Organic Photocatalytic Cyclization of Polyenes: A Visible-Light-Mediated Radical Cascade Approach

Article abstract of DOI:10.1002/chem.201503118

A visible-light-mediated, organic photocatalytic stereoselective radical cascade cyclization of polyprenoids is described. The desired cascade cyclization products are achieved in good yields and high stereoselectivities with eosinY as photocatalyst in hexafluoro-2-propanol. The catalyst system is also suitable for 1,3-dicarbonyl compounds, which require only catalytic amounts of LiBr to promote the formation of the corresponding enols.

Full text of DOI:10.1002/chem.201503118

DOI: 10.1002/chem.201503118
Communication
&
Photocatalysis
Organic Photocatalytic Cyclization of Polyenes: A Visible-Light-
Mediated Radical Cascade Approach
Zhongbo Yang,[a, c] Han Li,[b] Long Zhang,[a, c] Ming-Tian Zhang,*[b] Jin-Pei Cheng,[b] and
Sanzhong Luo*[a, c]
velopment of mild and catalytic methodologies for radical cyc-
Abstract: A visible-light-mediated, organic photocatalytic
stereoselective radical cascade cyclization of polyprenoids
lization of polyprenes remains highly desirable.
Simple alkenes are known to undergo facile functionalization
is described. The desired cascade cyclization products are
through photoinduced electron transfer (PET) and, in recent
achieved in good yields and high stereoselectivities with
years, elegant visible-light-mediated processes have been ach-
eosin Y as photocatalyst in hexafluoro-2-propanol. The cat-
ieved.[15] However, the extension of the PET process to a poly-
alyst system is also suitable for 1,3-dicarbonyl compounds,
ene cyclization seems non-trivial due to the easily conceived
which require only catalytic amounts of LiBr to promote
nucleophilic interception of the radical cation intermediate,
the formation of the corresponding enols.
thus interrupting the cascade. Demuth and co-workers[14g,16]
previously reported a UV-induced radical polyene cyclization,
wherein the initially PET-generated radical cation was firstly
Owing to their biological activities and structural diversity, the
synthesis of terpenoids has been and remains an attractive
goal for the synthetic organic community.[1] In nature, poly-
cyclic terpenes are constructed through cationic cyclizations of
the corresponding polyprenes.[2] Inspired by natural cationic
approach, chemical catalysts,[3] including Brønsted/Lewis
acids[4–6] and organocatalysts[7–9] as well as transition
metals,[10–12] have been developed toward the straightforward
synthesis of diverse polycyclic skeletons in a stereocontrolled
manner. Compared with the extensively explored ionic strat-
egy, radical approaches for polyene cyclizations have been
much less explored, even though Breslow et al. proposed and
tested their feasibility more than fifty years ago.[13] Recently,
a few stereoselective radical polyene cyclizations have been
developed,[8,14] showing significant potentials in complement-
ing the cationic processes. Despite these advances, most of
the radical approaches suffer from the need for stoichiometric
amounts of metals or radical initiators and, in many cases,
result in low yields with complicated reaction mixtures, thus
limiting further development along this line. Therefore, the de-
trapped with H2O and the resultant radical intermediate under-
went cascade cyclization. The overall process is a redox-neutral
radical addition requiring an electron-withdrawing alkene as
a terminator (Scheme 1). Herein, we report a visible light-pho-
Scheme 1. Photoinduced electron transfer (PET) in polyene cyclizations:
A) Previous work: UV-mediated radical cyclization; B) this work: visible-light-
mediated radical cyclization.
toinduced electron transfer (PET) method for radical cation
polyene cyclization with an organic photocatalyst.[17] In con-
trast to Demuth’s approach, we found that the use of an or-
ganic dye in hexafluoro-2-propanol allowed for facile genera-
tion of the olefin radical cation under visible light irradiation
and facilitated the subsequent cationic radical cyclization
(Scheme 1). In this process, the OH moiety of either aliphatic
alcohols, phenols, or enols can be employed as a terminator,
enabling an unprecedented scope for PET-type polyene cycli-
zations.
[a] Z. Yang, Dr. L. Zhang, Prof. Dr. S. Luo
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry, The Chinese Academy of Sciences
Beijing, 100190 (China)
Fax: (+86)-10-6255-4449http://luosz.iccas.ac.cn
E-mail: luosz@iccas.ac.cn
[b] H. Li, Prof. M.-T. Zhang, Prof. Dr. J.-P. Cheng
Center of Basic Molecular Science (CBMS) Department of Chemistry
Tsinghua University, Beijing, 100084 (China)
E-mail: mtzhang@mail.tsinghua.edu.cn
[c] Z. Yang, Dr. L. Zhang, Prof. Dr. S. Luo
Collaborative Innovation Center of Chemical Science and Engineering
Tianjin, 300071 (China)
Our proposed polycyclization strategy was first evaluated for
the direct cyclization of substrate 1a with dichloromethane
(CH2Cl2) as solvent, 450 nm light emitting diode (LED) irradia-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503118.
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Table 1. Screening and optimization.[a]
Table 2. Control reactions.
Entry
Conditions
Conversion [%][a] Yield [%][b] d.r.[a]
1
2
3
4
5
6
Standard conditions I
No light
No photocatalyst
33W CFL (72 h)
TfOH[c]
TsOH[d]
> 95
<5
<5
> 95
> 95
> 95
93
–
–
83
mixture
mixture
>19:1
–
–
>19:1
–
–
Entry[b] Catalyst Solvents Conversion [%][c] Yield[d] [%] d.r.[c]
[a] Determined by 1H NMR spectroscopy; [b] yields of compounds isolat-
ed by chromatographic purification; [c] 0.025 equivalent of TfOH;
[d] 0.025 equivalent of TsOH.
1
2
3
4
5
6
7
8
9[e]
10
11
3a
3a
3a
3a
3a
3a
3a
3b
3b
3c
[Ir][f]
[Ru][g]
3b
DCE
> 95
> 95
–
> 95
<5
> 95
> 95
> 95
> 95
> 95
<5
21
24
9
31
–
79
83
93
93
83
–
2:1
3:1
2:1
DCM
toluene
CH3CN
DMF
TFE
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
8:1
--
9:1
version but a complicated reaction mixture with only minor
desired adduct detected[19] (Table 2, entries 5 and 6). Taken to-
gether, these results indicated that the current reaction was
through a photocatalytic process but not promoted by HFIP
acting as an acid.
>19:1
>19:1
>19:1
>19:1
--
12
13[h]
<5
>95
–
47
--
>19:1
Having optimized a viable catalyst system, we next investi-
gated the newly developed radical protocol in a series of cas-
cade cyclizations (Table 3). Various electron-withdrawing sub-
stituents on the phenol ring were well tolerated (Table 3, en-
tries 1–6). Moreover, electron-rich arenes, such as methoxy-sub-
stituted phenol, could also afford the desired product (Table 3,
entry 7). In addition, when applying this cyclization strategy to
extended ring systems, tricyclization products were furnished
in moderate yields (Table 3, entries 8 and 9). In accordance
with our strategy, simple alkyl hydroxy groups could also serve
as suitable terminators to give the cyclized adducts under
these mild conditions (Table 3, entries 11 and 12). The reaction
with a terminal olefin without methyl substitution (1j) was
also examined and ‘incomplete’ polycyclization product was
observed (Table 3, entry 10). It seemed that unsubstituted ter-
minal vinyl moiety was not a viable initiator and the reaction
tended to initialize with 1,2-disubstituted C=C bond, probably
due to the higher oxidation potential of the monosubstituted
olefin.[20] To our delight, the catalyst system performed well
when the reaction was scaled up to 1.0 mmol scale (Table 3,
entry 1).
[a] Unless otherwise noted, the reactions were performed with 1a
(0.05 mmol) in the presence of photocatalysts 3 (5 mol%) in solvent
(0.5 mL) at room temperature under argon atmosphere; [b] entries 1–7
and 10 were irradiated by Blue LEDs; entries 8–9 and 13 were irradiated
by Green LEDs; entries 11–12 were irradiated by 33 W CFL; [c] determined
by 1H NMR spectroscopy; [d] yields of compounds isolated by chromato-
graphic purification; [e] the loading of 3b was 2.5 mol%; [f] [Ir]=tris(2-
phenylpyridinato)iridium(III); [g] [Ru]=tris(2,2’-bipyridyl)ruthenium chlo-
ride hexahydrate; [h] the reaction was open to air.
tion, and 5 mol% 9-mesityl-10-methylacridinium perchlorate
(3a)[18] as a photoredox catalyst at room temperature (Table 1,
entry 1), affording product 2a in 21% yield and 2:1 d.r. In
screening a variety of solvents, a dramatic solvent effect was
noted (Table 1, entries 1–7). The use of polar protic solvent led
to not only clean reaction, but also significantly improved dia-
stereoselectivity. Hexafluoro-2-propanol (HFIP) was identified
as the optimal solvent (Table 1, entry 7). When examining
other photocatalysts, we found the yield of product was raised
to 93%, alongside >19:1 d.r., with eosin Y (EY; 3b) as the pho-
tocatalyst (Table 1, entries 8 and 9). Organic photocatalyst 3c
also worked comparably well (entry 10). In contrast, inorganic
[Ir]- or [Ru]-based photosensitizers were ineffective (Table 1, en-
tries 11 and 12). In addition, the reaction was more favorably
conducted under inert gas conditions, and the presence of
excess O2 was found to decrease the product yield (Table 1,
entry 13) mainly due to the overoxidation of substrate.
We further examined the reactions with polyenes bearing
1,3-ketocarbonyls as terminators based on their known ten-
dency to equilibrate into the enol forms. However, the initial
experiments with 1,3-diketone 5a under the standard condi-
tions led to no reaction at all. To our delight, when a weak
Lewis acid LiBr was added, the reaction proceeded smoothly
to give the desired cyclic adduct 6a in high yield and good se-
lectivity (Table 4A, entry 1). Further control reactions showed
the use of LiBr only could not promote the reaction, indicating
no acid catalysis was involved (Table 4A, entry 3) and that the
reaction did not proceed without light irradiation, further veri-
fying the photocatalytic nature (Table 4A, entry 4). Other 1,3-
ketocarbonyls were also examined. Both aromatic and aliphatic
Further control reactions confirmed that both light and
eosin Y were essential for significant conversion to the cyclic
product (Table 2, entries 2 and 3). The reaction worked less ef-
fectively under compact fluorescent lamp (CFL) irradiation
(Table 2, entry 4). The use of acid catalysts such as TfOH or p-
toluene sulfonic acid was also examined, giving complete con-
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Table 3. Substrate scope.[a]
Table 3. (Continued)
Entry
Substate
Product
Yield [%][b]
12
70[e]
[a] Unless otherwise noted, the reactions were performed with
1 (0.05 mmol) in the presence of photocatalysts 3b (2.5 mol%) in HFIP
(0.5 mL) at room temperature under argon atmosphere; [b] yields of com-
pounds isolated by chromatographic purification; [c] the reaction was
performed with 1a (1 mmol) in the presence of 3b (1.0 mol%) in HFIP
(3.0 mL); [d] single isomer; [e]>95% conversion; due to the volatility of
this compound, the yield was determined by 1H NMR spectroscopy with
diphenyl as standard.
Entry
1
Substate
Product
Yield [%][b]
93
90[c]
2
3
4
5
6
7
79
85
Table 4. Cyclization of 1,3-diketone substrates.[a]
85
A. Control reactions.
Entry Condition
Conversion [%][b] Yield [%][c] d.r.[b]
83[d]
1
2
3
4
Standard conditions II > 95
88
–
–
>19:1
no LiBr
<5
<5
<5
–
–
–
no photocatalyst 3b
no light
–
83
B. Substrate scope.[c]
83
8
9
72
40
[a] Unless otherwise noted, the reactions were performed with
5
(0.05 mmol) in the presence of photocatalysts 3b (2.5 mol%) in HFIP
(0.5 mL) at room temperature under argon atmosphere; diasteresoselec-
tivities of 6a–e exceeded 19:1 as determined by 1H NMR spectroscopy;
1
[b] determined by H NMR spectroscopy; [c] yields of compounds isolated
by chromatographic purification.
10
11
93
with excellent diasteresoselectivity (>19:1 d.r.). To our knowl-
edge, such polyene cyclizations with b-ketocarbonyls as termi-
nators have rarely been reported.[21]
65[e]
Mechanistically, PET-generated radical cation species derived
from the end-alkene moiety can be invoked to initialize the
cyclization based on known precedents,[22] as well as our ex-
perimental observations. However, a competing pathway may
also occur when there is a second redox moiety and this is the
case with easily oxidized phenol substrates, wherein the pho-
togenerated phenol radical cation may lose a proton, leading
ketones worked very well in the reactions (Table 4B). The reac-
tion also reacted regioselectively with the aromatic keto
moiety when asymmetric 1,3-diketone was applied (Table 4B,
6c). Notably, all of the products in this survey were obtained
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to a typical acid-mediated polyene cyclization (Scheme 2A).
Stern–Volmer experiments, with phenol-containing substrate
1a, para-bromophenol, as well as aliphatic alcohol 1l were
conducted to shed light on the mechanistic pathway of the
photocatalytic cascade cyclization. The lifetime of the excited
state of EY in HFIP was determined with the laser flash photo-
lysis spectrometer at 550 nm corresponding to the T–T absorp-
process, whereas the possible photoinduced acid catalytic
pathway, if not completely ruled out, should be minor under
the present conditions. In addition, the fact that aliphatic alco-
hols and ketocarbonyls, for which PET oxidation is not com-
petitive, worked equally well in the reactions, clearly shows
that the phenol group is not a prerequisite for the reaction
and is supportive of a radical cascade pathway.
3
tion of the triplet EY*.[23] Stern–Volmer analysis showed that
On the basis of the above results, a plausible mechanism for
this photoreaction is proposed (Scheme 3). Initially, eosin Y is
the quenching constant of the pure olefin 1l and para-bromo-
phenol was comparable, and both of which are much less than
that of substrate 1a (Figure 1). These results strongly suggest
that PET of the olefin moiety is feasible and also competitive
under our conditions. Further control experiments indicated
that the reaction didn’t occur in the presence of a radical in-
hibitor TEMPO, but proceeded smoothly in the presence of
stoichiometric sodium bicarbonate with maintained diastereo-
selectivity and reduced yield (Scheme 2B). Taken together,
these observations are consistent with a PET-induced radical
3
excited by green LEDs, then forms triplet EY* through inter-
system crossing (ISC). Subsequently, PET from substrate 7 to
3
the triplet EY* results in the formation of the radical cation
Scheme 3. Possible pathways for the cyclization of polyenes.
7+· and EY radical anion (EYÀ·).[24] The former undergoes radi-
cal cascade cyclization alongside simultaneous accompanying
hydrogen shift to give the radical cation intermediate 8. Finally,
intermediate 8 is reduced and leads to the desired product. At
the same time, EY radical anion (EYÀ·) is oxidized, regenerating
EY and completing the photocatalyst circulation. Within this
process, we believe that the polar solvent HFIP plays a dual
role: a) HFIP has a significant stabilizing effect on radical cation
intermediates,[25] whereas other solvent effects, such as hydro-
gen bonding, dramatically influence the redox potential of EY
and substrates, hence facilitating a stereoselective cascade pro-
cess; b) HFIP participates and accelerates the aforementioned
hydrogen shifting process.
Scheme 2. A) Potential photoinduced acid catalysis; B) control reactions.
In summary, we have realized a visible-light-mediated, or-
ganic photocatalytic cyclization of polyenes under mild condi-
tions. Good yields and high stereoselectivity are achieved. The
reaction proceeds smoothly at room temperature, and requires
no additives except a catalytic amount of photocatalyst. The
photocatalyst eosin Y is commercially available and cheap, and
the reaction can be readily scaled up. Further investigations
into the mechanism of the reaction and the enantioselective
application of this catalytic system are ongoing in our labora-
tory.
3
Figure 1. Stern–Volmer plots of quenching of EY* (in HFIP) lifetime for each
3
quencher studied. EY* lifetime was measured with the laser flash photolysis
spectrometer at 550 nm. The Stern–Volmer quenching constant, KSV, was de-
termined by the slope of the linear regression, where the bimolecular
quenching constant, kq, is equal to KSV/t0. Conditions: [Eosin Y]=42 mm;
lex =510 nm; Ar atmosphere.
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Acknowledgements
The project was supported by the Ministry of Science and
Technology (2012CB821600), the Natural Science Foundation
of China (NSFC 21390400, 21202171 and 21025208), and the
Chinese Academy of Sciences. S.L. is supported by the National
Program for Support of Top-Notch Young Professionals.
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Keywords: cyclization
· organocatalysis · photocatalysis ·
polyenes · radical reactions
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Received: August 7, 2015
Published online on September 1, 2015
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