| Abstract
| - The mechanism and diastereoselectivity of synthetically useful sulfur ylide promoted cyclopropanationreactions have been studied using the density functional theory method. Addition of different substitutedylides (Me2S+CH-R) to enone ((E)-pent-3-en-2-one, MeHCCH−COMe) has been investigated. Thenature of the substituent on the ylidic carbon brings about subtle changes in the reaction profile. Thestabilized (R = COMe) and semistabilized (R = Ph) ylides follow a cisoid addition mode, leading to1,2-trans and 1,2-cis cyclopropanes, respectively, via syn and anti betaine intermediates. The simplestand highly reactive model ylide (R = H) prefers a transoid addition mode. Diastereoselectivity is controlledby the barrier for cisoid-transoid rotation in the case of stabilized ylides, whereas the initial electrophilicaddition is found to be the diastereoselectivity-determining step for semistabilized ylides. High selectivitytoward trans cyclopropanes with stabilized ylides are predicted on the basis of the relative activationenergies of diastereomeric torsional transition states. The energy differences between these transitionstates could be rationalized with the help of weak intramolecular as well as other stereoelectronicinteractions.
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