Carbenoid insertion into organozirconium species'

Abstract

Zirconium-mediated synthesis of mono- and bi- cyclic zirconacyclopent-anes, -enes and - adienes is well established and the potential for their elaboration has begun to be realised in recent years by carbenoid insertion into carbon-zirconium bonds. Synthetic methods and the history of carbenoid insertion into organozirconocenes, including zirconacycles, is reviewed herein. The novel work in this thesis is then presented in three sections. Firstly, regioselective co-cyclisation between 1-stannyl, 1-thio or 1-silyl substituted 1-propynes and zirconocene- ethene has furnished monocyclic zirconacyclopentenes with a-substituents. Subsequent insertion of allyl carbenoids into the sp^C-Zr bond leads to cyclic zirconocene ri^-allyl species, reaction of which with aldehydes constitutes a concise route to homoallylic alcohols. The nature of the a-substitution is found to influence geometry about the alkene introduced and zirconacyclopentenes to the synthesis of naturally occurring terpenoids has been made. 9- Hydroxyfamesoic acid, 9-hydroxyfamesol, crinitol and 9-hydroxysargaquinone, all of which contain (E)-homoallylic alcohol functionality, are synthesised via this one-pot protocol followed by Stille coupling methods for functional group interconversion at the alkenyl stannyl group. In the same way, a precursor compound to p-D-galactose and - altrose linked sesquiterpenoids Moritoside and Euplexide A has been furnished. A second area, carbenoids into zirconacycles, demonstrates synthesis of zirconacyclohexenes bearing an a-exocyclic alkene by 1-chloro-l-lithio enyne insertion into five-membered zirconacycles. Further ring-expansion via isocyanide insertion yields a zirconacycloheptene-iminoacyl complex in a multi-component synthesis of a, P-unsaturated aldehydes and imines. Finally, investigation of the regiochemistry of carbenoid insertion into non-symmetrical zirconacycles required the preparation of an a-ethyl substituted zirconacyclopentane, found to arise from either (E) or (Z) co-cyclisation precursor alkenes via P-H elimination and re-addition to give a five- and not six-membered zirconacycle. Ring junction stereochemistry of P-methyl substituted zirconacyclopentanes can also be predicted. The regiochemistry of insertion of 1-chlor-l-lithio enynes, 1 -chloro-1 -lithio-silanes and 1- chloro-l-lithio ethers into these a-alkyl and P-alkyl zirconacyclopentanes is found to be consistent, taking place exclusively or predominantly into the a-cyclohexyl and P-methyl substituted side of the zirconacycle and into the C-Zr bond opposite to an cc-ethyl substituent. Discussion of the mechanism of carbenoid insertion in terms of nucleophilic attack upon zirconium and interconversion of regioisomeric zirconate complexes is made.

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