LETTER
Ytterbium Triflate Promoted Coupling Reaction
553
Table 1 Yb(OTf)3 Catalyzed Coupling Reaction between Aryl
Alkynes and Aldehydes (continued)
Reaction times and yields were not significantly depen-
dent neither on the position and electronic properties of
aryl substituents nor on the alkyne structure. Using ali-
phatic aldehydes, yields were similar to those obtained us-
ing aromatic ones, except for n-hexanal for which the
yield was lower, probably because of dimerization of the
aldehyde by aldol condensation. Using ketones or aliphat-
ic alkynes as starting materials resulted in no reaction,
even after a prolonged time. The regioselectivity giving
exclusively trans-adducts may be explained by the mech-
anism proposed by Yamaguchi and coworkers.4
Aldehyde Alkyne
Product
Yield (%)
84
4-Phenyl- R1 = -H
benzalde-
hyde
613
712
812
4-Nitro-
benzal-
dehyde
R1 = -H
71
79
71
After completion, a simple addition of few mL of CH2Cl2
to the reaction mixture precipitated the catalyst, which
simply recovered by filtration in nearly quantitative yield.
Thus the catalyst could be recycled and reused several
times without appreciable loss of activity: the reaction
giving compound 1 was actually repeated three times,
through the catalyst washed with CH2Cl2 and dried at
70 °C for 2 hours, with the following yields: 76%, 74%,
71%.
p-Anisal- R1 = -H
dehyde
3-Nitro-
benzal-
dehyde
R1 = -H
In conclusion, we have shown that Yb(OTf)3 is an effec-
tive catalyst in promoting the coupling reaction between
aryl alkynes and aldehydes affording selectively trans-
benzylideneacetophenones. The present method com-
pares favourably with earlier reported methodologies, es-
pecially in terms of easy work-up, absence of solvent,
simple recovery and very high recyclability of the cata-
lyst.
1012
o-Tolual- R1 = -H
72
79
dehyde
1114
2-Nitro-
benzal-
dehyde
R1 = -H
R1 = -H
R1 = -Me
Acknowledgment
Financial support from Università degli Studi di Perugia is grate-
fully acknowledged.
1212
2-Chlo-
robenzal-
dehyde
76
References
(1) Han, Y.; Huang, Y. Z. Tetrahedron Lett. 1995, 36, 7277; and
references cited therein.
(2) Frantz, D. E.; Fässler, R.; Carreira, E. M. J. Am.Chem. Soc.
2000, 122, 1806; and references cited therein.
(3) Anand, N. K.; Carreira, E. M. J. Am.Chem. Soc. 2001, 123,
9687; and references cited therein.
(4) Hayashi, A.; Yamaguchi, M.; Hirama, M. Synlett 1995, 195;
and references cited therein.
(5) Viswanathan, G. S.; Li, C. J. Tetrahedron Lett. 2002, 43,
1613.
(6) Kobayashi, S.; Sugiura, M.; Kitagawa, H.; Lam, W. W. L.
Chem. Rev. 2002, 102, 2227.
(7) Curini, M.; Epifano, F.; Marcotullio, M. C.; Rosati, O.
Tetrahedron Lett. 2001, 42, 3193.
(8) Curini, M.; Epifano, F.; Marcotullio, M. C.; Rosati, O.
Heterocycles 2001, 55, 1599.
(9) Curini, M.; Epifano, F.; Marcotullio, M. C.; Rosati, O. Eur.
J. Org. Chem. 2002, 1562.
1312
1412
1512
1615
171
Benzal-
dehyde
81
77
70
50
p-Tolual- R1 = -Me
dehyde
Cyclohex- R1 = -H
ancarboxy-
aldehyde
n-Hexanal R1 = -H
(10) Curini, M.; Epifano, F.; Maltese, F.; Rosati, O. Tetrahedron
Lett. 2002, 43, 4895.
(11) Typical Procedure: To a mixture of aldehyde (1.0 mmol)
and aryl alkyne (2.0 mmol), Yb(OTf)3 (0.1 mmol) was added
and stirring was continued at 90 °C for 12 h. To this mixture
CH2Cl2 (2 mL) was added at r.t., the solid precipitated was
filtered under vacuum, the filtrate dried over Na2SO4 and
a Yields in pure isolated products.
Synlett 2003, No. 4, 552–554 ISSN 0936-5214 © Thieme Stuttgart · New York