Allylic Ta n ta lu m s a s High ly
Im in e-Selective Rea gen ts
SCHEME 1. Allyla tion of N-Ar om a tic Im in es
Ikuya Shibata,* Kyo Nose, Kazuya Sakamoto,
Makoto Yasuda, and Akio Baba*
Department of Molecular Chemistry,
Science and Technology Center for Atoms,
Molecules and Ions Control, Graduate School of
Engineering, Osaka University, Osaka, J apan
Received November 12, 2003
Abstr a ct: A pratical method for the allylation of low
electrophilic N-aliphatic imines was established by allylic
tantalum. This is a superior methodology compared with
conventional ones in terms of wide applicability to imines.
In particular, the reaction using aliphatic aldehydes
having R-protones incurs severe side reactions that
1
decrease the yields in the allylation of imines, and such
examples of three-compornent reactions have scarcely
been reported.2 Second, imines are less reactive com-
pared with the corresponding carbonyl compounds be-
cause of their low electrophilicity. Most allylations of
imines are performed on N-aromatic imines. In particu-
lar, no effective methods of the allylation of imines
derived from aliphatic aldehydes and N-aliphatic amines
have been reported so far. Recently, we have reported
the first preparation of active allylic tantalum reagents
by the transmetalation between allylic tri-n-butyltin with
a,e
Although the allylation of imines is a representative
1
method to give homoallylic amines, there are still
limitations in the method. First, the scope of application
to imines is still narrow because of their stability. The
three-component reaction in which aldehydes, amines,
and allylating reagents are treated in one portion is
assumed as the method of choice to generate labile imines
in situ, in which allylating reagents must bear high
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2
tantalum(V) chloride and the application to selective
imine-selectivity over carbonyls. However, most of the
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addition to enones. Herein we report the characteristic
reported three-component reactions include the limitation
of the use of aromatic aldehydes as starting substrates.
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reactivity of allylic tantalums toward imines. Thus, high
imine-selectivity over carbonyls was observed, and the
allylation of low electrophilic imines was established.
Initially, as shown in Scheme 1, we performed the
*
To whom correspondence should be addressed. Tel: +81-6-6879-
386. Fax: +81-6-6879-7387.
1) Allylation of imines, see for example: (a) Yamamoto, Y.; Asao,
7
(
allylation of benzylidene N-aromatic amines 1 in CH
Cl at -78 °C for 2 h by using allyltantalum generated
by the transmetalation between allyltri-n-butyltin and
2
-
N. Chem. Rev. 1992, 93, 2207-2293. (b) Bloch, R. Chem. Rev. 1998,
9
1
8, 1407-1438. (c) Kobayashi, S.; Ishitani, H. Chem. Rev. 1999, 99,
069-1094.
2
(2) The three-component synthesis of homoallylic amines has been
6
pentachlorotantalum (TaCl ) at -78 °C for 0.5 h. As a
5
carried out by using aromatic amines. (a) Kobayashi, S.; Butsujima,
T.; Nagayama, S. Chem. Commun. 1998, 19-20. (b) Akiyama, T.; Iwai,
J . Synlett 1998, 273-274. (c) Aspinall, H. C.; Bissett, J . S.; Greeves,
N.; Levin, D. Tetrahedron Lett. 2002, 43, 323-325. (d) Choudary, B.
M.; Chidara, S.; Sekhar, C. V. R. Synlett 2002, 1694-1696. (e) Yadav,
J . S.; Reddy, B. V. S.; Reddy, P. S. R.; Rao, M. S. Tetrahedron Lett.
result, homoallylamines 2a was obtained in 92% yield.
It was confirmed that complete transmetalation occurred
at -78 °C for 0.5 h to form a quantitative yield of tri-n-
3
butyltin chloride (nBu SnCl), which was measured by
2
002, 43, 6245-6247. (f) Yadav, J . S.; Reddy, B. V. S.; Krishnam Raju,
A. Synthesis 2003, 43, 883-886.
3) Allyltri-n-butyltin is a good precursor of active allylic metals such
119
GLC and Sn NMR.
The generated allyltantalum reacted with other N-
aromatic imines. Imines having electron-donating (p-
MeOC H ) and -withdrawing (p-O NC H ) nitrogen sub-
6 4 2 6 4
(
3
a
3b
3c
3d
as titanium, boron, tin, and indium. (a) Keck, G. E.; Abbott, D.
E.; Borden, E. P.; Enholm, E. J . Tetrahedron Lett. 1984, 25, 3927-
3
930. Marshall, J . A.; DeHoff, B. S. J . Org. Chem. 1986, 51, 863-872.
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2
92, 311-318. Yamamoto, Y.; Nishii, S.; Maruyama, K.; Komatsu, T.;
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(5) Generation and synthetic use of tantalum reagents5a have
scarcely been reported so far, although moderate reactivity of the Ta-
1
431. Hoppe, D. In Stereoselective Synthesis; Helmchen, G., Hoffmann,
5
b,c
R. W., Mulzer, J ., Schaumann, E., Eds.; Georg Thieme: Stuttgart,
carbon bonds to electorophiles is revealed.
(a) For example, see:
1
996; Vol. 3, Chapter 1.3.3.8, pp 1551-1583. (b) Tanigawa, Y.;
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son, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982;
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M.; Takai, K.; Utimoto, K. Tetrahedron 1992, 48, 3495-3502. Kataoka,
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Bull. Chem. Soc. J pn. 1992, 65, 1543-1549. (c) Yasuda, H.; Arai, T.;
Okamoto, T.; Nakamura, A. J . Organomet. Chem. 1989, 361, 161-
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Moritani, I.; Nishida, S. J . Organomet. Chem. 1971, 28, 73-79. Hagen,
G.; Mayr, H. J . Am. Chem. Soc. 1991, 113, 4954-4961. Marton, D.;
Tagliavini, G.; Zordan, M.; Wardell, J . L. J . Organomet. Chem. 1990,
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90, 127-138. Harston, P.; Wardell, J . L.; Marton, D.; Tagliavini, G.;
Smith, P. J . Inorg. Chim. Acta 1989, 162, 245-250. Corey, E. J .; Kim,
S. S. Tetrahedron Lett. 1990, 31, 3715-3718. (c) Keck, G. E.; Abott,
D. E. Tetrahedron Lett. 1984, 25, 1883-1886. Denmark, S. E.; Wilson,
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5
(6) The use of 2 equiv of allyltin to TaCl gave superior results
4
5, 1053-1065. Keck, G. E.; Andrus, M. B.; Castellino, S. J . Am. Chem.
compared to the case using 1 equiv of allyltin; the yield of 2a was
decreased to 57% when 1 equiv of allyltin was used. As we have already
Soc. 1989, 111, 8136-8141. (d) Marshall, J . A.; Hinkle, K. W. J . Org.
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A. Synlett 1997, 699-700.
reported, 2 equiv of allyltin was consumed in the reaction with TaCl
Thus, the generation of diallyltantalum species can be considered.
5
.
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0.1021/jo0356659 CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/20/2004
J . Org. Chem. 2004, 69, 2185-2187
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