Dedicated to Prof. Oleg Kulinkovich on the occasion of his 70th birthday
Asymmetric Kulinkovich cyclopropanation of carboxylic esters with prochiral alkenes
is reported. The process is mediated by titanium(IV) (4R,5R)-TADDOLate complexes and affords correspondingly (Z)- or (E)-cyclopropanols with up to 84–87% ee in the event of intra- or intermolecular olefin
ligand exchange in intermediate titanacyclopropane [titanium(II)-alkene] species.
Configuration of the olefin double bond is preserved in the cyclopropane products,
pointing out on total retention of configuration at Ti–C bond in the cyclopropane
forming step. The results have been interpreted in the framework of ate complex mechanism,
suggesting formation of pentacoordinated titanium ate species as a prerequisite of
high enantiocontrol.
Key words
titanium - Kulinkovich reaction - cyclopropanation - esters - cyclopropanols - asymmetric
synthesis - reaction mechanisms