Cascade Photoredox/Iodide Catalysis: Access to Difluoro-γ-lactams via Aminodifluoroalkylation of Alkenes

Article abstract of DOI:10.1021/acs.orglett.6b01515

The novel cascade photoredox/iodide catalytic system enables the alkene to serve as a radical acceptor capable of achieving aminodifluoroalkylation of alkenes. Cheap iodide salts play a vital role in this reaction, which could tune carbocation reactivity

Full text of DOI:10.1021/acs.orglett.6b01515

Letter
pubs.acs.org/OrgLett
Cascade Photoredox/Iodide Catalysis: Access to Difluoro-γ-lactams
via Aminodifluoroalkylation of Alkenes
Muliang Zhang,† Weipeng Li,† Yingqian Duan,† Pan Xu,† Songlin Zhang,† and Chengjian Zhu*,†,‡
†State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: The novel cascade photoredox/iodide catalytic system enables the alkene to serve as a radical acceptor capable of
achieving aminodifluoroalkylation of alkenes. Cheap iodide salts play a vital role in this reaction, which could tune carbocation
reactivity through reversible C−I bond formation for controlling reaction selectivity, and a series of competitive reactions are
completely eliminated in the presence of multiple reactivity pathways. The present dual catalytic protocol affords a very
convenient method for direct synthesis of various difluoro-γ-lactams from simple and readily available starting materials under
mild reaction conditions.
iven the great significance of the difluoromethylene group
(CF2) in drug discovery, medicinal chemistry, and life
reaction energy barrier is higher than others in the presence of
multiple reaction paths. As a proof of concept, we herein
describe photoredox/iodide cascade catalysis reaction that
comprises aminodifluoroalkylation of alkenes to difluoro-γ-
lactams, which are commonly present in natural products and
pharmacological compounds and largely improve a broad range
of biological activities.7
G
science,1 the development of a new synthetic approach for the
assembly of fluorinated small compounds has become a hot
area of chemical research, and increasingly general and practical
methods are now accessible for these transformations.2 Among
the various developed routes thus far, the addition of a CF2
radical to a π acceptor (alkene, alkyne, arene, isocyanides,
imines) is considered a useful and economical synthesis
strategy.3,4 However, most difunctionalization-type difluoroal-
kylation of alkenes with bromodifluoro reagents is still limited
to the modes of hydrogen atom abstraction, halogen atom
attack, and deprotonation (Scheme 1).5 This is due mainly to
The greatest challenge that requires circumvention for the
validation of the concept is how to overcome these developed
reaction pathways as shown in Scheme 1. Recently, dual
catalysis realized by merging visible-light-induced photoredox
catalysis with other catalytic modes has been found to be
attractive, and it garners great interest and concern from
chemists.8 These dual-catalytic strategies have been widely used
to develop various valuable chemical transformations, which are
not easily available, if not impossible, by a single catalytic
system. In spite of these impressive advances, further
exploration of combination of photoredox catalysis with other
catalytic forms to search for other useful chemical reactions,
especially in carbon−heteroatom bond formation, remains a
great challenge. It is well-known that iodide salts are an
inexpensive and readily available reagents and could be usually
applied as the nucleophilic catalyst in carbon−heteroatom bond
formation reactions for iodide ion properties of nucleophilic
and leaving performance.9 We envisaged that reversible C−I
bond formation might represent another promising approach
for tuning reactivity of carbocation produced by a CF2 radical
Scheme 1. Developed Difunctionalization-Type
Difluoroalkylation of Alkenes with Bromodifluoro Reagents
the fact that the highly reactive and unstable radical or
carbocation intermediates make controlling reaction selectivity
very challenging in organic transformations.6 Consequently, it is
a desirable, yet difficult, task to continue to explore new
catalytic strategy to tune the reactive intermediates for
achieving the target reaction, in particular, of which the
Received: May 26, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01515
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
added to an olefinic substrate under visible-light photoredox
catalysis conditions, and other alternative reaction ways were
suppressed throughout the course of the reaction (Scheme 2).
Table 1. Optimizing Reaction Conditions
Scheme 2. Cascade Photoredox/Iodide Catalysis for
Aminodifluoroalkylation of Alkenes
e
e
b
entry
photocat
halide
base
solvent
yield (%)
1
2
A
A
A
B
C
A
A
A
A
A
A
A
A
A
NaI
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaHCO3
k2CO3
CH3CN
CH2Cl2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
52
NaI
trace
73
3
NaI
4
NaI
45
5
NaI
36
6
NaI
79
7
NaI
60
8
NaI
Na2HPO4
NaOAc
Et3N
40
9
NaI
87
To validate the feasibility of a photoredox/iodide cascade
catalytic system, the readily accessible bromodifluoroacetamide
1a was chosen as the model substrate. As a preliminary
experiment, 1a was reacted with 2 equiv of styrene (2b) in the
presence of 2 equiv of sodium carbonate, the photocatalyst fac-
[Ir(ppy)3], and iodide catalyst sodium iodide at room
temperature, we observed the formation of the desired in
degassed CH3CN (0.1 M) under irradiation with visible light
from blue LEDs (5 W, λmax = 455 nm). Delightfully, after 36 h,
product 3ab, with a moderate yield of 52% (Table 1, entry 1),
showed that the novel catalytic strategy worked well. Solvent
screening showed that strong polar solvents such as DMF
provided better results for this reaction and the reaction could
not occur with a nonpolar solvent (entries 2 and 3). The
catalysts [Ir(ppy)2(dtbpy)3]PF6 and [Ru(bpy)3]PF6 showed
inferior reactivity toward the conversion of 1a (entries 4 and 5).
It was observed that the use of inorganic bases such as
NaHCO3, K2CO3, and Na2HPO4 promoted the reactions,
NaOAc gave the best result, and the use of organic base (Et3N)
completely inhibited the reaction (entries 6−10). In this
system, the amount of styrene (2b) employed could be further
reduced to 1.5 equiv without lowering the yield (86%, entry
11). Importantly, when sodium iodide was not added to the
reaction system or sodium bromide was added, the reaction
occurred in a state of complete confusion and the desired
product 3ab was not detected (entries 12−14). It can be seen
that added iodine anion exerts a powerful catalytic effect on the
reaction. Finally, control experiments demonstrated that the
photocatalyst fac-[Ir(ppy)3], and visible light were all important
(entries 15 and 16).
With the optimized conditions in hand, we explored the
scope of the bromodifluoroacetamide, and the representative
examples are shown in Scheme 3. This cascade catalytic strategy
has good substrate compatibility and showed excellent
chemoselectivity. Bromodifluoroacetamide bearing either elec-
tron-donating (methyl, methoxy) or electron-withdrawing
groups such as halide substituents furnished the corresponding
products in moderate to good yields (3aa−fa). The position of
the substituents on the phenyl ring has no obvious influence on
this reaction. Ortho-substituted (methyl, phenyl) derivatives
also worked well, providing the desired products with good
yields, which seems to have a lower steric effect (3ma−oa).
Substrates with more substituents on the phenyl ring were
used, and they all underwent the aminodifluoroalkylation
reaction to yield difluoro-γ-lactam products in 50−65% yield
(3pa−ta). However, when we used the bromodifluoroaceta-
10
NaI
c
11
NaI
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
86
12
13
14
15
NaBr
nBu4NI
trace
73
NaI
NaI
d
16
A
a
The reactions were carried out with 1a (0.2 mmol), 2b (0.4 mmol,
2.0 equiv), base (0.4 mmol, 2.0 equiv), photocatalyst (0.004 mmol, 2
mol %), halidecatalyst (0.04 mmol, 20 mol %), solvent (2 mL), at
room temperature, 5 W LEDs, 36 h. isolated yields. 1.5 equiv 2b was
used. In dark. DMF = N,N-dimethylformamide, CH3CN =
acetonitrile, CH2Cl2 = dichloromethane.
b
c
d
e
mide of an aliphatic substituent with replacement of the phenyl
ring, the desired product could not be obtained (3ua),
presumably owing to the reduced nucleophilicity of nitrogen.
Furthermore, various alkenes were studied in Scheme 4. The
presence of an electron-donating (methyl or tert-butyl)
electron-withdrawing (halide or methyl formate) substituent
on the benzene ring was well tolerated (3ab−ah). Nonterminal
olefins including β-methylstyrene, indene, and 1,2-dialin could
react smoothly, delivering the corresponding products 3al−an,
respectively. The vinyl ether compound can also be applied to
the catalytic system (3ao).
To gain mechanistic insight into the cascade photoredox/
iodide catalysis for aminodifluoroalkylation of alkenes, the
radical scavenger TEMPO (2,2,6,6-tetramethyl-1-piperidiny-
loxy) was introduced into the photochemical system, and the
reaction was completely restrained under these conditions,
indicating that a single-electron-transfer radical process is
operating (Scheme 5a). When we did not add base to this
photoredox/iodide cascade catalytic system, interestingly, the
adduct 5a was formed as estimated by ESI-HRMS analysis (SI),
which implied 5a may be the reaction intermediate (Scheme
5b). Additionally, we also prepared 2,2-difluoro-2-iodo-N-
phenylacetamide 4a and replaced 2-bromo-2,2-difluoro-N-
phenylacetamide 1a as a CF2 radical source. The desired
B
DOI: 10.1021/acs.orglett.6b01515
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Organic Letters
Letter
Scheme 3. Scope of Bromodifluoroacetamides in the
Aminodifluoroalkylation reaction.
Scheme 4. Scope of Alkenes in the Aminodifluoroalkylation
Reaction
a
a
a
Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol, 1.5 equiv), NaOAc
(0.4 mmol, 2 equiv), fac-Ir(ppy)3 (0.004 mmol, 2 mol %), NaI (0.04
mmol, 20 mol %), DMF (2 mL), at room temperature, 5 W LEDs,
36−48 h.
Scheme 5. Mechanistic Investigations of
Aminodifluoroalkylation of Alkenes
a
Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol, 1.5 equiv), NaOAc
(0.4 mmol, 2 equiv), fac-Ir(ppy)3 (0.004 mmol, 2 mol %), NaI (0.04
mmol, 20 mol %), DMF (2 mL), at room temperature, 5 W LEDs,
30−48 h.
product 3ab was obtained in the absence of iodide catalyst in
78% yield (Scheme 5c). This provided straightforward evidence
of the reaction process through the intermediate 5a.
On the basis of the experimental results and literature,3j,4d
a
possible mechanism is shown in Scheme 6. The photocatalyst
fac-[Ir3+(ppy)3] 1 undergoes a metal-to-ligand charge-transfer
(MLCT) process under visible-light irradiation to transfer from
this Ir species to 3 to generate the CF2 radical precursor 4 and
the Ir intermediate Ir4+ (+0.77 V vs SCE in CH3CN). The
electrophilic radical added to alkenes to form intermediate 6,
which was oxidized to the β-difluoroalkylated carbocation
intermediate 7 by the Ir intermediate 5 followed by rapid iodide
ion nucleophilic trapping. The nitrogen of the amide as a
nucleophile performs intramolecular nucleophilic attack to
form the intermediate 9, and the iodide anion could be
regenerated. Finally, difluoro-γ-lactam 10 was produced by
deprotonation of the nitrogen in the presence of base.
Scheme 6. Proposed Mechanism
In conclusion, we have developed a novel photoredox/iodide
cascade catalytic system for achieving aminodifluoroalkylation
of alkenes. The use of iodide catalyst is the key to achieve
specific selectivity for obtaining various α,α-difluorinated γ-
lactams in the presence of multiple reaction pathways. We
anticipate that this dual-catalytic protocol will be applied to the
development of other reactions to achieve more valuable
transformations. Further exploration of this photoredox/iodide
C
DOI: 10.1021/acs.orglett.6b01515
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Organic Letters
Letter
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterization data. The Supporting
Information is available free of charge on the ACS Publications
Web site. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.orglett.6b01515.
Experiemental procdures (PDF)
1H, 13C, and 19F NMR spectra (PDF)
Additional 1H, 13C, and 19F NMR spectra and ESI-
HRMS spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: cjzhu@nju.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Natural Science
Foundation of China (21474048 and 21372114).
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