Hydride-accepting abilities

As expected on the basis of the higher stability of sulfonium salts as compared with oxonium salts, thiopyrylium cations are more stable and less reactive than pyrylium cations hydride-accepting ability decreases in the order pyrylium> selenopyrylium> thiopyrylium [95], 2,4,6-Triphenyl-thiopyrylium salts react with ammonia and primary alkylamines forming the corresponding pyridine and pyridinium salts, respectively, but they do not react with aniline or its derivatives [96-99], As described below, the Se- or Te-analogs are less stable than the thiopyrylium salts. 

TEMPO+ abstracted hydride anions from the hydrides of aldehydes and ketones in acetonitrile without any side products. The hydride affinity of aldehydes and ketones in acetonitrile was defined as fhe enthalpy change of the aldehydes and ketones. Several conclusions regarding the hydride-accepting abilities of aldehydes and ketones, based on thermodynamic data, were listed." " ... 

By isoelectronic replacement of the nitrosyl ligand with a carbyne unit, we derived a new type of system, which was realized in the form of the complex W(sCMes)(dmpe)2H [41]. A strong 7t-accepting ability is also attributed to the carbyne ligand, so that a electronically related environment of the hydride could be expected, compared to related nitrosyl substituted species. It turns out that W(NO)(dmpe)2H reacts with excess benzophenone about five times faster than the corresponding carbyne complexes. This factor of enhancement, however, is in kinetic terms a minor one, which means that these complexes have overall comparable reactivity. For the insertion of benzophenone into the W-H bond of W(CMes)(dmpe)2H, an inverse and very pronounced deuterium isotope effect is observed (kj /k = 1/3). This points to existence of a late transition state in the rate determining step, which must have to do with the actual H-transfer. 

As shown above (Section 14.3.3) the proton accepting ability of the hydride ligand (expressed as the Ej factor) determines the thermodynamic drive toward proton transfer from different acids. For weak basicity hydrides (Ej < 0.8), proton transfer is possible only from strong acids like HBF4 or CF3COOH, while those... 

The ability of coenzyme NAD(P)+ (57, R = sugar) and its mimics to accept and deliver hydride ion stereoselectively via 1,4-dihydropyridine intermediates has spurred much interest. Much work involving NADIP)" " and its simpler derivatives (56 R = PhCH2, BNAH R = C12H25, DNAH) as chiral reducing agents has been reported. ... 

The enthalpy of the standard complex (AH,) is solvent dependent, i.e. -4.6 kcal/mol for CH2CI2 and -5.7 kcal/mol for hexane, so the enthalpy values to be used in Eq. (10) should be measured in the corresponding solvent [34a,b]. The Ej factor obtained in this manner should be independent not only on the nature of proton donors but also on the medium. As one can see from Table 1, the basicity factor for a given hydride is really independent on the acidity (proton donating ability) of the proton donor. Comparison of the Ej values in Table 1 shows that the proton accepting properties increase from rhenium to tungsten and to ruthenium complexes. These Ej values enable us to define a scale of basicity factors for the... 

Many other molecules can act either as acids or bases depending on the other molecules they interact with, as illustrated, again, by the reactions of sulfuric acid and ammonia. Sulfuric acid can donate proton to bases such as carbonyl compounds (eq. 9) and can accept proton from stronger acids such as fluorosul-furic acid (eq. 10). The protonation of ammonia with the stronger acid water and its proton donor ability against sodium hydride are given in equations 11 auid 12, respectively. 

It is generally accepted that in rhodium-catalyzed hydroformylation the formation of the most stable linear and branched alkyl rhodium complexes is assumed to be the step that determines the regioselectivity when there are no P-elimination processes (Figure 1), (see chapter 2). But the reverse process, the dissociation of the metal-alkyl via P hydride elimination, plays an important role in the regioselectivity of the reaction, because it helps to equilibrate the alkyl species and can produce isomerization. In this case the regioselectivity is determined by the relative migratory ability of both metal-alkyl intermediates. 

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