Lewis acid-catalyzed three-component condensation reactions of aldehydes, alkoxysilanes, and propargylsilane: Synthesis of α-allenyl ethers

Article abstract of DOI:10.1016/S0040-4039(00)02333-9

For simultaneous construction of a polyether backbone and the allenyl side chains, Lewis acid-catalyzed three-component condensation reactions of aldehydes, alkoxytrimethylsilanes, and 1-trimethylsilyl-2-butyne were studied. The reaction of these three compounds took place in the presence of a catalytic amount of TrClO₄ at -78°C to yield the corresponding α-allenyl ethers in good yields. This reaction was also applied to the synthesis of a polyether having allenyl side chains.

Full text of DOI:10.1016/S0040-4039(00)02333-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1721–1724
Lewis acid-catalyzed three-component condensation reactions of
aldehydes, alkoxysilanes, and propargylsilane: synthesis of
a-allenyl ethers
Lui Niimi, Keitaro Shiino, Shuichi Hiraoka and Tsutomu Yokozawa*
Department of Applied Chemistry, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
Received 13 November 2000; revised 11 December 2000; accepted 15 December 2000
Abstract—For simultaneous construction of a polyether backbone and the allenyl side chains, Lewis acid-catalyzed three-compo-
nent condensation reactions of aldehydes, alkoxytrimethylsilanes, and 1-trimethylsilyl-2-butyne were studied. The reaction of these
three compounds took place in the presence of a catalytic amount of TrClO4 at −78°C to yield the corresponding a-allenyl ethers
in good yields. This reaction was also applied to the synthesis of a polyether having allenyl side chains. © 2001 Elsevier Science
Ltd. All rights reserved.
Many condensation polymers have been synthesized by
the two-component polycondensation of bifunctional
nucleophilic monomers and electrophilic monomers.
Recently, we have reported three-component polycon-
densation of dialdehydes, alkylene bis(trimethylsilyl)
ethers, and various silyl nucleophiles.1 In these poly-
merizations, the use of trialkylsilanes as a silyl nucleo-
phile gave ordinary polyethers,1a whereas the use of
allyltrimethylsilane, cyanotrimethylsilane, and silyl enol
ethers afforded polyethers having the allyl,1b cyano,1c
and keto1d,e side chains, respectively. In the latter cases,
we have demonstrated that the polyether backbone and
functional side chains are simultaneously constructed in
one pot. The allenyl group is also interesting as a
functional side chain because allene derivatives2 under-
go a variety of reactions, such as cycloaddition,3a–c
radical addition,3d,e photooxygenation,3f polymerization
by transition metals,3g–k etc. A polymer having the
allenyl side chains was synthesized by the polymeriza-
tion of a monomer bearing an allenyl group,4 but, to
OR2
SiMe3
cat. TrClO4
•
R2O-SiMe3
R3
R1-CHO
+
+
R1
CH2Cl2, -78 ˚C
R3
1
2
3
5
Scheme 1.
OHC
CHO
SiMe3
OSiMe3
+
+
Me3SiO
3b
2b
1b
•
•
cat. TrClO4
CH2Cl2
O
O
n
Scheme 2.
Keywords: propargylsilane; 1-trimethylsilyl-2-butyne; trityl perchlorate; polyether.
* Corresponding author. Tel.: +81-45-481-5661; fax: +81-45-491-7915; e-mail: yokozt01@kanagawa-u.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02333-9

1722
L. Niimi et al. / Tetrahedron Letters 42 (2001) 1721–1724
OMe
SiMe3
Lewis acid
CH2Cl2
+
R
Ph
OMe
4a
3
OMe
Cl
OMe
OMe
SiMe3
•
Ph
+
+
+
Ph
Ph
Ph
R
6
7
8
9
Table 1.
Reaction of 4a with 3
Entry
R in 3
Lewis acid (mol%)
Temp. (°C)
Time (h)
Yield (%)c
6
7
8
9
1a
2b
3b
4a
H:3a
3a
3a
TrClO4 (5)
TMSOTf (50)
TiCl4 (50)
−78
−78
−78
−50
4.0
7.5
1.0
2.0
2
0
0
0
3
0
0
0
1
0
0
3
2
0
14
Me:3b
TrClO4 (5)
100d
a The reaction was carried out with 5 mol% of TrClO4 in CH2Cl2 ([4a]0=[3]0=0.5 M).
b The reaction was carried out with 50 mol% of Lewis acid in CH2Cl2 ([4a]0=[3a]0=0.67 M).
c Isolated yield.
d Determined by 1H NMR.
the best of our knowledge, it has not been hitherto
reported that an allenyl group is directly introduced to
a polymer by allenyl reagents. One might think that
propargyl metallic reagents would serve as the allenyl
reagents. However, the reactions of propargylmagne-
sium, -zinc, and -aluminum reagents with electrophiles
mainly afford acetylenic derivatives.5a,b On the other
hand, the reactions of propargylsilanes in the presence
of Lewis acid with electrophiles such as acetals,5a,c–e
dialkyl alkylidenemalonates,5b aldehydes, ketones,5f–h
and a,b-unsaturated acyl cyanides5i yield allenic deriva-
tives selectively. Furthermore, trichloropropargylsilane
also reacts with aldehydes in the presence of a catalytic
amount of cuprous chloride to yield a-allenyl alco-
hols.5j,k However, the three-component reaction of
propargyl silanes, aldehydes, and alkoxysilanes, where
a-allenyl ethers would be obtained in one pot, has not
been reported. Furthermore, this reaction would be
applicable to polycondensation that yields polyethers
having the allenyl side chains. In this paper, we report
the Lewis acid-catalyzed three-component condensation
reaction of aldehydes 1, alkoxytrimethylsilanes 2, and
propargylsilanes 3, and its application to polycondensa-
tion using bifunctional aldehydes and alkoxysilanes
(Schemes 1 and 2).
When 50 mol% of TiCl4 was used, not only the
reported4a a-allenyl ether 6a and chloroprene derivative
7 but also a-propargyl ethers 8 and 9 were obtained
(entry 3). Surprisingly, the reaction of 3b with 4a
proceeded smoothly with 5 mol% of TrClO4 to yield 6b
in quantitative yield (entry 4). It should be noted that
the reaction of methyl-substituted 3b with 4a was
totally different from that of unsubstituted 3a with 4a,
and the former reaction gave selectively a-allenyl ether
6b.
Next we carried out the three-component condensation
of benzaldehyde 1a, methoxytrimethylsilane 2a, and 3b
in the presence of 5 mol% of TrClO4 or TMSOTf
(Table 2). Both reactions gave 6b in good yields; both
catalysts were effective for this reaction. The fact that
no a-allenyl alcohol was produced as a result of a direct
attack of 3b on 1a shows that the reaction of 3b with an
intermediate acetal, which is generated by 1a and 2a,
proceeds selectively even in the presence of 1a in the
reaction mixture.
With the optimized conditions in hand, we carried out
the reactions of a variety of 1, 2, and 3b (Table 3).
Thus, a round-bottomed flask, equipped with a three-
way stopcock, was charged with TrClO4 (0.025 mmol)
and purged with argon. Dry dichloromethane (0.5 mL)
was added to the flask, followed by cooling the solution
We first studied the reaction of benzaldehyde dimethyl-
acetal 4a with propargylsilanes, such as 3-trimethylsilyl-
1-propyne 3a6a and 1-trimethylsilyl-2-butyne 3b,6b
because acetals were found to be an intermediate in the
Lewis acid-catalyzed reactions of aldehydes, alkoxysil-
anes, and silyl nucleophiles.1 The results are shown in
Table 1. When 3a was used, the reaction only partly
proceeded with 5 mol% of triphenylmethyl perchlorate
(TrClO4)7 to give 6a in very low yield and 45% recovery
of 4a (entry 1). The use of 50 mol% of trimethylsilyl
trifluoromethanesulfonate (TMSOTf)8 resulted in com-
plete consumption of 4a, but 6a was not obtained and
a-propargyl ether 9 was afforded in low yield (entry 2).
Table 2. Three-component condensation of 1a, 2a, and
3ba
Catalyst
Time (h)
Yield of 6b (%)b
TrClO4
TMSOTf
18
24
95
97
a The reaction was carried out with 5 mol% of catalyst in CH2Cl2
([1a]0=[2a]0=[3b]0=0.5 M) at −78°C.
b Determined by GC.

L. Niimi et al. / Tetrahedron Letters 42 (2001) 1721–1724
Table 3. Three-component condensation of 1, 2, and 3ba
1723
Yield of 5 (%)b
Entry
1
Temp (˚C)
-78
Time (h)
18
1
2
95c
MeOSiMe3 : 2a
CHO : 1a
2
2a
2a
-78
24
72
Cl
CHO
3
4
-78
-78
24
24
0
MeO
CHO
1a
86
OSiMe3
OSiMe3
5
1a
-78
24
80
6
7
2a
-78 to 25
-78 to 25
30
30
0
2
CHO
2a
2a
2a
CHO
O
8
9
-78 to -20
-78
48
72
0
9
O
a The reaction was carried out with 5 mol% of TrClO4 in CH2Cl2 ([1]0=[2]0=[3b]0=0.5 M). b Isolated yield. c Determined by GC.
to −78°C. A solution of 1 (0.5 mmol) and 2 (0.5 mmol)
in dichloromethane (0.25 mL) was added at −78°C.
In summary, the present work has demonstrated that
1-trimethylsilyl-2-butyne 3b is available for Lewis acid-
catalyzed three-component condensation reactions of
a-allenyl ethers. Furthermore, the three-component
polycondensation of dialdehydes, bifunctional silyl
ethers, and 3b yielded a functional polyether with the
allenyl side chains.
After
5 min, a solution of 3b (0.5 mmol) in
dichloromethane (0.25 mL) was added. The solution
was stirred at the indicated temperature in Table 3. The
reaction mixture was quenched with a few drops of
ammoniacal methanol. The solvent was evaporated and
the residue was purified by column chromatography on
silica gel to afford the corresponding a-allenyl ethers 5.
Unsubstituted 1a and an aromatic aldehyde having an
electron-withdrawing group reacted with 2a and 3b to
yield the corresponding 5 in high yields (entries 1 and
2). However, an aromatic aldehyde having an electron-
donating group did not afford 5 in this reaction (entry
3). Although the exact structure of the obtained com-
pound has not been revealed, the desired 5 was
assumed to react with one more equivalent of highly
reactive anisaldehyde on the basis of the 1H NMR
spectrum showing the two kinds of signals of the
methoxyphenyl groups and the absence of the signal of
the allenyl group. Regarding 2, not only primary 2 but
also secondary 2 resulted in 5 in good yields (entries 1,
4, and 5). Compared to aromatic aldehydes, aliphatic
aldehydes and ketones gave 5 in low yields. Even at
higher temperatures and longer reaction times, the
yields of 5 were not improved (entries 6–9).
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This reaction was also applied to polycondensation.
Thus,
isophthalaldehyde
1b,
decamethylene
bis(trimethylsilyl) ether 2b, and 3b were reacted with 10
mol% of TrClO4 in CH2Cl2 at −50°C for 24 h to yield
a polyether having the allenyl side chains with the
number-averaged molecular weight (Mn) of 6600
(Scheme 2).
.

1724
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.
.