Intermolecular [3+2] cycloaddition of cyclopropylamines with olefins by visible-light photocatalysis

Article abstract of DOI:10.1002/anie.201106162

It's the power of light! A visible-light-mediated intermolecular [3+2] cycloaddition of mono- and bicyclic cyclopropylamines with olefins catalyzed by [Ru(bpz)₃](PF₆)₂·H₂O has been developed to furnish aminocyclopentane derivatives in good yields (see scheme, bpz=2,2′-bipyrazine). Saturated 5,5- and 6,5-fused heterocycles are obtained in synthetically useful yields and diastereoselectivity.

Full text of DOI:10.1002/anie.201106162

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Communications
DOI: 10.1002/anie.201106162
Visible-Light Photocatalysis
Intermolecular [3+2] Cycloaddition of Cyclopropylamines with Olefins
by Visible-Light Photocatalysis**
Soumitra Maity, Mingzhao Zhu, Ryan Spencer Shinabery, and Nan Zheng*
Solar energy is clean, abundant, and more importantly,
renewable. As such, any reaction that efficiently harvests
and converts solar energy into chemical energy is more
important than ever as the world turns to its scientists to meet
the challenge of environmental sustainability. Visible light
(390–750 nm) accounts for 43% of the overall solar
spectrum. However, many organic molecules are unable
to absorb visible light efficiently, thereby limiting the use of
visible light in organic synthesis. A possible solution to this
problem involves the use of visible-light photoredox
catalysts such as ruthenium[1] or iridium[2] polypyridyl
complexes to channel energy from visible light into organic
molecules. The groups of MacMillan,[3] Yoon,[4] Stephen-
son,[5] Akita,[6] and others[7] have recently published seminal
that new transformations of cyclopropylamines catalyzed by a
RuII or IrIII polypyridyl complex could be developed
(Scheme 1). Herein we report an intermolecular [3+2] cyclo-
addition of olefins with mono- and bicyclic cyclopropylani-
lines under visible light photocatalysis.
À
works on visible-light-promoted C C bond-formation
reactions catalyzed by these complexes. Amines are often
used as a sacrificial electron donor to reduce the photo-
excited RuII and IrIII complexes to RuI and IrII complexes.[8]
Recently, amines have been also explored as a substrate in
these processes.[9] We were intrigued by the potential of
using amines as both the sacrificial donor and the substrate,
thus making the process more atom economical.
Scheme 1. [3+2] Cycloaddition of cyclopropylamines with olefins.
Bn=benzyl, CAN=ceric ammonium nitrate, TIPS=triisopropylsilyl.
We envisioned a class of amines that are capable of
initializing a downstream irreversible reaction upon oxidation
by the photoexcited RuII or IrIII complexes. Cyclopropyl-
amines have been shown to undergo irreversible opening of
the cyclopropyl ring upon their oxidation to the nitrogen
radical cations. Based on this mode of action, cyclopropyl-
amines have been used to probe amine oxidation in biological
systems.[10] Cyclopropylamines have seen limited use in
organic synthesis to date.[11] All these applications focus on
intramolecular reactions, except the formation of the endo-
peroxides.[11a] Furthermore, the generation of nitrogen radical
cations requires UV light with a photosensitizer or a strong
oxidant (e.g., ceric ammonium nitrate), thus limiting the
substrate scope and/or the type of the products being formed.
Since visible-light photocatalysis has been shown to be a mild
and chemoselective method to oxidize amines, we envisioned
Cyclopropylaniline 1a and styrene 2a were chosen as the
model substrates to optimize the reaction conditions
(Table 1). Using a 13W GE fluorescent lightbulb, irradiation
of a solution of 1a and 2a in CH3NO2 with [Ru(bpz)3]-
(PF6)2·2H2O[12,13] (4a) and air afforded the desired cyclo-
pentane product 3a as a 1:1 mixture of cis and trans isomers in
21% yield (Table 1, entry 1). Degassing the reaction mixture
Table 1: Optimization of the catalytic system.
Entry Conditions[a]
t [h]
Conv. of
Yield of
1a [%][b]
3a [%][b]
[*] Dr. S. Maity, Dr. M. Zhu, R. S. Shinabery, Prof. Dr. N. Zheng
Department of Chemistry and Biochemistry
University of Arkansas, Fayetteville, AR 72701 (USA)
E-mail: nzheng@uark.edu
1
2
3
4
4a (2 mol%), Air, CH3NO2
12
3
12
12
100
100
25
21
96
16
9
4a (2 mol%), CH3NO2
without 4a, CH3NO2
4a (2 mol%), CH3NO2,
lightbulb off
35
[**] We thank the University of Arkansas, the Arkansas Bioscience
Institute, and the NIH NCRR COBRE grant (P30 RR031154) for
generous support of this research. R.S.S thanks the Division of
Organic Chemistry of the ACS for a SURFaward. We also thank Prof.
Bill Durham for insightful discussions on photochemistry and Prof.
Jim Hinton for obtaining NOESY spectra.
5
6
4b (2 mol%), CH3NO2
4c (2 mol%), CH3NO2
12
12
100
100
79
73
[a] Reaction conditions: 1a (0.2 mmol, 0.1m in degassed CH3NO2), 2a
(1 mmol), irradiation with a 13 W fluorescent lightbulb at RT.
[b] Measured by GC using dodecane as an internal standard. bpz=2, 2’-
bipyrazine.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106162.
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minimized the decomposition of cyclopropyl amine[14] and
dramatically improved the yield of 3a to 96% (Table 1,
entry 2). Control studies showed that both the catalyst 4a
(Table 1, entry 3) and light (entry 4) with a suitable wave-
length range were necessary for efficient conversion into 3a.
Other photoredox catalysts such as [Ru(bpy)3](PF6)2 (4b;
Table 1, entry 5) and [Ir(dtbbpy)(ppy)2]PF6·H2O[15] (4c;
Table 1, entry 6) were not as effective in the cycloaddition
as 4a. The effectiveness of the catalysts correlates with the
redox potentials of RuII*/RuI and IrIII*/IrII.[16] Catalyst 4a,
which has the highest redox potential among the three, gave
the highest yield of 3a.
Having identified 4a in degassed CH3NO2 as the optimal
catalytic system, we next sought to apply it to other
monocyclic cyclopropylamines (Table 2). The catalytic
system was generally effective for secondary cyclopropylani-
lines, which were prepared by the Buchwald–Hartwig ami-
nation[17] or Cu-catalyzed amination[18] of cyclopropylamine.
An aryl group, which lowers the redox potential of the amine,
is necessary for the initial oxidation to occur.[19] Substitution
on the aromatic ring is tolerated (Table 2, entries 2 and 3).
Cyclopropylamines substituted with other arenes such as
biphenyl and naphthalene worked equally well in the cyclo-
addition (Table 2, entries 4 and 5). Cyclopropylamines sub-
stituted with pyridine also worked well albeit taking a longer
time to complete the cycloaddition (Table 2, entry 6). The six
cyclopropylanilines either failed to show any diastereoselec-
tivity (1a–c and 1 f) or gave modest diastereoselectivity (1d
and 1e, d.r. = 3:2) in the cycloaddition with styrene. Tertiary
cyclopropylanilines were found to be ineffective in the
cycloaddition, possibly because the ring opening was too
slow to be competitive.[20]
To address the issue of lack of the diastereoselectivity in
the cycloaddition with monocyclic cyclopropylamines, we
subsequently applied the optimized reaction conditions to
bicyclic cyclopropylamines, which could impart a steric bias
toward the cycloaddition. Bicyclic cyclopropylamines 5a–f
were readily prepared from their corresponding amides by the
Kulinkovich–de Meijere reaction.[11a,21] In contrast to mono-
cyclic tertiary cyclopropylanilines, bicyclic tertiary cyclopro-
pylanilines 5a–f successfully underwent the cycloaddition
with styrene (2a) to provide fused saturated heterocycles 6a–f
in synthetically useful yields and diastereoselectivities
(Table 3). The drastic difference in reactivity between these
two classes of cyclopropylamines was likely as a result of the
higher ring strain in the bicyclic compounds. Bicyclic cyclo-
propylamines 5a–c afforded 5,5-fused bicyclic heterocycles
6a–c in 69–77% yields with diastereomeric ratios (d.r.)
Table 3: [3+2] Cycloaddition of styrene (2a) with bicyclic cyclopropyl-
amines 5.[a]
Table 2: [3+2] Cycloaddition of styrene (2a) with monocyclic cyclo-
propylamines (1).[a]
Entry Substrate
Product[b]
t
Yield [%][c]
[h] d.r.[d]
Entry
1[b]
Substrate
Product
t [h]
Yield [%][d]
87
77
5
1
2
3
1a, R=H
3
4:1
74
8
2[b]
1b, R=4-Cl
6
4
82
82
5:1
3[b]
1c, R=4-CF3
69
36
5:1
28
12 (64)[e]
>25:1
4[c]
24
80
4
72
6
5
6
5[c]
6
75
71
3:1
58
12
6[b]
48
4:1
[a] Reaction conditions: substrate (0.2 mmol, 0.1m in degassed
CH3NO2), 2a (1 mmol), 4a (2 mol%), irradiation with a 13 W
fluorescent lightbulb at RT. [b] d.r.=1:1 and [c] d.r.=3:2 as determined
by 1H NMR spectroscopy of crude products. [d] Yield of the combined
isomers after isolation.
[a] Reaction conditions: 5a–f (0.2 mmol, 0.1m in degassed CH3NO2), 2a
(1 mmol), 4a (2 mol%), irradiation with a 13 W fluorescent lightbulb at
RT. [b] Only the major diastereoisomer shown. [c] Combined yields of the
two isomers after chromatography. [d] Determined by 1H NMR analysis
of the crude products (a/b). [e] Based on recovered 5d.
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ranging from 4:1 to 5:1 (Table 3, entries 1–3). Replacement of
the methyl group of 5a by a tert-butyl group dramatically
increased the diastereoselectivity as only a single diastereo-
mer of 6d was obtained (Table 3, entry 4). However, the
cycloaddition was much slower than that of 5a–c. Addition-
ally, the five-membered ring of [3.1.0] bicyclic cyclopropyl-
amines was tolerated in the cycloaddition, as 5e reacted as
well as 5a–c with only a slightly diminished diastereoselec-
tivity (Table 3, entry 5). Furthermore, [4.1.0] bicyclic cyclo-
propylamine 5 f participated in the cycloaddition in a similar
fashion to 5a–c to afford 6,5-fused bicyclic heterocycles 6 f in
58% yield with a d.r. of 4:1 (entry 6). The relative config-
urations of 6a–f were established by NMR spectroscopy. The
relative configuration of compound 6d was further confirmed
by X-ray crystallography (see the Supporting Information).[22]
Given the success of the cycloaddition with styrene, we
turned our attention to other olefins to explore the potential
of this method (Table 4). Terminal olefins substituted with
electron-withdrawing groups underwent the cycloaddition
with mono- and bicyclic cyclopropylamines, while internal
olefins failed to do so. Acrylonitrile gave a lower yield of the
cycloaddition product (7a) than styrene (Table 4, entry 1).
The introduction of a methoxy group to the benzene ring
(Table 4, entry 2; 7b versus 6d) or replacement of the phenyl
ring in styrene by a larger naphthalene group (Table 4,
entry 3; 7c versus 6a) had little effect on the yield and d.r. of
the cycloaddition. Substitution of the ortho hydrogen of
styrene by bromine rendered the cycloaddition modestly
diastereoselective, with the cis isomer of 7d being favored
(Table 4, entry 4). Moreover, the conjugated diene, 1-phenyl-
1,3-butadiene participated in the cycloaddition, which oc-
curred only at the terminal double bond to provide cyclo-
pentane 7e with the trans isomer being favored (Table 4,
entry 5).
The cyclopentane products obtained from the cycloaddi-
tion with monocyclic cyclopropylamines are useful building
blocks for preparing fused heterocycles (Scheme 2).[23] For
example, the major isomer of 7d was subjected to the Pd-
Scheme 2. Synthesis of indoline and octahydro-1H-cyclopenta[b]pyri-
dine. dba=dibenzylideneacetone, Tol-binap=2,2’-bis(di-p-tolylphos-
phanyl)-1,1’-binaphthyl.
catalyzed Buchwald–Hartwig amination reaction to furnish
fused indoline 8[24] in 89% yield. Separately, the major isomer
of 7e was converted into octahydro-1H-cyclopenta[b]pyri-
dine 10[25] in 71% overall yield over three steps. This sequence
is complementary to the [3+2] cycloaddition of bicyclic
cyclopropylamines with olefins (see above).
A possible catalytic cycle is shown in Scheme 3. Upon
irradiation, the RuII complex enters the photoexcited state
and cyclopropylamine 11 is oxidized to the nitrogen radical
cation 12 with the concomitant formation of RuI. The
nitrogen radical cation 12 subsequently undergoes ring open-
ing to generate b-carbon radical iminium ion 13, which then
adds intermolecularly to the olefin (“Giese Reactions”)[26] to
produce the stabilized radical 14. The intramolecular addition
of the stabilized radical to the iminium ion furnishes the
nitrogen radical cation 15 with the formation of a cyclo-
pentane ring; this compound is reduced by RuI to complete
the catalytic cycle.
Table 4: Scope of olefins in the [3+2] cycloaddition.
Entry Substrate Olefin
Product[a]
t
Yield [%][b]
[h] d.r.[c]
52
2
1[d]
1a
5d
1:1
30
2
12 (67)[e]
>25:1
72
6
3
5a
3:1
82
6
4
5
1a
1a
2:1
40
6
2:1
[a] Only the major diastereoisomer shown. [b] Combined yield of the two
isomers after chromatography. [c] Determined by 1H NMR analysis of the
crude products. [d] Two isomers shown. [e] Based on recovered 5d.
Scheme 3. Proposed catalytic cycle.
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Scheme 4. Diastereoselectivity model.
In summary, we have developed a visible-light-mediated
[3+2] cycloaddition of alkenes with cyclopropylamines cata-
lyzed by [Ru(bpz)3](PF6)2·2H2O (4a). The method features
excellent regiocontrol with respect to the alkene. A variety of
functional groups are tolerated and the reactions occur under
mild reaction conditions. The configuration at the carbon
bearing R1 in 11 is preserved in 16. Diastereoelectivity is high
when the cyclopropylamine is bicyclic. Further investigation
of the diastereoselectivity of the cycloaddition and its
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À
application to other C C bond-forming reactions is ongoing
and will be reported in due course.
Experimental Section
A representative procedure: [Ru(bpz)3](PF6)2·2H2O 4a (3.6 mg,
0.004 mmol, 2 mol%) and phenyl cyclopropylamine 1a (26 mg,
0.2 mmol) were added to a screw-capped oven-dried test tube
equipped with a stir bar. The tube was evacuated and backfilled
with nitrogen before styrene (110 mL, 1.0 mmol) and CH3NO2 (2 mL)
were added. The reaction mixture was degassed by the freeze-pump-
thaw method and then irradiated at room temperature by a 13 W
fluorescent lightbulb for 3 h. After the reaction was complete
(monitored by TLC), the mixture was filtered through a short pad
of silica gel and eluted with Et2O (10 mL). The solution was
concentrated and the residue was purified by flash chromatography
on silica gel (1.5% Et2O in hexane) to afford 3a (42 mg, 87%) as a
colorless 1:1 mixture of cis and trans isomers.
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Received: August 31, 2011
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Published online: November 23, 2011
Keywords: cycloaddition · cyclopropylamines · photochemistry ·
.
ruthenium · visible light
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