Configuration geometric, retention

The mechanism of 1,3-dipolar cycloaddition can be found in Ref. 63 and the references within. The reaction of nitrone with 1,2-disubstituted alkenes creates three contiguous asymmetric centers, in which the geometric relationship of the substituents of alkenes is retained. The synthetic utility of nitrone adducts is mainly due to their conversion into various important compounds. For instance, P-amino alcohols can be obtained from isoxazolidines by reduction with H2-Pd or Raney Ni with retention of configuration at the chiral center (Eq. 8.44). 

Kharasch has shown by the decomposition of an optically active peroxide that no alkyl radical involved in the formation of at least ope product, the ester, becomes sufficiently free to racemize during the decomposition.62 In fact, the geometrical result is retention of the original configuration. 

Whereas the phosphetanium (Sect. 2.2) derivatives undergo base-catalyzed hydrolysis with essentially complete retention of configuration, the reaction with the related thietanium salts proceeds with complete inversion of configuration and can be described simply as a direct nucleophilic substitution 35. As far as the author is aware, no exchange evidence of the type er countered in the acid-catalyzed hydrolysis of ethylene phosphate, has been found for the cyclic sulfate esters. Consequently despite the geometrical similarities between the cyclic sulfates and phosphates and the related hybridization of the central atoms, the situation in which a finite pentacovalent intermediate sulfur species exists has not been delineated (restricting the discussion to esters). 

The geometric limitation for migration with retention of configuration should be lifted in the first excited state of the homotropenylium ions. Childs and Rogerson showed that this was indeed the case and reported a number of examples where photochemically induced circumambulations take place. An example is given in Scheme 27164. 

In a simple membrane reactor, basically the membrane divides the reactor into two compartments the feed and the permeate sides. The geometries of the membrane and the reaction vessel can vary. The feed may be introduced at the entrance to the reactor or at intermediate locations and the exiting retentate stream, for process economics, may be recycled back to the reactor. Furthermore, the flow directions of the feed and the sweep (including permeate) streams can be co-current or counter-current or some combinations. It is obvious that there are numerous possible process and equipment configurations even for a geometrically simple membrane reactor. 

The reaction of T with the geometrical isomers of DMC lead mainly to the T-substituted isomer with configuration similar to the reactant however, there is a 5% trans contamination in the case of T 4- cis-DMC and an 8 % cis contamination in the case of T -f- trans-DMC. In other hot T for H substitution, it was found " that there is retention of the configuration. Here, however, substitution with inversion or late isomerization of the T-substituted parent molecule cannot be distinguished. The activation energy for the thermal isomerization of cis - trans and trans - cis is about 2 kcal mol lower than that of isomerization of pentenes. 

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