Palladium-Catalyzed Direct Intramolecular C-N Bond Formation: Access to Multisubstituted Dihydropyrroles

Article abstract of DOI:10.1021/acs.orglett.7b00072

A palladium-catalyzed intramolecular amination of alkenes with retention of olefin functionalization was achieved under mild reaction conditions. In the presence of palladium catalyst, the tosyl-protected amine can directly couple with a double bond to provide versatile dihydropyrrole derivatives in moderate to excellent yields.

Full text of DOI:10.1021/acs.orglett.7b00072

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed Direct Intramolecular C−N Bond Formation:
Access to Multisubstituted Dihydropyrroles
Bing Jiang,† Fei-Fan Meng,† Qiu-Ju Liang,† Yun-He Xu,*,† and Teck-Peng Loh*,†,‡
†Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
‡Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371
S
* Supporting Information
ABSTRACT: A palladium-catalyzed intramolecular amination of
alkenes with retention of olefin functionalization was achieved
under mild reaction conditions. In the presence of palladium catalyst,
the tosyl-protected amine can directly couple with a double bond to
provide versatile dihydropyrrole derivatives in moderate to excellent
yields.
Scheme 1. Oxidative Amination Reactions of Alkenes
evelopment of economical and practical synthetic methods
Dfor the construction of a carbon−nitrogen (C−N) bond is
an important goal in organic synthesis because nitrogen-
containing compounds are featured prominently in many
pharmaceuticals and functional materials.1,2 Therefore, many
methodshavebeendevelopedfortheconstructionofaC−Nbond
especially in the area of palladium-catalyzed olefin amination.3,4
Thesemethods usuallyinvolvetheuseofvarious nitrogensources
to couple with electrophilic or nucleophilic carbon compounds.
Amongthesemethods,transition-metal-catalyzeddirectoxidative
amination is the most efficient due to its atom economy in nature
and the easy accessibility of the substrates (Scheme 1). It is
important to note that pioneering works on oxidative amination
(sp2) of o-allylaniline and olefinic tosamides to form indoles and
non-aromatic nitrogen heterocycles were elegantly demonstrated
byHegedus.5 Another example of palladium-catalyzed cyclization
of vinylanilines to indoles was reported by Stille and co-workers.6
screening of different solvents, the results suggested that 1,4-
dioxane was the most efficient solvent for this transformation,
affording the desired product 2a in 58% yield (Table 1, entry 6).
Among the palladium catalysts screened in this reaction,
Pd(CH3CN)2Cl2 was found to be the most efficient when the
reaction was conducted at 80 °C (Table 1, entry 13). Therefore,
the optimal reaction conditions can be summarized as follows:0.5
mmol substrate in 1,4-dioxane (2 mL) with Pd(CH3CN)2Cl2
catalyst(10mol%)andchloranil(1equiv)at80°Cfor24hunder
air atmosphere.
Under the optimized reaction conditions, we next explored the
substratescopeofthisreaction.Initially,westudiedtheeffectofthe
substituents on the double bond. A substrate with an electron-
withdrawing group such as CF3− on the phenyl ring could afford
the desired product in 75% yield (2b, Chart 1).
Thereafter, Tamaru,7 Larock,8 Stahl,9 Chemler,10 Muniz,11
̃
Obora,12 Mizuno,13 and Jimen
́
ez14 et al.15 independently
developed oxidative amination reactions. Simultaneously, Beller
and colleagues also reported a Rh-catalyzed intermolecular
oxidative amination of secondary amines with styrenes.16
Nonetheless, these methods are mainly limited to aniline with
styrene derivatives and/or intermolecular amination processes.
We envisage that an intramolecular amination of a double bond
with simple amines will provide easy access to versatile dihydro-
nitrogen-containing compounds. In this paper, we focus on the
palladium-catalyzedintramoleculardirectaminationthatprovides
easy access to multisubstituted dihydropyrroles.17
We initiated our study by investigating the Pd-catalyzed
intramolecular sp2 5-endo cyclization of 1a in THF at 50 °C
under air atmosphere. It was found that the desired product could
be obtained in 8% yield in the presence of BQ as oxidant (Table 1,
entry 1) after being stirred for 24 h. Encouraged by this result, we
screened a variety of oxidants (Table 1, entries 1−5), and it was
found that the product yield could be improved to 28% when
chloranil was used as the oxidant (Table 1, entry 3). After further
Unfortunately,thepresenceofanelectron-donatingsubstituent
such as Me− on the phenyl ring reduced the product’s yield
significantly(2c,Chart1).Similarresultswerealsoobservedinthe
formation of products 2d and 2e (which were generated via
Received: January 9, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00072
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Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions for Palladium-Catalyzed 5-endo-trig Cyclization of 1a
b
entry
catalyst (10 mol %)
oxidant (equiv)
BQ (2.0)
solvent
THF
temp (°C)
yield (%)
1
2
Pd(CH3CN)2Cl2
Pd(CH3CN)2Cl2
Pd(CH3CN)2Cl2
Pd(CH3CN)2Cl2
Pd(CH3CN)2Cl2
Pd(CH3CN)2Cl2
Pd(CH3CN)2Cl2
Pd(CH3CN)2Cl2
Pd(CH3CN)2Cl2
PdCl2
50
50
50
50
50
50
50
50
50
50
50
80
80
8
22
28
17
20
58
57
48
53
38
75
69
82
p-toluquinone (2.0)
chloranil (2.0)
tBuOOtBu (2.0)
dicumyl peroxide (2.0)
chloranil (2.0)
chloranil (2.0)
chloranil (2.0)
chloranil (1.0)
chloranil (1.0)
chloranil (1.0)
chloranil (1.0)
chloranil (1.0)
THF
3
THF
4
THF
5
THF
6
1,4-dioxane
toluene
7
8
CH2CI2
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
9
10
11
12
13
Pd(PhCN)2Cl2
Pd(PhCN)2Cl2
Pd(CH3CN)2Cl2
a
Reaction conditions: To a mixture of 0.5 mmol 1a, catalyst (10 mol %), and oxidant was added solvent (2 mL), and the mixture was allowed to stir
b
for 24 h under air atmosphere. Yield was determined by 1H NMR using mesitylene as internal standard. Chloranil is tetrachloro-1,4-benzoquinone.
Chart1.InvestigationofSubstrateswithDifferentSubstituents
on the Double Bond
substituted phenyl ring. It was also observed that a wide scope of
functional substituents onthe phenylring could bewell-tolerated,
except for the cyano and hydroxyl groups, which afforded the
desired products only in moderate yields, possibly due to their
coordinative effect with the palladium catalyst. It should be noted
that the reaction with the 2,4,6-trimethylphenyl ring proceeded
smoothly to give the desired product in 88% yield. However, the
electron-rich substrate 1n with a 3,4,5-trimethoxylphenyl ring
only afforded the desired product 2s in 37% yield. Another
product, 2s′, formed via intramolecular oxidative coupling and
aromatization processes, was isolated in 43% yield. Next, the
substrates bearing other aryl groups such as naphthyl and furyl
groups were tested in this cyclization reaction, and both provided
the corresponding products in high yields. When the styryl-
substituted substrate (1v, Chart 2) was subjected to the reaction,
only a moderate yield was observed. Furthermore, all reactions
proceededsmoothlytogivethedesiredproductsinexcellentyields
when R2 was changed toalkyl groups. However, the steric effectof
R2 fortheproduct’syieldwassignificantbecauseonly41%yieldof
2y could be obtained using the substrate with a sterically
demanding tert-butyl group. Finally, it was found that the
substituents at the R3 position were less influential on the
product’s yield. Under the optimized reaction conditions, the 5-
endo-trig cyclization reaction can be scaled up to 5 mmol without
compromising the product yield (2g, Chart 2).
a b
,
a
The reaction was performed under the following reaction conditions:
1 (0.5 mmol), Pd(CH3CN)2Cl2 (10 mol %), and chloranil (1.0 equiv)
b
in 2 mL of dioxane at 80 °C under air atmosphere for 24 h. Isolated
To explore the possible mechanistic pathway of this intra-
molecularoxidativeaminationreaction,thefollowingexperiments
(Scheme 2, eq 1−4) were carried out. When the reaction was
conducted without any oxidant, only about 7% of the desired
c
d
e
yield. Reaction was run for 12 h. Reaction was run for 48 h. Starting
material is N-(2-(3,4-dihydronaphthalen-1-yl)-1-phenylethyl)-4-meth-
f
ylbenzenesulfonamide. 5 mol % of Pd(CH3CN)2Cl2 was used.
1
product was detected according to crude H NMR spectrum.
cyclization and aromatization processes). No reaction took place
when the substituent R1 was replaced by a long-chain alkyl group
(2f,Chart1).WhenR1 waschangedtodifferentestergroups,itwas
also found that all of the desired products could be obtained in
excellent yields, except the tert-butyl ester substituted 2i, possibly
due to its steric hindrance. These results implied that electron-
withdrawing functional groups at the R1 position favor the 5-endo-
trig cyclization.
However, reaction without palladium catalyst or those with other
Lewis acids (10 mol %) instead of palladium, gave no desired
product. Therefore, the hypothesis which suggests the role of
palladium catalyst as mere Lewis acid could be omitted. Next,
isotopically labeled experiments were conducted to gain some
preliminary understandings of this coupling process. Intermo-
lecular competition experiments between 1g and its dideuterated
analogue 1g-d2 exhibited a kinetic isotopic effect value (KIE) of
1.2, which suggested that β-hydride elimination step was not the
turnover limiting step in this catalytic cycle.
Next,weturnedourattentiontoexpandthesubstratescopewith
the R1 group as ethyl ester. Expectedly, the desired products were
obtainedinmoderatetoexcellentyieldswhentheR2 groupwasthe
B
DOI: 10.1021/acs.orglett.7b00072
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Organic Letters
Letter
Chart 2. Scope of Different Substrates with an Ester Group on
the Double Bond
Scheme 3. Derivation of Compound 2g
ab
,
Scheme 4. Proposed Plausible Mechanism for the Palladium-
Catalyzed 5-endo-trig Cyclizaton
a
The reaction was run under the following reaction conditions: 1 (0.5
mmol), Pd(CH3CN)2Cl2 (10 mol %), and chloranil (1.0 equiv) in 2
b
mL of dioxane at 80 °C under air atmosphere for 24 h. Isolated yield.
c
Starting material is ethyl 2-methylene-4-(4-methylphenylsulfonami-
do)-4-(3,4,5-trimethoxyphenyl)butanoate.
Scheme 2. Control Experiments
sonication (Scheme3, eq1). Moreover, hydrogenated product 4g
could be afforded in high yield with excellent stereoselectivity
(Scheme 3, eq 2).
Basedonthepreviousreports,7b apossiblemechanisticpathway
is proposed (Scheme 4). First, the Pd(II) coordinates with a
double bond followed by nucleophilic attack on the coordinated
olefin to form the C−N and σ-Pd bonds. After β-hydride
elimination of intermediate B, the desired product is formed
together with the release of the palladium catalyst for the next
catalytic cycle.
In conclusion, we have developed a palladium-catalyzed
intramolecular oxidative amination of alkenes. In this work,
various tosyl-protected amines can be coupled with multi-
substituted vinylarene and acrylate fragments efficiently. The
corresponding products can be obtained in moderate to excellent
yieldsundersimplereactionconditions.Applicationofthecurrent
methodologyinorganicsynthesisofnaturalproductsisongoingin
our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00072.
Furthermore, derivatization reactions of compound 2g were
investigated. First, the tosyl protecting group could be easily
removed using magnesium turnings in methanol solution under
Experimental procedures and spectral data for all new
compounds (1H NMR, 13C NMR, HRMS) (PDF)
C
DOI: 10.1021/acs.orglett.7b00072
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
́
689.(o)Iglesias,A.;Per
8109. (p) Genet, J. P.; Balabane, M. Tetrahedron Lett. 1983, 24, 2745.
(q) Backvall, J. E.; Bjorkman, E. E.; Bystrom, S. E.; Solladie-Cavallo, A.
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ez,E.G.;Muniz,K.Angew.Chem.,Int.Ed.2010,49,
̃
AUTHOR INFORMATION
Corresponding Authors
■
́
̈
̈
̈
*E-mail: xyh0709@ustc.edu.cn.
*E-mail: teckpeng@ntu.edu.sg.
ORCID
Tetrahedron Lett. 1982, 23, 943. (r) Backvall, J.-E.; Bystrom, S. J. Org.
Chem. 1982, 47, 1126. (s) Backvall, J. E.; Nordberg, R. E.; Nystrom, J. E.;
Hogberg, T.; Ulff, B. J. Org. Chem. 1981, 46, 3479. (t) Backvall, J.-E.;
Nystrom,J.E.J.Chem.Soc.,Chem.Commun.1981,0,59.(u)Backvall,J.E.;
̈
̈
̈
̈
̈
̈
Yun-He Xu: 0000-0001-8817-0626
Teck-Peng Loh: 0000-0002-2936-337X
Notes
Bjorkman, E. E. J. Org. Chem. 1980, 45, 2893.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the funding support of the National
NaturalScienceFoundationofChina(21372210, 21672198), the
State Key Program of National Natural Science Foundation of
China (21432009), and the Collaborative Innovation Center of
Chemistry for Energy Materials (2011-iChEM) for financial
support. We appreciate Miss Jun Kee Cheng (Nanyang
Technological University, Singapore) for her help on the
manuscript.
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