Research - Organometallics


Organometallic Chemistry Planar Chiral Metallocenes Pincer Complexes Molecular Gears
We have devised a number of methods for the enantioselective synthesis of planar chiral metallocenes, compounds that lack a plane of symmetry due to the presence of two or more different substituents on a metal coordinated cyclopentadienyl ring. Oxazolines, readily generated from amino acids such as (S)-valine, have proven to be excellent auxiliaries to control the diastereoselective lithiation of ferrocene. Both 1,2-disubstituted ferrocenes 1,^1,2 and 1,1’,2,2’-tetrasubstituted ferrocenes 2³ may be isolated, essentially diastereoisomically pure, following introduction of a suitable electrophile.

Oxazolines 3,4 and imidazoles 5 have also been employed in the diastereoselective palladation of bulky cobalt metallocenes.^4-6 A notable and suprising feature of this work was the complete reversal of diastereoselectivity observed on changing the isopropyl substituent of 3⁴ to the tert-butyl substituent of 4,⁵ a consequence of these reactions being under thermodynamic and kinetic control respectively.⁷

The resultant cobalt oxazoline palladacycles have recently been utilised for the first highly enantioselective transcyclopalladation reactions.⁸ Both enantiomers of the resulting phosphapalladacycles 5 are available, the extra methyl group of 4 making all the difference to the enantioselectivity of this two step process.

An alternative approach to the synthesis of planar chiral cobalt metallocenes is to exploit the diastereoselective complexation of chiral linked diynes 6 to give metallocenes 7 and/or 8. The viability of this approach has been demonstrated with the synthesis of the cyclopentadienone complexes 9 and 10.⁹.<sup>8</sup>⁸

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