NbCl3-catalyzed three-component [2 + 2 + 2] cycloaddition reaction of terminal alkynes, internal alkynes, and alkenes to 1,3,4,5-tetrasubstituted 1,3-cyclohexadienes

Article abstract of DOI:10.1021/ol200662e

Three-component [2 + 2 + 2] cycloaddition of terminal alkynes, internal alkynes, and terminal alkenes is achieved using an NbCl₃(DME) catalyst, leading to 1,3,4,5-substituted 1,3-cyclohexadienes in excellent yields with high chemo- and regioselectivity.

Full text of DOI:10.1021/ol200662e

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2568–2571
NbCl3-Catalyzed Three-Component [2 þ 2þ 2]
Cycloaddition Reaction of Terminal
Alkynes, Internal Alkynes, and Alkenes to
1,3,4,5-Tetrasubstituted 1,3-
Cyclohexadienes
Yasushi Satoh and Yasushi Obora*
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials
and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan
obora@kansai-u.ac.jp
Received March 12, 2011
ABSTRACT
Three-component [2 þ 2 þ 2] cycloaddition of terminal alkynes, internal alkynes, and terminal alkenes is achieved using an NbCl3(DME) catalyst,
leading to 1,3,4,5-substituted 1,3-cyclohexadienes in excellent yields with high chemo- and regioselectivity.
Transition-metal-catalyzed [2 þ 2 þ 2] cycloaddition of
unsaturated compounds has been extensively studied for
the formation of various aromatic and unsaturated cyclic
compounds.1 In general, this protocol is selectively accom-
plished by intramolecular reactions using diynes and
dienes as substrates. Chemo- and stereocontrolled inter-
molecular [2 þ 2 þ 2] cycloadditions, in particular from
three different substrates, are, therefore, highly desirable
and a challenging target. To date, several examples of
chemoselective transition-metal-catalyzed or -mediated
alkyne cyclotrimerizations from three different alkynes,
leading to multisubstituted aromatic compounds, have
been reported.2,3
1,3-Cyclohexadiene derivatives are an important class of
compounds and are widely used in organic and poly-
mer chemistry.4,5 Among the various synthetic methods
for the synthesis of 1,3-cyclohexadienes reported so far,6
(3) Example of Ru-catalyzed cycloaddition of three different alkynes:
Ura, Y.; Sato, Y.; Tsujita, H.; Kondo, T.; Imachi, M.; Mitsudo, T. J. Mol.
Catal. A: Chem. 2005, 239, 166 and references cited therein.
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Santelli, M. Tetrahedron Lett. 2008, 49, 4630. (c) Boyd, D. R.; Sharma,
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2005, 4741. (d) Yamamoto, Y. Curr. Org. Chem. 2005, 9, 503. (e)
Nakamura, I.; Yamamoto, Y. Chem. Rev. 2004, 104, 2127. (f) Aubert,
C.; Buisine, O.; Malacria, M. Chem. Rev. 2002, 102, 813. (g) Saito, S.;
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M.; Li, Z.; Donovan, R. J. Chem. Rev. 1996, 96, 635. (i) Negishi, E.;
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Thorarensen, A.; Palmgren, A.; Itami, K.; Backvall, J.-E. Tetrahedron
Lett. 1997, 38, 8541. (j) Hartsough, D.;Schuster, G. B.J. Org. Chem. 1989,
54, 3 and references cited therein.
(5) For example, see: (a) Heiser, D. E.; Okuda, J.; Gambarotta, S.;
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Coperet, C.; Ma, S.; Liou, S.-Y.; Liu, F. Chem. Rev. 1996, 96, 365. (j)
Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49. (k) Vollhardt,
K. P. C. Angew. Chem., Int. Ed. 1984, 23, 539.
(2) Example of Zr-mediated cycloaddition of three different alkynes:
Takahashi, T.; Xi, Z.; Yamazaki, A.; Liu, Y.; Nakajima, K.; Kotora, M.
J. Am. Chem. Soc. 1998, 120, 1672. (b) Takahashi, T.; Tsai, F.-Y.; Li, Y.;
Nakajima, K.; Kotora, M. J. Am. Chem. Soc. 1999, 121, 11093.
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Muelhaupt, R. Macromol. Chem. Phys. 2005, 206, 195. (b) Natori, I.
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C.; Arnould, M. A. Macromolecules 2004, 37, 1234 and references cited
therein.
r
10.1021/ol200662e
Published on Web 04/14/2011
2011 American Chemical Society

transition-metal-catalyzed [2 þ 2 þ 2] cycloaddition from
easily accessible unsaturated compounds, such as simple
alkynes and alkenes, is the most favorable. However, in
contrast to the case of cycloaddition by alkyne trimeriza-
tion, only a limited amount of work on the synthesis of 1,3-
cyclohexadienes by intermolecular cycloaddition of al-
kynes and alkenes has been reported because of the
difficulty in controlling the chemo- and regioselectivity.7,8
NbCl3(DME) is a thermally stable low-valent early
transition-metal complex, which has been used as a reagent
in the reactions of alkynes with several electrophiles.9ꢀ12
We recently reported that NbCl3(DME) serves as an
efficient catalyst for intermolecular cycloaddition of two
molecules of terminal alkynes and one alkene or R,ω-diene
molecule to afford 1,4,5-trisubstituted-1,3-cyclohexadiene
derivatives.13
Table 1. NbCl3-Catalyzed [2 þ 2 þ 2]-Cycloaddition of
Trimethylsilylacetylene (1a), 5-Decyne (2a), and Styrene (3a)a
yield (%)
entry
catalyst
NbCl3(DME) Cl(CH2)2Cl quant (97) trace nde
solvent
4ab
5ac
6ad
1
In this paper, we report the three-component [2 þ 2 þ 2]
cycloaddition of terminal alkynes, internal alkynes, and
terminal alkenes, catalyzed by NbCl3(DME), leading to
1,3,4,5-tetrasubstituted 1,3-cyclohexadienes in excellent
yields with high chemo- and regioselectivity. This reaction
provides an unprecedented, selective, and atom-economic-
al methodology for the formation of tetrasubstituted 1,3-
cyclohexadienes from three different simple unsaturated
feedstocks via intermolecular [2 þ 2 þ 2] cycloaddition.
The reaction of trimethylsilylacetylene (1a), 5-decyne
(2a), and styrene (3a) was chosen as a model reaction and
was carried out under various conditions (Table 1).
For instance, 1a (2 mmol) was allowed to react with
2a (1 mmol) and 3a (4 mmol) under the influence of
2f
3g
4h
5
NbCl3(DME) Cl(CH2)2Cl 62
NbCl3(DME) Cl(CH2)2Cl 56
NbCl3(DME) Cl(CH2)2Cl 72
NbCl3(DME) Cl(CH2)4Cl 82
5
trace
trace trace
9
5
12
8
6
NbCl3(DME) toluene
NbCl3(DME) dioxane
NbCl3(DME) THF
NbCl3(DME) DME
77
58
3
nde
trace 10
7i
8
trace trace
nde
nde
nde
13
trace
nde
nde
6
9
10
11
12
TaCl3(DME)
NbCl5
Cl(CH2)2Cl nde
Cl(CH2)2Cl trace
Cl(CH2)2Cl nde
Cp2NbCl2
nde
nde
a Reaction conditions: 1a (2 mmol), 2a (1 mmol), 3a (4 mmol), and
catalyst (0.2 mmol) in solvent (1 mL) at 60 °C for 16 h. b Determined by
GC based on 2a used. The number in parentheses shows isolated yield.
c Determined by GC based on 1a used. Compound 5a was exclusively
obtained as 1,4,5-adduct. d Determined by GC based on 1a used.
Compound 6a was exclusively obtained as 1,4,5-adduct. e Not detected
by GC. f Reaction was performed using NbCl3(DME) (0.2 mmol), 1a (1
mmol), 2a (1 mmol), and 3a (1 mmol). g Reaction was performed using
NbCl3(DME) (0.2 mmol), 1a (2 mmol), 2a (2 mmol), and 3a (2 mmol).
h Reaction was performed using NbCl3(DME) (0.2 mmol), 1a (4 mmol),
2a (2 mmol) and 3a (2 mmol). i The reaction was performed at 80 °C.
(6) (a) Lautens, M.; Ma, S.; Belter, R. K.; Chiu, P.; Leschziner, A.
J. Org. Chem. 1992, 57, 4065. (b) Berchtold, G, A.; Ciabattoni, J.; Tunick,
A. A. J. Org. Chem. 1965, 30, 3679. (c) Dyachenko, V. D.; Dyachenko,
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NbCl3(DME) (0.2 mmol) in 1,2-dichloroethane (1 mL)
at 60 °C for 16 h. 1-Trimethylsilyl-3,4-n-dibutyl-5-phenyl-
1,3-cyclohexadiene (4a) was obtained in quantitative
yield with excellent chemo- and regioselectivity (Table 1,
entry 1). Itisnoteworthythatthe reaction ledexclusivelyto
intermolecular three-component cross-cycloaddition pro-
ducts from three different substrates, in preference to the
cyclotrimerization of terminal alkynes (leading to 6a)11
and cross-cycloaddition reactions of terminal alkynes with
alkenes (leading to 5a).7,8,13
€
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The substrates ratio markedly influenced the selectivity
and yield of the desired product 4a. On screening the
reaction, we found that the optimized reaction ratio of
1a:2a:3a is 2:1:4 (entry 1). Nevertheless, even if 1a, 2a, and
3a were allowed to react in a stoichiometric ratio (1a:2a:3a =
1:1:1), the yield of the product was still acceptable (62%)
(entry 2). In addition, chemoselective (entry 3) and high-
yield formation (entry 4) of 4a was achieved by tuning the
ratio of the substrates (1a:2a:3a) with smaller amounts (10
mol %) of NbCl3 catalyst. With regard to the solvent, 4a
was obtained in high yields when halogenated solvents
such as 1,2-dichloroethane and 1,4-dichlorobutane were
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Org. Lett., Vol. 13, No. 10, 2011
2569

such as NbCl5 and Cp2NbCl2, were totally ineffective as
catalysts for the cycloaddition reactions (entries 11 and 12).
Under the optimized conditions shown in Table 1, entry
1, the reactions of various terminal alkynes 1, internal
alkynes 2, and alkenes 3 were examined (Table 2). The
chemo- and regioselectivities of the adducts were signifi-
cantly influenced by the bulkiness of the substituents on the
terminal alkynes 1 used in the reaction. Thus, reactions
using trialkylsilylacetylenes, such as trimethylsilylacetylene
(1a), triethylsilylacetylene (1b), and dimethylphenylsilyla-
cetylene (1c), provide the corresponding 1,3,4,5-tetrasub-
stituted-1,3-cyclohexadiene derivatives 4aꢀc in excellent
yield, as the sole product, and with excellent chemo- and
regioselectivity (entries 1ꢀ3). tert-Butylacetylene wasalso a
good substrate for the formation of three-component
cycloaddition products in high yield, along with the forma-
tion of 5d (12%) and 6d (4%) as byproduct. The regios-
electivity for the 1,3,4,5-adduct 4d was good but not
completely selective (71%) (entry 4). The use of less bulky
terminal acetylenes such as phenylacetylene and 1-octyne
did not give the desired three-component coupling product
selectively, but alkyne cyclotrimerization products were
obtained in considerable yields (entries 5 and 6).
Table 2. NbCl3-Catalyzed Reactions of Various Terminal Al-
kynes 1, Internal Alkynes 2, and Alkenes 3 Leading to 1,3,4,5-
Tetrasubstituted 1,3-Cyclohexadienes 4a
Similarly, 4-octyne and 1-phenyl-1-propyne were al-
lowed to react with 1a and 3a, affording the corresponding
1,3,4,5-tetrasubstituted-1,3-cyclohexadiene derivatives 4g
and 4h in excellent yields with excellent chemo- and
regioselectivity (entries 7 and 8). In addition, the reaction
tolerated the use of simple alkenes, such as 1-octene and
1-decene, giving the desired 1,3,4,5-substituted 1,3-cyclo-
hexadienes in high yields with high regioselectivity (entries
9 and 10). It is noteworthy that the reactions of R,ω-dienes
with 1a and 2a took place exclusively at one side, affording
5-ω-alkenyl-1,3,4,5-tetrasubstituted-1,3-cyclohexadienes
in high yield with high regioselectivity (entry 11).
In addition, we found that 3,4,5-trisubstituted 1,3-cy-
clohexadieneshavebeensynthesizedbydesilylation14 from
the 1,3,4,5-tetrasubstituted 1,3-cyclohexadienes obtained
in this study. Thus, addition of KF (2 mmol) and THF (1
mL) to a reaction mixture prepared under the reaction
conditionsshown in Table 1, entry 1, gave3,4-di-n-butyl-5-
phenyl-1,3-cyclohexadiene (7) ingood isolated yields (74%
with R1 = SiMe3 and 88% with R1 = SiEt3; eq 1). This
one-pot and high-yield synthesis provides an efficient and
versatile protocol for the synthesis of various 3,4,5-trisub-
stituted 1,3-cyclohexadienes.
a Reaction conditions: 1 (2 mmol), 2 (1 mmol), 3 (4 mmol), and
NbCl3(DME) (0.2 mmol) in 1,2-dichloroethane (1 mL) at 60 °C, 16 h.
b Isolated yields unless otherwise noted. c Compound 4 was obtained
exclusively as the 1,3,4,5-adducts. d Regioisomers (entries 4ꢀ6) of 4 were
identified by GC and GCꢀMS, and the yields were determined by GC.
The structures of the major regioisomers (1,3,4,5-addducts) are shown in
the table. The products were isolated as a mixtureof 4ꢀ6 due to difficulty
in completely separating them. e The major regioisomer (1,3,4,5-adduct)
of 4d was isolated in 63% yield. f In addition to the product 4d, 5d (12%)
and 6d (4%) were formed. g Regioselectivity (%) of the 1,3,4,5-adducts
in the total regioisomers of 4. The regiochemistry of the other isomers
was not determined. h In addition to the product 4e, 6e (18%) was
formed. i In addition to the product 4f, 5f (8%) and 6f (59%) were
formed. j In addition to the product 4i, 5i (8%) and 6i (12%) were
formed. k In addition to the product 4j, 6i (13%) was formed. l In
addition to the product 4k, 5k (8%) and 6k (10%) were formed.
used (entries 1ꢀ5). However, toluene and dioxane were
tolerated as solvents (entries 6 and 7). The catalyst precursor
significantly influenced the reaction activity: the low-valent
Nb(III) complex, NbCl3(DME), is highly efficient (entry 1).
When the Ta(III) analogue, TaCl3(DME), was used as a
catalyst, all the substrates were evidently converted in the
course of the reaction, but only an intractable mixture of
oligomeric products was obtained under these conditions
(entry 10). Other Nb(IV), Nb(V), and V(III) complexes,
Although it is not possible to confirm a detailed reaction
mechanism at this stage, the present three-component
(14) Nelson, T. D.; Crouch, R. D. Synthesis 1996, 1031.
Org. Lett., Vol. 13, No. 10, 2011
2570

cycloaddition of 1a, 2a, and 3a is thought to proceed by the
following pathway (Scheme 1).
internal alkyne 2a into the Nbꢀvinyl bond or the Nbꢀalkyl
bond occurs to form B and B0, respectively.15,16 Insertion
into the less-hindered Nbꢀvinyl bonds would be favored,
affording the niobacyclic intermediate B.17 B then exclu-
sively produces the three-component coupling product
1,3,4,5-tetrasubstituted 1,3-cyclohexadiene 4a.
Scheme 1. Plausible Reaction Pathway
In conclusion, we have found a new highly active catalytic
system for chemo- and regioselective [2 þ 2 þ 2] intermo-
lecular cycloaddition reactions of terminal alkynes, internal
alkynes, and alkenes to 1,3,4,5-tetrasubstituted-1,3-cyclo-
hexadiene derivatives. Further investigation with regard to
the detailed reaction mechanism, scope, and applications of
this reaction is currently in progress.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan, and
the Strategic Project to Support the Formation of Research
Bases at Private Universities (2010-2014), matching fund
subsidy from the Ministry of Education, Culture, Sports,
Science and Technology.
The reaction initiates oxidative cyclometalation of the
terminal alkyne (1a) and terminal alkene (3a); coordination
of the internal alkyne (2a) to the Nb center gives a nioba-
cyclopentene intermediate (A). Bulky substituents such as a
trimethylsilyl group would lead to preferential formation of
A rather than the sterically congested form A0.15 Subse-
quently, divergent migratory insertion of the coordinated
Supporting Information Available. Experimental proce-
dure and compound characterization data (1H NMR, 13C
NMR, HMQC, HMBC, IR, and MS spectra) of the com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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