Visible-light-facilitated 5-exo-trig cyclization of 1,6-dienes with alkyl chlorides: Selective scission of the C(sp3)-H bond in alkyl chlorides

Article abstract of DOI:10.1002/ejoc.201301849

A general and conceptually novel method for preparing polychloro- substituted pyrrolidin-2-ones and indeno[2,1-c]pyrrol-3(3aH)-ones is established by visible-light-facilitated 5-exo-trig cyclization of 1,6-dienes with alkyl chlorides through selectively splitting the C(sp³)-H bond adjacent to the chloride atom to form an alkyl radical. The 5-exo-trig cyclization of 1,6-dienes with alkyl chlorides through a visible-light-facilitated radical strategy is presented. This is achieved by selectively splitting the C(sp ³)-H bond adjacent to the chloride atom to form an alkyl radical. bpy = 2,2′-bipyridyl. Copyright

Full text of DOI:10.1002/ejoc.201301849

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301849
Visible-Light-Facilitated 5-exo-trig Cyclization of 1,6-Dienes with Alkyl
Chlorides: Selective Scission of the C(sp3)–H Bond in Alkyl Chlorides
Yu Liu,[a] Jia-Ling Zhang,[a] Ren-Jie Song,[a] and Jin-Heng Li*[a]
Keywords: Radicals / Cyclization / Dienes / C–H activation / Nitrogen heterocycles
A general and conceptually novel method for preparing po-
lychloro-substituted pyrrolidin-2-ones and indeno[2,1-c]pyr-
rol-3(3aH)-ones is established by visible-light-facilitated 5-
exo-trig cyclization of 1,6-dienes with alkyl chlorides through
selectively splitting the C(sp3)–H bond adjacent to the chlor-
ide atom to form an alkyl radical.
edge, however, a method for the formation of carbon-cen-
tered radicals from organohalides by selectively abstracting
the hydrogen atom, not the halide atom, has not been estab-
lished. The reason may be that in organohalides the reactiv-
ity of the carbon–halogen bond is far higher than that of
the carbon–hydrogen bond.
Herein we report an unprecedented visible-light-facili-
tated method for 5-exo-trig cyclization of 1,6-dienes with
alkyl chlorides through selective scission of the C(sp3)–H
bond adjacent to the chloride atom to form a chloro-substi-
tuted alkyl radical in the presence of aryldiazonium tetra-
fluoroborate radical initiators[4,5] (Scheme 1b). The results
demonstrate that both visible-light and photoredox cata-
lysts can facilitate this reaction.[4f–4i,5] Importantly, this new
method provides an efficient way to synthesize various po-
lychloro-substituted pyrrolidin-2-ones and indeno[2,1-c]-
pyrrol-3(3aH)-ones, which form the nuclei of many natural
products and bioactive molecules.[6,7]
Introduction
The cyclization of dienes is a continuing active field, be-
cause it is widely utilized for the construction of carbocyclic
and heterocyclic compounds that are important compo-
nents of multitudinous natural products, pharmaceutical
molecules, and functional materials.[1–3] In this context, the
cyclization of dienes through a radical strategy, particularly
the generation of a carbon-centered radical from organo-
halides as the initial step, has attracted considerable atten-
tion.[2,3] Generally, organohalides (often alkyl halides) are
transferred to the carbon-centered radicals by selectively
splitting the carbon–halogen bonds by using a thermo- or
photoinitiation strategy wherein radical initiators such as
azobis(isobutyronitrile) (AIBN) and organotin reagents
(often Bu3SnH) are usually used to accomplish these trans-
formations (Scheme 1a).[2,3a–3e] To the best of our knowl-
Results and Discussion
We began our investigation with the reaction of N-allyl-
N-phenylmethacrylamide (1a) with dichloromethane (2a) to
optimize the reaction conditions (Table 1).[8] Gratifyingly,
treatment of diene 1a with chloride 2a, 4-MeOC6H4N2BF4,
and Na2CO3 at 50 °C for 20 h afforded desired pyrrolidin-
2-one 3 in 25% yield (Table 1, Entry 1), and its stereochem-
istry was determined by analysis by 2D NMR spec-
troscopy.[8] However, the reaction did not take place with-
out a diazonium tetrafluoroborate (Table 1, Entry 2). The
results suggest that the generation of the 4-methoxyphenyl
radical from 4-MeOC6H4N2BF4 is the key step in the 5-
exo-trig cyclization reaction. It is well known that visible
light combined with a photoredox catalyst can trigger the
transformation of aryldiazonium salts into aryl radi-
cals.[4f–4i,5] Therefore, the effect of both visible light and the
photoredox catalyst was tested: the yield of 3 was indeed
enhanced to 35% under a 36 W compact fluorescent light
Scheme 1. Cyclization of 1,6-dienes with alkyl chlorides. bpy =
2,2Ј-bipyridyl.
[a] State Key Laboratory of Chemo/Biosensing and Chemometrics,
College of Chemistry and Chemical Engineering, Hunan
University,
Changsha 410082, China
E-mail: jhli@hnu.edu.cn
http://cc.hnu.cn/index.php?option=com_content&task=
view&id=1054&Itemid=228
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301849.
Eur. J. Org. Chem. 2014, 1177–1181
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1177

Y. Liu, J.-L. Zhang, R.-J. Song, J.-H. Li
SHORT COMMUNICATION
alone (Table 1, Entry 3) and was further increased to 58% OC6H4N2BF4 (Scheme 2 versus Table 1, Entry 4). However,
in the presence of both Ru(bpy)3Cl2 and a 36 W compact the common radical initiator, AIBN, had no effect on the
fluorescent light (Table 1, Entry 4). We found that the base reaction.
affected the reaction, and Na2CO3 was the most effective
(Table 1, Entries 4–7). Notably, the absence of a base re-
sulted in no detectable pyrrolidin-2-one 3 (Table 1, Entry 8).
Among the reaction temperatures examined, it turned out
that the reaction at 50 °C gave the best results (Table 1, En-
tries 4, 9 and 10). To our delight, a satisfactory yield was
still achieved at an Ru(bpy)3Cl2 loading of 2 mol-%
(Table 1, Entry 11). Notably, the reaction proceeded in
MeCO2nBu medium smoothly, although the product was
obtained in a slightly lower yield (Table 1, Entry 12). Two
other visible light photoredox catalysts, that is, Ir(ppy)3
(ppy = 2-phenylpyridine) and Eosin Y, were tested (Table 1,
Scheme 2. Screening of the diazonium salts.
With the optimal conditions in hand, the scope of dienes
Entries 13 and 14): Whereas Ir(ppy)3 also displayed high 1 and alkyl halides 2 for the 5-exo-trig cyclization reaction
catalytic activity, Eosin Y disfavored the reaction. Screening was investigated (Table 2 and Scheme 3). In the presence
revealed that the amount of 4-MeOC6H4N2BF4 had a fun- of Ru(bpy)3Cl2, 4-MeOC6H4N2BF4, Na2CO3, and a 36 W
damental influence on the reaction, and the yield decreased compact fluorescent light, diene 1a successfully reacted
to 33% with 1 equiv. of the diazonium salt (Table 1, En- with various alkyl chlorides, that is, 1,1-dichloroethane (2b),
tries 2, 4, and 15). Notably, the presence of an Ru catalyst chloroform (2d), 1,1,2-trichloroethane (2e), and 1,1,2,2-tet-
alone affected the reaction slightly without light (Table 1, rachloroethane (2f), to provide pyrrolidin-2-ones 4, 6, 8,
Entry 16).
and 9, respectively, in good yields (Table 2, Entries 1, 3, 5,
and 6). However, 1-chlorobutane (2c) was unreactive in this
reaction (Table 2, Entry 2). By using diene 1b, a moderate
yield of the product was still achieved (Table 2, Entry 4).
Interestingly, the selectivity of the products from dienes 1c
and 1d bearing a phenyl group at the 2-position depended
on chlorides 2 (Table 2, Entries 7–11). For example, diene
1c reacted with chlorides 2a and 2f to afford indeno[2,1-c]-
pyrrol-3(3aH)-ones 10 and 11 through a cascade of three
C–C bond-formation reactions (Table 2, Entries 7 and 9),
but chloride 2d gave only pyrrolidin-2-one 12 in moderate
yield (Table 2, Entry 8). Notably, treatment of diene 1d with
chloride 2a provided a mixture of tricyclic product 13 and
spirocyclic product 14 in 41 and 36% yield, respectively
(Table 2, Entry 10). By using chloroform (2d), only pyrrol-
idin-2-one 15 was isolated from diene 1d in moderate yield
(Table 2, Entry 11).
To understand the mechanism, some control experiments
were performed (Scheme 3). By using CDCl3 (2d-D1) in-
stead of CHCl3 (2d), both monodeuterated pyrrolidin-2-one
6-D1 and anisole 16-D1 were observed by GC–MS analysis
[Equation (1)].[8] This suggests that the reaction is triggered
by the 4-methoxyphenyl radical from 4-MeOC6H4N2BF4
and is finished by abstraction of a hydrogen atom from the
alkyl halide. To verify this, the reaction between 4-Me-
OC6H4N2BF4 and CDCl3 (2d-D1) was tested. Without Ru,
base, or light, 4-MeOC6H4N2BF4 was stable. However, 90%
of this diazonium salt was decomposed by Na2CO3 into
monodeuterated anisole 16-D1, which was obtained in 75%
yield with 83% deuterium, because the diazonium salt (neat
at 50 °C) decomposed smoothly with the aid of Na2CO3 to
generate the aryl radical. This may also justify the 25%
yield of pyrrolidin-2-one that was found in the absence of
Table 1. Screening for the optimal conditions.[a]
Entry
[M] (mol-%)
Base
Solvent
T
[°C]
Yield
[%]
1[b]
2[b,c]
3
4
5
–
–
–
Na2CO3
Na2CO3
Na2CO3
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
50
50
50
50
50
50
50
50
30
70
50
50
50
50
50
50
25
0
35
58
50
21
17
0
50
32
40
48
50
10
33
28
Ru(bpy)3Cl2 (5) Na2CO3
Ru(bpy)3Cl2 (5) NaHCO3
Ru(bpy)3Cl2 (5) Cs2CO3
6
7
8
Ru(bpy)3Cl2 (5)
Ru(bpy)3Cl2 (5)
Et3N
–
9
10
11
Ru(bpy)3Cl2 (5) Na2CO3
Ru(bpy)3Cl2 (5) Na2CO3
Ru(bpy)3Cl2 (2) Na2CO3
12[d] Ru(bpy)3Cl2 (5) Na2CO3 MeCO2nBu
13
14
Ir(ppy)3 (5)
Eosin Y (5)
Ru(bpy)3Cl2 (5) Na2CO3
Na2CO3
Na2CO3
neat
neat
neat
neat
15[e]
16[b] Ru(bpy)3Cl2 (5) Na2CO3
[a] Reaction conditions: 1a (0.3 mmol), 2a (15 mmol), [M], 4-Me-
OC6H4N2BF4 (2 equiv.), and base (2 equiv.) with 36 W compact
fluorescent light under argon for 20 h. Two diastereoisomers were
observed by GC–MS analysis of the crude products, and the dr
value was 9.8:1; however, only the cis isomer was isolated upon
purification. Yields of the isolated products are given. [b] Without
additional light. [c] Without 4-MeOC6H4N2BF4. [d] Compound 2a
(6 mmol) and MeCO2nBu (anhydrous, 0.5 mL). [e] 4-Me-
OC6H4N2BF4 (1 equiv.).
We next set out to examine the effect of the diazonium both [Ru] and visible light (Table 1, Entry 1). As expected,
salt (Scheme 2). Three other aryldiazonium salts displayed the decomposition was selectively promoted by using Ru
high reactivity, but they were less efficient than 4-Me- and light: 95% of the diazonium salt was converted into
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Visible-Light-Facilitated 5-exo-trig Cyclization of 1,6-Dienes
Table 2. 5-exo-trig cyclization of 1,6-dienes 1 with alkyl chlorides
2.[a]
Scheme 3. Control experiments.
16-D1 (91% yield). These deuteration experiments imply
that the yield of the present reaction is based on the selec-
tivity of the generation of the aryl radical, and the photo-
catalysts/visible-light system mainly effects the reaction by
facilitating the generation of the aryl radical. Notably, a
stoichiometric amount of two radical inhibitors (2 equiv.),
that is, TEMPO (2,2,6,6-tetramethylpiperidine N-oxide)
and 2,6-di-tert-butylphenol, was added to the reaction mix-
ture, and this resulted in no conversion of diene 1a. All of
the results described above support that the reaction in-
cludes a radical process.
Consequently, possible mechanisms outlined in Scheme 4
are proposed.[2–4] Initially, 4-MeOC6H4N2BF4 is readily
converted into the 4-methoxyphenyl radical by base/
Ru(bpy)3Cl2/visible light under heating conditions.[4,5] Sub-
sequently, selective hydrogen-atom abstraction of CH2Cl2
cation radical A, in situ generated from CH2Cl2 (2a) by vis-
ible-light catalysis, by the 4-methoxyphenyl radical offers
·
·
the CHCl2 radical. Addition of the CHCl2 radical to the
activated alkene yields radical intermediate B; cyclization
of B with another alkene through a single-electron-transfer
process gives rise to radical intermediate C. Finally, hydro-
gen-atom abstraction of A by C takes place to yield the
·CHCl2 radical and product 3. The photoinduced mecha-
nism is also supported by the quantum yield (Φx = 0.073).[9]
·
Notably, the CHCl2 radical can be directly generated from
the reaction of CH2Cl2 (2a) with the 4-methoxyphenyl radi-
cal if the reaction is conducted without an Rh catalyst or
light.
[a] Reaction conditions: 1 (0.3 mmol), 2 (15 mmol), Ru(bpy)3Cl2
(5 mol-%), 4-MeOC6H4N2BF4 (2 equiv.), and Na2CO3 (2 equiv.)
with a 36 W compact fluorescent light under argon at 50 °C for
20 h. The dr (cis/trans) values are given in parentheses, and they
were determined by GC–MS analysis of the crude products. The
yields are those of the cis isomers only, because only the cis isomers
were isolated after purification. Moreover, some side products, in-
cluding C–N bond-decomposition products and noncyclizing prod-
ucts, were observed by GC–MS analysis.
Scheme 4. Possible mechanisms.
Eur. J. Org. Chem. 2014, 1177–1181
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Y. Liu, J.-L. Zhang, R.-J. Song, J.-H. Li
SHORT COMMUNICATION
of Higher Education (No. 20120161110041), and the Hunan Prov-
incial Natural Science Foundation of China (No. 13JJ2018) for fin-
ancial support.
Conclusions
We illustrated a novel, general method for the synthesis
of polychloro-substituted pyrrolidin-2-ones and indeno[2,1-
c]pyrrol-3(3aH)-ones by a visible light photoredox catalyst
facilitated 5-exo-trig cyclization of dienes with alkyl chlor-
ides involving a C(sp3)–H functionalization process. Im-
portantly, this method represents a significant conceptual
advance that is achieved by selective scission of the C–H
bond adjacent to the chloride atom of the carbon-centered
radical by using a single-electron-transfer strategy. Investi-
gation of the applications of diazonium salts as potential
triggers in organic synthesis are currently underway in our
laboratory.
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Experimental Section
General Methods: All the materials and solvents were purchased
from commercial suppliers and used without additional purifica-
tion. IR measurements were performed with an FTIR Shimadzu
DR-8000 spectrometer fitted with a Pike Technologies MIRacle
single reflection ATR adapter. H and 13C NMR spectra were re-
1
corded with a Bruker DRX-500 spectrometer (1H at 500 MHz and
13C at 125 MHz) or a Bruker DRX-400 spectrometer (1H at
400 MHz and 13C at 100 MHz). NMR spectroscopic data were ob-
tained in CDCl3 unless otherwise noted. High-resolution mass
spectra were recorded with a Bruker microTOF-QII (ESI) spec-
trometer. Preparative thin-layer chromatography was performed on
silica gel plates with PF254 indicator. Flash column chromatog-
raphy was performed with silica gel 60N unless otherwise noted.
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For selected papers on the radical cyclization of dienes, see: a)
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Typical Experimental Procedure for the Visible-Light-Facilitated 5-
exo-trig Cyclization of 1,6-Dienes with Alkyl Chlorides: A Schlenk
tube was charged with N-allyl-N-phenylmethacrylamide (1a,
0.3 mmol), dichloromethane (2a, 15 mmol), Ru(bpy)3Cl2 (5 mol-
%), Na2CO3 (2 equiv.), and 4-MeOC6H4N2BF4 (4-methoxybenz-
enediazonium tetrafluoroborate, 2 equiv.). Then, the tube was
charged with argon, and the mixture was stirred at 50 °C under
visible light for the indicated time until complete consumption of
starting material, as monitored by TLC and/or GC–MS analysis.
Upon completion of the reaction, the mixture was diluted with
diethyl ether and concentrated in vacuo, and the resulting residue
was purified by silica gel column chromatography (hexane/ethyl
acetate, 20:1) to afford desired product cis-3.
[4]
Representative papers and reviews on using aryldiazonium
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cis-3-(2,2-Dichloroethyl)-3,4-dimethyl-1-phenyl-pyrrolidin-2-one (3):
1
Yellow oil. H NMR (400 MHz, CDCl3): δ = 7.61 (d, J = 4.8 Hz,
2 H), 7.38 (t, J = 3.6 Hz, 2 H), 7.16 (d, J = 7.2 Hz, 1 H), 6.14 (t,
J = 6.0 Hz, 1 H), 3.81 (dd, J = 8.0, 9.6 Hz, 1 H), 3.42 (t, J = 9.6 Hz,
1 H), 2.76 (dd, J = 6.4, 15.2 Hz, 1 H), 2.70–2.64 (m, 1 H), 2.50
(dd, J = 6.0, 15.6 Hz, 1 H), 1.13–1.12 (m, 6 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 176.8, 139.2, 128.9 (2 C), 124.6, 119.8,
70.1, 52.0, 49.2, 47.6, 33.6, 17.7, 12.0 ppm. IR (KBr): ν = 1693,
˜
1597, 1461 cm–1. LRMS (EI, 70 eV): m/z (%) = 287 (5) [M + 2]+,
285 (7) [M]+, 190 (6), 189 (47), 175 (13), 174 (100). HRMS (ESI):
calcd. for C14H1835Cl2NO [M + H]+ 286.0760; found 286.0751.
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Supporting Information (see footnote on the first page of this arti-
1
cle): H and 13C NMR spectra for all products 3, 4, and 6–16.
Acknowledgments
We thank the National Natural Science Foundation of China (No.
21172060), the Specialized Research Fund for Doctoral Program
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Visible-Light-Facilitated 5-exo-trig Cyclization of 1,6-Dienes
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See the Supporting Information for detailed data, including 2D
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Received: December 12, 2013
Published Online: January 17, 2014
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