N-heterocyclic carbenes of indazole as reagents: Indazol-3-ylidene-mediated syntheses of amidines from thiolactams of pyrrolobenzodiazepines

Article abstract of DOI:10.1055/s-0028-1083626

On decarboxylation, 1,2-dimethylindazolium-3-carboxylate forms in situ the N-heterocyclic carbene 1,2-dimethylindazol-3-ylidene, which proved to be a suitable reagent for amidinations of the monothiolactam of benzo[e]pyrrolo[1,2- a][1,4]diazepine. A comparison of this metal-free approach to amidines to HgCl₂- and Bi(NO₃)₃-mediated reactions is presented. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1083626

LETTER
2961
N-Heterocyclic Carbenes of Indazole as Reagents: Indazol-3-ylidene-Mediated
Syntheses of Amidines from Thiolactams of Pyrrolobenzodiazepines
N-Heterocyclic
Ca
n
rbene
s
of Ind
i
azole
a
k
s
R
eagents a Lindner, Andreas Schmidt*
Institute of Organic Chemistry, Clausthal University of Technology, Leibnizstr. 6, 38678 Clausthal-Zellerfeld, Germany
Fax +49(5323)723861; E-mail: schmidt@ioc.tu-clausthal.de
Received 5 August 2008
RNH2
Abstract: On decarboxylation, 1,2-dimethylindazolium-3-carbox-
ylate forms in situ the N-heterocyclic carbene 1,2-dimethylindazol-
3-ylidene, which proved to be a suitable reagent for amidinations of
the monothiolactam of benzo[e]pyrrolo[1,2-a][1,4]diazepine. A
comparison of this metal-free approach to amidines to HgCl2- and
Bi(NO3)3-mediated reactions is presented.
(strong bases)
– H2S
RNH2, Hg2+
– HgS
NR
S
R
R
R
N
H
R
N
H
Key Words: thiolactam, bismuth(III) nitrate, mercury(II) chloride,
mesomeric betaine, decarboxylation
Et
S
Et3O+BF4
–
RNH2
R
R
N
Thioamides not only possess a great practical and synthet-
ic applicability, but also constitute a class of compounds
with continuously growing significance as biologically
active or technically interesting compounds.1 Thus, thio-
amides have been described as drugs, plant protection
agents, flotation and vulcanization agents, and additives
to lubricating oils and greases.1 Consequently, several re-
views dealing with the syntheses2 and reactions1,3 of thio-
amides have been published. Among many other
applications, thioamides are starting materials for the
preparation of amidines; they are more reactive than
amides and do not generally require activation. Thus,
strong bases react immediately at room temperature gen-
erating gaseous hydrogen sulfide (Scheme 1).4 Activation
is necessary for less reactive thioamides and weaker basic
amines, and this activation usually is accomplished by
Meerwein’s reagent or iodomethane. In the latter case,
thioimidates are formed as isolable intermediates. By this
method, unsubstituted,5 N-substituted6 or N1,N1-disubsti-
tuted amidines7 are formed.3 The reaction of N-mono- or
N,N-disubstituted thioamides with primary or secondary
amines is usually carried out in the presence of toxic mer-
cury(II) salts such as HgO,8 HgCl2,9 Hg(OAc)2,10 or
Scheme 1 Syntheses of amidines from thioamides
and they are representatives of the family of privileged
structures.13 Certain pyrrolo[2,1-c][1,4]benzodiazepines
(PBD) recognize and bind to specific sequences of DNA
and are potential regulators of gene expression.14 They are
also applied as affinity-cleavage reagents in molecular bi-
ology.15 The PBD ring system is also found in natural an-
titumor antibiotics from Streptomyces species such as
anthramycin,16 tomaymycine,17 abbeymycin,18 chica-
mycin A,19 DC-81,20 mazethramycin,21 neothramycin A
and B,22 prothracarcin,23 sibanomicin,24 sibiromycin,25
and porothramycin B.26 A pentacyclic system was postu-
lated recently as structure of circumdatin A and B from
Aspergillus species.27
In the course of our studies we compared the mercury(II)
chloride mediated amidinations at position 11 of the PDB
ring system with those accomplished in the presence of
bismuth(III) nitrate. Searching for a metal-free and effi-
cient reagent, we thought of the in situ generated N-het-
erocyclic carbene indazol-3-ylidene 2 which we examined
recently in trapping reactions with heterocumulenes,28 re-
dox reactions,29 debrominations,30 dehydrohalogena-
tions,30 and cycloadditions.31 This carbene is formed by
thermal decarboxylation of the pseudo-cross-conjugated
heterocyclic mesomeric betaine 1,2-dimethylindazolium-
3-carboxylate (1) and readily reacts with sulfur to the
thione 328 (Scheme 2).
11
Hg(OTf)2 which trap the sulfur as mercury(II) sulfide.
Mercury oxide sometimes give amides from the thio-
amides as undesirable byproducts.8 For ample reasons,
there is an urgent need for new environmentally benign
reagents which allow the replacement of mercury reagents
in these syntheses.
We became interested in thioamides of benzodiazepines
as starting materials for potentially biologically active
compounds. Benzodiazepines form a well-known and
widely applied class of biologically active compounds,12
We report here that the in situ generated N-heterocyclic
carbene indazol-3-ylidene 2 is indeed a suitable reagent to
replace the metals in amidinations of thioamides.
The monothiolactam of benzo[e]pyrrolo[1,2-a][1,4]diaz-
epine, (S)-11-thioxo-2,3,11,11a-tetrahydro-1H-benzo[e]-
pyrrolo[1,2-a][1,4]diazepin-5(10H)-one (4),32 reacted
with the amines depicted in Scheme 3 and Table 1 to the
amidines 5a–k.33 We compared the yields on conducting
SYNLETT 2008, No. 12, pp 2961–2964
x
x
.x
x
.2
0
0
8
Advanced online publication: 12.11.2008
DOI: 10.1055/s-0028-1083626; Art ID: G24608ST
© Georg Thieme Verlag Stuttgart · New York

2962
A. Lindner, A. Schmidt
LETTER
O
Table 1 Preparation of Amidines from Thiolactam 4
O–
–
Entry
1
Amine
BuNH2
Product
Method
Yielda,b (%)
Δ
+
N
+
N
Me
Me
– CO2
5a
A
B
C
63
68
87
N
N
Me
Me
1
2
3
s-BuNH2
i-BuNH2
5b
5c
A
B
C
92
n.p.
46
S
S8
N
Me
N
Me
A
B
C
90
n.p.
88
N
N
Me
Me
3
2
4
Me(CH2)5NH2
CyNH2
5d
5e37
5f
A
B
C
86
n.p.
95
Scheme 2 Reaction of the in situ generated NHC 2 with sulfur
the reaction in the presence of HgCl2 (method A),34
Bi(NO3)3 (method B),35 and the indazolium-3-carboxylate
1 (method C) either at reflux temperature of the amine or
at 80 °C. In our reactions, liquid amines were taken as re-
agents and solvents, and solid amines were first molten
before thiolactam 4 and the corresponding reagent were
added. As a consequence of the formation of HgS the re-
action mixtures changed rapidly to black on addition of
HgCl2, and the same color is observed on addition of
Bi(NO3)3. In the latter case, the reactions suffered from
difficult chromatographic workup procedures. In the pres-
ence of the N-heterocyclic carbene 2, 1,2-dimethylindazo-
lium salt is formed as byproduct (vide infra) which can
very easily be removed from the reaction mixture by silica
gel filtration. All results are summarized in Table 1. Con-
ducting the reaction of 4 with amines without additional
reagents gave considerably lower yields, required harsher
reaction conditions and much longer reaction times. As an
example, 4-chloroaniline gave a maximum yield of 5i of
15% after 3 hours at reflux temperature.
5
A
B
C
81
37
98
6
BnNH2
A
B
C
93
95
96
7
PhNH2
5g37
5h
5i
A
B
C
60
n.p.
70
8
TolNH2
A
B
C
83
57
54
9
4-ClC6H4NH2
4-BrC6H4NH2
PMPNH2
A
B
C
34
n.p.
38
10
11
5j
A
B
C
43
n.p.
72
5k
A
B
C
94
62
92
O
O
a Yields refer to isolated products characterized by spectroscopic data
(1H NMR, 13C NMR, EIMS, IR, CHN analyses). Analytical data for
5e,g are in agreement with those reported in the literature;37 all other
amidines are new.
N
N
RNH2
Method A, B, or C
H
H
NH
N
N
b n.p. = experiment not performed.
H
S
R
5a–k
4
The pyrrolobenzodiazepines 5a–k were detected as tau-
tomers A in the NMR spectra taken in CDCl3 at room tem-
Scheme 3 Amidination of thiolactam 4. Reagents and conditions:
Method A: HgCl2, reflux or 80 °C, 1 h; Method B: Bi(NO3)3, reflux
or 80 °C, 1 h; Method C: 1, reflux or 80 °C, 1 h.
1
perature (Figure 1).36 All H NMR spectra in CDCl3
display only one NH signal. We found that 13C NMR res-
onance frequencies of C-9a of 5a–k between d = 142 and
146 are characteristic for this tautomer. All pyrrolobenzo-
diazepines possessing an N10–H group give resonance
frequencies of C-9a at approximately d = 133. In contrast
to tetracyclic systems which we prepared earlier,37,38 no
C(11)–C(11a) double bonds of tautomer C were detected,
although all reactions of aromatic amines lead to almost
complete racemization of the pyrrolobenzodiazepines 5g–
k which can – at least in part – be explained by the inter-
mediary existence of this tautomer.
O
O
O
3
6
9
2
5a
9a
N
N
11a
N
5
7
8
1
H
H
N
N
10
N
11 NH
R
N
NH
H
H
R
R
A
B
C
Figure 1 Possible tautomers of 5
Synlett 2008, No. 12, 2961–2964 © Thieme Stuttgart · New York

LETTER
N-Heterocyclic Carbenes of Indazole as Reagents
2963
We propose the following mechanism for the indazol-3- In the presence of amines these products were not detect-
ylidene-mediated amidination (Scheme 4). The N-hetero- able in the crude reaction mixtures by NMR and mass
cyclic carbene 2, which is a very strong base,28–31 depro- spectrometric methods.
tonates the thiolactam 4 to give a salt I consisting of an
In summary, we present an efficient N-heterocyclic car-
indazolium cation and a thioimidate anion. This salt can
bene mediated metal-free approach to amidines of the
be detected by electrospray ionization mass spectrometry
pyrrolobenzodiazepine ring system starting from a thi-
(ESI-MS) on spraying a sample of the crude reaction mix-
olactam.
ture from methanol. The cation gives a peak at m/z = 147.1,
and the thioimidate is detected in the anion detection
Acknowledgment
mode at m/z = 231.6 as prominent peak. As indazolium
cations proved to be electrophilic species toward a broad
range of nucleophiles,29,31 an attack of the nucleophilic
thiolate on the iminium bond of the indazolium cation can
be postulated. Indeed, a prominent peak of a 1:1 adduct
such as II was detected at m/z = 756.8 [2·II + H]+, when
the sample was sprayed from a freshly prepared reaction
mixture. Reaction of I or II with amines then yielded the
amidates 5a–k and indazolium thiolate 6 which can very
easily be separated from the reaction mixture by silica gel
filtration. No evidence for the existence of a 2,3-dihydro-
1H-indazole-3-thiol III was found in the NMR spectra.
The Deutsche Forschungsgemeinschaft DFG is gratefully acknow-
ledged for financial support.
References and Notes
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Scheme 4 Proposed mechanism for the NHC-mediated amidination
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O
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Scheme 5 Reaction between 4 and 1 without amine
Synlett 2008, No. 12, 2961–2964 © Thieme Stuttgart · New York

2964
A. Lindner, A. Schmidt
LETTER
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Method A
Thiolactam (4, 0.232 g, 1 mmol) was suspended in n-BuNH2
(4 mL) and heated to 60 °C. At this temperature HgCl2
(0.272 g, 1 mmol) was added, and the mixture was heated
over a period of 1 h at reflux temperature. After cooling, the
reaction mixture was filtered through a plug of Celite and
washed with CHCl3. The filtrate was washed with aq
Na2S2O3 and dried over Na2SO4. The solvent and excess
amine were distilled off in vacuo. The residue was then
chromatographed (SiO2, PE–EtOAc, 1:5).
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Method B
Bi(NO3)3 (0.395 g, 1 mmol) was used.
Method C
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Compound 1 (0.190 g, 1 mmol) was used and heated as
described. Then, the reaction mixture was concentrated in
vacuo and chromatographed. The amidine was obtained as a
faintly yellow oil, respectively; [a]D 518 (c 1, CDCl3). 1H
NMR (200 MHz, CDCl3): d = 0.96 (t, J = 7.17 Hz, 3 H,
16-H), 1.31–1.49 (m, 2 H, 15-H), 1.54–1.69 (m, 2 H, 14-H),
2.00–2.14 (m, 3 H, 1-H, 2-H), 2.18–2.30 (m, 1 H, 1-H),
3.34–3.48 (m, 2 H, 13-H), 3.51–3.64 (m, 1 H, 3-H), 3.82–
3.94 (m, 1 H, 3-H), 4.02 (t, J = 4.93 Hz, 1 H, 11a-H), 4.76
(br s, 1 H, NH), 7.02–7.14 (m, 2 H, 7-H, 9-H), 7.35–7.44 (m,
1 H, 8-H), 7.93 (dd, J = 7.9, 1.4 Hz, 1 H, 6-H). 13C NMR (50
MHz, CDCl3): d = 13.9 (C-16), 20.3 (C-15), 23.8 (C-2), 26.7
(C-1), 31.1 (C-14), 41.2 (C-13), 46.3 (C-3), 54.3 (C-11a),
122.1, 126.4, 126.8, 129.9, 131.5, 147.4, (6C, Carom.), 156.1
(C-11), 166.7 (C=O). IR (KBr) = 3345, 3057, 2956, 2871,
1607, 1536, 1456, 1412, 1337, 1236, 1202, 1148, 1034, 835,
762 cm–1. MS (EI): m/z (%) = 271 (80) [M+], 242 (20), 229
(20), 215 (10), 199 (15), 187 (25), 173 (30), 160 (70), 146
(30), 119 (25), 90 (30), 70 (100). Anal. Calcd for C16H21N3O:
C, 70.82; H, 7.80; N, 15.49. Found: C,70.58; H, 7.41; N,
14.91.
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Synlett 2008, No. 12, 2961–2964 © Thieme Stuttgart · New York