Nitrogen atomic structure

Isocyanates are derivatives of isocyanic acid, HN=C=0, in which alkyl or aryl groups, as well as a host of other substrates, are direcdy linked to the NCO moiety via the nitrogen atom. Structurally, isocyanates (imides of carbonic acid) are isomeric to cyanates, ROC N (nitriles of carbonic acid), and nitrile oxides, RO Nf—>0 (derivatives of carboxylic acid). 

Apart from the various tautomeric forms, there are four kinds of parent thiadiazine structures, having molecular formula C3H4N2S, in which the sulfur atom is adjacent to at least one ring nitrogen atom (structures I-IV). In order to name a particular tautomer, the extra hydrogen requires designation. Thus, using Chemical Abstracts nomenclature, structure I is named as 6//-1,2,3-thiadiazine, and structures V, VI and VII are named as... 

Compound I must be activated by the strongly acidic medium. One possible protonation site is the basic pyrrolidinium nitrogen atom (structure X), which would favor the formation of carbenium ion XI (see Scheme 16.3). This intermediate would gain additional stabilization from the polar solvent. Conditions for an SnI substitution would prevail to yield XII, whose fragmentation would resemble, formally speaking, the retro cycloaddition process used in the synthesis of I (i.e., VIII - I). The resulting ketone (XIII) would contain the elements required for the construction of a five-membered carbocycle with the desired incorporation of the functional carbon of one of the nitrile groups, as... 

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

Figure 2-52. a) Two semipolar resonance structures are needed in a correct VB representation of the nitro group, b) Representation of a nitro group by a structure having a pentavalent nitrogen atom, c) The RAMSES notation of a nitro group needs no charged resonance structures. One jr-system contains four electrons on three atoms. 

In spite of the diverse nature of alkaloid structures, two structural units, i.e. fused pyrrolidine and piperidine rings in different oxidation states, appear as rather common denominators. We therefore chose to give several examples for four types of synthetic reactions which have frequently been used in alkaloid total synthesis and which provide generally useful routes to polycyclic compounds with five- or six-membered rings containing one nitrogen atom. These are ... 

When the nitrogen atom is substituted by a nitrophenacyl group, OH attack gives the betainic zwitterion (Scheme 13). which is soluble in organic solvents (32). The stability of the C-betainic or ylid structure has been explained as an effect of resonance of the negative charge in the molecule (33, 34). 

In 1877, Maly (45) discussing formula 34 applied to thiohydantoine found it unable to explain the basic properties of the compound. He preferred a structure in which the -CH2-CO- group would be bonded to only one nitrogen atom. Meyer (46) prepared a monophenyl thiohydantoin (m.p. 178°C) by condensing diloroacetanilide with thiourea and proposed 42 for its structure. 

The bonding in the mononuclear metal complexes is considered to involve the two nitrogen atoms of the ligand structure, giving the familiar... 

Figure 8.38 Curve fitting of Mo extended X-ray absorption fine structure (EXAFS) for Mo(SC6H4NH)3, taking into account (a) sulphur and (b) sulphur and nitrogen atoms as near neighbours. (Reproduced, with permission, trom Winnick, H. and Doniach, S. (Eds), Synchrotron Radiation Research, p. 436, Plenum, New York, 1980)...

The effect of forming a more rigid structure in fluorescent dyes of the rhodamine series has been clearly demonstrated (18) with the remarkable dye designated Rhodamine 101 [41175A3-3] (19). This dye has its terminal nitrogen atoms each held in two rings and has a fluorescence quantum yield of virtually 100% independent of the temperature. 

Although all the rings in Figure 1 contain six tt-electrons, the accumulation of electronegative nitrogen atoms in the polyaza structures leads to hydrolytic as well as thermal instability. This is noticeable in pyrimidine, and marked in the triazines and tetrazine. Some stability can be conferred by appropriate substitution, as we shall outline later. 

Since the electrophilic reagent attacks the multiply-bonded nitrogen atom, as shown for (68) and (69), the orientation of the reaction product is related to the tautomeric structure of the starting material. However, any conclusion regarding tautomeric equilibria from chemical reactivity can be misleading since a minor component can react preferentially and then be continually replenished by isomerization of the major component. 

Mesomeric shifts of the types shown in structures (82) and (83) increase the electron density on the nitrogen atom and facilitate reaction with electrophilic reagents. However, the heteroatom Z also has an adverse inductive effect the pK, of NH2OH is 6.0 and that of N2H4 is 8.0, both considerably lower than that of NH3 which is 9.5. 

A multiply bonded nitrogen atom deactivates carbon atoms a or y to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (110) and (111). Pyrazoles (110 Z = NH), isoxazoles (110 Z = 0), isothiazoles (110 Z = S), imidazoles (111 Z = NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (111 Z = S) do indeed undergo electrophilic substitution as expected. Little is known of the electrophilic substitution reactions of oxazoles (111 Z = O) and compounds containing three or more heteroatoms in one ring. Deactivation of the 4-position in 1,3-dihetero compounds (111) is less effective because of considerable double bond fixation (cf. Sections 4.01.3.2.1 and 4.02.3.1.7), and if the 5-position of imidazoles or thiazoles is blocked, substitution can occur in the 4-position (112). 

This method of ring closure is the most convenient for the synthesis of ring-fused 1,2,4-triazoles with this arrangement of nitrogen atoms. It has been used extensively to form the analogous [5,6] ring-fused systems, and structural ambiguity is only encountered... 

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