Activation energy imines

On the basis of the high. 4-factors, low activation energies, and relative insensitivity of rates with respect to solvents and substituents, Talaty and Fargo consider a linear transition state in which one or both nitrogen atoms acquire an sp hybrid state, as the most likely one for the isomerization reaction. A similar transition state has been postulated for the cis - trans isomerization of imines and di-fluorodiimide. ... 

Which electrophile is lost from the amino acid residue is, of course, controlled by the enzyme. One way this may occur is by the enzyme binding the PLP imine so that the electrophile is in close proximity to a suitable or base to aid abstraction and also so that the a orbital of the bond to be broken is periplanar with the p r acceptor system, i.e. orthogonal to the plane of the pyridine ring (XXXI). Maximal orbital overlap, stereoelectronic control, will lower the activation energy for the reaction. Aldol-type reactions can also occur with PLP as in the laboratory the key to making carbon-carbon bonds is the formation of a stabilised carbanion. Proton abstraction from the initially formed imine gives a masked carbanion which can nucleophili-... 

The basic concept of activation in hetero-Diels-Alder reactions is to utilize the lone-pair electrons of the carbonyl and imine functionality for coordination to the Lewis acid. The coordination of the dienophile to the Lewis acid changes the FMOs of the dienophile and for the normal electron-demand reactions a decrease of the LUMO and HOMO energies is observed leading to a better interaction with... 

The majority of transformations reported within the literature using the concept of LUMO energy lowering iminium ion activation have nsed secondary amines as the catalyst. Under the aqueous acidic reaction conditions inherent to this mode of activation it is also possible to nse primary amines as efficient catalysts where the active species is the protonated imine 141 (Fig. 13). Althongh this is a somewhat less explored avenne of research, initial results suggest it will become an equally fruitful area with broad application. 

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