Chiral Bronsted acid-catalyzed asymmetric Friedel-Crafts alkylation of pyrroles with nitroolefins

Article abstract of DOI:10.1021/jo9013029

(Chemical Equation Presented) A highly efficient Friedel-Crafts reaction of pyrroles with nitroolefins by a chiral phosphoric acid was realized. With 5 mol % of catalyst, reactions conducted at rt afforded the 2-substituted or 2,5-disubstituted pyrroles i

Full text of DOI:10.1021/jo9013029

pubs.acs.org/joc
products3 and pharmaceuticals.4 However, the enantioselec-
Chiral Bronsted Acid-Catalyzed Asymmetric
Friedel-Crafts Alkylation of Pyrroles with
Nitroolefins
tive Friedel-Crafts alkylations of pyrroles are relatively less
explored.5 Meanwhile, the asymmetric Friedel-Crafts reac-
tion employing nitroolefin as the electrophilic partner is very
attractive6,7 since the nitro group of the products allows the
subsequent versatile transformations.8 Consequently, the
efficient asymmetric Friedel-Crafts reactions between pyr-
roles and nitroolefins are very useful and highly desirable in
organic synthesis. Surprisingly, only two asymmetric cata-
lytic protocols have been documented in the literature by
Trost and Du, and in both cases chiral zinc complexes were
used as the efficient catalyst.9 As part of our ongoing
program in exploring chiral Bronsted acids as suitable
catalysts for asymmetric Friedel-Crafts alkylation reacti-
ons,10 we recently found that the chiral phosphoric acids11,12
were efficient catalysts for the asymmetric Friedel-Crafts
Yi-Fei Sheng, Qing Gu, An-Jiang Zhang, and Shu-Li You*
State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Lu, Shanghai 200032, China, and
College of Chemistry and Material Engineering, Wenzhou
University, Wenzhou 325027, China
slyou@mail.sioc.ac.cn
Received June 17, 2009
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A highly efficient Friedel-Crafts reaction of pyrroles
with nitroolefins by a chiral phosphoric acid was realized.
With 5 mol % of catalyst, reactions conducted at rt
afforded the 2-substituted or 2,5-disubstituted pyrroles
in up to 94% ee for a wide range of substrates.
Org. Chem. 2002, 1877. (b) Almas-i, D.; Alonso, D. A.; Najera, C. Tetra-
hedron: Asymmetry 2007, 18, 299.(c) Vicario, J. L.; Badía, D.; Carrillo, L.
Synthesis 2007, 2065. With chiral metal complexes: (d) Bandini, M.; Garelli, A.;
Rovinetti, M.; Tommasi, S.; Umani-Ronchi, A. Chirality 2005, 17, 522. (e) Sui,
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N.; Yanagisawa, A. Chem.;Eur. J. 2008, 14, 2052. (j) Jia, Y.-X.; Zhu, S.-F.;
Yang, Y.; Zhou, Q.-L. J. Org. Chem. 2006, 71, 75. (k) Lu, S.-F.; Du, D.-M.; Xu, J.
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Friedel-Crafts alkylation is one of the most efficient
methods for the derivatization of aromatic compounds.1
Asymmetric Friedel-Crafts reactions provide an efficient
way to synthesize optically active aromatics bearing benzylic
chiral centers and have thus attracted considerable interest
and witnessed significant recent progress.2 Pyrroles exist
extensively as the structure core of biologically active natural
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Friedel-Crafts and Related Reactions; Wiley-Interscience: New York, 1964;
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S.; Umani-Ronchi, A. Synlett 2005, 1199. (c) Poulsen, T.; Jorgensen, K. A.
Chem. Rev. 2008, 108, 2903. (d) You, S.-L.; Cai, Q.; Zeng, M. Chem. Soc. Rev.
2009, 38, 2190.
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€
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preparation of the manuscript, a Cu-catalyst was reported by Yokoyama and Arai,
see: (c) Yokoyama, N.; Arai, T. Chem. Commun. 2009, 3285.
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349, 2169. (c) Kang, Q.; Zheng, X.-J.; You, S.-L. Chem.;Eur. J. 2008, 14,
3539. (d) Sheng, Y.-F.; Li, G.-Q.; Kang, Q.; You, S.-L. Chem.;Eur. J. 2009,
15, 3351. (e) Kang, Q.; Zhao, Z.-A.; You, S.-L. Tetrahedron 2009, 65, 1603.
(f) He, Q.-L.; Sun, F.-L.; Zheng, X.-J.; You, S.-L. Synlett 2009, 1111.
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Int. Ed. 2006, 45, 1520. (b) Akiyama, T. Chem. Rev. 2007, 107, 5744. (c) Yu,
X.; Wang, W. Chem. Asian J. 2008, 3, 516. (d) Terada, M. Chem. Commun.
2008, 4097.
€
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Angew. Chem., Int. Ed. 2005, 44, 2777. (b) Furstner, A. Angew. Chem., Int.
Ed. 2003, 42, 3582. (c) Johnson, J. A.; Ning, L.; Sames, D. J. Am. Chem. Soc.
2002, 124, 6900.
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1753. (b) Balme, G. Angew. Chem., Int. Ed. 2004, 43, 6238. (c) Baran, P. S.;
Richter, J. M.; Lin, D. W. Angew. Chem., Int. Ed. 2005, 44, 609.
DOI: 10.1021/jo9013029
r
Published on Web 07/29/2009
J. Org. Chem. 2009, 74, 6899–6901 6899
2009 American Chemical Society

Sheng et al.
JOCNote
TABLE 1. Screening the Catalystsa
TABLE 2. Optimization of the Reaction Conditionsa
entry
solvent
additive x (mol %) time (h) yield (%) ee (%)b
˚
˚
˚
1
2
3
4
5
6
7
8
9
CH2Cl2/benzene 4 A MS
10
5
2
1
0.5
5
5
5
5
5
2
3
6
25
48
7
6
4
5
5
94
93
91
89
66
89
91
92
93
91
89
88
81
72
23
35
45
85
83
86
CH2Cl2/benzene 4 A MS
CH2Cl2/benzene 4 A MS
˚
CH2Cl2/benzene 4 A MS
entry
(S)-1, R
(S)-1a, H
(S)-1b, Ph
(S)-1c, 4-NO2C6H4
(S)-1d, biphenyl
(S)-1e, 3,5-(CF3)2C6H3
(S)-1f, 1-naphthyl
(S)-1g, 2-naphthyl
(S)-1h, 2,4,6-(iPr)3C6H2
(S)-1i, SiPh3
time
yield (%)
ee (%)b
˚
CH2Cl2/benzene 4 A MS
˚
˚
˚
CH2Cl2/benzene 3 A MS
1
2
3
4
5
6
7
8
9
2 d
2 d
2 d
2 d
2 d
2 d
2 d
2 d
2 d
16 h
15 h
3 h
2d
79
81
80
83
72
79
73
77
76
89
93
93
15d
2 (R)
4 (R)
13 (S)
24 (S)
1 (R)
6 (S)
4 (S)
3 (R)
5 (S)
33 (R)
87 (R)
88 (R)
0
CH2Cl2/benzene 5 A MS
CH2Cl2
toluene
4 A MS
˚
4 A MS
˚
10 CH2Cl2/toluene 4 A MS
aReaction conditions: 3 equiv of 2a, x mol % of (S)-1k,
0.25 mol/L of 3a in CH2Cl2/benzene. bDetermined by chiral
HPLC analysis (Chiralcel OD-H column).
10
11
12c
13
a
(S)-1j, 9-phenanthryl
(S)-1k, 9-anthryl
(S)-1k, 9-anthryl
none
between pyrrole 2a and nitroolefin 3a (Table 1). With 5 mol %
˚
of the catalyst and 4 A molecular sieves (MS), all the
catalysts led to alkylation product with good yields but
mostly poor ee values. To our delight, chiral phosphoric
acid (S)-1k bearing 9-anthryl groups was found to be the
optimal catalyst, affording alkylation product 4aa in 93%
yield and 87% ee (entry 11, Table 1). The ee was further
increased to 88% by adding the nitroolefin with a syringe
pump over 2 h (entry 12, Table 1). Notably, the reaction
between 2a and 3a could proceed slowly in the absence of
the catalyst, affording 4aa in 15% conversion in 2 d (entry
13, Table 1).
Reaction conditions: 3 equiv of 2a, 5 mol % of (S)-1,
0.25 mol/L of 3a in CH2Cl2/benzene. bDetermined by chiral
HPLC analysis (Chiralcel OD-H column). cA syringe pump was
used to add 3a for 2 h. dConversion was determined by 1H
NMR.
alkylation reaction of nitroolefins with pyrroles, providing
2-substituted pyrrole derivatives with high ee values. To our
knowledge, this study represents the first example of an
enantioselective organocatalytic Friedel-Crafts reaction of
pyrroles with nitroolefins. In this paper, we report our
detailed studies on this subject.
We recently found that the (S)-1k bearing 9-anthryl
groups could catalyze the reaction between 4,7-dihydroin-
dole and nitroolefin with high yields and ee values.10d Since
4,7-dihydroindole is a substituted pyrrole derivative, the
success of the reaction of 4,7-dihydroindole prompted us to
explore simple unprotected pyrrole as a suitable electro-
philic partner in the asymmetric Friedel-Crafts reaction
with nitroolefins. We began our study by examining differ-
ent chiral phosphoric acids in the Friedel-Crafts reaction
Then the reaction conditions were further optimized as
summarized in Table 2. Increasing the catalyst loading to 10
mol % did not show notable beneficial effects. With 2 mol %
of the catalyst, the reaction was also able to proceed to
completion within 6 h with 81% ee (entry 3, Table 2).
However, dramatic decrease of both the reaction rate and
ee was observed with 0.5 or 1 mol % catalyst. Different
˚
molecular sieves were tested. Replacing 4 A MS with either 3
˚
˚
A MS or 5 A MS led to the decrease of ee (entries 6 and 7,
Table 2). Further examinations on the solvents showed that
dichloromethane/benzene (1/1) was the best solvent combo
in this reaction (entries 8-10, Table 2).
(12) For recent examples, see: (a) Chen, X.-H.; Zhang, W.-Q.; Gong,
L.-Z. J. Am. Chem. Soc. 2008, 130, 5652. (b) Wang, X.-W.; Reisinger, C. M.;
List, B. J. Am. Chem. Soc. 2008, 130, 6070. (c) Hu, W.; Xu, X.; Zhou, J.; Liu,
W.-J.; Huang, H.; Hu, J.; Yang, L.; Gong, L.-Z. J. Am. Chem. Soc. 2008, 130,
7782. (d) Li, G.; Fronczek, F. R.; Antilla, J. C. J. Am. Chem. Soc. 2008, 130,
12216. (e) Cheng, X.; Vellalath, S.; Goddard, R.; List, B. J. Am. Chem. Soc.
2008, 130, 15786. (f) Guo, Q.-S.; Du, D.-M.; Xu, J.-X. Angew. Chem., Int. Ed.
2008, 47, 759. (g) Wang, X.-W.; List, B. Angew. Chem., Int. Ed. 2008, 47,
1119. (h) Jiang, J.; Yu, J.; Sun, X.-X.; Rao, Q.-Q.; Gong, L.-Z. Angew.
Chem., Int. Ed. 2008, 47, 2458. (i) Xu, S.; Wang, Z.; Zhang, X.; Zhang, X.;
Ding, K. Angew. Chem., Int. Ed. 2008, 47, 2840. (j) Sickert, M.; Schneider, C.
Angew. Chem., Int. Ed. 2008, 47, 3631. (k) Terada, M.; Soga, K.; Momiyama,
N. Angew. Chem., Int. Ed. 2008, 47, 4122. (l) Cheng, X.; Goddard, R.; Buth,
G.; List, B. Angew. Chem., Int. Ed. 2008, 47, 5079. (m) Rueping, M.;
Antonchick, A. P. Angew. Chem., Int. Ed. 2008, 47, 5836. (n) Rueping, M.;
Antonchick, A. P.; Sugiono, E.; Grenader, K. Angew. Chem., Int. Ed. 2009,
48, 908. (o) Schrader, W.; Handayani, P. P.; Zhou, J.; List, B. Angew. Chem.,
Int. Ed. 2009, 48, 1463. (p) Terada, M.; Tanaka, H.; Sorimachi, K. J. Am.
Chem. Soc. 2009, 131, 3430. (q) Lu, M.; Zhu, D.; Lu, Y.; Zeng, X.; Tan, B.;
Xu, Z.; Zhong, G. J. Am. Chem. Soc. 2009, 131, 4562. (r) Liu, H.; Dagousset,
G.; Masson, G.; Retailleau, P.; Zhu, J. J. Am. Chem. Soc. 2009, 131, 4598. (s)
Terada, M.; Machioka, K.; Sorimachi, K. Angew. Chem., Int. Ed. 2009, 48,
2553. (t) Terada, M.; Toda, Y. J. Am. Chem. Soc. 2009, 131, 6354.
Under optimized reaction conditions, a wide array of
nitroolefins 3 and pyrroles 2 were investigated in the asym-
metric Friedel-Crafts alkylation reaction. The results are
summarized in Table 3.
Several substituted nitroolefins 3b-e, containing elec-
tron-donating groups, have been tested in the reaction
with pyrrole 2a. In all cases, excellent yields and ee values
were obtained for the desired alkylation products (87-
93% yield, 81-94% ee, entries 2-5, Table 3). The reaction
also proceeded well with substituted nitroolefins 3f-i,
containing an electron-withdrawing group, affording
their corresponding alkylation products in 90-92% yield
with 90-92% ee (entries 6-9, Table 3). 2-Naphthyl-
and heteroaryl-substituted nitroolefins 3j-l were well
tolerated under the optimized reaction conditions (89-
91% yield, 89-93% ee, entries 10-12, Table 3). Un-
fortunately, the reaction gave low enantioselectivity for
6900 J. Org. Chem. Vol. 74, No. 17, 2009

Sheng et al.
JOCNote
TABLE 3. Enantioselective Friedel-Crafts Reaction of Pyrroles with
Nitroolefinsa
FIGURE 1. Proposed catalyst working model.
entry
2
3, R
time (h) 4, yield (%) ee (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2a 3a, C6H5
3
6
6
6
24
6
6
5
5
5
5
6
12
24
6
4aa, 93
4ab, 93
4ac, 91
4ad, 90
4ae, 87
4af, 91
4ag, 90
4ah, 92
4ai, 92
4aj, 90
4ak, 91
4al, 89
4am, 81
<5%
88
87
94
81
92
91
92
92
90
92
93
89
11
2a 3b, 4-CH3-C6H4
2a 3c, 4-MeO-C6H4
2a 3d, 3-CH3-C6H4
2a 3e, 3,4,5-MeO-C6H2
2a 3f, 4-Br-C6H4
2a 3g, 4-Cl-C6H4
2a 3h, 4-F-C6H4
2a 3i, 4-CF3-C6H4
2a 3j, 2-furyl
2a 3k, 2-thienyl
2a 3l, 2-naphthyl
2a 3m, 2-Br-C6H4
2a 3n, cyclohexyl
2b 3j, 2-furyl
In conclusion, we have developed the enantioselective
Friedel-Crafts alkylation reaction of pyrroles with nitroo-
lefins by utilizing chiral phosphoric acid as an efficient
catalyst. This metal-free process, together with mild reaction
conditions and excellent yields and enantioselectivities, pro-
vides a practical method to synthesize highly enantioen-
riched 2-substitued and 2,5-disubstituted pyrroles.
4bj, 94
4ca, 91
93
86
2c 3a, C6H5
2
aReaction conditions: 3 equiv of 2, 5 mol % of (S)-1k, rt,
0.25 mol/L of 3 in CH2Cl2/benzene. bDetermined by chiral
HPLC analysis.
Experimental Section
General Procedure for the Catalytic Asymmetric Friedel-
Crafts Reaction. In a dry Schlenk tube, pyrrole 2 (0.9 mmol),
˚
chiral phosphoric acid (S)-1k (11 mg, 0.015 mmol), 4 A molecular
sieves (powder, activated, 50 mg), and CH2Cl2/benzene (0.6 mL,
1:1) were added under argon. The reaction mixture was stirred at
room temperature, and then a solution of nitroolefin 3 (0.3 mmol)
in CH2Cl2/benzene (0.6 mL, 1:1) was added by syringe pump over
2 h. When the reaction was complete, the reaction mixture was
concentrated, and the residue was purified by flash chromatogra-
phy (dichloromethane /petroleum ether=1/10 to 1/1) to afford the
product. 4aa was obtained as a brown crystal (60 mg, 93% yield,
88% ee) following silica gel chromatography (dichloromethane/
petroleum ether=1/4, v/v). Analytical data for 4aa: [R]20D þ61.7
2-Br-phenyl substituted nitroolefin 3m and very low con-
version for cyclohexyl-substituted nitroolefin 3n (entries
13 and 14, Table 3). Substituted pyrroles were also tested.
The pyrroles bearing either an alkyl or aryl substituent at
the 2-position were well tolerated (entries 15 and 16,
Table 3). As seen from above, the current methodology
is suitable for a wide range of substrates for both pyrroles
and aryl-substituted nitroolefins. With this protocol,
highly enantioenriched 2-substituted or 2,5-disubstituted
pyrroles could be easily obtained.
1
(c 0.48, MeOH). H NMR (300 MHz, CDCl3) δ 4.74-4.99 (m,
3H), 6.07 (s, 1H), 6.15 (s, 1H), 6.66 (s, 1H), 7.20-7.37 (m, 5H), 7.85
(br s, 1H). The enantiomeric ratio was determined by Daicel
The absolute stereochemistry of the products was deter-
mined by comparison of the optical rotations with those
reported by Du and co-workers.9b The absolute configura-
tion of the products was determined to be R.
Chiralcel OD-H (25 cm), hexanes/IPA = 80/20, 1.0 mL min-1
,
3
λ=254 nm, t(major)=13.3 min, t(minor)=15.9 min.
As shown in Figure 1, a similar working model to that
previously reported is proposed.10d Chiral phosphoric
acid acts as a bifunctional catalyst, and the acidic proton
and phosphoryl oxygen of the catalyst form the hydrogen
bond with nitroolefin and pyrrole nitrogen proton, respec-
tively. This proposed working model can be partially
supported by the fact that the NH of pyrrole is critical
for the reaction. When the N-methylpyrrole was used
under the optimal reaction conditions, a significant
decrease of reaction rate and ee was observed compared
to pyrrole itself (eq eq 1).
Acknowledgment. We thank the National Natural Science
Foundation of China (20732006, 20821002), the Natio-
nal Basic Research Program of China (973 Program
2009CB825300), and the Science and Technology Commis-
sion of Shanghai Municipality (07pj14106, 07JC14063) for
generous financial support. We thank Professor Christian A.
Sandoval (SIOC) for help with the manuscript preparation.
Supporting Information Available: Experimental proce-
dures and analysis data for 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 17, 2009 6901