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

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660 NITRONATES<br />

R = OSiMe2Bu-t<br />

nonequivalent<br />

Me Me<br />

R = OSiMe2Bu-t<br />

nonequivalent<br />

Me Me Me Me<br />

Si O<br />

N<br />

O Si<br />

t-Bu<br />

Si N Si<br />

Bu-t<br />

t-Bu<br />

O O Bu-t<br />

B<br />

N<br />

R = Me, OSiMe 3<br />

N<br />

R R<br />

Equivalent<br />

Fast rotation<br />

slow <strong>in</strong>version<br />

R N<br />

R<br />

R<br />

R<br />

R = OSiMe3<br />

N<br />

Me 3SiO OSiMe 3<br />

G*<br />

nonequivalent<br />

G* = Object conta<strong>in</strong><strong>in</strong>g<br />

asymmetric center<br />

(1)<br />

(2)<br />

R<br />

N<br />

R<br />

R<br />

A<br />

N R<br />

R<br />

Me<br />

N R<br />

Me<br />

R<br />

R<br />

R<br />

N<br />

B C<br />

N<br />

R<br />

R<br />

B A,C B<br />

R = Me, OSiMe 3<br />

R<br />

N R<br />

Equivalent<br />

B<br />

Fast <strong>in</strong>version<br />

slow rotation<br />

N R<br />

R = Me, OSiMe3, OSiMe2Bu-t<br />

R<br />

N<br />

R<br />

A, C<br />

Fig. 3.3 Conformation analysis of BENA stereodynamics<br />

nonequivalent<br />

Evidently, <strong>in</strong> the crystall<strong>in</strong>e state, BENA can adopt one of two conformations<br />

(A or B) presented <strong>in</strong> Fig. 3.3. Nevertheless, the nitrogen atom is characterized<br />

by a high degree of pyramidalization <strong>in</strong> both conformations, <strong>and</strong> the C–N<br />

bond is substantially elongated compared to similar bond <strong>in</strong> “classical” enam<strong>in</strong>es<br />

(see, e.g., (417)). It should also be noted that the C,C double bond <strong>in</strong> BENA is<br />

substantially shortened compared to the analogous bond <strong>in</strong> st<strong>and</strong>ard enam<strong>in</strong>es.<br />

Therefore, X-ray diffraction data provide unambiguous evidence that the structural<br />

characteristics of BENA are substantially more similar to those of nitroso<br />

acetals (418) (Table 3.26) than to enam<strong>in</strong>es <strong>and</strong>, consequently, the dynamic process<br />

observed by NMR spectroscopy is most likely a slow nitrogen <strong>in</strong>version.<br />

Low efÞciency of the n→π conjugation <strong>in</strong> BENA is conÞrmed by a strong<br />

hypsochromic shift of the absorption b<strong>and</strong> <strong>in</strong> the UV spectrum of nitroso acetal<br />

(416b) compared to product (417) (for (416b), λmax = 324 nm (logε = 4.1); for<br />

(417), <strong>and</strong> λmax = 404 nm (logε = 4.0)) (467).<br />

Quantum chemical calculations by the DFT PBE/TZP method for model<br />

nitroso acetals <strong>and</strong> related compounds (467) are also <strong>in</strong>dicative of nitrogen <strong>in</strong>version<br />

as the slowest dynamic process <strong>in</strong> the ene nitroso acetals studied (see Fig.<br />

3.4, Table 3.27).

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