Transfer processes intermolecular, reversible

Reaction (64) demonstrates the production of a metal formyl complex by intermolecular hydride transfer from a metal hydride which is expected to be regenerable from H2 under catalytic conditions. Further, it provides a plausible model for the interaction of [HRu(CO)4] with Ru(CO)4I2 during catalysis, and suggests a possible role for the second equivalent of [HRu(CO)4]- which the kinetics indicate to be involved in the process (see Fig. 23). Since the Ru(CO)4 fragment which would remain after hydride transfer (perhaps reversible) from [HRu(CO)4] is eventually converted to [HRu3(CO)),] [as in (64)] by reaction with further [HRu(CO)4], the second [HRu(CO)4]- ion may be involved in a kinetically significant trapping reaction. 

Intramolecular hydride transfer under MPV reduction conditions occurs in substrate (25) with complete stereospecificity to generate (26).275 A 2 1 mixture of product to reactant was observed, irrespective of reaction time or relative excess of Al(0 Pr)3, indicative of an equilibrium. Intermolecular hydride transfer to give (27) does not occur and the absence of the epimer of (25) implies that complete stereodifferentiation also occurs in the reverse process (Oppenhauer oxidation). Stereodifferentiation under... 

The assumption that the azomethine 34 is formed by an intermolecular hydride transfer from aldehyde 30 to the nitrilium salt 31 is not confirmed because the different Schiff bases 34 obtained carry the group R which are originated from the aldehyde 30. In this way, the process described in equation 14 can be reversed and applied as an enamide synthesis by acylation of imines2,3. However, the 7V-ethylbenzonitrilium salt 35 reacts with benzaldehyde to give the more stable Af-benzoyl-7V-ethyliminium ions 3647,49 which add to trimethylethylene to form 5,6-dihydro-4i/-l,3-oxazinium salt 37. On the other hand, the reaction of ions 36 with phenylacetylene leads to another type of 1,3-oxazinium heterocycle, namely to 4i/-l,3-oxazinium hexachloroantimonate 38 (equation 15). 

Both reactions, namely tte intermolecular process (131) and the intramolecular one (132) can be either reversible or irreversible (termination). In the case of reversible reactions true chain transfer takes place vdien the rate constant of the backward reaction (kj ) becomes comparable with the rate constant of M-opaptun. This applies to the polymerization of cyclic acetals where the product of chain traiKfer is equally active in propaption. 

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