Asymmetric transformation of the first kind

The same equilibria are attained if to a solution of racemate in an initially achiral medium (equal concentrations of d and I species) there is added another chiral species d or L. The equilibrium is then displaced in favour of one or other of the constituents of the racemic mixture. This process has recently been termed enantiomerization, although examples of optically labile systems in equilibria sensitive to the presence of other chiral molecules or ions have long been recognized. A typical example of what was earlier termed an asymmetric transformation of the first kind (no second-phase involved) is that of Read and McMath (1925) in which solutions in dry acetone of (—) or ( ) chlorobromomethanesulphonic acid d-l ) together with (—)-hydroxyhydrindamine (l+) showed a change of optical rotation interpreted in terms of an equilibrium... 

Asymmetric transformation The conversion of a mixture (usually 1 1) of stereoisomers into a single stereoisomer or a mixture in which one isomer predominates. An asymmetric transformation of the first kind involves such a conversion without separation of the stereoisomers. An asymmetric transformation of the second kind also involves separation, such as an equilibration accompanied by selective crystallization of one stereoisomer [76]. The terms first- and second-order asymmetric transformations to describe these processes are inappropriate. See also stereoconvergent. 

Scheme 7.8. Asymmetric reduction of chiral P-keto esters may be used in an asymmetric transformation of the first kind (dynamic kinetic resolution) [78],...

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

It may be of interest to note that the stereospecific transformation shown in equation 15 has been cited as the first reported observation of an 1 - 3 chirality transfer. It is evident that on rearrangement of optically active 6d to 7d, the chiral center at C-a is eliminated and a new one created at C-y. The term self-immolative asymmetric synthesis has also been used to describe syntheses of this kind. As pointed out by Hoffmann , quantitative 1 - 3 chirality transfer will follow from the suprafacial - course of rearrangement, provided the reactant has a uniform configuration at the j8, y-double bond. This stereochemical prediction has also been confirmed by the results obtained in several other [2,3]sigmatropic rearrangements, subsequently reported " . 

Whenever x is greater than or less than 0.5, a rotation will be observed. Although the Pfeiffer and the Harris explanations appear to be identical, Harris contends, in opposition to Pfeiffer but in agreement with the present authors, that an A-B association of some kind is essential to the development of the Pfeiffer-type activity. The term first-order asymmetric transformation is used for a system in which no optically active phase other than that originally introduced can be isolated. If such a new phase is isolatable, the transition is called second order. Turner and Harris have used the former term (first order) to describe the observations of Pfeiffer, Dwyer, and others. ... 

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