Pd-Catalyzed α-Selective C-H Functionalization of Olefins: En Route to 4-Imino-β-Lactams

Article abstract of DOI:10.1021/jacs.5b13353

Pd-catalyzed α-olefinic C-H activation of simple α,β-unsaturated olefins has been developed. 4-imino-β-lactam derivatives were readily synthesized via activation of α-olefinic C-H bonds with excellent cis stereoselectivity. A wide range of heterocycles at the β-position are compatible with this reaction. The product of 4-imino-β-lactam derivatives can be readily converted to 2-aminoquinoline which exists extensively in pharmaceutical drugs and natural products.

Full text of DOI:10.1021/jacs.5b13353

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Communication
Pd-Catalyzed #-Selective C–H Functionalization
of Olefins: en Route to 4-Imino- #-Lactams
Wei-Jun Kong, Yue-Jin Liu, Hui Xu, Yan-Qiao Chen, Hui-Xiong Dai, and Jin-Quan Yu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b13353 • Publication Date (Web): 08 Feb 2016
Downloaded from http://pubs.acs.org on February 8, 2016
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Pd-Catalyzed -Selective C–H Functionalization of Olefins: en Route to 4-Imino--
Lactams
Wei-Jun Kong,† Yue-Jin Liu,† Hui Xu,† Yan-Qiao Chen,‡ Hui-Xiong Dai,*,† Jin-Quan Yu*,†,‡
†State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, China; ‡Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines
Road, La Jolla, California 92037, USA
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Abstract: Pd-catalyzed -olefinic C−H activation of simple ,-
unsaturated olefins has been developed. 4-imino--lactam
derivatives were readily synthesized via activation of -olefinic
C−H bonds with excellent cis stereoselectivity. A wide range of
heterocycles at the -position are compatible with this reaction.
The product of 4-imino--lactam derivatives can be readily
converted to 2-aminoquinoline which exists extensively in
pharmaceutical drugs and natural products.
Driven by this idea, we initiated our study by using N-methoxy
cinnamamide 1a as the substrate, and we found that -C−H
activation occurred selectively in the presence of 5 mol%
Pd2(dba)3, 2 equiv of t-BuNC in dioxane to give two products 3a
and 3a’ in 16% and 53% yields respectively (Table 1, entry 2).
The structures of the four-membered ring 4-imino--lactam 3a
and 3a’ are confirmed by X-ray crystallography (Table 1). For the
major product 3a’, the acyl group is cis to the benzene ring in
contrast to the trans relationship in substrate 1a.
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Directed C(sp2)-H functionalization of various aromatic
systems has emerged as a promising tool for arene synthesis.1
Similarly, -olefinic C−H activation involving the same five-
membered cyclometalation intermediates have been extensively
exploited (eq 1).2-5 To the best of our knowledge, directed -C−H
activation of simple olefins has not been reported except for non-
directed functionalization of electron-rich -olefinic C−H bonds
(eq 2).6 In addition, construction of β-lactams through olefinic
C−H functionalization has not been achieved despite recent
reports on β-lactam formation from C(sp3)–H functionalization.7
Herein, we report a directed -selective olefinic C−H activation to
construct 4-imino--lactam derivatives using air as the sole
oxidant (eq 3). The C−H activation step proceeds through a five-
membered cyclopalladation of an intermediate formed from the
Pd(II)-catalyzed reaction of amide substrate and t-BuNC. The
tolerance of a wide range of heterocycles at the -positions
renders this reaction valuable for medicinal chemistry.
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Table 1. Optimization of Reaction Conditions a, b
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In directed C–H activation reactions, the hetero atoms in
heterocyclic substrates could coordinate with metal catalysts and
lead to catalyst poisoning or direct C–H activation at undesired
positions thus limiting its application in heterocycle containing
substrates. Recently, we have developed a strategy to tackle this
problem by using N-methoxy amide group as the directing group,
air as sole oxidant and low valent Pd(0) to initiate the reaction to
avoid the undesired influence from the heterocycles (eq 4).8 Given
our success in aromatic systems, we wondered whether -C−H
activation of ,-unsaturated olefins could take place to give five-
membered imino--lactams.
While both the regioselectivity and the stereochemistry of the
products are intriguing, we initially focused on improving the
yield of this reaction. In the course of reaction optimization, we
found that temperature has significant impact on the yield and the
product ratio of Z/E (Table 1, entries 1-4). When the temperature
was increased to 100 oC, yield of 3a’ was improved to 63% with
only trace 3a. However, further increasing the temperature to 120
oC decomposed the substrate (Table 1, entry 4). Scaling up the
reaction to 0.2 mmol afforded 3a’ in 58% yield (Table 1, entry 5).
Since we observed some decomposition of the starting material,
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we attempted several modifications of the conditions and found
corresponding 5-imino--lactams products (Table 4). A variety of
that decreasing the catalyst loading to 2 mol% improved the yield
to 72% (Table 1, entry 6). However, the use of 1 mol% Pd2(dba)3
resulted in low conversion (Table 1, entry 7).
substituents on the aryl ring are well tolerated (5a-5d). The cyclic
olefinic substrates also give good yields (5e and 5f). Moderate
yield is obtained with acyclic dialkyl substituted substrate (5g).
Table 2. -Olefinic C–H Activation to Construct 4-Imino-β-
Table 3. -Olefinic C–H Activation with Heterocyclesa,b
lactamsa,b
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Table 4. -Olefinic C–H Activation to Construct 5-Imino--
Lactamsa,b
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With optimized conditions in hand, we subsequently explored
the scope with regard to substitutions on the aryl ring (Table 2).
Substitution at the o-, m-, and p- position with a range of electron
withdrawing and donating groups are tolerated (3a’-3i). For
electron withdrawing substituted substrates such as trifloromethyl,
chloro and bromo, 5 mol % Pd2(dba)3 and prolonged time are
needed. For these N-OMe cinnamic amides, C–H activation all
occur at the - position of the acyl group. Importantly, various
3,3-disubstituted acrylamides are also compatible with this
reaction. ,-Dialkyl substituted 1k and 1l give 3k and 3l in 55%
and 36% yield, respectively. However, mono-alkyl substituted
substrate (E)-N-methoxyhex-2-enamide gave 17% yield only. For
mono- and di- phenyl substituted acrylamides 1j and 1m, the
corresponding 4-imino-β-lactams are obtained with good yields
up to 86%.
Next, we examined the compatibility of this reaction with
substrates containing common heterocycles at the-positions
(Table 3). Pyridine, furan and thiophene substituted acrylamides
give the corresponding 4-imino--lactams in yields from 48% to
71% (3n-3t). Meanwhile, indole substituted acrylamide gives two
separable products in total yield of 67% (3u and 3u’, 44% and
23%, respectively).
Based on these results and our previous work,8 a probable
mechanism is proposed (Scheme 2). First, Pd(0) is converted to
Pd(II)X2 (X = PhCH=CHCONOMe) in the presence of oxygen
and 1a, followed by 1,1-insertion of t-BuNC into the Pd-N bond
and acyl migration to form intermediate B. Subsequently, -
olefinic C−H is activated to form the five-membered
cyclopalladated complex C. Reductive elimination of C produces
the initial product 3a. 3a is isomerized to give the more stable
product 3a’. This isomerization of 3a to 3a’ is verified by
subjecting 3a to typical reaction conditions giving 3a’ in 94%
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When the substrates without -C−H are employed, C−H
activation can also take place at the -position to give the
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yield (see Supporting Information). Furthermore, reaction of the
procedure in the synthesis of 2-aminoquinoline-based
pharmaceuticals and natural products is evident from the literature
reports9. The reaction was found to be compatible with electron
withdrawing and donating groups on the aryl ring, and the desired
quinoline derivatives are formed in moderate to excellent yields
(6b and 6c). For -methyl or phenyl substituted substrates, this
transformation occurs at room temperature to afford the products
in good yields (6d and 6e). In some cases, the t-butyl group is
deprotected in one-pot (6f-h). While detailed mechanism of this
reaction remains unclear, one possible pathway involves Lewis
acid-induced ring open and nitrene insertion into C−H bond (see
Supporting Information).10
Z-isomer 1a’ under the same conditions gives the same product
3a’ being consistent with the proposed mechanism (see
Supporting Information).
Scheme 2. Proposed Mechanism
In conclusion, we have developed a directed regioselective -
olefinic C–H activation to construct 4-imino--lactam derivatives
using air as the sole oxidant. This operationally simple
transformation
features
excellent
regioselectivity
and
compatibility with heterocycles. Further transformations of the
products also indicates potential synthetic utility.
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Acknowledgement. We gratefully acknowledge Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, the
CAS/SAFEA International Partnership Program for Creative
Research Teams, NSFC-21121062, NSFC-21472211 and The
Recruitment Program of Global Experts for financial support. We
gratefully acknowledge The Scripps Research Institute, for
financial support. This work was supported by NSF under the CCI
Center for Selective C−H Functionalization, CHE-1205646, and
Novartis for an unrestricted grant.
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Table 5. Lewis Acid Promoted Rearrangementa,b
Supporting Information Available: Experimental procedures
and spectral data for all new compounds (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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