Tetrakis(dimethylamino)ethylene (TDAE) as a potent organic electron source: Alkenylation of aldehydes using an Ni/Cr/TDAE redox system

Article abstract of DOI:10.1016/S0040-4039(99)02006-7

Tetrakis(dimethylamino)ethylene (TDAE) was found to be a potent organic electron source which could promote alkenylation of several kinds of aldehydes in the presence of a catalytic amount of NiBr₂ and CrCl₃.

Full text of DOI:10.1016/S0040-4039(99)02006-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 81–84
Tetrakis(dimethylamino)ethylene (TDAE) as a potent organic
electron source: alkenylation of aldehydes using an Ni/Cr/TDAE
redox system
Manabu Kuroboshi, Muneaki Tanaka, Suguru Kishimoto, Kentaro Goto, Misato Mochizuki
∗
and Hideo Tanaka
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530,
Japan
Received 24 September 1999; revised 12 October 1999; accepted 18 October 1999
Abstract
Tetrakis(dimethylamino)ethylene (TDAE) was found to be a potent organic electron source which could promote
alkenylation of several kinds of aldehydes in the presence of a catalytic amount of NiBr2 and CrCl3. © 1999 Elsevier
Science Ltd. All rights reserved.
Keywords: reduction; enamines; alkenylation; aldehydes.
A combination of a transition metal salt with a proper electron source (reductant) constitutes an
electron transfer system in which the transition metal behaves as an electron transfer catalyst. The
reactivity and product selectivity are highly dependent on the components of the system, e.g., electron
1
a
1b
1c
source, transition metal, and solvent. Metals such as Zn, Mg, and Mn and metal hydrides such
1
d
as LiAlH4 have been used as an electron source. Organic reductants are expected to have several
advantages over inorganic reductants: (1) the redox potential is easily tunable by designing and modifying
the molecular structure; and (2) organic reductants are soluble in organic solvents, thus shortening the
induction period. Tetrakis(dimethylamino)ethylene (TDAE) is known as an organic reductant having
reducing power close to that of zinc, and is used in reduction of organofluorine compounds. However,
there has been no report on a combination of metal catalysts and TDAE used in organic synthesis. Herein
we describe that the catalytic Nozaki–Hiyama reaction proceeds by use of TDAE as a reductant for
regeneration and recycled use of the Cr(II) reagent.
2
2,3
∗
Corresponding author. E-mail: tanaka95@cc.okayama-u.ac.jp (H. Tanaka)
0
040-4039/00/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02006-7
tetl 15934

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2
The Cr–Ni promoted alkenylation of carbonyl compounds (Nozaki–Hiyama reaction) proceeds in
4
a highly chemoselective manner under very mild conditions, and has been used in the synthesis of
5
complex natural products. The serious drawback of this system, however, is that a large excess (400
mol%) of CrCl is usually used, which may cause environmental problems. Therefore, the regeneration
2
and recycled use of a catalytic amount of a Cr reagent are subjects of keen interest, and have been
successfully performed by reduction with metals such as Mn6 and Al and by electro-reduction.
a
6b
6c,d
In
most of these procedures, CrCl was used, which is expensive and difficult to handle because of its high
2
7
sensitivity to moisture and O . Systems using stable CrCl are more preferable.
2
3
A typical procedure for alkenylation of aldehydes is as follows (Table 1, Entry 1): To a mixture of β-
bromostyrene 1a (2.0 mmol), p-anisaldehyde 2a (1 mmol), CrCl (30 mol%, Katayama Chemical Co.),
3
NiBr (2 mol%, Katayama Chemical Co.), Me SiCl (150 mol%) in freshly distilled DMF (5 mL) was
2
3
added TDAE (150 mol%). The mixture was stirred at 50°C under argon atmosphere for 6 h. After usual
workup, the corresponding allyl alcohol 3a was obtained in 96% yield. Alkenylation hardly proceeded
at room temperature even with excess TDAE (Entry 2). A notable feature of this process is that the
alkenylation proceeded with CrCl . The yield of 3a decreased to 51% when 100 mol% of TDAE was
3
used (Entry 3). The yield of 3a was decreased to 40% when CrCl was decreased to 0.15 mmol (Entry
3
4
). When no CrCl was used, homo-coupling of β-bromostyrene occurred predominantly to give 1,4-
3
diphenyl-1,3-butadiene 5a (Entry 5).
Table 1
Cross-coupling of p-anisaldehyde 2a and β-bromostyrene 1a
Representative results of alkenylation of aldehydes 1 in the TDAE/CrCl /NiBr /DMF system are
2
2
shown below. The alkenylation of aromatic aldehydes proceeded smoothly to give the corresponding
allyl alcohol derivatives (3a, 3b, 3c) in good yields, whereas electron-poor aromatic aldehydes such as
p-cyanobenzaldehyde gave poor results (3e), resulting in predominant formation of the corresponding
pinacol derivatives. These results are almost comparable to those obtained in Al/Cr/Ni-promoted6b

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3
alkenylation. It is noteworthy that alkenylation of p-chlorobenzaldehyde gave the product 3d in better
8
9
yields, whereas aliphatic aldehydes such as 3-phenylpropanal gave no alkenylated product 3f, most
of which was recovered unchanged. This significant change would be caused by a Lewis-acidic species
generated in situ: in Al-promoted alkenylation, Lewis-acidic Al salts accumulate which would facilitate
either alkenylation or pinacolization of aldehydes. On the other hand, a strong Lewis-acidic species would
not be produced in the TDAE/Cr/Ni system.
Although the reaction mechanism is not clear at present, taking the well-established mechanism of
the Hiyama–Nozaki reaction into consideration, a plausible catalytic cycle of this reaction is illustrated
in Scheme 1. Standard redox potential of TDAE was measured in DMF to be −0.62 V versus SCE (a
3
+
2+
2+
two-electron reversible peak), which is less than those of Al /Al, Zn /Zn, and Mn /Mn but is adequate
3
+
2+
2+
0
for reduction of both Cr(III) (Cr /Cr −0.41 V) and Ni(II) (Ni /Ni −0.23 V) (Scheme 2). The purple
CrCl suspension in DMF immediately turned into a green solution on addition of TDAE, suggesting that
3
TDAE smoothly reduces Cr(III) into Cr(II). Ni(II) would be reduced to Ni(0) by either Cr(II) or directly
by TDAE, which would react with alkenyl halide to produce alkenylnickel species 6. Transmetallation
between 6 and Cr(III) would afford alkenylchromium 7 which would react with aldehyde to yield Cr-
alkoxide 8. Subsequent reaction of the alkoxide 8 with Me SiCl would liberate the Cr(III) salt with
3
forming silyl ether 9, which would be hydrolyzed during workup to give the corresponding homoallyl
alcohol 3. The thus-generated Cr(III) salt would be reduced with TDAE to regenerate Cr(II).
Scheme 1. A plausible mechanism of Cr/Ni/TDAE promoted alkenylation of aldehyde
In conclusion, TDAE can act as a potent electron source, and an electron transfer system consisting of
TDAE and a catalytic amount of CrCl /NiBr effectively promotes alkenylation of aromatic aldehydes.
3
2
CrCl can be used as a chromium source. As TDAE shows a reversible redox peak in its cyclic
3
voltammogram, TDAE itself is expected to be an electron transfer catalyst. Electrochemical recycling
of TDAE is now under study in our laboratory.

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4
Scheme 2. Redox potentials of metals, metal salts, and TDAE
Acknowledgements
This work was supported in part by the Ministry of Education, Science and Culture, Japan.
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7
8
9
. In the previous paper (Ref. 6b), we reported that in the presence of a small amount of Zn, CrCl
reaction using Al as an electron source.
3
can promote the alkenylation
. Upon reaction of p-chlorobenzaldehyde and vinyl bromide under Al/CrCl
-ol was obtained in 25% (Ref. 6b).
. 5-Phenyl-1-propen-3-ol was obtained in 34 and 64% yield from 3-phenylpropanal and vinyl bromide under electro-reduction
Ref. 6d) and Al/CrCl /NiBr (Ref. 6b) conditions, respectively.
2 2
/NiBr conditions, 1-(4-chlorophenyl)-2-propen-
1
(
2
2