Tetrakis(dimethylamino)ethylene (TDAE) as a potent electron source for Cr-mediated allylation of aldehydes and ketones

Article abstract of DOI:10.1055/s-1999-2995

A new combination of organic reductant, tetrakis(dimethylamino)ethylene, and transition metal catalysts is reported for allylation of aldehydes and ketones.

Full text of DOI:10.1055/s-1999-2995

1930
LETTER
Tetrakis(dimethylamino)ethylene (TDAE) as a Potent Electron Source for Cr-
Mediated Allylation of Aldehydes and Ketones
Manabu Kuroboshi, Kentaro Goto, Miisato Mochizuki, Hideo Tanaka*
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan
E-mail: tanaka95@cc.okayama-u.ac.jp
Received 16 September 1999
TDAE (200 mol%)
Abstract: A new combination of organic reductant, tetrakis(di-
NiBr2 (5 mol%)
methylamino)ethylene, and transition metal catalysts is reported for
allylation of aldehydes and ketones.
OH
O
CrCl3 (30 mol%)
Br
+
R1
R2
R1
R2
Me3SiCl (150 mol%)
DMF, room temp
Key words: allylations, cross-coupling, chromium, nickel, reduc-
tions
1
2
3
(Me2N)2C C(NMe2)2
(TDAE)
E
ox = -0.62 V
vs. SCE in DMF
For various carbon-carbon bond formation reactions,
electron-transfer systems consisting of an electron source
and a transition metal catalyst are often used. In these sys-
tems, very active low-valent transition metal species can
be generated in situ and used repeatedly. Reactivity and
product selectivity are highly dependent on the electron
source, transition metal catalysts, and solvents. Although
metals, low-valent metal salts, or metal hydrides are often
employed as electron sources, organic reductants have
several advantages over such inorganic reductants: (1) the
redox potential is tunable by designing the molecular
structure and (2) organic reductants are usually soluble in
conventional organic solvents.
Figure 1
The reaction was accelerated effectively by adding a cat-
alytic amount of NiBr2; indeed, in the absence of any Ni
salt, the yield of 3a decreased to 57% (Entry 2) even when
large excess of TDAE (400 mol%) and Me3SiCl (450
mol%) were used. Methylated product, 1-(4-methoxyphe-
nyl)ethanol 4a, was obtained as a by-product (8%). When
the reaction was performed at 50 °C, the yield of 3a was
increased to some extent (Entry 3). The desired product 3a
was generated in only low yield when the reaction was
performed in THF even in the presence of Ni salts (Entries
4 and 5).
Tetrakis(dimethylamino)ethylene (TDAE, Eox = -0.62 V
vs. SCE) is considered to be a potent organic reductant
close to zinc (Eox = -0.76 V).1 However, it has been
scarcely used in reductive molecular transformations ex-
cept for reduction of a few organofluorine compounds.2
There has been no report on a combination of metal cata-
lyst and TDAE used in organic synthesis.
OH
O
TDAE
Br
+
Ar
Ar
Ar
H
The Cr-Ni promoted allylation of carbonyl compounds
proceeds under very mild conditions,3 and has been used
in the synthesis of complex natural products. The draw-
back of this system is, however, that large excess (> 300
mol%) of CrCl2 is usually needed, which may cause seri-
ous environmental problems. Generation and recycle use
of low-valent Cr(II) reagents in situ with metals4a,4b and by
electroreduction4c,4d have been recently reported.
NiBr2
CrCl3 (30 mol%)
3a
1a
2a
OH
(1.0 mmol) (2 mmol)
Me3SiCl
23 h
Ar = p-Anis
4a
Figure 2
Table Allylation of p-anisaldehyde 1a with CrCl3/NiBr2/TDAE Sy-
stem
We report herein that TDAE can be used as an electron
source for the Ni/Cr redox-catalyzed allylation of carbon-
yl compounds.
Yield/%b
TDAE Me3SiCl NiBr2
/mmol /mmol /mmol
temp.
/°Ca
Entry
Solvent
A typical procedure is as follows: To a mixture of p-anis-
aldehyde 1a (1 mmol), allyl bromide 2a (2 mmol), CrCl3
(0.3 mmol, Katayama Chemical Co.),5,6 NiBr2 (0.05
mmol, Katayama Chemical Co.), and Me3SiCl (1.5 mmol)
in freshly distilled DMF (5 mL) was added TDAE (Tokyo
Kasei Co., Ltd., 2 mmol). The mixture was stirred at room
temperature for 23 h. After usual work-up, homoallyl
alcohol 3a was obtained in 94% yield (Table, Entry 1).
3a 4a 1a
1
2
3
4
5
2.0
4.0
4.0
4.0
4.0
1.5
4.5
4.5
4.5
4.5
0.05
0
DMF
DMF
DMF
THF
THF
r.t. 94
-
-
2
-
r.t. 57
50 76
8
1
0
0.05
0.05
r.t. 20 4 59
50 44 5 32
a. r.t. = room temp. b. Isolated yields
Synlett 1999, No. 12, 1930–1932 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Tetrakis(dimethylamino)ethylene (TDAE) as a Potent Electron Source
1931
Representative results of allylation of aldehydes and ke- allylation of carbonyl compound 1 would afford Cr-
tones 1 in the TDAE/CrCl2/NiBr2/DMF system are shown alkoxide 7. Subsequent reaction of alkoxide 7 with chlo-
below. The allylation proceeded with aromatic (3a ~ 3e), rotrimethylsilane would liberate Cr(III) with forming silyl
aliphatic (3g), and a,b-unsaturated aldehydes (3h) and ke- ether 8 which would be hydrolyzed during work-up to
tone (3i). Aromatic aldehydes bearing an electron-donat- give homoallyl alcohol 3. Thus the generated Cr(III)
ing substituent gave homoallyl alcohol derivatives in would be reduced with TDAE to regenerate Cr(II).
good yields, whereas electron-poor aromatic aldehyde p-
cyanobenzaldehyde gave no desired compound (3f), af-
fording instead a corresponding pinacol derivative pre-
dominantly. Carbonyl addition occurred with trans-
cinnamaldehyde to give 1,2-adduct 3h in 73% yield with-
out formation of any 1,4-adduct.
OCr(III)
R'CHO 1
R X
2
R Cr(III)
R Ni(II)
5
R
R'
6
7
Cr(III) Ni(II)
Ni(0)
Me3SiCl
OSiMe3
Cr(III)
+
Cr(III)
OH
OH
OH
2 Cr(II)
2 e-
(TDAE)
O
O
R
R'
8
MeO
H+
3c (3.5 h: 70%)
3a (3 h: 83%, 23 h: 94%) 3b (1.5 h: 89%)
OH
OH
OH
OH
R
R'
3
Cl
NC
Me
Figure 5
3d (4h: 75%)
3f (2 h: 0%)
3e (3 h: 86%)
OH
OH
OH
In conclusion, TDAE is shown to act as a potent electron
source, and the electron-transfer system consisting of
TDAE and cat. Cr-Ni works smoothly to promote the
allylation of aldehydes.
3h (3 h: 73%)
3i (4 h: 36%)
3g (2 h: 72%)
Figure 3
Acknowledgement
Regiochemistry of the allylation in the Ni/Cr/TDAE sys-
tem was also examined. When p-anisaldehyde 1a was
treated with 3-methyl-2-propenyl bromide (2b), the reac-
tion occurred selectively at the g-position of bromide 2b
to afford 1-(4-methoxyphenyl)-2,2-dimethyl-3-buten-1-
ol 3j in 48% yield, no a-allylating product being obtained.
This work was supported in part by the Ministry of Education, Sci-
ence and Culture, Japan.
References and Notes
(1) Burkholder, C.; Dolbier, W. R. Jr.; Médebielle, M. J. Org.
Chem. 1998, 63, 5385.
(2) Chambers, R. D.; Nishimura, S.; Sandford, G. J. Fluorine
Chem. 1998, 91, 63. Chambers, R. D.; Gray, W. K.; Korn, S.
R. Tetrahedron 1995, 51, 13167. Chambers, R. D.; Vaughan,
J. F. S.; Mullins, S. J.; Nakamura, T.; Roche, A. J. J. Fluorine
Chem. 1995, 72, 231. Briscoe, M. W.; Chambers, R. D.;
Mullins, S. J.; Nakamura, T.; Vaughan, J. F. S.; Drakesmith,
F. G. J. Chem. Soc., Perkin Trans. 1 1994, 3115. Buerger, H.;
Moritz, P. Organometallics 1993, 12, 4930. Burkholder, C.;
Dolbier, W. R., Jr.; Medebielle, M. Tetrahedron Lett. 1997,
38, 821. Pawelke, G. J. Fluorine Chem. 1991, 52, 229.
Pawelke, G. J. Fluorine Chem. 1989, 42 429.
TDAE (200 mol%)
OH
NiBr2 (5 mol%)
CrCl3 (30 mol%)
O
Br
+
Ar
Ar
H
Me3SiCl (150 mol%)
DMF, room temp, 6 h
3j (48%)
+
4a (3%)
1a
2b
Ar = p-Anis
(3) Wessjohann, L. A.; Scheid, G. Synthesis 1999, 1. Takai, K.;
Kimura, K.; Kuroda, T.; Hiyama, T.; Nozaki, H. Tetrahedron
Lett. 1983, 24, 5281. Nicholas, A. S. in Comprehensive
Organic Synthesis; Trost, B. M.; Fleming, I.; Schreber, S. L.,
Eds.; Pergamon Press: Oxford, 1991; Vol. 1, Chapter 1.6.
Cintas, P. Synthesis 1992, 248. Hoppe, D. in Methods of
Organic Chemistry; Georg Thieme Verlag: Stuttgart, 1995;
Vol. E21, p. 1584.
(4) a) Fürstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118, 12349.
b) Kuroboshi, M.; Tanaka, M.; Kishimoto, S.; Goto, K.;
Tanaka, H.; Torii, S. Tetreahedron Lett. 1999, 40, 2785. c)
Grigg, R.; Putnikovic, B.; Urch, C. J. Tetrahedron Lett. 1997,
Figure 4
Taking well-established mechanism of the Hiyama-Noza-
ki reaction into consideration, a plausible catalytic cycle
of this reaction can be suggested as follows. Purple CrCl3
suspension in DMF immediately turned into green solu-
tion on addition of TDAE, suggesting that TDAE smooth-
ly reduced Cr(III) into Cr(II). The Cr/Ni mediated
Synlett 1999, No. 12, 1930–1932 ISSN 0936-5214 © Thieme Stuttgart · New York

1932
M. Kuroboshi et al.
LETTER
38, 6307. d) Kuroboshi, M.; Tanaka, M.; Kishimoto, A.;
Tanaka, H.; Torii, S. Synlett 1999, 69.
(6) When the amount of CrCl3 was reduced to 0.2 and 0.1 mmol,
yields of the desired compound 3 gradually decreased to 58
and 50%, respectively.
(5) Stable CrCl3 can be used in place of CrCl2. CrCl2 is expensive
and difficult to handle because of its high sensitivity to
moisture and O2. When Al and CrCl3 were used as an electron
source and Cr source, a catalytic amount of Zn was
indispensable to promote the reaction. See 4b.
Article Identifier:
1437-2096,E;1999,0,12,1930,1932,ftx,en;Y18399ST.pdf
Synlett 1999, No. 12, 1930–1932 ISSN 0936-5214 © Thieme Stuttgart · New York