Racemization during hydrogenation

Because of the possibility of racemization during the transesterification reaction (strong basic conditions) alternative methods are reported. These include transesterification in the presence of the KF/18-crown-6 ether 461 or the use of titanium tetraalkoxides. 471 The methods are efficient and represent a route to any required dialkyl ester using diphenyl esters as starting materials. Diphenyl groups can also be removed by hydrogenation in the presence of platinum dioxide (Adams catalyst) to provide the free phosphonic acid moiety directly)46 ... 

Similar intermediates seem to be involved in the net trans addition in hydrogenation, in racemization during exchange, and in transfer of the isotopic exchange process from one side of the cyclopentane ring to the other. 

Shvo s dimthenium complex 1 is activated by heat and dissociates into the two catalytically active monomthenium complexes an isolable 18 electron complex 2 and a proposed highly reactive 16 electron complex (A) that has never been observed directly (Scheme 1). The 18 electron species acts as a hydrogenation catalyst, whereas the 16 electron species acts as a dehydrogenation catalyst [19,20]. The two catalytically active complexes interconvert during racemization and hydrogen transfer (Scheme 1). 

Hydrogenation of ethyl pyruvate in the presence of cinchonidine. In our previous studies [3, 4,14] variety of experimental data were obtained, which could not be explained by existing models [1,2] proposed earlier. These results are as follows [3,4,12] (i) the monotonic increase type behaviour of the optical yield - conversion dependencies, (ii) the complexity of the reaction kinetics, (iii) side reactions catalyzed by CD. It was also demonstrated that the enantio-differentiation can be induced if the modifier is injected into the reactor during racemic hydrogenation. 

Upon injection of CD into the reactor during racemic hydrogenation the rate acceleration was always instantaneous, while the optical yield vs conversion dependencies showed a monotonic increase type behaviour as seen in Figure 7. In acetic acid the increase part of the above dependence is so fast that it hardly can be followed by our sampling technique. At low concentration of modifier the optical yield passes through a maximum In this case the... 

We have compared, the rate acceleration effect induced either by the CD and different moieties originated from CD, i.e quinuclidine and quinoline. These experiments were carried out in ethanol If the relative rate of racemic hydrogenation is equal to one the following relative rates has been measured quinoline = 2, quinuclidine = 3, cinchonidine = 40. In the presence of quinoline a short induction period was needed to observe the rate acceleration. It is suggested that during this period quinoline was partly hydrogenated. Other tertiary nitrogen bases, such as triethylamine, triethylenediamine, etc. resulted also rate acceleration with relative rate = 2-4. 

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