Hydrogen state transitions

Factor b above is discussed in Sections II, B, 1 II, B, 4 and II, C. A hydrogen-bonded structure such as 221 can account for the facile reaction of 5-bromouracil or for the unique, so-called hydrolyzability of carboxymethylthio-azines (237). The latter may also react via the intramolecular mechanism indicated in 136. The hydrogen-bonded transition state 238 seems a reasonable explanation of the fact that 3,4,6- and 3,4,5-trichloropyridazines react with glacial acetic acid selectively to give 3-pyridazinones while other nucleophiles (alkoxides, hydrazine, ammonia, or sulfanilamide anion) react at the 4- and 5-positions. In this connection, 4-amino-3,5-dichloro-pyridazine in liquid hydrazine gives (95°, 3hr, 60%yield)the isomer-... 

Also worthy of consideration is the more highly organized, doubly hydrogen-bonded transition state suggested by Capon and Rees32. This has been represented in G as involving R2NH as the catalyst, but it can be... 

It is known that the transition from the a state to the ji state is accompanied by a lattice increase [9] of about 5%. This transition occurs at about 18 mm Hg (or about 2.37% H2 concentration). It takes a higher increasing level of hydrogen to transition from a phase to ji phase... 

The fact that compounds with an 8,14 double bond (VIII) cannot be hydrogenated implies that the isomerization cannot proceed via a half-hydrogenated species, an essentially saturated structure. To avoid the excessive compression between the angular methyl groups at C-10 and C-13 which is enforced by the required geometry of the transition to the half-hydrogenated state (IX) the isomerization proceeds via an allylic intermediate (X) which permits the carbon atom at C-8 to retain its hybridization (Fig. 13). 

D. The Geometry of the Transition State for the Formation of THE Half-Hydrogenated State ... 

Although the transition state for the exchange reaction may be described as the critical complex for the conversion of the half-hydrogenated state to either a jr-complexed olefin or an eclipsed vicinal diadsorbed alkane, the stereochemistry of hydrogenation of cycloalkenes on platinum at low pressures can be understood if the transition state has a virtually saturated structure. 

Fig. 17. Preferred conformations of the transition state which yields the half-hydrogenated state from a 4-substituted methylcyclohexene and a methylenecyclohexane.

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

P-ketoacid, but these compounds are especially susceptible to loss of carbon dioxide, i.e. decarboxylation. Although P-ketoacids may be quite stable, decarboxylation occurs readily on mild heating, and is ascribed to the formation of a six-membered hydrogen-bonded transition state. Decarboxylation is represented as a cyclic flow of electrons, leading to an enol product that rapidly reverts to the more favourable keto tautomer. 

Decarboxylation of 1,1-diacids (ge i-diacids) is a similar reaction involving a hydrogen-bonded transition state. 1,1-Diacids may be stable entities, e.g. 

Fig. 3. Calculated j8-hydrogen-transfer transition states for Me2Al(Et)(CH2=CH2) and H2SiCp2Zr(Et)(CH2=CH2)+ [29]...

Calculated transition-state geometries (Fig. 5) show a clear similarity with the alkyl -I- olefin system described in the previous section. Transition states are somewhat earlier, since these reactions are much more exothermic this can be seen, e.g., from the noticeably different C- H bond lengths of the hydrogen transfer transition state. 

Scheme 8. Direct hydrogen-transfer transition states for (a) Ir(COD) (amino alcohol) and (b) Ir(COD)(amino sulfide) complexes.

Another characteristic feature of the hydrogen-reduced transition metal zeolites is their acidic properties, as demonstrated by their catalytic behavior (7). Naccache and Ben Taarit (8) gave IR evidence of the subsequent formation of protons on hydrogen-reduced Cu(II)-Y zeolite. Furthermore, transition metal ions have various oxidation states. Owing to the shielding effect caused by the zeolite network and the electric fields, the transition metal ions may be stabilized in unusual oxidation states—i.e. Ni(I) (9). 

Scheme 5. Stabilization of dinuclear species in aqueous solution by intramolecular hydrogen-bond formation (VI) is well established. Stabilization of the mononuclear species in aqueous solution by intermolecular hydrogen bonds (IV) may be important in some systems. Interconversion between mononuclear and dinuclear species may occur via non-hydrogen-bonded and hydrogen-bonded transition states, respectively, as schematically shown in II and V. Dashed lines denote hydrogen bonds dotted lines denote bond making and bond breaking.

R. B. King (ed.), Encyclopedia of Inorganic Chemistry, Wiley, New York, 1994 (a) K. Yvon, Hydrides solid state transition metal complexes, pp. 1401-20 (b) M. Kakiuchi, Hydrogen inorganic chemistry, pp. 1444-71. 

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