Visible Light Driven and Copper-Catalyzed C(sp3)-H Functionalization of O-Pentafluorobenzoyl Ketone Oximes

Article abstract of DOI:10.1021/acs.orglett.1c02133

The C(sp3)-H functionalization of O-pentafluorobenzoyl ketone oximes was implemented under visible light irradiation with copper complexes as catalysts. The reactions involve iminyl-radical-mediated intramolecular hydrogen atom transfer as the key step, w

Full text of DOI:10.1021/acs.orglett.1c02133

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Letter
Visible Light Driven and Copper-Catalyzed C(sp3)−H
Functionalization of O‑Pentafluorobenzoyl Ketone Oximes
Danhua Wu, Shuang-Shuang Cui, Fengling Bian,* and Wei Yu*
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ABSTRACT: The C(sp3)−H functionalization of O-pentafluorobenzoyl ketone oximes was implemented under visible light
irradiation with copper complexes as catalysts. The reactions involve iminyl-radical-mediated intramolecular hydrogen atom transfer
as the key step, with the iminyl radicals being generated via copper-effected N−O cleavage. The reaction afforded 3,4-dihydro-2H-
pyrroles under the conditions of [Cu(DPEphos)(bcp)]PF6 and DABCO, while γ-pentafluorobenzoyloxy ketones were produced
predominantly when [Cu(dpp)2]PF6 and InCl3·4H2O were used as catalysts.
Scheme 1. C(sp3)−H Functionalization via Intermediary of
Oxime-Derived Iminyl Radicals
minyl-radical-mediated reactions are of current research
interest because of their potential usefulness in the synthesis
I
of nitrogen-containing compounds.1 Apart from the intra-
molecular addition that has long been recognized as a valuable
tool for the preparation of dihydropyrrole derivatives,2 iminyl-
radical-mediated intramolecular hydrogen atom transfer
(HAT)3 and fragmentation4 are highly useful for the
preparation of functionalized ketones and nitriles as well as
nitrogen heterocycles. The employment of visible light
photoredox catalysis enables convenient generation of iminyl
radicals from readily accessible precursors and thus largely
promotes the applications of iminyl radicals in organic
synthesis.5
Intramolecular HAT mediated by nitrogen-centered radicals
constitutes a valuable tool for the site-selective C(sp3)−H
functionalization of organic compounds.3,6 In this context,
much attention has been focused recently toward applying the
iminyl-radical-mediated intramolecular 1,5-HAT to the prep-
aration of benzene-fused cyclohexanones,7 nitrogen hetero-
cycles,8 β-amino alcohols,9 and γ-functionalized ketones.10
Encouraged by these achievements, we set out to develop new
C(sp3)−H functionalization methods on the basis of this
chemistry. Previous study by Nevado showed that 3,4-dihydro-
2H-pyrroles can be prepared from O-acyl ketone oximes by
employing visible light photoredox catalysis with [Ir(dtbbpy)
(ppy)2]PF6 as catalyst, but this protocol has only been applied
to gem-dimethyl-substituted oxime precursors (Scheme 1A,
reaction 1).7a It was expected that O-acyl oximes without a
gem-dimethyl group can be converted in the same way.
Moreover, we envisioned that O-acyl oximes would undergo
acyloxy transfer to give γ-acyloxy ketones, a transformation that
has not been reported in spite of its apparent feasibility
(Scheme 1, reaction 2).11 Indeed, we found that by using
[Cu(DPEphos)(bcp)]PF6 as the photocatalyst, O-pentafluor-
obenzoyl ketone oximes can be converted to 3,4-dihydro-2H-
pyrroles in good yield. When [Cu(dpp)2]PF6 and InCl3·4H2O
were used as catalysts, on the other hand, the same substrates
reacted to afford γ-acyloxy ketones (Scheme 1B). Herein, we
report these results.
Copper(I) complexes have recently risen into prominence as
an economical and environmentally benign alternative for the
precious ruthenium or iridium complexes in photocatalysis.12
The capacity of copper complexes to enable transformations
via inner-sphere ligand exchange provides an effective means
for carbon-heteroatom coupling that may not be fulfilled easily
with other commonly used photocatalysts. We believed that
Received: June 24, 2021
Published: July 19, 2021
https://doi.org/10.1021/acs.orglett.1c02133
© 2021 American Chemical Society
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these merits of copper complexes could benefit the iminyl-
radical-mediated C−H functionalization. With this consid-
eration in mind, we applied several reported Cu(I) complexes
to the cyclization of O-pentafluorobenzoyl ketone oxime 1a.
Among these complexes, [Cu(DPEphos)(bcp)]PF613 exhibited
the best catalytic capacity. As shown in Table 1, 1a can be
yield. The substituent at the phenyl ring neighboring to the
imino group has some influence on the reaction, but this effect
does not correlate with their electron-donating or electron-
withdrawing capacity.
When gem-dimethyl-lacking 3a was subjected to these
conditions, cyclization product 4a was generated in a moderate
yield of 32%. To improve this result, we explored more
conditions (Tables S4 and S5 in SI) and found that by adding
10 mol % of Cu(TMHD)2 into the system (in CH3CN), the
yield of cyclization product 4a can be raised to 63% (Scheme
3). Scaling up the reaction to 10 mmol scale resulted in a slight
loss in yield (Scheme 3, footnote a).
a
Table 1. Screening of Conditions for the Cyclization of 1a
b
entry
mol % of [CuI]
base (equiv)
yield of 2a (%)
1
2
3
4
5
6
7
8
9
−
DABCO (4.0)
DABCO (4.0)
DABCO (3.0)
DABCO (4.0)
−
DABCO (4.0)
Et3N (4.0)
i-Pr2NEt (4.0)
DBU (4.0)
DMAP (4.0)
N.R.
92
80
Scheme 3. Scope of the Modified Protocol
5.0
5.0
2.5
5.0
5.0
5.0
5.0
5.0
5.0
76
N.R.
N.R.
c
80
80
28
d
10
0
a
The reaction was conducted on 0.1 mmol scale under an argon
atmosphere in 2 mL of dichloroethane (DCE) at ambient
temperature (<28 °C). An 18 W blue LED apparatus was used as
b
c
d
the light source. Isolated yield. Control experiment in the dark. 1a
decomposed. [CuI] = [Cu(DPEphos)(bcp)]PF6.
converted to 3,4-dihydro-2H-pyrrole 2a in high yield in
dichloroethane (DCE) by blue light irradiation with [Cu-
(DPEphos)(bcp)]PF6 as the photocatalyst. The presence of an
organic base such as 1,4-diazabicyclo[2.2.2]octane (DABCO),
Et3N, or i-Pr2NEt is necessary for the reaction to proceed, but
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was much less
effective and no reaction took place when 4-dimethyl-
aminopyridine (DMAP) was used (details on optimization of
reaction conditions are presented in the Supporting
Information (SI); see Tables S1−S3).
The optimal conditions (Table 1, entry 2) were then applied
to a variety of gem-dimethyl-substituted O-pentafluorobenzoyl
oximes 1, and the result is depicted in Scheme 2. The desired
transformations took place uneventfully, and the correspond-
ing cyclization products were obtained in good to excellent
The scope of the modified protocol was examined next by
applying it to more substrates. As shown in Scheme 3,
variously substituted 3,4-dihydro-2H-pyrroles 4 were generated
Scheme 2. Reaction of gem-Dimethyl O-Pentafluorobenzoyl Oximes
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from the corresponding O-pentafluorobenzoyl ketone oxime
precursors (3) in moderate to good yield under the indicated
conditions. Compound 4af, which incorporates an ethox-
ycarbonyl rather than an aryl group at the 2-position, was
prepared as well. However, this protocol does not work for
substrates having an alkyl attached to the iminyl carbon (4ag
and 4ah) or lacking an aryl ring at the γ-position (4ad), and it
is invalid for the preparation of pyridyl-bearing 4ak.
These cyclization reactions are believed to proceed via
iminyl radical-mediated 1,5-HAT, which was confirmed by
radical trapping and inhibition experiment with 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) (Scheme S1, reactions
1 and 2 in SI). A luminescence quenching experiment indicates
that [Cu(DPEphos)(bcp)]PF6* can be quenched by 3a, while
DABCO has no such effect (Figures S3 and S4 in SI).
Apparently the iminyl radical was generated by the
interaction of O-pentafluorobenzoyl oximes with [Cu-
(DPEphos)(bcp)]PF6*, which probably involves a single-
electron transfer (SET) process. The reduction potential of 3a
was measured to be −1.3 V vs SCE in MeCN (Figure S6 in
SI), lower than the oxidation potential of [Cu(DPEphos)-
(bcp)]PF6* (−1.02 V vs SCE13c). However, it has been proven
that SET reaction with an endothermicity of 1.0 V can still take
place rapidly as long as the subsequent steps are
thermodynamically favorable,14 and thus SET oxidation of
[Cu(DPEphos)(bcp)]PF6* by O-pentafluorobenzoyl ketone
oximes would be a viable pathway under the current
circumstances.
oxidizing capacity of [CuII] would be compromised if the
C6F5CO2 anion is bonded to it.
−
Having acquired the above results, we went on to explore
conditions to engender the acyloxy transfer from the oxime
nitrogen in 3 to the γ-carbon. After extensive screening of the
reaction conditions, we found that the anticipated trans-
formation can be enabled with the assistance of InCl3·4H2O.
Under the optimal conditions which employed 5 mol % of
16
[Cu(dpp)2]PF6 and 1.0 equiv of InCl3·4H2O as the catalyst,
3a reacted in DCE under blue light irradiation to give 5a in a
yield of 70% (Scheme 5). The structure of 5a was confirmed
by X-ray crystallographic analysis (CCDC No. 2055827). The
optimal reaction temperature was found to be 38 °C. Notably,
the reaction can also take place in the dark, albeit in a lower
yield (43%) (details on optimization of reaction conditions are
listed in Tables S6−S8 in SI).
This new protocol (Condition C) provides an atom-
economical means for the preparation of γ-acyloxy ketones
(Scheme 5). This protocol not only allows aryl ketones to be
functionalized at the γ-position but also is suitable for the
acyloxylation of aliphatic ketones (5ag and 5ah). However,
substrates bearing a methoxy group at the para position of the
phenyl ring did not react well. In the case of 3g,
dihydronaphthalen-1(2H)-one 6g was generated in 17% yield
along with 5g. The reaction of 3z, on the other hand, resulted
in the formation of a complex mixture.17 This protocol is also
unsuitable for preparing compounds 5ai, 5aj, and 5ak. When
1a was treated with this protocol, 7al was generated besides
the acyloxy transfer product 5al.
On the basis of above analyses, the cyclization reaction is
rationalized with a mechanism shown in Scheme 4. With 3a as
The radical trapping experiment with TEMPO indicates that
this acyloxy transfer also involves intermediacy of the iminyl
radical (Scheme S1, reactions 3 and 4 in SI). Under the
condition of blue light irradiation, the iminyl radical should
derive from SET reduction of 3 (Ered for 3a = −1.5 V vs SCE
in DCE, Figure S7 in SI) by photoexcited [Cu(dpp)2]PF6
(Eox° = −1.11 V vs SCE18), as indicated by the quenching of
luminescence of [Cu(dpp)2]PF6* by 3a (Figures S5 in SI).
However, this mechanism cannot explain why the reaction also
took place in the dark. [Cu(dpp)2]PF6 at the ground state is a
poor reductant (Eox° = +0.69 V vs SCE18); it cannot reduce 3
via SET. We assume that, under the latter circumstance, the
only way to engender the iminyl radical from 3 is through
oxidative N−O insertion by [CuI] followed by homolytic N−
[CuIII]O2CC6F5 cleavage (Scheme 6). The radical trapping
experiment lent support to this mechanism (Scheme S1,
reaction 4 in SI). When the reaction of 3a was conducted in
the dark, 5a was formed in 21% yield along with the TEMPO-
trapped product (37% yield), whereas only the TEMPO-
trapped product was obtained under the condition of blue light
irradiation. The fact that 5a can still be formed in the presence
of TEMPO indicates the proximity of the C6F5CO2-[CuII]
species and iminyl radical B, which might be incorporated in
the same solvent cage. As a result, a certain amount of B was
coupled to C6F5CO2-[CuII] before they diffused away from
each other.
Scheme 4. Proposed Mechanism for the Formation 3,4-
Dihydro-2H-pyrroles
an example, the initial SET between photoexcited [Cu-
(DPEphos)(bcp)]PF6 and 3a generates iminyl radical A,
which undergoes 1,5-HAT to form carbon radical B. B is
converted to 4a via intermediacy of carbocation C (path (a)),
or through copper-mediated C−N coupling (path (b)). Both
path (a) and path (b) require the participation of [CuII]
species, and it explains why the yield of 3a was improved by
the addition of Cu(TMHD)2. That 4ad and 4ak were not
obtained is more consistent with the mechanism of path (a), as
in these two cases a carbocation-stabilizing aryl group is lacking
at the γ-position.
From Table 1 it can be seen that DABCO plays a critical
role in the reaction. Apart from its function as a base to remove
the proton from intermediate B, we assume that DABCO
might also serve as a ligand to prevent the binding of
C6F5CO2− with the [CuII] species. The copper catalyst is liable
to dissociate after being oxidized to [CuII],13c,15 and the
InCl3·4H2O was found to be necessary for the acyloxy
transfer reaction to take place. As illustrated in Scheme 6, we
believe that InCl3·4H2O facilitates the reaction by enhancing
the hydrogen abstraction capacity of the iminyl radical7a,10f as
well as by promoting hydrolysis of the HAT-derived imine.
Formation of 5ag and 5ah under these conditions confirms the
beneficial effect of InCl3·4H2O on the HAT step. In addition,
InCl3·4H2O might also play an important role for [Cu(dpp)2]-
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Scheme 5. Scope of the Pentafluorobenzoyloxy Transfer Reaction
Scheme 6. Proposed Mechanism for the Formation of γ-
Pentafluorobenzoyloxy Ketones
Experimental procedures; optimization of reaction
conditions; luminescence quenching experiment; cyclic
voltammetry measurement; characterization data; crys-
tallographic data for compound 5a (CCDC 2055827);
copies of NMR spectra (PDF)
Accession Codes
CCDC 2055827 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Fengling Bian − College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou, Gansu 730000,
China; Email: bianfl@lzu.edu.cn
Wei Yu − State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou,
Gansu 73000, China; orcid.org/0000-0002-3131-3080;
Email: yuwei@lzu.edu.cn
PF6 regeneration by preventing the carbonyl in 5 from binding
with the copper.
In summary, we have developed two new photocatalytic
protocols for the site-selective C(sp3)−H functionalization of
ketone O-acyl oximes. These methods employ copper
complexes to convert O-pentafluorobenzoyl oximes to iminyl
radicals under visible light irradiation. By using different
copper catalysts and adopting different conditions, the same
precursors can be guided to follow two different pathways to
deliver 3,4-dihydro-2H-pyrroles and γ-pentafluorobenzoyloxy
ketones, respectively. Besides their synthetic implications, the
present results demonstrate the versatility of copper complexes
as photocatalysts, which is worthy of further exploitation for
iminyl radical-mediated coupling reactions.
Authors
Danhua Wu − College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou, Gansu 730000,
China
Shuang-Shuang Cui − College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou, Gansu 730000,
China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c02133
ASSOCIATED CONTENT
* Supporting Information
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Notes
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c02133.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (No. 21772077) and State Key Laboratory of Applied
Organic Chemistry for financial support.
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