Combining Ru-Catalyzed C-H Functionalization with Pd-Catalyzed Asymmetric Allylic Alkylation: Synthesis of 3-Allyl-3-aryl Oxindole Derivatives from Aryl α-Diazoamides

Article abstract of DOI:10.1021/acs.orglett.6b02423

Ruthenium-catalyzed C-H functionalization was successfully combined with palladium-catalyzed asymmetric allylic alkylation in one pot. The novel dual-metal-catalyzed reaction provides a variety of 3-allyl-3-aryl oxindoles from the corresponding α-diazoamides in up to 99% yield with up to 85% ee. The appropriate ligand choice is important to promote the sequential reaction, avoiding undesired metal interaction or ligand exchange.

Full text of DOI:10.1021/acs.orglett.6b02423

Letter
pubs.acs.org/OrgLett
Combining Ru-Catalyzed C−H Functionalization with Pd-Catalyzed
Asymmetric Allylic Alkylation: Synthesis of 3‑Allyl-3-aryl Oxindole
Derivatives from Aryl α‑Diazoamides
Kosuke Yamamoto, Zafar Qureshi, Jennifer Tsoung, Guillaume Pisella, and Mark Lautens*
Davenport Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada
M5S 3H6
S
* Supporting Information
ABSTRACT: Ruthenium-catalyzed C−H functionalization
was successfully combined with palladium-catalyzed asymmet-
ric allylic alkylation in one pot. The novel dual-metal-catalyzed
reaction provides a variety of 3-allyl-3-aryl oxindoles from the
corresponding α-diazoamides in up to 99% yield with up to
85% ee. The appropriate ligand choice is important to promote
the sequential reaction, avoiding undesired metal interaction or
ligand exchange.
Scheme 1. Synthetic Approaches for the Synthesis of Chiral
Oxindoles: (A) Rh-Catalyzed C−H Insertion Followed by
Addition to Activated Imines; (B) Ru-Catalyzed C−H
Functionalization Followed by Pd-Catalyzed Asymmetric
Allylic Alkylation
he development of synthetic methods to construct
enantioenriched molecules in an efficient manner
T
continues to be a goal for synthetic chemists.1 More recently,
significant effort has been made to develop novel one-pot
asymmetric multicatalyst reactions.2−5 The exploitation of
combinations of multimetal catalysts to achieve C−C and C−
X bond formation in a single operation can afford more
efficient processes by reducing the number of isolation and
purification steps and can also offer valuable reactivity and
selectivity. In this context, our group has demonstrated a
number of multimetal-catalyzed tandem reactions, including
asymmetric and racemic reactions.2h−k,6
Chiral oxindole derivatives are important skeletons found in
a wide variety of biologically active molecules and natural
products.7 To date, considerable effort has been devoted to the
catalytic construction of oxindoles bearing a chiral quaternary
carbon center using both transition metal catalysts8a−g and
organocatalysts.8h−k We envisioned that these targets could be
readily accessed in a novel enantioselective dual-metal-catalyzed
process by trapping the oxindole enolate intermediate from a
metal-catalyzed C−H functionalization with a chiral electro-
philic allylpalladium complex (Scheme 1). In 2012, Hu and co-
workers reported a related process whereby an enantioselective
synthesis of oxindoles and indoles was designed using a
synergistic dual-catalytic system consisting of Rh-catalyzed C−
H functionalization followed by 1,2-addition to a chiral
phosphoric acid-activated imine.9
Herein we report a dual-metal-catalyzed enantioselective
one-pot synthesis that combines ruthenium-catalyzed C−H
functionalization and palladium-catalyzed asymmetric allylic
alkylation (AAA) to afford chiral 3-allyl-3-aryl oxindoles. We
found that the appropriate palladium source and ligand were
crucial for both promoting the tandem reactions and providing
the products with high enantiopurity. To the best of our
knowledge, this is the first example of an enantioselective
tandem reaction involving a ruthenium carbenoid and an
allylpalladium complex to form two new C−C bonds with a
chiral quaternary carbon center.10,11
We began this study with an optimization of a transition-
metal-catalyzed C−H functionalization of 4-(trifluoromethyl)-
phenyl-substituted α-diazoamide 1a (Table 1). In 2012, Yu and
co-workers had demonstrated the use of [Ru(p-cymene)Cl2]2
in the synthesis of 3-alkylideneoxindoles from acyl α-
diazoamides.12 We found that [Ru(p-cymene)Cl2]2 could also
be employed in constructing 3-aryl oxindoles. With 2 mol %
ruthenium catalyst in toluene at −25 °C, 2a was isolated in 90%
yield (entry 1).6f Dirhodium(II) catalysts are among the most
powerful reagents for generating metal carbenoid species from
diazo compounds.13 Under the same conditions, Rh(II)
catalysts gave a complex product mixture and provided 2a in
poor yields (entries 2−4). When a solution of 1a was stirred at
Received: August 12, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02423
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the Pd catalyst led to unreacted 1a in 93% yield (entry 1). The
result indicated that the ruthenium catalyst might be
deactivated in the presence of free phosphine ligands (PPh3
dissociates in solution from Pd(PPh3)4). To minimize the
detrimental Ru−phosphine interaction, we premixed the Pd
phosphine prior to starting the reaction.
Table 1. Catalyst Optimization of C−H Functionalization of
a
Aryl α-Diazoamide 1a
Different palladium sources were employed in the tandem
reaction with ligand L1 developed by the Trost group (Table 2,
entries 2−4). We found that the use of Pd2dba3·CHCl3 with L1
restored the catalytic activity of the ruthenium step but afforded
2a in 80% yield along with 3a in 11% yield with poor
enantioselectivity (entry 4). On the basis of these results, we
selected Pd2dba3·CHCl3 as the optimal palladium source for
ligand screening. Ligand L2, the naphthyl variant of L1, did not
improve the yield of 3a (entry 5). The use of L3,14 bearing a
more sterically hindered dimethyltetrahydronaphthalene back-
bone, slightly improved the yield and enantioselectivity (entry
6). Among the various chiral ligands, ligand L4 developed by
Trost showed superior reactivity and enantioselectivity,
affording 3a in 97% yield with 82% ee (entry 7; see Table S1
for the full ligand screening).15 Although both Ru-catalyzed
allylic substitution reactions16 and Pd-catalyzed diazo decom-
position17 have been reported, we did not observe any of these
reactions under our conditions when one or more components
were removed (entries 8−10).
b
yield (%)
entry
cat.
2a
1a
1
2
3
4
5
[Ru(p-cymene)Cl2]2
Rh2(OAc)4
Rh2(TFA)4
Rh2((S)-PTV)4
−
91 (90)
0
8
26
0
32
50
1
0
96
a
Representative procedure: in a 2 dram vial, 1a (0.1 mmol) and
catalyst (2 mol %) were dissolved in toluene (1 mL) at −78 °C, and
b
the mixture was stirred at −25 °C for 24 h. Yields were determined
by 1H NMR spectroscopy. The yield in parentheses is an isolated yield.
−25 °C in the absence of catalyst, we observed unreacted 1a
and a negligible amount of 2a by H NMR analysis (entry 5).
1
Next, we investigated the combination of the Ru(II)-
catalyzed C−H functionalization and a Pd(0)-catalyzed AAA
reaction (Table 2). Initial reactions were performed with
[Ru(p-cymene)Cl2]2 (2 mol %), Pd(PPh3)4 (5 mol %), or a Pd
catalyst/ligand (5 mol % [Pd], [Pd]/L = 1:1.2) and allyl tert-
butyl carbonate (2 equiv) as an allyl source. Using Pd(PPh3)4 as
It is noteworthy that when isolated intermediate 2a was
subjected to the AAA reaction using L4 without the ruthenium
catalyst (eq 1, route B), we obtained 3a in 98% NMR yield with
Table 2. Optimization of Pd Catalyst and Ligand in the One-
a
Pot Dual-Metal Reaction
b
c
entry
[Pd]
ligand yields of 1a/2a/3a (%) ee of 3a (%)
1
2
3
4
5
6
7
8
9
Pd(PPh3)4
Pd(OAc)2
−
93/0/0
n.d.
L1
L1
L1
L2
L3
L4
−
66/22/2
2/92/3
n.d.
Pd(allyl)Cl2
Pd2dba3·CHCl3
Pd2dba3·CHCl3
Pd2dba3·CHCl3
Pd2dba3·CHCl3
−
n.d.
81% ee, indicating that the inherent enantioselectivity of L4 in
the AAA reaction was maintained regardless of the presence of
the ruthenium catalyst (route A vs route B). However, in the
case of L1, the yield and also the ee value decreased in the dual-
metal-catalyzed reaction (92% yield, 25% ee in route B vs 11%
yield, 5% ee in route A). Although neither [Ru(p-cymene)Cl2]2
nor [Ru(p-cymene)Cl2]2/L1 catalyzed the allylic alkylation
reaction under our conditions,18 we also observed a decrease in
ee when the AAA of 2a was conducted using Pd/L1 in the
presence of 2 mol % [Ru(p-cymene)Cl2]2. From the results
obtained with L4, we proposed that each catalyst participates in
one of two independent reactions via sequential catalysis.
With a successful Ru/Pd-catalyzed sequence toward chiral
oxindoles in hand, we examined the effect of the catalyst
loading and the amount of allyl tert-butyl carbonate (Table 3).
When the reaction was performed at −25 °C from the outset,
we obtained 3a without any loss of the yield and
enantioselectivity (entry 1). Halving the Ru or Pd catalyst
loading resulted in incomplete conversion after 24 h (entries 2
and 3). Decreasing the ratio of the ligand to palladium from 1.2
to 1.0 and halving the amount of the carbonate also resulted in
a decrease in yield (entries 4 and 5). Importantly, the catalyst/
ligand/allyl carbonate loadings did not affect the ee, and thus,
we chose the conditions in entry 1 for our studies.
1/80/11
0/82/2
5 (S)
n.d.
0/68/20
0/0/89 (92)
0/92/0
30 (S)
82 (S)
n.d.
−
L4
92/0/0
n.d.
d
10
Pd2dba3·CHCl3
L4
92/0/0
n.d.
a
Representative procedure: [Ru(p-cymene)Cl2]2 (2 mol %) and 1 (0.1
mmol) were dissolved in the premixed (20 min at rt) solution of
Pd2dba3·CHCl3/L in toluene (1.5 mL); allyl tert-butyl carbonate (2
equiv) was added at −78 °C, and the mixture was stirred at −25 °C for
b
24 h. Yields were determined by 1H NMR spectroscopy. The yield in
c
parentheses is an isolated yield. Determined by chiral HPLC analysis.
The absolute configuration was determined by comparison of the
optical rotation with that from the literature (ref 8b). n.d. = not
determined. The reaction was performed without [Ru(p-cymene)-
d
Cl2]2 at −25 °C.
B
DOI: 10.1021/acs.orglett.6b02423
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the desired oxindoles 3 in good to excellent yields with high
enantioselectivity (3a−d, 3i). A 3-cyano group afforded 3j with
lower ee, probably as a result of coordination to the Pd or Ru
metal center. Although the substrates bearing a 3- or 4-formyl
group did not participate in the AAA reaction under the
standard protocol, a one-pot telescoping protocol, namely, the
Ru-catalyzed C−H functionalization reaction at room temper-
ature followed by the Pd-catalyzed AAA reaction at −25 °C,
afforded the desired products in excellent yield with high
enantioselectivity (3e, 3k). Electron-neutral or electron-
donating groups led to incomplete reaction even with
prolonged reaction time (3f, 3g, 3l). The results indicate that
the acidity of the intermediate oxindole is crucial for the AAA
reaction. Substrates with lower pKa values facilitate enolization,
resulting in faster allylation. A substrate with an o-tolyl group
(R1 = 2-Me) was incompatible in this protocol, affording a 7%
yield of intermediate 2m and an 85% yield of 1m as measured
Table 3. Effect of the Catalyst and the Carbonate Loading on
the Dual-Metal-Catalyzed Reaction
a
b
yields (%)
c
entry
a
x
y
z
3a
2a
ee of 3a (%)
1
2
3
4
5
2
2
2
2
1
2
1
2
2
2
2.5
2.5
1.25
2.5
2.5
6
6
3
5
6
(97)
54
0
28
54
7
82
82
82
82
82
32
88
67
22
a
See Table 2 for the representative procedure. The reaction was
b
performed at −25 °C from the outset. Yields were determined by 1H
1
NMR spectroscopy. The yield in parentheses is an isolated yield.
by H NMR spectroscopy. The steric hindrance obstructs the
c
Determined by chiral HPLC analysis.
ruthenium step, and unreacted starting material was recovered.
In terms of R2, halogen atoms (3n−p, 3r, 3s) at the 5- or 6-
position were tolerated, which could be utilized for further
functionalization of the oxindole scaffold. The substituents on
the nitrogen atom were also examined (3t−y). A linear alkyl
group afforded 3x in 89% yield with 73% ee. More sterically
demanding substituents such as cyclohexyl (3w) or cyclopropyl
groups (3x) diminished the reactivity, affording the products in
moderate yields. An ester group was also tolerated, affording 3y
in excellent yield with moderate enantioselectivity. The
absolute configuration of 3h was unambiguously determined
to be S by single-crystal X-ray diffraction.20
In summary, we have developed a Ru/Pd-catalyzed
enantioselective dual-catalyst reaction that provides a series of
chiral 3-allyl-3-aryl oxindoles in good to excellent yields with
moderate to high enantioselectivity. The combination of the
appropriate palladium source and ligand enables the Ru(II)/
Pd(0)-catalyzed sequential reaction in a one-pot manner
without any loss of catalytic activity and enantioselectivity.
Further investigation of Ru(II)/Pd(0)-catalyzed reactions is
underway in our laboratory.
We next synthesized and applied a variety of electronically
disparate diazo compounds 1 in the Ru/Pd-catalyzed reaction
(Scheme 2). The starting α-diazoamides 1 were synthesized by
either the palladium-catalyzed arylation of the corresponding
acyl α-diazoamides19 or a condensation reaction of aniline
derivatives with carboxylic acid counterparts. An electron-
withdrawing group (R1) at the 4- or 3-position of the aryl ring
attached to the diazo-bearing carbon was tolerated, affording
Scheme 2. Substrate Scope of the C−H Functionalization/
a
Asymmetric Allylic Alkylation of Aryl α-Diazoamides
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02423.
Detailed experimental procedures and full compound
characterization data (PDF)
Crystallographic data for 3h (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: mlautens@chem.utoronto.ca.
Notes
The authors declare no competing financial interest.
a
See Table 2 for the representative procedure. The reaction was
performed at −25 °C from the outset. Isolated yields after column
ACKNOWLEDGMENTS
■
chromatography are reported. The ee values were determined by chiral
b
The authors thank the Natural Sciences and Engineering
Research Council (NSERC), the University of Toronto, and
Alphora Research Inc. for financial support. K.Y. acknowledges
support from a JSPS research fellowship. Z.Q. thanks the
Ontario Graduate Scholarship for funding. The authors
HPLC analysis. The reaction was performed using a telescoping
c
protocol. See the Supporting Information for details. The reaction
d
was performed for 48 h. 4 equiv of allyl tert-butyl carbonate was used.
e
f
The reaction was performed for 72 h. The reaction was performed
g
for 96 h. After a single recrystallization.
C
DOI: 10.1021/acs.orglett.6b02423
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Am. Chem. Soc. 2011, 133, 18566. For the combination of Grubbs’
first-generation catalyst with a Pd catalyst, see: (b) Grigg, R.;
Sridharan, V.; York, M. Tetrahedron Lett. 1998, 39, 4139. (c) Grigg,
R.; York, M. Tetrahedron Lett. 2000, 41, 7255. For Pd/Ru cooperative
catalysis, see: (d) Misumi, Y.; Ishii, Y.; Hidai, M. J. Mol. Catal. 1993,
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455. For a Ru(III)/Pd(II)-catalyzed reaction, see ref 2j.
acknowledge the Canadian Foundation for Innovation and the
Ontario Research Fund for the Centre for Spectroscopic
Investigation of Complex Organic Molecules and Polymers. We
thank Dr. Alan Lough (University of Toronto) for single-crystal
X-ray analysis of 3h.
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D
DOI: 10.1021/acs.orglett.6b02423
Org. Lett. XXXX, XXX, XXX−XXX