External-oxidant-free amino-benzoyloxylation of unactivated alkenes of unsaturated ketoximes with: O -benzoylhydroxylamines

Article abstract of DOI:10.1039/d1cc01565f

A new copper-catalyzed two-component amino-benzoyloxylation of unactivated alkenes of unsaturated ketoximes with O-benzoylhydroxylamines as the benzoyloxy sources is developed. Chemoselectivity of this method toward amino-benzoyloxylation or oxy-benzoyloxylation of alkenyl ketoximes relies on the position of the tethered olefins, and provides an external-oxidant-free alkene difunctionalization route that directly utilizes O-benzoylhydroxylamines as the benzoyloxy radical precursors and internal oxidants for the divergent synthesis of cyclic nitrones and isoxazolines.

Full text of DOI:10.1039/d1cc01565f

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External-oxidant-free amino-benzoyloxylation
of unactivated alkenes of unsaturated ketoximes
with O-benzoylhydroxylamines†
abcd
Cite this: DOI: 10.1039/d1cc01565f
Received 23rd March 2021,
Accepted 19th April 2021
Jiangfei Chen,a Yan-Ping Zhu, *b Jin-Heng Li
*
and Qiu-An Wang*a
DOI: 10.1039/d1cc01565f
rsc.li/chemcomm
A new copper-catalyzed two-component amino-benzoyloxylation of suffer from the limited external functional reagents and/or
unactivated alkenes of unsaturated ketoximes with O-benzoylhydro- requirement of noble transition-metal catalysts (such as palla-
xylamines as the benzoyloxy sources is developed. Chemoselectivity of dium, and nickel).3,4 Alternatively, radical-mediated two-
this method toward amino-benzoyloxylation or oxy-benzoyloxylation of component difunctionalization of alkenyl ketoximes has been
alkenyl ketoximes relies on the position of the tethered olefins, and developed recently and is particularly attractive because this
provides an external-oxidant-free alkene difunctionalization route that method proceeds via the intramolecular addition of oxime
directly utilizes O-benzoylhydroxylamines as the benzoyloxy radical radicals across the CQC double bonds to generate the
precursors and internal oxidants for the divergent synthesis of cyclic carbon-centered radicals and then cross coupling with a wide
nitrones and isoxazolines.
range of external functional radical trapping functional
reagents wherein oxime radicals can selectively serve as the
The alkene difunctionalization reaction has been considered as iminoxyl oxygen-centered radicals or iminoxyl nitrogen-
one of the most powerful methodologies for increasing mole- centered radicals to divergently access isoxazolines and cyclic
cular complexity in synthesis because it can introduce two nitrones relying on the position of the tethered olefins
functional groups across the CQC double bond in a single (Scheme 1a).5 In 2012, Han and coworkers first reported
reaction step.1,2 In particular, difunctionalization of alkenes of dioxygenation, oxyamination, and diamination of b,g- and
unsaturated ketoximes is appealing and has attracted consider- g,d-unsaturated ketoximes with external radical trapping func-
able attention over the past few decades because it can assem- tional reagents, such as 2,2,6,6-tetramethyl-1-piperidinyl-oxy
ble highly valuable functionalized O- and N-heterocycles (TEMPO) and diethyl azodicarboxylate (DEAD).5a This dual
through intramolecular cyclization and external functional radicals-enabled alkene difunctionalization strategy has been
group incorporation cascades.2–5 Classical methods for the proven to be synthetically useful and has been widely applic-
difunctionalization of alkenes of unsaturated ketoximes focus able to dioxygenation, oxyamination, diamination, carbooxy-
on electrophile addition across the CQC bonds followed by genation, carboamination, oxyhalogenation, oxysulfonylation
intramolecular cyclization3 or transition-metal-catalyzed intra- and oxythioetherization of alkenyl ketoximes with various radical
molecular cyclization followed by nucleophile addition.4 trapping functional reagents.5 However, these approaches are
Despite significant advances in the field, these approaches limited to the requirement of stoichiometric classical radical
t
scavengers (such as TENPO, DEAD and BuONO) or external
a State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,
Changsha 410082, China. E-mail: jhli@hnu.edu.cn, wangqa@hnu.edu.cn
b School of Pharmacy, Key Laboratory of Molecular Pharmacology and Drug
Evaluation, Ministry of Education, Collaborative Innovation Center of Advanced
Drug Delivery System and Biotech Drugs in Universities of Shandong,
Yantai University, Shandong, Yantai, 264005, China.
E-mail: chemzyp@foxmail.com
c Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources
Recycle, Nanchang Hangkong University, Nanchang 330063, China
d State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, China
† Electronic supplementary information (ESI) available. CCDC 2057134 and
2057127. For ESI and crystallographic data in CIF or other electronic format
Scheme 1 Radical-mediated difunctionalization of alkenyl oximes.
see DOI: 10.1039/d1cc01565f
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oxidants to trigger these processes. Moreover, to our knowledge, temperature efficiently afforded the desired cyclic nitrone pro-
radical-mediated two-component amino-benzoyloxylation or duct 3aa in 76% yield along with a trace of 4aa (entry 1). It is
oxy-benzoyloxylation of alkenyl ketoximes with the external benzoy- noted that no desired reaction occurs in the absence of a Cu
loxy radical precursors remains an unexplored area. Thus, the catalyst (entry 2), and a higher loading of CuCl (20 mol%)
development of new radical-mediated strategies, especially involving slightly decreased the yield (entry 3). A series of Cu catalysts,
an external-oxidant-free strategy using internal oxidants as the such as CuBr, Cu(MeCN)4PF4, CuOAc and Cu(OAc)2, were
radical trapping functional reagents, to allow new alkenyl ketoxime examined: the CuBr catalyst showed identical catalytic activity
difunctionalization modes is desirable.
to CuCl (entry 4), but the other Cu catalysts (e.g., Cu(MeCN)4
Herein, we report a new radical-mediated two-component PF4, CuOAc, Cu(OAc)2) were less efficient than CuCl along with
amino-benzoyloxylation of unactivated alkenes of unsaturated no side-reactions (entries 5–7). The reason might be that such
ketoximes with O-benzoylhydroxylamines6 using copper cataly- three Cu catalysts (entries 5–7) are more basic than CuCl (entry
sis for accessing benzoyloxy-possessing cyclic nitrones 1) or CuBr (entry 4), thus resulting in a decrease of the Lewis
(Scheme 1c). This reaction employs O-benzoylhydroxylamines acid properties to coordinate and activate the in situ generated
as both the benzoyloxy sources and internal oxidants to enable radical intermediates. The results showed that ligands could
selective amino-benzoyloxylation or oxy-benzoyloxylation of affect the reaction but were not the key to the success of the
alkenyl ketoximes for producing carbon-quaternary-center- reaction, as the omission of L1 resulted in the formation 3aa in
containing cyclic nitrones and isoxazolines7 which depends 49% yield and 4aa in 6% yield (entry 8). While both acidic
on the position of the tethered olefins.
ligands L2–L3 and a basic bipyridine L5 could improve the
We investigated the amino-benzoyloxylation reaction reaction (entries 9–10 and 12), phosphine L3 completely sup-
between 2,2,4-trimethyl-1-phenylpent-4-en-1-one oxime (1a) pressed the reaction (entry 11) and 1,10-phenanthroline L6 had
and morpholino benzoate (2a) for optimization of the reaction no effect (entry 13). Other solvents, including CH2Cl2, toluene
conditions (Table 1). Upon surveying various reaction para- and MeCN, were inferior to ClCH2CH2Cl (entries 14–16).
meters, we found that treatment of oxime 1a with benzoate 2a, A higher temperature (50 1C) gave identical results to those at
10 mol% CuCl, and 20 mol% 1,1-binaphthyl-2,2-diyl hydrogen- room temperature (entry 17). Notably, the reaction with a scale
phosphate (BNPA) ligand L1 in ClCH2CH2Cl at room up to 1 mmol of 2a was performed smoothly in good yield
(entry 18).
After establishing the optimized reaction conditions, the
substrate scope of this alkene amino-benzoyloxylation protocol
with respect to oximes 1 and morpholino benzoates 2 was
explored (Table 2). The substitution effect on the aryl ring of
1-arylpent-4-en-1-one oximes was initially examined in the
presence of morpholino benzoate (2a), CuCl, and BNPA L1
(3ba–fa). We found that substituents, including Me, i-Pr, Cl
and CF3, were well tolerated, and both the electronic effect and
steric hindrance had a fundamental influence on the reaction.
While oximes 1b–c bearing an electron-donating Me or i-Pr
group furnished 3ba–ca, respectively, in moderate yields, oxi-
mes 1d–f bearing an electron-withdrawing Cl or CF3 group
afforded 3da–fa in good yields. However, oxime 1f having an
m-Cl group was less reactive than oxime 1d having a p-Cl group
1d bearing a p-Cl group in terms of yields (3da, 3fa). Strikingly,
heteroaryl and naphthalen-2-yl-substituted pent-4-en-1-one
oximes 1g–i accommodated to the optimized conditions, giving
3ga–ia in moderate yields. This alkene amino-benzoyloxylation
protocol could be applicable to the construction of spiro-cyclic
rings and polycyclic compounds (3ja–la). For example, using
(1-(2-methylallyl)cyclohexyl)(phenyl)methanone oxime 1j smoo-
thly afforded 2-azaspiro[4.5]dec-1-ene 2-oxide 3ja in 53% yield.
For 2-(2-methylallyl)-3,4-dihydronaphthalen-1(2H)-one oxime 1l
the reaction efficiently executed to construct functionalized
3,3a,4,5-tetrahydro-2H-benzo[g]indole 1-oxide 3la. The reaction
was compatible with 4-methyl-1-phenylpent-4-en-1-one oxime
1m, furnishing 3ma in 62% yield.8 Replacement of the Me
group at the 4 position of the alkene moiety by a Ph group
accommodated to the reaction (3na). Delightfully, the use of
2,2-dimethyl-1,3-diphenylbut-3-en-1-one oxime 1o resulted in
Table 1 Optimization of reaction conditionsa
Isolated yield (%)
3aa
4aa
Entry
Variation from the standard conditions
1
2
None
Without CuCl
76
0
Trace
0
3
4
5
6
7
8
CuCl (20 mol%)
70
69
55
44
66
49
65
63
Trace
61
45
58
48
53
73
70
Trace
Trace
Trace
Trace
Trace
6
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
CuBr instead of CuCl
Cu(MeCN)4PF4 instead of CuCl
CuOAc instead of CuCl
Cu(OAc)2 instead of CuCl
Without L1
9
L2 instead of L1
L3 instead of L1
L4 instead of L1
L5 instead of L1
10
11
12
13
14
15
16
17
18b
L6 instead of L1
CH2Cl2 instead of ClCH2CH2Cl
Toluene instead of ClCH2CH2Cl
MeCN instead of ClCH2CH2Cl
At 50 1C
None
a
Standard reaction conditions: 1a (0.2 mmol), 2a (0.1 mmol), CuCl
(10 mol%), L1 (20 mol%), ClCH2CH2Cl (2 mL), argon, room tempera-
b
ture and 12 h. 2a (1 mmol) and 24 h.
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Table 2 Variations of the unsaturated oximes (1) and morpholino benzo-
ates (2)a
Scheme 2 Control Experiments.
isomerization cascades (equation 1), suggesting that the reaction
involves a radical process. The control experiments also support a
radical process and confirm the generation of the oxygen-centered
radical (the iminoxyl radical)/the resonance nitrogen-centered
radical cation as the reaction was completely inhibited in the
presence of a radical scavenger (such as TEMPO and BHT) and
afforded the phthalimido-N-oxyl-containing 3,4-dihydro-2H-
pyrrole 1-oxide 7 in 68% yield (equation 2). The results of entry
10 in Table 1 showed that the presence of PivOH could improve
the reaction. However, no reaction of oxime 1a with benzoic acid
was observed in the presence/absence of L1 (equation 3). These
results imply that the formation of the BzOꢀ radical intermediate
is crucial to the accomplishment of the desired reaction.
Consequently, the possible mechanisms for this alkene
amino-benzoyloxylation reaction were proposed on the basis
of the present results and precedent literature (Scheme 3).5,6
The highly reactive O–CuIIILn–N intermediate A is formed from
the reaction between the active CuILn species and morpholino
benzoate 2a through the insertion of the C–O bond, which also gives
resonance structures B/C and/or D/E.6 Under the external-oxidant-
free conditions, the intermediates B/C are more reactive than the
intermediates D/E. Subsequently, oxidative single electron transfer
between oxime 1a and the N–CuIILn intermediate B occurs to afford
the nitrogen-centered radical cation F (n = 1) or the resonance
oxygen-centered radical (the iminoxyl radical) F0 (n = 0). Intra-
molecular 5-exo-trig N-cyclization of the intermediate F occurs
to form the N-heterocyclic radical intermediates G, followed by
coupling with the intermediate C and reductive elimination to forge
the desired N-heterocycle product 3 and regenerate the active CuILn
species.5 When R4 = H in unsaturated oxime 1, the b-H elimination
of the intermediate I takes place to afford the desired product 6.
a
Reaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), CuCl (10 mol%),
L1 (20 mol%), ClCH2CH2Cl (2 mL), argon, room temperature and 12 h.
the formation of 4,5-dihydroisoxazole 5oa, a five-membered
O-heterocyclic ring, through intramolecular oxygen atom addi-
tion across the CQC bond.
Next, we set out to investigate the scope of morpholino
benzoates 2 under the optimized conditions. Morpholino
benzoates 2b–e bearing an electron-donating (e.g., MeO) or an
electron-withdrawing group (e.g., F, CF3, Cl) on the aryl ring of
the benzoate moiety were successfully converted to 3ab–ae in
moderate to good yields, leaving these groups intact for further
derivatization. Notably, the electron-donating group (3ab) dis-
played higher reactivity than the electron-withdrawing groups
(3ac–ae). To our surprise, both monosubstituted terminal
alkenes 1p–q and internal alkenes 1r–t underwent intra-
molecular nitrogen addition across the CQC bond and iso-
merization to afford alkenyl 3,4-dihydro-2H-pyrrole 1-oxides
6p–t.8
As shown in Scheme 2, internal alkene 1u containing a
cyclopropyl ring was converted to highly valuable (E)-2-(buta-1,
3-dien-1-yl)-3,4-dihydro-2H-pyrrole 1-oxide 6u via intramolecular
nitrogen addition across the CQC bond, ring-opening and Scheme 3 Proposed reaction mechanisms.
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5
For papers on dual radical enabled difunctionalization of alkenyl
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On the other hand, intramolecular 5-exo-trig O-cyclization of
the intermediate F0 affords the O-heterocyclic radical inter-
mediates G0, which sequentially undergoes coupling with the
intermediate C to deliver the C–CuIII–OBz intermediate H0 and
then reductive elimination to access the desired O-heterocycle
product 5 and regenerate the active CuILn species.5
In summary, we have developed a new copper-catalyzed two-
component amino-benzoyloxylation of unactivated alkenes of
unsaturated ketoximes with O-benzoylhydroxylamines for selec-
tively producing cyclic nitrones and isoxazolines under
external-oxidant-free conditions. This method enables the pre-
ferential formation of the benzoyloxyl radical intermediates to
form two new carbon–heteroatom bonds in a single reaction
step, and represents the first external-oxidant-free copper-catalyzed
alkene amino-benzoyloxylation route through intermolecular cross
coupling with the benzoyloxyl radicals using O-benzoylhydroxyl-
amines as the benzoyloxyl sources and internal oxidants.
The authors thank the National Natural Science Foundation
of China (No. 21625203 and 21871126) and the Talent Induc-
tion Program for Youth Innovation Teams in Colleges and
Universities of Shandong Province for financial support.
6 O-Benzoylhydroxylamines are well-known organic building blocks,
especially serving as the amino sources and internal oxidants, in
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Conflicts of interest
There are no conflicts to declare.
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crystallographic data for this paper†.
Chem. Commun.
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