Hydride transfer concept

Adolfsson, H. Organocatalytic Hydride Transfers A New Concept in Asymmetric Hydrogenations. Angew. Chem. Int Ed. 2005, 44, 3340-3342. 

A further modification of the reaction was evolved, based on the concept that hydride transfer could be split into the two processes of electron transfer and protonation (80CC766). Thus 2-ethylthiophene is reduced by zinc and trifluoroacetic acid to a mixture of 2-ethyl-2,5-dihydrothiophene (70%) and 2-ethyltetrahydrothiophene (6%). The suggested mechanism for this reduction is shown in Scheme 44. 

Although the remaining steps required for the conversion of 463 to haemanthidine (382) were conceptionally straightforward, it was an experimentally difficult task owing to the poor stereoselectivity encountered in the reduction of the neopentyl ketone function at C-l 1. Moreover, the more forcing conditions required in some experiments to effect the reduction of the sterically hindered carbonyl function at C-ll resulted in an internal Cannizzaro hydride transfer process that produced nortazettine (467). The best results were eventually ob-... 

Truly, the redox reaction concept in its simplest form, transfer of one or several electrons between two species, is much easier to apply to the central atoms of inorganic complexes and relatively simple covalent inorganic compounds with their well-defined oxidation states than to the carbon atoms of organic molecules. Nevertheless oxidation states of the latter can be defined using very simple rules (see, e.g. Hendrickson et al., 1970) and immediately reveal the possible redox nature of any transformation at a carbon atom. It is also true that redox mechanisms other than electron transfer—hydrogen atom or hydride transfer, oxygen transfer, displacement, etc.—should by their very... 

Many other important theoretical concepts of paramount importance for biomimetic photochemistry are still in their infancy. This includes the strategies to mediate and accelerate proton-coupled single and multiple electron transfer catalysis (171-173), and the fundamental aspects of catalytic hydride transfer and hydrogen tunneling processes (174—176) among several others. 

Figure 4.9. Structural evidence favoring a hydride-transfer mechanism for the action of the PQQ cofactor in the soluble glucose dehydrogenase of Acinetobacter calcoaceticus (structure and concept of Oubrie et al. [71] ...

Olah s investigation of norbomyl cation provided him with an example illustrating a concept he had advanced in an independent line of research, namely, formation of pentacoordinate carbocation intermediates by attack of electrophiles on C-C and C-H bonds. Space does not permit discussion of the impressive experimental work associated with this concept, but it established that species analogous to the well-known CHs, so common in electron impact mass spectrometry, could be generated in solution. Attack of an electrophile on a C-H bond probably involves a triangular transition state, and results in hydride abstraction if a stable tricoordinate carbocation can be produced. Hydride transfer reactions between carbonium ions are quite common in media of low nucleophilicity ... 

If it is assumed that the adsorbed carbonium ion has a tetrahedral configuration, it is evident that the cis-adsorbed species is sterically more favorable than the trans species. Thus, hydride ion transfer to the adsorbed charged species from the catalyst would lead to predominant cis-product formation. In contrast to the Weidlich proposal, these mechanisms call for a 1,4-addition of hydrogen in acidic media. The concept of a hydride ion transfer from the catalyst is not unique to these mechanistic proposals. Hydride ions have been proposed to take part in the catalytic hydrogenolysis of certain substituted cyclopropanes... 

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