Nonmetathesis Heterocycle Formation by Ruthenium-Catalyzed Intramolecular [2 + 2] Cycloaddition of Allenamide-enes to Azabicyclo[3.1.1]heptanes

Article abstract of DOI:10.1021/acscatal.6b00628

We have developed a novel nonmetathesis reaction, namely, ruthenium-catalyzed intramolecular [2 + 2] cycloaddition of allenamide-enes, to give heterocycles (i.e., azabicyclo[3.1.1]heptanes). This is the first example of [2 + 2] cycloaddition using a ruthe

Full text of DOI:10.1021/acscatal.6b00628

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Letter
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Nonmetathesis Heterocycle Formation by Ruthenium-Catalyzed
Intramolecular [2 + 2] Cycloaddition of Allenamide-enes to
Azabicyclo[3.1.1]heptanes
Tomomi Nada,†,‡ Yusuke Yoneshige,†,§ Yasuhiro Ii,§ Takashi Matsumoto,# Hiromichi Fujioka,§
Satoshi Shuto,‡ and Mitsuhiro Arisawa*,‡,§
‡Faculty of Pharmaceutical Sciences, Hokkaido University, Kita 12, Nishi 6, Kita-ku, Sapporo 060-0812, Japan
§Graduate School of Pharmaceutical Sciences, Osaka University, Yamada-oka 1-6, Suita, Osaka 565-0871, Japan
#X-ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan
S
* Supporting Information
ABSTRACT: We have developed a novel nonmetathesis reaction,
namely, ruthenium-catalyzed intramolecular [2 + 2] cycloaddition of
allenamide-enes, to give heterocycles (i.e., azabicyclo[3.1.1]heptanes). This
is the first example of [2 + 2] cycloaddition using a ruthenium carbene
catalyst. The reaction proceeds at room temperature, but not under
thermal or radical conditions.
KEYWORDS: metathesis, heterocycles, ruthenium, cycloaddition, allenamide
lefin metathesis using a ruthenium carbene catalyst (see
Figure 1, compounds A−G) is a versatile carbon−carbon
We have made several contributions to this research field and
have already reported several nonmetathesis reactions, i.e.,
isomerizations7n of terminal olefins, cycloisomerizations,18a,c
and one-pot metathesis and subsequent nonmetathesis
reactions to give novel heterocyclic dyes.16c
O
Allenamides are important functional groups in organic
synthesis, and many allenamides reactions have been
developed.20 Allenamides reactions using organometallic
catalysts have been reported, but nonmetathesis reactions
with ruthenium carbene catalysts are limited to isomerizations
and cycloisomerizations (see eqs 119 and 218b in Scheme 1 ).21
These reactions both proceed via a ruthenium hydride species
with a nitrogen-containing heterocyclic carbene ligand
generated from a Grubbs II catalyst.
Here, we report intramolecular [2 + 2] cycloadditions of
terminal allenes, N-allenyl-o-vinylaniline (1), at room temper-
ature, catalyzed by Grubbs I, to give novel bicyclic heterocycles:
azabicyclo[3.1.1]heptanes. The reaction does not proceed
under thermal or radical conditions or via ruthenium hydride
species. As far as we know, there have been no reports of [2 +
2] cycloadditions involving a nonmetathesis reaction.22
Figure 1. Ruthenium catalysts.
t
Compound 2a was treated with BuOK (0.5 equiv) in THF
double-bond-forming reaction; it is widely used to prepare
complex organic compounds.1 Ruthenium alkylidenes such as
A,2 B,3 C,4 and D,5 which are widely used for olefin metathesis,
have been shown to function as procatalysts6 in olefin
isomerizations,7 hydrogenations,8 radical reactions,9 activation
of silanes,10 cyclopropanations,11 cyclopropane epimeriza-
tions,12 oxidations,13 hydrovinylations,14 [4 + 2] cyclo-
additions,15 [3 + 2] cycloadditions,16 [2 + 2 + 2] cyclo-
additions,17 and cycloisomerizations.18
at 0 °C for 10 min to form 1a, which was treated, without
purification, with a catalytic amount of one of the ruthenium
carbene catalysts in Figure 1 for 1.5 h. The [2 + 2]
cycloaddition proceeded at room temperature with Grubbs I
Received: March 1, 2016
Revised: March 30, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acscatal.6b00628
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ACS Catalysis
Letter
Scheme 1. Reactions of Allenes Using Ruthenium Carbene
Catalysts
Figure 2. X-ray structure of 3a. (Space group: P21/n. R1 [I > 2.0σ(I)]:
4.60%. wR [all data]: 12.13%. Goodness of fit (GOF): 1.033.)
Based on these results, we next examined the effect of a
substituent on nitrogen (Table 2). A 2-mesitylenesulfonyl
Table 2. Effect of the Nitrogen-Protecting Group
b
entry
substrate
R
yield of 3 (%, two steps)
catalyst A to give 3a in 62% yield in two steps (Table 1, entry
1). The crystal structure of 3a was determined using single-
1
2
3
4
5
2a
2b
2c
2d
2e
Ts
76
78
43
0
a
Mts
Ms
Ac
Table 1. Reaction of N-allenyl-o-vinylaniline 1a with a
Ruthenium Carbene Catalyst
Boc
23
a
b
Mts = 2,4,6-trimethylphenylsulfonyl. Isolated yield (two steps).
(Mts) derivative 2b and methanesulfonyl (Ms) derivative 2c
gave the corresponding [2 + 2] cycloadducts in yields of 78%
and 43%, respectively; the yield difference is probably the result
of steric effects. Derivatives with weaker electron-withdrawing
groups on nitrogen, i.e., acetyl derivative 1d and tert-
butoxycarbonyl derivative 1e, did not take part in this reaction;
1d was unchanged and 1e was converted to 3e in 23% yields. In
these experiments, we observed that allenamides with weaker
electron-withdrawing groups gradually decomposed.
a
b
entry “Ru” (mol %) solvent temp (°C) yield of 3a (%, two steps)
1
2
A (10)
B (10)
C (10)
D (10)
E (10)
F (10)
G (10)
H (10)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
rt
rt
62
0
3
rt
14
0
4
rt
5
rt
10
3
6
rt
7
rt
31
0
The effects of substituents on the benzene ring are
summarized in Table 3; the data show the scope, limitation,
and mechanism of this [2 + 2] cycloaddition. All the substrates
8
rt
9
110
rt
0
10
11
12
A (10)
A (10)
A (10)
56
76
71
rt
Table 3. Effect of Benzene Ring Substituents
c
THF
rt
a
b
The abbreviation “rt” denotes room temperature. Isolated yield (two
c
steps). Here, galvinoxyl (10 mol %) was added.
a
crystal X-ray diffraction (XRD); it contained four planar rings
(Figure 2).23 Only catalyst A was effective in this novel
nonmetathesis reaction, and other ruthenium carbene catalysts
in Figure 1, including Grubbs II catalyst (B), Hoveyda−Grubbs
I catalyst (C), Hoveyda−Grubbs catalyst II (D), and
RuHCl(CO) (PPh3)3 (H), did not work well (see Table 1,
entries 2−8). Solvent screening was performed, and we
obtained 3a from 2a in 76% yield, in two steps, when we
used THF as the solvent in the second step, i.e., the [2 + 2]
cycloaddition (entry 11). Although [2 + 2] cycloadditions
proceed under heating or radical conditions, control experi-
ments (Table 1, entries 9 and 12) clearly showed that our [2 +
2] cycloaddition does not require thermal or radical conditions.
entry
substrate
R
yield (%, two steps)
1
2
2b
2f
H
78
70
69
73
16
68
70
69
3-OMe
4-OMe
5-OMe
6-OMe
3-Cl
3
2g
2h
2i
4
5
6
2j
7
2k
2l
4-Cl
8
5-Cl
b
9
2m
2n
6-Cl
0
10
4-CO2Me
63
a
b
Isolated yield (two steps). Starting material was recovered.
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2b−2n were prepared as Mts amides and were subjected to the
reaction conditions used for Table 1, entry 7 (see Table 3).
Allenamides 2f, 2g, and 2h and allenamides 2j, 2k, and 2l,
which have a substituent at the 3-, 4-, or 5-position,
respectively, were converted to the corresponding [2 + 2]
cycloadducts 3 in good yields. However, allenamides 1i and
1m, with a substituent at the 6-position, were respectively
converted to 3i in 16% yield and unchanged (see Table 3,
entries 5 and 9). There were sharp contrasts between 1,2,3-
trisubstituted and 1,2,6-trisubstituted substrates, although both
have substituents at the ortho position. These results suggest
that some ruthenium species24 react with the allene moiety
faster than the styrene moiety on allenamide 1 in this [2 + 2]
cycloaddition.
carbene catalyst. It will be of interest because ruthenium
carbene catalysts are widely used in functional molecular
synthesis.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00628.
■
S
Experimental procedures and full characterizations of
compounds (PDF)
X-ray structure report for compound 3a (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: arisaw@phs.osaka-u.ac.jp.
■
Finally, we used NMR spectroscopy to obtain information on
the active ruthenium species in this [2 + 2] cycloaddition. We
1
compared the H and 31P NMR spectra in C6D6 of catalyst A
Author Contributions
†T.N. and Y.Y. contributed equally to this work.
and the reaction mixture for the [2 + 2] cycloaddition. For
catalyst A, we observed signals for the benzylidene proton at
Notes
1
20.6 ppm in the H NMR spectrum, and tricyclohexylphos-
The authors declare no competing financial interest.
phine at 36.82 ppm in the 31P NMR spectrum (Figure 3). In
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan, ACT-C from the Japan
Science and Technology Agency, and the Takeda Science
Foundation.
Figure 3. Summary of NMR spectroscopic data for catalyst A.
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this reaction was drawn in Figure 4.
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