Secondary deuterium kinetic isotope effects SDKIE

Another important observation was provided even before the Dewar papers, namely that the secondary deuterium kinetic isotope effect (SDKIE), at C4 of a 3,3-dicyano substituted-1,5-hexadiene was a larger normal effect in an absolute sense than the inverse SDKIE at C6. This must represent more bond breaking at C4, i.e. rehybridization from sp to sp, than bond formation at C6 involving rehybrization from sp to sp, a result inconsistent with a diyl-like transition state whose SDKIEs would be opposite in magnitude. Subsequent SDKIE determination by Conrad on 3-aIkyl substituted -1,5-hexadiene and 2-phenyl-l,5-hexadiene and on carbon-13 labeled 2,5-diphenyl-1,5-hexadiene revealed a change in transition state structure from a species with more bond making than breaking to one with a nearly a fully formed 1,4-cyclohexane diyl in that order. 

For the mechanism of the metal-mediated Grignard-type reactions in water, Li previously proposed a carbanion/allylmetal/radical triad, in which the specific mechanism of the reaction is dependent on the metal being used (Fig. 4.6). Recently, mechanistic studies of the aUylation detected secondary deuterium kinetic isotope effects in the metal-mediated aUylation ofbenzaldehyde in aqueous media.The inverse SDKIE observed for the indium and tin cases are consistent with the polar addition mechanism. For magnesium and antimony, normal SDKIE were observed. These were interpreted as single electron transfer processes on metal surface in the magnesium case, or between the allyhnetal and the carbonyl compound in the antimony case. 

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