3
970
J. Am. Chem. Soc. 1996, 118, 3970-3971
Communications to the Editor
A New Palladium-Catalyzed Benzannulation of
Conjugated Enynes
Shinichi Saito, Matthew M. Salter, Vladimir Gevorgyan,
Norie Tsuboya, Kazushi Tando, and Yoshinori Yamamoto*
Department of Chemistry, Graduate School of Science
Tohoku UniVersity, Sendai 980-77, Japan
ReceiVed January 5, 1996
The transition metal catalyzed synthesis of aromatic rings is
1-3
mainly accomplished by cyclotrimerization of alkynes (eq 1).
We wish to report an entirely new route; the Pd(0)-catalyzed
reaction of conjugated enynes gives the corresponding disub-
stituted aromatic compounds in good to high yields.
To help clarify the mechanism of this unprecedented cy-
clodimerization, the reaction of enyne 1e in which the deuterium
content was 90% at the 4-position was carried out. Disubstituted
benzene 2e, which contained deuterium mainly at the 2- and
6
-positions (83% deuterium content at each position), was
We have recently reported that the palladium (Pd2dba3‚
CHCl3-dppf) catalyzed reaction of conjugated enynes with
pronucleophiles affords the corresponding 1,4-addition products
obtained in high yield. Deuterium was not distributed to other
positions except for the protons attached to the benzene ring.
The formation of 2e indicates that the acetylenic C-H(D) bond
is not cleaved in the present benzannulation (eq 3). Transition
metal catalysts effective for the cyclotrimerization of acetylenes
(
allenes) in high yields.4 We also examined the reaction
between conjugated enynes and pronucleophiles by using the
Trost catalyst system (bis π-allylpalladium chloride dimer-KO-
1
7
(CpCo(CO)2 or RhCl(PPh3)2 ) were not effective for the
t
5
Bu-dppf). Very interestingly, an aromatic compound derived
cyclodimerization of enynes. Taken together, it seems difficult
to explain the unprecedented cyclodimerization by the ordinary
accepted mechanisms.
from dimerization of enyne was obtained as a minor product
along with the major product (allenes)! Accordingly, we
investigated the reaction of 2-hexyl-1-buten-3-yne (1a) under
various catalyst systems to find the optimum condition for
obtaining this unprecedented reaction product in higher yields.
Among the catalyst systems we examined, Pd(PPh3)4/toluene
or Pd2dba3‚CHCl3-tris(2,6-dimethoxyphenyl)phosphine/toluene
gave better results. The best solvent for the reaction was
toluene, benzene, or acetonitrile; the rate of the reaction was
slower in THF, DMSO, and DMF.
The results of the reactions of several enynes 1 under Pd-
PPh3)4 catalyst are shown in eq 2. The reaction proceeded
Two plausible mechanisms are shown in eqs 4 and 5, although
they are highly speculative. The reaction of 1 with the Pd(0)
species would produce a π-complex intermediate 3, which might
act as a nucleophilic diene and undergo (formal) Diels-Alder
(
very smoothly and rapidly to give 1,4-disubstituted benzene 2
in good to high yield. Isomeric products such as 1,3-disubsti-
tuted benzene or trisubstituted benzenes, which may be formed
via cyclotrimerization, were not detected. Even in the presence
of functional groups such as the hydroxyl (1c) and carbonyl
group (1d) in the alkyl chain at the 2-position, the reaction
proceeded smoothly to give 2c and 2d, respectively, in high
yields.6 However, 1- and/or 4-substituted enynes did not
dimerize under the present catalyst systems. The preparation
of 2a is representative. To a solution of 1a (1 mmol) in dry
toluene (2 mL) was added Pd(PPh3)4 (20 µmol, 2 mol %) under
Ar at rt. The yellow solution was heated to 65 °C for 1 h, and
the product was isolated by column chromatography to give
reaction (eq 4).8,9
Alternatively, this reaction may involve a
10
metallacycle such as 4 as an intermediate (eq 5).
The wide applicability of this new benzannulation procedure
is demonstrated in the synthesis of cyclophanes (eq 6). The
bis-enyne 5, synthesized in one step from 1,8-dibromooctane
(
7) Neeson, S. J.; Steverson, P. J. Tetrahedron 1989, 45, 6239-6248
and references cited therein.
8) The structure and reactivity of the corresponding alkyne-Pt(0)
(
complex are well established. See: Glanville, J. O.; Stewart, J. M.; Grim,
S. O. J. Organomet. Chem. 1967, 7, P9-P10. Mann, B. E.; Shaw, B. L.;
Tucker, N. I. J. Chem. Soc. A 1971, 2667-2673. Greaves, E. O.; Lock, C.
J. L.; Maitlis, P. M. Can. J. Chem. 1968, 46, 3879-3891. Tripathy, P. B.;
Renoe, B. W.; Adzamli, K.; Roundhill, d. M. J. Am. Chem. Soc. 1971, 93,
1
,4-disubstituted benzene 2a in 77% yield.
4
406-4410. Chatt, J.; Rowe, G. A.; Williams, A. A. Proc. Chem. Soc.
(
1) Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984, 23, 539-
1957, 208-209. Cook, C. D.; Allen, A. D. Can. J. Chem. 1964, 42, 1603-
6
44.
1608.
(
2) Schore, N. E. Chem. ReV. 1988, 88, 1081-1119.
(9) For examples of transition metal catalyzed [4 + 2] cycloadditions
and Diels-Alder reactions, see: Wender, P. A.; Jenkins, T. E.; Suzuki, S.
J. Am. Chem. Soc. 1995, 117, 1843-1844. McKinstry, L.; Livinghouse, T.
Tetrahedron 1994, 50, 6145-6154. Jolly, R. S.; Luedtke, G.; Sheehan, D.;
Livinghouse, T. J. Am. Chem. Soc. 1990, 112, 4965-4966. Wender, P. A.;
Jenkins, T. E. J. Am. Chem. Soc. 1989, 111, 6432-6434. Matsuda, I.;
Shibata, M.; Sato, S.; Izumi, Y. Tetrahedron Lett. 1987, 28, 3361-3362.
Mach, K.; Antropiusov a´ , Petrusov a´ , L.; Turecek, F.; Hanus, V.; Sedmera,
P.; Schraml, J. J. Organomet. Chem. 1985, 289, 331-339. tom Dieck, H.;
Diercks, R. Angew. Chem., Int. Ed. Engl. 1983, 22, 778-779. van Leeuwen,
P. W. N. M.; Roobeek, C. F. Tetrahedron 1981, 50, 1973-1983.
(
3) Collman, J. P.; Hegedus, L. S.; Norton, J.; R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Vally, CA, 1987; Section 18.3c, pp 870-879.
(4) Salter, M. M.; Gevorgyan, V.; Saito, S.; Yamamoto, Y. J. Chem.
Soc., Chem. Commun. 1996, 17-18.
5) (a) Trost, B. M.; Zhi, L. Tetrahedron Lett. 1992, 33, 1831-1834.
b)Trost, B. M.; Gerusz, V. J. J. Am. Chem. Soc. 1995, 117, 5156-5157.
(
(
(6) The reaction of 2-tert-butyl-1-buten-3-yne and 2-(trimethylsilyl)-1-
buten-3-yne afforded corresponding cyclodimerized products in lower yields,
together with inseparable linear oligomers.
0
002-7863/96/1518-3970$12.00/0 © 1996 American Chemical Society