Sequential Au(i)/chiral tertiary amine catalysis: A tandem C-H functionalization of anisoles or a thiophene/asymmetric Michael addition sequence to quaternary oxindoles

Article abstract of DOI:10.1039/c5cc10096h

We report an unprecedented sequential Au(i)/bifunctional tertiary amine catalysis, which enables a tandem C-H functionalization of weak nucleophiles (anisoles or thiophenes) and asymmetric Michael addition for the highly enantioselective synthesis of quaternary oxindoles from diazooxindoles and nitroenynes.

Full text of DOI:10.1039/c5cc10096h

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Sequential Au(I)/chiral tertiary amine catalysis:
a tandem C–H functionalization of anisoles or
a thiophene/asymmetric Michael addition
sequence to quaternary oxindoles†
Cite this:DOI: 10.1039/c5cc10096h
Received 8th December 2015,
Accepted 23rd December 2015
Zhong-Yan Cao,‡a Yu-Lei Zhao‡a and Jian Zhou*ab
DOI: 10.1039/c5cc10096h
www.rsc.org/chemcomm
We report an unprecedented sequential Au(I)/bifunctional tertiary
amine catalysis, which enables a tandem C–H functionalization of
weak nucleophiles (anisoles or thiophenes) and asymmetric Michael
addition for the highly enantioselective synthesis of quaternary
oxindoles from diazooxindoles and nitroenynes.
Asymmetric sequential metal/organo catalysis has emerged as a
promising strategy to build up molecular complexity from
simple materials.1 It nicely takes advantage of metal-catalyzed
reactions to generate suitable functionalities for the subsequent
asymmetric organocatalytic reactions, which brings into full
play the potential of two distinct catalysts to enlarge the reaction
types for the development of new tandem sequences.2 While a
number of elegant protocols have been developed by this
strategy, it is still highly desirable to exploit new catalyst
combinations to enable novel one-pot tandem reactions for the
efficient creation of all-carbon quaternary carbon stereocenters
from easily available substrates in an operationally friendly way, a
very challenging task in organic synthesis.3
Scheme 1 Aromatic C–H functionalization based tandem reactions.
In this context, the coupling of a metal-catalyzed carbenoid
reaction4 with organocatalytic asymmetric reactions is a fruitful
strategy to elaborate diazo reagents to create fully substituted Hu et al. exploited several elegant asymmetric multicomponent
carbon stereocenters,5 since the pioneering work of Hu & Gong reactions7 to create chiral quaternary carbons based on C–H
in merging Rh catalysis with chiral phosphoric acid catalysis to functionalization of indoles, pyrroles or N,N-dialkylanilines (A).
develop a tandem O–H insertion/Mannich sequence.6 When Gong et al. instead pioneered the combination of Ru or Rh
starting from C–H functionalization of arenes, it allows facile catalysis with chiral bifunctional base catalysis, allowing facile
construction of all-carbon quaternary stereocenters (Scheme 1). access to quaternary 3-indolyloxindoles from diazooxindole 1
By merging Rh or Pd-catalysis with phosphoric acid catalysis, and indole (B).8 Despite progress, weak nucleophiles such as
anisoles and thiophenes have not been coupled in such tandem
reactions, possibly because they are less active than pyrroles
and N,N-dialkylanilines, by at least five orders of magnitude
according to Mayr’s nucleophilicity scale.9 In light of this, it is
important to identify new catalyst combinations to develop the
corresponding reactions based on C–H functionalization of less
active aromatics. Here, we wish to report a sequential gold(I)/
chiral tertiary amine catalysis for the highly enantioselective
synthesis of quaternary oxindoles 5 and 6 from diazooxindoles
1, anisoles 2 or thiophene 3 and nitroenynes 4.
a Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, 3663N,
Zhongshan Road, Shanghai 200062, China. E-mail: jzhou@chem.ecnu.edu.cn
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
† Electronic supplementary information (ESI) available: Experimental procedures and
characterization data of new compounds. CCDC 1024590 and 1024591. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc10096h
‡ These authors contributed equally to this work.
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Table 1 Condition optimization
Entry Solvent T (1C) Additive Time (d) Yielda (%) Eeb (%) Drc
1
2
3
4
5
CH2Cl2 À10
—
—
—
1.0
1.0
3.0
95
90
89
94
93
97
98
98
98
99
2 : 1
4 : 1
5 : 1
6 : 1
7 : 1
Et2O
Et2O
Et2O
Et2O
À10
À40
À40
À40
4 Å MS 3.0
5 Å MS 3.0
Scheme 2 Conditions for C–H functionalization.
a
c
Isolated yield. b Determined by chiral phase HPLC analysis. Determined
by 1H NMR analysis of the crude reaction mixture.
Recently, with our interest in oxindole chemistry,10 we tried using
diazooxindoles 1 to develop asymmetric reactions, a research in
its infancy,11 and developed a highly stereoselective olefin cyclo- based bifunctional phosphoramide C120 was suitable for Michael
propanation of 1 catalyzed by Au(I).11a During this study, we reaction of oxindole 7b to nitroenyne 4a (for details, see the ESI†).
noticed that in the presence of Au(I), C–H insertion of anisole However, the merging of the two distinct catalytic reactions into
occurred. Investigation revealed that under the catalysis of only one-pot operation was not trivial as it first appeared. After careful
1.0 mol% Ph3PAuOTf, the C–H functionalization of anisole 2a optimization, it turned out that solvent CH2Cl2 must be removed
using diazooxindole 1a could complete within 2 h at À10 1C, after the initial step finished, as the use of Et2O as solvent was
and that of 3,4-dimethylthiophene 3 finished within 0.5 h at important to obtain higher diastereoselectivity in the Michael
room temperature, which represented the first Au-catalyzed C–H addition (entry 1 vs. 2, Table 1). Further lowering the temperature
bond insertion of thiophene using diazo reagents (Scheme 2).12 In to À40 1C, with the addition of 5 Å MS, the dr value increased to
contrast, Rh, Pd and Ru catalysts that Hu and Gong used all failed 7 : 1 (entries 3–5). Finally, a simple procedure for the one-pot tandem
in both reactions, although Ag(I) catalyzed both reactions slowly, so sequence was established. After the initial C–H functionalization
did Cu(I) in the reaction of 3 (for details, see the ESI†). The step finished, CH2Cl2 was removed under vacuum, followed by the
efficiency of this low catalyst loading method not only showed successive addition of Et2O, MS 5 Å, 10 mol% C1 and nitroolefin.
the potential of Au catalysis in C–H functionalization of less active Then the reaction was run at indicated temperature till completion.
aromatics,13 but also encouraged us to consider an unexplored
By this procedure, we evaluated the substrate scope with
sequential Au(I)/chiral tertiary amine catalysis for tandem synthesis respect to differently substituted aromatics and nitroenynes, as
of quaternary oxindoles from diazooxindoles.14 First, chiral tertiary shown in Table 2. Very impressively, under gold(I) catalysis,
amines were powerful in deprotonative activation of 3-aryloxindoles anisole and its derivatives even with a bromo or an iodo group
for asymmetric syntheses of quaternary oxindoles, a privileged worked well with diazooxindoles 1 to produce 3-aryloxindoles 7
scaffold in natural products and drugs.15 Second, while Dixon for the next Michael addition catalyzed by phosphoramide C1
reported the incompatibility of the two catalysts in their pioneering to furnish quaternary oxindoles 5 in good to excellent yield and
work of sequential chiral tertiary amine/Au(I) catalysis,16 we good dr value, with 99% ee in all the cases. However, while
speculated that the presence of 1.0 mol% Au(I) might have no Michael adduct 6 derived from 3,4-dimethylthiophene was also
severe detrimental effect on the potency of a chiral tertiary obtained in 82% yield and 99% ee, the diastereoselectivity was
amine that was typically used in 10 mol%, based on our work in poor. By replacing Ph3PAuOTf with (2,4-tBu2C6H3O)3PAuSbF6,12c
multicatalyst promoted tandem reactions,17 which was later acetanilide derived quaternary oxindole 9 was also prepared via
confirmed by control experiments.18
this sequence in good yield, dr and 93% ee. On the other hand,
With this hypothesis, we first tried combining a Michael the alkyne substituent of nitroenynes could be varied from an
reaction using nitroenynes14c with the C–H functionalization for alkyl, aryl to trimethylsilyl (TMS) group. The relative and absolute
sequential synthesis of quaternary oxindoles, as the conjugate configuration of product 5h was determined by X-ray analysis. In
addition of 3-substituted oxindole to nitroenynes has not been addition to nitroenynes, ordinary nitrostyrenes also worked well in
attempted, although this synthetic valuable reaction had been this tandem sequence, as shown by the synthesis of compound 10
intensively studied.19 A merit of using nitroenynes was to install in 84% yield, 6 : 1 dr and 97% ee.
an alkyne moiety as a versatile synthetic handle for further
elaboration of the Michael adducts.
The synthetic value of the resulting adducts 5 was exempli-
fied by the facile cyclization of 5g to give spirocyclic oxindole 11
Condition screenings suggested CH2Cl2 as the best solvent in the presence of 1 mol% of Ph3PAuOTf, without erosion of the
for the Au-catalyzed C–H functionalization, and our cinchonidine ee value. Our method constituted a new method for the
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Table 2 Tandem sequence to quaternary oxindoles
Table 3 Sequential catalysis for 3-heteroatom oxindoles
electrophiles would lead to this subunit. This was exemplified
by a tandem C–H insertion/amination sequence and an O–H
insertion/Michael sequence to aminooxindole 13 and 3-alkoxy-
oxindole 14, bearing a C3 tetrasubstituted carbon stereocenter,
respectively (Table 3). In addition, it was possible to synthesize
oxindole derivatives with two C3 heteroatoms, as shown by
highly enantioselective synthesis of oxindole O,N-ketal 15 via a
tandem O–H insertion/amination sequence.
For details, ESI.
synthesis of spirocyclic oxindoles,15 a type of privileged scaffold
in bioactive compounds.
It is worth mentioning that the insertion/amination sequence
was a novel type of tandem sequence starting from diazo reagents
(entries 1 and 3). By choosing a suitable chiral tertiary amine
catalyst, it was possible to obtain the desired amination products21
in high to excellent enantioselectivity. This result further
demonstrates the flexibility of sequential Au(I)/chiral tertiary
amine catalysis in elaborating diazo reagents to value added
compounds featuring a tetrasubstituted carbon stereocenter.
In conclusion, we have developed an asymmetric sequential
gold(I)/chiral tertiary amine catalysis as a flexible strategy for
the one-pot enantioselective synthesis of 3,3-disubstituted oxindoles
from diazooxindoles. Importantly, this research further highlights
the potency of cationic Au(I) catalysis in C–H functionalization of
weak nucleophiles such as anisoles and thiophenes, which plays a
crucial role in developing these tandem reactions. The use of this
new catalyst combination to develop other types of asymmetric
tandem reactions is now in progress in our laboratory.
In addition, an Au(I)-catalyzed hydration could be coupled to
form a one-pot triple asymmetric sequence for the facile synthesis of
quaternary oxindole 12 with a ketone moiety in excellent dr and ee
values. The enrichment of the dr value was due to the fact that the
minor diastereomer of 5a was difficult to be hydrated.
We thank the 973 program (2015CB856600) and NSFC
(21222204, 21472049) for financial support.
This sequential Au(I)/chiral tertiary amine catalysis constituted a
flexible strategy to prepare 3,3-disubstituted oxindoles with C3
heteroatom substitution as well, a prominent structural motif
in drugs and bioactive compounds. Basically, the reaction of
3-aryloxindoles with a heteroatom based electrophile, or the
reaction of 3-heteroatom substituted oxindoles with carbon
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