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Title: New insights into asymmetric hydrogenation using monodentate phosphorus ligands
Author: Jankowski, Piotr
ISNI:       0000 0004 2718 4575
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2011
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The synthesis and coordination chemistry of monodentate phosphorus ligands that span a range of stereoelectronic effects is the topic of this thesis. It is shown that many Rh complexes of the type [Rh(L)2(olefin)]BF4 (L = ligand) exhibit restricted Rh-P bond rotation, which is slow enough to be observed on the NMR timescale. Variable temperature NMR studies of a range of Rh complexes of mono ph os ligands reveal similarities in the observed pattern, suggesting that the M-P restricted rotation is a more general phenomenon of the cis-ligated square planar complexes. OFT calculations and solid state analysis are employed to shed light on the structures of the Cl- and C2-symmetrical rotamers observed in the NMR spectra. The improvement of the separation of enantiomers of optically active 1,3,5,7- tetramethyl-4,6,8-trioxa-2-phospha-adamantane, CgPH is reported, allowing the recovery of the previously inaccessible a-CgPH. Optically active a-CgPH yields a highly selective catalyst in the asymmetric hydrogenation of various olefins, producing ee of up to 95%. Rh complexes of CgPH are extensively studied by NMR techniques and OFT calculations in order to understand the unexpectedly high enantioselectivity obtained with Rh-CgPH catalysts; it is proposed that the enantioselectivity arises from the presence of a Cl-symmetrical rotamer in solution. A series of 3,3' -dimethylphosphonite ligands of a type (binolate)PR (R = Me, iPr, Cy, tBu) are synthesised and their coordination chemistry is investigated. Most of the ligands upon reaction with [Rh(cod)2]BF 4 produce complexes of a type [Rh(L)2(cod)]BF4• The tBu-phosphonite does not follow this reactivity pattern, presumably due to the steric bulk of the ligand, making the incorporation of two ligands in the cis arrangement unfavourable. Of all the above mentioned ligands, the iPr- and Cy-phosphonite are the best catalysts for the hydrogenation of a- dehydroaminoacid derivative substrates, methyl-(Z)-a-acetamidocinnamate (up to 94%) and methyl-a-acetamidoacrylate (up to 97%), but they are less selective for dimethylitaconate (up to 75%). A series of ligands bearing extended aromatic systems are synthesised in order to probe the possibility of the backbone formation using pi-pi interactions in cis square planar complexes. Some circumstantial evidence for the interactions have been observed and the presence of the extended pi systems is shown not to be beneficial on the asymmetric hydrogenation outcome.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available