Transition metal-mediated diastereoselective synthesis of metallophosphinates

Zhou, Zhongxin (1994) Transition metal-mediated diastereoselective synthesis of metallophosphinates. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Transition metal-mediated diastereoselective synthesis is one of the most important and extensively used methods to create chiraiity at carbon and other heteroatom centres. This thesis focuses on chiral induction from an asymmetric, pseudo- octahedral metal centre in the Arbuzov-like dealkylation of prochiral phosphonites. A series of η⁵-cyclopentadienyl, η⁵-pentamethylcyclopentadienyl, and η⁵-indenyl three- legged piano-stool complexes have been studied as chiral auxiliaries. The effects of ligand sphere steric and electronic properties on M*→P optical induction have been examined. -- The synthesis, structure, spectroscopic properties of a series of chiral η⁵-indenyl cobalt complexes are described. X-ray crystal structure and solution conformational analysis showed that all η⁵-indenyl complexes have a propensity to distort away from η⁵ toward η³ coordination, and that the indenyl ring always has certain conformational preferences with respect to the ligands on the metal. Distortion and conformational preferences of indenyl complexes are fully discussed on the basis of extended Huckel molecular orbital (EHMO) calculations, and fragment molecular orbital (FMO) analysis. -- Arbuzov reactions of η⁵-indenyl, η⁵-cyclopentadienyl, and η⁵-pentamethyIcyclopentadienyl cobalt iodide auxiliaries with PR'(OMe)₂ (R' = OMe, Ph) proceed via an ionic mechanism to form the corresponding cobaltophosphonates or phosphinates. Ionic intermediates were observed by ¹H NMR spectroscopy at ambient temperature in most cases and isolated in some cases. -- The following conclusions can be drawn from the results presented in this thesis: (i) The chiral metal center is essential for stereoselectivity, (ii) The steric requirements of the spectator ligands is crucial in determining chiral induction. The bulkier the spectator ligands, the higher the chiral induction. (iii) The stereochemical properties of the entering phosphonites, PR'(OMe)₂ is also important for diastereoselectivity. Large substituent R’ result in high diastereoselectivity. (iv) Intramolecular hydrogen bonding plays a significant role in controlling the stereoselectivity in the Arbuzov reaction by restricting population of solution conformations, maximizing the preference of one diastereomer over another.

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