Pd(II)- and Pt(II)-catalyzed ring-expansion of enantiomerically pure alkylidenecyclopropane derivatives leads to the formation of cyclobutene species with a complete preservation of the stereogenic center.

Enantiomerically enriched cyclopropene derivatives, the smallest possible unsaturated carbocycles, are of great synthetic interest since they serve as versatile reactive building blocks. Their reactivity results from the relief of the ring strain in the small molecule. They can be transformed into a wide variety of complex chiral structures and a special emphasis will be directed towards the preparation of enantiomerically enriched methylene- and alkylidenecyclopropane derivatives. The ready availability of a wide range of these chiral entities now provides an excellent opportunity to discover new and unique transformations that can further enrich mainstream synthetic methodology.

Enantiomerically enriched cyclopropene derivatives, the smallest possible unsaturated carbocycles, are of great synthetic interest since they serve as versatile reactive building blocks. Their reactivity results from the relief of the ring strain in the small molecule. They can be transformed into a wide variety of complex chiral structures and a special emphasis will be directed towards the preparation of enantiomerically enriched methylene- and alkylidenecyclopropane derivatives. The ready availability of a wide range of these chiral entities now provides an excellent opportunity to discover new and unique transformations that can further enrich mainstream synthetic methodology.

With just a nudge (in the form of silica gel, an acidic ion-exchange resin, or heating at about 40 °C), the acetate derivatives of enantiomerically enriched cyclopropenyl alcohols undergo sigmatropic rearrangement to give alkylidenecyclopropanes with high ee  values (see scheme). Similarly, the rearrangement of phosphinite derivatives at room temperature leads to phosphine oxide precursors of unusual chiral phosphine ligands. R¹, R²=alkyl, aryl.

{With just a nudge (in the form of silica gel, an acidic ion-exchange resin, or heating at about 40 °C), the acetate derivatives of enantiomerically enriched cyclopropenyl alcohols undergo sigmatropic rearrangement to give alkylidenecyclopropanes with high ee values (see scheme). Similarly, the rearrangement of phosphinite derivatives at room temperature leads to phosphine oxide precursors of unusual chiral phosphine ligands. R1

The straightforward approach: The copper-catalyzed carbomagnesiation reaction of chiral cyclopropenylcarbinol derivatives, obtained through kinetic resolution of secondary allylic alcohols, leads to the preparation of enantiomerically pure alkylidenecyclopropane derivatives. The reaction mechanism is composed of a syn carbometalation followed by a syn elimination reaction.

The straightforward approach: The copper-catalyzed carbomagnesiation reaction of chiral cyclopropenylcarbinol derivatives, obtained through kinetic resolution of secondary allylic alcohols, leads to the preparation of enantiomerically pure alkylidenecyclopropane derivatives. The reaction mechanism is composed of a syn carbometalation followed by a syn elimination reaction.