C O M M U N I C A T I O N S
Scheme 3
Figure 1
Scheme 2
CSD2006-0003), the MEC (predoctoral fellowship to V.L.-C.), the
AGAUR (2009 SGR 47), and the ICIQ Foundation. We also thank
Dr. S. Lo´pez for preliminary results.
complex E was not effective, whereas F led to 6a in 58% yield
after 4 h (Table 1, entries 8 and 9). In this case, longer reaction
times led to lower yields.14
Reaction of terminal alkynes 4a-i with alkenes 5a-f led
regioselectively to cyclobutenes 6a-q in moderate to good yields
using catalysts B (Table 2). The reaction proceeds satisfactorily
with alkynes with both electron-rich and electron-poor substituents,
including a free OH group (Table 2, entry 7).15
Supporting Information Available: Additional data, experimental
details, and characterization data. This material is available free of
References
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Biscyclobutenes 6r and 6s were also obtained from p- and
m-diethynylbenzene, respectively (Figure 1). Similarly, reaction of
m-di(prop-1-en-2-yl)benzene (5g) with 4a gave biscyclobutene 6t.
These results are consistent with a reaction of cationic
Au(I)-alkyne complexes 8 with the alkenes 5 to form intermediates
9/9′,16 which give cyclobutenes 6/6′ via carbocations 10/10′
(Scheme 2). Selective formation of regioisomers 6 is probably due
to the faster formation of intermediate 9, which is an analogue of
the exo-type intermediates in the gold(I)-catalyzed cyclization of
1,n-enynes.2
Gold(I)-catalyzed reaction of terminal alkynes with 1,5-diene 5h
gave biscyclopropyl derivatives 11a-e with an anti-relative con-
figuration, in addition to cyclobutenes 6u-y (Scheme 3). Formation
of 11a-e and 6u-y could be explained by the different evolution
of stereoisomeric intermediates 9 by intramolecular cyclopropana-
tion17 or ring expansion. Reaction of 4a with 5h-d2 gave stereospe-
cifically 11a-d2 and 6u-d2 (1:1 ratio), which suggests that the
reaction proceeds through intermediate 9a in which no free rotation
occurs around the C3-C4 bond. However, reaction 4a with (E)-
5a-d1 gave cycloadduct 6a-d1 as a 1:1 mixture of diastereomers.
These results are consistent with a [2+2] cycloaddition proceeding
stepwise through intermediates 918 in which rotation around the
C3-C4 bond can occur if the alkene bears electron-donating
substituents.19
In summary, this work shows that in the absence of the constrains
imposed by the tethers in intramolecular processes, the gold(I)-
catalyzed reaction of alkynes with alkenes leads to cyclobutenes.
Key for the success of this [2+2] cycloaddition is the use of gold(I)
complexes with bulky ligands that selectively activate alkynes in
the presence of alkenes, which opens new opportunities for the
invention of related intermolecular gold(I)-catalyzed processes.
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H.-B.; Li, B.; Hua, R.; Yin, Y. Eur. J. Org. Chem. 2006, 4231–4236.
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(13) A rapid equilibrium was immediately established between catalyst A and
[AuL(alkene)]SbF6 (L ) phosphine) in CD2Cl2 (1H and 31P NMR analysis).7
However, even at 23 °C, this equilibrium was slow with catalyst B (Keq
)
0.02 (5a) and 0.17 (5d)). Isomerization of the alkenes (i.e., 5e to
1-methylcyclohexene) was observed with complex A at 23 °C, whereas
this isomerization was very slow with B.
Acknowledgment. This work was supported by the MICINN
(Projects CTQ2007-60745/BQU, Consolider Ingenio 2010 Grant
(14) See Supporting Information for additional details.
9
J. AM. CHEM. SOC. VOL. 132, NO. 27, 2010 9293