The zirconocene-mediated synthesis of amines

Probert, Gareth David (1997) The zirconocene-mediated synthesis of amines. University of Southampton, Doctoral Thesis.

Abstract

The recent appearance of zirconium in organic synthesis is discussed. The formation and elaboration of Group (IV) metallacycles is reviewed, with emphasis on the formation of zirconocene η²-imine complexes. An introduction to the use of silicon as a removable molecular tether, and its oxidative removal to give a hydroxy group is given.

The zirconocene(1-butene) cyclisation of 1, n-dienes and enynes is used to prepare a number of different zirconacycles which react with trimethylsilylcyanide to afford zirconocene iminoacyl complexes. A rearrangement to zirconocene η²-imine complexes occurs, and trapping with a range of unsaturated species gives functionalised cyclic amines on work up. The highly functionalised amines are produced in good overall yield from the starting materials in a one-pot transformation. Zirconaindanes were prepared from aryl zirconocenes via formation of zirconocene benzyne complexes through a C-H activation process and trapping with alkenes, and used in the trimethylsilylcyanide insertion reactions. Substituted alkynes insert regiospecifically into the zirconocene η²-imine complexes, giving allylic amines. Alkenes also insert, but show a lower reactivity. Isocyanates insert readily into zirconocene η²-imine complexes to give α-aminoamides on work up, and ketones insert to give 1,2-aminoalcohols.

The insertions of isocyanates into zirconocene η²-imine complexes formed using a C-H activation route give a range of α-aminoamides.

A range of 1, n-dienes and -enynes containing a silicon in the linking chain were prepared and successfully cyclised using zirconocene(1-butene). Intramolecular trapping os a zirconocene benzyne complex (generated by a C-H activation route) with a silicon tethered alkene was also achieved. Protic work up followed by oxidative cleavage of the silyl group using the Tamao oxidation gives hydroxyl containing products. Further elaboration of the intermediate zirconacycles before protonation/oxidation gave a range of highly functionalised cyclic and acyclic products in good overall yield. This is the first example of the use of a cleavable silicon tether in early transition metal-mediated cyclisations.

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