A novel synthesis of allenes by alkenylation of magnesium alkylidene carbenoids with lithium α-sulfonyl carbanions

Article abstract of DOI:10.1016/S0040-4039(02)00103-X

Treatment of magnesium alkylidene carbenoids, which were generated from 1-chlorovinyl p-tolyl sulfoxides with a Grignard reagent by sulfoxide-magnesium exchange, with lithium α-sulfonyl carbanions gave allenes in moderate to good yields. This procedure offers a novel synthetic method for allenes with three-component coupling in relatively short steps.

Full text of DOI:10.1016/S0040-4039(02)00103-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2043–2046
A novel synthesis of allenes by alkenylation of magnesium
alkylidene carbenoids with lithium a-sulfonyl carbanions
Tsuyoshi Satoh,* Tatsuya Sakamoto and Masanori Watanabe
Department of Chemistry, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
Received 10 December 2001; accepted 11 January 2002
Abstract—Treatment of magnesium alkylidene carbenoids, which were generated from 1-chlorovinyl p-tolyl sulfoxides with a
Grignard reagent by sulfoxide–magnesium exchange, with lithium a-sulfonyl carbanions gave allenes in moderate to good yields.
This procedure offers a novel synthetic method for allenes with three-component coupling in relatively short steps. © 2002
Elsevier Science Ltd. All rights reserved.
Allenes are quite important and interesting compounds
in organic and synthetic organic chemistry, and a large
number of studies have been reported on their chem-
method for the synthesis of allenes 6 from three compo-
nents, ketones 1, chloromethyl p-tolyl sulfoxide 2, and
sulfones 5, in relatively short steps.
1
istry and synthesis. The general methods for the syn-
thesis of allenes are, for example, isomerization of
In a previous paper, we reported the generation of
2
3
acetylenes, ring-opening of cyclopropylidenes, the
reaction of propargylic derivatives with organocopper
magnesium alkylidene carbenoids 4 from 1-chlorovinyl
6
p-tolyl sulfoxides 3 and their properties. In order to
4
5
reagents, and b-elimination of olefins.
use the carbenoids 4 in a new synthetic method, we
reinvestigated an improved method for the generation
of the magnesium alkylidene carbenoids 4 from 3 as
follows (Scheme 2).
We previously reported the generation of magnesium
6
alkylidene carbenoids 4 from 1-chlorovinyl sulfoxides
3
by a sulfoxide–magnesium exchange reaction and an
application of the methodology to a new synthesis of
In a flame-dried flask, 0.5 equivalent of t-BuMgCl was
added to a solution of 7 (1 equivalent) in dry THF with
stirring at −78°C. Using this treatment no reaction of 7
with t-BuMgCl took place, but a trace of moisture in
the solution was removed. After 10 min, 3 equivalents
of EtMgCl or i-PrMgCl (both Grignard reagents work
equally well) were added to the reaction mixture. A
quite rapid reaction took place to give the magnesium
alkylidene carbenoid 8 in over 92% chemical yield, and
the protonation was suppressed to less than 10% (the
protonation product of this reaction is the desulfinyl-
ated chloroalkene).
6
tetra-substituted alkenes. In continuation of our study
on the use of the magnesium alkylidene carbenoids
generated by the sulfoxide–metal exchange reaction in
7
8
organic synthesis, we investigated the reaction of car-
benoids 4 with some carbanions and found that lithium
a-sulfonyl carbanions 5 gave allenes 6 in moderate to
good yields (Scheme 1). This procedure offers a novel
TolS(O)CH2Cl
R1
R1
RMgCl
Cl
2
O
R2
in three steps
lit. 6
R2
S(O)Tol
1
3
PhSO2C(Li)R R4
3
To this carbenoid a solution of the lithium a-sulfonyl
carbanion 10 (3 equivalents) derived from benzyl
phenyl sulfone was added, and the temperature of the
reaction mixture was gradually allowed to warm to
room temperature. We obtained a crystalline com-
pound which showed sharp absorption at 1954 cm in
IR spectrum. Other spectral data showed that the
product was the allene 12. We propose a plausible
mechanism of this reaction, as shown in Scheme 2.
R1
R1
R2
R3
R4
5
.
:
MgCl2
R2
4
6
Scheme 1.
−
1
Keywords: allene; magnesium alkylidene carbenoid; sulfoxide; sulfox-
ide–magnesium exchange; sulfone.
*
Corresponding author.
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00103-X

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044
T. Satoh et al. / Tetrahedron Letters 43 (2002) 2043–2046
O
O
Cl
1) t-BuMgCl (0.5 eq.)
Cl
:
MgCl2
S(O)Tol
2) EtMgCl (3 eq.)
THF, -78 °C
MgCl
9
8
7
PhSO2C(Li)HPh
O
O
Ph
10
-
.
SO2Ph
Ph
:
H
-
PhCHSO2Ph
H
1
2
1
1
Scheme 2.
1
First, the lithium a-sulfonyl carbanion 10 attacks the
electron-deficient carbenoid carbon of 9 to give the
vinyl anion 11. As the sulfonyl group is a good leaving
group, b-elimination takes place to afford the allene 12.
group as R gave no allenes but only a complex
mixture.
One difference between benzyl phenyl sulfone and the
sulfones in entries 2–4 is the acidity of the hydrogen on
the carbon bearing the sulfonyl group. The pKa values
We next investigated this reaction using various kinds
of sulfones, and the results are summarized in Table 1.
The result in entry 1 has been described above. The
results in entries 2–4 are quite interesting. In contrast to
the result with benzyl phenyl sulfone (entry 1), the
a-sulfonyl carbanion having a hydrogen or an alkyl
9
of these hydrogens are estimated to be as follows:
PhSO CH6 Ph (23.4), PhSO CH6 (29), PhSO CH6 2CH3
2
2 2 3 2
(31). These values imply that some carbanion-stabiliz-
ing group is essential for the reaction of the a-sulfonyl
carbanion with the magnesium alkylidene carbenoid.
Table 1. Synthesis of allenes 13 from 1-chlorovinyl p-tolyl sulfoxide 7 and various lithium a-sulfonyl carbanions
1
) t-BuMgCl (0.5 eq.)
2) EtMgCl (3 eq.), THF, -78 °C
2(3 eq.)
O
O
Cl
O
O
R1
R2
.
1
3) PhSO2C(Li)R R
S(O)Tol
THF, -78 °C to room temp.
1
3
7
Sulfone
Yield
%
Entry
1
Allene
R1
R2
Ph
H
Ph
H
.
63
2
3
4
H
H
H
H
complex mixture
complex mixture
complex mixture
CH (CH )
3
2 8
5
H
.
5
5
1
1
H
H
H
.
6
7
CH2=CH
C H
H
H
H
C5H11
.
65
5
11
H
Ph
8
9
Ph
Ph
H
.
4
1
Ph
PhS
SPh
14a)
41b)
.
H
COOCH3
COOCH3
CH3
.
1
0
CH3
a) The reaction was carried out in the presence of DMPU. b) The reaction was carried
out in the presence of HMPA.

T. Satoh et al. / Tetrahedron Letters 43 (2002) 2043–2046
2045
Table 2. Synthesis of allenes 15 from 1-chlorovinyl p-tolyl sulfoxide 14 and various lithium a-sulfonyl carbanions
1
2
) t-BuMgCl (0.5 eq.)
R1
R2
) i-PrMgCl (3 eq.), THF, -78 °C
Cl
.
1
2(3 eq.)
S(O)Tol 3) PhSO2C(Li)R R
THF / HMPA, -78 °C to room temp.
1
5
14
Sulfone
Yield
%
Entry
1
Allene
R1
R2
Ph
H
Ph
H
.
54
2
3
H
.
.
56
H
Ph
Ph
Ph
2
4
Ph
H
H
H
.
4
CH2=CH
H
61
Cl
1) t-BuMgCl
H
R
H
R
H
R
H
.
:
2
3
) EtMgCl (3 eq.),
THF, -78 °C
) PhSO2C(Li)HPh
S(O)Ph
Ph
1
7
16
THF / HMPA
-78 °C to room temp.
R
H
O
1
1
8a 87%
8b 79%
16a R=
O
16b R=CH3(CH2)14-
Scheme 3.
Actually, the sulfones having a carbanion-stabilizing
group on the a-carbon gave the desired allenes, as
shown in entries 5–10, though the yields were variable.
It is interesting to note that tetra-substituted allenes,
the synthesis of which is recognized to be difficult, is
also obtained by this method (entries 8 and 10).
hydrogen in the magnesium alkylidene carbenoid pro-
ceeds faster than the reaction with the a-sulfonyl
carbanion. From these results it is emerged that we
cannot synthesize allenes from aldehydes by this
procedure.
We are continuing to study the scope and limitation of
this procedure and improving the yields of the allenes.
Table 2 shows the results of the reaction of 1-
chlorovinyl p-tolyl sulfoxide 14 derived from
6
cyclopentadecanone with some sulfones. We antici-
pated some problems with the Fritsch–Wiechell
1
0
rearrangement of the intermediate, magnesium alkyli-
dene carbenoid; however, 14 gave allenes in a similar
yields and no rearranged product, cyclohexadecyne,
was observed.
Acknowledgements
This work was supported by a Grant-in-Aid for Scien-
tific Research No. 11640545 from the Ministry of Edu-
cation, Science, Sports, and Culture, Japan, which is
gratefully acknowledged.
Finally, we studied this reaction with the 1-chlorovinyl
p-tolyl sulfoxides (16a and 16b) derived from aldehydes
(
Scheme 3). The reaction was conducted in the same
way as described above and we obtained completely
different results. By this reaction acetylenes 18a and 18b
were obtained in good yields and the desired allenes
were not observed. These results clearly indicated to us
that the Fritsch–Wiechell rearrangement of the b-
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