Hydrogen exchange reaction intermediates

The present article is a review of tt complex adsorption which has recently been proposed in catalytic reaction mechanisms (2-11). The main evidence for this intermediate has been obtained from isotopic hydrogen exchange reactions with aromatic compounds where an interpretation according to classical theories has met with increasing difficulties. The limitations of the classical associative and dissociative exchange mechanisms originally proposed by Horiuti and Polanyi (12) and Farkas and Farkas (13-15) re discussed. This is followed by a... 

The hydrogen-exchange reaction between C2H4 and C2D4 via ethyl intermediates is the same type of reaction as the cis-trans isomerization reaction of 2-butene, because these reactions both involve the elementary processes of alkyl intermediate formation followed by internal rotation around C—C bonds, as described in Scheme 5. Then the hydrogen exchange... 

So far the intermediates of the hydrogenation of olefins have been estimated by a hydrogen exchanged between D2 and olefins during the hydrogenation reaction, where the steps of the alkyl formation in Scheme 14 are assumed to be reversible and to be the main route of the hydrogen-exchange reaction. A historical reaction of olefins with D2 on nickel catalyst... 

The a-hydrogen exchange, Reaction 1, and the e-hydrogen exchange, Reaction 2, are interrelated by a 1,2-hydrogen exchange as shown in Reaction 3. Each of these reactions may be more complicated than that shown above, since they probably involve solvent or coal radical intermediates. In reactions 1 and 2, for example, the coal probably abstracts 2H from the naphthalene-ds to form a naphthyl radical, and subsequently the naphthyl radical abstracts lH from a different part of the coal as shown in Reactions 4 and 5 ... 

E = H) resulting from a lithium—hydrogen exchange in intermediate 198. This hydrogen abstraction from the reaction medium, probably from THF at the a-position, has already been observed in other cases for very reactive organolithium intermediates. The use of Barbier-type reaction conditions did not improve the obtained results124. 

Reductive elimination of H2 occurs when [Co(H)2 P(OR)3 4]+ salts are treated with CO or P(OR )3 and the reaction is not measurably reversible, p(H2) = 1 atm (equation 68).383 This contrasts with that found for isoelectronic [Fe(H)2 P(OR)3 4]. Also, the hydrogen exchange reaction (69) occurs without H-D scrambling and both reactions have been interpreted in terms of a dissociative [Co P(OR)3 4]+ intermediate.383... 

The species H3 occurs as an intermediate in the hydrogen exchange reaction... 

Hydrogen exchange reactions of heteroaromatics carried out in strongly alkaline media, such as potassium amide/liquid ammonia, alcoholic solutions of alkoxides, or solutions of potassium <-butoxide in dimethyl sulfoxide, proceed through an entirely different mechanism (sometimes called protophilic ) involving a carbanion-type intermediate they are not electrophilic substitutions as such and will not be treated in this review. 

This work resulted, over the period 1948-1952, in eight publications dealing with the isomerization of paraffins and their hydrogen-exchange reactions in acidic systems. The interfaces involved were either solid, with catalysts based on aluminum chloride, or liquid, with concentrated sulfuric acid. The results provided strong support for the utility of the carbonium-ion model and new information about the properties of the postulated transient intermediates. 

This genera] scheme could be used to explain hydrogen exchange in the 5-position, providing a new alternative for the reaction (466). This leads us also to ask whether some reactions described as typically electrophilic cannot also be rationalized by a preliminary hydration of the C2=N bond. The nitration reaction of 2-dialkylaminothiazoles could occur, for example, on the enamine-like intermediate (229) (Scheme 141). This scheme would explain why alkyl groups on the exocyclic nitrogen may drastically change the reaction pathway (see Section rV.l.A). Kinetic studies and careful analysis of by-products would enable a check of this hypothesis. 

Reactions at the a-carbons have been of considerable kiterest because it is at these positions that enzymatic oxidation, which is beheved to initiate the events leading to carcinogenic metaboUtes, generally occurs (5,7,8,73). The a-hydrogens exchange readily as shown in the following where D represents H. This exchange apparentiy results from stabilization of an anionic intermediate by electron delocalization (74,75). 

---