Catalysts lability effect

Ion concentration is important in oxidative processes because the metal ions can act as catalysts. The effects of ions in urine can be reduced by the ad tion of a protein sudi as bovine soiim albumin. Solutions of some labile drugs are more stable in plasma than the same concentration in wat. The decomposition of chlorambucil in urine increases with the concentration of chloride ion. 

As Olah et al. have reported (81JOC2706), iV-nltropyrazole in the presence of Lewis or Brpnsted acid catalysts is an effective nitrating agent for aromatic substrates. The greater lability of the N—NO2 bond in iV-nltropyrazole compared with aliphatic nltramines was discussed on the basis of its molecular structure as determined by X-ray crystallography. 

The use of well-defined complexes has been widespread in this reaction, despite intriguing studies by Beller and others that have shown that in situ catalytic systems often give better yields in comparison to isolated carbene-Pd(O) complexes [147-149]. Since the mechanism consists of an oxidative addition on a Pd(0)-monocarbene species, efforts in catalyst synthesis have been directed towards Pd(ll)-monocarbene complexes with other labile groups that can be easily released leading to the formation of Pd(0). This is the case for dimers of the type [Pd( j,-C1)C1(NHC)]2, a family of pre-catalysts effective under aerobic conditions [150], the [Pd(acac)Cl(NHC)] complexes [151] and related palladacycles [152-154],... 

The effectiveness of an organic chloride in activating the catalyst appears to be related to the lability of the C—Cl bonds and probably also to their coordinating ability with the Rh. For example, CH30—CH2C1 and CH2=CH—CH2C1 were found to be extremely effective activators presumably, the presence of the ether or the allylic donor sites adds to the ease of the oxidative addition reaction ... 

Olefin hydrogenation catalyzed by Fe(CO)s normally requires somewhat severe conditions, typically 150°C and 10 atm H2 (/, p. 64). With near-ultraviolet irradiation the carbonyl becomes effective at ambient conditions for hydrogenation (and isomerization) of olefins (448, 449). Photoinduced labilization of carbonyls is thought to give tricarbonyl species as the active catalysts, e.g.,... 

An alternative mechanistic scheme in which TMCS first reacts with alcohols to form hydrogen chloride which then activates the phosphoroamidite in situ seems to be unlikely. It is known that TMCS reacts very slowly with alcohols unless a catalyst is present [36]. Formation of HCl would effect the removal of an acid labile group like DMTr attached to a base or tert-hutyl... 

However, beeause of the kinetie lability of eobalt(ll), heterogenized catalysts based on Co are suseeptible to metal leaehing during liquid phase reaetions and thus repeated use of such catalysts is not practical from a chemical point of view. In order to avoid this problem, use of catalysts based on cobalt(lll), whieh is substitutionally inert, may be expected to show more attraetive eatalytic properties for the same reaetions. As expeeted, substitutionally inert cobalt(lll) eomplexes have been shown to be eatalytieally very aetive, and henee attractive, for alkylaromatie oxidation [26]. Also, as we shall see later, a series of tetramerie eobalt(lll) complexes eapable of cycling oxidation states between 111 and IV has also been foimd to be effective as eatalysts for the oxidation of alkyl aromaties, aleohols and alkenes [5,15,27]. 

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