Fused indolines by palladium-catalyzed asymmetric C-C coupling involving an unactivated methylene group

Article abstract of DOI:10.1002/anie.201102639

Selectivity at high temperatures: Bulky, thermally stable, chiral palladium complexes generated from N-heterocyclic carbenes (NHCs; see picture) were successfully applied to the synthesis of highly enantioenriched trans-fused indolines. Remarkably, this r

Full text of DOI:10.1002/anie.201102639

Communications
DOI: 10.1002/anie.201102639
À
C H Activation
À
Fused Indolines by Palladium-Catalyzed Asymmetric C C Coupling
Involving an Unactivated Methylene Group**
Masafumi Nakanishi, Dmitry Katayev, Cꢀline Besnard, and E. Peter Kꢁndig*
In memory of Keith Fagnou
The field of palladium-catalyzed cross-coupling reactions was
honored by the 2010 Nobel Prize in Chemistry which was
awarded to the pioneers R. Heck, E. Negishi, and A. Suzuki.
This thus reflected the tremendous importance of this family
elements in a monodentate ligand at appropriate positions
near the metal center represent major challenges. We have
reported new chiral NHC ligands derived from chiral o-
substituted a-alkylphenethyl amines. They perform very well
in the intramolecular arylation of amides[9] to give highly
enantioenriched 3-aryl-3-alkyl-, 3-aryl-3-alkoxy-, and 3-aryl-
3-amino-oxindoles.[10] We report here on the highly enantio-
selective synthesis of fused indolines using members of the
same family of chiral NHC ligands.
We started our study with the N-cyclohexyl-substituted
carbamate 1a. Under the conditions we used previously for
reactions of oxindoles (Pd(dba)2/NaOtBu/DME/RT) indoline
2a was formed in traces only. The first encouraging result was
obtained using the protocol developed by Fujii, Ohno, and co-
workers[6a] (Table 1, entry 1). trans Fusion in product 2a was
indicated by the 12 Hz coupling constant in the 1H NMR
spectrum, confirming the previous assignment.[6a,11] Both the
yield and enantiomeric ratio of products increased strongly
when, in place of the h3-allyl complex 3, the more reactive h3-
cinnamyl palladium complex (S,S)-4 was used (Table 1,
entry 2). It is noteworthy that in the absence of pivalic acid
as an additive, only traces of product were observed.[12] The
reaction did not proceed at lower temperature (1108C).
Complex (R,R)-5, incorporating the enantiomeric NHC
ligand with o-OMe aryl groups, afforded ent-2a as the
major product (Table 1, entry 3).
À
of reactions in organic synthesis. Initially focused on C C
bond formation between C(sp) and C(sp2) centers, cross-
coupling reactions subsequently evolved to include the
2
2
À
À
formation of C(sp ) N and C(sp ) O bonds and most
3
À
recently the formation of C C bonds involving C(sp ) centers.
C C bond formation typically requires the activation of the
coupling partners either in the form of a C X and/or a C M
precursor. The past decade has seen the emergence of
À
À
À
À
coupling reactions with C H centers, which thus bypasses
the preparation of functionalized substrates.[1] The use of
3
À
unactivated C(sp ) H alkyl groups remains a major chal-
lenge, however, even though some progress has been made
recently in this area.[2] Asymmetric reactions would be an
additional exceptionally powerful tool in this area, but to our
knowledge efficient coupling reactions that distinguish
À
between two enantiotopic C H bonds in an unactivated
methylene group have not been reported.[3] Herein we
describe a successful approach to this transformation.
The indoline motif is found in a large number of natural
products and pharmaceuticals, and diverse strategies for their
synthesis have been developed.[4] The important subgroup of
fused indoline-containing natural products has received much
attention.[5] New routes of access to indolines by palladium-
In a previous paper, we reported the X-ray structure of
complex 3. The arrangement of the Pd–carbene ligand
fragment is shown in Figure 1. The minimization of allylic
(A1,3) strain, both in the Pd-C-N-C-H part and in the H-
3
À
catalyzed functionalization of unactivated C(sp ) H alkyl
groups are detailed in very recent literature reports.[6]
N-heterocyclic carbenes (NHCs) are outstanding nucleo-
philic catalysts in organic synthesis and electron-rich ligands
in transition-metal-catalyzed transformations.[7] Conversely,
chiral NHC ligands that lead to high asymmetric induction are
still scarce.[8] The wedge shape of NHC ligands, the absence of
a chiral backbone (which controls chiral induction in chelat-
ing ligands), and the difficulty of placing stereocontrol
C
benzylic-CAr-Cortho-CH3 parts of the ligand, position the stereo-
control elements of the NHC ligand in the complex (Fig-
ure 1).[10c] We hypothesized that the substitution of the ortho-
Me group by a fused aromatic ring could make the ligand
even better. Indeed the naphthyl-NHC complex showed
improved stability at elevated temperatures, and this had a
beneficial effect on the yield of the coupling reaction (Table 1,
entry 4). Conversely, we found that 4,5-dihydroimidazolium-
derived NHC–palladium complexes 7 and 8 lacked robustness
at high temperature and gave product 2a in very poor yield
(Table 1, entries 5 and 6).[13]
[*] Dr. M. Nakanishi, D. Katayev, Dr. C. Besnard, Prof. Dr. E. P. Kꢀndig
Department of Organic Chemistry, University of Geneva
30 Quai Ernest Ansermet, 1211 Geneva, 4 (Switzerland)
Fax: (+41)22-379-3215
Consequently, compound 9, the precursor of the most
efficient NHC ligand, was chosen for the in situ generation of
a palladium–NHC catalyst. This protocol was investigated
because it would simplify the operational procedure. Exper-
imentation showed that using an in situ generated catalyst in
the presence of cesium pivalate and cesium carbonate in
xylenes as solvent gave 2a in good yield with high asymmetric
E-mail: peter.kundig@chiorg.unige.ch
Homepage: http://www.unige.ch/sciences/chiorg/kundig/
[**] Support of this work by the Swiss National Foundation and the
Univeristy of Geneva is gratefully acknowledged. We thank a referee
who drew our attention to Ref. [3].
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102639.
7438
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7438 –7441

Table 1: Screening of chiral NHC–palladium precursors in the asym-
metric synthesis of fused indolines.
Table 2: Chiral palladium–NHC catalyzed asymmetric synthesis of fused
indolines 2 from carbamates 1.
Entry[a]
Pd–NHC
Yield [%][b]
e.r.[c]
1
2
3
4
5
6
(S,S)- 3
(S,S)- 4
(R,R)- 5
(S,S)- 6
(S,S)- 7
(S,S)- 8
36
73
56
89
9
93:7
97.5:2.5
10:90
97.5:2.5
78:22
Entry
Substr.
R
Cond.[a]
T [8C]/t [h]
Prod. Yield
[%][b]
e.r.[c]
9
n.d.[d]
1
2
3
4
5
6
7
8
1a
1a
1a
1b
1c
1d
1e
1e
1 f
1 f
1g
1h
1i
H
140/24
140/24
160/3
160/3
160/3
160/3
140/24
160/3
140/24
160/3
140/24
160/3
140/24
160/3
160/3
160/3
140/24
160/3
2a
2a
2a
2b
2c
2d
2e
2e
2 f
2 f
2g
2h
2i
84
39
78
82
84
88
81
59
30
13
0[e]
82
55
24
70
62
68
88
97.5:2.5
96:4
95:5
95:5
94.5:5.5
96:4
[a] Reaction conditions: 1a (0.2 mmol), Pd–NHC complex (0.02 mmol),
pivalic acid (0.06 mmol), cesium carbonate (0.3 mmol), dry xylenes
(2 mL). [b] Yield of product isolated after flash column chromatography.
[c] The enantiomeric ratio was determined by HPLC on a chiral stationary
phase (Chiralcel OD-H, n-hexane/iPrOH 99:1, 0.5 mLminÀ1). The left
column refers to the enantiomer shown. [d] n.d.=not determined.
H[d]
H
4-Me
4-MeO
4-Ph
4-F
4-F
4-Cl
96:4
90:10
63:37
55:45
n.d.
89:11
96:4
90:10
93:7
90:10
92:8
9
10
11
12
13
14
15
16
17
18
4-Cl
4-Br
4-CO2Me
5-MeO
5-MeO
5-F
5-CF3
6-F
1i
1j
1k
1l
1m
2i
2j
2k
2l
2m
4,6-Me2
96:4
[a] Conditions for reaction carried out at 1408C : 1 (0.2 mmol), [{Pd(h3-
cinnamyl)Cl}2] (0.01 mmol), NHC-HI (0.02 mmol), cesium pivalate
(0.2 mmol), cesium carbonate (0.3 mmol), dry xylenes; conditions for
reactions carried out at 1608C: 1 (0.2 mmol), [{Pd(h3-cinnamyl)Cl}2]
(0.005 mmol), NHC-HI (0.01 mmol), cesium pivalate (0.2 mmol),
cesium carbonate (0.3 mmol), dry mesitylene. [b] Yield of product
isolated after flash column chromatography. [c] Enantiomeric ratios were
determined by HPLC on a chiral stationary phase. The left column refers
to the enantiomer shown. [d] [{Pd(h3-cinnamyl)Cl}2] (2.5 mol%), NHC-
HI (5 mol%). [e] 0.1 mmol scale.
entry 8) is tolerated, but, not surprisingly, chloro and bromo
groups are not (Table 2, entries 9 and 11). Before extending
this study further, we reduced the catalyst loading and
reaction time to 5% and 3 h, respectively, and increased the
reaction temperature to 1608C (mesitylene as solvent).
Gratifyingly, the yields and asymmetric induction remained
high (Table 2, entries 3–6).
Substrates with donor substituents at C4 gave improved
product yields (Table 2, entries 4–6). In contrast, electron-
withdrawing groups at C4 decreased both product yields and
enantioselectivities (Table 2, entries 7–12). A methoxy group
at C5 reduced the product yield (Table 2, entries 13 and 14).
Fluoro and trifluoromethyl groups are reasonably well
tolerated (Table 2, entries 15 and 16). Finally, entries 17 and
18 show that the reaction can be performed with 6-substituted
carbamates.
Figure 1. Part of the X-ray structure of (S,S)-3 showing the Pd–NHC
ligand arrangement. In pink, the modeled modification of the ligand in
which a naphthyl fragment is used to extend the aryl plane.
induction (Table 2, entry 1). The results closely match those
obtained with the pre-assembled NHC–Pd catalyst (Table 1,
entry 6). The yield dropped when catalyst loading was
reduced (entry 2).
The size of the cycloalkyl ring is crucially important in this
À
Extension to substituted aniline precursors showed that
under these reaction conditions a fluoro group (Table 2,
system. Thus, C H activation of the N-cyclopentyl carbamate
derivative did not proceed at all (Table 3, entry 2). By
Angew. Chem. Int. Ed. 2011, 50, 7438 –7441
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7439

Communications
Table 3: Asymmetric synthesis of fused indolines 2 from carbamates 1.
Entry[a]
Substr.
n
t [h]
Product (yield %)[b]
e.r.[c]
1
2
1a
1n
1o
1o
2
1
3
3
3
24
3
2a (78)
2n (0)
2o (94)
2o (88)
95:5
–
97.5:2.5
97.5:2.5
3
4[d]
3
[a] Reaction conditions:
1
(0.2 mmol), [{Pd(h3-cinnamyl)Cl}2]
(0.005 mol%), NHC-HI (0.01 mol%), cesium pivalate (0.2 mmol),
cesium carbonate (0.3 mmol), mesitylene. [b] Yield of isolated product.
[c] Enantiomeric ratios were determined by HPLC on a chiral stationary
phase. The left column refers to the enantiomer shown. [d] 1 mmol scale.
contrast, N-cycloheptyl-substituted substrates gave better
yields and an even higher asymmetric induction than the N-
cyclohexyl (Table 3, entries 3 and 4).
The absolute configuration of the fused indolines was
determined by an X-ray structural analysis of 10, which was
obtained by hydrolysis of carbamate 2p. All other indolines
were assigned configurations by analogy. As 2p was not
accessible directly, it was formed from 2o by reaction with
tetra-n-butylammonium tribromide (Scheme 1). The X-ray
structure shows 10 to have the 5aS,10aR configuration
(Figure 2).[14]
Scheme 2. Proposed catalytic cycle for the synthesis of fused indolines
2.
cesium pivalate. Nucleophilic addition of pivalate to the
cinnamyl ligand followed by alkene dissociation generates the
Pd0–NHC catalyst.[15] Oxidative addition of carbamate 1 and
À
bromide/pivalate exchange is followed by C H bond activa-
We propose the reaction mechanism shown in Scheme 2.
The palladium dimer, [{Pd(h3-cinnamyl)Cl}2, is cleaved by the
NHC ligand. The latter is generated in situ from NHC-HI and
tion and reductive elimination to produce the fused indoline 2
and regenerate the catalyst. An excellent mechanistic analysis
À
of the C H bond-activation step has been reported by
Rousseaux et al.[2f]
In summary, chiral NHC ligands derived from chiral 2,2-
dimethyl-1-arylpropane-1-amines were successfully applied
to the synthesis of highly enantioenriched trans-fused indo-
À
lines by palladium-catalyzed C H activation. The reaction
requires temperatures of 140–1608C. It is extraordinary that
despite the high temperature, this transformation occurs
À
through high asymmetric recognition of an enantiotopic C H
bond in an unactivated methylene unit.
Scheme 1. Bromination of fused indoline 2o.
Experimental Section
Representative procedure for the asymmetric synthesis of indoline 2:
[{Pd(h3-cinnamyl)Cl}2] (2.5 mol%), cesium pivalate (0.2 mmol), and
NHC-HI (5 mol%) were introduced to a Schlenk tube. After
evacuation of the tube and backfilling with nitrogen, anhydrous
mesitylene (2 mL) was added and the mixture was stirred for 30 min.
Methyl carbamate 1 (0.2 mmol) and cesium carbonate (0.3 mmol)
were added next and the reaction mixture was heated to 1608C and
stirred at this temperature for 3 h. The reaction mixture was cooled to
room temperature, diluted with dichloromethane (2 mL), and then
filtered through a pad of celite. Volatiles were removed in vacuo and
the crude product was purified by flash column chromatography
(eluent: ethyl acetate/pentane) to afford the fused indoline 2.
Received: April 16, 2011
Published online: July 1, 2011
Figure 2. X-ray structure of fused indoline (À)-(5aR,10aS)-10.[15]
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Angew. Chem. Int. Ed. 2011, 50, 7438 –7441

À
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Keywords: asymmetric catalysis · C H activation · indolines ·
N-heterocyclic carbenes · palladium
.
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Angew. Chem. Int. Ed. 2011, 50, 7438 –7441
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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