Palladium-catalyzed enantioselective α-arylation and α-vinylation of oxindoles facilitated by an axially chiral p-stereogenic ligand

Article abstract of DOI:10.1021/ja903880q

(Chemical Equation Presented) The enantioselective α-arylation and α-vinylation of oxindoles catalyzed by Pd and a biarylmonophosphine ligand with both axial and phosphorus-based chirogenicity is reported. The resultant quaternary carbon stereocenters are

Full text of DOI:10.1021/ja903880q

Published on Web 07/06/2009
Palladium-Catalyzed Enantioselective r-Arylation and r-Vinylation of
Oxindoles Facilitated by an Axially Chiral P-Stereogenic Ligand
Alexander M. Taylor, Ryan A. Altman,† and Stephen L. Buchwald*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received May 18, 2009; E-mail: sbuchwal@mit.edu
All-carbon quaternary centers are found in numerous biologically
active small molecules, and their efficient construction remains a
challenge in organic synthesis.1 In that context, methods for the
asymmetric R-arylation and R-vinylation of carbonyl enolates hold
particular promise because of their ability to form highly substituted
centers adjacent to a functional group that can be readily manipu-
lated. Recent reports have described the asymmetric R-arylation
of enolates derived from ketones2 and, in an intramolecular reaction,
aldehydes.3 Despite the progress in this field, however, the substrate
scope remains limited in intermolecular reactions; there has been
only one example of enantioselective R-arylation of lactones,4 for
example, and to the best of our knowledge, there have been no
reports to date of general methods for the intermolecular enanti-
oselective R-arylation or R-vinylation of amide, ester, and other
non-ketone carbonyl enolates.5
our results, we hypothesized that a ligand similar to KenPhos but
with an additional asymmetric element would lead to a more
enantioselective coupling process. Indeed, we observed that 1
facilitated the coupling under the same conditions in 76% yield
and 97% ee. Ligand 1 was reported by our group several years
ago as the first example of an axially chiral, P-stereogenic ligand,11
and although it was examined in several cross-coupling reactions
at that time, including R-arylation and R-vinylation reactions, it
was not superior to simpler ligands and has not been reported in
an application since that time.
Table 1. Enantioselective R-Arylation and R-Vinylation of
1,3-Dimethyloxindolea
Recently, we reported conditions that allow for the selective N-
or C-arylation of oxindoles based on the application of either Cu
or Pd catalysts.6 The oxindole core and its derivatives are found in
many natural products and other biologically active compounds,7
and methods for their asymmetric formation and transformation
are of considerable interest.8,9 The Pd-catalyzed conditions we
described were capable of forming quaternary centers from
3-substituted oxindoles in racemic fashion, and we set out to explore
an asymmetric variant of that method. Herein we describe the highly
enantioselective Pd-catalyzed intermolecular coupling of oxindoles
and aryl and vinyl bromides facilitated by a biaryl monophosphine
ligand that contains two sources of asymmetry.
Figure 1. Pd-catalyzed enantioselective R-arylation of 1,3-dimethyloxindole
with an axially chiral, P-chirogenic ligand.
Following an initial survey of ligands and optimization of Pd
sources, we found that KenPhos and (iPr)2MOP promoted the
coupling of 1,3-dimethyloxindole with 3-bromoanisole in the
a Reaction conditions: oxindole (0.65 mmol, 1.3 equiv), aryl/vinyl
bromide (0.5 mmol, 1 equiv), TMEDA·PdMe2 (4 mol %), 1 (4 mol %),
NaOtBu (1.0 mmol, 2 equiv) in cyclohexane (1 mL) at the temperature
shown. Each result shown is the average of two runs in which all of the
starting material was consumed. b Yield of isolated material. c Determined
by chiral HPLC. d Using a reaction time of 18 h. e Vinyl bromide was used
as a 4.5:1 trans/cis mixture. f Determined following reduction with H2 and
Pd/C.
10
presence of TMEDA ·PdMe2 and NaOtBu in good yield with
promising levels of enantiomeric excess (Figure 1, with the
conditions described in Table 1). Interestingly, of all the ligands
screened, only biaryl monosphosphines were found to promote the
reaction in appreciable yield or enantioselectivity. On the basis of
Having identified 1 as the optimal ligand, we explored the
substrate scope with regard to the aryl bromide coupling partner.
† Current address: Department of Chemistry, University of California, Irvine,
1102 Natural Sciences II, Irvine, CA 92697-2025.
9
9900 J. AM. CHEM. SOC. 2009, 131, 9900–9901
10.1021/ja903880q CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Both electron-rich and electron-deficient aryl bromides reacted in
good yield with high enantioselectivity (Table 1, entries 1 and 2),
as did 2-bromonaphthalene (Table 1, entry 3). Substituted aryl
bromides, including those with 3-chloro, 2-dioxolanyl, and alkyl
substituents, also were transformed to the corresponding products
with high selectivity (Table 1, entries 4-6). In general, aryl halides
with substituents positioned meta or para to the bromine reacted
effectively, whereas reactions of those with ortho substituents led
to low yields; this trend is often seen in intermolecular asymmetric
enolate arylation.2a,12
Vinyl bromides were also efficient coupling partners under these
conditions. For instance, we found that application of a cis/trans
mixture of ꢀ-bromostyrene formed a separable mixture of the
corresponding cis- and trans-styrenyl oxindole products under the
reaction conditions, although the cis isomer was formed with
significantly higher enantioselectivity (Table 1, entry 7). Similarly,
cis-1-bromo-1-propene gave product more enantioselectively than
trans-1-bromo-1-propene (Table 1, entries 8 and 9). Use of
2-bromopropene formed the corresponding isopropenyl oxindole
in good yield and high enantiomeric excess (Table 1, entry 10).
Single-crystal X-ray diffraction of the enantiomer of that product
(ent-4, formed with the enantiomer of ligand 1) was used to
determine the absolute stereochemistry of that compound and, by
inference, those of all of the products of this reaction.
only asymmetric example of such a coupling with a heterocyclic
aryl halide of which we are aware.
As shown in Scheme 1, vinyl oxindole 4 was readily converted
into either the related saturated compound 5 or the 3-acetyl
derivative 6 by reduction or ozonolysis, respectively. Access to
enantiomerically enriched compounds of this type would be difficult
using conventional methodology. 3-Aryl oxindole 2 was converted
into the corresponding indoline 7 with LiAlH4, and 3 was employed
in a Pd-catalyzed C-N cross-coupling reaction to give 8.13 All of
these reactions took place with no loss of optical activity.
In conclusion, we have developed conditions for the Pd-catalyzed
enantioselective R-arylation and R-vinylation of oxindoles using a
ligand with both axial and phosphorus-based chirogenicity.
Acknowledgment. We thank the National Institutes of Health
(NIH) (GM46059) for funding this project. R.A.A. acknowledges
an NIH Predoctoral Fellowship (F31GM081905). We thank Amgen,
Boehringer-Ingelheim, Merck, Nippon Chemical, and BASF (Pd
compounds) for additional support. We thank Dr. Patrick Bazinet
for obtaining the crystal structure of ent-4.
Supporting Information Available: Experimental procedures,
characterization data for all new compounds, spectral data, and
crystallographic data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
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Figure 2. Products of reactions to form other substituted oxindoles.
Reactions were run at 50 °C using the same conditions as shown in Table
1. Results shown are averages of two runs. The first value below each
structure is the yield of isolated material, and the second is the enantiomeric
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To examine the generality of these reaction conditions, we
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oxindole backbone (Figure 2). R-Arylation proceeded in good yield
and excellent enantioselectivity with an N-aryl oxindole. A 3-ben-
zyl-containing substrate was well-tolerated, as was one with a
5-methoxy group, a motif commonly found in bioactive oxindole-
based compounds, although this substrate reacted with lower
enantioselectivity. Also, R-arylation with 3-bromothiophene formed
the corresponding product in high enantiomeric excess; this is the
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Scheme 1. Derivatization of R-Aryl and R-Vinyl Oxindole Products
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