The Cleavage of a C-C Bond in Cyclobutylanilines by Visible-Light Photoredox Catalysis: Development of a [4+2] Annulation Method

Article abstract of DOI:10.1002/anie.201504076

We report the first example of an intermolecular [4+2] annulation of cyclobutylanilines with alkynes enabled by visible-light photocatalysis. Monocyclic and bicyclic cyclobutylanilines successfully undergo the annulation with terminal and internal alkynes to generate a wide variety of amine-substituted cyclohexenes including new hydrindan and decalin derivatives with good to excellent diastereoselectivity. The reaction is overall redox neutral with perfect atom economy.

Full text of DOI:10.1002/anie.201504076

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201504076
German Edition: DOI: 10.1002/ange.201504076
Photocatalysis
À
The Cleavage of a C C Bond in Cyclobutylanilines by Visible-Light
Photoredox Catalysis: Development of a [4+2] Annulation Method**
Jiang Wang and Nan Zheng*
Abstract: We report the first example of an intermolecular
[4+2] annulation of cyclobutylanilines with alkynes enabled
by visible-light photocatalysis. Monocyclic and bicyclic cyclo-
butylanilines successfully undergo the annulation with terminal
and internal alkynes to generate a wide variety of amine-
substituted cyclohexenes including new hydrindan and decalin
derivatives with good to excellent diastereoselectivity. The
reaction is overall redox neutral with perfect atom economy.
Scheme 1. Proposed [4+2] annulation of cyclobutylaniline with alkyne.
T
he release of ring strain has often been exploited as
a driving force to promote the cleavage of typically unreactive
bonds such as carbon–carbon bonds.[1] This strategy has been
successfully applied to the ring opening of cyclopropanes,
which leads to the wide use of cyclopropanes in organic
synthesis.[2] We recently reported visible-light-promoted
[3+2] annulation reactions of cyclopropylanilines with vari-
ous p bonds.[3] In these reactions, the cleavage of the
cyclopropyl rings is promoted by photooxidation of the
parent amine to the corresponding amine radical cation.
Since the oxidation potentials of cyclopropylaniline and
cyclobutylaniline were found to be similar,[4] we were
intrigued by the possibility of using cyclobutylanilines in
a similar manner in the annulation reactions. We were further
encouraged by the fact that cyclobutaneꢀs strain energy is
almost identical to that of cyclopropane.[5,6]
the best of our knowledge, there have been no reports of using
cyclobutylanilines in the tandem ring opening and annulation
sequence to construct six-membered carbocycles. This is in
contrast to the increasing use of other types of cyclobutanes in
the tandem sequence.[10] Successful ring opening of these
cyclobutanes usually relies on the decoration of the cyclobutyl
ring with donor–acceptor[11] or ketone/hydroxy functionali-
ties.[12] Herein we describe the successful development of the
[4+2] annulation of cyclobutylanilines with alkynes by visible-
light photoredox catalysis, which greatly broadens the use of
cyclobutanes as a four-carbon synthon in organic synthesis.
We hope that this method will add to the growing pool of
synthetic methods based on photocatalysis.[13]
We chose 4-tert-butyl-N-cyclobutylaniline 1a and phenyl-
acetylene 2a as the standard substrates to optimize the [4+2]
annulation. After extensive screening (see the Supporting
Information, SI), [Ir(dtbbpy)(ppy)2](PF6) 4a in MeOH under
Still, the ring opening of cyclopropanes is generally much
faster than that of cyclobutanes. Newcomb et al. reported that
the rate constant for ring opening of a cyclobutylcarbinyl
radical was 1.5 ” 103 sÀ1 at 208C compared to 7 ” 107 sÀ1 at
208C for ring opening of a cyclopropylcarbinyl radical.[7]
Ingold et al. reported the rate constant for ring opening of
the cyclobutyl-n-propylaminyl radical to be 1.2 ” 105 sÀ1 at
258C whereas the rate constant was estimated to be greater
than 107 sÀ1 at 258C for ring opening of the cyclopropyl-n-
propylaminyl radical (Scheme 1).[8] The proposed ring open-
ing of the cyclobutylaniline radical cation was expected to be
faster than the neutral aminyl radical,[9] which lent further
credence to the proposed annulation reaction. Surprisingly, to
Table 1: Optimization of the reaction conditions.
Entry[a] Conditions
t [h] Conv. of
1a [%][b]
Yield of
3a [%][b]
1
2
3
4
4a (2 mol%), MeOH
4a (2 mol%), MeOH, air
without 4a, MeOH
12 100
16 100
97 (90)[c]
42
3
7
16
7
[*] J. Wang, Prof. Dr. N. Zheng
4a (2 mol%), MeOH, light 16
10
Department of Chemistry and Biochemistry, University of Arkansas
Fayetteville, AR 72701 (USA)
E-mail: nzheng@uark.edu
bulb off
4a (2 mol%), MeOH
4a (2 mol%), MeOH
5[d]
6[e]
12
12
70
29
68
27
Homepage: http://www.zhenggroup.org
[**] We thank the University of Arkansas, the Arkansas Bioscience
Institute, the National Institutes of Health (NIH) (grant number
P30 GM103450) from the National Institute of General Medical
Sciences, and NSF Career Award (Award Number CHE-1255539) for
generous support of this research.
[a] Reaction conditions: 1a (0.2 mmol, 0.1m in degassed solvent), 2a
(1 mmol), irradiation with two 18 W LED light bulbs at room temper-
ature. [b] Yield determined by GC analysis using dodecane as an internal
standard unless noted otherwise. [c] Yields of isolated products are
shown. [d] One 18 W LED light, reaction tube in a 558C water bath. [e]
One 18 W LED light. dtbbpy=4,4’-di-tert-butyl-2,2’-bipyridine, ppy=2-
phenylpyridine.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504076.
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Chemie
irradiation with two 18 W LEDs was identified as the optimal
conditions, providing the desired product 3a in 97% yield as
determined by gas chromatography (GC; 90% yield of the
isolated product; Table 1, entry 1). Conducting the experi-
ment without degassing the reaction mixture led to a signifi-
cant decrease in the yield (entry 2; see SI). Two control
studies showed that omitting either the photocatalyst or light
produced a negligible amount of 3a (entries 3 and 4). The
beneficial effect of using two LED lights could be due to an
increased exposure to light and to raising the reaction
temperature. An internal temperature of 558C was measured
with two bulbs whereas 258C was recorded with only one
bulb. To further probe this phenomenon, we performed
a temperature control study in which the test tube was placed
in a 558C water bath with one LED light (entry 5). Though
the yield was not as high as when two LED lights were used,
the product 3a was detected in 68% yield. This improvement,
in comparison to using one LED light at room temperature
(entry 6), suggests that higher temperature and photon output
are both beneficial to the reaction.
alkyl) on the aryl ring. The low yield of 3d (entry 3) was due
to the low solubility of 1d in MeOH. Use of a cosolvent, such
as DMF, helped to solubilize 1d but failed to improve the
yield. Steric hindrance was also well tolerated as ortho
substituents with various sizes (1e–1g) showed little effect on
the reaction. Moreover, it is worth noting that heterocycles
can be easily incorporated into the annulation products. The
cyclobutylanilines (1i and 1j) substituted by a pyridyl group
at the 2- or 3-position underwent the annulation reaction
uneventfully, affording the desired products (3i and 3j) in
good yields.
Encouraged by the success in the initial scope studies, we
turned our attention to the generality of alkynes (Table 3).
The reactivity pattern displayed by alkynes in the annulation
reaction generally resembles that in the intermolecular
addition of nonpolar nucleophilic alkyl radicals to alkynes.[15]
Terminal alkynes substituted with a functional group that is
capable of stabilizing the incipient vinyl radical, were found to
be viable annulation partners. The list of functional groups
includes quite diverse groups such as naphthyl (2b), 1,3-
benzodioxole (2c), methyl ester (2d), and thiophene (2e).
The annulation of these alkynes with several cyclobutylani-
We next examined the scope of cyclobutylaniline deriv-
atives with phenylacetylene 2a as annulation partner under
optimized conditions (Table 2). Cyclobutylanilines were read-
ily prepared by the Buchwald–Hartwig amination of cyclo-
butylamine with aryl halides.[14] The annulation reaction
generally tolerated both electron-withdrawing (e.g., CF3 and
CN) and electron-donating substituents (e.g., OMe, Ph, and
Table 3: Scope of alkynes in the [4+2] annulation.
Entry[a] Substrate Alkyne
Product
t [h] Yield [%][b]
Table 2: [4+2] Annulation of phenylacetylene (2a) with monocyclic
cyclobutylanilines.
1
1a
1a
1j
16 71
2[c]
16 57
16 72
14 42
12 66
14 61
Entry[a]
1
Substrate
Product
t [h]
Yield [%][b]
76
1b, R=H
12
3
2b
2
3
4
5
6
1c, R=4-CF3
1d, R=4-OMe
1e, R=2-CN
1 f, R=2-phenyl
1g, R=2-iPr
24
18
20
20
24
79
27
84
78
73
4
1 f
1a
1a
5
6
7
8
1h
1c
24 42
7
1h,
12
87
8
9
1i,
1j,
14
18
76
83
12 92
[a] Reaction conditions: substrate (0.2 mmol, 0.1m in degassed MeOH),
2b–2h (0.6 mmol), 4a (2 mol%), irradiation with two 18 W LED light
bulbs. [b] Yield of the isolated product. [c] Mixed solvent of MeOH and
CH3NO2 (1:1).
[a] Reaction conditions: substrate (0.2 mmol, 0.1m in degassed MeOH),
2a (1 mmol), 4a (2 mol%), irradiation with two 18 W LED light bulbs.
[b] Yield of the isolated product.
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Table 4: [4+2] Annulation of alkynes with bicyclic cyclobutylanilines.
lines showed complete regioselectivity, affording
a range of six-membered carbocycles in good yields. A
notable side reaction occurred when less hindered
cyclobutylanilines were used in conjunction with
methyl propiolate 2d. 1,4-Addition of cyclobutylaniline
1k to 2d completely suppressed the desired [4+2]
annulation reaction.[16] An ortho substituent on the N-
aryl ring (e.g., 1 f) was required to inhibit the side
reaction. Typically, internal alkynes are less reactive
than terminal alkynes in the intermolecular addition of
carbon radicals to alkynes due to steric hindrance.[15]
Internal alkynes were unsuccessful in the annulation
with cyclopropylanilines.[3b, c] Surprisingly, divergent
reactivity emerged between cyclopropylanilines and
cyclobutylanilines with respect to this class of alkynes.
Several internal alkynes (2 f–h) successfully underwent
the annulation reaction with cyclobutylanilines under
complete regiocontrol. A limitation of utilizing internal
alkynes is that at least one of the two substituents must
be capable of stabilizing the vinyl radical. The regio-
chemistry of the annulation products (5 f–h) was
assigned based on 2D NMR spectroscopy. The structure
of the annulation product (5h) was further confirmed by
X-ray crystallography (see SI).[17] The regioselectivity
can be rationalized based on the substituentꢀs ability to
stabilize the incipient vinyl radical.
Entry[a] Substrate
Alkyne
Product
Yield [%][b]
(d.r.[c])
major
minor
95
(13:1)
1
2
2a
2a
n.a.
97
(11:1)
n.a.
n.a.
89
(20:1)
3
98
(4:1)
4
5
6
2h
2h
2a
89
(4:1)
94
(6:1)
Bicyclo[4.3.0]nonane (hydrindan) and bicyclo-
[4.4.0]decane (decalin) are two common structural
motifs in small organic molecules. Yet, only a handful
of methods such as the Diels–Alder reaction[18] and the
Robinson annulation[19] are available for their prepara-
tion. Hence, these structures are ideal targets to test the
scope of the [4+2] annulation reaction (Table 4). The
requisite starting materials, cis-fused 5,4-membered
(6a–d) and 6,4-membered (6e and 6 f) bicycles, were
readily accessible in four steps from commercially available
cyclopentene and cyclohexene, respectively.[20,21] Under the
optimized conditions, a pair of diastereomeric 5,4-membered
bicycles (6a and 6b), which differ in the stereochemistry at
C6, underwent the annulation with phenylacetylene 2a to
provide 7a as the major product in similar yields and almost
identical d.r.s (Table 4, entries 1 and 2). High diastereoselec-
tivity was achieved (> 10:1) with 7a being trans-fused. This
data is consistent with regioselective ring opening of 6a or 6b
92
(4:1)
7
2c
[a] Reaction conditions: substrate (0.2 mmol, 0.1m in degassed MeOH), 2a, 2c,
2h, 2i (0.6 mmol), and 4a (2 mol%), irradiation with two 18 W LED light bulbs
for 12 h. [b] Combined yields of the two isomers after column chromatography.
1
[c] Determined by H NMR analysis of the crude product. n.a.=not available.
lectivity was observed in comparison to 5,4-membered
bicycles (entries 6 and 7). Two cis-fused decalin derivatives
were obtained along with a third diastereomer whose relative
configuration was unidentified in both examples (6e and
6 f).[23] The structure and stereochemistry of the annulation
products were assigned by 2D NMR spectroscopy. X-ray
crystallographic analysis of 7d was also performed to further
support our assignments.[24]
A catalytic cycle similar to the [3+2] annulation is
proposed for the [4+2] annulation (see SI).[3b,c] The oxidation
peak potential of 1a was found to be 0.8 V versus SCE, which
is more positive than the reduction potential of the photo-
excited 4a (Ir3+*/Ir2+: 0.66 V vs. SCE). Although thermody-
namically unfavorable, such SET processes have been
reported.[25] Stern–Volmer quenching studies revealed that
cyclobutylaniline 1a quenches the photoexcited 4a whereas
alkynes 2a and 2h showed little quenching (see SI).
À
at the C5 C6 bond, which leads to formation of the identical
distonic radical iminium ion[22] and subsequent loss of the
stereochemical integrality of the C6 stereocenter. The
observed regioselective ring opening was probably driven
by the formation of a more stable secondary carbon radical
versus a primary radical. Incorporation of a strong electron-
withdrawing group (e.g., CF3; entry 3) into the N-aryl ring
showed little effect on both the yield and diastereoselectivity.
Internal alkyne 2h successfully participated in the annulation
reaction, affording the annulation products 7c and 7c’ in
excellent yield albeit lower diastereoselectivity when com-
pared to terminal alkynes (entries 1–3). The annulation of 6,4-
membered bicycles (6e and 6 f) with terminal alkynes (2a and
2c) furnished cis-fused decalin derivatives (7e, 7e’ and 7 f,
7 f’) in excellent yields, although a decrease in diastereose-
In conclusion, we have accomplished the first example of
cleaving C C bonds of cyclobutylanilines enabled by visible-
À
light photoredox catalysis. Monocyclic and bicyclic cyclo-
butylanilines undergo the [4+2] annulation with terminal and
internal alkynes to produce amine-substituted six-membered
carbocycles. Good to excellent diastereoselectivity is
observed for the latter class of the compounds, yielding new
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J. Am. Chem. Soc. 2005, 127, 6932 – 6933; e) N. Ishida, S. Sawano,
hydrindan and decalin derivatives. Finally, the approach we
have developed to overcome cyclobutylanilinesꢀ lower pro-
pensity for ring cleavage can be potentially applied to open
rings larger than three- and four-membered rings with
suitable built-in ring strain.
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Keywords: cyclohexenes · iridium catalyst ·
N-aryl cyclobutyl amines · photocatalysis · small ring systems
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[23] The third isomer from the annulation of 6e and 2a could not be
1
detected by H NMR spectroscopy or GC analysis of the crude
product. The diastereomer ratio for the three isomers (7 f/7 f’/the
third isomer) from the annulation of 6 f and 2c was measured by
GC to be 13:3:1.
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Received: May 4, 2015
Published online: July 24, 2015
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