Hydrogen exchange base catalysis

The reaction takes place extremely rapidly and if D2O is present in excess all the alcohol is con verted to ROD This hydrogen-deuterium exchange can be catalyzed by either acids or bases If D30 is the catalyst in acid solution and DO the catalyst in base wnte reasonable reaction mech anisms for the conversion of ROH to ROD under conditions of (a) acid catalysis and (b) base catalysis... 

Base catalysis is most effective with alkali metals dispersed on solid supports or, in the homogeneous form, as aldoxides, amides, and so on. Small amounts of promoters form organoalkali comnpounds that really contribute the catalytic power. Basic ion exchange resins also are usebil. Base-catalyzed processes include isomerization and oligomerization of olefins, reactions of olefins with aromatics, and hydrogenation of polynuclear aromatics. 

For exchange of non-labile organic hydrogen atoms, acid-base catalysis (or some other catalytic hydrogen-transfer agent such as palladium or platinum) is required. The method routinely gives tritiated products having a specific activity almost 1000 times that obtained by the Wilzbach method shorter times are required (2-12h) and subsequent purification is easier. 

Mechanism Kinetic" Order P-Hydrogen Exchange Faster Than Elimination General or Specific Base Catalysis hAd Electron Withdrawal atCp Electron Release at C Leaving- Group Isotope Effect or Element Effect... 

As discussed in the previous section, metal oxides have both acidic and basic properties. The acid-base properties of metal oxides have led to many interesting catalytic reactions. Catalytic reactions such as H2-D2 exchange, hydrogenation, isomerization, dehydrogenation, dehydrohalo-genation, and benzylation can be considered as examples of acid-base catalysis reactions.31-36 These reactions will be briefly discussed in the following section. The remarkable properties of MgO as a catalyst have been well documented in the literature and we shall discuss some of these unique catalytic properties. 

Selected entries from Methods in Enzymology [vol, page(s)] Add-base catalysis [with site-directed mutants, 249, 110-118 altered pH dependencies, 249, 110] commitment to [in determination of intrinsic isotope effects, 249, 343, 347-349 in interfacial catalysis, 249, 598-599 equilibrium isotope exchange in, 249, 443-479 hydrogen tunneling in, 249, 373-397] interfacial [competitive inhibitors, kinetic characterization, 249, 604-605 equilibrium parameters, 249, 587-594 forward commitment to, 249, 598-599 interpretation, 249, 578-586 (constraining variables for high processivity, 249, 582-586 kinetic variables at interface,... 

Mechanism Kinetic order p-hydrogen exchange faster than elimination General or specific base catalysis ku ko Electron withdrawal at Cff Electron release at CJ Leaving- gronp isotope effect or element effect... 

In addition to performing acid/base catalysis, zeolite structures can serve as hosts for small metal particles. Transition metal ions, e.g., platinum, rhodium, can be ion exchanged into zeolites and then reduced to their zero valent state to yield zeolite encapsulated metal particles. Inside the zeolite structure, these particles can perform shape selective catalysis. Joh et al. (16) reported the shape selective hydrogenation of olefins by rhodium encapsulated in zeolite Y (specifically, cyclohexene and cyclododecene). Although both molecules can be hydrogenated by rhodium supported on nonmicroporous carbon, only cyclohexene can be hydrogenated by rhodium encapsulated in zeolite Y since cyclododecene is too large to adsorb into the pores of zeolite Y. 

The isotopic exchange between molecular hydrogen and a hydroxylic solvent under base catalysis poses a rather intriguing mechanistic problem. The two main mechanisms that had been proposed (Symons and Buncel, 1972) involve rate determining... 

Exchange of hydrogen in alkaline solution was reported by Franke and M5nch and subsequently confirmed but attributed to exchange of phosphite formed by alkaline decomposition. A study by proton nmr methods reports simple first-order dependence on the concentrations of HjPOJ and OD. At 25°C and H = 2.0, the rate parameters are k = (6.0+0.7)10 l.mole . sec ( a = 18.9 0.5 kcal.mole ). Reaction of D2POJ and OH has k = (1.92 0.1)10 l.mole sec ( a = 18.7 kcal.mole ). It is suggested that there may be general base catalysis, from qualitative observations of the effect of phosphate. 

The hydrogen exchange of phosphine in aqueous solution was studied by deuterium labelling. The reaction shows little sensitivity to bases other than hydroxide ion, though it shows general acid catalysis, viz. 

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