The use of the n2-aldehyde, ketone and benzyne complexes of Ti and Zr in organic synthesis

The chemistry of the η2-aldehyde, ketone and benzyne complexes of the early transition metals is reviewed. An intramolecular cocyclisation of unsaturated aldehydes and ketones mediated by the low valent titanium complex [Cp2Ti(PMe3)2] to produce the novel 2-bis(cyclopentadienyl)-1-oxa-2-titanabicyclo[3.3.0]octane and oct-3-ene complexes has been investigated. Elaboration of these complexes with electrophilic reagents was examined as a potential synthetic method for the production of highly functionalised cyclopentanes. The novel zirconocene naphthalyne complexes were found to carbometallate unactivated alkenes and alkynes; when the naphthalyne complexes were trapped in situ with alkenes poor regioselectivity was observed, whilst the corresponding carbometallation of alkynes failed totally. When [PMe3] stabilised zirconocene naphthalyne complexes were treated with alkenes and alkynes good regiocontrol was observed. The synthesis of the A,B,C,D rings of aromatic steroids utilising the [PMe3] stabilised zirconocene naphthalyne complexes was examined; the B,C,D rings of aromatic steroids could also be synthesised using zirconocene benzyne complexes derived from 1-bromoanisole.