COMMUNICATION
Highly efficient [2 + 2] intramolecular cyclizations of allenynes under
microwave irradiation: construction of fused bicyclic compounds{
Chang Ho Oh,*a Arun Kumar Gupta,a Dai In Parka and Nakjoong Kimb
Received (in Cambridge, UK) 13th June 2005, Accepted 19th September 2005
First published as an Advance Article on the web 17th October 2005
DOI: 10.1039/b508306k
A palladium [2 + 2] cycloaddition of 1,6- and 1,7- allenyne
carboxylates and microwave-mediated [2 + 2] cycloaddition of
various 1,n-allenynes were developed and, particularly, the
microwave irradiated [2 + 2] cycloaddition of allenynes can
provide a simple, general and eco-friendly synthetic method to
fused bicyclo[m,2,0]alkadienes.
ð1Þ
We have demonstrated that 1,7-allenynes have three different
cyclization modes with different palladium catalysts.
Cycloreduction occurred at the triple bond to give alkenyl-
palladium species that underwent carbopalladation to give six-
membered cycles.7 Arylative cyclization occurred at the allene
functionality to form a p-allylpalladium intermediate which
underwent carbocyclization to give the corresponding five-
membered rings. In continuing our research on allenyne cycliza-
tion, we describe a highly economical as well as environmentally
benign microwave-irradiated [2 + 2] cycloaddition in comparison
with Pd-catalyzed [2 + 2] cycloaddition of 1,n-allenynes.8
Microwaves have become a new energy source that is powerful
enough to complete reactions in minutes rather than hours or even
days.9 This may be because of the so-called ‘‘microwave effect’’
generated within the medium.10 In fact, there are many reports
concerning improvement of stereo-, regio- or chemoselectivity and
of chemical efficacy when microwaves were used instead of
conventional heating. However, allenyne reactions in the absence
of metal catalysts have not been reported in the literature. Upon
optimization of the [2 + 2] cycloaddition of a model substrate 1a,
we found that the use of a catalytic amount of PdCl2(PPh3)2
in toluene, or microwave irradiation afforded the correspond-
ing fused bicycle 2a in good to excellent yields (eqn (2) and
Table 1).
Bicyclic compounds fused with a cyclobutene ring have gained
much interest in recent years, because of their importance in
theoretical and photochemical studies.1 These compounds have
been mostly prepared from intramolecular [2 + 2] cycloaddition of
1,n-enynes by photo-irradiation. Since such fused compounds are
prone to facile thermal isomerizations to the corresponding acyclic
compounds, few synthetic studies have been reported before Trost
reported the formation of fused bicyclo[3,2,0]heptenes from the
Pd-catalyzed cyclization of 1,6-enynes.2 Since then, there were a
few reports dealing with intramolecular metal-mediated [2 + 2]
cycloadditions,3 none of which involved direct cycloadditions of
allenynes in synthetic points. Allenynes have witnessed tremendous
developments on their use for diverse synthetic applications as well
as for precursors for searching new reactions of unsaturated
systems.4 We envisioned that a [2 + 2] cycloaddition of
1,n-allenynes could provide an easy access to bicyclo[m,2,0]alk-
adienes as shown in Fig. 1.
Recently, 1,n-allenynes have been shown to exhibit two different
mechanistic pathways depending on the amount of Mo(CO)6 used.
When using a stoichiometric amount of Mo(CO)6, 1,7-allenynes
underwent Pauson–Khand reactions.5 However, Hammond
reported that a catalytic amount of Mo(CO)6 could catalyze the
[2 + 2] cycloaddition of 1,7-allenynes to bicyclo[3,2,0]octadienes via
metallocyclopentene intermediates (eqn (1)).6 Hammond postu-
lated that the electron-withdrawing gem-difluoro substituent of
allenynes would be expected to favor reductive elimination of the
molybdenum metallocycle rather than a CO insertion.
At first, using Hammond’s conditions on 1a resulted in
decomposition at 110 uC, but gave the [2 + 2]-cyclization product
in 40% yield at 85 uC (entry 1). This [2 + 2] cycloaddition was
generally catalyzed by various transition metal catalysts such as
Ni(PPh3)4, RuPhCl2 dimer, RhCl(PPh3)3 or PtCl2 in varying yields
(entries 2–5). Pd(PPh3)4, Pd(OAc)2, or p-allylpalladium chloride
dimer, however, resulted in decomposition of allenyne 1a
(entries 6–8). In a sharp contrast, PdCl2, Pd(dppe)Cl2 and
Pd(PPh3)2Cl2 catalyzed this [2 + 2]-cycloaddition smoothly to give
2a in 47, 61 and 76% yields, respectively (entries 9–11). Being
gratified by the good catalyst in hand, we then surveyed reaction
solvents such as dichloroethane, THF, p-dioxane, acetonitrile and
DMF (entries 12–16). The optimal conditions for this reaction
were found to be a use of a mixture of Pd(PPh3)2Cl2 (5 mol%) in
refluxing toluene, where the allenyne 1a underwent smooth [2 + 2]
cycloaddition to give 2a in 76% yield without forming any other
side products (entry 11). To our surprise, this allenyne 1a thermally
underwent [2 + 2]-cyclization to give the product in 64% yield
after a prolonged heating for 20 h (entry 17). In view of
Fig. 1
aGreen Organic Synthesis Lab., Department of Chemistry, Hanyang
University, Sungdong-Gu, Seoul, 133-791, Korea.
E-mail: changho@hanyang.ac.kr
bCenter for Photorefractive Materials, Department of Chemistry,
Hanyang University, Sungdong-Gu, Seoul, 133-791, Korea
{ Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b508306k
5670 | Chem. Commun., 2005, 5670–5672
This journal is ß The Royal Society of Chemistry 2005
Oh, Chang Ho
