Reversal of enantioselective Friedel-Crafts C3-alkylation of pyrrole by slightly tuning the amide units of N,N′-dioxide ligands

Article abstract of DOI:10.1039/c4cc10055g

Chiral Ni(ii)-complexes of N,N′-dioxides show high catalytic activity and enantioselectivity in catalysing the asymmetric Friedel-Crafts C3-alkylation of 2,5-dimethyl pyrrole to β,γ-unsaturated α-ketoesters. A dramatic reversal of enantioselectivity is re

Full text of DOI:10.1039/c4cc10055g

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Reversal of enantioselective Friedel–Crafts
C3-alkylation of pyrrole by slightly tuning the
Cite this:DOI: 10.1039/c4cc10055g
amide units of N,N0-dioxide ligands†
Received 17th December 2014,
Accepted 5th February 2015
Yulong Zhang,a Na Yang,a Xiaohua Liu,a Jing Guo,a Xiying Zhang,a Lili Lin,a
Changwei Hua and Xiaoming Feng*ab
DOI: 10.1039/c4cc10055g
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Chiral Ni(II)-complexes of N,N0-dioxides show high catalytic activity both enantiomers of the chiral compounds bearing the pyrrole unit
and enantioselectivity in catalysing the asymmetric Friedel–Crafts is demanded for the purpose of evaluation of biological and
C3-alkylation of 2,5-dimethyl pyrrole to b,c-unsaturated a-ketoesters. pharmaceutical activity. Generally, a switch in enantioselectivity is
A dramatic reversal of enantioselectivity is realized with ligands derived achieved by the use of enantiomeric ligands. Unfortunately, the two
from the same type of chiral source of L-ramipril, by slightly tuning enantiomers of a chiral ligand are not always readily available or
the amide units.
economically feasible to synthesize. Alternatively, enantioselectivity
of reactions could also be changed with a single ligand by changing
Substituted pyrroles are abundant in both natural products and the metal sources, solvents, temperature or by other methods.6
biologically active molecules.1 The enantioselective Friedel–Crafts There are also some examples of the reversal of enantioselectivity
type alkylation reactions of pyrrole are powerful transformations using ligands with the same chiral backbone simply by modifying
that introduce a substituent onto the C2- or C3-position of this subunits of the ligands.5,7 Prior investigations of chiral N,N0-dioxide–
important heterocycle. Despite advances in asymmetric Friedel– metal complex catalysts by our group revealed that subtle modifica-
Crafts reactions of similar nucleophiles of indoles, sporadical tion of the ligand moieties, such as the amide substituents, and the
reactions of pyrrole have been documented,2 likely due to the amino acid backbone, obviously affects the stereoselection of the
regioselectivity and reactivity issues. The first enantioselective reactions.6v,w,8 As an extension to this approach, we report herein
Friedel–Crafts reaction of pyrrole with a,b-unsaturated aldehydes the asymmetric Friedel–Crafts alkylation of 2,5-dimethyl pyrrole with
was reported by MacMillan.2i After that, both chiral Lewis acid b,g-unsaturated a-ketoesters,9 where the reaction occurred at the
complexes and organocatalysts have been explored in the nucleo- C3-position of pyrrole. A dramatic switch in the enantioselectivity was
philic addition of pyrrole at its C2-position.2h–l Nevertheless, realized by slight modification of the substituent at the aniline units
rare examples were reported using the nucleophilicity of the of the N,N0-dioxides. It showed that N,N0-dioxide–Ni(II) complexes10
C3-position of pyrroles. Recently, the You group presented an that contain a 2,6-diisopropylaniline or 3,5-ditertbutylaniline substi-
elegant intermolecular asymmetric allylic dearomatization reac- tuent imparted good reactivity and reversal of enantioselectivity.
tion using 2,5-disubstituted pyrroles.3 Given the regioselective
At the outset of this study, 2,5-dimethyl-pyrrole 2a and
control of the Friedel–Crafts reaction with 3-substituted indoles,4 b,g-unsaturated a-ketoester 1a were employed as the model
we expect to realize the asymmetric Friedel–Crafts alkylation substrates to study the asymmetric Friedel–Crafts alkylation
of pyrrole derivatives at the C3-position by the employment of reaction (Table 1). In the presence of the chiral N,N0-dioxide
2,5-dimethyl pyrrole. The multisubstituted pyrrole derivatives, ligand L-PiPh, derived from L-pipecolic acid and aniline, only
therefore, could be formed regio- and enantioselectively.
the Ni(OTf)2 complex could give the desired C3-alkylation pro-
Control of the absolute configuration of newly created stereo- duct 3a in moderate yield, albeit the enantioselectivity was low
centers is of special interest in asymmetric catalysis.5 Synthesis of (74% yield and 15% ee; entries 1–3). To improve the enantio-
selectivity of the reaction, a series of chiral N,N0-dioxide ligands
a Key Laboratory of Green Chemistry & Technology, Ministry of Education,
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China.
E-mail: xmfeng@scu.edu.cn; Fax: +86 28 85418249; Tel: +86 28 85418249
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
P. R. China
was examined. Interestingly, the reaction catalyzed by Ni(OTf)2
complexes of N,N0-dioxides derived from 2,6-disubstituted
anilines displayed the opposite sense of stereoinduction to
those observed in the reactions with 3,5-disubstituted ones
(entries 3–10 vs. entries 11–14). The enantioselectivity gradually
increased when the hindrance of the substituents on anilines
was raised (entries 4–6). Changing the backbone of the ligands
† Electronic supplementary information (ESI) available: Additional experimental
and spectroscopic data. CCDC 1035849 and 1035929. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4cc10055g
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Table 1 Optimization of the reaction conditions
Table 2 Substrate scope of b,g-unsaturated a-ketoesters 1
Entrya
R1
R2
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
C6H5
C6H5
C6H5
C6H5
Me
Et
Bn
92(95)
84(85)
91(89)
89(89)
84(91)
80(78)
89(93)
94(95)
92(89)
90(93)
92(91)
82(95)
81(78)
90(92)
79(80)
89(89)
84(95)
94(95)
92(95)
95(96)
92(96)
95(93)
99(67)
98(60)
90(90)
93(93)
88(92)
93(93)
91(94)
93(93)
96(94)
91(92)
95(91)
93(94)
92(93)
92(96)
t-Bu
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
2-ClC6H4
2-BrC6H4
3-ClC6H4
4-ClC6H4
4-O2NC6H4
4-MeC6H4
4-PhC6H4
3-MeOC6H4
4-MeOC6H4
3,4-Cl2C6H3
1-Naphthyl
2-Naphthyl
2-Thienyl
2-Furanyl
9
10
11
12
13
14
15
16
17
18
Entrya
Ligand
Metal
t (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
L-PiPh
L-PiPh
L-PiPh
L-PiMe2
L-PiEt2
L-PiPr2
L-PrPr2
L-RaPr2
L-RaPr2
L-RaPr2
L-PiMe2
L-PimBu2
L-PrmBu2
L-RamBu2
Sc(OTf)3
Cu(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
12
12
12
12
12
12
12
12
18
24
15
24
24
24
11
—
74
83
84
85
60
79
70
92
62
92
75
95
14
—
15
16
22
55
70
80
90
92
51(ꢀ)
92(ꢀ)
93(ꢀ)
95(ꢀ)
9d
10d,e
11
12e
13e
14e
19
Me
68(95)
94(91)
20
Me
Me
85(90)
78(84)
91(95)
86(20)
21
c-Hexyl
a
Unless otherwise noted, the reaction was carried out with 1 (0.1 mmol),
2a (3.0 equiv.) and L-RaPr2/Ni(OTf)2 (10 mol%, 1/1) in toluene–DCM
(0.5 mL) at ꢀ20 1C for 48 h. The data in parentheses was obtained by
a
Unless otherwise noted, the reaction was carried out with 1a (0.1 mmol),
2a (3.0 equiv.) and L-metal (10 mol%, 1 : 1) in CH2Cl2 (0.5 mL) at
b
c
b
c
25 1C. Isolated yield of 3a. Determined by chiral HPLC, and (ꢀ) refers
using L-RamBu2 instead. Isolated yield of 3. Determined by chiral
d
to the rotation sign that is opposite to others. In toluene (0.5 mL).
HPLC. The data in parentheses had opposite configuration.
e
At ꢀ20 1C.
from L-pipecolic acid to L-proline or L-ramipril, resulted in further also a suitable candidate for this catalytic system giving 78%
improved ee values and yields (Table 1, entries 6–8, and 12–14). The yield with 86% ee (Table 2, entry 21).
ligand L-RaPr2 of choice prepared from 2,6-diisopropylaniline and
We next proceeded to prepare the antipodes of the adducts 3
L-ramipril afforded the (+)-3a in 79% yield with 80% ee (entry 8). by the employment of the L-RamBu2–Ni(OTf)2 complex as
Better results were given when the reaction was performed in the catalyst instead. The Friedel–Crafts alkylation reaction
toluene and at ꢀ20 1C (92% yield, 92% ee; Table 1, entry 10). with 2,5-dimethylpyrrole was tolerable to various substituted
On the other hand, ligand L-RamBu2, appropriately combined b,g-unsaturated a-ketoesters, independent of the electron-rich
with 3,5-ditertbutylaniline and L-ramipril, generated (ꢀ)-3a or electron-deficient characteristic of the substituents (Table 2,
with 95% ee and 95% yield (entry 14).
data in the parentheses). It is noteworthy that the enantio-
To test the generality of the catalytic systems, firstly, the selectivity was fairly similar to its corresponding enantiomer in
substrate scope for the (+)-enantiomer of pyrrole esters 3 was almost all the cases. The yield was slightly higher than the corres-
investigated by using the L-RaPr2–Ni(OTf)2 complex as the catalyst. ponding enantiomer, such as for 3-methoxylphenyl, 2-thienyl, and
Various b,g-unsaturated a-ketoesters with 2,5-dimethylpyrrole (E)-phenylethenyl substituted ones (entries 12, 17 and 19). Thus, the
were evaluated, giving the corresponding adducts with excellent two enantiomers of the 2,3,5-trisubstituted pyrrole derivatives could
yields and enantioselectivities (up to 94% yield and 99% ee). be readily formed from chiral N,N0-dioxides with subtle modification
The ester group had a slight influence on the outcome (Table 2, of the amide units. Scale-up experiments under these reaction
entries 1–4). g-Aryl ketoesters bearing a halo-group at the ortho- conditions with 5 mmol of 1a afforded both enantiomers in 90%
position underwent the reaction with higher enantioselectivity yield, and 92% and 96% ee. Finally, the reaction of N-methyl
than the others (up to 99% ee; entries 5 and 6). Electron-donating 2,5-dimethyl pyrrole worked well under the standard reaction
substituents afforded the products sluggishly in comparison with conditions, thus affording the related enantiomers in 76% yield
the electron-withdrawing ones (entries 12 and 13 vs. 5–10). with 90% ee, and 72% yield with 92% ee. It indicated that there
In addition, naphthyl and 2-thienyl or 2-furanyl substituted was no interaction between the NH of pyrrole and the catalyst
ketoesters reacted well with 2,5-dimethyl-pyrrole, delivering the (see ESI† for details).
desired products in 92% to 95% ees and 79% to 94% yields,
Encouraged by the above results, we also explored the reaction
(Table 2, entries 15–18). Remarkably, an aliphatic ketoester was with 2-substituted indole derivatives as the nucleophile. A reversal
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Scheme 1 Substrate scope for indole derivatives.
of enantioselectivity was found upon the ligands and substituents
on indoles. As shown in Scheme 1, 2-methyl indole 4b was a
suitable substrate for both catalytic systems, delivering 95% yield
with 93% ee, and 99% yield with 94% ee, respectively (Scheme 1,
5b). The L-RaPr2–Ni(OTf)2 complex catalytic system was compatible
for the Friedel–Crafts alkylation of indole 4a and 2-phenyl
indole 4c, giving the desired products in excellent yield and
enantioselectivity. Moderate reversed or unreversed enantio-
selectivity was observed in the presence of the L-RamBu2–Ni(OTf)2
complex catalyst (Scheme 1, 5a11 and 5c). Additionally, when pyrrole
was subjected to the catalytic system of L-RaPr2–Ni(OTf)2, the
Friedel–Crafts reaction with b,g-unsaturated a-ketoester 1a occurred
at the C2-position, delivering the 2-substituted pyrrole derivative
in 76% yield and 99% ee. A small amount of 2,5-disubstituted
pyrrole byproduct was detected.
Fig. 1 X-ray crystal structures of the N,N0-dioxide–Ni(II) complexes (top view).
Fig. 2 Possible catalytic models for the switch in enantioselectivity (side
view). Left: L-RaPr2–Ni(II)–1a; right: L-RamBu2–Ni(II)–1a.
However, the reaction catalyzed by L-RamBu2–Ni(OTf)2 was
less enantioselective, and the adduct was given in 72% yield
and 11% ee with the major enantiomer maintained. It implied substituent of the forward amide in L-RamBu2 efficiently
0
that the steric hindrance of the nucleophiles was also crucial shields the Re-face of the substrate (dX–C2 = 4.51 Å). The other
for the facial selection (see ESI† for details).
amide unit blocks the Si-face a little since it is far away from the
We have been able to obtain the crystal structure of the g-position of the substrate (dX–C3 = 6.33 Å). As a result, the
hydrates of both the L-RaPr2 and L-RamBu2 complexes of Ni(II)12 corresponding (S)-5a is generated as the major enantiomer
that show a six-coordinate distorted octahedral geometry. Both from the Si-face attack. Density functional theory (DFT) calculations
N,N0-dioxides act as neutral tetradentate ligands that bind the at the UM06/[6-31G(d), LanL2DZ] level indicate the preference for
Ni(II) cation securely through two amine oxide oxygens and two the products of L-RaPr2-Re and L-RamBu2-Si over the products of
amide oxygens. There are two coordination sites in the equatorial L-RaPr2-Si and L-RamBu2-Re, with the differences of the activation
plane that can capture ancillary substrates or solvents. The energy barriers for the two corresponding competing pathways
available spaces in the seesawed amide units of the two catalyst being 2.5 (L-RaPr2-Re vs. L-RaPr2-Si) and 2.9 kJ molꢀ1 (L-RamBu2-Si
complexes are markedly different (Fig. 1). The torsion angle vs. L-RamBu2-Re) (see ESI† for details).
a (C1–C2–C10–C20) of the two amide units of L-RaPr2 is 147.041,
In summary, we have developed a novel Friedel–Crafts
whereas the corresponding angle in L-RamBu2 is ꢀ155.741. The C3-alkylation of 2,5-dimethyl-pyrrole to b,g-unsaturated a-keto-
distance between X and C3 of the center carbon in the sub- esters catalysed by chiral N,N0-dioxide–Ni(OTf)2 complexes. Dramatic
stituent in L-RaPr2 is much shorter than in L-RamBu2 (4.65 Å vs. reversal of enantioselectivity was accomplished by slightly modifying
6.33 Å), but the distance between X and C20 in L-RaPr2 is much the subunit of the N,N0-dioxide. The two enantiomers of
longer than in L-RamBu2 (5.51 Å vs. 4.51 Å).
2,3,5-trisubstituted pyrroles were given in high yield and enantio-
Fig. 2 shows the side view of two possible catalytic models selectivity. A variety of b,g-unsaturated a-ketoesters, 2,5-dimethyl-
that rationalize the reversal of the enantioselectivity. The cis- pyrrole, pyrrole, and 2-methyl indole could be tolerated in the
auxiliary ligands X/Y (Fig. 1) can be replaced by the bidentate Friedel–Crafts reaction. X-ray analysis of the two catalysts as
b,g-unsaturated a-ketoester 1a. As shown in Fig. 2 (left), the well as DFT calculations of the transition states provided a
steric hindrance of the ortho-isopropyl substituent of the back- rational explanation for the controllable enantioselectivity.
ward amide in L-RaPr2 is adverse for the Si-face attack of indole
We appreciate the National Natural Science Foundation of China
(dX–C3 = 4.65 Å). Therefore, the alkylation reaction dominates (No. 21372162, 21432006, and 21321061), and the National Basic
from the Re-face of the substrate, giving (R)-5a as the major Research Program of China (973 Program: No. 2011CB808600)
isomer. In contrast, in the right view (Fig. 2), the tert-butyl for financial support.
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12 CCDC 1035849 (L-RaPr2–Ni(BF4)2ꢁ6H2O) and CCDC 1035929
(L-RamBu2–Ni(BF4)2ꢁ6H2O).
Chem. Commun.
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