Protonation of the heteroatom

DFT studies on heteroatom diallenes have found, in line with previous experimental data, that biradical cyclization to a five-membered heteroaromatic ring is the preferred reaction pathway, although protonation of the heteroatom has been found to promote a competing cyclization to a six-membered, initially biradical, ring.59... 

The reduction of an a, i3-unsaturated nitro compound [29], such as /3-nitrostyrene (V), occurs in two steps. The first reduction of (V) is a four-electron reaction that yields phenylacetaldoxime (VI) the reaction has been formulated as a reduction of the nitro group followed by a rearrangement of the unsaturated hydroxylamine (VII), but might also be regarded as a 1,4-reduction of the intermediate unsaturated nitroso compound. There is no conclusive evidence for either route, but the former has been chosen here, as the protonation of the heteroatom would be expected to be faster than protonation of the carbon atom ... 

An interesting example of the N(9)-C(8) prototropic tautomerism has been reported for the caffeine radical by pulse radiolysis studies in aqueous solution the transformation of the heteroatom-protonated electron adduct 25 into the carbon-protonated tautomer 26 occurred spontaneously in neutral media [95JCS(F)615]. 

Numerous colorless organic compounds with extended jt-electron systems can be converted to colored cations or anions with polymethyne chromophore by protonation or deprotonation. The intense coloration of the corresponding salts is usually attributable to the fact that the lone pairs of electrons of the heteroatoms participate in the mesomerism of the conjugated n-electron systems [4]. 

By contrast base-catalyzed mechanisms are generally fast, provided, of course, that one of the heteroatoms defining the tetrahedral intermediate has an ionizable proton. 

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

The pH-independence of Tdhpz at Pt indicates that the driving force for coordination of the nitrogen heteroatom to the Pt surface is much larger than that for protonation even in molar acid. This behavior is in contrast to that of pyridine, where protonation of the nitrogen heteroatom in molar acid hinders N-coordination to the surface (H). Such a difference in chemisorption characteristics may be related to the fact that the basicity of the nitrogen heteroatom in pyridine (pKb =8.8) is much greater than that of the nitrogens in pyridazine (pKb = 11.8) (23.). ... 

More generally, double bonds between two carbons or one carbon and a heteroatom, possibly conjugated with other unsaturated moieties in the molecule, are eligible for two-electron/two-proton reactions according to Scheme 2.20. Carbonyl compounds are typical examples of such two-electron/two-proton hydrogenation reactions. In the case of quinones, the reaction that converts the quinone into the corresponding hydroquinone is reversible. With other carbonyl compounds, the protonation of the initial ketyl anion radical compete with its dimerization, as discussed later. 

The Bronsted correlation for five-membered rings shows that effects of structure on reactivity and on acidity are related. Variations in rate constants for quaternization and in pKa values (Table III) are understandable in terms of resonance and inductive effects of the heteroatom X.120 The effects on the energy of a transition state leading to quaternized product are similar but smaller than those on the energy of protonated material. The following considers in more detail the influence of benzo-fusion. 

Apart from pyrrole, the chemical shifts for the [3-protons increase with decreasing electronegativity of the heteroatom. In contrast, the chemical shifts of the a-protons do not display any obvious regularity, probably due to paramagnetic shielding contributions which will become more important with increasing availability of 7-orbitals. The chemical shift of the pyrrole N-H is solvent dependent. 

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