Synthesis, Structure and Applications of Cationic Phosphonites

by Leo David Mwenya Nicholls
Doctoral thesis
Date of Examination:2018-10-22
Date of issue:2018-11-07
Advisor:Prof. Dr. Manuel Alcarazo
Referee:Prof. Dr. Lutz Ackermann
Referee:Prof. Dr. Dietmar Stalke
Referee:Prof. Dr. Franc Meyer
Referee:Dr. Max Hansmann
Referee:Dr. Franziska Thomas
Persistent Address: http://dx.doi.org/10.53846/goediss-7124

 

 

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Abstract

English

Phosphorus based ligands are a hugely important class of ligands for homogenous catalysis, owing to the easy fine-tuneability of their steric and electronic properties. In α-cationic phosphines, the incorporation of a cationic charge adjacent to the phosphorus donor atom renders the resulting ligand an excellent π-acceptor and is thus ideal for applications of corresponding metal complexes in catalysis where an electron deficient metal center has a beneficial effect on the rate determining step. The Au(I) catalysed intramolecular hydroarylation to access phenanthrenes is one such reaction and benefits greatly from the use of cationic phosphines as ligands. In this thesis the synthesis of cationic phosphonites is described. These are excellent π-acceptor ligands owing to the cumulative electron withdrawing effect of two oxo- and one cationic substituent. A variety of derivatives of this ligand class are synthesised, encompassing different chiral backbones as well as different cationic substituents. The structure and donor properties are studied and the corresponding Au(I) complexes are evaluated in enantioselective hydroarylation reactions to afford [6]carbohelicenes and naturally occurring biphenanthrenes. In the synthesis of [6]carbohelicenes, careful tailoring of the cationic substituent enabled a highly selective method. Specifically, 1,2,3 and 1,2,4-triazolium substituents were found to be highly reactive, in addition to affording a variety of [6]carbohelicenes in excellent enantio- and regiocontrol.
Keywords: Ligand design; Gold catalysis; Asymmetric catalysis; Carbohelicenes; Biphenanthrenes
 
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