Intermolecular [3+2] Annulation of Cyclopropylanilines with Alkynes, Enynes, and Diynes via Visible Light Photocatalysis

Article abstract of DOI:10.1002/adsc.201400742

One-step syntheses of carbocycles substituted with amines from simple starting materials remain rare. We recently developed an intermolecular [3+2] annulation of cyclopropylanilines with alkenes and alkynes that enables this one-step synthesis. Herein, we

Full text of DOI:10.1002/adsc.201400742

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DOI: 10.1002/adsc.201400742
Intermolecular [3+2]Annulation of Cyclopropylanilines with
Alkynes, Enynes, and Diynes via Visible Light Photocatalysis
Theresa H. Nguyen,a Scott A. Morris,a and Nan Zhenga,*
a
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, USA
Fax : (+1)-479-575-4049; phone: (+1)-479-575-5078; e-mail: nzheng@uark.edu
Received: July 29, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400742.
Abstract: One-step syntheses of carbocycles substi- kynes has been achieved. A range of structurally di-
tuted with amines from simple starting materials verse carbocycles substituted with amines is pre-
remain rare. We recently developed an intermolecu- pared.
lar [3+2]annulation of cyclopropylanilines with al-
kenes and alkynes that enables this one-step synthe-
sis. Herein, we report our findings for a full-scale Keywords: alkynes;
study of the annulation. Significant expansion of the anilines; diynes; enynes; intermolecular annulation;
substrate scope for both cyclopropylanilines and al- photocatalysis; visible light
ACHTUNGTRNE[NUNG 3+2]annulation; cyclopropyl-
AHCTUNGTRENNUNG
Introduction
lation. Herein we report our findings towards expan-
sion of these reactions.
Cycloaddition or annulation reactions have been pri-
marily used to construct carbo- or heterocycles with
various sizes.[1] These types of reactions have gained Results and Discussion
increasing prominence in diversity-oriented synthesis
(DOS) because it can efficiently increase the structur- Reaction Optimization
al diversity and complexity of small molecules.[2] Both
features are critical for the success of small molecule Although we previously optimized the conditions for
libraries as they enable small molecules to occupy the [3+2]annulation of cyclopropylanilines with al-
more chemical space. Recently, the renaissance of kynes using ruthenium polypyridyl complexes, the de-
visible light photocatalysis has brought new life to sired annulation products were obtained in only
some existing reactions, including cycloaddition or an- modest yields.[5b] We were motived to examine more
nulation reactions.[3] Since the interaction of photoex- conditions in order to improve the yields, as shown in
cited catalysts and small molecules is often specific,[4] Table 1. Using the [3+2]annulation of cyclopropylani-
the photocatalyzed transformations are usually highly line 1a with phenylacetylene 2a as the model reaction,
chemoselective and thus tolerate a variety of func- we first investigated the annulation in a continuous
tional groups. Therefore, in principle, photocatalyzed flow format[6] (entry 2). To our surprise, comparing
cycloaddition or annulation reactions are ideal reac- against the batch format (entry 1), although the reac-
tions for making small molecule libraries. We have re- tion time was shortened from 8 h to 3 h as expected,
cently developed a [3+2]annulation of cyclopropyl- the yield did not improve. We also examined the use
ACHTUNGTRENNUNG
anilines with alkenes and alkynes.[5] Although our pre- of cyclometalated iridium complexes to catalyze the
liminary results in these reactions gave a glimpse of annulation. These complexes are another important
their potential in DOS, constraints with respect to class of photocatalysts that finds rising numbers of
both reacting partners, cyclopropylanilines and al- synthetic applications in photochemistry.[7] Two iridi-
kenes or alkynes, existed. An aryl group was required um complexes were examined and neither provided
on the cyclopropylamine moiety while only terminal a better yield than RuACHTNUGTRENN(UG bpz)3ACHTUNTGREN(NGUN PF6)2 (entries 3 and 4).
alkenes and alkynes were reactive.[5] We intended to Lastly, we performed the reaction in the presence of
address both limitations with the ultimate goal of TEMPO (entry 5). We previously proposed distonic
thoroughly investigating the scope of the [3+2]annu- ion 1aC+ as one of the key intermediates in the
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Table 1. Reaction optimization.[a]
Entry
Catalyst
Ru(bpz)
Solvent
Light
GC Yield [%][b]
1
2
3
G
G
CH3NO2
CH3NO2
CF3CH2OH
CF3CH2OH
CH3NO2
18 W LED
flow[d]
18 W LED
18 W LED
18 W LED
69 (68)[c]
Ru(bpz)
Ir(ppy)2A(dtb-bpy)
Ir[dF(CF3)ppy]2A(dtb-bpy)
Ru(bpz)3A(PF6)2
T
60
60
36
7
A
U
E
4
E
G
ACHTUNGRTEN(NUNG PF6)
5[e,f]
C
[a]
A solution of 1a (0.2 mmol), catalyst (2 mol%), and 2a (5 equiv.) in 2 mL of solvent was degassed via freeze-pump-thaw
cycles. The resulting solution was irradiated using an 18 W white LED for 8 h.
Using dodecane as an internal standard.
Isolated yield.
Flow reaction time of 3 h.
19 hour reaction.
[b]
[c]
[d]
[e]
[f]
Using 1 equiv. of TEMPO.
(DOS). The expected annulation products would pos-
sess a 1,3-conjugated enyne moiety from 1,3-conjugat-
ed diynes or a 1,3-conjugated diene moiety from 1,3-
conjugated enynes. Both moieties can be further en-
gaged in cycloaddition reactions such as the Diels–
Alder reaction, and thereby enable cycloaddition cas-
cades to rapidly assembly complex fused carbocycles
or heterocycles. However, 1,3-conjugated diynes and
enynes present more challenges than simple alkynes
Scheme 1. Proposed intermediate distonic ion in the in the annulation. We previously reported that the an-
[3+2]annulations.
nulation was sensitive to substitution on alkynes as
well as their electronic characters. For example, alkyl-
substituted terminal alkynes and internal alkynes
[3+2]annulation of cyclopropylanilines with alkenes were found to be unreactive.[5b] It was not clear to us
and alkynes (Scheme 1). This ion presumably results whether addition of one more p bond would enhance
from ring opening of cyclopropylanilines that is in- the original p bondꢁs reactivity enough to take part in
duced by one-electron oxidation of the parent amine the annulation. Moreover, prediction of regiochemis-
(PF6)2.[5a] The use of try in 1,3-conjugated diynes and enynes could be chal-
by the excited state of RuACHTNUGRTNENUG(bpz)3ACHTUNGTRENNUGN
TEMPO was designed to intercept the radical moiety lenging. Finally, for unsymmetric diynes and enynes,
of the ion and thereby perturb the annulation. since there are two different p bonds, the issue of che-
Indeed, the annulation product was isolated in a negli- moselectivity may arise as addition to either or both
gible yield, which lent credence to our argument for p bonds can occur.
+
the involvement of distonic ion 1aC . Since no im-
To our delight, under the standard conditions, sym-
provement in the yield was achieved using the flow or metrical and asymmetrical 1,3-conjugated diynes suc-
the Ir catalysts, we settled for the previously opti- cessfully underwent the intermolecular [3+2] annula-
mized conditions (entry 1, Table 1) as the standard tion with monocyclic cyclopropylaniline 1b to afford
conditions for the scope studies.
the conjugated cyclopentene adducts (Table 2). Sym-
metrical diynes bearing phenyl 2b, methyl 2c, and hy-
droxymethyl 2d groups proceeded in fair yields (en-
tries 1–3). Asymmetrical diynes also produced the an-
nulation products in similar yields (entries 4–7). These
Diyne and Enyne Studies
We were intrigued by the possibility of using 1,3-con- diynes 2e–h all bore a phenyl group on one end, while
jugated diynes and enynes in the [3+2]annulation as a variety of moieties, such as n-Bu and hydroxymethyl
they offer the potential to further enhance the value groups, were tolerated on the other end. A quaternary
of the annulation in diversity-oriented synthesis center adjacent to the reactive carbon center of diyne
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Table 2. Scope studies using symmetrical and unsymmetrical diynes.[a]
[a]
A solution of 1b (0.2 mmol), RuACHTNUTRGENN(UG bpz)3ACHTUGNTRNE(NUGN PF6)2 (2 mol%), and 2 (5 equiv.) in 2 mL of
solvent was degassed via freeze-pump-thaw cycles. The resulting solution was irradiat-
ed using an 18 W white LED.
Ratio >20:1, determined using GC.
Isolated yields.
Ratio 12:1, determined using GC.
Unidentified minor isomer.
[b]
[c]
[d]
[e]
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Theresa H. Nguyen et al.
yields (entries 2 and 3). In both cases, the annulation
favored the terminal alkyne moiety of enyne 2j. It is
worth noting that for these two enynes (2i and 2j),
the catalyst system composed of IrACHTNUTRGENNUG(ppy)2ACHTUNGTRENN(UGN dtb-bpy)
(PF6) in a 1:1 mixture of CH3NO2 and CF3CH2OH
proved to be more effective than the standard catalyst
system [RuACHTNUGTRENNU(G bpz)3ACHTUNGTERN(NUNG PF6)2 in CH3NO2]. Likewise for
synthetically prepared enyne 2k[8] bearing a terminal
alkyne and a 1,2-disubstituted alkene, the furnished
annulated product 6d was isolated in 59% combined
yields, with the 1,3-diene as the major isomer
(entry 4). Lastly, enyne 2l, possessing both an internal
alkyne and a 1,2-disubstituted alkene, was prepared
according to a literature protocol[9] and then treated
with cyclopropylaniline 1b under the standard condi-
tions. The major products 6e, composed of four dia-
Figure 1. Rationale for the observed selectivity using DFT
calculations.
2e was also well tolerated. Although the yields for the stereomers derived from the annulation on the alkene
annulation products were modest, complete regiocon- moiety, was obtained in 66% combined yields
trol was observed universally in all but one example (entry 5).
(entry 7). Even with the exception 3h, the regioselec-
tivity was excellent (12:1). We rationalized the ob-
served excellent regioselectivity based on the stability Substituted Cyclopropanes
À
of radicals generated after the first C C bond forma-
tion. DFT calculations on two regioisomeric radicals Next, we shifted our attention to focus on the scope
(5a and 5b) that model the proposed regioisomeric of cyclopropylanilines. To maximize the potential of
radicals (4a and 4b) in the annulation supported this the [3+2]annulation, it was imperative for us to ex-
argument (Figure 1). Radical 5a was found to be amine substituents on the nitrogen atom and the cy-
44 kJmolÀ1 more stable than radical 5b due to reso- clopropyl ring. Particularly for the latter, if succeeded,
nance. The stability trend of the two regioisomeric it would allow us to decorate the cyclopentene ring of
radicals was also observed in diyne 2b (see the Sup- the annulation products. Using phenylacetylene 2a as
porting Information).
the model alkyne, we first investigated the annulation
The annulation with enynes was expected to be of N-cyclopropyl-N-methylaniline 1d to form 7a and
more complicated than with diynes. In addition to re- no reaction was observed (entry 1, Table 4). This is in
gioselectivity, chemoselectivity has to be dealt with accordance with the result reported by Tanko and co-
because of the competition between the alkene and workers in which the ring opening of the amine radi-
alkyne moiety in the annulation. We found that in our cal cation derived from 1d was found to be very slug-
previous work, alkenes are more reactive than al- gish.[10] Despite this observation, substitution on the
kynes in general.[5b] However, because of steric ef- cyclopropyl ring was generally tolerated. N-(1-methyl-
fects, this reactivity trend can be reversed if the cyclopropyl)aniline 1e provided the annulation prod-
alkene is more substituted than the alkyne. Further- uct 7b bearing a quaternary carbon center in 33%
more, if the alkene moiety is more reactive than the yield when subjected to the standard conditions
alkyne moiety, the diastereoselectivity also needs to (entry 2). The [3+2]annulation was also effective
be addressed.
using 2-substituted cyclopropyl rings, as demonstrated
Indeed, 1,3-conjugated enynes 2i–l produced a mix- with methyl 1f and phenyl 1g substituents (entries 3
ture of isomeric products in the annulation (Table 3). and 4). In both examples, the ring opening was com-
Commercially available enyne 2i, possessing both pletely regioselective, presumably forming the more
a terminal alkyne and a terminal alkene, underwent substituted carbon radical. Modest diastereoselectivity
the [3+2]annulation predominately on the alkene was observed, providing the cis isomer as the major
moiety to provide the annulation products in 50% product in both cases.
combined yields (entry 1), with the minor isomer
being identified as the annulation on the alkyne. The
chemoselectivity was 10 to 1, favoring the alkene Cleavage of N-Aryl Groups
moiety. Another commercially available enyne, 2j,
bearing a terminal alkyne and a trisubstituted alkene, The requirement of an aniline moiety has been a limi-
was subjected to the annulation with cyclopropylani- tation to the [3+2]annulation, as it lowers the gener-
lines 1b and 1c, respectively, to afford the correspond- ality of the reaction. To circumvent this limitation, we
ing conjugated 1,3-dienes 6b and 6c in moderate were interested in installing a removable aryl or het-
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Table 3. Scope studies using enynes.[a]
[a]
A solution of cyclopropylaniline (0.2 mmol), IrACTHUNGTREN(NUNG ppy)2ACHTUNRTGEG(NNNU dtb-bpy)ACHTNGUTNER(NUGN PF6) (2 mol%), and enyne (5 equiv.)
in 2 mL CH3NO2:CF3CH2OH (1:1), was degassed via freeze-pump-thaw cycles. The resulting solu-
tion was irradiated using an 18 W white LED.
Major isomer shown.
Isolated yields.
Isomer ratios (determined by GC-MS) shown in parentheses.
7:3 dr.
Unidentified minor isomers.
Using RuACTHUNGTRENNG(U bpz)3ACHTUNTGERN(NUGN PF6)2.
Identified as the cis-alkene of 6d.
Ratio 10:1 trans:cis.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
[j]
All identified as diastereomers of 6e. See the Supporting Information for details.
eroaryl group that is also capable of mediating the an-
ACHTUNGTRENaNUGN mine 1h (entry 1). Surprisingly, 2-pyridyl-substituted
nulation. Some of these potential candidates include cyclopropylamine 1i afforded the annulation product
pyrimidine 1h,[11] pyridine 1i,[12] and para-methoxy- in a low yield (13%), which was not synthetically
phenyl (PMP, 1k)[13] groups. With this in mind, we useful (entry 2). In comparison, a much higher yield
synthesized N-arylcyclopropylamines 1h–k bearing (48%) was obtained with 3-pyridyl-substituted cyclo-
these groups and subjected them under the standard propylamine 1j (entry 3). Unfortunately, the 3-pyridyl
conditions to reaction with phenylacetylene 2a. The group was not cleavable. A similar yield (47%) was
results are summarized in Table 5. No reaction was achieved with PMP-substituted cyclopropylamine 1k
observed with 2-pyrimidyl-substituted cyclopropyl-
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Table 4.
lines.[a]
[3+2] Annulation of substituted cyclopropylani-
Table 5. ACTHNGUTERNNUG
[3+2]Annulation of various arylamines.[a]
ACHTUNGTRENNUNG
[a]
A solution of 1 (0.2 mmol), RuACHTUNRGTENNGU(bpz)3ACHTUNTGRNE(NUGN PF6)2 (2 mol%),
and 2a (5 equiv.) in 2 mL CH3NO2 was degassed via
freeze-pump-thaw cycles. The resulting solution was irra-
diated using an 18 W white LED.
Monitored by TLC.
Isolated yields.
Determined using GC-MS.
[a]
A solution of 1 (0.2 mmol), RuACHTUNRGTENNGU(bpz)3ACHTUNTGRNE(NUGN PF6)2 (2 mol%),
[b]
[c]
[d]
and 2a (5 equiv.) in 2 mL of CH3NO2 was degassed via
freeze-pump-thaw cycles. The resulting solution was irra-
diated using an 18 W white LED.
Isolated yields.
Determined using GC-MS.
Major isomer shown.
Ratio 2:1 cis:trans, determined by GC-MS.
Ratio 4:1 cis:trans, determined by GC-MS.
[b]
[c]
[d]
[e]
[f]
(entry 4). Therefore, we opted to focus on studying
the cleavage of the PMP group in 8d.
The PMP group is generally cleaved under oxida-
tive conditions, and a number of oxidants are suitable
for the cleavage.[13] We subjected 8d to these oxidants,
including ceric ammonium nitrate (CAN), periodic
Scheme 2. Removal of the para-methoxyphenyl (PMP)
group.
acid (H5IO6), trichloroisocyanuric acid (TCCA), and Conclusions
CAN/Fe(bpy)3A
A
CHTUNGTRENNUNG
ined, CAN in sulfuric acid gave the highest yield. The In summary, we have significantly expanded our stud-
cleavage product was initially isolated as the HCl salt ies on the [3+2]annulation of cyclopropylanilines
9 and then immediately acylated in the presence of with alkynes. These studies were conducted on multi-
Et3N and acetyl chloride to give the corresponding ple fronts including catalyst optimization, mechanistic
acetamide 10 in 68% isolated yield over 2 steps studies, expansion of the substrate scope for both cy-
(Scheme 2). The successful removal of the PMP clopropylanilines and alkynes, and identification of
group circumvents the previous limitation of our a removable N-aryl group for cyclopropylamines.
chemistry and allows for more structural diversifica- Using simple building blocks, the [3+2]annulation en-
tion of the [3+2]annulation products.
ables rapid assembly of diverse cyclic allylic amine
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Intermolecular [3+2]Annulation of Cyclopropylanilines
eral Medical Sciences of the National Institutes of Health
(NIH) (Grant Number P30 GM103450), NSF Career Award
under Award Number CHE-1255539. We thank Jiang Wang,
Soumitra Maity, and Mingzhao Zhu for early experimental
assistance of this work as well as Derrel Walters and Profes-
sor Peter Pulay for the DFT calculations.
derivatives. Most of these amines possess embedded
functional groups that allow for further structural di-
versification. We anticipate that this method will find
usage particularly in diversity-oriented synthesis
(DOS).
Experimental Section
References
General Procedure for the [3+2] Annulation of
Cyclopropylanilines with Alkyne, Enyne, and Diyne
[1] Comprehensive Organic Synthesis, 2ednnd ednVol. 5,
(Eds.: P. Knochel, G. A. Molander), Elsevier, Amster-
dam, 2014.
An oven-dried test tube (16ꢂ125 mm) equipped with a stir
bar was charged with catalyst (2 mol%), cyclopropylaniline
(0.2 mmol), alkyne, enyne, or diyne (1.0 mmol), and dry sol-
vent (2 mL). The test tube was sealed with a Teflon screw
cap. The reaction mixture was degassed by freeze-pump-
thaw cycles and then irradiated at room temperature with
one white LED (18 W) positioned 8 cm from the test tube.
After the reaction was complete as monitored by TLC, the
mixture was diluted with diethyl ether and filtered through
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vacuum and purified by silica gel flash chromatography to
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a
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(51 mg, 0.2 mmol) in 3:1 CH3CN:H2O (2 mL) was slowly
added concentrated H2SO4 (22 mL, 0.385 mmol) at 08C.
Ceric ammonium nitrate (220 mg, 0.4 mmol) was then added
in one portion, and the mixture was stirred for one hour at
08C. The resulting mixture was then diluted with water
(2 mL) and separated. The aqueous phase was then washed
with Et2O (3ꢂ5 mL). The combined organic phase was then
extracted with 0.1N HCl (1ꢂ15 mL), and the obtained
aqueous phase was added to the previous aqueous mixture,
which was immediately basified to pH 14 using 5N KOH.
The basic aqueous layer was then extracted with Et2O (2ꢂ
30 mL). The combined organic layer was then acidified to
pH 1 using hydrogen chloride (2M in Et2O). The resulting
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CH2Cl2 (5 mL) was added Et3N (56 mL, 0.4 mmol) dropwise
at room temperature. Acetyl chloride (16 mL, 0.22 mmol)
was slowly added and the reaction mixture was stirred for
6 h at room temperature. The reaction was quenched with
water (10 mL) and the layers were separated. The aqueous
phase was extracted with CH2Cl2 (3ꢂ10 mL). The combined
organic layers were dried over MgSO4 and concentrated to
give the crude product. Purification using silica gel chroma-
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Acknowledgements
This publication was supported by the University of Arkan-
sas, Arkansas Bioscience Institute, National Institute of Gen-
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8 Intermolecular [3+2]Annulation of Cyclopropylanilines
with Alkynes, Enynes, and Diynes via Visible Light
Photocatalysis
Adv. Synth. Catal. 2014, 356, 1 – 8
Theresa H. Nguyen, Scott A. Morris, Nan Zheng*
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