene-1,4-dione, followed by addition of
methanethiol.
In summary, the reactions of thiobenzoylketene S,N-acetals
1 having C᎐S, alkylamino, and alkylsulfanyl functionalities
᎐
with α-diazo carbonyl compounds in the presence of a Rh()
catalyst gave thiophene derivatives as major products, which
indicates that the carbene or carbenoid generated interacts
preferentially with the thione sulfur of 1.
Experimental
1
The H and 13C NMR spectra were recorded at 300 MHz in
CDCl3 solution containing tetramethylsilane as an internal
standard; J-values are given in Hz. IR spectra were recorded in
KBr or for thin-film samples on KBr plates. Mass spectra were
obtained by electron impact at 70 eV. Elemental analyses were
determined by the National Center for Inter-University
Research Facilities, Seoul National University. Column
The mechanism for the formation of 4 may be rationalized
by an intramolecular nucleophilic attack of the carbanion 11,
produced by deprotonation from the resonance form of a thio-
carbonyl ylide 10, to the imino carbon to give dihydrothiophene
J. Chem. Soc., Perkin Trans. 1, 2002, 2414–2417
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Scheme 3
chromatography was performed using silica gel (Merck, 70–230
mesh, ASTM). Mps were determined on a Fisher-Johns melting
point apparatus and are uncorrected. 3-Methylamino-3-methyl-
sulfanyl-1-phenylpropenethione 1a was prepared according to
the literature procedures.9b Ethyl,18 tert-butyl,19 and cyano-
methyl diazoacetates20 were prepared according to the liter-
ature procedures. α-Diazoacetophenone, and other α-diazo-
ketones were prepared from the corresponding aroyl chloride
and diazomethane.21
2H2O and α-diazo-m-bromoacetophenone (98 mg, 0.437 mmol)
was isolated a reaction mixture, which was chromatographed to
give unreacted 1a (9 mg, 14%) and 2-(m-bromobenzoyl)-3-
methylamino-5-phenylthiophene 4i (69 mg, 64%), mp 111–112
ЊC (from CH2Cl2–n-hexane) (Found: C, 58.0; H, 3.7; N, 3.8; S,
8.6. C18H14BrNOS requires C, 58.1; H, 3.8; N, 3.8; S, 8.6 %);
νmax (neat)/cmϪ1 3328, 1699, 1584, 1539, 1459, 1414, 1363, 1222,
1158 and 1017; δH 3.14 (3H, d, J 5.2, NCH3), 6.95 (1H, s, =CH),
7.37 (1H, d, J 7.9, ArH), 7.42 (3H, m, ArH), 7.66 (3H, m, ArH),
7.76 (1H, d, J 7.9, ArH), 7.95 (1H, t, J 7.5, ArH) and 8.65 (1H,
br d, J 4.4, NH); m/z (EI) 373 (100%), 372 (Mϩ, 91), 356 (38),
275 (18), 216 (32), 188 (20), 155 (16) and 115 (21). (ii) From the
reaction of 1a (80 mg, 0.385 mmol), Rh2(OAc)4ؒ2H2O and
α-diazo-m-bromoacetophenone (201 mg, 0.895 mmol) was isol-
ated 1-(m-bromophenyl)-2-methylsulfanylethanone 6i (70 mg,
80%) by eluting the reaction mixture with a mixture n-hexane
and EtOAc (8 : 1). Compound 6i was purified by HPLC
(µPorasil, 10 µm, 7.8 × 300 mm id) using acetonitrile, pale
yellow liquid (Found: C, 44.0; H, 3.7; S, 13.2. C9H9BrOS
requires C, 44.1; H, 3.7; S, 13.1%); νmax (neat)/cmϪ1 2928, 1670,
1558, 1414, 1254, 1190, 1132, 1062, 761 and 675; δH 2.15 (3H, s,
SCH3), 3.74 (2H, s, CH2), 7.34 (1H, t, J 7.9, ArH), 7.71 (1H, m,
ArH), 7.92 (1H, m, ArH) and 8.13 (1H, m, NH); m/z (EI) 246
(Mϩ ϩ 2, 28%), 244 (Mϩ, 21), 200 (16), 198 (16), 185 (100), 183
(100), 157 (31), 155 (30), 76 (15) and 74 (14). (ii) Unreacted
1a (10 mg, 14%) and 4i (82 mg, 76%) were isolated from the
reaction of 1a (65 mg, 0.291 mmol), Rh2(OAc)4ؒ2H2O and
α-diazo-m-bromoacetophenone (169 mg, 0.756 mmol) after
43 h.
Reaction of 1a with ethyl diazoacetate
(i) To a solution of 1a (75 mg, 0.336 mmol) in CH2Cl2 was
added 1 mol% of Rh2(OAc)4ؒ2H2O (8 mg). The mixture was
stirred for 5 min at rt, followed by dropwise addition of a
solution of ethyl diazoacetate (57 mg, 0.504 mmol) in CH2Cl2
(6 ml). The mixture was stirred for 72 h at rt. Removal of the
solvent in vacuo gave a deep reddish, and sticky residue, which
was chromatographed on a silica gel (1.2 × 18 cm) using a
mixture of n-hexane and EtOAc (4 : 1) to give ethyl 1-methyl-5-
methylsulfanyl-3-phenylpyrrole-2-carboxylate 5 (15 mg, 16%):
yellow liquid (Found: C, 65.3; H, 6.1; N, 5.2; S, 11.7.
C14H17NO2S requires C, 65.4; H, 6.2; N, 5.1; S, 11.6%); νmax
(neat)/cmϪ1 2960, 1689, 1408, 1260, 1180, 1097 and 732; δH 1.04
(3H, t, J 7.1, CH3), 2.39 (3H, s, SCH3), 3.97 (3H, s, NCH3), 4.11
(2H, q, J 7.1, CH2), 6.28 (1H, s, =CH), 7.28 (2H, m, ArH)
and 7.33 (3H, m, ArH); m/z (EI) 275 (Mϩ, 100%), 260 (37), 232
(41), 203 (28), 147 (19) and 102 (11). Subsequent elution with
the same solvent mixture (4 : 1) gave ethyl 3-methylamino-5-
phenylthiophene-2-carboxylate 4a (46 mg, 53%), mp 56–57 ЊC
(from CH2Cl2–MeOH) (lit.9b,12 55–57 ЊC) (Found: C, 64.3; H,
5.7; N, 5.3; S, 12.2. C14H15NO2S requires C, 64.3; H, 5.8; N, 5.4;
S, 12.3%); νmax (neat)/cmϪ1 3392, 1654, 1574, 1260, 1091 and
761; δH 1.29 (3H, t, J 7.1, CH3), 2.96 (3H, d, J 5.2, NCH3), 4.23
(2H, q, J 7.1, CH2), 6.61 (1H, s, NH), 6.79 (1H, s, =CH), 7.35
(3H, m, ArH) and 7.57 (2H, m, ArH); m/z (EI) 261 (Mϩ, 100%),
215 (35), 187 (38), 160 (10) and 115 (14). (ii) From the reaction
of 1a (70 mg, 0.313 mmol), Rh2(OAc)4ؒ2H2O and ethyl diazo-
acetate (89 mg, 0.783 mmol) were isolated 5 (18 mg, 21%) and
4a (51 mg, 61%).
Acknowledgements
We are grateful to the Korea Research Foundation for financial
support (KRF-2001-OP0261).
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