Silyl nitronates bonding

A series of 3-substituted-2-isoxazoles are prepared by the following simple procedure in situ conversion of nitroalkane to the silyl nitronate is followed by 1,3-dipolar cycloaddition to produce the adduct, which undergoes thermal elimination during distillation to furnish the isoxazole (Eq. 8.74). 5 Isoxazoles are useful synthetic intermediates (discussed in the chapter on nitrile oxides Section 8.2.2). Furthermore, the nucleophilic addition to the C=N bond leads to new heterocyclic systems. For example, the addition of diallyl zinc to 5-aryl-4,5-dihydroi-soxazole occurs with high diastereoselectivity (Eq. 8.75).126 Numerous synthetic applications of 1,3-dipolar cycloaddition of nitronates are summarized in work by Torssell and coworker.63a... 

Elimination Reactions with Silyl Nitronates Most elimination reactions of SENA involve cleavage of the weak N-0 bond or cleavage of the O-Si bond. In the latter case, the reactions could occur with the participation of hyper-valent silicon in the transition state, that requires the presence of an external nucleophile. 

The X-ray crystallographic data for an acyl nitronate (25) is compiled in Table 2.14 (56). Because the phenyl group attached to the nitronate, the bond angles about the sp hybridized nitrogen are similar to those of nitronates 9 and 10. However, the 0(1)—N bond is slightly shorter than those in either the alkyl or silyl nitronates, while the 0(2)—N is slightly longer. This is in line with the observed instability of monosubstituted acyl nitronates, which eliminate readily to the nitrile oxides. 

N—0(2) bonds are 1.274 and 1.424 A, respectively. The X-ray crystallographic data for 24 corresponds well with those of 22, however, the bond angles of 23 are drastically different from the other two alkyl nitronates. Interestingly, these bond angles agree better with those observed in the silyl nitronates, however, this may be due to the constraints imposed by the fused seven- and six-membered rings. 

Keywords Catalyst, Alkylation, Allylation, Arylation, Mannich reaction, Carbon-nitrogen double bond, Imine, Nitrone, Aldimine, Organozinc reagents, Silyl ketene acetal, Silyl enol ether, Amine, (3-Amino acid... 

Nitro compounds are oxidized at carbon bonded to nitro group 2-Fluoronitro compounds and silyl nitronates are converted to 2-fluorocarbonyl compounds by ceric ammonium salt [71] (equation 73)... 

Reactions of Silyl Nitronates with C-C Bond Formation. 

The second most important synthetic application of silyl nitronates in C-C bond-forming reactions is their fluoride-mediated addition to aldehydes. Silyl nitronates from secondary nitroalkanes lead to free nitro aldols such as (4), while those from primary nitro alkanes give silylated products. In contrast to the classical Henry reaction, the silyl variant is highly diastereose-lective with aldehydes, furnishing e yfAro-0-silylated nitro aldols (e.g. 5). It is important that the reaction temperature does not rise above 0 °C, otherwise threo/erythro equilibration takes place. The same erythro-nitio aldol derivatives are available by diastere-oselective protonation of silyloxy nitronates (eq 3) (usually the dr is >20 1), while the nonsilylated fAreo-epimers (R = H, dr = 7 3-20 1) are formed by kinetic protonation of lithioxy lithio nitronates in THF/DMPU (eq 4). Other recent modifications of the nitroaldol addition using titanium nitronates or ClSiRs in situ are less selective. It should also be mentioned that there are recent reports about the enantioselective addition of nitromethane to aldehydes in the presence of rare earth binaphthol complexes. 

The N—O bond in silyl nitronates can also be cleaved through oxidation, see Ref. [114],... 

It should be emphasized that in no case were derivatives of AN containing the C-Si bond detected, although the formation of these compounds in certain silylation reactions as kinetic products cannot be ruled out. In particular, this can be expected for nitronates (51 h,i) (Scheme 3.57) generated in the reactions of silylated dimethylformamides with conjugated nitro olefins (202), because this reaction with other Michael substrates affords products with the C-Si bond. 

First, it should be noted that the starting nitronates (342) or target nitroso acetals (343) can undergo stereoisomerization under the action of electrophiles, which can be accompanied by reversible cleavage of the endocyclic N-0 bond reactions (342=342 ) or (343=343 ). To suppress these processes, it is necessary to perform the transformation (342=343) under conditions as mild as possible and to use a silylating agent containing a small excess of a base. 

As demonstrated in a series of kinetic experiments by Wittkopp and Schreiner, nitrone N-benzylideneanihne N-oxide can be activated for 1,3-dipolar cycloadditions through double hydrogen-bonding 9 [Ij. Takemoto and co-workers, in 2003, published the nucleophilic addition of TMSCN and ketene silyl acetals to nitrones and aldehydes proceeding in the presence of thiourea organocatalyst 9 (Figure 6.4) [147]. 

Similarly, for the purpose of studying silyl radicals in the gas phase in the presence of a spin trap, it has been established that the trimethylsilyl radical added to the double bond of a nitrone to yield the a-trimethylsilylated hydroxylamine.204 Other studies have been... 

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