Photoredox-Catalyzed Intermolecular Remote C-H and C-C Vinylation via Iminyl Radicals

Article abstract of DOI:10.1021/acs.orglett.8b02540

A unified strategy for intermolecular remote C(sp³)-H and C-C vinylation of O-acyl oximes with vinyl boronic acids has been achieved. This strategy is enabled by photoreductive generation of iminyl radicals from O-acyl oximes under irradiation by visible light. The translocated carbon-centered radicals, which are generated from the iminyl radicals through 1,5-hydrogen atom transfer or C-C cleavage, can be vinylated with vinyl boronic acids. This strategy opens up a new approach to remote functionalization via C(sp³)-H and C-C cleavage and provides an efficient and versatile solution to the synthesis of ?-vinylation of ketones and nitriles.

Full text of DOI:10.1021/acs.orglett.8b02540

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Photoredox-Catalyzed Intermolecular Remote C−H and C−C
Vinylation via Iminyl Radicals
Xu Shen, Jia-Jia Zhao, and Shouyun Yu*
State Key Laboratory of Analytical Chemistry for Life Science, Jiangsu Key Laboratory of Advanced Organic Materials, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China
S
* Supporting Information
ABSTRACT: A unified strategy for intermolecular remote
C(sp3)−H and C−C vinylation of O-acyl oximes with vinyl
boronic acids has been achieved. This strategy is enabled by
photoreductive generation of iminyl radicals from O-acyl
oximes under irradiation by visible light. The translocated
carbon-centered radicals, which are generated from the iminyl
radicals through 1,5-hydrogen atom transfer or C−C cleavage,
can be vinylated with vinyl boronic acids. This strategy opens
up a new approach to remote functionalization via C(sp3)−H
and C−C cleavage and provides an efficient and versatile
solution to the synthesis of γ-vinylation of ketones and nitriles.
liphatic C−H and C−C bonds are ubiquitous in organic
molecules and are chemically inert.1,2 Consequently, C−H
alkenes in pharmaceuticals and natural products, we sought to
develop a unified strategy for intermolecular remote C(sp3)−H
and C−C vinylation of O-acyl oximes with vinyl boronic acids
promoted by visible light (Figure 1). This strategy could provide
an efficient and versatile means for the synthesis of γ-
functionalization of ketones and nitriles.
A
and C−C functionalization is a long sought after goal in organic
synthesis, but it remains a formidable synthetic challenge due to
issues of selectivity and activity.3
As pioneered by Forrester4 and Zard,5 iminyl radicals are
found to be valuable intermediates for diverse chemical
transformations.6 Oxidants, radical initiators, and transition
metal catalysts are often used as promoters for the generation of
iminyl radicals.7 Recently, visible light photoredox catalysis has
emerged as a powerful method for the generation of iminyl
radicals.8 As a result of these mild conditions, C−H and C−C
functionalization mediated by iminyl radicals has become
feasible.9−14 We reported that iminyl radicals can be generated
from O-acyl oximes under photoredox catalysis and that
subsequent intramolecular homolytic aromatic substitution
(HAS) leads to intramolecular C(sp2)−H iminylation.9 It was
found that the iminyl radicals can undergo 1,5-hydrogen atom
transfer (HAT) followed by intramolecular HAS to achieve
intramolecular C(sp3)−H arylation.4,10 In these two reports,
iminyl radicals were produced by reductive single electron
transfer (SET) through an oxidative quenching cycle of the
photocatalyst. Studer11 and Leonori12 reported that photo-
redox-induced single-electron oxidation of α-imino-oxy acid
derivatives leads to iminyl radicals. The translocated carbon-
centered radicals (C-radicals) generated from the iminyl radicals
through 1,5-HAT can be further alkylated and halogenated,
respectively. Groups led by Leonori12a and Xiao13 described
photoredox-induced C−C cleavage mediated by iminyl radicals.
The resultant C-radicals could be further functionalized with
Selectfluor and activated alkenes, respectively. These results
show that iminyl radicals can serve as a platform for C−H and
C−C functionalization. In view of the importance of remote
C(sp3)−H and C−C functionalization and the ubiquity of
Figure 1. Unified strategy for remote C(sp3)−H and C−C vinylation
via iminyl radicals. PC = photocatalyst.
Our strategy is shown in Figure 2. For the C−H vinylation, the
catalytic cycle begins with the visible-light promoted photo-
excitation of a photocatalyst (IrIII) to its excited state (IrIII*).15
SET reductive cleavage of an acyl oxime (I) assisted by PC*
produces an iminyl radical (V), an acyl anion (ArCO2−), and an
oxidized-state photocatalyst (IrIV).9,10 1,5-HAT15 from the
distal C−H bond to the iminyl radical provides C-radical
(VI), which is intermolecularly trapped by a vinyl boronic acid
(II) giving the corresponding radical (VII). This radical (VII) is
then oxidized by IrIV, delivering a cationic intermediate (VIII)
Received: August 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02540
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Figure 2. Mechanism for intermolecular remote C(sp3)−H and C−C vinylation.
and regenerating the photocatalyst (IrIII). Ultimately, deboro-
nation16 followed by hydrolysis of VIII yields the remote
C(sp3)−H vinylation product (III). Likewise, C-radical (VI′)
can be generated from iminyl radical (V′) through C−C
cleavage. C-Radical (VI′) then follows a similar pathway as C-
radical (VI) to give C−C vinylation product (IV).
Our initial investigation focused on the intermolecular remote
C(sp3)−H vinylation of the O-acyl oxime 1a with (E)-
styrylboronic acid (2a). Investigation of the reaction parameters
showed that remote C−H vinylation of 1a by 2a could be
achieved in good isolated yield (for details, see the Supporting
Information (SI)). When a mixture of 1a (1 equiv) and 2a (3
equiv) in 1,2-dichloroethane (DCE) was irradiated with blue
LED strips in the presence of 0.5 mol % Ir(ppy)3 as the
photocatalyst at 22 °C, the desired product (3a) was isolated in
77% yield with a 20:1 E/Z ratio (Table 1, entry 1). Control
experiments revealed that photocatalyst and light were necessary
for the reaction to proceed (entry 2) and increased loading of
the photocatalyst led to lower E/Z ratios (entries 3 and 4). A
ruthenium-based photocatalyst was not suitable in this reaction
(entry 5). Acetonitrile and dichloromethane were suitable as
solvents but gave lower yields (entries 6 and 7). Reduced levels
of boronic acid (2a) led to lower yields and reduced
stereoselectivity (entry 8). The reaction is sensitive to
temperature, and when it was carried out at 30 °C, the yield
decreased to 58%.
We next evaluated the generality of this photoredox-catalyzed
remote C(sp3)−H vinylation reaction. The scope of the reaction
of O-acyl oximes with (E)-styrylboronic acid (2a) was examined
(Scheme 1). O-Acyl oximes with tertiary γ-C−H bonds were
well tolerated, giving the γ-vinyl ketones (3a−3f) with good
yields and E/Z ratios (Scheme 1a). Unactivated secondary C−H
bonds could be vinylated, but with low yields (e.g., 21% for 3g).
Substrates bearing activated secondary C−H bonds (benzylic
bonds or bonds adjacent to oxygen) functioned well in the
reaction (3h−3j) (Scheme 1b). Primary C−H bonds remained
a challenge and only moderate yields (27% for 3k and 42% for
3l) could be achieved (Scheme 1c). We next varied the vinyl
boronic acids using an O-acyl oxime (1a) as the substrate
(Scheme 1d). Various substituted styrylboronic acids showed
good reactivity, and the corresponding γ-vinyl ketones (3m−3t)
were obtained in up to 78% yield. Para-substituted styrylboronic
acids with electron-neutral (Me, t-Bu), -rich (MeO), and
-deficient (F, Br) groups underwent the transformation
smoothly (53−78% yields). Meta-substituted styrylboronic
acids were also suitable, and the desired products 3r and 3s
were isolated in 61% and 56% yields, respectively. Ortho-
substituted styrylboronic acid gave the γ-vinyl ketone (3t) in
only 34% yield. The reaction could be scaled up to the 2 mmol
scale giving comparable isolated yield (77% for 0.2 mmol vs 71%
for 2 mmol).
Encouraged by the successful results in the case of remote
C(sp3)−H vinylation, we next sought to explore the possibility
of photoredox-catalyzed remote C−C vinylation of cyclic O-acyl
oximes with vinyl boronic acids. After a quick screening (for the
condition optimization, see the SI), the reaction of cyclo-
butanone O-4-CF3-benzoyloxime (4a) with (E)-styrylboronic
acid (2a) was achieved with 77% isolated yield and a 20/1 E/Z
ratio for nitrile 5a in DMSO using Ir(ppy)3 as the photocatalyst
irradiated by blue LEDs. After determining the optimized
reaction conditions, the synthetic potential of this remote C−C
vinylation was investigated, and the results are summarized in
a
Table 1. Variation of Reaction Parameters
b
c
entry
deviation from standard conditions
yield (%)
E/Z
1
2
3
4
5
6
7
8
9
none
77
0
20/1
−
17/1
14/1
−
>20/1
>20/1
11/1
20/1
w/o photocatalyst or light
1 mol % Ir(ppy)3
2 mol % Ir(ppy)3
Ru(phen)3(PF6)2, instead of Ir(ppy)3
CH3CN, instead of DCE
CH2Cl2, instead of DCE
1a/2a = 1/2
82
75
trace
64
71
64
58
30 °C, instead of 22 °C
a
Standard conditions A: A mixture of 1a (0.2 mmol, 1.0 equiv), 2a
(0.6 mmol, 3.0 equiv), and Ir(ppy)3 (0.001 mmol, 0.5 mol %) in DCE
(2.0 mL) was irradiated by blue LED strips at 22 °C for 24 h,
b
c
followed by aqueous workup. Isolated yield. Determined by 1H
NMR analysis.
B
DOI: 10.1021/acs.orglett.8b02540
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a
a
Scheme 1. Scope of Remote C(sp3)−H Vinylation
Scheme 2. Scope of Remote C−C Vinylation
a
Standard conditions A: A mixture of 1 (0.2 mmol, 1.0 equiv), 2 (0.6
mmol, 3.0 equiv), and Ir(ppy)3 (0.001 mmol, 0.5 mol %) in DCE (2.0
mL) was irradiated by blue LED strips at 22 °C for 13−48 h, followed
by aqueous workup. Isolated yield. E/Z ratios were determined by 1H
a
Standard conditions B: A mixture of 4 (0.2 mmol, 1.0 equiv), 2a (0.6
mmol, 3.0 equiv), Na2HPO4 (0.4 mmol, 2.0 equiv), and Ir(ppy)3
(0.001 mmol, 0.5 mol %) in DMSO (2.0 mL) was irradiated by blue
LED strips at 22 °C for 9−52 h, followed by aqueous workup.
Isolated yield.
b
NMR analysis. Two mmol scale (1a).
Scheme 2. The scope of the reaction was explored with respect
to cyclic O-acyl oximes (Scheme 2a). A variety of cyclo-
butanone-derived O-acyl oximes bearing different functional
groups at the C3-position of the cyclobutanone reacted with 2a
to give the corresponding nitriles 5b−5f in a range of 71−87%
yields. The piperidine derivative also showed comparable
reactivity and gave the product 5g in 83% yield. Furthermore,
both 1-Boc-3-azetidinone and oxetan-3-one derived O-acyl
oximes participated in this reaction very well to deliver products
5h and 5i with good yields. α-Branched cyclobutanone-derived
O-acyl oximes delivered nitriles 5j and 5k in 83% and 71% yields
in which C−C bond cleavage occurred selectively at the more
substituted position. In addition, less-strained O-acyl oximes
derived from five-membered cyclic ketones could also
participate in this transformation well to give the desired
products 5i and 5m in moderate yields. Notably, benzocyclo-
butenone derivatives worked well, forming the benzonitrile
product 5n in 71% yield with E/Z = 14/1. To further explore the
synthetic potential of this C−C vinylation reaction, a series of
(E)-styrylboronic acid derivatives (2) were examined. As
highlighted in Scheme 2b, para-substituted styrylboronic acids
on the phenyl ring were tolerated, giving the corresponding
nitriles (5o−5v) in range of 43−78% yields. Meta-substituted
styrylboronic acids (such as Me, Br, and Cl) also gave the
desired products in 55−68% yields. ortho-Cl styrylboronic acid
proceeded smoothly to give the target product 5z in 49% yield.
Cyclopentanone and cyclohexanone-derived O-acyl oximes
failed to give the corresponding ring opening products under
our conditions.
In order to investigate the chemoselectivity of this remote C−
H and C−C vinylation reaction, O-acyl oximes with several
reactive sites were synthesized as the probe molecules. When the
aryl ketone-derived O-acyl oxime 4o was reacted under the
standard C−H vinylation conditions A, no remote C−H
vinylation product (3u) was isolated, and instead, the intra-
molecular HAS product (3u′) was obtained in 61% yield
(Scheme 3a).10 This phenomenon suggests that intramolecular
HAS is faster than intermolecular vinylation under our
established conditions. O-Acyl oxime 4p was then subjected
C
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a
Scheme 3. Chemoselectivity Investigation
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a
For standard conditions A and B, see footnotes of Schemes 1 and 2.
Isolated yield. Ar = 4-CF3Ph.
into C−H (conditions A) and C−C (conditions B) vinylation
conditions. Only C−C vinylation product 6 was isolated under
both conditions, and no C−H vinylation product 3v was
observed (Scheme 3b). Furthermore, proline-derived O-acyl
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(Scheme 3c). These experiments imply that C−C bond cleavage
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boronic acids. These reactions are enabled by iminyl radical-
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applicable to a wide range of O-acyl oximes and vinyl boronic
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C−C functionalization and is currently in progress in our
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02540.
Experimental details, NMR spectra, and details of
experiments (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yushouyun@nju.edu.cn.
ORCID
Shouyun Yu: 0000-0003-4292-4714
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science Founda-
tion of China (21472084, 21672098, and 21732003) and the
Fundamental Research Funds for the Central Universities
(020514380131 and 020514380092) is acknowledged.
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