Copper(I)-catalyzed aryl or vinyl addition to electron-deficient alkenes cascaded by cationic cyclization

Article abstract of DOI:10.1021/ol500033w

An exoselective copper-catalyzed arylation- and vinylation-carbocyclization of electron-deficient alkenes was developed to provide rapid and efficient access to a variety of functionalized 3,3-disubstituted oxindoles. With this method, a highly efficient and concise formal synthesis of (±)- physostigmine and (±)-physovenine has been completed.

Full text of DOI:10.1021/ol500033w

Letter
pubs.acs.org/OrgLett
Copper(I)-Catalyzed Aryl or Vinyl Addition to Electron-Deficient
Alkenes Cascaded by Cationic Cyclization
Bing Zhou,* Wei Hou, Yaxi Yang, Huijin Feng, and Yuanchao Li*
Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu Chong Zhi Road, Shanghai 201203, PR China
S
* Supporting Information
ABSTRACT: An exoselective copper-catalyzed arylation− and vinylation−carbocyclization of electron-deficient alkenes was
developed to provide rapid and efficient access to a variety of functionalized 3,3-disubstituted oxindoles. With this method, a
highly efficient and concise formal synthesis of ( )-physostigmine and ( )-physovenine has been completed.
elective conjugated addition of aryl and alkenyl groups to
electron-deficient alkenes was conventionally achievable
report the first example of copper-catalyzed arylation− and
vinylation−carbocyclization of electron-deficient alkenes with
diaryliodonium and vinyl(aryl)iodonium salts. The reaction
yields a class of highly functionalized 3,3′-dicarbonsubstituted
oxindoles,9,10 which feature motifs found in a number of
bioactive natural products and pharmaceutical molecules.11
Our synthetic attempts began with reaction of 1a and
diphenyliodonium salt (2a) under different catalysts (Table S1,
Supporting Information). Only a trace amount of anticipated
product 3a (for the structure, see Scheme 1) was detected using
CuCl, CuTc, or CuOTf as catalyst. To our delight, when
phenylmesityliodonium salt (2b) was used as the aryl donor
and CuCl as the catalyst, 3a was isolated in 30% yield.
Subsequently, various Cu- and Fe-based catalysts were
screened. CuCl gave a relatively higher yield than FeCl2 and
other Cu(I) salts such as CuOAc, CuOTf, and CuTc. We also
examined the effect of bases because previous studies showed
that an addition of 2,6-di-tert-butylpyridine was crucial for Cu-
catalyzed aryl transfer.3−6 Surprisingly, the desired product 3a
was not detected when 2,6-di-tert-butylpyridine (DTBP) or
pyridine was added. Interestingly, a dramatic increase of
reaction yield (57%) was observed when NaHCO3 was
employed . In order to further improve the yield, a series of
solvents were screened, and DCE was found to be optimal
(77% yield). No product 3a was observed in the absence of
CuCl or in the presence of Cu(II) salts such as Cu(OAc)2 and
CuCl2, indicating the importance of Cu(I) species for successful
aryl transfer.
S
through nucleophilic addition of aryl or alkenyl anions such as
aryl cuprates and other organometallic reagents.1 These
reactions are the foundations and very important for organic
synthesis. However, these approaches require the use of
stoichiometric organometallic reagents and often suffer from
harsh conditions and low functional group tolerance. Catalytic
addition to alkenes using aryl radicals has certain advantages,
but using alkenyl radicals is very rare.2 Therefore, the further
development of catalytic methods for introduction of aryl and
vinyl groups to alkenes is highly desirable and of prime
synthetic value.
A combination of Cu catalyst and diaryliodonium salts3 has
recently emerged as a very useful and efficient strategy for
arylation via a high oxidation state Cu(III)−aryl intermediate
which behaves as an activated aromatic electrophile.4 Despite
great advances, arylation of alkenes remain relatively unex-
plored.5−7 In 2012, Gaunt and co-workers reported the first
example of Cu(I)-catalyzed arylation of simple alkenes with
diaryliodonium salts.5 When cyclic electron-rich enamides were
employed as substrates, the aryl groups of diaryliodonium salts
were transferred to C-3 position of enamides.6 Very recently, a
CuCl-catalyzed arylation of allylic amide followed by trapping
carbocation with oxygen atom of amide carbonyl group was
reported to furnish 5,6-dihydro-4H-1,3-oxazines.7
Despite the significance of the aforementioned works, the
scope of the reported alkene substrates is limited to electron-
rich or simple alkenes, and there is no report on arylation and
vinylation of electron-deficient alkenes, thus proving to be a
formidable challenge. In continuation of our interest in
transition-metal-catalyzed C−H functionalization,8 we herein
Received: January 6, 2014
Published: February 19, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
a
a
Scheme 1. Substrate Scope
Scheme 2. Substrate Scope of Aryliodonium Salts
a
Isolated yields. See the Supporting Information for details.
were readily transferred, affording the corresponding vinyl-
substituted oxindoles in good yields (74% of 3w and 60% of
3x). To the best of our knowledge, this is the first example of
Cu-catalyzed 1,2-vinylarylation of an electron-deficient al-
kene.13
To gain additional mechanistic insights, we carried out
several control experiments. First, we performed reactions of 1a
and 2b in the presence of radical scavengers (2 equiv) under
optimal reaction conditions (eq 1 and 2). In the presence of
a
Isolated yields. See the Supporting Information for details.
With the optimized reaction conditions in hand, the substrate
scope of phenylacrylamide was subsequently explored, and the
results are summarized in Scheme 1. Generally, phenyl-
acrylamides tethering electron-donating groups on the phenyl
ring were successfully converted to the desired products (3b−
d) in good yield (66−88%). The phenylacrylamides tethering
an electron-withdrawing CF3 group on the phenyl ring
provided 3e in relatively low yield (58%). Notably, the halo
substitutions (F, Cl, Br, and I) on N-phenylacrylamides were
well tolerated, providing the corresponding oxindoles in 50−
64% yield (3f−i). The substrate bearing a meta methyl group
on phenylacrylamide gave a mixture of two regioisomers in 67%
yield with poor selectivity (3j:3j′ = 1.8:1). The phenyl-
acrylamide with methyl groups at the ortho position produced a
relatively low yield of 3k (42%). To our delight, a novel tricyclic
oxindole derivative 3l was successfully obtained in 45% yield
from the corresponding tetrahydroquinoline substrate. When
the N-methyl substituent of phenylacrylamide was replaced by a
phenyl group, the reaction yield of 3m was increased to 93%.
The N-benzyl derivative was also compatible with the reaction
conditions, affording the corresponding product 3n in 55%
yield. To our delight, when the R3 group was CH2OMe or
phenyl, the corresponding acrylamides were also compatible
with the optimal conditions, affording 3o and 3p in moderate
yield.
The substrate scope of diaryliodonium salts was also
examined (Scheme 2). Diaryliodonium salts tethering a methyl
group at the para and meta position of the phenyl ring were
tolerable, affording 3q and 3r in 66% and 73% yield,
respectively. However, diaryliodonium salt bearing a methyl
group at ortho position of the phenyl ring produced 3s in low
yield (34%), suggesting the steric hindrance of the aryl group
attenuates the reactivity of its transfer. Halogen (3t and 3u) and
electron-withdrawing (3v) substituents were well tolerated,
emphasizing the generality of this method. Furthemore, we
tested whether alkenyl transfer to phenylacrylamide 1a was
possible using vinylmesityliodonium triflates as reagents.12 To
our great delight, styrenyl- and cyclohexyl-substituted vinyl
radical scavenger 2,6-di-tert-butyl-4-methylphenol (BHT), the
product 3a was isolated in 65% yield. On the contrary, we did
not isolate any 3a in the presence of 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO) (2 equiv). We surmised that the
detrimental effect of TEMPO could result from its interaction
with Cu(I), leading to Cu(II)14 which has proven ineffective for
this reaction. In addition, reaction of phenylacrylamides 4 and 5
with 2b under the standard reaction conditions provided no
products of 6−9 but recovery of 4 and 5 (eqs 3 and 4). If an
electrophilic Cu(III)−phenyl species was involved, the
monosubstituted olefin of 4 and the phenol ester of 5 make
both substrates less nucleophilic than 1a, which could account
for the observed lower reactivity. In addition, when phenyl-
acrylamide 1y was used as the substrate, we isolated 3y in 75%
yield with a second arylation on the phenyl ring of 1y (eq 5).
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Letter
The Cu(III)−phenyl species undergoes a electrophilic addition
to the very electron-rich phenyl ring of 1y followed by
reductive elimination and tautomerization, which could account
for the formation of 3y rather than a radical pathway.
In view of our data presented above, we tentatively proposed
a cationic reaction mechanism as shown in Scheme 3. Initially,
Ozonolysis of 3z followed by reductive amination in the
presence of methylamine and LiAlH4 furnished ( )-esermet-
hole 10 in 86% yield over two steps. Additional two-step
modification17 of 10 yielded ( )-physostigmine in 63% yield.
On the other hand, ozonolysis of 3z followed by direct
reduction with LiAlH4 at room temperature gave tricyclic amine
11 in 88% yield over two steps. ( )-Physovenine has been
synthesized from 11 in two steps.17 Thus, Cu(I)-catalyzed
vinylation−carbocyclization enables the formal total synthesis
of ( )-physostigmine and ( )-physovenine in a very efficiant
manner.17
Scheme 3. Proposed Reaction Mechanism
In conclusion, we have developed a novel copper(I) chloride
catalyzed 1,2-diarylation or vinylarylation of electron-deficient
alkenes. This transformation tolerates a wide range of diaryl-
and vinyl(aryl)iodonium triflates and versatile substitutions on
phenylacrylamides, providing efficient access to a variety of
functionalized 3,3-disubstituted oxindoles. Using the vinyl
addition−cyclization method, highly efficient and concise
formal syntheses of ( )-physostigmine and ( )-physovenine
have been accomplished in six steps in 41% and 36% overall
yield, respectively. Further applications of the method in the
syntheses of complex bioactive natural products and analogues
are currently in progress and will be reported in due course.
an oxidative insertion of Cu(I) into the diaryliodonium salt
gives an electrophilic Cu(III)−aryl intermediate which under-
goes an electrophilic addition to afford intermediate A. Finally,
rearomatization and reductive elimination provide the desired
oxindole 3a and regenerate Cu(I) catalyst.
In order to demonstrate the strength of our methodology, it
was applied to accomplish concise formal total syntheses of
( )-physostigmine and ( )-physovenine (Scheme 4).
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization of products, and
1
copies of H and 13C NMR spectra. The material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
Scheme 4. Formal Total Synthesis of ( )-Physovenine and
( )-Physostigmine
a
*E-mail: zhoubing2012@hotmail.com.
*E-mail: ycli@mail.shcnc.ac.cn.
Notes
The authors declare no competing financial interest.
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