Quasi-symmetrical reaction mechanisms

The substituent parameters for the condition k, = 2 be predicted for quasi-symmetrical reactions. The decomposition steps of a quasi-symmetrical intermediate to reactants (, ) and to products therefore obey identical linear free energy relationships. A change in rate-limiting step will occur when the intermediate is perfectly symmetrical (and k. In the addition-elimination mechanisms of Scheme 9 the ratio k Jk2 becomes unity when entering and leaving groups are identical. 

Substituted phenolate anions react with 4-nitrophenyl diphenyl-phosphinate in a quasi-symmetrical reaction so that the effective charges can be determined for both forming and breaking bonds from the two quantities and Peq without any special experiments made on the leaving group substituents. The transition structure for the reaction, which has a concerted mechanism, has ... 

Quasi-symmetrical bimolecular reactions involving dissociative mechanisms are illustrated in Scheme 10. These mechanisms involve the intervention of pre-association complexes (such as Nu. A-Nu,). 

Scheme 10 Quasi-symmetrical bimolecular reactions with dissociative mechanisms and termolecular intermediates...

The mechanism of the disproportionation reaction is currently receiving attention. Two basic pathways have been suggested. The first proposes the concerted transformation via a symmetrical intermediate, variously described as quasi-cyclobutane (11), tetramethylene (12), etc., of one pair of olefins into another. The second mechanism postulates a nonconcerted process involving metallocyclopentane intermediates (13). Experimental evidence for both mechanisms has been presented, but the issue is still unresolved. 

In order to distinguish between a mechanism proceeding via a symmetrical cyclopropanone intermediate (Favorskii reaction) and a mechanism closely related to the benzilic acid rearrangement and called semibenzilic (or quasi-Favorskii) rearrangement, the ring contraction of 2-bromocyclobutanone was studied in deuterium oxide using sodium carbonate as base (50 C) or in boiling deuterium oxide only. 

Band and Freed have criticized the quasi-diatomic approximation and emphasized that any complete theory of dissodation must involve the use of the correct sets of normal modes Q and O of the molecule in the initial and final states (> and /> respectively. The two sets are not independent, but are related by a co-ordinate transformation. A detailed, quantum mechanical description has been developed in which the set Q in state ( > are taken to be the normal modes of the unexdted parent molecule for direct photodissodation, or the metastable photo-excited molecule for indirect predissociation, and the set Q ) in the state /> are separated into QUQi, wh e Qi is the reaction co-ordinate on the final repulsive surface and IQi are the normal modes in the photofragments. For a linear, triatomic molecule, Qi is simply the vibrational mode of the diatomic fragment and Q) indudes the symmetric and antisymmetric stretching modes (if collinearity is preserved). The matrix elements for the transition from... 

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