Relative Reactivity and Stereoselectivity in the Wittig Reactions of Substituted Benzaldehydes with Benzylidenetriphenylphosphorane

Article abstract of DOI:10.1021/jo00036a021

Carbonyl carbon-14 kinetic isotope effects and substituent effects on the relative reactivity and on the cis-trans product ratio were determined in the Wittig reaction of XC6H4CHO with Ph3P=CHC6H4Y in THF at 0 deg C.The isotope effect and the Hammett ρ value were positive under both Li salt-free (¹²k/¹⁴k = 1.060 +/- 0.003 and ρ_x = 2.77 +/- 0.15) and Li salt-present (¹²k/¹⁴k = 1.015 +/- 0.004 and ρ_x = 1.38 +/- 0.12) conditions, although they were much larger in the former case.These, together with the absence of enone isomerization for the benzylidene ylide reported previously, suggested that the reactions proceed via a polar cycloaddition transition state of considerable nucleophilic character.The cis-trans ratio of the product stilbene was essentially unchanged (40:60 in the salt-free and 60:40 in the Li salt-present reaction) by the change in concentration, the mode of addition, and the molar ratio of aldehyde and ylide, and it was varied only slightly for most substituents X and Y.However, the ratio was significantly varied when o-MeO or o-Cl was introduced as X.The results could be rationalized by assuming a chelating interaction between the lone pair of the ortho substituents and the phosphorous of the ylide.