Nitrogen-centred

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

The concern over ecological consequences of the atmospheric input of oxidized and reduced nitrogen centres on soil acidification by the oxidized nitrogen directly, and by the reduced nitrogen following its transformation in the soil into NO/ or due to its uptake by vegetation (Figure 5). ... 

Monomeric thiazyl halides can be stabilized by coordination to transition metals and a large number of such complexes are known (Section 7.5). In addition, NSX monomers undergo several types of reactions that can be classified as follows (a) reactions involving the n-system of the N=S bond (b) reactions at the nitrogen centre (c) nucleophilic substitution reactions (d) halide abstraction, and (e) halide addition. Examples of each type of behaviour are illustrated below. 

The third class of compounds to be discussed in this chapter are those in which an RE group (E = S, Se, Te) is attached to a nitrogen centre. This category includes amines of the type (REfsN and the related radicals [(RE)2N] , as well as organochalcogen(ir) azides, REN3, and nitrenes REN (E = S, Se). Covalent azides of the type RTe(N3)3 and R2Te(N3)2, in which the chalcogen is in the +4 oxidation state, have also been characterized. 

Stable S-nitrosothiols and S-nitrososelenols have also been generated by attaching bowl-shaped aryl substituents to the nitrogen centre.193,194 The alkyl derivative (Me3Si)3CSeNO is prepared by nitrosation of (Me3Si)3CSeH with tert-butylnitrite.195... 

Imino-fe(phosphine chalcogenide)s can in theory exist in two different tautomeric states an N-H tautomer 41a and a E-H tautomer 41b. However, extensive studies in both solution and the solid state have shown that in all cases (with the exception of some dioxo E = O species) the N-H tautomer predominates.75 Structural studies have revealed the N(PE)2 group in 41a to be planar or close to planar, with P-N-P bond angles in the range 122-133°, indicative of substantial sp2 character at the nitrogen centre. Planarity of the nitrogen is also observed in the anionic dichalcogenoimidodiphosph(in)ates 42. 

Acid and base catalysis of a chemical reaction involves the assistance by acid or base of a particular proton-transfer step in the reaction. Many enzyme catalysed reactions involve proton transfer from an oxygen or nitrogen centre at some stage in the mechanism, and often the role of the enzyme is to facilitate a proton transfer by acid or base catalysis. Proton transfer at one site in the substrate assists formation and/or rupture of chemical bonds at another site in the substrate. To understand these complex processes, it is necessary to understand the individual proton-transfer steps. The fundamental theory of simple proton transfers between oxygen and nitrogen acids and... 

An attempt has been made to predict the general requirements for the operation of a non-dissociative mechanism of tautomerisation between oxygen and nitrogen centres (Bensaude et al., 1977). The proximity of the two centres between which the proton is transferred is obviously of critical importance but more experimental results are required before a clear picture will emerge. 

Data for some 90 compounds with the general structure [9] were analysed in terms of the twist-torsion angle r and the out-of-plane bending, x- which measures the extent of pyramidalization at the carbon and nitrogen centres. [Scheme 1. For full details see the definitions in the original paper (Kaftory et al., 1990).] In all cases is small (<5-6°), so pyramidalization at carbon... 

Fig. 12 Dependence of the rate constant for alkaline hydrolysis for three amides [58] (x = 1, y = 2 x = 2, y = 1 and x = y = 2), and [59] on three different geometrical parameters the twist-torsional angle r (A), and the out-of-plane bending at the amide carbonyl (O) and nitrogen ( ) centres. [The geometrical...

The above description implies that the nitrogen centre is planar, but it is clear that in many nitroxides the energy required to introduce quite substantial pyramidal distortion is small (Lajzerowicz-Bonneteau, 1976). Nevertheless, the discussion in the following pages generally assumes planar nitrogen. 

Optically pure P-ethanolamines react with dichlorocarbene under phase-transfer catalytic conditions to produce epoxides of high configurational retention [30]. Initial reaction occurs at the tertiary nitrogen centre (Scheme 7.29) with subsequent cleavage of the C-N bond. The reaction is configurationally controlled, as shown by the reaction of the conformationally rigid cyclic systems epoxide formation occurs with the equatorial hydroxyl system (50%), but not with the axial hydroxyl compound. 

Free-radical cyclization on to unsaturated CN bonds and also the cyclization of a range of nitrogen-centred radicals have continued to attract interest and have been reviewed. Aryl radicals, generated from BusSnH- or TTMSS-mediated homolytic cleavage of aryl-bromide bonds, have been shown to cyclize on to the nitrogen atom of imidate esters in the 5-exo mode (Scheme 9). Loss of an ethyl radical leads to the observed A-acylindolines. No cyclization in the 6-endo mode was detected. 

The reaction of amino acids with HOCl was smdied using EPR spin trapping and UV-vis spectroscopy. Some nitrogen-centred radicals, which then undergo a variety of abstraction, rearrangement, and fragmentation reactions, were detected (Scheme 27). 

A paper has reported that nitrogen-centred radicals (29) have been generated by one-electron reduction (at carbon or metal electrodes) of stable nitrenium ions of the general structure (28). The reactivity of the radicals is influenced by the substituents attached to the two nitrogen atoms that are directly linked to the ion nitrenium centre." ... 

Diastereoselective tandem conjugate addition of both oxygen- and nitrogen-centred nucleophiles (potassium phthalimide, TsNHK, MeONa, and MesSiOK) to the novel (IN)-lO-camphorsulfonic acid-derived nitroalkenes (139 R = Me, Pr, and... 

C-2. As with protonation, electrophiles do not react at the nitrogen centre. 

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