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

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664 NITRONATES<br />

Table 3.28 The barriers of nitrogen <strong>in</strong>version of BENA <strong>and</strong> related compounds<br />

determ<strong>in</strong>ed with dynamic NMR<br />

X<br />

N R 2<br />

R 1<br />

Nitrogen <strong>in</strong>version<br />

X<br />

N<br />

R2 R1<br />

Entry X R 1 <strong>and</strong> R 2 ΔG �= 298, kJ/mol<br />

1<br />

2<br />

Ph<br />

Ph O<br />

Ph<br />

(427)<br />

MeO2C CO2Me (428)<br />

OSiMe3<br />

OSiMe3<br />

> 80 a<br />

> 80 a<br />

3 C6H5 (429) OSiMe2Bu-t 52 (471)<br />

4 CH2=CH ((423)) OSiMe2Bu-t 49 b<br />

5 MeCH=CH OSiMe2Bu-t 63<br />

6 CH2=C(Me) OSiMe2Bu-t 56<br />

7 MeO2CCH=CH OSiMe3 – c<br />

8 CH2=C(CO2Et) OSiMe2Bu-t 58<br />

9 MeO2CCH=C(Me) OSiMe2Bu-t – d<br />

10 CH(OsiMe3)Pr-i OSiMe3 59<br />

11 CH(OsiMe3)Pr-i OSiMe2Bu-t 56<br />

12 EtO2CCH(Me)CH=CH OSiMe3 59<br />

13 EtO2CCH(Me)CH=CH OSiMe2Bu-t 52<br />

14 4-NO2C6H4CH=CH OSiMe2Bu-t 37<br />

15 4-NO2C6H4CH=CH (424) Me 45<br />

a Tentative data.<br />

b Calculated for (423) ΔG �= 298 of nitrogen <strong>in</strong>version 52 kJ/mol.<br />

c No broaden<strong>in</strong>g of signals of 1 H<strong>and</strong> 29 Si over 210 K.<br />

d No broaden<strong>in</strong>g of signals of 1 H, 13 C, <strong>and</strong> 29 Si over 170 K.<br />

h<strong>in</strong>drances <strong>in</strong> the trialkylsilyl group have only a slight effect on the nitrogen<br />

<strong>in</strong>version barrier <strong>in</strong> BENAs (cf. entries 10 <strong>and</strong> 11 or entries 12 <strong>and</strong> 13, Table<br />

3.28), <strong>and</strong> their <strong>in</strong>ßuence is ambiguous. In most cases, not only the barriers but<br />

also the activation parameters were determ<strong>in</strong>ed for dynamic processes <strong>in</strong> BENAs<br />

by dynamic NMR spectroscopy (467). As a rule, ΔS �= of nitrogen <strong>in</strong>version is<br />

positive.<br />

The stereodynamics of N-siloxy-ene- nitroso acetals related to 3-alkylensubstituted<br />

4H -tetrahydro-1,2-oxaz<strong>in</strong>es (e.g., see (416 d)) differs from that of<br />

BENA <strong>in</strong> that free rotation about the C,N bond <strong>in</strong> oxaz<strong>in</strong>es cannot occur, but the<br />

r<strong>in</strong>g <strong>in</strong>version should take place (process IR) (see Scheme 3.228).<br />

Analysis of the published data on the stereodynamics of cyclic nitrogen compounds<br />

(510) as well as the results of studies of the stereodynamics of BENA

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