1b. The generated dienolate was reacted with 4 to give the
dienyl triflate 2b in a moderate yield (Table 1, entry 2). The
scope of the present method was broad, and dienyl triflates
of a wide range of substitution patterns could be prepared.
It should be noted that all of the dienyl triflates except 2a
could not be prepared by direct deprotonation of the vinyl
ketones 1 (the route shown in Scheme 2).12
In most of the cases, silyl dienol ethers 7 were obtained
as mixtures of (E)- and (Z)-isomers. The (E)/(Z) ratios in
7c-f were nearly retained during the transformation to 2c-f
via 3c-f (Table 1, entries 3-6). The triflate 2a prepared by
the method shown in Scheme 2 was (E)-isomer exclusively.
On the other hand, 2a obtained via the silyl dienol ether 7a
was a mixture of the two isomers with the (Z)-isomer as the
major component (Table 1, entry 1).
in the presence of a Pd catalyst (2 mol %) generated in situ
from [PdCl(π-allyl)]2 and dpbp13 to give the corresponding
allene 9am in 97% yields (Table 2, entry 1). Multisubstituted
allenes could be prepared with appropriate triflates 2. For
example, the Pd-catalyzed reactions of 1,1-disubstituted 1,3-
butadien-2-yl triflates, such as 2b or 2d, afforded trisubsti-
tuted allenes in excellent yields (Table 2, entries 2-5, 7,
and 8). Similarly, tetrasubstituted allenes 9em-9eo were
obtained from the 1,1,3-trihydrocarbyl-1,3-butadien-2-yl tri-
flate 2e in >90% isolated yields (Table 2, entries 9-11). A
variety of soft nucleophiles, such as stabilized carbanions
(8m and 8n) and an N-nucleophile (8o), could be used for
the reactions with the triflates (Table 2, entries 4, 5, 10, and
11). In all cases, the reactions proceeded cleanly, and no
appreciable byproducts were detected in the reaction mix-
tures.
The (E)/(Z)-isomers of 2d were separated by careful
column chromatography of the mixture. Both isomers reacted
with 8m smoothly under the Pd catalysis and yielded the
identical allene 9dm in nearly quantitative yields (Table 2,
entries 7 and 8). Apparently, the stereochemistry in 2 was
not critical for the Pd-catalyzed reaction. Indeed, the allenic
products were obtained in excellent yields even with the (E)/
(Z)-mixtures of the triflates (Table 2, entries 6, 9-12).
The 1,3-dien-2-yl triflates 2 obtained here were applied
in the Pd-catalyzed reaction with soft nucleophiles. As
expected, 2 were found to be excellent substrates for a variety
of multisubstituted functionalized allenes. The results of the
Pd-catalyzed reaction are summarized in Table 2. The triflate
Table 2. Palladium-Catalyzed Synthesis of Multisubstituted
Allenes 9 from 1,3-Dien-2-yl Triflates 2 and Nucleophiles 8a
Table 3. Pd-Catalyzed Asymmetric Synthesis of Axially Chiral
Allenes from Dienyl Triflates 2 or Bromodienes 10a
entry
triflate 2
(E)-2a
(Z)-2a
2b
nucleophile 8 T (°C) yield of 9b (%)
1
2
3
4
5
6
7
8
9
8m
8m
8m
8n
23
23
23
50
50
23
23
23
23
50
50
23
97 (9am)
92 (9am)
99 (9bm)
96 (9bn)
85 (9bo)
87 (9cm)
96 (9dm)
95 (9dm)
93 (9em)
95 (9en)
90 (9eo)
90 (9fm)
2b
2b
8o
(E)- and (Z)-2cc
(E)-2d
8m
8m
8m
8m
8n
(Z)-2d
(E)- and (Z)-2ec
(E)- and (Z)-2ec
(E)- and (Z)-2ec
(E)- and (Z)-2fc
10
11
12
8o
8m
entry diene Nu-M solvent
L*
yieldb (%) % eec
1d
(Z)-2a
8o
8o
8n′
8n′
THF
THF
CH2Cl2 BINAP
CH2Cl2 BINAP
tms-BINAP 72 (9ao)
tms-BINAP 98 (9ao)
22
77
79
89
a Reaction was carried out with 2 (1.0 mmol) and 8 (1.1 mmol) in THF
in the presence of the catalyst (2 mol %) generated from [PdCl(π-allyl)]2
and dpbp. b Isolated yield by silica gel chromatography. c As (Z)- and
(E)-mixtures.
2d,e (Z)-10
3f
4f,g
(Z)-2a
(Z)-10
89 (9an)
75 (9an)
a All of the reactions were carried out with 2a or 10 (0.50 mmol) and 8
(0.55 mmol) in a given solvent (5.0 mL) for 24 h in the presence of a Pd
catalyst generated from Pd(dba)2 and (R)-L*. b Isolated yield by column
chromatography. c Determined by chiral HPLC analysis (9ao: Chiralpak
OD-H; 9an: Chiralcel OJ-H). d With 5 mol % of the Pd catalyst. e Taken
from ref 15. f With 10 mol % of the Pd catalyst. g Taken from ref 3b.
(E)-2a was reacted with Na[CMe(COOMe)2] (8m) in THF
(11) Cazeau, P.; Duboudin, F.; Moulines, F.; Babot, O.; Dunogues, J.
Tetrahedron 1987, 43, 2089.
(12) Several 1,3-dien-2-yl triflates have been reported so far. However,
none of the reported procedures could be generally applicable for preparation
of dienyl triflates. See:, (a) Luan, L.; Song, J.-S.; Bullock, R. M. J. Org.
Chem. 1995, 60, 7170. (b) Wipf, P.; Xu, W. J. Org. Chem. 1996, 61, 6556.
(c) Nicolaou, K. C.; Postema, M. H. D.; Miller, N. D.; Yang, G. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2821. (d) Alvarez, R.; Iglesias, B.; Lo´pez,
S.; de Lera, A. R. Tetrahedron Lett. 1998, 39, 5659. (e) Bio, M. M.;
Leighton, J. L. Org. Lett. 2000, 2, 2905. (f) Bio, M. M.; Leighton, J. L. J.
Org. Chem. 2003, 68, 1693.
Since the dienyl triflates 2 were demonstrated as useful
precursors to the allenic compounds, their application in
(13) dpbp ) 2, 2′-bis(diphenylphosphino)-1, 1′-biphenyl. See: Ogasawara,
M.; Yoshida, K.; Hayashi, T. Organometallics 2000, 19, 1567, and
references therein.
Org. Lett., Vol. 7, No. 25, 2005
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