Nbcl5/zN/PCy3-system-catalyzed intramolecular [2 + 2 + 2] cycloadditions of diynes and alkenes to form bicyclic cyclohexadienes

Article abstract of DOI:10.1021/acs.orglett.7b02672

A NbCl₅, Zn, and PCy₃ catalytic system that generated low-valent Nb species is used for the synthesis of bicyclic cyclohexadienes from diynes and simple alkenes. A phosphine ligand is important for stabilizing low-valent Nb in the cycloaddition. The bicyclic cyclohexadiene skeleton is important in transition-metal-catalyzed intramolecular cycloadditions.

Full text of DOI:10.1021/acs.orglett.7b02672

Letter
pubs.acs.org/OrgLett
NbCl /Zn/PCy ‑System-Catalyzed Intramolecular [2 + 2 + 2]
5
3
Cycloadditions of Diynes and Alkenes To Form Bicyclic
Cyclohexadienes
Keisuke Watanabe, Yasushi Satoh, Motofumi Kamei, Hirohisa Furukawa, Maito Fuji,
and Yasushi Obora*
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita,
Osaka 564-8680, Japan
*
S Supporting Information
ABSTRACT: A NbCl , Zn, and PCy catalytic system that
5
3
generated low-valent Nb species is used for the synthesis of
bicyclic cyclohexadienes from diynes and simple alkenes. A
phosphine ligand is important for stabilizing low-valent Nb in
the cycloaddition. The bicyclic cyclohexadiene skeleton is
important in transition-metal-catalyzed intramolecular cyclo-
additions.
ow-valent early transition metals are efficient in organic
Scheme 1. Synthesis of Bicyclic Hexadiene Derivatives from
Diynes and Terminal Alkenes
L
syntheses because of their electron-rich and Lewis acidic
1
properties; for example, Ti−alkyne complexes react with
1a
ketones to give allyl alcohols and Zr−alkyne complexes
1b,e−h
produce benzene derivatives.
species need to be generated at low temperatures (−78 to 0
C). Low-valent Nb species have greater thermal stability than
However, these low-valent
°
other early transition-metal species and can be used in organic
2
syntheses under ambient conditions. Previously, we reported
the Nb-mediated synthesis of indenes from aliphatic ketones
3
and aromatic internal alkynes. We also reported Nb−alkyne
4
complex mediated coupling reactions of aryl iodides. Recently,
diynes and terminal alkenes for preparing bicylic cyclo-
hexadienes without double-bond migration. For example, Ni-
the use of early transition metals in many catalytic organic
5
6
reactions, e.g., cycloaddition, reductive coupling, and C−H
12a
7
catalyzed reaction of diynes and α,β-enones and Ti-mediated
bond activation, have been reported. We previously developed
1c
reaction of diynes and homoallylic alcohols were reported.
Therefore, to the best of our knowledge, there is no report of
selective synthesis of bicyclic cyclohexadienes from intra-
molecular [2 + 2 + 2] cycloaddition between diynes and
simple alkenes, such as aliphatic- and aromatic alkenes, without
double-bond migration.
Nb-catalyzed intermolecular [2 + 2 + 2] cycloadditions of
8
,9
alkynes with alkenes or nitriles and isocyanates. In these
reactions, Nb-catalyzed cycloadditions of alkynes and alkenes
produce cyclohexadienes with high chemo- and regioselectivi-
8
,9a
ties.
A few examples of transition-metal-catalyzed inter-
molecular cycloadditions to give cyclohexadienes have been
1
0
In this paper, we report the intramolecular [2 + 2 + 2]
cycloaddition of diynes and simple alkenes catalyzed by low-
reported.
The selectivities of transition-metal-catalyzed intramolecular
cycloadditions are easier to control than those of intermolecular
valent Nb species formed from NbCl /Zn/tricyclohexylphos-
5
1
1
phine (PCy ) to give bicyclic cyclohexadienes in high yields and
cycloadditions. However, the chemo- and regioselective
preparation of bicyclic cyclohexadienes from diynes and alkenes
is difficult because cyclohexadiene aromatization or double-
3
with high selectivities, without CC bond migration (Scheme
1
). In this reaction, the phosphine ligand significantly affects the
1
2
13
cycloaddition. Ti−phosphine catalysts have been used in
bond migration can occur. Enynes and alkynes or diynes
and cyclic alkenes
15
1
2b,14
organic synthesis, but this is the first reported example of
the use of a Nb−phosphine catalyst in organic synthesis.
Initially, dimethyl 2,2-di(pent-2-ynyl)malonate (1a) and 1-
octene (2a) were used as model substrates for optimization of
are used for transition-metal-catalyzed
syntheses of bicyclic cyclohexadienes to prevent these
5a
14a,b
processes. Rothwell and Saa
́
reported the formation of
bicyclic cyclohexadienes from diynes and terminal alkenes using
Ti and Ru catalysts, respectively. These cyclohexadiene
skeletons are formed via double-bond migration (Scheme 1).
There are some reports on intramolecular cycloaddition of
Received: August 28, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02672
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the cycloaddition reaction conditions; the results are shown in
Table 1. We investigated various phosphine ligands. When 1a
stoichiometric ratio of 2:1 and 4:1 was used, 3a was obtained
in good yields (71% and 73%, respectively, entries 9−10). The
product yield was poor when the catalyst loading was 10 mol %
(entry 11). The substrate was not converted when other early
transition-metal compounds, i.e., NbCl (DME), TaCl , and
Table 1. NbCl /Zn/PCy -Catalyzed Reaction of Diyne 1a
5
3
a
with 1-Octene (2a) under Various Conditions
3
5
ZrCl , were used (entries 12−14).
4
The reactions of various alkenes 2 under the optimum
conditions, shown in Table 1, entry 1, were investigated
(
Scheme 2, 3a−h). Alkyne 1a was reacted with various aliphatic
alkenes (2a−d) using the optimum catalytic system (Scheme 2,
3a−d).
b
c
entry
catalyst
NbCl5
ligand
yield of 3a (%)
d
1
2
3
4
5
PCy3
none
PPh3
P(Oct)3
dppe
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
83 (82, 84 )
Scheme 2. NbCl /Zn/PCy -Catalyzed Cycloadditions of
e
5
3
NbCl5
NbCl5
NbCl5
NbCl5
NbCl5
NbCl5
NbCl5
NbCl5
NbCl5
NbCl5
nd
a
Diynes 1 and Alkenes 2
24
e
nd
3
f
e
6
nd
g
7
40
46
71
73
9
h
8
i
9
j
1
1
1
1
1
0
1
2
3
4
k
e
NbCl (DME)
nd
3
e
TaCl5
ZrCl4
nd
e
nd
a
Reaction conditions: 1a (0.5 mmol), 2a (3.0 mmol), catalyst (0.1
mmol, 20 mol %), Zn (0.5 mmol), and ligand (0.6 mmol) in 1,2-
b
dichloroethane (2 mL) at 40 °C for 2 h under Ar. Cone angle (deg):
c
PCy (170), PPh (132); see ref 16. Yields were determined by GC
3
3
based on 1a used. GC yields, except the value in parentheses. The
d
numbers in parentheses show isolated yields. Isolated yield from
e
f
larger scale reaction (1a: 1.0 mmol). Not detected by GC. Without
g
h
i
Zn. Zn (0.25 mmol) was used. 2a (0.5 mmol) was used. 2a (1.0
mmol) was used. 2a (2.0 mmol) was used. NbCl (0.05 mmol), Zn
j
k
5
(
0.25 mmol), and PCy (0.1 mmol) were used.
3
(
0.5 mmol) was reacted with 2a (3 mmol) in the presence of
NbCl (0.2 mmol), Zn (0.5 mmol), and PCy (0.2 mmol) in
5
3
1,2-dichloroethane (2 mL) at 40 °C for 2 h, the reaction
provided bicyclic cyclohexadiene 3a in 82% yield (entry 1). It is
noteworthy that side products, such as cyclotrimerization
products of alkynes, in the aromatization of 3a are not
obtained. Takai and co-workers reported that NbCl /Zn
5
catalysis is effective for generating low-valent Nb species in
2
c
situ. We therefore used NbCl and Zn in this reaction;
5
a
however, 3a was not obtained, and none of the diyne was
converted (entry 2).
Reaction conditions: see optimum conditions (Table 1, entry 1).
c
b
Reaction temperature was 60 °C. P(o-tolyl)
was used as ligand.
3
d
Not detected by GC.
We then investigated the effects of other phosphine ligands,
namely PPh , P(n-Oct) , and 1,2-bis(diphenylphosphino)-
3
3
ethane (dppe) (compare entries 1 and 3−5). When PCy3
was used for the cycloaddition, the reaction proceeded
quantitatively. Other phosphine ligands were not efficient in
the reaction. These ligand-screening results imply that the cone
The 1-alkene 1-hexene (2b), α,ω-diene 1,5-hexadiene (2c),
and an aliphatic alkene bearing a benzene ring, i.e., allylbenzene
(2d), gave the corresponding bicyclic cyclohexadienes 3b−d in
70−86% yields, with excellent chemo- and regioselectivities.
Various styrene derivatives (2e−h) were tolerated in the
cycloaddition. Styrene (2e) and p-methylstyrene (2f) provided
the desired products 3e and 3f in excellent yields. When styrene
derivatives with functional groups, e.g., p-methoxystyrene (2g)
and p-chlorostyrene (2h), were used in the reaction, the bicyclic
hexadienes 3g and 3h were obtained in 66% and 51% yields,
respectively. Internal alkenes, e.g., cis-4-octene and cyclohexene,
did not give any cycloadducts.
1
6
angle affects stabilization of the low-valent Nb species and the
reactivity of the cycloaddition. Recently, we found that NbCl5/
Zn/Ph Si(OMe) catalyzes the cycloaddition of alkynes and
2
2
9b
nitriles to give pyridines. However, Ph Si(OMe) was used
2
2
instead of PCy in the reaction, and a complex mixture of
3
isomers was obtained. When the reaction was performed
without Zn, the cycloaddition did not proceed (entry 6). This
shows that Zn is necessary to activate the catalyst. Compounds
1
a and 2a reacted in a 1:1 stoichiometric ratio to give the
We next investigated the scope of the reaction using various
diynes (Scheme 2, 3i−l). When an ethyl group on the alkyne
product in moderate yield (entry 8). When a 2a:1a
B
DOI: 10.1021/acs.orglett.7b02672
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
skeleton was changed to a methyl group [1b, dimethyl 2,2-
di(but-2-ynyl)malonate], the reaction afforded the correspond-
ing product 3i in 69% yield. Similarly, the sulfonamide- and
aniline-substituted diynes 1c and 1d, respectively, reacted with
derivatives. A phosphine ligand is critical in promoting the
intramolecular cycloaddition. This is the first example of the
transition-metal-catalyzed synthesis of bicyclic cyclohexadienes
from the reaction of various diynes with terminal alkenes
without double-bond migration. Further investigations regard-
ing the scope and application of this reaction are underway.
1
a to provide the desired products 3j and 3k in moderate
yields. However, in the reaction of an α,ω-diyne [1e, dimethyl
,2-di(prop-2-ynyl)malonate] with 2a under the standard
2
conditions, the desired product was not obtained and the
diyne was converted to intractable oligomeric products. We
ASSOCIATED CONTENT
■
*
S
Supporting Information
have recently reported that the NbCl /Zn/alkoxysilane system
5
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02672.
indicated high catalytic activity to intermolecular cycloaddition
of tert-butylacetylene and benzonitrile to afford pyridine
9
b
derivatives. However, the reaction was sluggish, and no
catalytic activity was observed by using the present NbCl /Zn/
Experimental procedures and compound characterization
(PDF)
5
PCy system under these conditions.
3
In previous studies, the low-valent Nb complex
NbCl (DME) reacted with internal alkynes to give cis-alkenes
3
AUTHOR INFORMATION
2a,b,3,4
■
by hydrolysis via a Nb−alkyne complex.
We therefore
Corresponding Author
*E-mail: obora@kansai-u.ac.jp.
ORCID
examined the stoichiometric reaction of NbCl , Zn, and PCy
5
3
with 4-octyne (4) (0.5 mmol); cis-4-octene was obtained by
9c
hydrolysis or deuteriolysis. This result suggests that NbCl5,
Zn, and PCy efficiently generated low-valent Nb species in situ
3
Yasushi Obora: 0000-0003-3702-9969
Notes
(Scheme 3).
Scheme 3. Hydrolysis/Deuteriolysis of Low-Valent Niobium
Complex Generated from NbCl /Zn/PCy and 4-Octyne (4)
The authors declare no competing financial interest.
5
3
ACKNOWLEDGMENTS
■
1
This work was supported partly by JSPS KAKENHI Grant No.
6K05784. High-resolution mass spectra were performed at the
Global Facility Center, Hokkaido University.
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Organic Letters
Letter
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D
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