Iminoxyl radical-based strategy for intermolecular C=O bond formation: Cross-dehydrogenative coupling of 1,3-dicarbonyl compounds with oximes

Article abstract of DOI:10.1002/adsc.201400143

Cross-dehydrogenative C=O coupling of 13-diketones and 1,3-keto esters with oximes was realized for the first time. The reaction proceeds in the presence of the oxidants [KMnO₄, Mn(OAc)₂/KMnO₄, Mn(OAc)₃2 H₂O, MnO₂, Mn(acac)₃, Fe(ClO₄)₃, Cu(ClO₄)₂6 H ₂O, Cu(NO₃)₂2.5 H₂O, and (NH ₄)₂Ce(NO₃)₆]. Twenty cross-coupling products were synthesized using potassium permanganate (KMnO₄), manganese(II) acetate dihydrate [Mn(OAc)₃2 H₂O], or the manganese(II) acetate/potassium permanganate [Mn(OAc)₂/KMnO ₄] system; yields are 27-92%. The synthesis can be easily scaled up to gram quantities of the target products. Apparently, the reaction proceeds via a radical mechanism in which the oxidizing agent serves to generate radicals from oximes and perform the one-electron oxidation of 1,3-dicarbonyl compounds. The formation of oxime radicals was confirmed quantitatively by electron spin resonance (ESR) spectroscopy. The coupling described in the present study is the first example of the selective intermolecular reaction involving unstable iminoxyl radicals generated in situ.

Full text of DOI:10.1002/adsc.201400143

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DOI: 10.1002/adsc.201400143
À
Iminoxyl Radical-Based Strategy for Intermolecular C O Bond
Formation: Cross-Dehydrogenative Coupling of 1,3-Dicarbonyl
Compounds with Oximes
Igor B. Krylov,a Alexander O. Terent’ev,a,* Vladimir P. Timofeev,b
Boris N. Shelimov,a Roman A. Novikov,a,b Valentina M. Merkulova,a
and Gennady I. Nikishina
a
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospekt 47, 119991 Moscow,
Russian Federation
Fax : (+7)-499–135–5328; phone: (+7)-499–137–2944; e-mail: terentev@ioc.ac.ru
V. A. Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, 32 Vavilova st., 119991 Moscow, Russian
b
Federation
Received: February 7, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400143.
À
Abstract: Cross-dehydrogenative C O coupling of rently, the reaction proceeds via a radical mechanism
1,3-diketones and 1,3-keto esters with oximes was re- in which the oxidizing agent serves to generate radi-
alized for the first time. The reaction proceeds in the cals from oximes and perform the one-electron oxi-
presence of the oxidants [KMnO4, Mn
KMnO4, Mn(OAc)3·2H2O, MnO2, Mn
Fe(ClO4)3, Cu(ClO4)2·6H2O, Cu(NO3)2·2.5H2O, and electron spin resonance (ESR) spectroscopy. The
(NH4)2Ce(NO3)6]. Twenty cross-coupling products coupling described in the present study is the first
were synthesized using potassium permanganate example of the selective intermolecular reaction in-
(KMnO4), manganese(II) acetate dihydrate volving unstable iminoxyl radicals generated in situ.
[Mn(OAc)3·2H2O], or the manganese(II) acetate/po-
tassium permanganate [Mn(OAc)2/KMnO4] system;
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
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yields are 27–92%. The synthesis can be easily scaled Keywords: cross-coupling; 1,3-dicarbonyl com-
up to gram quantities of the target products. Appa- pounds; iminoxyl radicals; oxidation; oximes
Introduction
cause the starting compounds are prone to side oxida-
tion and fragmentation reactions giving, for example,
Cross-dehydrogenative coupling, as a rule, is a reac- alcohols and carbonyl compounds. This gives rise to
tion in which two different starting molecules are con- a problem of searching for oxidizing agents and reac-
nected by a new bond with the elimination of one hy- tion conditions suitable for the cross-coupling of dif-
drogen atom from each of the molecules. In the last ferent types of substrates.
decade, these reactions have attracted great atten-
The literature data were thoroughly analyzed to de-
tion[1–11] because they can be used to form a new bond termine the scope of the C O cross-dehydrogenative
with almost the maximum possible atom economy coupling reactions. Aldehydes,[12,56,57] formamides,[58]
and do not require additional synthetic steps for the alkylarenes,[56,59–63] isochromanes,[64a] alcohols,[57,65,66]
introduction of functional groups (Hal, OTf, BR2, compounds containing the allyl moiety,[59,64b,67–83] 1,3-
SiR3, SnR3, ZnHal, etc.) that are necessary in other dicarbonyl compounds,[84–90] malononitriles and cyano-
À
approaches to the cross-coupling.
acetic esters,[88,91] monocarbonyl compounds[66,92–96]
The cross-dehydrogenative C C coupling has been and pyrrolidines[97] were used as C H reagents for the
À
À
[9–42]
[7,8,10,43–55]
studied in most detail;[1–11] the C N,
C P,
coupling.
Alcohols,[12,65,84–86,97]
N-hydroxy
À
À
À
C O
[10–12,56–97]
cross-coupling reactions are less imides,[12,57,59,63,88] hydroxamic acids,[88,90] oximes,[64] sul-
À
and C O
well developed. It is difficult to achieve high selectivi- fonic acids,[66,84,85,92–95] carboxylic acids,[60–62,67–83,85,87,96]
À
ty in the cross-dehydrogenative C O coupling be-
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Igor B. Krylov et al.
phosphonic acids,[85] t-BuOOH,[89,91] phenols,[58] and involving unstable iminoxyl radicals generated in situ
1,3-keto esters[58] were used as O H reagents.
were realized only recently: the intramolecular hydro-
À
À
À
Depending on the nature of C H and O H re- gen atom abstraction by oxime radical followed by
agents for the cross-dehydrogenative C O coupling, cyclization[107] and the intramolecular addition to the
À
various catalysts, oxidants, oxidizing systems and con- C=C double bond[108] were performed. The drawback
ditions
(n-Bu)4NI/t-BuOOH,[12,61,67,87,96]
I2/MCPBA,[93] DDQ,[64] iodine
compounds,[57,84–86]
CuCl/PhI
(OAc)2,[59] Cu2+/t-BuOOH,[56,58,89,91] CuCl2/
LiCl,[69] Cu(OTf)2/ligand/MnO2,[90] ruthenium com- oximes decompose to give a complex mixture of prod-
plexes,[65] Mn(OAc)3,[88] Co2+/KMnO4,[88] NaBrO3/ ucts, in particular, these radicals can dimerize to form
NaHSO3,[60] (NH4)2Ce
In many cases, the oxi-
were
used:
(n-Bu)4NBr/t-BuOOH,[12] of these protocols is the use of expensive reagents
G
NH4I/MCPBA,[92] such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy,
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[102]
(NO3)6,[63] Pd(II)-based sys- N N, N O, or O C bonds.
À
À
À
tems,[62,68–83] and anodic oxidation.[97]
dation of oximes affords the corresponding ke-
To the best of our knowledge, the only one type of tones.[109–118] In known intramolecular cyclization reac-
À
the cross-dehydrogenative C O coupling involving tions of iminoxyl radicals, the latter act both as O rad-
oximes was described in the literature, i.e., the cross- icals[108] and N radicals[101,108] to form a C O or C N
coupling with isochromanes[64a] or 1-phenylpropene bond, respectively.
À
À
derivatives[64b] promoted by 2,3-dichloro-5,6-dicyano-
The cross-dehydrogenative coupling products,
para-benzoquinone (DDQ). It was suggested that the oxime ethers containing carbonyl groups near the C=
À
À À
C O bond is formed via the nucleophilic attack of N O C moiety, are structurally similar to compounds
oxime on the carbocation generated from the C H exhibiting neuroprotective,[119] AMPA-antagonis-
À
reagent.[64]
tic,[119,120] anti-inflammatory,[121] fungicidal,[122] and anti-
viral activities.[123] The C=N O C moiety is also in-
À
À À
In the present work, the cross-dehydrogenative C
O coupling of 1,3-dicarbonyl compounds with oximes volved in the antimicrobial agents ceftazidime and
was performed for the first time. In previous studies, roxithromycin, the insect growth regulator flucyclox-
we found a series of unusual cross-dehydrogenative uron, and other biologically active compounds.
À
À
C O coupling reactions in which the C O bond is
formed by a homolytic mechanism involving O radi-
cals.[63,88,89,91] The oximes used in the present study also Results and Discussion
give O radicals in the presence of oxidants.[98–103] How-
ever, only a few reactions with oxime (iminoxyl) radi- The starting reagents used for the study were 1,3-keto
cals were described in the literature. The synthetic ap- esters 1a–f, 1,3-diketones 1g–k and diethyl etylmalo-
plication of these radicals has long been limited to re- nate 1l combined with oximes 2a–i (Scheme 1).
actions of the stable di-tert-butyliminoxyl radical:[99,100]
In the first step, we studied the cross-dehydrogena-
the oxidation of amines to imines,[104] the oxidative tive coupling of ethyl 2-methyl-3-oxobutanoate 1a
addition to phenols,[105] and the replacement of allylic with 3-(hydroxyimino)pentane-2,4-dione 2a in order
hydrogen in cyclohexene.[106] First selective reactions to find the optimal reaction conditions (Table 1). The
À
Scheme 1. Cross-dehydrogenative C O coupling of 1,3-dicarbonyl compounds 1a–l with oximes 2a–i.
2
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Iminoxyl Radical-Based Strategy for Intermolecular C O Bond Formation
Table 1. Effect of the nature of the oxidant, the solvent, the temperature, and the reaction time on the yield of cross-dehy-
drogenative coupling product 3.[a]
Run
Oxidant (ratio: mole per mole of 1a)
Solvent
Temperature
[8C]
Time
[min]
Isolated yield
of 3 [%]
N
1
2
3
4
5
6
7
8
KMnO4 “macrocrystalline” (0.4)
KMnO4 “macrocrystalline” (0.4)
KMnO4 “macrocrystalline” (0.4)
KMnO4 “macrocrystalline” (0.4)
KMnO4 “macrocrystalline” (0.4)
KMnO4 “powder” (0.4)
AcOH
AcOH
AcOH
AcOH
AcOH/H2O 20/1
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
CHCl3
MeCN
MeOH
AcOH
MeCN
MeCN
MeCN
AcOH
MeOH
EtOAc
CHCl3
MeCN
MeCN
MeCN
AcOH
MeOH
MeCN
AcOH
AcOH
AcOH
MeCN
MeCN
AcOH
MeCN
MeCN
AcOH
AcOH
MeCN
MeCN
AcOH
AcOH
AcOH
AcOH
60
40
40
24
40
40
40
40
40
40
60
60
80
40
24
60
80
60
60
80
24
80
24
24
24
60
80
80
80
60
60
60
60
60
80
80
80
60
80
80
60
60
80
80
80
80
80
80
20
10
5
90
91
90
84
74
90
64
90
92
90
79
92
92
91
80
77
74
85
74
84
84
25
0
40
10
10
10
10
10
10
20
20
10
20
20
20
20
20
20
20
5
KMnO4 “fine powder” (0.4)
KMnO4 “fine powder” (0.4)[b]
9
KMnO4 “fine powder” (0.4), Mn
KMnO4 “fine powder” (0.4)[c]
MnO2 (1)
ACHTUNGTRENN(NUG OAc)2·4H2O (1.6)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Fe
Fe
Fe
Fe
Fe
Fe
Fe
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
T
ACHTUNGTRENNUNG
N
T
A
20
5
5
0
5
14
55
45
49
0
55
17
12
0
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
60
60
60
60
Cu
Cu
Cu
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
A
U
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
8
6
G
N
15
40
6
10
35
0
0
0
0
0
Cu
Cu
Cu
Cu
Co
Co
Co
Co
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
T
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
N
R
A
PhI
A
BzOOBz 75% (1)
DDQ (1)
DIAD (1)
0
0
0
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Figure 1. Samples of KMnO4 used in the study: “macrocrystalline”, “powdered,” and “fine powdered”.
reaction was performed in CH3COOH, MeCN, perimental Section). An increase in the addition time
MeOH, EtOAc and CHCl3 using different oxidants at of “fine powdered” KMnO4 from 5–10 s to 8 min (run
temperatures from 24 to 808C.
8) leads to an increase in the yield of 3 from 64% to
Compounds of Mn(III, IV, and VII), FeAHCNUTGTERG(NNUN III), 90%. Apparently, the effect of the slow addition of
Cu(II), Ce(IV), Pb(IV), and Cr(VI), combinations of “fine powdered” KMnO4 is similar to the effect of the
Mn(II), Fe(III), Cu(II), and Co(II) salts with perox- slow dissolution of larger KMnO4 particles in runs 6
ides or oxygen, and non-metal agents, such as (diace- and 2.
toxyiodo)benzene [PhI(OAc)2], dibenzoyl peroxide The ESR study showed that KMnO4 oxidizes oxime
(BzOOBz), DDQ, and diisopropyl azodicarboxylate 2a to form iminoxyl O-centered radicals (see below).
(DIAD), were examined as oxidants. Since manganese forms stable compounds in all oxi-
The cross-coupling proceeds in the presence of dation states from +2 to +7, the oxidation of 2a with
various manganese-based oxidants [Mn(OAc)3, potassium permanganate can be accompanied by the
Mn(acac)3, KMnO4, MnO2; runs 1–19] under a wide formation of different manganese compounds in the
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
variety of reaction conditions: at 24–808C in different reaction mixture, the composition of which obviously
solvents [AcOH, MeCN, MeOH, CHCl3; runs 12 and depends on the rate of supply of KMnO4 to the reac-
16–18; the oxidant Mn
were 5–20 min. KMnO4 and Mn
the most efficient oxidants; the yield of 3 was up to 8) corresponds to an excess of the reducing agent (2a
90–92% (runs 1–3, 6, 8–10, and 12–14). and 1a) in the reaction zone over the oxidant
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
The yield of 3 depends on the particle size of (KMnO4) and facilitates the formation of manganese
KMnO4 (runs 2, 6 and 7). Photomicrographs of three compounds in lower oxidation states than in the case
manganese permanganate samples are shown in of the fast addition of small particles (run 7). Based
Figure 1. “Powdered” and “fine powdered” KMnO4 on our previous experience in the study of the cross-
samples were prepared by grinding “macrocrystalline” dehydrogenative coupling involving 1,3-dicarbonyl
KMnO4 in a mortar.
compounds and O radicals,[88] we suggested that, for
Despite a large difference in the KMnO4 particle the coupling to proceed, the reaction mixture should
size of samples 1 and 2, the reactions with their use contain a one-electron oxidant for the oxidation of
give product 3 in the same yield (runs 2 and 6, yield the dicarbonyl compound. An MnACTHNUTRGNEUGN(III) compound, for
90–91%). The reaction with the use of sample 3 (run example, manganese triacetate, can be such oxi-
7) affords cross-coupling product 3 in substantially dant.[124–128] The conditions of the following experi-
lower yield (64% versus 90–91%). After the standard ments are favorable for the formation of MnACHTUNGTRENNUNG(III)
treatment, the mixture of reaction products obtained compounds: run 2 (the use of large KMnO4 particles),
in run 7 contained unconsumed keto ester 1a (approx- run 8 (the slow addition of “fine powdered” KMnO4),
1
imately 29% based on the H NMR data, see the Ex- run 9 [in situ generation of Mn
N
[a]
General reaction conditions: an oxidant [in runs 9 and 35–44, the salt Mn
CAHUTGTNERN(NGU OAc)2·4H2O, FeACHTUNTGRNE(NUGN ClO4)3·nH2O,
Cu(ClO4)2·6H2O, Co(OAc)2·4H2O, or Co(NO3)2·6H2O was added one minute prior to the addition of the oxidant] was
A
R
ACHTUNGTRENNUNG
added to a stirred mixture of ethyl 2-methyl-3-oxobutanoate 1a (200 mg, 1.39 mmol), 3-(hydroxyimino)pentane-2,4-dione
2a (179 mg, 1.39 mmol), and a solvent (5 mL) at a specified temperature for 5–10 s, and then the reaction mixture was
stirred at the same temperature for a specified time.
KMnO4 was added for 8 min, and then the reaction mixture was stirred for 2 min.
The amount of oxime 2a was doubled (358 mg, 2.78 mmol)
[b]
[c]
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Iminoxyl Radical-Based Strategy for Intermolecular C O Bond Formation
of the extra reducing agent Mn
mixture, the reaction conditions are similar to those 1,3-dicarbonyl compounds 1a–l with oximes 2a–i were
used for the synthesis of Mn(OAc)3 from Mn(OAc)2 carried out under conditions similar to those used in
and KMnO4[124]], and run 10 (the use of the double runs 2 (oxidant KMnO4, method A), 12 [oxidant
amount of oxime 2a, which reduces KMnO4). In these Mn(OAc)3, method B], and 9 [oxidant Mn(OAc)2/
A
The cross-dehydrogenative coupling reactions of
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
experiments, cross-coupling product 3 was obtained in KMnO4, method C] presented in Table 1. The results
high yields, and the complete conversion of keto ester are summarized in Table 2.
1a was observed. On the contrary, the conditions of
1,3-Diketones and keto esters with different struc-
run 7 facilitate a higher concentration of KMnO4 in tures, including compounds containing easily oxidiza-
the reaction zone and, correspondingly, the formation ble allyl and benzyl moieties, are involved in the
of products of the reduction of manganese in higher cross-dehydrogenative coupling reactions with oximes.
oxidation states, resulting in a decrease in the efficien- Diethyl ethylmalonate 1l does not react with oxime
cy of the reaction. The suggestion about the involve- 1a (Methods A–C, reaction temperature 808C).
ment of Mn
G
The cross-coupling of a wide range of 1,3-dicarbon-
ative coupling of 1a with 2a in the presence of yl compounds with 3-(hydroxyimino)pentane-2,4-
KMnO4 is confirmed also by the results of run 5. dione 2a proceeds efficiently. The cross-coupling of
Thus, the addition of small amounts of water to the oxime 2b containing only one electron-withdrawing
reaction mixture leads to a decrease in the yield of 3 carbonyl group with keto ester 1a gives lower yields
by 17% compared to run 2 and to the incomplete con- compared to the cross-coupling of oxime 2a contain-
version of keto ester 1a (run 5, about 16% of 1a were ing two electron-withdrawing groups (runs 2 and 1,
present in the mixture of reaction products according respectively). The cross-coupling reactions of oximes
to the 1H NMR data). It is known that in water 2c–e and 2h and 2i containing no electron-withdraw-
Mn
Mn(IV).[129] Therefore, the results of runs 2 and 5–10 pling product increases with increasing bulkiness of
can be attributed to the fact that Mn(III) formed in the substituents at the C=NOH moiety (runs 14–16).
the course of the reaction between 1a, 2a and KMnO4 Except for the cross-coupling reactions with oxime
plays an important role in the cross-coupling of 1a 2a, the efficiency of the oxidants Mn(OAc)3 and
with 2a. Mn(OAc)2/KMnO4 is higher compared to KMnO4
Since substantially different results can be obtained (runs 2, 3, 4, 9, and 14–16).
in the reactions with the use of powdered KMnO4 Apparently, the reaction proceeds through the for-
ACHTUNGTRENNUNG(III) tends to disproportionate into Mn(II) and ing groups also take place. The yield of the cross-cou-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
samples, depending on the particle size, we used mation of iminoxyl O radicals from oximes, and the
“macrocrystalline” KMnO4 in subsequent experi- stability of these radicals has a key effect on the yield
ments for the cross-dehydrogenative coupling of 1,3- of the cross-coupling product. It is known that 2-imi-
dicarbonyl compounds with oximes.
noxy-1,3-dicarbonyl radicals, particularly diacetylimi-
Iron(III) perchlorate in acetonitrile proved to be noxyl radical A, which is formed by the oxidation of
ACHTUNGTRENNUNG
the iron salt of choice (runs 20 and 21, yield 84%). 3-(hydroxyimino)pentane-2,4-dione 2a, are relatively
The reactions with the use of iron nitrate (run 22) in- stable compared to the iminoxyl radicals generated
stead of Fe
methanol, ethyl acetate, or chloroform (runs 23–26, ents.[103,130]
respectively) instead of acetonitrile afford the target The hypothesized pathway of the cross-dehydrogen-
product in substantially lower yields. Fe(ClO4)3·nH2O ative coupling of 1,3-dicarbonyl compounds with
ACHTUNGTRENNUNG(ClO4)3 or with the use of acetic acid, from oximes containing alkyl and aryl substitu-
AHCTUNGTRENNUNG
is a slightly less effective reagent for the coupling oximes is depicted in Scheme 2 for the cross-coupling
than manganese-based oxidants, the other drawbacks of 1a with 2a.
of this reagent are high molecular weight and hygro-
scopicity.
In the presence of an oxidant, iminoxyl radicals A
are generated from oxime 2a. The reaction of radical
The reactions with the use of copper(II) salts or A with metal complex B or radical C affords cross-
cerium(IV) ammonium nitrate as oxidants give 3 in coupling product 3. The metal ion is necessary for the
yields of no higher than 55% (runs 27–32); lead tet- one-electron oxidation of the dicarbonyl compound
raacetate and ammonium dichromate are inefficient 1a. The probability of the one-electron oxidation of
in this reaction (runs 33 and 34). The cross-dehydro- 1,3-dicarbonyl compounds with MnACTHUNGTRENNNUG
genative coupling with the use of metal/peroxide or Fe
metal/oxygen oxidizing systems (runs 35–44) affords CuACTHNUGTRNE(UNG ClO4)2
(ClO4)3,[131,132]
(NH4)2CeACHUTNGTRENNUNG
[135]
the target products in yields of at most 40%. The generation of iminoxyl radicals A under the condi-
target product is not formed in the presence of the tions similar to the reaction conditions was confirmed
non-metal oxidants used in our experiments (runs 45– by ESR spectroscopy.
48).
Most of the iminoxyl radicals with alkyl and aryl
substituents at the carbon atom, except for the di-tert-
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Table 2. Products 3–22 of the cross-dehydrogenative coupling of 1,3-dicarbonyl compounds 1a–l with oximes 2a–i.
[a]
Method A (oxidant KMnO4): macrocrystalline KMnO4 (0.8 mmol) was added to a stirred mixture of a 1,3-di-
carbonyl compound (2 mmol), oxime (2 mmol), and acetic acid (7 mL) at 408C for 5–10 s, and then the mix-
ture was stirred at 408C for 10 min.
Method B [oxidant MnACHTUNGTRNNEUG(OAc)3]: MnACHTUNGTNERN(UNG OAc)3·2H2O (4 mmol) was added to a stirred mixture of a 1,3-dicarbonyl
[b]
compound (2 mmol), oxime (2 mmol), and acetic acid (7 mL) at 608C for 5–10 s, and then the mixture was
stirred at 608C for 5 min.
[c]
[d]
Method C [oxidant Mn
ACHTUNGTRENNUNG
of a 1,3-dicarbonyl compound (2 mmol), oxime (2 mmol), MnAHCTUNGTRENNUNG
(7 mL) at 408C for 5–10 s, and then the mixture was stirred at 408C for 5 min.
The synthesis was performed (method A) was scaled up to 1.00 g of ethyl 2-methyl-3-oxobutanoate 1a, see
the Experimewntal Section.
butyliminoxyl radical,[99,100,104–106] are unstable at room
temperature even in solutions.[98,102,106] It is necessary generated from model oxime 2a. 3-(Hydroxyimino)-
Hence, in ESR experiments, the latter radical was
to use flow techniques[98] or generate radicals by irra- pentane-2,4-dione
diation of the sample in the cavity of the spectrome- Mn(OAc)3·2H2O,
ter[102] in order to detect these radicals by ESR spec- system,
KMnO4,
troscopy. The iminoxyl radicals R1COC(=NOC)COR2,
(NH4)2Ce(NO3)6, Pb
particularly the 2,4-pentanedione-3-iminoxyl radical or MeCN at room temperature (Table 3). In all cases,
A,[103,130] are substantially more stable.
the ESR spectrum corresponding to the diacetylimin-
2a
the
Fe
was
Mn(OAc)2·4H2O/KMnO4
(ClO4)3, Cu(ClO4)2·6H2O,
(OAc)4, and PhI(OAc)2 in AcOH
oxidized
with
G
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
A
U
G
N
ACHTUNGTRENNUNG
6
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FULL PAPERS
Iminoxyl Radical-Based Strategy for Intermolecular C O Bond Formation
Figure 2. ESR spectrum of diacetyliminoxyl radical A gener-
ated by the oxidation of 3-(hydroxyimino)-2,4-pentanedione
2a with potassium permanganate in AcOH (Table 3, run 3,
2–3 min after the mixing).
Scheme 2. Plausible pathway of the cross-dehydrogenative
coupling of 1,3-dicarbonyl compounds with oximes.
ACHTUNGTRENNUNG
AHCTUNGTREN(GUNN NO3)6; in the presense of this oxidant radi-
ramethyl-1-piperidinyloxy (TEMPO) as a concentra- cal A is generated, but decomposes rapidly (run 5).
tion standard. An example of the ESR spectrum of Lead(IV) acetate and (diacetoxyiodo)benzene (runs 8
radical A is shown in Figure 2. The values of the g and 9) also generate radicals A from oxime 2a; how-
factor and 14N hyperfine coupling constant (g= ever, the cross-coupling of 2a with 1a does not take
2.0047; aN =28.5 G) are in agreement with literature place in the presence of these oxidants (Table 1, runs
data.[103,130]
33, 45). This observation is in agreement with the as-
As can be seen from runs 1–7, the oxidants, which sumption that the oxidant not only generates iminoxyl
were active in the cross-dehydrogenative coupling of radicals A but is also involved in the one-electron oxi-
oxime 2a with keto ester 1a, generate considerable dation of dicarbonyl compounds.
amounts of iminoxyl radicals A from oxime 2a even
To the best of our knowledge, all cross-dehydrogen-
at room temperature. The only exception is ative coupling products 3–22 are new. The products
Table 3. Generation of iminoxyl radical A from oxime 2a under conditions similar to the conditions of the cross-dehydrogen-
ative coupling of 2a with keto ester 1a. Estimation of the yield of radical A by ESR spectroscopy of reaction solution.
Run
Oxidant (oxidant/oxime molar ratio)
Solvent
Yield[a] of A [%]
2–5 min[b]
10 min[b]
20 min[b]
1
2
3
4
5
6
7
8
9
Mn
Mn
KMnO4 (0.2)
KMnO4 (0.4)
N
AcOH
AcOH
AcOH
AcOH
AcOH
MeCN
MeCN
AcOH
AcOH
34
49
55
96
3
81
19
20
2
40
52
77
86
2
81
19
22
5
39
52
72
–
–
–
–
–
8
T
(NO3)6 (1)
N
R
A
N
[a]
Calculated as follows: [concentration of A estimated by ESR spectroscopy]/[starting concentration of 2a]ꢁ100%.
Time elapsed after mixing of 2a with oxidant
[b]
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FULL PAPERS
Igor B. Krylov et al.
1
were characterized by H and 13C NMR, IR spectros-
IR spectra were recorded on a Bruker ALPHA FT-IR
spectrometer.
copy, elemental analysis, EI mass spectrometry, and
HR-MS. The configurations of the double bond in the
cross-coupling products 4, 5, 7, 14, and 15 and their
structures were determined by 2D NOESY, HMBC,
editing-HSQC and DOSY NMR.
The scanning electron microscopy of the KMnO4 samples
(see Figure 1 and the Supporting Information) was carried
out using a Hitachi SU8000 field-emission scanning electron
microscope (FE-SEM).[137] Before measurements the sam-
ples were mounted on a 25 mm aluminum specimen stub
and fixed by conductive carbon tape. Sample morphology
was studied under native conditions to exclude metal coat-
ing surface effects.[138] Images were acquired in secondary
electron mode at 2 kV accelerating voltage and at working
distance 8–10 mm.
For ESR experiments, 100 mL glass micropipettes
equipped with a stopper at the bottom were used as ESR
tubes (inner diameter 1.2 mm). Spectra were recorded on an
X-band ESR instrument (Varian-E104 A) using the follow-
ing adjustments: HF (100 kHz) field modulation amplitude
1 G; microwave power 5 mW; scan range 200 G; scan time
15 s; time constant 8 ms, temperature 23–248C.
Conclusions
À
Cross-dehydrogenative C O coupling of 1,3-diketones
and 1,3-keto esters with oximes is reported for the
first time. The best results were obtained with the
use of the widely available manganese-based
oxidants KMnO4, Mn
Mn(OAc)3·2H2O. The method is applicable for the
cross-coupling of a wide range of oximes with b-dicar-
bonyl compounds; the yields are up to 92%.
ACHTUGNTRENUN(NG OAc)2·4H2O/KMnO4, and
ACHTUNGTRENNUNG
Column chromatography was performed on silica gel
(0.060–0.200 mm, 60 A, Acros). CH2Cl2, CHCl3, ethyl ace-
tate, MeOH, and acetic acid of high purity grade were used
as obtained from commercial sources. MeCN was distilled
over P2O5.
The results are unusual because of the fact that
Mn-containing oxidants, including MnACTHNUGRTNEUNG(OAc)3·2H2O
and permanganates, were previously used for the
deeper oxidation of oximes with selective formation
of carbonyl compounds.[110–118]
Apparently, the oxidant serves two functions in the
cross-dehydrogenative coupling reaction: the genera-
tion of iminoxyl radicals from oximes and the one-
electron oxidation of 1,3-dicarbonyl compounds. This
reaction is the first example of the selective intermo-
lecular transformation of unstable iminoxyl radicals.
KMnO4 99%, MnO2 99%, Mn
Fe(ClO4)3 hydrate (10.0–12.5% Fe), Fe
Cu(ClO4)2·6H2O 98%, Cu(NO3)2·2.5H2O
Cu(OAc)2·H2O 99%, (NH4)2Ce(NO3)6 99%, Pb(OAc)4 95%,
K2Cr2O7 99%, Mn(OAc)2·4H2O 99%, (NH4)2S2O8 98%,
Co(NO3)2·6H2O 99%, Co(OAc)2·4H2O 98%, t-BuOOH
(70% aqueous solution) H2O2 (34% aqueous solution),
PhI(OAc)2 98%, dibenzoyl peroxide (BzOOBz, 75%, re-
mainder water), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) 98%, diisopropyl azodicarboxylate (DIAD) 94%,
ethyl 2-methyl-3-oxobutanoate 1a 95%, diethyl acetylsucci-
nate 1c 99%, ethyl 2-oxocyclohexane-1-carboxylate 1f 95%,
2-acetylcyclopentanone 1j 99%, 2-methyl-1,3-cyclohexane-
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Experimental Section
AHCTUNGTRENNUNG
dione 1k 98%, and diethyl ethylmalonate 99% 1l were com-
General Methods and Materials
mercial reagents.
1H and 13C NMR spectra were recorded on Bruker AMX-
III 400 (400.1 and 100.6 MHz, respectively) and Bruker
AVANCE II 300 (300.1 and 75.5 MHz, respectively) spec-
MnACTHNUTRGNEUNG
(acac)3,[139] 2-substituted 1,3-dicarbonyl compounds
1b,[140] 1d,[141] 1e,[142] 1g,[143] 1h,[144] 1i,[145] oximes 2a[146] and
2f[147] were synthesized according to the literature. Oximes
2c–e, 2h and 2i were synthesized from corresponding car-
bonyl compounds, NH2OH·HCl and NaHCO3, 2b was syn-
thesized from ethyl pyruvate and NH2OH·HCl; oxime 2g
was synthesized from propanal in 3 steps (oxime formation,
chlorination and substitution of chlorine with cyanide
anion). For experimental details of the synthesis of oximes
2b–e, 2g–i see the Supporting Information.
1
trometers in CDCl3 and DMSO-d6. Assignments of H and
13C signals and determinations of the structures and their
stereochemistry were made with the aid of 2D NOESY,
HMBC, editing-HSQC, and DOSY spectra where necessary.
High resolution mass spectra were recorded on a Bruker
maXis instrument equipped with electrospray ionization
(ESI) ion source.[136,137] All measurements were performed in
a positive (MS+) ion mode (interface capillary voltage:
4500 V) with scan range m/z: 50–3000. External calibration
of the mass spectrometer was performed with Electrospray
Calibrant Solution (Fluka). A direct syringe injection was
used for the all analyzed solutions in MeCN (flow rate:
3 mLminÀ1). Nitrogen was used as nebulizer gas (0.4 bar)
and dry gas (4.0 LminÀ1); interface temperature was set at
1808C. MeCN (HPLC grade) for ESI-HR-MS experiments
was ordered from Merck and used as supplied. All samples
for ESI-HR-MS experiments were prepared in 1.5 mL Ep-
pendorf tubes. All plastic disposables (Eppendorf tubes and
tips) used in sample preparation were washed with MeCN
before use. All spectra were processed by using Bruker Da-
taAnalysis 4.0 software package.
The composition of Fe
was taken as Fe
(ClO4)3·8H2O (MW=498.4 gmolÀ1) to cal-
culate the weight of this reagent, which corresponds to the
specified Fe(III) content (10.0–12.5%, titration by Na2S2O3).
CAHUTGTNRNENGU(ClO4)3 hydrate [FeACHTUNTGERN(NUGN ClO4)3·nH2O]
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Experiments for Table 1
The oxidant (87.7–1520 mg, 0.555–3.47 mmol, 0.4–2.5 mol/
mol 1a) was added to a stirred mixture of ethyl 2-methyl-3-
oxobutanoate 1a (200 mg, 1.39 mmol), 3-(hydroxyimino)-
pentane-2,4-dione 2a (179 mg, 1.39 mmol), and the solvent
(5 mL) at a specified temperature for 5–10 s. Then the mix-
ture was stirred at the same temperature for a specified
8
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Iminoxyl Radical-Based Strategy for Intermolecular C O Bond Formation
time. In runs 9 and 35–44, the salts Mn
Fe(ClO4)3·nH2O, Cu(ClO4)2·6H2O, Co(OAc)2·4H2O, and
Co(NO3)2·6H2O (17.0–544 mg, 0.0693–2.22 mmol, 0.05–
1.6 mol/mol 1a) were added one minute prior to the addi-
tion of the oxidant. In runs 40, 41, and 44, a stream of
oxygen (0.3 mLsÀ1) was passed through the mixture for
20 min.
ACHTUNGTRNEN(UNG OAc)2·4H2O,
Ethyl 2-(2,4-Dioxopentan-3-ylideneaminooxy)-2-
methyl-3-oxobutanoate 3 (Experiment with an
Increase in the Amounts of the Reagents, Table 2,
run 1, note [d])
G
E
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Macrocrystalline KMnO4 (438 mg, 2.77 mmol, 0.4 mol/mol
of 1a) was added to a mixture of ethyl 2-methyl-3-oxobutan-
AHCTUNGTREGoNNNU ate 1a (1.00 g, 6.93 mmol), 3-(hydroxyimino)pentane-2,4-
The reaction mixture was cooled to room temperature,
CHCl3 (10 mL) and a solution of Na2S2O4 (200 mg) in H2O
(30 mL) were added, the mixture was shaken, the organic
layer was separated, and the aqueous layer was extracted
with CHCl3 (2ꢁ10 mL). All organic extracts were combined
and washed with H2O (3ꢁ20 mL). The mixture was dried
with MgSO4, and the solvent was removed on a rotary evap-
orator. An oily mixture of products was obtained. Product 3
was isolated by silica gel column chromatography using gra-
dient elution with 0% to 20% (v/v) of ethyl acetate in
CH2Cl2.
dione 2a (895 mg, 6.93 mmol), and CH3COOH (10 mL) at
408C for 10 s. Then the mixture was stirred at the same tem-
perature for 10 min.
The reaction mixture was cooled to room temperature,
CHCl3 (15 mL) and a solution of Na2S2O4 (200 mg) in H2O
(30 mL) were added, the mixture was shaken, the organic
layer was separated, and the aqueous layer was extracted
with CHCl3 (2ꢁ10 mL). All organic extracts were combined
and washed with a saturated aqueous NaHCO3 solution
(15 mL) and then with H2O (20 mL). The mixture was dried
with MgSO4, and the solvent was removed on a rotary evap-
orator. Ethyl 2-(2,4-dioxopentan-3-ylideneaminooxy)-2-
methyl-3-oxobutanoate 3 was obtained as a slightly orange
In runs 1–15, a part of the mixture of products was ana-
lyzed by 1H NMR spectroscopy with the addition of 1,4-dini-
trobenzene as a standard for the determination of the
amount of unconsumed ethyl 2-methyl-3-oxobutanoate 1a
based on the signal of the CH fragment (d=3.49, q, J=
1
oil; yield: 1.74 g (92%) (see H NMR spectrum in the Sup-
porting information); elemental analysis: calcd. (%) for
C12H17NO6: C 53.13, H 6.32, N 5.16; found: C 53.09, H 6.17,
N 5.40.
1
7.1 Hz, the H NMR spectrum of 1a is given in the Support-
ing Information). Unconsumed 1a was detected only in runs
5 (16%) and 7 (29%).
Ethyl 2-(2,4-dioxopentan-3-ylideneaminooxy)-2-methyl-3-
oxobutanoate (3): Colorless oil; 1H NMR (300.13 MHz,
CDCl3): d=4.29–4.13 (m, 2H, OCH2), 2.36 (s, 3H, CH3),
2.30 (s, 3H, CH3), 2.20 (s, 3H, CH3), 1.68 (s, 3H, CH3), 1.24
(t, J=7.1 Hz, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): d=
201.1, 197.2, 193.6 (C=O), 167.4 (COO), 157.3 (C=N), 91.5
(CON), 62.4 (CH2O), 30.4, 25.8, 25.6, 19.3, 14.1 (CH3); IR
(thin layer): nmax =2988, 2943 (CH2, CH3), 1753, 1728, 1695
(C=O), 1362, 1297, 1270, 1130, 1109, 978, 959 cmÀ1; MS
(70 eV) m/z (%)=271 [M+] (47), 159 (100), 117 (95), 112
(60), 89 (53), 55 (95); HR-MS (ESI): m/z=294.0949, calcd.
for C12H17NO6 +Na+: 294.0948; elemental analysis: calcd.
(%) for C12H17NO6: C 53.13, H 6.32, N 5.16; found: C 53.05,
H 6.08, N 5.18.
Ethyl 2-methyl-3-oxobutanoate (1a): commercial reagent,
1
95%; slightly yellow oil; H NMR (300.13 MHz, CDCl3): d=
4.19 (q, J=7.1 Hz, 2H, OCH2), 3.49 (q, J=7.1 Hz, 1H,
CHMe), 2.23 (s, 3H, CH3C=O), 1.33 (d, J=7.1 Hz, 3H,
CH3), 1.27 (t, J=7.1 Hz, 3H, CH3).
Experiments for Table 2
Method A (oxidant KMnO4): “Macrocrystalline” KMnO4
(126.4 mg, 0.8 mmol) was added to a stirred mixture of
a 1,3-dicarbonyl compound (228–441 mg, 2 mmol), oxime
(196–378.4 mg, 2 mmol), and acetic acid (7 mL) at 408C for
5–10 s. The reaction mixture was stirred at 408C for 10 min.
Ethyl 2-{[(1-ethoxy-1-oxopropan-2-ylidene)amino]oxy}-2-
methyl-3-oxobutanoate (mixture of E and Z isomers 10:1)
Method B [oxidant MnACHTUNGTRENNG(U OAc)3]: MnACHTUNTGERN(NUGN OAc)3·2H2O 95%
(1.13 g, 4.0 mmol) was added to a stirred mixture of a 1,3-di-
carbonyl compound (228–441 mg, 2 mmol), oxime (196–
428 mg, 2 mmol), and acetic acid (7 mL) at 608C for 5–10 s.
The reaction mixture was stirred at 608C for 5 min.
Method C [oxidant MnACHTNUTRGNE(UGN OAc)2/KMnO4]: “Fine powdered”
KMnO4 (126.4 mg, 0.8 mmol) was added to a stirred mixture
1
(4): Colorless oil; H NMR (300.13 MHz, CDCl3): d=4.38–
4.12 (m, 4H, OCH2), 2.25 (s, 2.73H, CH3), 2.22 (s, 0.27H,
CH3), 2.15 (s, 2.73H, CH3), 2.05 (s, 0.27H, CH3), 1.68 (s,
2.73H, CH3), 1.61 (s, 0.27H, CH3), 1.40–1.18 (m, 6H, CH3);
13C NMR (75.47 MHz, CDCl3): d=202.8 (C=O), 168.2, 163.5
(COO), 151.4 (C=N), 90.8 (CON), 62.1, 61.9 (OCH2), 25.9,
19.7, 14.2, 14.1, 11.9 (CH2, CH3); IR (CHCl3): nmax =1728
(C=O), 1135 cmÀ1; MS (70 eV): m/z (%)=228 (72), 159
(86), 117 (100), 87 (94), 84 (95); HR-MS (ESI): m/z=
296.1108, calcd. for C12H19NO6 +Na+: 296.1105; elemental
analysis: calcd. (%) for C12H19NO6: C 52.74, H 7.01, N 5.13;
found: C 52.65, H 7.13, N 5.21.
of a 1,3-dicarbonyl compound (288–441 mg, 2 mmol), oxime
(230–378 mg,
2 mmol),
Mn
G
(784 mg,
3.2 mmol), and acetic acid (7 mL) at 408C for 5–10 s, The re-
action mixture was stirred at 408C for 5 min.
Product isolation for methods A–C: The reaction mixture
was cooled to room temperature, CHCl3 (10 mL) and a solu-
tion of Na2S2O4 (200 mg) in H2O (30 mL) were added, the
mixture was shaken, the organic layer was separated, and
the aqueous layer was extracted with CHCl3 (2ꢁ10 mL). All
organic extracts were combined and washed with a saturated
aqueous NaHCO3 solution (15 mL) and then with H2O
(20 mL). The mixture was dried with MgSO4, and the sol-
vent was removed on a rotary evaporator. Products 3–22
were purified by silica gel column chromatography using
gradient elution with 0% to 20% (v/v) of ethyl acetate in
CH2Cl2.
Ethyl
2-{[(3,3-dimethylbutan-2-ylidene)amino]oxy}-2-
methyl-3-oxobutanoate (mixture of E and Z isomers 12:1)
1
(5): Colorless oil; H NMR (300.13 MHz, CDCl3): d=4.28–
4.12 (m, 2H, OCH2), 2.22 (s, 3H, CH3), 1.90 (s, 3H, CH3),
1.59 (s, 3H, CH3), 1.24 (t, J=7.1 Hz, 3H, CH3), 1.07 (s, 9H,
t-Bu); 13C NMR (75.47 MHz, CDCl3): d=204.7 (C=O),
169.2, 165.9 (COO, C=N), 89.3 (CON), 61.6 (OCH2), 37.7
(CH3), 27.7 (3CH3), 25.8, 19.7, 14.2, 10.8 (CH3); IR (thin
layer): nmax =2971, 2940, 2909, 2873 (CH3, CH2), 1754, 1733
Adv. Synth. Catal. 0000, 000, 0 – 0
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Igor B. Krylov et al.
(C=O), 1368, 1264, 1170, 1133, 1108, 907 cmÀ1; MS (70 eV):
m/z (%)=117 (34), 98 (56), 84 (100), 59 (51), 55 (56), 45
(69), 43 (47); HR-MS (ESI): m/z=280.1524, calcd. for
C13H23NO4 +Na+: 280.1519.
Ethyl 2-acetyl-2-{[(2,4-dioxopentan-3-ylidene)amino]oxy}-
pent-4-enoate (10): Slightly yellow oil; 1H NMR
(300.13 MHz, CDCl3): d=5.73–5.51 (m, 1H, C=CH), 5.21–
5.04 (m, 2H, C=CH2), 4.24 (q, J=7.1 Hz, 2H OCH2), 2.96
(d, J=7.0 Hz, 2H, CH2), 2.39 (s, 3H, CH3), 2.33 (s, 3H,
CH3), 2.20 (s, 3H, CH3), 1.26 (t, J=7.1 Hz, 3H, CH3);
13C NMR (75.47 MHz, CDCl3): d=200.7, 197.2, 193.6 (C=
O), 166.5 (COO), 157.5 (C=N), 130.1, 120.5 (C=C), 93.6
(CON), 62.5 (OCH2), 37.6, 30.4, 26.7, 25.9, 14.2 (CH2, CH3);
Ethyl
2-{[(2,4-dimethylpentan-3-ylidene)amino]oxy}-2-
methyl-3-oxobutanoate (6): Slightly yellow oil; 1H NMR
(300.13 MHz, CDCl3): d=4.27–4.11 (m, 2H, OCH2), 3.10
(heptet, J=7.0 Hz, 1H, CHMe2), 2.58–2.42 (m, 1H,
CHMe2), 2.22 (s, 3H, CH3), 1.58 (s, 3H, CH3), 1.24 (t, J=
7.1 Hz, 3H, CH3), 1.17 (d, J=7.0 Hz, 6H, CH3), 1.10–0.96
(m, 6H, CH3); 13C NMR (75.47 MHz, CDCl3): d=204.8 (C=
O), 170.4, 169.2 (COO, C=N), 89.3 (CON), 61.6 (OCH2),
31.4, 28.6, 25.9, 21.30, 21.28, 19.5, 19.0, 18.9, 14.2 (CH3, CH);
IR (thin layer): nmax =2969, 2938, 2875 (CH, CH2, CH3),
1754, 1733 (C=O), 1366, 1265, 1132, 1105, 910 cmÀ1; MS
(70 eV): m/z (%)=271 (2) [M+], 155 (25), 112 (35), 89 (33),
85 (30), 71 (38), 70 (100); HR-MS (ESI): m/z=294.1672,
calcd. for C14H25NO4 +Na+: 294.1676; elemental analysis:
calcd. (%) for C14H25NO4: C 61.97, H 9.29, N 5.16; found: C
61.71, H 9.10, N 5.16.
IR (CHCl3): nmax =1752, 1728, 1695 (C=O) cmÀ1
; MS
(70 eV): m/z (%)=255 (100), 143 (95), 115 (74); HR-MS
(ESI): m/z=320.1099, calcd. for C14H19NO6 +Na+: 320.1105;
elemental analysis: calcd. (%) for C14H19NO6: C 56.56, H
6.44, N 4.71; found: C 56.85, H 6.67, N 5.04.
Diethyl 2-{[(2-benzyl-1-ethoxy-1,3-dioxobutan-2-yl)oxy]-
imino}malonate (11): Slightly yellow oil; 1H NMR
(300.13 MHz, CDCl3): d=7.33–7.16 (m, 3H, ArH), 7.16–
7.03 (m, 2H, ArH), 4.47–4.08 (m, 6H, OCH2), 3.55 (s, 2H,
PhCH2), 1.95 (s, 3H, CH3), 1.36 (t, J=7.1 Hz, 3H, CH3),
1.28 (t, J=7.2 Hz, 3H, CH3), 1.22 (t, J=7.1 Hz, 3H, CH3);
13 NMR (75.47 MHz, CDCl3): d=202.1 (C=O), 166.2, 160.0,
159.2 (COO), 146.3 (C=N), 133.9, 130.6, 128.4, 127.2 (CAr),
94.8 (CON), 62.9, 62.6, 62.4 (OCH2), 38.9, 27.2, 14.13, 14.05
(CH2, CH3); IR (thin layer): nmax =2985, 1752, 1725, 1258,
1095 cmÀ1; MS (70 eV): m/z (%)=407 (3) [M], 217 (57), 193
(59), 192 (59), 131 (76), 119 (100), 91 (86), 56 (72); HR-MS
(ESI): m/z=430.1469, calcd. for C20H25NO8 +Na+: 430.1472;
elemental analysis: calcd. (%) for C20H25NO8: C 58.96, H
6.19, N 3.44; found: C 58.99, H 6.27, N 3.38.
Ethyl (E)-2-{[(1-(4-bromophenyl)ethylidene)amino]oxy}-
2-methyl-3-oxobutanoate (7): Slightly yellow oil; 1H NMR
(300.13 MHz, CDCl3): d=7.48 (m, 4H, ArH), 4.32–4.18 (m,
2H, OCH2), 2.33 (s, 3H, CH3), 2.28 (s, 3H, CH3), 1.70 (s,
3H, CH3), 1.26 (t, J=7.1 Hz, 3H, CH3); 13C NMR
(75.47 MHz, CDCl3): d=203.8 (C=O), 168.8, 156.0 (COO,
C=N), 134.7, 131.8, 127.8, 124.2 (CAr), 90.1 (CON), 61.9
(OCH2), 25.8, 19.8, 14.2, 12.8 (CH3); IR (thin layer): nmax
=
2984, 2939 (CH2, CH3), 1751, 1731 (C=O), 1368, 1266, 1132,
Ethyl
2-benzyl-2-{[(2,4-dimethylpentan-3-ylidene)ami-
1105, 1009, 999, 916, 827 cmÀ1
; HR-MS (ESI): m/z=
no]oxy}-3-oxobutanoate (12): Colorless oil; 1H NMR
(300.13 MHz, CDCl3): d=7.25–7.15 (m, 3H, ArH), 7.15–
7.03 (m, 2H, ArH), 4.28–4.05 (m, 2H, OCH2), 3.57 (d, J=
14.1 Hz, 1H, PhCH2), 3.43 (d, J=14.1 Hz, 1H, PhCH2),
3.15–2.96 (m, 1H, CHMe2), 2.68–2.50 (m, 1H, CHMe2), 1.99
(s, 3H, CH3C=O), 1.21 (t, J=7.1 Hz, 3H, CH3), 1.17–1.09
(m, 12H, CH3); 13C NMR (75.47 MHz, CDCl3): d=204.8
(C=O), 170.6, 168.1 (COO, C=N), 135.7, 130.6, 128.1, 126.7
(CAr), 92.5 (CON), 61.6 (OCH2), 38.5, 31.6, 28.8, 27.2, 21.4,
18.8, 18.7, 14.2 (CH, CH2, CH3); IR (thin layer): nmax =2968,
2935, 2874 (CH3, CH2, CH), 1754, 1722 (C=O), 1467, 1455,
378.0306, calcd. for C15H18BrNO4 +Na+: 378.0311.
Ethyl 2-acetyl-2-{[(2,4-dioxopentan-3-ylidene)amino]oxy}-
hexanoate (8): Colorless oil; 1H NMR (300.13 MHz,
CDCl3): d=4.25 (q, J=7.0 Hz, 2H, OCH2), 2.42 (s, 3H,
CH3), 2.33 (s, 3H, CH3), 2.27–2.13 (m, 2H, CH2), 2.22 (s,
3H, CH3), 1.42–1.11 (m, 7H, CH2, CH3), 0.88 (t, J=7.1 Hz,
3H, CH3); 13C NMR (75.47 MHz, CDCl3): d=201.3, 197.4,
193.8 (C=O), 167.1 (COO), 157.4 (C=N), 94.4 (CON), 62.4
(OCH2), 32.8, 30.5, 26.6, 25.9, 25.2, 22.7, 14.2, 13.8 (CH2,
CH3); IR (thin layer): nmax =2963, 2936, 2854 (CH2, CH3),
1754, 1729, 1695 (C=O), 1363 cmÀ1; MS (70 eV): m/z (%)=
313 (12) [M+], 201 (62), 159 (71), 85 (100), 42 (66); HR-MS
(ESI): m/z=336.1409, calcd. for C15H23NO6 +Na+: 336.1418;
elemental analysis: calcd. (%) for C15H23NO6: C 57.50, H
7.40, N 4.47; found: C 57.48, H 7.51, N 4.37.
1366, 1353, 1260, 1231, 1198, 1085, 1067, 1018, 912, 702 cmÀ1
;
MS (70 eV): m/z (%)=219 (26), 112 (28), 91 (93), 70 (100);
elemental analysis: calcd. (%) for C20H29NO4: C 69.14, H
8.41, N 4.03; found: C 69.28, H 8.44, N 4.21.
Ethyl 1-{[(2,4-dioxopentan-3-ylidene)amino]oxy}-2-oxocy-
clohexane-1-carboxylate (13): Colorless oil; 1H NMR
(300.13 MHz, CDCl3): d=4.35–4.18 (m, 2H, OCH2), 2.69–
2.51 (m, 2H, CH2), 2.51–2.12 (m, 2H, CH2), 2.37 (s, 3H,
CH3), 2.29 (s, 3H, CH3), 2.00–1.60 (m, 4H, CH2), 1.28 (t, J=
7.0 Hz, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): d=202.6,
197.7, 193.9 (C=O), 168.2 (COO), 156.9 (C=N), 91.2 (CON),
62.1 (OCH2), 40.0, 34.9, 30.4, 27.2, 25.8, 20.8, 14.2 (CH3,
CH2); IR (thin layer): nmax =2945, 2871 (CH2, CH3), 1749,
1728, 1693 (C=O), 1365, 1298, 1282, 1249, 1096, 1052, 998,
962, 943 cmÀ1; MS (70 eV): m/z (%)=185 (84), 111 (100), 83
(72), 55 (95); HR-MS (ESI): m/z=320.1105, calcd. for
C14H19NO6 +Na+: 320.1105; elemental analysis: calcd. (%)
for C14H19NO6: C 56.56, H 6.44, N 4.71; found: C 56.59, H
6.55, N 4.86.
Diethyl 2-acetyl-2-{[(2,4-dioxopentan-3-ylidene)amino]-
oxy}succinate (9): Colorless oil; 1H NMR (300.13 MHz,
CDCl3): d=4.25 (q, J=7.1 Hz, 2H, OCH2), 4.10 (q, J=
7.2 Hz, 2H, OCH2), 3.47 (d, J=17.2 Hz, 1H, CH2), 3.32 (d,
J=17.2 Hz, 1H, CH2), 2.39 (s, 3H, CH3), 2.33 (s, 3H, CH3),
2.32 (s, 3H, CH3), 1.34–1.18 (m, 6H, CH3); 13 C NMR
(75.47 MHz, CDCl3): d=200.8, 196.8, 193.4 (C=O), 168.7,
165.7 (COO), 157.8 (C=N), 91.7 (CON), 62.9, 61.4 (OCH2),
38.6, 30.4, 26.4, 25.9, 14.14, 14.09 (CH3, CH2); IR (thin
layer): nmax =2986, 2942, 2911, 2876 (CH2, CH3), 1731, 1696
(C=O), 1366, 1358, 1297, 1276, 1232, 1214, 1194, 1055, 1022,
965, 943 cmÀ1; MS (70 eV): m/z (%)=189 (100), 143 (84),
115 (92), 86 (70), 69 (62); HR-MS (ESI): m/z=366.1147,
calcd. for C15H21NO8 +Na+: 366.1159; elemental analysis:
calcd. (%) for C15H21NO8: C 52.47, H 6.17, N 4.08; found: C
52.51, H 6.15, N 4.20.
Ethyl 2-{[(3-methyl-2,4-dioxopentan-3-yl)oxy]imino}pro-
panoate (mixture of E and Z isomers 15:1) (14): Colorless
10
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Iminoxyl Radical-Based Strategy for Intermolecular C O Bond Formation
1
oil; H NMR (300.13 MHz, CDCl3): d (E-isomer)=4.24 (q,
J=7.1 Hz, 2H, OCH2), 2.19 (s, 6H, O=CCH3), 2.17 (s, 3H,
N=CCH3), 1.61 (s, 3H, CH3), 1.28 (t, J=7.1 Hz, 3H, CH3);
d (Z-isomer, minor)=4.33 (q, J=7.1 Hz, 2H, OCH2), 2.23
(s, 3H, CH3), 1.34 (t, J=7.1 Hz, 3H, CH3); 13C NMR
(75.47 MHz, CDCl3): d=203.3 (C=O), 163.3 (COO), 151.8
(C=N), 95.8 (CON), 62.0 (OCH2), 26.2, 19.4, 14.1, 12.0
(CH3); IR (thin layer): nmax =2988, 2940 (CH2, CH3), 1739,
1720 (C=O), 1367, 1359, 1322, 1178, 1155, 1132, 1120,
1000 cmÀ1; MS (70 eV): m/z (%)=243 (1) [M+], 199 (36),
159 (32), 130 (47), 88 (69), 87 (59), 84 (100); HR-MS (ESI):
m/z=266.1008, calcd. for C11H17NO5 +Na+: 266.0999; ele-
mental analysis: calcd. (%) for C11H17NO5: C, 54.31; H,
7.04; N, 5.76. found: C, 54.51; H, 7.16; N, 5.84.
1441, 1420, 1354, 1141, 1113, 1100, 893 cmÀ1; MS (70 eV): m/
z (%)=297 (4) [M+], 169 (52), 99 (100), 71 (57); HR-MS
(ESI): m/z=320.2196, calcd. for C17H31NO3 +Na+: 320.2198;
elemental analysis: calcd. (%) for C17H31NO3: C 68.65, H
10.51, N 4.71; found: C 68.60, H 10.65, N 4.74.
3-Methyl-3-[(pentan-3-ylideneamino)oxy]pentane-2,4-di-
1
one (18): Slightly yellow oil; H NMR (300.13 MHz, CDCl3):
d=2.41 (q, J=7.6 Hz, 2H, CH2), 2.29–2.14 (m, 2H, CH2),
2.18 (s, 6H, CH3), 1.51 (s, 3H, CH3), 1.13 (t, J=7.6 Hz, 3H,
CH3), 1.03 (t, J=7.4 Hz, 3H, CH3); 13C NMR (75.47 MHz,
CDCl3): d=205.1 (C=O), 165.8 (C=N), 93.9 (CON), 27.3,
26.2, 22.0, 19.1, 10.8, 10.4 (CH2ꢂ, CH3); IR (thin layer):
nmax =2976, 2940, 2881 (CH2, CH3), 1739, 1715 (C=O), 1355,
1141, 1119, 1100, 922, 890 cmÀ1; MS (70 eV): m/z (%)=213
(30) [M+], 102 (58), 99 (88), 87 (89), 84 (100); HR-MS
(ESI): m/z=236.1258, calcd. for C11H19NO3 +Na+: 236.1257.
3-Butyl-3-{[(2,4-dioxopentan-3-ylidene)amino]oxy}pent-
ane-2,4-dione (19): Colorless oil; 1H NMR (300.13 MHz,
CDCl3): d=2.42 (s, 3H, CH3), 2.31 (s, 3H, CH3), 2.25–2.11
(m, 2H, CH2), 2.17 (s, 6H, CH3), 1.38–1.22 (m, 2H, CH2),
1.22–1.07 (m, 2H, CH2), 0.86 (t, J=7.2 Hz, 3H, CH3);
13C NMR (75.47 MHz, CDCl3): d=201.8, 197.5, 193.7 (C=
O), 157.7 (C=N), 100.0 (CON), 32.9, 30.3, 27.0, 25.8, 25.2,
22.8, 13.8 (CH2, CH3); IR (CHCl3): nmax =2963, 2934, 2875,
Pure major E-isomer and a fraction containing concen-
trated Z-isomer in mixture with E-isomer were isolated by
additional column chromatography. H NMR (300.13 MHz,
1
DMSO-d6): d (E-isomer)=4.22 (q, J=7.1 Hz, 2H, OCH2),
2.18 (s, 6H, CH3), 2.16 (s, 3H, CH3), 1.50 (s, 3H, CH3), 1.23
(t, J=7.1 Hz, 3H, CH3); d (Z-isomer)=4.32 (q, J=7.1 Hz,
2H, OCH2), 2.13 (s, 6H, CH3), 2.06 (s, 3H, CH3), 1.42 (s,
3H, CH3), 1.30 (t, J=7.1 Hz, 3H, CH3). The structures and
configurations of 14-E and 14-Z were confirmed by HMBC
and NOESY 2D NMR (see the Supporting Information).
N-[(3-Methyl-2,4-dioxopentan-3-yl)oxy]propionimidoyl
cyanide (Mixture of E and Z isomers 4:1) (15): Colorless
2866 (CH2, CH3), 1725, 1717, 1695 (C=O), 1358, 977 cmÀ1
;
MS (70 eV): m/z (%)=173 (68), 172 (72), 130 (100), 129
(71), 85 (77); HR-MS (ESI): m/z=306.1319, calcd. for
C14H21NO5 +Na+: 306.1312; elemental analysis: calcd. (%)
for C14H21NO5: C 59.35, H 7.47, N 4.94; found: C 59.54, H
7.61, N 4.77.
1
oil; H NMR (300.13 MHz, CDCl3): d (E-isomer)=2.64 (q,
J=7.7 Hz, 2H, CH2), 2.18 (s, 6H, CH3), 1.64 (s, 3H, CH3),
1.28 (t, J=7.7 Hz, 3H, CH3); d (Z-isomer)=2.52 (q, J=
7.5 Hz, 2H, CH2), 2.23 (s, 6H, CH3), 1.60 (s, 3H, CH3), 1.22
(t, J=7.5 Hz, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): d
(E-isomer)=201.9 (C=O), 143.3 (C=N), 113.4 (CN), 96.6
(CON), 26.02, 21.9, 19.2, 9.7 (CH3, CH2); d (Z-isomer)=
202.3 (C=O), 137.1 (C=N), 109.8 (CN), 96.2 (CON), 26.2,
25.95, 19.0, 10.7 (CH3, CH2); IR (thin layer): nmax =2985,
2943, 2886 (CH2, CH3), 1740, 1718, 1358, 1120, 993 cmÀ1; MS
(70 eV): m/z (%)=169 (48), 87 (100), 71 (66); HR-MS
(ESI): m/z=233.0889, calcd. for C10H14N2O3 +Na+:
233.0897; elemental analysis: calcd. (%) for C10H14N2O3: C
57.13, H 6.71, N 13.33; found: C 57.17, H 6.55, N 13.39.
3-{[(2,4-Dimethylpentan-3-ylidene)amino]oxy}-3-methyl-
Diethyl 2-{[(3-benzyl-2,4-dioxopentan-3-yl)oxy]imino}mal-
onate (20): Slightly yellow oil; 1H NMR (300.13 MHz,
CDCl3): d=7.32–7.18 (m, 3H, ArH), 7.12–7.00 (m, 2H,
ArH), 4.46–4.30 (m, 4H, OCH2), 3.50 (s, 2H, PhCH2), 1.96
(s, 6H, CH3), 1.38 (t, J=7.1 Hz, 3H, CH3), 1.30 (t, J=
7.1 Hz, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): d=201.3
(C=O), 160.0, 159.1 (COO), 146.9 (C=N), 134.0, 130.5,
128.5, 127.3 (CAr), 100.0 (CON), 63.1, 62.8 (OCH2), 38.9,
27.4, 14.2, 14.1 (CH2, CH3); IR (CHCl3): nmax =1745, 1719
(C=O), 1260, 1094 cmÀ1; MS (70 eV): m/z (%)=377 (13)
[M+], 293 (79), 217 (100), 187 (46), 174 (66); HR-MS (ESI):
m/z=400.1362, calcd. for C19H23NO7 +Na+: 400.1367; ele-
mental analysis: calcd. (%) for C19H23NO7: C 60.47, H 6.14,
N 3.71; found: C 60.91, H 6.48, N 4.06.
pentane-2,4-dione
(16):
Colorless
oil;
1H NMR
(300.13 MHz, CDCl3): d=3.11 (heptet, J=7.1 Hz, 1H, CH),
2.52 (heptet, J=6.8 Hz, 1H, CH), 2.17 (s, 6H, O=CCH3),
1.49 (s, 3H, CH3), 1.19 (d, J=7.1 Hz, 6H, CH3), 1.04 (d, J=
6.8 Hz, 6H, CH3); 13C NMR (75.47 MHz, CDCl3): d=205.1
(C=O), 170.9 (C=N), 94.1 (CON), 31.5, 28.7 (CH), 26.3,
21.3, 19.1, 19.0 (CH3); IR (thin layer): nmax =2969, 2936,
2875 (CH, CH3), 1740, 1715 (C=O), 1355, 1117, 1101,
900 cmÀ1; MS (70 eV): m/z (%)=88 (16), 72 (18), 71 (100);
HR-MS (ESI): m/z=264.1559, calcd. for C13H23NO3 +Na+:
264.1570; elemental analysis: calcd. (%) for C13H23NO3: C
64.70; H 9.61, N 5.80; found: C 64.41, H 9.51, N 5.71.
3-Methyl-3-[(undecan-6-ylideneamino)oxy]pentane-2,4-di-
one (17): Colorless oil; H NMR (300.13 MHz, CDCl3): d=
2.45–2.30 (m, 2H, N=CCH2), 2.24–2.08 (m, 2H, N=CCH2),
2.18 (s, 6H, CH3), 1.64–1.40 (m, 4H, CH2), 1.50 (s, 3H,
CH3), 1.40–1.17 (m, 8H, CH2), 1.00–0.80 (m, 6H, CH3);
13C NMR (75.47 MHz, CDCl3): d=205.1 (C=O), 164.1 (C=
N), 93.9 (CON), 34.1, 32.2, 31.5, 28.6, 26.2, 25.9, 25.6, 22.48,
3-{[(1-Acetyl-2-oxocyclopentyl)oxy]imino}pentane-2,4-di-
1
one (21): Colorless oil; H NMR (300.13 MHz, CDCl3): d=
2.73–2.59 (m, 1H, CH2), 2.54–2.31 (m, 2H, CH2), 2.37 (s,
3H, CH3), 2.33 (s, 3H, CH3), 2.31–2.16 (m, 1H, CH2), 2.24
(s, 3H, CH3), 2.14–1.99 (m, 2H, CH2); 13C NMR
(75.47 MHz, CDCl3): d=209.1, 203.2, 197.2, 193.5 (C=O),
157.9 (C=N), 95.9 (CON), 37.0, 31.9, 30.5, 26.6, 25.9, 18.1
(CH2, CH3); IR (thin layer): nmax =2974, 2925 (CH2, CH3),
1758, 1724, 1694 (C=O), 1421, 1359, 1296, 1167, 1029, 989,
964, 940 cmÀ1; MS (70 eV): m/z (%)=141 (100), 113 (19),
100 (16); HR-MS (ESI): m/z=276.0836, calcd. for
C12H15NO5 +Na+: 276.0842; elemental analysis: calcd. (%)
for C12H15NO5: C 56.91, H 5.97, N 5.53; found: C 56.81, H
5.99, N 5.46.
1
2-{[(2,4-Dioxopentan-3-ylidene)amino]oxy}-2-methylcy-
clohexane-1,3-dione (22): White crystals; mp 115–1168C;
1H NMR (300.13 MHz, CDCl3): d=2.87–2.62 (m, 4H, CH2),
2.37 (s, 3H, CH3), 2.25 (s, 3H, CH3), 2.02–1.84 (m, 2H,
22.45, 19.2, 14.07, 14.06 (CH2, CH3); IR (thin layer): nmax
2958, 2932, 2872, 2862 (CH2, CH3), 1740, 1716 (C=O), 1460,
=
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Igor B. Krylov et al.
CH2), 1.57 (s, 3H, CH3); 13C NMR (75.47 MHz, CDCl3): d=
203.1, 197.3, 193.8 (C=O), 157.0 (C=N), 94.5 (CON), 38.3,
30.4, 25.8, 18.9, 17.9 (CH2, CH3); IR (KBr): nmax =2972,
2916, 2871 (CH2, CH3), 1742, 1715, 1691 (C=O), 1130, 1016,
969, 571 cmÀ1; MS (70 eV): m/z (%)=114 (62), 99 (68), 72
(65), 56 (100); HR-MS (ESI): m/z=276.0833, calcd. for
C12H15NO5 +Na+: 276.0842; elemental analysis: calcd. (%)
for C12H15NO5: C 56.91, H 5.97, N 5.53; found: C 57.00, H
5.99, N 5.51.
gy of organic synthesis and design of compounds with valua-
ble applied properties” of the Presidium of the Russian Acad-
emy of Sciences. High resolution mass spectroscopy and elec-
tron microscopy were performed in the Department of Struc-
tural Studies of Zelinsky Institute of Organic Chemistry,
Moscow.
References
Experiments for Table 3
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1292.
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2013, 355, 269–273.
Since MnACHTUNGTRENNUNG(OAc)3·2H2O, KMnO4 and (NH4)2CeCAHTUNGTREN(NUGN NO3)6 are
poorly soluble in AcOH, it was impossible to prepare solu-
tions of these oxidants at concentrations appropriate for ex-
periments. Hence, different procedures were used in runs 1–
5 (heterogeneous reaction mixture) and runs 6–9 (homoge-
neous reaction mixture). In all experiments, the complete
conversion of oxime 2a into iminoxyl radical A corresponds
to the concentration of the latter equal to 0.005 mol/L. The
concentration of radical A was determined by the double in-
tegration of the ESR spectrum. Solutions of the stable ni-
troxyl radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)
were used as concentration standards in MeCN (to estimate
the concentration of A in runs 6 and 7 in MeCN) and in
EtOAc (to estimate the concentration of A in runs 1–5 and
8 and 9 in AcOH). It is incorrect to use a solution of
TEMPO in AcOH as a concentration standard because this
nitroxyl radical can disproportionate in an acidic
medium.[148,149] Hence, ethyl acetate was used instead of
AcOH because these solvents have similar relative permit-
tivities (dielectric constants):[150] 6.02 for EtOAc and 6.15
for AcOH.[151]
[9] S. H. Cho, J. Y. Kim, J. Kwak, S. Chang, Chem. Soc.
Rev. 2011, 40, 5068–5083.
[10] C. Zhang, C. Tang, N. Jiao, Chem. Soc. Rev. 2012, 41,
3464–3484.
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[12] B. Tan, N. Toda, C. F. Barbas III, Angew. Chem. 2012,
124, 12706–12709; Angew. Chem. Int. Ed. 2012, 51,
12538–12541.
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14, 3280–3283.
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124, 6594–6598; Angew. Chem. Int. Ed. 2012, 51,
6488–6492.
[15] S. Wiese, Y. M. Badiei, R. T. Gephart, S. Mossin, M. S.
Varonka, M. M. Melzer, K. Meyer, T. R. Cundari,
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J. Org. Chem. 2008, 73, 6207–6212.
In runs 1–5, a solution of 3-(hydroxyimino)pentane-2,4-
dione 2a (32.3 mg, 0.25 mmol) in AcOH (50 mL) was added
to the oxidant (the amounts are given below). Then the mix-
ture was continuously shaken, and samples were taken at
certain time intervals for ESR measurements. In run 2, a so-
lution of 2a in acetic acid contained Mn
(49.0 mg, 0.2 mmol). The amounts of the oxidant were as
follows: Mn(OAc)3·2H2O 95% (70.6 mg, 0.25 mmol, run 1),
KMnO4 (7.90 mg, 0.05 mmol, runs and 3), KMnO4
(15.8 mg, 0.1 mmol, run 4), (NH4)2Ce(NO3)6 (137 mg,
ACHTUNGTERN(NUNG OAc)2·4H2O
ACHTUNGTRENNUNG
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Acknowledgements
This work was supported by the Russian Foundation for
Basic Research (Grant 13-03-12074) and the Programs “Mo-
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16 Iminoxyl Radical-Based Strategy for Intermolecular C O
Bond Formation: Cross-Dehydrogenative Coupling of 1,3-
Dicarbonyl Compounds with Oximes
Adv. Synth. Catal. 2014, 356, 1 – 16
Igor B. Krylov, Alexander O. Terent’ev,*
Vladimir P. Timofeev, Boris N. Shelimov,
Roman A. Novikov, Valentina M. Merkulova,
Gennady I. Nikishin
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