Photocatalytic decarboxylative [2 + 2 + m] cyclization of 1,7-enynes mediated by tricyclohexylphosphine and potassium iodide

Article abstract of DOI:10.1021/acs.orglett.0c03182

A novel photocatalytic decarboxylative [2 + 2 + m] cyclization of 1,7-enynes with alkyl N-hydroxyphthalimide (NHP) esters, using tricyclohexylphosphine and potassium iodide as redox catalysts, is reported for the construction of functional polycyclic compounds. This protocol tolerates primary, secondary, and tertiary alkyl NHP esters through a single reaction via decarbonylation, radical addition, C-H functionalization, and cyclization under mild conditions.

Full text of DOI:10.1021/acs.orglett.0c03182

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Letter
Photocatalytic Decarboxylative [2 + 2 + m] Cyclization of 1,7-Enynes
Mediated by Tricyclohexylphosphine and Potassium Iodide
Hui-Yuan Liu,∥ Yuan Lu,∥ Yang Li,* and Jin-Heng Li*
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ABSTRACT: A novel photocatalytic decarboxylative [2 + 2 + m] cyclization of 1,7-enynes with alkyl N-hydroxyphthalimide (NHP)
esters, using tricyclohexylphosphine and potassium iodide as redox catalysts, is reported for the construction of functional polycyclic
compounds. This protocol tolerates primary, secondary, and tertiary alkyl NHP esters through a single reaction via decarbonylation,
radical addition, C−H functionalization, and cyclization under mild conditions.
Scheme 1. Photocatalytic Decarboxylative Cyclization of
1,n-Enynes with N-Hydroxyphthalimide Esters
n recent years, radical-triggered cyclization of 1,n-enynes
Ihas been developed as the widely used strategy for
preparing polycyclic compounds.1−5 Generally, these radical-
triggered cyclizations require oxidants, such as peroxides,2 tert-
butyl nitrite,3 pyrosulfates,4 and so on,5 which enable the
generation of various radicals by the oxidation of the
corresponding bonds. Despite impressive progress in this
field, some disadvantages such as the necessity for stoichio-
metric oxidants and elevated temperatures, limit the
application of this strategy in large-scale industrial processes.
Therefore, the development of mild synthetic strategies for the
radical-triggered cyclization of 1,n-enynes is highly desirable.
Visible-light photocatalysis has been demonstrated as a
powerful method for preparing functional compounds, because
of its unique catalytic mechanisms and environmental
friendliness.6−8 In 2016, Xia’s group and ours simultaneously
reported the visible-light photocatalytic cascade alkylations of
α-carbonyl alkyl bromides as one-carbon units for the synthesis
of carbocyclic and heterocyclic frameworks.8a,b In 2018, the
group of Jiang, Tu, and Hao described an analogous cyclization
for the construction of syn-fluoren-9-ones.8c In addition to α-
bromides, alkyl NHP esters also proved to be efficient, with
regard to C-centered radical precursors in the photocatalytic
catalysts and have limited scope. This inspired us to establish
a new strategy for the photocatalytic decarboxylative
cyclization of 1,n-enynes.
Herein, we report a novel photocatalytic decarboxylative [2
+ 2 + m] cyclization of 1,7-enynes with alkyl NHP esters in
combination with KI and PCy3 for accessing functional
cyclization of 1,n-enynes. In 2019, Paixao and co-workers
̃
developed a photoredox decarboxylative [2 + 2 + 1] cyclization
of 1,7-enynes with alkyl NHP esters in the presence of
Ir(ppy)2(dtbbpy)PF6 and pyridine (Scheme 1a).8d Soon after,
the groups of Hu and Xu reported a similar decarboxylative [2
+ 2 + 1] cyclization of 1,6-enynes with alkyl NHP esters using
fac-Ir(ppy)3 as the photocatalyst and trifluoroacetic acid as the
additive (Scheme 1a).8e Although these two methods serve as a
straightforward approach for preparing complex polycyclic
compounds, both of these methods require noble-metal
Received: September 22, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03182
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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Letter
polycycles (see Scheme 1b). Compared with previous studies
for the radical cyclization of 1,n-enynes, this strategy has three
significant advantages. First, the use of KI and PCy3 as co-
catalysts, instead of oxidants or noble metals, facilitates a low-
cost and environmentally friendly approach. Second, different
primary, secondary, and tertiary alkyl NHP esters including
derived those from α-amino acids, as one or two carbon units
can be used for the cyclization of 1,n-enynes, thereby
highlighting the broad substrate scope of this strategy. Finally,
the novel reaction mechanism involves the assembly of KI and
PCy3 with alkyl N-hydroxyphthalimides to form a chromo-
phore capable of absorbing visible light in the blue range. In
addition, the photoirradiated combination of iodide and
phosphine has been developed by Shang and Fu for
decarboxylative alkylations;9c however, this approach has not
yet been reported for preparing cyclic compounds, particularly
functional polycycles.
8−10). Performing the reaction in other polar aprotic solvents
such as acetone, MeCN, DMF and DMA resulted in a lower
yield than in DMSO (Table 1, entries 11−14). Gratifyingly,
scaling the reaction up to 1 mol of enynes 1a gave the desired
product 3aa in 71% yield within 36 h (Table 1, entry 15).
Under the optimized reaction conditions, we investigated
the substrate scope of this reaction by using various 1,7-enynes
1 and NHP esters 2 (see Schemes 2 and 3). Replacement of N-
a
Scheme 2. Variation of the 1,7-Enynes (1)
Initially, N-methyl-N-(2-(phenylethynyl)phenyl)-methacry-
lamide 1a and tert-butyl substituted NHP ester 2a were
chosen for the optimization of the reaction for the photo-
catalytic decarboxylative reaction (see Table 1). A mixture of
a
Table 1. Variation of Reaction Parameters
entry
variation from the standard conditions
none
without KI
yield (%)
1
2
75
0
3
without PCy3
0
4
without blue LEDs
0
5
6
7
8
NaI instead of KI
LiI instead of KI
65
51
<5
60
36
<5
54
45
60
70
71
KF, KCl, KBr instead of KI
PPh3 instead of PCy3
tricyclopentylphosphine instead of PCy3
tris(perfluorophenyl)phosphine instead of PCy3
acetone instead of DMSO
MeCN instead of DMSO
DMF instead of DMSO
DMA instead of DMSO
none
a
Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol; 2 equiv), KI (1.5
9
equiv), PCy3 (20 mol %), DMSO (2 mL), rt, argon, and 16 h.
10
11
12
13
14
Me by N-H or N-Bn resulted in good reactivity and afforded
the desired products (3ba−3ca) in 61%−70% yield. Even the
strong electron-withdrawing N-Ts group was isolated in
moderate yield. Several electron-donating and electron-with-
drawing substituentsnamely, Me, MeO, Cl, and CF3on
the aromatic ring of the aniline moiety allowed for smooth
conversion to the desired products (3ea−3ia). It was found
that the presence of electron-donating substituents resulted in
higher reactivity, compared to electron-withdrawing substitu-
ents. Next, we examined various aryl groups at the alkyne
terminus, such as 4-MeC6H4, 4-CNC6H4, 3-MeOC6H4, and 3-
BrC6H4. The presence of these groups on various positions of
the aryl ring did not have a detrimental effect on the reaction
(3ja−3ma). Heterocyclic enynes 1n and 1o were successfully
converted to 3na and 3oa. Notably, aliphatic alkyne 1p was
converted to 3pa in 62% yield. Interestingly, when a Ph group
was located at the α-position of the CC bond, the reaction
selectivity of 1,7-enyne 1q shifted toward the functionalization
of the phenyl C(sp2)−H bond to deliver 3qa.
b
15
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol; 2 equiv), KI (1.5
b
equiv), PCy3 (20 mol %), DMSO (2 mL), rt, argon, and 16 h. 1a (1
mmol) for 36 h.
1a (0.2 mmol) with 2 equiv of 2a, using 20 mol % PCy3 and
1.5 equiv of KI under blue LEDs in dimethylsulfoxide
(DMSO) for 16 h at 25 °C, delivered 3aa in 75% isolated
yield (Table 1, entry 1). In contrast to the optimal conditions,
the reaction did not occur in the absence of iodide or
phosphine or irradiation (entries 2−4). Replacement of KI
with NaI or NaLi resulted in high catalytic activity, but the
reaction was less efficient (Table 1, entries 5 and 6), while the
use of KF, KCl, and KBr (halide anion screening) led to a
complete loss of reactivity (Table 1, entry 7). PCy3 was found
to be critical for stabilizing the iodine radical as an R3P−I•
species. Other phosphines, such as PPh3 or tricyclopentyl-
phosphine, afforded low yields, and tris(perfluorophenyl)-
phosphine was found to be entirely ineffective (Table 1, entries
Next, we explored the reaction scope for alkyl NHP esters.
This mild protocol was suitable for primary, secondary, and
tertiary alkyl NHP esters, including derived those from α-
amino acids. First, primary NHP esters bearing alkyne and
B
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On the basis of control experiments and literature survey,9
we proposed a plausible mechanism for this photocatalytic
reaction. The first step involves the generation of chromophore
I by the assembly of tricyclohexylphosphine, potassium iodide,
and alkyl NHP esters 2 in DMSO. Then, chromophore I
absorbs visible light in the blue range to induce electron
transfer from iodide to alkyl NHP esters 2 to form a tert-butyl
radical, CO2, and a phthalimide anion. PCy3 stabilizes the
iodine radical by forming Cy3P−I• species II. The addition of
the radical across the CC bond in 1,7-enyne 1a affords
radical intermediate A, followed by 6-exo-dig cyclization to
form vinyl radical intermediate B. Subsequently, vinyl radical
intermediate B undergoes an intramolecular 1,6-H atom shift
and cyclization leading to radical intermediate D, which is
directly oxidized by Cy3P−I• species II to produce
intermediate E. After a deprotonation process, intermediate
E is transformed to 3aa. (See Scheme 5.)
a
Scheme 3. Variation of NHP Esters (2)
Scheme 5. Possible Mechanism
a
Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol; 2 equiv), KI (1.5
equiv), PCy3 (20 mol %), DMSO (2 mL), rt, argon, and 16 h.
alkene groups exclusively furnished polycycles (3ab−3ac), as
the reaction proceeded with complete chemoselectivity.
Notably, the NHPs 2d−2e derived from amino acids, reacted
smoothly to afford the expected products (3ad−3ae). A range
of cyclic moietiesincluding cyclopentyl, cyclohexyl, and
cycloheptylwere compatible and underwent conversion to
the corresponding spiro-tetracyclic compounds (3af−3ah) in
76%−88% yield. Furthermore, the less-bulky acyclic ester 2i
successfully delivered 3ai in 80% yield. To our delight, several
alkanoic esters could be used as tertiary carbon-centered
radicals to synthesize six-membered carbocycle-fused poly-
cycles in moderate yield (3aj−3al).
We performed some control reactions, using 1,7-enyne 1a
and NHP ester 2a with several radical inhibitors, including
TEMPO, hydroquinone and BHT, resulting in complete
suppression of the reaction (eq 1 in Scheme 4). In addition,
the radical-clock experiment was conducted for the reaction
between 1,7-enyne 1a and cyclopropyl NHP ester 2m, giving
ring-opening product 3am (eq 2 in Scheme 4), which supports
the radical process.
In summary, an efficient photocatalytic decarboxylative
cyclization of 1,7-enynes with alkyl NHP esters was developed
for preparing carbocycle-fused polycycles. The protocol allows
for a broad substrate scope through a one-pot strategy by using
a combination of tricyclohexylphosphine and potassium iodide.
Further synthetic applications of this environmentally benign
process for the assembly of functional polycycles are currently
being investigated in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03182.
Scheme 4. Control Experiments
Experimental details, NMR spectra, and details of the
experiments (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Yang Li − Key Laboratory of Jiangxi Province for Persistent
Pollutant Control and Resource Recycling, Nanchang Hangkong
University, Nanchang 330063, China;
Email: liyang8825490@126.com
Jin-Heng Li − Key Laboratory of Jiangxi Province for Persistent
Pollutant Control and Resource Recycling, Nanchang Hangkong
University, Nanchang 330063, China; State Key Laboratory of
Applied Organic Chemistry, Lanzhou University, Lanzhou
C
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730000, China; orcid.org/0000-0001-7215-7152;
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Email: jhli@hnu.edu.cn
Authors
Hui-Yuan Liu − Key Laboratory of Jiangxi Province for
Persistent Pollutant Control and Resource Recycling, Nanchang
Hangkong University, Nanchang 330063, China
Yuan Lu − Key Laboratory of Jiangxi Province for Persistent
Pollutant Control and Resource Recycling, Nanchang Hangkong
University, Nanchang 330063, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03182
Author Contributions
∥These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Nos. 21625203, 21871126, and 22001108), Jiangxi Educa-
tional Committee Foundation of China (No. GJJ190535), and
Jiangxi Province Science and Technology Project (No.
20202BAB213004) for financial support.
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