Synthesis of trisubstituted pyrroles from rhodium-catalyzed alkyne head-to-tail dimerization and subsequent goldcatalyzed cyclization

Article abstract of DOI:10.1002/adsc.200800735

Dimerization of N-protected propargylic amines in a rather rare head-to-tail mode has been achieved under mild conditions with high selectivity using rhodium catalysts. The JV-protecting group could be a sulfonyl, carbamate, or carbonyl functionality and

Full text of DOI:10.1002/adsc.200800735

FULL PAPERS
DOI: 10.1002/adsc.200800735
Synthesis of Trisubstituted Pyrroles from Rhodium-Catalyzed
Alkyne Head-to-Tail Dimerization and Subsequent Gold-
Catalyzed Cyclization
Hong Mei Peng,a Jing Zhao,b,* and Xingwei Lia,c,*
a
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, Singapore 637371
b
State Key Laboratory of Pharmaceutical Biotechnology, School of Life Sciences, Nanjing University, Nanjing 210093,
Peopleꢀs Republic of China
E-mail: jingzhao@nju.edu.cn
Current address: The Scripps Research Institute, Scripps Florida, Jupiter, Florida 33458, USA
c
Fax : (+1)-561-228-3084; e-mail: xli@scripps.edu
Received: November 27, 2008; Revised: April 5, 2009; Published online: June 3, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800735.
Abstract: Dimerization of N-protected propargylic
ACHUTNGTRENpGUN hine)rhodium chloride [RhACHUTGNTREN(NNGU PPh3)3Cl] proved to be
amines in a rather rare head-to-tail mode has been active catalysts. In addition, these functionalized
achieved under mild conditions with high selectivity gem-enynes subsequently undergo selective gold-
using rhodium catalysts. The N-protecting group
could be a sulfonyl, carbamate, or carbonyl function- give trisubstituted pyrroles under mild conditions.
ality and (cyclooctadiene)rhodium chloride
dimer/1,1’-bis(diphenylphosphino)ferrocene {[Rh- Keywords: enynes; gold; head-to-tail dimerization;
(COD)Cl]2/dppf} as well as tris(triphenylphos- hydroamination; propargylic amines
ACHTUNGTREN(NGNU III)-catalyzed intramolecular hydroamination to
ACHTUNGTRENNUNG
Introduction
actions catalyzed by phosphine complexes,[3] control-
ling the selectivity to favor Z- and gem-enyne prod-
It remains a challenge to develop catalytic systems ucts remains a task. In general, late transition metal-
that efficiently promote the selective formation of catalyzed alkyne dimerization reactions tend to give
carbon-carbon bonds by combining several simple E-enynes,[4–7] while organolanthanide catalysts often
molecules.[1] 1-Alkyne dimerization reactions are at- give the corresponding Z-enyne products.[8] For in-
tractive in that enynes are atom-economically pro- stance, the yields of Z-enynes can even be as high as
duced which are important building blocks in organic 99% using lutetium half-metallocene catalysts, and
synthesis.[2] Three isomeric products, Z-, E-, and gem- mechanistic studies have shown that dimeric lutetium
enynes can be possible as a result of the regio- and intermediates are responsible for this selectivity.[8a] In
stereoselectivity [Eq. (1)]. While it has been realized addition, ruthenium catalysts have also been reported
that the identity of phosphine ligands and the steric for selective Z-dimerization.[9]
bulk of alkyne substrates play important roles in the
In sharp contrast, examples of selective head-to-tail
regio- and stereoselectivity of alkyne dimerization re- dimerization of alkynes are rather rare although re-
ports in this context span over two decades.[10] In most
cases the substrates are limited to simply alkyl- or ar-
ylalkynes (Table 1). Trost and co-workers reported
one of the few examples of efficient dimerization of
heteroatom-functionalized alkynes in this mode.[10b] In
view of the poor or under-explored functional group
compatibility in alkyne dimerization in this mode, we
now report a successful case of dimerization of N-pro-
tected propargylamines. Moreover, these gem-enyne
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Hong Mei Peng et al.
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Table 1. Examples of selective alkyne dimerization in a head-to-tail mode.
Catalyst
[Ru(dppm)
R
Yield [%]
Ref.
G
E
Ph or n-Bu
i-Pr
n-Bu, i-Pr, or Ph
Ph
not reported
75
>99
>99
62–63
[9e]
[9d]
[9c]
[9a]
[9b]
ACHTUNGTRENNUNG
R
A
Ph or n-hexyl
products can subsequently undergo Au-catalyzed in- es.[4b] We reason that TsNHCH2C CH might show su-
tramolecular hydroamination reactions to afford im- perior selectivity and reactivity in that the tosyl
portant 2-(aminomethyl)pyrrole derivatives which oxygen is potentially a weak donor,[14] and the intro-
have been used in molecular recognition[11a,b] and in duction of a withdrawing Ts group should tune the
ꢀ
ꢀ
the pharmaceutical industry as pharmacologically electronic effects of the C C bond. We also noted
active compounds or central nervous system depres- that
simple
rhodium
complexes
such
as
sants.[11c,d] Moreover, pyrroles are important building Rh
(PPh3)3Cl[15] and [Rh
G
blocks in organic synthesis[12] and are widely used for have been applied as catalysts for the head-to-tail di-
biological studies.[13]
merization of alkynes. We hope that the introduction
of a Ts group could switch the selectivity to the rare
head-to-tail mode even when using such simple rhodi-
um complexes.
Results and Discussion
Indeed, the desired gem-enyne product (1) was ob-
The dimerization of propargylamines is an important tained in 52% yield (toluene, 24 h, room temperature)
reaction in that the functionalized enyne products when catalyzed by a combination of [Rh(COD)Cl]2
AHCTUNGTRENNUNG
could allow further intramolecular elaboration. How- (2 mol%) and PPh3 (4 mol%), although 42% of the
ever, no selective head-to-tail dimerization has been starting material remained intact (entry 1, Table 2).
reported for such substates. Recent studies by Miura The NMR yield was improved to 64% when CH2Cl2
and co-workers seem to suggest that simple propar- is used as the solvent (entry 2, Table 2), but essentially
gylamines or alcohols have poor reactivity toward ho- no further increase of the reaction yield could be
modimerization when catalyzed by rhodium complex- achieved in CH2Cl2 under reflux. To our surprise,
Table 2. Optimization of reaction conditions.[a]
Entry
Catalyst
Solvent
NMR yield[b] [%]
SM[b] [%]
1
2
3
4
5
6
7
8
[Rh
[Rh
N
PhMe
52
64
42
31
10
30
5
23
75
95
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Rh
G
83 (77[c])
64
[Rh
[Rh
[Rh
[Rh
[Rh
90 (83[c])
54
<3
<3
[a]
ꢀ
The reaction was carried out at 238C for 24 h under argon, using TsNHCH2C CH (0.25 mmol), Rh
(PPh3)3Cl
(0.005 mmol) or [Rh
A
0.083 mmol), and CH2Cl2 (1.5 mL).
[b]
[c]
Yields and starting material percentage were determined using 1.3.5-trimethoxybenzene as an internal standard by
1H NMR spectroscopy.
Isolated yield.
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Synthesis of Trisubstituted Pyrroles from Rhodium-Catalyzed Alkyne Head-to-Tail Dimerization
FULL PAPERS
nearly no desired product was obtained when PPh3 (entries 7 and 8, Table 3). Interestingly, an amide with
was replaced by PCy3, P(p-OMeC6H4)3 or P(p- R=tert-butyl can dimerize with 78% isolated yield
C6H4F)3 (entries 3 and 4, Table 2). Rh(PPh3)3Cl was using the [Rh(COD)Cl]2/dppf catalyst at room tem-
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
then applied as a catalyst, which has been reported to perature (entry 9, Table 3). Although no detailed
catalyze the dimerization of silyl-substituted acetylene mechanism has been elucidated, it is possible that the
to the corresponding (E)-enynes,[15] and the isolated tert-butyl group renders the amide group a stronger
yield of 1 was improved to 77% (entry 3, Table 2). It donor such that head-to-tail dimerization is facilitat-
has been reported that bidentate phosphines have sig- ed.
nificant effects on the selectivity of alkyne dimeriza-
In addition, both catalysts showed poor activity and
tion,[4b,c] therefore, we screened dppb, dppf, dppp, selectivity for the dimerization of phenylacetylene,
dppm, and xantphos together with [RhACTHNURGTNEUNG(COD)Cl]2 in a ethyl propiolate, and trimethylsilylacetylene under
2:1 ratio,[16] among which dppf gave the best result the same conditions. N-Sulfonylated propargylamines
(entries 5–8, Table 2). Thus, the isolated yield was fur- seem to be the most efficient substrates in this head-
ther improved to 83% when [Rh
used as a catalyst (entry 5, Table 2).
We thus retained Rh(PPh3)3Cl and [Rh
dppf to further explore the scope of this reaction, and with moderate to high yields (67% to 83%).
the results are given in Table 3. Various N-propargyl- Importantly, in all these gem-enyne products the
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
carbamates and ureas smoothly undergo dimerization proper orientation of the NH group and the neighbor-
[17]
under the same conditions. Moderate to good yields ing C C bond allows further elaboration. Au, Pd,[18]
were obtained in the dimerization of BocNHCH2C
ꢀ
and Ir[19] complexes are known to activate alkenes
CH (62%) and CbzNHCH2C CH (74%) when cata- and alkynes to allow catalytic hydroamination. In ad-
ꢀ
ꢀ
lyzed by Rh
(PPh3)3Cl or [Rh
U
tries 3 and 4, Table 3). The Rh
G
N
,
shows an activity superior to that of RhACHTUNGTRENNNUG
ꢀ
the dimerization of Me2NC(O)NHCH2C CH (67%) nation (5-endo-dig cyclization) of enyne 1 using AuCl3
or EtOC(O)NHCH2C CH (66%) (entries 5 and 6, as a catalyst at room temperature. Although H NMR
1
ꢀ
Table 3). Dimerization of structurally related N-prop- analysis of the reaction mixture obtained showed no
argylamides is also achievable. However, in compari- starting material after 1 h, the reaction suffered from
son with propargylcarbamates or ureas, amides with poor selectivity in that product 18 was isolated in only
R=aryl or vinyl groups all showed lower activity and 40% yield (entry 1, Table 5). The selectivity was dra-
moderate to low yields (46–64%) were obtained in matically improved by lowering the reaction tempera-
toluene even at 808C using Rh
N
Table 3. Head-to-tail dimerization of functionalized 1-alkynes.[a]
Entry
R
Catalyst (2mol%)
[Rh(COD)Cl]2/2dppf
Solvent
Temperature [oC]
Isolated yield [%]
Product
1
2
3
4
5
6
7
Ts
G
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
PhMe
24
24
24
24
24
24
80
83
77
62
74
67
66
46
1
1
2
3
4
5
6
Ts
Rh(PPh3)3Cl
[Rh(COD)Cl]2/2dppf
Rh(PPh3)3Cl
ACHTUNGTRENNUNG
Boc[b]
ACHTUNGTRENNUNG
Cbz[c]
ACHTUNGTRENNUNG
Me2NC(O)-
EtOC(O)-
PhC(O)-
[Rh
[Rh
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Rh
ACHTUNGTRENNUNG
8
Rh
G
PhMe
80
24
64
78
8
9
9
t-BuC(O)-
[Rh
N
CH2Cl2
[a]
Reaction conditions: alkyne substate (0.5 mmol), [Rh
(0.01 mmol), CH2Cl2 or PhMe (1.5 mL), 24 h.
Boc=tert-butyloxycarbonyl.
ACHUTGTNRENN(UG COD)Cl]2 (0.01 mmol)/dppf (0.02 mmol) or RhACHTUNGTRENNUNG
[b]
[c]
Cbz=carbobenzoxy.
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Hong Mei Peng et al.
FULL PAPERS
Table 4. Head-to-tail dimerization of propargyl sulfon-
Cyclization of cabamate- or amide-functionalized
enynes is also applicable, and the results are given in
Table 5. Here the R group has pronounced effects on
the cyclization process. Pyrrole 27 was isolated in
80% yield from the Cbz-substitited enyne 3 (À408C,
1 h) (entry 11, Table 5), while Boc-substituted pyrrole
28 was obtained in 50% yield (À408C, 2 h) (entry 12,
Table 5). In line with the low yield of the dimerization
step (Table 3), cyclization of 8 gave pyrrole product
30 in only 36% yield after initial optimerization
(entry 14, Table 5). In sharp contrast, cyclization of
compound 9 (with R=t-Bu) gave two isomeric oxa-
zoles (Z)-31 and (E)-31 [Eq. (2)], instead of any pyr-
ACHTUNGTRENNUNG
amides.[a]
Entry
R
Time [h] Isolated yield [%] Product
1
2
Me
24
16
67
79
10
11
3
4
5
16
24
24
80
64
81
12
13
14
PhCH2
6
16
83
15
7
8
Ph
n-Bu
15
24
79
72
16
17
[a]
Reaction conditions: N-propargylamide (0.5 mmol), [Rh-
(COD)Cl]2 (0.01 mmol), dppf (0.02 mmol), CH2Cl2
ACHTUNGTRENNUNG
(1.5 mL), rt, 16-24 h.
to 88% after 0.7 h (entry 2, Table 5). The scope of the
enyne substrates was extended to those with alkyl- or
arylsulfonamide functionalities. Under differently op- role (see Supporting Information). The yield and the
timized conditions (entries 3–10, Table 5), the isolated identity of the cyclization product seem to correlate
yields range from 70% to 88%. We noted that gold- with the acidity of the NH proton (entries 11–14,
catalyzed intramolecular hydroamination reactions Table 5): a lower yield of pyrrole was obtained for an
have been reported to give substituted pyrroles.[23]
amide-substituted enyne (less acidic). In the cycliza-
Table 5. Au-catalyzed cyclization of N-functionalized enynes.[a]
Entry
R
Temperature [8C]
Time [h]
Yield [%]
Product
1
2
3
4
5
6
7
8
Ts (1)
Ts (1)
MeSO2 (10)
n-BuSO2 (17)
25
1
0.7
1
1
2
2
3
3
1
2
1
2
2
40
88
75
76
81
88
80
83
76
70
80
50
52
36
18
18
19
20
21
22
23
24
25
26
27
28
29
30
À20
25
25
0
0
0
p-Br
PhSO2 (16)
p-Cl(C6H4)SO2 (14)
p-MeO(C6H4)SO2 (11)
m-Me(C6H4)SO2 (11)
A
N
G
À20
9
U
25
10
11
12
13
14
PhCH2SO2 (13)
Cbz (3)
Boc (2)
EtOC(O)- (5)
PhC(O)- (8)
0
À40
À40
À40
25
20
[a]
Reaction conditions: enyne (0.5 mmol), AuCl3 (3 mol%), and CH2Cl2 (2 mL).
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Synthesis of Trisubstituted Pyrroles from Rhodium-Catalyzed Alkyne Head-to-Tail Dimerization
FULL PAPERS
Analytical thin layer chromatography (TLC) was performed
using Merck 60 F254 precoated silica gel plates (0.2 mm
thickness). TLC plates were visualized using UV radiation
(254 nm) or by staining with a basic solution of KMnO4. IR
spectra were recorded on a Horiba FT 300-S by the ATR
method and on a Shimazu IR Prestige-21 FT-IR spectrome-
ter. High-resolution mass spectra were obtained with a Fin-
nigan MAT 95 XP mass spectrometer (EI) and a Waters Mi-
cromass Q-Tof Premier mmass spectrometer (ESI). Melting
points were recorded on an SRS MPA100 automated melt-
ing point system. Microanalyses were performed by the Ele-
mental Analysis Laboratory of the Division of Chemistry
and Biological Chemistry, Nanyang Technological Universi-
ty. 1H and 13C NMR spectra were recorded on a Bruker
Avance DPX 300, Bruker AMX 400, or Bruker DRX 500
spectrophotometer. Chemical shifts for 1H and 13C NMR
spectra are reported as d in units of parts per million (ppm)
downfield from SiMe4.
tion of enyne 9, only O-attack took place as a result
of the low acidity of the NH group. Several examples
of metal-mediated carbonyl attack on terminal al-
kynes have been recently reported.[24] A plausible
mechanism of the overall transformation is proposed
ꢀ
in Scheme 1. The C C bond in 9 is activated by
AuCl3 towards carbonyl attack (5-exo-dig cyclization)
to give a gold vinyl intermediate, protonation of
which affords an exo-cyclic olefin intermediate. The
final product is proposed from the isomerization of
this olefin intermediate to the more stable aromatic
oxazole products.
Conclusions
Rare head-to-tail dimerization reactions of N-protect-
ed propargylic amines were achieved with moderate
to high yields under mild conditions. Both Rh-
General Procedure for the Synthesis of N-Protected
Propargylamines
ACHTUNGTRENNUNG(PPh3)3Cl and [RhACHTUGNTREN(NGUN COD)Cl]2/dppf showed high cata-
To a stirred solution of propargylamine in CH2Cl2 (8 mL)
was added a corresponding acyl chloride or a sulfonyl chlo-
ride (1.1 equiv.) at 08C. Et3N (1.5 equiv.) was then added via
a syringe. The reaction mixture was allowed to slowly warm
up to room temperature and was stirred for 10 h. The mix-
ture was then washed with a 1M HCl solution and subse-
quently with a brine solution. The combined organic layer
was dried by anhydrous Na2SO4. In most cases analytically
pure products were obtained after the solvent was removed.
If necessary, further purification was performed by silica gel
chromatography using hexane and ethyl acetate in a ratio of
5:1.
lytic activity with 2 mol% loading. The dimerization
works most efficiently for enynes with a sulfonamide
functionality, although still moderate to good yields
could be obtained for the dimerization of enynes teth-
ered to amide, carbamate, and urea functionalities.
The enyne products can further undergo Au-catalyzed
hydroamination to afford trisubstituted pyrroles. For
the hydroamination of amide-functionalized enynes,
either oxygen or nitrogen can attack the neighboring
triple bond, depending on the substituents attached to
the carbonyl group. Studies of factors that favor this
dimerization mode, the scope of cross-dimerization
and homodimerization of other substrates in this
mode, and product functionalization other than hy-
droamination are currently in progress in our labora-
tory.
General Procedure for Dimerization of N-Protected
Propargylamines(Table 3 and Table 4)
An N-protected propargylamine (0.5 mmol), 2 mol% [Rh-
AHCTUNGTREGNUN(N COD)Cl]2, and 4 mol% dppf were weighed into a flask,
which was kept under an argon atmosphere. Subsequently,
anhydrous CH2Cl2 or toluene (1.5 mL) was added via a sy-
ringe, and the mixture was stirred at room temperature (for
CH2Cl2) or 808C (for toluene) for 15–24 h. The solvent was
then removed under reduced pressure and the residue was
purified by silica gel column chromatography.
Experimental Section
General Remarks
All commercial solvents were dried using 4 ꢂ molecular
sieves, and all commercial reagents were used as received.
Scheme 1. Proposed mechanism for the cyclization of 9.
Adv. Synth. Catal. 2009, 351, 1371 – 1377
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Hong Mei Peng et al.
FULL PAPERS
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General Procedure for the Au(II)-Catalyzed
Cyclization of Enynes (Table 5)
The following procedure was followed to prepare com-
pounds 18–31. The corresponding enyne (0.5 mmol) and
AuCl3 (4.5 mg, 3 mol%) were weighed into a flask in a
glove box. CH2Cl2 (2 mL) was then slowly added at thee
temperatures indicated in Table 5. The mixture was then
stirred for another 40 min to 20 h (see Table 5). Removal of
the solvent gave a residue which was purified by silica gel
column chromatography.
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