Hypervalent Organophosphorus Compounds as Organocatalysts

Significance

Catechol-derived phosphonium salts (1) were used as novel metal-free Lewis acid catalysts. So far organophosphorus compounds were mostly employed as Lewis bases. The Diels-Alder reaction of different α,β-unsaturated amides (2) with cyclopentadiene was studied. Moderate to excellent yields (41-99%) with catechol-derived catalysts were achieved, producing endo-3 in >97% selectivity when using the (Z)-dienophile. When the (E)-dienophile was used, both yield (31-46%) and selectivity (endo/exo = 1:6) dropped dramatically, giving the exo-product predominantly. Performing the reaction in the absence of catalyst at room temperature for 17 hours gave 82% yield with a selectivity of 1:9.1 (endo/exo).