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Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

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SYNTHESIS OF NITRONATES 479<br />

decomposition of silylat<strong>in</strong>g agents (e.g., BSA is transformed <strong>in</strong>to a mixture of<br />

hexamethyldisiloxane <strong>and</strong> acetonitrile).<br />

To <strong>in</strong>hibit this decomposition, it is necessary to add a small amount of triethylam<strong>in</strong>e<br />

to BSA.<br />

Table 3.3 summarizes examples of the successful use of DPSU for silylation<br />

of AN.<br />

As can be seen from this table, it is advantageous to use DPSU only for<br />

silylation of α-functionalized AN.<br />

Pr<strong>in</strong>cipal results of the use of BSA <strong>and</strong> BSTFA are given <strong>in</strong> Table 3.4.<br />

The mechanism of silylation of AN with silylated amides was not systematically<br />

studied, but some data on this problem have been reported (204).<br />

It could be suggested that silylation of AN occurs through multicenter transition<br />

states A or B(Chart 3.1).<br />

However, certa<strong>in</strong> facts contradict this <strong>in</strong>terpretation. For example, the silylation<br />

of AN with BSA was accelerated after addition of triethylam<strong>in</strong>e, which<br />

facilitates AN ionization (185). Besides, the addition of 1,1-d<strong>in</strong>itroethane leads<br />

to an <strong>in</strong>crease <strong>in</strong> the rate of silylation of methyl nitroacetate with a deÞcient<br />

amount of DPSU. At the same time, the silylation of 1,1-d<strong>in</strong>itroethane by itself,<br />

taken separately with DPSU, occurs faster than silylation of MeO2CCH2NO2<br />

(204). Apparently, this is accounted for by the higher nucleophilicity of methyl<br />

nitroacetate compared to that of 1,1-d<strong>in</strong>itroethane. (A separate experiment demonstrated<br />

that silyl aci-d<strong>in</strong>itroethane does not react with methyl nitroacetate.)<br />

Table 3.3 The silylation of AN with DPSU<br />

Entry AN Conditions SENAs Yield,% Ref.<br />

1 MeNO235 ◦ C SiMe3ON=CHCH= 76 171<br />

2 HON=CHCH2NO2 35<br />

N(O)O SiMe3<br />

◦ C SiMe3ON=CHCH= > 90 171<br />

3 MeO2CCH2NO2 20<br />

N(O)O SiMe3<br />

◦ C, 15 h, MeO2CCH=N(O)O 90 203<br />

4 CH2(NO2)2<br />

C6H6<br />

20<br />

SiMe3<br />

◦ C, 0.3 h, NO2CH=N(O)O 90 203<br />

5 MeCH(NO2)2<br />

C6H6<br />

20<br />

SiMe3<br />

◦ C, 0.3 h, MeC(NO2)=N(O)O 90 204<br />

6 (MeO2C)2CHNO2<br />

C6H6<br />

20<br />

SiMe3<br />

◦ C, 0.6 h, (MeO2C)2C=N(O)O 97 175<br />

7 HC(NO2)3<br />

C6H6<br />

0<br />

SiMe3<br />

◦ C, 0.3 h, (NO2)2C=N(O)O 69<br />

176<br />

8 PhCH2NO2<br />

tolyene<br />

80<br />

SiMe3<br />

(by NMR)<br />

◦ C, 1 h, PhCH=N(O)O SiMe3 0 204<br />

C6H6<br />

9 PhCH=N(O)OH 20 ◦ C, 0.5 h,<br />

C6H6<br />

PhCH=N(O)O SiMe3 80 204

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