Heterolytic hydrogenation

Scheme 22.2 Formal heterolytic hydrogen activation via deprotonation of a dihydride intermediate.

The products of reductive cyclization incorporate two non-exchangeable hydrogen atoms. Homolytic and heterolytic hydrogen activation pathways may now be discriminated on the basis of hydrogen-deuterium crossover experiments. Reductive cyclization of the indicated nitrogen-tethered enyne under a mixed atmosphere... 

There are many examples where monohydridometal complexes are formed via homolytic or heterolytic hydrogen splitting.24 31 The basic difference between the... 

In the case of HDN, an additional interesting possibility also consistent with the heterolytic mechanism arises, since substrates like the pyridines -or intermediate alkyl or aryl amines- are sufficiently basic to promote the activation of hydrogen so as to form a metal hydride plus a protonated base (e.g. a pyridinium or an alkylammonium cation). Furthermore, some of the most widely accepted amine HDN mechanisms include the initial protonation of the amine nitrogen, followed by elimination of ammonia from the ammonium cation. Therefore, it is very easy to combine the idea of a heterolytic hydrogen activation promoted by, say n-pentylamine, with a subsequent degradation by a Hoffmann mechanism, to conform a reasonable HDN catalytic cycle. A simplified representation of this idea is given in Fig. E4. 

Rokob, T. A., Hamza, A., Stirling, A., Soos, T., Papai, I. (2008). Turning Frustration into Bond Activation A Theoretical Mechanistic Study on Heterolytic Hydrogen Sphtting by Frustrated Lewis Pairs. Angewandte Chemie International Edition, 47(13), 2435-2438. 

A general approach to the hydrogenation of alkynes to cw-alkenes under mild conditions using the ansa-aminohydroborane (2-[bis(pentafluorophenyl)boryl]-A, A-dimethylaniline) (77) as a catalyst. The approach combines several reactions as the elementary steps of the catalytic cycle hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated Lewis-pair reactivity) and facile intramolecular protodeborylation (product release). The mechanism is verified by experimental and computational studies." ... 

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