Hydrogen product distributions, role

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. 

D.J. Dwyer and G.A. Somoijai. The Role of Readsorption in Determining the Product Distribution During CO Hydrogenation over Fe Single Crystals. J. Catal. 56 249 (1979). 

In the case of 1-phenylpropyne a vinyl cation is suggested as intermediate. The kinetic law is role=h,[—C=C—] [HCl]. The second order dependence on HCl is explained by assuming that proton transfer to the triple bond results in the anion hydrogen dichloride, Hdi. The product distribution mid the stereochemistry, under kinetic control, have been explained by assuming that a c s-oriented intimate ion-pair, 14 is initially formed which, following Scheme 2, may either collapse to cis chloride, undergo anion displacement by acetic add to form trana acetate or a randomly oriented (solvent separated) ion-pair 15 which gives racemic material. 

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