Transition metals in organic synthesis, part 97: 1 Silver-catalyzed synthesis of hexahalogenated 2,2′-bipyrroles

Article abstract of DOI:10.1055/s-0031-1289563

We describe the synthesis of three hexahalogenated 1,1-dimethyl-2,2- bipyrroles using an efficient silver(I)-catalyzed cyclization as key step. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1289563

LETTER
2795
Transition Metals in Organic Synthesis, Part 97:1 Silver-Catalyzed Synthesis
of Hexahalogenated 2,2¢-Bipyrroles
S
ilver-Catalyzed
e
Synthesis
o
n
f
H
exahalog
é
e
nated2,2
¢
-Bipyr
M
roles artin, Anne Jäger, Hans-Joachim Knölker*
Department Chemie, Technische Universität Dresden, Bergstr. 66, 01069 Dresden, Germany
Fax +49(351)46337030; E-mail: hans-joachim.knoelker@tu-dresden.de
Received 12 September 2011
quent X-ray crystal structure analysis.11 The 2,2¢-bipyr-
Abstract: We describe the synthesis of three hexahalogenated 1,1¢-
dimethyl-2,2¢-bipyrroles using an efficient silver(I)-catalyzed cy-
clization as key step.
role core structure is found in several pharmaceutically
interesting natural products, e.g. prodigiosins,12 streptoru-
bin B13 and tambjamines.14 Diverse synthetic approaches
to 2,2¢-bipyrroles resulted from the strong interest in this
structural unit.15
Key words: alkaloids, catalysis, cyclization, halogenation, pyrroles
We have developed a short access to the compounds 1–3
by using our silver(I)-catalyzed cyclization as key step.
Condensation of the commercially available aldehyde 5
with p-toluenesulfonamide (6) afforded the N-tosylaldi-
mine 7 (Scheme 1).16 Generally, unactivated aldimines
need to be activated by a Lewis acid for alkylation with
Grignard reagents.17 In contrast, Grignard addition to N-
tosylaldimines proceeds smoothly without further activa-
tion of the imine carbon atom by a Lewis acid.18 Thus,
alkylation of 7 with trimethylsilylpropargylmagnesium
bromide provided the silyl-N-tosylhomopropargylamine
8. Removal of the trimethylsilyl protecting group with tet-
rabutylammonium fluoride (TBAF) led to the terminal
acetylene 9. Subsequent cyclization of 9 using 10 mol% of
silver(I) acetate afforded the 2,3-dihydro-1¢-methyl-1-to-
syl-2,2¢-bipyrrole (10) in 95% yield.19 The structure of the
2,3-dihydro-2,2¢-bipyrrole 10 was confirmed by an X-ray
crystal structure analysis (Figure 2).20 Aromatization of
the dihydropyrrole 10 was achieved by elimination of p-
toluenesulfinic acid. Thus, treatment of the 2,3-dihydro-
2,2¢-bipyrrole 10 with potassium tert-butoxide in dimeth-
yl sulfoxide followed by addition of iodomethane provi-
ded via N-alkylation of the intermediate anion 1,1¢-
dimethyl-2,2¢-bipyrrole (11).21 Alternative routes to com-
pound 11 have been reported previously by other
groups.11,22
The pyrrole ring is a pivotal substructure of many hetero-
cyclic natural products.2 Thus, a broad range of different
methods for the construction of pyrroles has been devel-
oped.3,4 In 2004, we reported a silver(I)-promoted oxida-
tive cyclization of homopropargylamines leading to
pyrroles.5 This procedure was applied to the total synthe-
sis of ( )-harmicine and ( )-crispine A.6,7 More recently,
we have used a silver(I)-catalyzed cyclization of N-tosyl-
homopropargylamines for the synthesis of the naturally
occurring pentabromopseudilin and pentachloropseudilin
and their synthetic analogues.8 For our ongoing studies on
polyhalogenated heterocycles,9 we became interested in
the synthesis and biological testing of the hexahalogena-
ted 1,1¢-dimethyl-2,2¢-bipyrroles 1–3 (Figure 1).
Br
Br
Cl
Cl
Me
N
Me
N
Br
Cl
Br
Cl
N
N
Me
Me
Br
Br
Cl
Cl
1
2
Br
Br
Br
Br
Me
N
H
N
Cl
Br
Cl
Br
N
N
H
Me
Br
Br
Br
Br
3
4
Figure 1 Hexahalogenated 1,1¢-dimethyl-2,2¢-bipyrroles 1–4
The hexabromo-2,2¢-bipyrrole 4 was found first by
Faulkner et al. in 1974 in the marine bacterium Chromo-
bacter I-L-33.10a In 1999, the hexahalogenated 2,2¢-bipyr-
roles 1 and 3 were detected using mass spectrometry in
seabird eggs and in samples of bald eagle liver.10b So far,
the hexachloro derivative 2 has not been found in nature.
Gribble et al. have confirmed the structural assignments
by total synthesis of the 2,2¢-bipyrroles 1–3 and subse-
Figure 2 Molecular structure of the 2,3-dihydro-2,2¢-bipyrrole 10
in the crystal
SYNLETT 2011, No. 19, pp 2795–2798
x
x
.x
x
.2
0
1
1
Advanced online publication: 31.10.2011
DOI: 10.1055/s-0031-1289563; Art ID: B17411ST
© Georg Thieme Verlag Stuttgart · New York

2796
R. Martin et al.
LETTER
by chiral HPLC into the stable atropisomers.24 The spec-
troscopic data for the hexahalogenated 1,1¢-dimethyl-2,2¢-
bipyrroles 1–3 were in full agreement with those reported
previously.10,11a,23
SO2NH2
Me
N
Me
N
H
a
b
Ts
+
N
O
Me
6
In conclusion, the silver(I)-catalyzed pyrrole cyclization
provides a facile access to 2,2¢-bipyrroles. In order to
demonstrate the utility of this chemistry for natural prod-
uct synthesis, we have developed a short access to the
hexahalogenated 1,1¢-dimethyl-2,2¢-bipyrroles 1–3. Fur-
ther applications of our methodology and biological stud-
ies with the compounds 1–3 are currently in progress.
5
7
SiMe3
H
Me
N
Me
N
c
d
Ts
Ts
N
N
H
H
8
9
Me
N
Me
e
N
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
N
N
Ts
Me
10
11
References and Notes
Scheme 1 Synthesis of 1,1¢-dimethyl-2,2¢-bipyrrole (11). Reagents
and conditions: (a) 6 (1.0 equiv), Si(OEt)4 (1.05 equiv), 160 °C, 6 h,
77%; (b) BrMgCH2C≡CSiMe3 (2.0 equiv), CH2Cl2, 25 °C, 15 h, 92%;
(c) TBAF (1.1 equiv), THF, 25 °C, 16 h, 99%; (d) AgOAc (10 mol%),
acetone, 56 °C, 3 d, 95%; (e) t-BuOK (5.2 equiv), DMSO, 50 °C, 4 h,
then MeI (10.0 equiv), 25 °C, 15 h, 76%.
(1) For Part 96, see: Thomas, C.; Kataeva, O.; Knölker, H.-J.
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Eds.; Elsevier: Oxford, 1996, 207.
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1975.
As demonstrated previously by Gribble and co-workers,
the 1,1¢-dimethyl-2,2¢-bipyrrole (11) represents the cru-
cial intermediate for the synthesis of the 2,2¢-bipyrroles 1–
3.11a Electrophilic bromination of 11 using N-bromosuc-
cinimide (NBS) provides the hexabromo derivative 1 in
an improved yield of 91% (Scheme 2).23 Chlorination of
11 with N-chlorosuccinimide (NCS) afforded the
hexachloro derivative 2 in 82% yield. For an access to the
mixed halogenated derivative 3, the bipyrrole 11 was ini-
tially treated with two equivalents of NCS to generate
5,5¢-dichloro-1,1¢-dimethyl-2,2¢-bipyrrole. After a short
chromatographic purification over silica gel, this labile in-
termediate was exhaustively brominated with NBS (5
equiv) to afford the tetrabromodichloro derivative 3 in
63% yield over two steps. It is interesting to note that
compound 3 due to its axial chirality could be separated
Br
Br
Me
N
Me
N
a
Br
Br
N
N
Me
Me
Br
Br
(9) (a) Fedorov, R.; Böhl, M.; Tsiavaliaris, G.; Hartmann, F. K.;
Taft, M. H.; Baruch, P.; Brenner, B.; Martin, R.; Knölker,
H.-J.; Gutzeit, H. O.; Manstein, D. J. Nat. Struct. Mol. Biol.
2009, 16, 80. (b) Martin, R.; Kirst, J.; Weidlich, S.; Schieb,
H.; Schlechtingen, G.; Jäger, A.; Rajendran, L.; Böhl, M.;
Richter, S.; Fedorov, R.; Manstein, D. J.; Gutzeit, H. O.;
Klafki, H.; Wiltfang, J.; Simons, K.; Knölker, H.-J. Med.
Chem. Res. 2010, 19, S23. (c) Martin, R.; Agarwal, S.;
Jäger, A.; Böhl, M.; Richter, S.; Tsiavaliaris, G.; Fedorov,
R.; Manstein, D. J.; Gutzeit, H. O.; Knölker, H.-J. In The
Chemistry and Biological Activity of Synthetic and Natural
Compounds, Modern Aspects of Chemistry of Heterocycles;
Kartsev, V. G., Ed.; ICSPF Press: Moscow, 2010, 110.
(d) Preller, M.; Chinthalapudi, K.; Martin, R.; Knölker,
H.-J.; Manstein, D. J. J. Med. Chem. 2011, 54, 3675.
(e) Chintalapudi, K.; Taft, M. H.; Martin, R.; Heissler, S. M.;
Preller, M.; Hartmann, F. K.; Brandstaetter, H.; Kendrick-
11
1
b
c, d
Cl
Cl
Br
Br
Me
Me
N
N
Cl
Cl
Cl
Cl
N
N
Me
Me
Cl
Cl
2
Br
Br
3
Scheme 2 Synthesis of the 2,2¢-bipyrroles 1–3. Reagents and con-
ditions: (a) NBS (7.0 equiv), MeCN, –40 °C to 25 °C, 15 h, 91%; (b)
NCS (6.2 equiv), MeCN, –40 °C to 25 °C, 90 min, 82%; (c) NCS (2.0
equiv), MeCN, –40 °C to 25 °C, 4 h, 83%; (d) NBS (5.0 equiv),
MeCN, –40 °C to 25 °C, 15 h, 76%.
Synlett 2011, No. 19, 2795–2798 © Thieme Stuttgart · New York

LETTER
Silver-Catalyzed Synthesis of Hexahalogenated 2,2¢-Bipyrroles
2797
Jones, J.; Tsiavaliaris, G.; Gutzeit, H. O.; Fedorov, R.; Buss,
F.; Knölker, H.-J.; Coluccio, L. M.; Manstein, D. J. J. Biol.
Chem. 2011, 286, 29700.
0.302 e Å–3. CCDC 829173 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge
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Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(21) Synthesis of 1,1¢-Dimethyl-2,2¢-bipyrrole (11) by
Aromatization and Alkylation of 10:
A solution of potassium tert-butoxide (926 mg, 8.25 mmol)
and the 2,3-dihydro-2,2¢-bipyrrole 10 (500 mg, 1.65 mmol)
in dimethyl sulfoxide (35 mL) was stirred at 50 °C for 3 h.
The solution was cooled to 0 °C, MeI (1.0 mL, 2.28 g, 16.1
mmol) was added and the reaction mixture was stirred at r.t.
overnight. After addition of H2O (50 mL), the reaction
mixture was extracted with Et2O (3 × 25 mL). The combined
organic layers were washed with H2O (2 × 25 mL) and dried
over MgSO4. Removal of the solvent and purification of the
crude product by flash chromatography (light petroleum
ether–Et2O, 10:1) on silica gel afforded 1,1¢-dimethyl-2,2¢-
bipyrrole (11) as a yellow oil; yield: 200 mg (76%). IR
(ATR): 3101, 2926, 1514, 1484, 1447, 1410, 1359, 1312,
1282, 1234, 1203, 1088, 1062, 968, 781, 706 cm–1. 1H NMR
(500 MHz, CDCl3): d = 3.52 (s, 6 H), 6.16–6.17 (m, 2 H),
6.19–6.21 (m, 2 H), 6.73 (m, 2 H). 13C NMR (DEPT; 125
MHz, CDCl3): d = 34.38 (2 × Me), 107.30 (2 × CH), 110.44
(2 × CH), 122.58 (2 × CH), 125.05 (2 × C). MS (EI): m/z (%)
= 160 (100) [M+], 159 (38), 145 (16), 118 (21), 117 (16).
HRMS: m/z [M+] calcd for C10H12N2: 160.1000; found:
160.0995.
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(b) Matsumoto, S.; Kobayashi, T.; Ogura, K. Heterocycles
2005, 66, 319. (c) Dohi, T.; Morimoto, K.; Maruyama, A.;
Kita, Y. Org. Lett. 2006, 8, 2007. (d) Jolicoeur, B.; Lubell,
W. D. Org. Lett. 2006, 8, 6107. (e) Jiao, L.; Hao, E.;
Vicente, G. H.; Smith, K. M. J. Org. Chem. 2007, 72, 8119;
and references cited therein.
(16) Love, B. E.; Raje, P. S.; Williams II, T. C. Synlett 1994, 493.
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Hino, T. Chem. Pharm. Bull. 1993, 41, 287.
(22) (a) Kauffmann, T.; Lexy, H. Chem. Ber. 1981, 114, 3674.
(b) Dohi, T.; Morimoto, K.; Ito, M.; Kita, Y. Synthesis 2007,
2913. (c) Sánchez-García, D.; Borrell, J. I.; Nonell, S. Org.
Lett. 2009, 11, 77.
(18) Sisko, J.; Weinreb, S. M. J. Org. Chem. 1990, 55, 393.
(19) Synthesis of 2,3-Dihydro-1¢-methyl-1-tosyl-2,2¢-
bipyrrole (10) by Silver(I)-Catalyzed Cyclization of the
N-Tosylhomopropargylamine 9: Silver(I) acetate (6 mg,
35.9 mmol) was added to a stirred solution of compound 9
(105 mg, 347 mmol) in anhyd acetone (12 mL) and the
resulting mixture was heated at reflux for 3 d. After cooling
to r.t., the solvent was removed in vacuum. Purification of
the crude product by flash chromatography (light petroleum
ether–Et2O, 5:1) on silica gel provided the 2,3-dihydro-2,2¢-
bipyrrole 10; yield: 100 mg (95%); yellow crystals; mp 125
°C. UV (MeOH): l = 221, 252, 277 nm. IR (ATR): 3099,
2925, 2857, 1619, 1597, 1494, 1451, 1344, 1295, 1160,
1089, 1049, 957, 911, 814, 706, 664 cm–1. 1H NMR (500
MHz, CDCl3): d = 2.41 (s, 3 H), 2.60 (ddt, J = 16.7, 7.6, 2.4
Hz, 1 H), 2.84 (ddt, J = 16.7, 11.2, 2.4 Hz, 1 H), 3.54 (s, 3
H), 4.86 (dd, J = 11.2, 7.6 Hz, 1 H), 5.14 (m, 1 H), 6.00 (m,
2 H), 6.47 (m, 1 H), 6.50 (t, J = 2.2 Hz, 1 H), 7.24 (m, 2 H),
7.51 (d, J = 8.3 Hz, 2 H). 13C NMR (DEPT; 125 MHz,
CDCl3): d = 21.55 (Me), 34.47 (Me), 38.43 (CH2), 56.76
(CH), 106.87 (CH), 108.74 (CH), 109.35 (CH), 123.51
(CH), 127.44 (2 × CH), 129.44 (2 × CH), 130.61 (CH),
130.92 (C), 134.30 (C), 143.49 (C). MS (EI): m/z (%) = 302
(33) [M+], 147 (100), 107 (11). HRMS: m/z [M+] calcd for
C16H18N2O2S: 302.1089; found: 302.1078. Anal. Calcd for
C16H18N2O2S: C, 63.55; H, 6.00; N, 9.26; S, 10.60. Found:
C, 63.72; H, 5.97; N, 9.15; S, 10.49.
(23) Synthesis of 3,3¢,4,4¢,5,5¢-Hexabromo-1,1¢-dimethyl-2,2¢-
bipyrrole (1) by Bromination of 11:
A solution of N-bromosuccinimide (449 mg, 2.52 mmol) in
anhyd MeCN (10 mL) was added at –40 °C to a solution of
1,1¢-dimethyl-2,2¢-bipyrrole (11) (57.5 mg, 0.36 mmol) in
MeCN (15 mL). The resulting green solution was warmed
slowly to r.t. and stirred for 15 h. Removal of the solvent in
vacuum and purification by flash chromatography (light
petroleum ether–Et2O, 7:1) on silica gel provided compound
1; yield: 208 mg (91%).
Spectroscopic data for the 3,3¢,4,4¢,5,5¢-Hexahalo-1,1¢-
dimethyl-2,2¢-bipyrroles 1–3:
3,3¢,4,4¢,5,5¢-Hexabromo-1,1¢-dimethyl-2,2¢-bipyrrole (1):
pale yellow solid; mp 237–238 °C. UV (MeOH): l = 256
nm. IR (ATR): 2923, 2853, 1478, 1462, 1436, 1402, 1384,
1364, 1321, 1228, 1187, 1087, 1042, 971, 753, 733, 677, 613
cm–1. 1H NMR (500 MHz, CDCl3): d = 3.46 (s, 6 H). 13
C
NMR (DEPT; 125 MHz, CDCl3): d = 35.37 (2 × Me), 101.21
(2 × C), 103.31 (2 × C), 106.77 (2 × C), 122.17 (2 × C). MS
(EI): m/z (%) = 640 (7), 638 (44), 636 (81), 634 (100), 632
(84), 630 (47), 628 (7) [M+], 516 (10), 514 (23), 512 (24),
510 (11), 476 (29), 474 (46), 472 (31), 395 (15), 393 (15).
HRMS: m/z [M+] calcd for C10H6Br6N2: 627.5631; found:
627.5619.
3,3¢,4,4¢,5,5¢-Hexachloro-1,1¢-dimethyl-2,2¢-bipyrrole (2):
colorless crystals; mp 208–209 °C (dec.). UV (MeOH): l =
230, 260 nm. IR (ATR): 2951, 2920, 2846, 1509, 1446,
1382, 1348, 1333, 1259, 1198, 1112, 1055, 1015, 981, 909,
794, 734, 708, 690, 627 cm–1. 1H NMR (500 MHz, CDCl3):
d = 3.41 (s, 6 H). 13C NMR (DEPT; 125 MHz, CDCl3): d =
32.79 (2 × Me), 108.38 (2 × C), 113.52 (2 × C), 116.39 (2 ×
C), 116.51 (2 × C). MS (EI): m/z (%) = 370 (30), 368 (78),
366 (100), 364 (47) [M+], 331 (11), 296 (16), 294 (17),
292 (16), 290 (26), 288 (16). HRMS: m/z [M+] calcd for
C10H6Cl6N2: 363.8662; found: 363.8658. Anal. Calcd for
(20) Crystallographic Data for the 2,3-Dihydro-2,2¢-bipyrrole 10:
C16H18N2O2S, M = 302.38 g mol–1, crystal size: 0.70 × 0.55
× 0.30 mm3, monoclinic, space group P21/n, a = 13.286 (1),
b = 8.603 (1), c = 13.580 (1) Å, V = 1494.6 (2) Å3, Z = 4,
r
calcd = 1.344 g cm–3, m = 0.223 mm–1, l = 0.71073 Å, T = 198
(2) K, q range = 3.04–30.00°, reflections collected: 73987,
independent: 4349 (Rint = 0.0256), 192 parameters. The
structure was solved by direct methods and refined by full-
matrix least-squares on F2; final R indices [I > 2s(I)]: R1 =
0.0353; wR2 = 0.1005; maximal residual electron density:
Synlett 2011, No. 19, 2795–2798 © Thieme Stuttgart · New York

2798
R. Martin et al.
LETTER
C10H6Cl6N2: C, 32.74; H, 1.65; N, 7.64. Found: C, 32.76; H,
1.75; N, 7.44.
(2 × C). MS (EI): m/z (%) = 550 (11), 548 (46), 546 (94), 544
(100), 542 (55), 540 (13) [M+], 428 (12), 426 (38), 424 (57),
422 (43), 420 (12), 388 (19), 386 (56), 384 (63), 382 (25).
Anal. Calcd for C10H6Br4Cl2N2: C, 22.05; H, 1.11; N, 5.14.
Found: C, 22.32; H, 1.21; N, 4.96.
3,3¢,4,4¢-Tetrabromo-5,5¢-dichloro-1,1¢-dimethyl-2,2¢-
bipyrrole (3): yellow solid; mp 220 °C (dec.). UV (MeOH):
l = 232, 253 nm. IR (ATR): 2943, 2923, 2851, 1726, 1491,
1476, 1440, 1408, 1375, 1323, 1190, 1105, 1049, 978, 764,
681, 665, 620 cm–1. 1H NMR (500 MHz, CDCl3): d = 3.43
(s, 6 H). 13C NMR (DEPT; 125 MHz, CDCl3): d = 33.50 (2
× Me), 97.37 (2 × C), 102.89 (2 × C), 118.81 (2 × C), 120.25
(24) Rosenfelder, N.; Ostrowicz, P.; Fu, L.; Gribble, G. W.;
Tittlemier, S. A.; Fey, W.; Vetter, W. J. Chromatogr. A
2010, 1217, 2050.
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