Protons addition

Table 1 Values for Proton Addition <63PMHiiii.7iPMHi3n.B-76Mi4fl2i)o.B-76Mi4n2i)i)...

Cationic rings are readily reduced by complex hydrides under relatively mild conditions. Thus isoxazolium salts with sodium borohydride give the 2,5-dihydro derivatives (217) in ethanol, but yield the 2,3-dihydro compound (218) in MeCN/H20 (74CPB70). Pyrazolyl anions are reduced by borohydride to pyrazolines and pyrazolidines. Thiazolyl ions are reduced to 1,2-dihydrothiazoles by lithium aluminum hydride and to tetrahydrothiazoles by sodium borohydride. The tetrahydro compound is probably formed via (219), which results from proton addition to the dihydro derivative (220) containing an enamine function. 1,3-Dithiolylium salts easily add hydride ion from sodium borohydride (Scheme 20) (80AHC(27)151). 

When a Br nsted plot includes acids or bases with different numbers of acidic or basic sites, statistical corrections are sometimes applied in effect, the rate and equilibrium constants are corrected to a per functional group basis. If an acid has p equivalent dissociable protons and its conjugate base has q equivalent sites for proton addition, the statistically corrected forms of the Br insted relationships are... 

FIGURE 19.6 The phosphoglncoisomerase mechanism involves opening of the pyranose ring (Step A), proton abstraction leading to enediol formation (Step B), and proton addition to the double bond, followed by ring closure (Step C). 

The mono- and some di-aminoacridines, 6- and 7-aminoquin-oline, and 3-aminopyridine form monocations in which protonation occurs preferentially at the ring nitrogen atom. However, ultraviolet spectral data show that 1,9-diaminoacridine (237, R = NH2) undergoes proton addition on the amino groups in preference to the cyclic nitrogen atom, whereas l-amino-9-methylacridine (237, R = Me) is monoprotonated principally at the amino group in ethanol... 

A solution of nitrous acid in sulfuric acid exists as the equilibrium indicated in Scheme 2-15, as was shown by Seel and Winkler (1960), and by Bayliss et al. (1963). These authors determined the equilibrium concentration of [HN02] and [NO+] in various acid concentrations spectrophotometrically. They found that in aqueous sulfuric acid containing more than about 57% w/w H2S04 the equilibrium of Scheme 2-15 is predominantly on the right-hand side. This equilibrium and the problem of the potential intermediate nitrosoacidium ion (H20-N0), which is the proton addition product of HN02, will be discussed in Section 3.2. 

The carbonyl group also possesses electrophilic properties at the carbon atom and nucleophilic properties at the oxygen atom. Nucleophilic attack of the carbonyl group is favored if this is attached to an aromatic ring (inductive effect) and there is also a methoxy or phenolic OH group present in the 4-position. Changing a neutral reaction medium by proton addition has the same effect. 

It is probable that the negative charge induced by these three electrons on FeMoco is compensated by protonation to form metal hydrides. In model hydride complexes two hydride ions can readily form an 17-bonded H2 molecule that becomes labilized on addition of the third proton and can then dissociate, leaving a site at which N2 can bind (104). This biomimetic chemistry satisfyingly rationalizes the observed obligatory evolution of one H2 molecule for every N2 molecule reduced by the enzyme, and also the observation that H2 is a competitive inhibitor of N2 reduction by the enzyme. The bound N2 molecule could then be further reduced by a further series of electron and proton additions as shown in Fig. 9. The chemistry of such transformations has been extensively studied with model complexes (15, 105). 

The P-N bond in phosphinous amides is essentially a single bond, so the lone pairs on N and P are available for electrophiUc reagents and for donor bonding towards metal atoms. Proton addition to the N atom of HjPNHj has been calculated to loosen the P-N bond, whereas protonation at P renders this bond stronger than in the parent molecule [26]. NH-Phosphinous amides are practically not associated by intermolecular hydrogen bonds [27]. 

To summarise, the electrochemistry of polyaniline shows a complex pattern of behaviour with evidence of both proton addition/elimination and anion inser-... 

By their very nature, their partial charge separation can make them fairly insoluble and the degree of this charge separation (and hence resultant NMR spectra) tends to be highly dependant on concentration and pH. For these reasons, we recommend dealing with such compounds by pushing them one way or the other, i.e., by adjusting the pH of your NMR solution so that the compound in question is either fully protonated (addition of a drop of DC1) or de-protonated (addition of a drop of saturated sodium carbonate in D20) ... 

The Metal. Proton addition to the metal rather than to the carbyne carbon has been observed, e.g.,... 

Because most species will release energy when a proton is added, the proton addition enthalpy is negative. The proton affinity is the heat associated with the reverse process, the removal of the proton,... 

No attempts at optimization have been made. Note that this reaction differs from the one in which cyclohexenone or maleic esters are acceptors in that the chirality-producing step involves proton addition to the a-carbon of the ester. This latter step is probably not rate determining. 

D. The Electron Excitation Spectra of Proton Addition Complexes. 222... 

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