In the modified Julia olefination, called Julia-Kocienski olefination (http://www.organic-chemistry.org/namedreactions/modified-julia-kocienski-olefination.shtm
), the initial step in the reaction mechanism is attack of the organometallic reagent on the aldehyde, either from Si or Re face, to yield (not respectively) the anti
adducts. The anti isomer evolves after rearrangement and beta-eilimination into the trans
-isomer, whereas the syn
isomer reacts to yield the cis
Studies have shown that the type of the metal (Li or K) has an influence on the direction of the initial attack, where with lithium, a chelate can be formed with the aldehyde's oxygen and the benzothiazole's nitrogen (see within the attached hyperlink, vide supra
) in a "closed transition state", leading to the syn
isomer; whereas the potassium
, apparently due to its relative larger atomic radius, does not form a chelate, but leading to the so called "open transition state" and the anti
isomer (see the attached picture file).
My question is: why does this chelate necessarily
evolve into the syn
isomer, whereas the non-chelate ends up in the anti
-isomer? Particularly in the potassium case, which factor here prevents any other orientation between the carbanion and the aldehyde which hypothetically could lead to the trans
Thanks in advance!