Applications of zirconium chemistry to organic synthesis

Tuckett, Mark William (1999) Applications of zirconium chemistry to organic synthesis. University of Southampton, Doctoral Thesis.

Abstract

Zirconocene mediated coupling of alkenes and alkynes has allowed access to a range of mon- and bi-cyclic zirconacyclo-pentanes, -pentenes and -pentadienes. The two carbon-zirconium bonds are relatively inert to conventional electrophiles. The productive elaboration methods which have overcome this limitation are reviewed in detail.

The reaction of zirconabicyclopentanes with an allyl lithium halo carbenoid resulting in an allyl zirconium complex which reacted with imminium salts or Lewis acid activated aldehydes was examined. A second electrophile: elemental iodine reacted with the other carbon zirconium bond giving a 5-component coupling sequence. The resulting homo-allylic amines and alcohols were eliminated to give stereodefined dienes. The halogen-carbon bond was elaborated to give an activated alkene or alkyne and an intramolecular Diels-Alder reaction used to give 5,6,6-; 6,6,6-; 5,7,7- and 6,7,6-tricyclic compounds including the key skeletons of the natural products Dolastane and Pisiferanol.

Three routes to the natural product 12-deoxypisiferanol were examined, including one utilising the novel zirconium mediated cyclisation of a methylene cyclopropane containing enyne.

The regiochemistry of insertion of allyl carbenoids into asymmetric zirconacyclo-pentanes and -pentenes was investigated and found to be highly chemoselective though not readily explicable in terms of sterics or electronics.

The insertion of the carbenoid derived from 2-phenyl allyl chloride into a zirconacyclopentane was investigated. The resulting allyl zirconium complex was found to be more reactive than the corresponding allyl complexes without the phenyl group, reacting with aldehydes without Lewis acid activation for the first time and with ketones, both with notable degrees of 1,6-diastereocontrol.

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