Nitronates structural properties

Relative contribution of each of these structures differs significantly and is determined by internal structural characteristics of the nitrones and by the influence of external factors, such as changes in polarity of solvent, formation of a hydrogen bond, and complexation and protonation. Changes in the electronic stmcture of nitrones, effected by any of these factors, which are manifested in the changes of physicochemical properties and spectral characteristics, can be explained, qualitatively, by analyzing the relative contribution of A-G structures. On the basis of a vector analysis of dipole moments of two series of nitrones (355), a quantum-chemical computation of ab initio molecular orbitals of the model nitrone CH2=N(H)0 and its tautomers, and methyl derivatives (356), it has been established that the bond in nitrones between C and N atoms is almost... 

The electron-donor N -oxide oxygen atom of a nitrone makes it suitable for com-plexation and protonation. Such properties of nitrones have been widely used to influence their reactivity, using Lewis acids and protonation in nucleophilic addition reactions (see Section 2.6.6). In this chapter, the chemistry of nitrones with various metal ions [Zn (II), Cu(II), Mn (II), Ni (II), Fe (II), Fe (III), Ru (II), Os (II), Rh (I), UO2 2 ] (375, 382, 442-445), and diarylboron chelates is described (234—237, 446). Accurate descriptions of the structures of all complexes have been established by X-ray analysis. 

Structure B is of most interest. It is responsible for the activity of nitronates as 1,3-dipoles in [3+ 2]-cycloaddition reactions. This is the most important aspect of the reactivity of nitronates determining the significance of these compounds in organic synthesis (see e.g., Ref. 267). In addition, this structure suggests that nitronates can show both, O -nucleophilic properties, that is, react at the oxygen atom with electrophiles, and a-C-electrophilic properties, that is, add nucleophiles at the a-carbon atom. 

The resonance structures of nitronates are most similar to those of nitrones, but nitronates have the additional structure D. Strange as it may seem, the contribution of this structure more likely slightly diminishes a-C-electrophilic activity of nitronates, move than is favorable for the appearance of the nucleophilic properties. In any case, no transformations, in which nitronates unambiguously act as C-nucleophiles, have been rigorously established. 

This chapter is divided into four major sections. The first (Section 2.1) will deal with the structure of both alkoxy and silyl nitronates. Specifically, this section will include physical, structural, and spectroscopic properties of nitronates. The next section (Section 2.2) describes the mechanistic aspects of the dipolar cycloaddition including both experimental and theoretical investigations. Also discussed in this section are the regio- and stereochemical features of the process. Finally, the remaining sections will cover the preparation, reaction, and subsequent functionalization of silyl nitronates (Section 2.3) and alkyl nitronates (Section 2.4), respectively. This will include discussion of facial selectivity in the case of chiral nitronates and the application of this process to combinatorial and natural product synthesis. 

The structure of nitrones is hidden in that of the oxaziridines (Figure 4)57. For both compounds the preparations and properties have been reviewed58-63. The main reactions... 

The insertion of two functional groups OH and >C=N—>0 into structure of one molecule has been carried out in the work [76]. As a spin trap, -(3,5-di- -butyl-4-hydroxyphenyl)N-t-butylnitrone was used. This bifunctional trap forms phenoxyl radicals in reaction with radicals having pronounced oxidising properties (RO, R C=0, PhCOO ) and ketones in triplet-exited states. The radicals with an unpaired electron on carbon atoms add to the P-carbon of the nitrone-forming AR ... 

New Cyclic Nitrone Spin Traps. - This area has seen the most explosive growth over the past few years and the structures of the majority of these new traps are shown in Scheme 2, together with the most widely employed abbreviations for these species. This builds on a previous compilation. Details on some of the characteristics of these new spin traps, their methods of synthesis and some key properties are given in Table 1. 

---