Eosin Y- and Copper-Catalyzed Dark Reaction to Construct Ene-γ-Lactams

Article abstract of DOI:10.1021/acs.orglett.8b03147

Eosin Y, a common organo-photocatalyst in visible-light photoredox processes, was found to show excellent catalytic activities for thermal redox reactions under a catalytic amount of Cu(OAc)₂. With this catalytic system, vinyl azides and ketene silyl acetals combine to form formal [3 + 2] cycloadducts by α-ester radical addition without light irradiation. This method provides a mild and straightforward paradigm to prepare important synthons of five-membered ene-γ-lactams and bridge ring lactams. It is the first example of an eosin Y-catalyzed redox reaction in the dark.

Full text of DOI:10.1021/acs.orglett.8b03147

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Eosin Y- and Copper-Catalyzed Dark Reaction To Construct Ene-γ-
Lactams
Wen-Long Lei,† Kai-Wen Feng,† Tao Wang,† Li-Zhu Wu,‡ and Qiang Liu*,†
†State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
‡Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry &
University of Chinese Academy of Sciences, the Chinese Academy of Sciences, Beijing 100190, P. R. China
S
* Supporting Information
ABSTRACT: Eosin Y, a common organo-photocatalyst in
visible-light photoredox processes, was found to show
excellent catalytic activities for thermal redox reactions
under a catalytic amount of Cu(OAc)2. With this catalytic
system, vinyl azides and ketene silyl acetals combine to form
formal [3 + 2] cycloadducts by α-ester radical addition
without light irradiation. This method provides a mild and
straightforward paradigm to prepare important synthons of five-membered ene-γ-lactams and bridge ring lactams. It is the first
example of an eosin Y-catalyzed redox reaction in the dark.
ne-γ-lactams are significant synthons as they are widely
partners might be activated simultaneously by a redox catalytic
Eused in synthesis and key structural elements in a wide cycle to afford the desired [3 + 2] cycloadducts. In order to
realize this idea, a simple but novel eosin Y/Cu(OAc)2
catalytic system was found to drive the cyclization without
light irradiation. Eosin Y is widely employed as a cheap, readily
accessible, and green photocatalyst in visible-light photoredox
catalysis, however its catalytic potential in thermal reaction is
still unexploited.
Our initial effort was focused on visible-light photoredox
catalysis because both vinyl azides and KSAs should be easily
activated by an organic dye-mediated photoredox cycle.
However, accompanied by the products of the desired ene-γ-
lactams, 2H-azirines were always obtained via an energy
transfer process (Scheme 1c).9c,11 After realizing that the
process is inevitable under light irridiation, we turned to
devising an organic dye-redox system that could catalyze the
formal [3 + 2] cyclization in the dark. Due to a large part of
xanthenes dyes in oxidative state having the potential to
oxidize KSAs,12 we anticipated that the combination of a
xanthene with a proper cocatalyst would drive the dye based
redox cycle, leading to the desired ene-γ-lactams without the
formation of 2H-azirines.
To validate this idea, our initial studies focused on the [3 +
2] cyclization of α-phenyl vinyl azide 1a with KSA 2a at room
temperature. After extensive screening (see the Supporting
Information), we were delighted to observe that the reaction of
1a with 2a in CH3CN solution containing catalytic eosin Y (3
mol %) and Cu(OAc)2 (5 mol %) provided the respective ene-
γ-lactam 3a in 96% yield. Control experiments established that
both eosin Y and Cu(OAc)2 were necessary for an efficient
reaction. Meanwhile, when a series of xanthenes dyes and
range of bioactive natural products and medicinally relevant
compounds.1 They could be transformed into chiral lactams
that are ubiquitous pharmacophore motifs by asymmetric
hydrogenation.2 As a highly versatile intermediate, the
electron-rich enamine scaffolds of ene-γ-lactams could
participate in hetero-Diels−Alder reaction3 or [3 + 2]
cyclization with α, β-unsaturated carbonyl compounds,
providing tetrahydropyranopyrrolones or enantioenriched
bicyclic γ-lactams.4 They could also react with electrophilic
aldehydes to afford the fused bicycles.4a In addition, ene-γ-
lactams could be activated by appropriate Brønsted or Lewis
acid via in situ formed N-acyliminium ions; further reaction of
the iminium motifs with proximal nucleophiles produces
various valuable nitrogen-bearing polycycles.5
Accordingly, several synthetic strategies for accessing ene-γ-
lactams have been developed. In 1960s, Wasserman and
Moon6 reported the synthesis of ene-γ-lactams through an
oxidative rearrangement of poly-substituted pyrroles. It is
known that ene-γ-lactams can be obtained from anhydrides in
four steps. Specifically, Vassilikogiannakis and co-workers3−5
explored an “all-in-one” reaction sequence to produce ene-γ-
lactams from the reaction of singlet oxygen with furans
followed by treatment with Me2S and primary amines. In these
transformations, most of the approaches require multistep
reactions to prepare the precursors or access the products.2a,7
Therefore, a concise, efficient and straightforward trans-
formation from readily available materials to ene-γ-lactams
under mild reaction conditions is highly desirable.
In conjunction with our efforts aimed at extending the utility
of α-carbonyl radical chemistry,8 we describe an intramolecular
redox strategy to prepare ene-γ-lactams from vinyl azides9 and
ketene silyl acetals (KSAs).10 We propose that both reaction
Received: October 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03147
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Scheme 1. Method of Constructed Ene-γ-lactams
carboxylate ligands was screened, the product 3a was obtained
in unsatisfactory yields, which indicated eosin Y plays an
irreplaceable role in the catalytic system.
With optimized conditions in hand, we evaluated the scope
of the vinyl azide component, utilizing 2a as the common KSA
partner. As shown in Figure 1, various substituted α-aryl vinyl
azides worked well to deliver the corresponding annulated
products in good to excellent yields. Among all the examples
presented, it was found that the electronic property slightly
affected the product yields. α-Aryl vinyl azides bearing an
electron-donating substituent on the phenyl motifs afforded
lower yields than those with electron withdrawing groups in
most cases. The absolute configuration of 3a was unambigu-
ously confirmed by single-crystal X-ray diffraction analysis.
Many versatile functional groups, such as methoxyl (3b),
halides (3c, 3d), alkyl (3e), formyl (3f), phenyl (3g),
trifluoromethyl (3h), vinyl (3i), esters (3j) or others, were
all suitable in this transformation, highlighting the potential of
the method in organic synthesis. Substitution at the ortho and
meta positions of the phenyl ring was successfully incorporated
as well (3k−3m, 65%−94%). Subsequently, the transformation
proceeded well with various polyaromatic and heteroaryl-
substituted vinyl azides to deliver the corresponding γ-ene-
lactams, such as naphthyl (3n), quinolyl (3p), pyridyl (3q),
pyrimidyl (3r), and thienyl (3s). Moreover, 4,5-disubstituted
γ-ene-lactams can be also obtained from α-phenyl β-
ethoxycarbonyl vinyl azide in good yield (3t, 62%). It is
worth noting that certain vinyl azides are beyond the reach of
the present catalytic systems such as 1u−1x.
We next turned our attention to the capacity of KSAs to
participate in [3 + 2] cycloaddition to synthesize γ-ene-lactams
(Table 1). We found that the alkoxyl group (2b−2c) of KSA
had less affect on the reactivity and the yield of the product
(85%−87%). The β,β-asymmetrically disubstituted KSA also
proceeded smoothly under the standard conditions (3u, 79%).
To further explore the advantage of the dye-catalyzed [3 + 2]
cycloaddition, the exocyclic KSAs were evaluated under the
standard conditions. Delightfully, the reactions proceeded well
and delivered spirocyclic lactams 2-azaspiro[4.5]dec-3-en-1-
ones in good yields (3v−3w, 72%−75%). In addition, the
exocyclic KSAs 2g can be utilized as an effective substrate for
Figure 1. Scope of vinyl azides. (a) Conditions: 1a (0.1 mmol, 1.0
equiv), 2a (0.2 mmol, 2.0 equiv), eosin Y (3 mol %), Cu(OAc)2 (5
mol %) in 1.0 mL of dry CH3CN under an argon atmosphere for 12 h
at room temperature. (b) Isolated yields. (c) Reaction heated to 60
°C. (d) For 36 h. (e) For 48 h.
this cyclization, producing nonplanar and hard-earned bicyclic
lactams 2-oxa-7-azabicyclo [3.2.1] octan-6-ones in good yield
(3x−3y, 69%−77%).13 However, the monosubstituted KSA
did not provide the desired ene-γ-lactams.
Ene-γ-lactams are versatile synthetic intermediates and could
be further converted to various compounds (Scheme 2).4−6
For example, the asymmetric hydrogenation of 3a successfully
delivered chiral γ-lactam 1014 in excellent yield and
enantioselectivity.2a Moreover, the reduction of 3a using
lithium aluminum hydride provided 1-pyrroline 7 in 72%
yield. Furthermore, the thiocyanation of 3a with potassium
thiocyanate and iron tribromide was carried out to afford β-
thiocyanted ene-γ-lactam 9 in excellent yield.15
Finally, by using a polypyridyl iridium(III) catalyst (Ir[dF-
(CF3)ppy]2(dtbbpy)PF6), the ene moiety of γ-lactam 3a easily
underwent visible-light driven intramolecular [2 + 2] cyclo-
B
DOI: 10.1021/acs.orglett.8b03147
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a
addition or crossed [2 + 2] cycloaddition with N-phenyl-
maleimide to form cyclobutanes 6 or 8 in high yields.16
Although a detailed mechanism of the eosin Y-catalyzed
cyclization is beyond the scope of this paper, Scheme 3
Table 1. Scope of Ketene Silyl Acetals
Scheme 3. Proposed Pathway of Ene-γ-Lactams
outlines a mechanistic picture on the basis of the above results
and further control experiments (see below). Thus, the initial
reaction between eosin Y and the Cu(OAc)2 would afford the
eosin Y radical cation,3b which would be capable of oxidizing
the KSAs via singlet electron transfer (SET) to α-ester radicals.
Addition of α-ester radicals to vinyl azides would produce the
iminyl radical with dinitrogen.9a The resulting iminyl radicals
then should be reduced by low-valent Cu(I), thus affording the
desired iminyl anions and regenerating Cu(II).9b Finally,
intramolecular nucleophilic substitution of the iminyl anion
moiety to ester followed by isomerization delivers the final
products.
We inferred the existence of an α-ester radical intermediate
because of the evidence provided by the radical trapping
experiment. The reaction can be fully suppressed when 2.0
equiv of TEMPO was added and the mixture was studied by
HRMS. The trapped peak was detected at 258.2062 in
accordance with calculation (see Scheme 4). We also
a
Conditions: 1a (0.1 mmol, 1.0 equiv), 2 (0.2 mmol, 2.0 equiv),
Scheme 4. TEMPO Trapping and Control Experiment
eosin-Y (3 mol %), Cu(OAc)2 (5 mol %) in 1.0 mL of dry CH3CN
b
under an argon atmosphere for 12 h at room temperature. Isolated
c
d
yields. 2 (0.3 mmol, 3.0 equiv). 48 h.
a
Scheme 2. Synthetic Applications of Compound 3a
conducted a spin trapping experiment using 5,5-dimethyl-
pyrroline N-oxide (DMPO) and observed the formation of
persistent nitroxide radical adduct with ESR spectroscopy (see
the Supporting Information). However, 2H-azirine, an
important intermediate of vinyl azide chemistry, could not
produce ene-γ-lactams 3a in standard conditions.
It is worth noting that equivalent Cu(OAc)2 could not solely
drive the [3 + 2] cycloaddition at room temperature. The
oxidation potential of KSAs is around 0.90 V (vs SCE),10a
which is much higher than the reduction potential of
Cu(OAc)2 (≈0.32 V vs Fc+/Fc),17 suggesting that the electron
transfer from Cu(OAc)2 to KSAs is thermodynamically
unfavorable. Therefore, the existence of eosin Y is crucial for
the cyclization.
Although the attempt to detect the short-lived eosin Y
radical cation18 in the mixture of eosin Y and Cu(OAc)2 failed
at this moment, the formation of Cu(I) was verified by X-ray
photoelectron spectroscopic (XPS) measurements. Upon
analysis of the mixture of Cu(OAc)2 and eosin Y, XPS
a
Conditions: (a) (1 mol %) Ir[dF(CF3)ppy]2(dtbbpy)PF6; (2.0
equiv) N-phenylmaleimide in anhydrous CH3CN irradiation with
LEDs at 455 nm; (b) (3.0 equiv) LiAlH4 in THF 70 °C; (c)
Ir[dF(CF3)ppy]2(dtbbpy)PF6 (1 mol %) in anhydrous CH3CN
irradiation with LEDs at 455 nm; (d) (0.5 equiv) FeBr3, (2.0 equiv)
KSCN in CH3CN 80°C; (e) ref 2a.
C
DOI: 10.1021/acs.orglett.8b03147
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Letter
detected the peak characteristic of Cu(I) at 932.5 eV (see
Scheme 5).19 We found that the maximum absorption peak of
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: liuqiang@lzu.edu.cn.
ORCID
Scheme 5. X-ray Photoelectron Spectrum
Li-Zhu Wu: 0000-0002-5561-9922
Qiang Liu: 0000-0001-8845-1874
Funding
We are grateful for financial support from the NSFC (No.
21572090 and 21871123) and the Fundamental Research
Funds for the Central Universities (lzujbky-2017-k05).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Nan Zheng (Department of Chemistry and
Biochemistry, University of Arkansas, United States) and Prof.
Hao Chen (Department of Chemistry and Biochemistry, Ohio
University, United States) for helpful discussions in exper-
imental mechanism.
eosin Y was increased and slightly blue-shifted with progressive
addition of Cu(OAc)2 and a new reduction peak at 0.88 V was
detected in acetonitrile solution containing Cu(OAc)2 and
eosin Y by cyclic voltammograms; this may contribute to the
chelation between copper(II) ions and the eosin Y (see the
Supporting Information). Meanwhile, the chelation has also
been detected by FTIR. The 1752.19 cm−1 resonance
(carbonyl group) of eosin Y disappeared with the addition of
Cu(OAc)2 (see the Supporting Information). Thus, it is
plausible that Cu(I) and the eosin Y radical cation could
simultaneously arise from the inner-sphere electron transfer of
the Cu(OAc)2−eosin Y complex.20
Xanthenes dyes have been widely used as organo-photo-
catalysts in visible-light driven synthetic transformations.12,21
In stark contrast, the redox abilities of these dyes in the ground
state were greatly ignored. The newly developed eosin Y−
Cu(OAc)2 system allows the [3 + 2] cyclization of vinyl azides
with KSAs and reveals real-world practical advantages
compared to the eosin Y based photoredox system, such as
the photo byproducts 2H-azirines are no longer produced.
With this system, the desired ene-γ-lactams were obtained
smoothly at room temperature. It is also the first report on
exploring xanthene dyes as the redox catalyst in thermal
reactions. Further expansions of dye-catalyzed thermal
reactions and full disclosure of the mechanism are now in
progress.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03147.
Detailed experimental procedures, characterization of
new compounds, UV−vis, IR, XPS, and NMR spectra,
and CVs (PDF)
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