On the NHC/DBU-Mediated Tosyl Group Transfer to Allenes: Is There an Organocatalytic Role for DBU?

Abstract

The reactions of allenylphosphonates/allenylphosphine oxides or an allenyl sulfone [4-Cl-C6H4-SO2C(R)=C=CMe2] with the aldimine PhCH=NTs in the presence of a carbene [NHC·HCl/DBU] afford tosylated allylic phosphonates/phosphine oxides or a sulfone. This reaction involves C-S bond formation and not C-O bond formation, and thus is different from similar reactions of allenes with PhCH=NTs in the presence of bases like DMAP, DBU or Ph3P/(alkyl)3P wherein cycloaddition products are generally observed. The reactions of phosphorylated allenes with DBU lead to the unprecedented formation of products in which the DBU ring is cleaved, as evidenced by single crystal X-ray crystallography. This observation suggests a ‘reactant’ role for DBU when using this base in the reactions of allenes.