Scheme 4
strongly suggests that path A works in the reaction. The
predominance of 3b at the end of the reaction should be due to
an equilibrium between 3b and 4b under the reaction conditions
(Scheme 4). Thus, formation of alkyl cation 9 by the reaction of
4b with a metal triflate and subsequent bond rotation followed
by elimination of the metal triflate would induce the isomeriza-
tion from 4b to thermodynamically favored 3b. Prolonged
reaction time (100 h) showed no further change in the isomer
ratio.
The drastic change in the isomer ratio around the point over
80% conversion of 1b, i.e., almost consumption of 1b, may
show that metal triflates prefer an alkyne rather than an alkene
for complexation under the reaction conditions. Actually, in the
absence of 1b, the isomerization catalysed by 10 mol% of
In(OTf)3 in p-xylene at 85 °C was completed within 20 min to
give a 91+9 ratio of 3b and 4b from a mixture of 3b and 4b in
a ratio of 45+55. Such salient character of metal triflates should
be ideal for the Friedel–Crafts alkenylation with alkynes to
reduce the possibility of side reactions through alkyl cation 9.
In summary, we disclose here the preliminary results on the
use of some metal triflates as efficient catalysts for the Friedel–
Crafts alkenylation of arenes with alkynes including internal
ones. Further investigation on the reaction of alkynes with
nucleophiles other than arenes is currently in progress.
Fig. 1 Plots of isomeric ratio of 3b+4b versus conversion (%) of 1b for
In(OTf)3 (-5-), Zr(OTf)4 (---), and Sc(OTf)3 (-ê-).
70% yield, whereas In(OTf)3 did not catalyse the reaction (entry
12). Treatment of halobenzenes, which can be used as a solvent
in the Friedel–Crafts alkylation, with 1-phenylprop-1-yne gave
the products in somewhat lower yields (entries 13 and 14).
Arenes attacked these alkynes exclusively at the carbon having
the aryl group. The reaction of 1,2-diphenylethyne with p-
xylene using In(OTf)3 as a catalyst also gave 3 in 79% yield
(entry 15). In all cases, the triarylalkane that would be produced
by the reaction of 3 and 4 with an arene was not observed.
Dependence of the isomer ratio between alkenylated products
3b and 4b on the conversion of 1-phenylprop-1-yne (1b) in the
reaction with p-xylene is illustrated in Fig. 1.9 Fig. 1 indicates
that (1) both stereoisomers 3b and 4b already exist in almost a
1+1 ratio at the early stage of the reaction, and (2) there is a
drastic change of the isomer ratio at the late stage of the
reaction.
Two routes (path A and path B) are possible for this
alkenylation reaction (Scheme 3). In path A, the attack of an
arene to initially formed zwitterionic intermediate 5 from both
X and Y sites affords a mixture of 7 and 8, which are
transformed by protonation to 3 and 4, respectively. As regards
path B, the activation of an alkyne by a metal triflate and the
attack of an arene to the alkyne proceeds through a concerted
mechanism. Therefore, an arene attacks 1b stereoselectively
from the side opposite to the metal triflate to furnish 7 and
subsequent protonation of 7 gives 3 as the sole stereoisomer.
The existence of both 3b and 4b at the early stage of the reaction
Notes and references
1 For reviews of Friedel–Crafts alkylation reactions: (a) W. E. Bachmann,
J. R. Johnson, L. F. Fieser and H. R. Snyder, Org. React., 1946, 3, 1; (b)
G. A. Olah, Friedel-Crafts and Related Reactions, Wiley-Interscience,
New York, 1964, vol. II, part 1; (c) R. M. Roberts and A. A. Khalaf,
Friedel-Crafts Alkylation Chemistry. A Century of Discovery, Dekker,
New York, 1984; (d) G. A. Olah, R. Krishshnamurit and G. K. S. Prakash,
Friedel-Crafts Alkylations in Comprehensive Organic Synthesis, ed.
B. M. Trost and I. Fleming, Pergamon Press, Oxford, 1991.
2 T. Takeda, F. Kanamori, H. Matsusita and T. Fujiwara, Tetrahedron Lett.,
1991, 32, 6563.
3 T. Kitamura, S. Kobayashi, H. Taniguchi and Z. Rappoport, J. Org.
Chem., 1982, 47, 5003.
4 For example: O. W. Cook and V. J. Chabers, J. Am. Chem. Soc., 1921, 43,
334; J. S. Reichert and J. A. Nieuwland, J. Am. Chem. Soc., 1923, 45,
3090; J. A. Reilly and J. A. Nieuwland, J. Am. Chem. Soc., 1928, 50,
2564; I. P. Tsukervanik and K. Y. Yuldashev, J. Gen. Chem. USSR, 1961,
31, 790.
5 G. Sartori, F. Bigi, A. Pastorio, C. Porta, A. Arienti, R. Maggi, N. Moretti
and G. Gnappi, Tetrahedron Lett., 1995, 36, 9177.
6 M. Yamaguchi, Y. Kido, A. Hayashi and M. Hirama, Angew. Chem., Int.
Ed. Engl., 1997, 36, 1313; Y. Kido, S. Yoshimura, M. Yamaguchi and T.
Uchimaru, Bull. Chem. Soc. Jpn., 1999, 72, 1445.
7 We have previously reported Sc(OTf)3-catalysed Friedel–Crafts alkyla-
tion reactions of arenes with alcohols, aldehydes or acetals as alkylating
agents: T. Tsuchimoto, K. Tobita, T. Hiyama and S. Fukuzawa, Synlett,
1996, 557; T. Tsuchimoto, T. Hiyama and S. Fukuzawa, Chem.
Commun., 1996, 2345; T. Tsuchimoto, K. Tobita, T. Hiyama and S.
Fukuzawa, J. Org. Chem., 1997, 62, 6997.
8 Addition of nitromethane to make the reaction system homogeneous
caused remarkable rate acceleration only in the reaction using Sc(OTf)3
as a catalyst. Use of AlCl3–MeNO2 as a soluble Friedel–Crafts alkylation
catalyst has been reported; L. Schmerling, Ind. Eng. Chem., 1948, 40,
2072; G. A. Olah, S. Kobayashi and M. Tashiro, J. Am. Chem. Soc., 1972,
94, 7448.
Scheme 3
9 Configuration of 3b and 4b was determined by NOESY.
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Chem. Commun., 2000, 1573–1574