Lewis acid-catalyzed three-component condensation reactions of aldehydes, N-silylcarbamates, and allylsilane: Synthesis of N-homoallylcarbamates

Article abstract of DOI:10.1016/S0040-4039(00)01214-4

For simultaneous construction of the polyurethane backbone and the allyl side chains, Lewis acid-catalyzed three-component condensation reactions of carbonyl compounds, N-trimethylsilylcarbamates, and allyl-trimethylsilane are studied. The reaction of these three compounds took place in the presence of a catalytic amount of TrClO₄ at 0°C to yield the corresponding N-homoallylcarbamates in good yields. This reaction was also applied to the synthesis of a polyurethane having the allyl side chains. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01214-4

Tetrahedron Letters 41 (2000) 7075±7078
Lewis acid-catalyzed three-component condensation reactions
of aldehydes, N-silylcarbamates, and allylsilane: synthesis of
N-homoallylcarbamates
Lui Niimi, Ken-ichi Serita, Shuichi Hiraoka and Tsutomu Yokozawa*
Department of Applied Chemistry, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
Received 14 June 2000; accepted 14 July 2000
Abstract
For simultaneous construction of the polyurethane backbone and the allyl side chains, Lewis acid-catalyzed
three-component condensation reactions of carbonyl compounds, N-trimethylsilylcarbamates, and allyl-
trimethylsilane are studied. The reaction of these three compounds took place in the presence of a catalytic
ꢀ
amount of TrClO at 0 C to yield the corresponding N-homoallylcarbamates in good yields. This reaction
4
was also applied to the synthesis of a polyurethane having the allyl side chains. # 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: N-homoallylcarbamate; Lewis acid; allylsilane; polyurethane.
Three-component condensation reactions are interesting and important, not only because two
bonds are formed in one pot, but also because the methodology would be useful for making a
broad variety of compound libraries. It has been reported that the three-component condensation
1
a
reactions of aldehydes, alkoxysilanes and silyl nucleophiles such as trialkylsilanes,
allyltrimethylsilane,1 cyanotrimethylsilane, and silyl enol ethers take place with a catalytic
amount of a Lewis acid to yield ethers, homoallyl ethers, a-alkoxynitriles, and b-alkoxyketones,
respectively. We have applied these reactions to polycondensations, which simultaneously
b,c
1d
1e,f
2a
1d
construct both the polyether backbone and functional side chains containing the allyl, cyano,
e,2b
and keto groups.1
However, it would be dicult to extend the Lewis acid-catalyzed three-
component condensation to the synthesis of amine derivatives by using N-silyl nucleophiles
instead of alkoxysilanes, because the strong anity of Lewis acids for the amino groups does not
regenerate the Lewis acid in the reaction. In general, the synthesis of amine derivatives from three
compounds is attained by the use of a stoichiometric amount of a Lewis acid in the three-
3
component condensation or by the reaction of organometallic nucleophiles with imines and
4
acylimines prepared from aldehydes and amino derivatives in advance. To the best of our
knowledge, there are a few reports of the catalytic systems of three-component condensations for
*
Corresponding author. Tel: +81-45-481-5661; fax: +81-45-491-7915; e-mail: yokozawa@cc.kanagawa-u.ac.jp
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01214-4

7
076
5
amine derivatives, which need a particular Lewis acid stable enough in water or a Brùnsted acid±
6
surfactant±combined catalyst in water. We have found that N-silyl nucleophiles having an
alkoxycarbonyl group adjacent to the amino group undergo three-component condensation
reactions by a catalytic amount of ordinary Lewis acids. In this paper, we report the condensation
reactions of aldehydes 1, N-silylcarbamates 2, and allylsilane 3 by a catalytic amount of
triphenylmethyl perchlorate (TrClO ), which yield N-homoallylcarbamates 4 in a one-pot procedure.
4
Furthermore, the reaction mechanism and the application to polymer synthesis are also described.
In order to establish the optimal conditions for the reactions of 1, 2, and 3, we ®rst studied the
reaction of benzaldehyde 1a, ethyl N-trimethylsilylcarbamate 2a, and 3 by various Lewis acids.
The results are shown in Table 1. The reaction was found to proceed with a catalytic amount of
Me SiOTf and TrClO . In particular, TrClO a€orded N-homoallylcarbamate 4 in the highest
3
4
4
ꢀ
yield at 0 C. However, ZnCl was not e€ective for this reaction even when using an excess amount.
2
Table 1
Three-component condensation of 1a, 2a, and 3a
With the optimized conditions in hand, we next carried out the reactions of a variety of 1, 2,
and 3. Thus, a round-bottomed ¯ask, equipped with a three-way stopcock, was charged with
TrClO (0.017 g, 0.05 mmol) and purged with argon. Dry dichloromethane (1.0 ml) was added to
4
ꢀ
the ¯ask, followed by cooling the solution to 0 C. A solution of 1 (1.0 mmol) and 2 (1.0 mmol) in
ꢀ
dichloromethane (0.5 ml) was added at 0 C. After 5 min, a solution of 3 (1.0 mmol) in
ꢀ
dichloromethane (0.5 ml) was added. The solution was stirred at 0 C for 24 h. The reaction
mixture was quenched with a few drops of ammoniacal methanol. The solvent was evaporated
and the residue was puri®ed by column chromatography on silica gel to a€ord the corresponding
N-homoallylcarbamates 4. The results are summarized in Table 2. Aromatic and aliphatic
aldehydes reacted with 2a and 3 to yield the corresponding 4 in good yields (entries 1 and 3±7).
The electron-donating and electron-withdrawing groups on the benzene ring of aromatic
aldehydes did not a€ect the yields of 4 (entries 3 and 4). Ketones resulted in low yield of 4 for
the same reaction time (24 h), but the yield increased up to 97% in 96 h (entry 9 versus 10).
N-Alkylated 2b a€orded 4 in poor yield (entry 2).

7
077
Table 2
Three-component condensation of various 1, 2, and 3a
To discuss the reaction mechanism, the reaction of 1a with 2a was carried out in the presence of
mol% of TrClO in CDCl at ambient temperature, and then the reaction mixture was directly
5
4
3
1
observed by H NMR. The spectrum showed the signal of the aldehyde proton of 1a at ꢀ 10.00
and a broad signal of the benzylic proton at ꢀ 6.45±6.11, with a signal intensity ratio of 1:1. At
this point, the reaction was quenched to a€ord 1a and bisurethane 6,3a the benzylic proton signal
of which appeared at ꢀ 6.23 in CDCl . However, when a catalytic amount of TrClO was added
3
4
to a solution of 6 in CDCl , the benzylic proton signal of 6 was changed to a broad signal, which
3
was identical with the broad signal of the above reaction mixture. Accordingly, the broad signal
at ꢀ 6.45±6.11 would be assigned to a fast equilibrium mixture of 6 and cationic intermediate 7
generated by the coordination of TrClO to 6. On the basis of the above results and the reported
4
mechanism of a similar reaction using 1 equiv. of Lewis acid,3a a proposed mechanism is shown
in Scheme 1. A half-amount of 1a reacts with all of 2a to yield 6; 1a and 6 are in the reaction
mixture in the ratio 1:1. When 3 is added to this reaction mixture, 3 reacts with 7, which is
generated by the activation of 6 with TrClO , to yield 4 and 2a. Regenerated 2a reacts with 1a to
4
supply 6. The fact that no homoallyl alcohols were produced as a result of a direct attack of 3 on
a shows that the reaction of 3 with 6 proceeds selectively even in the presence of 1a in the
1
reaction mixture. It should also be noted that hexamethyldisiloxane as a byproduct in this
condensation reaction does not quench the Lewis acid catalyst, and this enables the reaction to
proceed in a catalytic fashion.

7
078
Scheme 1.
This reaction was also applied to polycondensation. Thus, bis(4-formylphenyl) succinate 1c,
0
octamethylene N,N -bis(trimethylsilyl)carbamate 2c, and 3 were reacted with 10 mol% of TrClO
ꢀ
4
in CH Cl at 0 C for 24 h to yield a polyurethane having the allyl side chains with a number-
2
2
averaged molecular weight (Mn) of 12,000.
In summary, the present work has demonstrated that N-trimethylsilylcarbamates are available
for Lewis acid-catalyzed three-component condensation reactions for N-homoallylcarbamates.
0
Furthermore, the three-component polycondensation using bifunctional N,N -bistrimethyl-
silylcarbamate yielded a functional polyurethane with allyl side chains.
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