PIDA-Mediated Formal Olefinic C=C Bond Cleavage of α-Oxo-Ketene N,N-Acetals toward Substituted Oxazolines

Article abstract of DOI:10.1002/chem.201803466

The hypervalent iodine reagent PhI(OAc)₂ (PIDA) mediated the formal oxidative C=C bond cleavage and subsequent cyclization of internal olefins, that is, α-oxo-ketene N,N-acetals, which afforded substituted oxazolines. Isothiazoline derivatives were obtained from the reactions of α-thioxo-ketene N,N-acetals with PIDA under the same conditions. Hydrolysis of the resultant oxazoline derivatives led to highly functionalized oxazolones. A plausible mechanism was proposed based upon the formation of isothiazoline-type intermediates.

Full text of DOI:10.1002/chem.201803466

A Journal of
Accepted Article
Title: PIDA-Mediated Formal Olefinic C=C Bond Cleavage of α-Oxo
Ketene N,N-Acetals toward Substituted Oxazolines
Authors: Tenglong Guo, Fei Huang, Quanbin Jiang, and Zhengkun Yu
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To be cited as: Chem. Eur. J. 10.1002/chem.201803466
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10.1002/chem.201803466
Chemistry - A European Journal
PIDA-Mediated Formal Olefinic CC Bond Cleavage of -Oxo
Ketene N,N-Acetals toward Substituted Oxazolines
Tenglong Guo,[a] Fei Huang,[a] Quanbin Jiang,[a] and Zhengkun Yu*[a,b]
of regioselectivity or structural diversity of the products.
Abstract: Hypervalent iodine reagent PhI(OAc)2 (PIDA)
mediated formal oxidative CC bond cleavage and
subsequent cyclization of internal olefins, that is, α-oxo
ketene N,N-acetals, affording substituted oxazolines.
Isothiazoline derivatives were obtained from the
reactions of α-thioxo ketene N,N-acetals with PIDA
under the same conditions. Hydrolysis of the resultant
oxazoline derivatives led to highly functionalized
oxazolones. A plausible mechanism is proposed based
upon the formation of isothiazoline-type intermediates.
Owing to the easiness of preparation and low toxicity as
compared with transition-metal oxidants, hypervalent iodine
reagents have been extensively used in modern organic
synthesis.[13] Regarding iodo(III)-promoted oxidative transfor-
mations of amino-functionalized alkenes, Zhao, et al. develop-
ped a CC bond formation strategy for indole synthesis via
PIDA-mediated oxidization of N-aryl enamines (Scheme 1a).[14]
Recently, Loh and Jiang group[15] reported highly site-selective
CH acyloxylation of enamines with PIDA, giving solvent-
dependent α- and β-site-selective products (Scheme 1b). In a
similar manner, synthesis of amides assisted by vicinal alkylthio
migration of α-oxo ketene N,S-acetals was achieved by our
group (Scheme 1c).[16]
Functionalization of olefins is an essential organic transformation
in the fields of pharmaceuticals, agrochemicals, and materials
science.[1] Among the diverse olefinic transformation
methodologies, CC bond cleavage occupies a particularly
significant position in terms of building complex molecules from
relatively simple raw materials.[2] For example, ring-closing
metathesis (RCM) processes of olefins offer an efficient
approach to cyclic hydrocarbons under transition-metal
catalysis.[3] Various carbonyl-containing products were also
accessed through the oxidative CC bond cleavage.[4] Although
significant progresses have been made in this area, the relevant
synthesis of heterocyclic compounds has seldomly been
documented under transition metal-free conditions. Substituted
oxazolines are one of the common substructures in a wide
variety of biologically active compounds, synthetic intermediates,
and pharmaceuticals.[5] Consequently, various synthetic
methods have been developed to access an oxazoline core.[6]
However, there have been only a few scattered reports on the
formation of 2-imino-1,3-oxazolines, which are regarded as the
precursors of oxazolones widely existing in a number of natural
products
and
pharmacological
active
compounds.[7]
Condensation of α-haloketones and symmetrical diarylureas in
the presence of bromine gives 2-aryl-imino-3-aryl-1,3-
oxazolines.[8] Cathodic reduction of mono-imines with N-
arylcarbonimidoyl dichlorides affords tetraaryl-iminooxazolines.[9]
Reacting ketenimines with hydroxylamino derivatives to produce
2-imino-1,3-oxazolines.[10] Transition-metal-catalyzed reactions
of alkyl diazoacetates with carbodi-imides or α-hydroxyketones
can also be applied for the same purpose.[11,12] Unfortunately,
these methods often encounter the issues such as low atom
efficiency, high cost of toxic transition-metal catalysts, and lack
Scheme 1. PIDA-Mediated Transformations of Functionalized Enamines.
During our ongoing investigation of functionalized internal
olefins, that is, α-oxo ketene S,S-acetals[17] and N,S-
acetals,[18 ]as versatile building blocks in organic synthesis,[16,19]
we reasonably envisioned that their analogs α-oxo ketene N,N-
acetals[20] might be utilized for the synthesis of functionalized
heterocycles. Herein, we disclose PIDA-mediated olefinic CC
bond cleavage and cyclization of α-oxo ketene N,N-acetals for
the synthesis of substituted oxazolines (Scheme 1d).
[a] T. L. Guo, F. Huang, Q. B. Jiang, Prof. Dr. Z. K. Yu
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023 (P. R. China)
E-mail: zkyu@dicp.ac.cn
[b] Prof. Dr. Z. K. Yu
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (P. R. China)
Initially, the reaction of α-benzoyl ketene N,N-acetal (1a) with
PIDA was conducted to optimize the reaction conditions (Table
1). Treatment of 1a with PIDA (1.2 equiv) in CH2Cl2 at ambient
Supporting information for this article is given via a link at the end of the
document.
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10.1002/chem.201803466
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temperature afforded the target product 2a in 32% yield (Table 1,
entry 1). Elevating temperature obviously improved the reaction
efficiency to form 2a (64%) at 80 oC (Table 1, entries 1-3).
CH2Cl2 acted as the most suitable solvent among those
screened, i.e., DCE, dioxane, and DMF (Table 1, entries 4-6).
Both PhI(TFA)2 and K2S2O8 could not initiate the reaction, and
the reaction did not occur without PIDA (Table 1, entry 7).
Variation of the amount of PIDA revealed that 1.2 equiv of PIDA
was the suitable loading of the oxidant for the desired reaction
(Table 1, entries 3, 8, and 9). A base was beneficial to the
reaction, and 2a was obtained in 74% yield in the presence of
K2CO3 (Table 1, entries 10-12).
Table 2. Scope of -Oxo Ketene N,N-Acetals 1.[a,b]
Table 1. Screening of reaction conditions.[a]
Temp
[oC]
Yield[b]
[%]
Entry
Solvent
Oxidant
Base
1
2
25
60
80
80
80
80
80
80
80
80
80
80
CH2Cl2
CH2Cl2
PhI(OAc)2
PhI(OAc)2
32
42
64
48
50
48
0
CH2Cl2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
3
4
DCE
5
dioxane
6
DMF
CH2Cl2
7
8[c]
9[d]
10
11
12
62
58
69
74
71
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
Na2CO3
K2CO3
K3PO4
[a] Conditions: 1a (0.2 mmol), oxidant (0.24 mmol), base (0.4 mmol), solvent
(1.5 mL), in air, 2 h. The reaction was performed in a sealed 10-mL Pyrex
glass screw-cap tube. [b] Isolated yield. [c] PIDA (0.2 mmol). [d] PIDA (0.3
mmol). PIDA = PhI(OAc)2.
Next, the substrate scope of α-oxo ketene N,N-acetals (1)
was explored under the optimized conditions (Table 2). A steric
effect of methyl on the aryl moiety of the arylamino (NHAr)
functionality was observed, leading to 2-, 3-, and 4-methyl-
substitued products 2b (55%), 2c (72%), and 2d (81%),
respectively. The electronic effect from both electron-donating 4-
OMe and 4-OEt groups was remarkable, which diminished the
reaction efficiency in the formation of 2e (57%) and 2f (50%),
while electron-withdrawing 4-Cl and 4-F-substituted aniline-
derived N,N-acetals reacted well with PIDA to give 2g (70%) and
2h (66%) in good yields. 1-Naphthyl group exhibited an obvious
steric effect on the yield of product 2i which could only be
obtained in 30% yield by extending the reaction time to 6 h.
However, the methyl group on the aryl ring of the α-aroyl
functionality in 1 exhibited an steric effect opposite to that on the
NHAr moiety, facilitating the formation of 2j (91%), 2k (78%),
and 2l (73%). 4-OMe group in the aroyl moiety did not favor the
reaction, resulting in 2m in 51% yield. The 2-Cl and 4-Cl substi-
[a] Conditions: 1 (0.3 mmol), PhI(OAc)2 (0.36 mmol), K2CO3 (0.6 mmol),
CH2Cl2 (2.5 mL), 80 oC, air, 2 h. The reactions were performed in a sealed
10-mL Pyrex glass screw-cap tube. [b] Yields refer to the isolated products.
[c] 6 h.
2
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tuents promoted the formation of 2n (80%) and 2o (76%), while
4-Br and 4-F groups behaved less efficiently to render the
production of 2p and 2q in 62-70% yields. However, the strong
electron-withdrawing groups such as ester and trifluoromethyl on
the aryl group diminished the formation of 2r (32%) and 2s
(46%). 2-Naphthoyl did not exhibit a negative impact on the yield
of 2t (73%). To our delight, -(2-thienoyl) N,N-acetal also
efficiently reacted with PIDA to form 2u (80%). However, as
compared to the corresponding -aroyl substrates 1l and 1o, -
alkenoyl N,N-acetals 1v and 1w only exhibited a lower reactivity
to generate 2v (46%) and 2w (54%), respectively. Although -
acetyl N,N-acetal 1x could not effectively react with PIDA to
afford 2x (47%), its -pivaloyl analog 1y demonstrated a good
reactivity to form 2y (75%). It should be noted that N-benzyl and
other aliphatic amine-derived -oxo ketene N,N-acetals did not
undergo the same type of reactions.
Figure 1. Molecular structure of compound 2d.
Table 3. Synthesis of isothiozolines 4.[a]
Figure 2. Molecular structure of compound 4b.
2-Imino-functionalized oxazolines 2 were regarded as the
precursors to oxazolones. Thus, compounds 2 were treated with
10% aqueous H2SO4 to be hydrolyzed [Eq. (1)], efficiently
affording the corresponding oxazolone products 5a-5b (80-85%)
that are potentially useful building blocks in organic synthesis.[7]
Such a transformation has been demonstrated that the present
method is an alternative to access diverse oxazolone derivatives.
[a] Conditions: 3 (0.2 mmol), PhI(OAc)2 (0.24 mmol), K2CO3 (0.4 mmol),
CH2Cl2 (1.5 mL), 80 oC, air, 2 h. The reactions were performed in a
sealed 10-mL Pyrex glass screw-cap tube. Yields refer to the isolated
products.
To further demonstrate the synthetic application of the present
protocol, α-thioxo N,N-acetals of type 3 were employed to react
with PIDA under the same conditions (Table 3). Unexpectedly,
isothiazoline derivatives 4a-4e that are potentially biologically
active and pharmaceutically useful[21] were obtained in 81-95%
while the analogs of oxazolines 2, that is, thiazolines 4’, were
not detected from the reaction mixtures of 3 with PIDA. It is
noteworthy that the molecular structures of compounds 2 and 4
were further confirmed by the X-ray single crystal structural
determinations of compounds 2d and 4b (Fig.s 1 and 2, see the
Supplementary Information for details).
To further demonstrate the application potential of this
synthetic strategy, a gram-scale reaction was conducted. The
3
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10.1002/chem.201803466
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reaction of 1a (5 mmol, 1.56 g) proceeded smoothly under the
standard conditions to afford 2a (1.06 g) in 68% yield [Eq. (2)].
A plausible mechanism is proposed in Scheme 2. Initially, an
amidine intermediate is formed upon enolization of the carbony
or thiocarbonyl of substrate 1a or 3a, then interacts with PIDA to
generate intermediate A[22] by loss of one molecule of acetic acid.
Nucleophilic attack of the anilide nitrogen atom at the
hypervalent iodo(III) center forms species B. The in situ
generated HOAc was neutralized by K2CO3 base to prevent the
intermediates and/or products 2a or 4a from decomposition
under the acidic conditions. Subsequent reductive elimination of
PhI yields iminoisoxazoline C or isothiazoline 4a. Due to the
instability of the NO bond under the reaction conditions,
intermediate C undergoes further Baldwin-rearrangement[23] to
afford oxazoline 2a through olefinic CC bond cleavage/
cyclization of iminoacyl-aziridine species D. The formation of
isothiazoline has suggested that the N-S bond in 4a is stable
under the stated conditions, whereas the NO bond in
isoxazoline C can not withstand the reaction conditions and thus
undergoes further NO cleavage reaction to form oxazoline 2a.
mmol) in 2.5 mL CH2Cl2 was stirred at 80 oC under an air atmosphere for
2 h. After cooled to ambient temperature, 5 mL CH2Cl2 was added and
the resultant mixture was filtered through a short pad of celite, followed
by rinsing with 10 mL CH2Cl2. The combined filtrate was concentrated
under reduced pressure. The resulting residue was purified by silica gel
column chromatography (eluent: petroleum ether (60-90 oC)/EtOAc/
CH2Cl2 (15:1:1, v/v/v)) to afford 2a as a white solid (69 mg, 74%).
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (21472185) and the National Basic Research Program of
China (2015CB856600) for support of this research.
Keywords: PIDA • CC Bond Cleavage • Internal Olefins •
Oxazolines • Isothiazoline
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In summary, an efficient method has been developed to
synthesize substituted oxazolines via PIDA-mediated olefinic
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methodology can also be applied to access isothiazolines from
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economy with use of cheap PIDA as the oxidant under transition
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environmentally benign.
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Experimental Section
General procedure for the synthesis of 2
Synthesis of 2a: In a sealed 10-mL Pyrex glass screw-cap tube, a
mixture of 1-phenyl-3,3-bis-(phenylamino)prop-2-en-1-one (1a) (94 mg,
0.3 mmol), PhI(OAc)2 (116 mg, 0.36 mmol) and K2CO3 (83 mg, 0.6
4
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5
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Entry for the Table of Contents
FULL PAPER
T. L. Guo, F. Huang, Q. B. Jiang, and Z.
K. Yu*
Page No. – Page No.
PIDA-Mediated Formal Olefinic CC
Bond Cleavage of -Oxo Ketene N,N-
Acetals toward Substituted
Oxazolines
Formal Olefinic CC Bond Cleavage! PIDA mediated oxidative formal CC bond
cleavage and subsequent cyclization of α-oxo ketene N,N-acetals, affording
substituted oxazolines. Isothiazoline derivatives were obtained from the reactions of
α-thioxo ketene N,N-acetals with PIDA under the same conditions. Hydrolysis of the
resultant oxazoline derivatives led to highly functionalized oxazolone.
6
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