Quasi-intermediate

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

After 28 years the perepoxide quasi-intermediate was supported by a two-step no intermediate mechanism [71, 72]. The minimum energy path on the potential energy surface of the reaction between singlet molecular oxygen ( A and dg-teramethylethylene reaches a vaUey-ridge inflection point and then bifurcates leading to the two final products [73]. 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

HOMO Amplitudes, Quasi-Intermediate Structures, and Mode Selectivities... 

The geometrical structure of the perepoxide quasi-intermediate was suggested to play critical roles in determining diverse selectivities of the reactions of O with substituted olefins [92]. ... 

Scheme 16 HOMO polarized by X deforms pere- R poxide quasi-intermediates ...

The argument of the directing effect of lone pairs on the substiment [92] easily extends to the alkyl cases. The orbital interaction (Scheme 20) [103] in the pere-poxide quasi-intermediate suggests the stabilization occurs by the simultaneous interaction of O with two allylic hydrogens on the same side of the alkene. Photooxygenation of trisubstituted olefins revealed a strong preference for H-abstraction from disubstituted side of the double bond [104, 105],... 

Hydrogen bonding to the pendant (tailing) oxygen (Scheme 21) in the perepoxide quasi-intermediates controls the facial/diastereoselectivty of the ene reactions of... 

Scheme 20 HOMO-LUMO interaction in the perepoxide quasi-intermediate for the cis-effect and the regioselectivity (percent) of the hydrogen abstractions...

The Potential Bole of Quasi-Intermediates of Monofunctional Reaction... 

We have noted in Section II,B,3 that a single monofunctional catalyst while generating a desired product may give rise to species which we have termed quasi-intermediates. The introduction of a different catalyst component Y may then lead to interception of the usual reaction path, diverting old product C into a new product D ... 

It is important to recall (see Section II,B,2 and II,B,3) the limitation we must ascribe to the meaning of quasi-intermediates as to their being true intermediates in the monofunctional Z-system. We must recognize that similar coupling of a second catalytic system (F) to a previously monofunctional system can be attained where the latter system can be pictured as one generating a low level side-product... 

The conversion of cyclohexanes to aromatics is a classical dehydrogenation reaction which will readily take place on many transition metals and metal oxides. On chromia-alumina Herington and Eideal (S) have demonstrated the occurrence of cyclo-olefin intermediate products. Weisz and Swegler 25) have demonstrated the effect on benzene yield of allowing early diffusional escape of cyclo-olefin from the porous catalyst particle. Prater et al. 26) have developed evidence that cyclohexene occurs as a quasi-intermediate in aromatization catalysis over platinum catalyst also, although at a smaller concentration, because of a larger ratio of effective rate constants fe/Zci in the scheme... 

The quasi-intermediate cyclo-olefin of the monofunctional dehydrogenation reaction thus becomes a true intermediate in the dual functional naphthene ring isomerization, by a process of interception, as discussed in Sections II,B,3 and II,E,1. 

Certain catalyst materials that may not have been considered to operate as multifunctional catalyst composites may in fact be operative via distinct and separate catalytic sites. Keulemans and Schuit (41) have discussed such possibilities briefly. In this article, we shall not look further into these possibilities however, inasmuch as we have examined various reactions and reaction sequences, it is very appropriate to point out that there undoubtedly exist reactions which have been considered to be catalyzed essentially by monofunctional systems, but which may in fact undergo polystep conversion. Also, quasi-intermediates may exist, appearing as traces of side products or remaining undetectable, that carry a ready... 

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