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Asymmetric Ligands Derived from Carbohydrates

This thesis describes the incorporation of carbohydrate groups in thiourea, 1, or iminophosphorane, 2, ligands. Several carbohydrate acyl thioureas, 3, were synthesised from the reaction of protected and unprotected D‐glucosamine with acyl isothiocyanates, RCONCS. Internal hydrogen‐bonding forms a planar six‐membered ring, which locks the thiourea into an Z,E,Z‐anti conformation as shown by 1H NMR spectroscopy and confirmed by X‐ray structure determination of two examples. As neutral ligands to Rh(III), Rh(I), Ru(II), Pd(II), Pt(II) and Au(I) they bond through the sulfur atom with retention of the six‐membered ring and the Z,E,Z‐anti conformation. This was confirmed by two X‐ray structure determinations on Rh(III) and Pd(II) examples which also showed the new hydrogen‐bond formed by HN2 to a halide on the metal. Deprotonation of the ligands 3 with NaOAc gave anionic species which formed bidentate chelating complexes with metals. When attached to Rh(III), Ru(II) or with two thiourea bonding to Pd(II) it forms a four‐membered ring through the S and N2 atoms which retains the hydrogen‐bond. If Rh(I) or (C,Ndimethylbenzylamine‐ Pd) is used a six‐membered ring is formed through the S and O atoms which disrupts the hydrogen‐bond. This disruption of the hydrogenbond is apparent from the chemical shift of HN1. For examples where coordination led to a chiral metal complex (e.g. for Cp*RhCl(N,S‐thiourea)) NMR measurements showed that the natural chirality of the ligand did not provide any selectivity with equal proportions of the two diastereoisomers formed. Iminophosphoranes were produced by the reaction of protected sugar azides with phosphines via the Staudinger reaction. Cyclometalated complexes, 4, could not be formed directly which an X‐ray structure determination suggested was because of steric crowding of the nitrogen. An indirect transmetalation route was developed where the sugar azides were reacted with the mercurated diphosphine, Hg(2‐C₆H₄PPh₂)₂, and the resulting iminophosphoranes transmetalated with [NMe₄][AuCl₄] to produce cyclometalated Au(III) dichloride complexes. Two X‐ray structure determinations showed that the five‐membered metalocyclic ring was in an envelope conformation. The chlorides were labile and able to be displaced by PPh3 and thiosalicylic acid. A series of N‐sugar and N‐phenyl iminophosphorane Au(III) complexes were shown to catalyse the addition of 2‐methyl furan to methyl vinyl ketone.
Type of thesis
Jarman, B. P. (2011). Asymmetric Ligands Derived from Carbohydrates (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/5620
University of Waikato
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