Reactions of solvate dihydrides with prochiral substrates

The hydrogenation of the catalyst-substrate complex was substantially slower it could not be hydrogenated at all at -100°C, and it required 2 h at -80°C to achieve approximately 90% conversion to the same monohydride intermediate.  

Essentially the same results were later obtained with various prochi-ral substrates a-dehydro amino acids, enamides, - P-dehydroamino acid, unsaturated phosphonate, ° and itaconic ester. All studied substrates reacted instantaneously with 28a, b at -100°C yielding high ee s of the hydrogenation products—either equal or comparable to the ee s observed in the corresponding catalytic reactions. -  

Important results were obtained when the reaction of tetrahydride 33 with MAC (4) was studied. Despite the fact that there were two solvate 

in this experiment the quantitative formation of the nonchelating dihydride-substrate complexes 81 and 82 has been observed. Obviously, the coordination of the double bond with the formation of chelating tetrahydride-substrate complex 83 must occur before the migratory insertion. However, due to an essentially negligible barrier of the migratory insertion (less than 1 kcal/mol), chelating dihydride intermediate 83 was not observed directly. 

It is clear that, if the complete reversal of the sense of enantioselection was observed, such cases must exist that the competition between the alternative pathways would lead to the low optical yields. Indeed, with o-methoxyphenylenamide 94 both types of monohydride intermediates 

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Reactions of solvate dihydrides with prochiral substrates The accessibility of the solvate dihydrides for some catalysts (Schemes 1.10 and 1.12) makes possible the study of their direct reactions with prochiral substrates. These experiments can provide information on the late intermediates in the catalytic cycle. Besides, since the substrate is introduced in the system at very low temperature after the hydrogen activation already took place, the input from unsaturated mechanism is effectively excluded, and checking the ee of the recovered product gives direct information about the effectiveness of the enantioselection by coordination of a substrate to the octahedral solvate dihydride. 

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