A computational mechanistic investigation of calcium catalysed hydrophosphination

• Main Author: Ward, Bryan James
• Other Authors: Hunt, Patricia
• Published: Imperial College London 2015
• Subjects: 547
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.702790

The calcium-catalysed intermolecular hydrophosphination reaction is an atom efficient synthetic route to organophosphines. Early experimental studies indicated a correspondence between the hydroamination and hydrophosphination reactions. However, we recognise a need for a theoretical investigation of the intermolecular hydrophosphination reaction particularly as many aspects of the mechanism remain poorly understood. The work within this thesis describes a theoretical investigation of the intermolecular hydrophosphination reaction catalysed by the [beta]-diketiminate stabilised Ca complex, [{HC(C(Me)N-2,6-iPr2C6H3)2}Ca{PPh2}(THF)]. Preliminary DFT calculations were performed on a simplified model of the experimental system. The intermolecular hydrophosphination of ethene is a stepwise mechanism involving alkene insertion and protonolysis, consistent with experimental results. Calculations demonstrate the importance of secondary stabilising interactions, particularly the Ca–[pi] interaction, which coordinates preferentially over a Ca–P interaction. Calculations involving the substitution of ethene for styrene and vinylpyridine were also performed (Chapter 4). Stabilisation of the alkylphosphine fragment through the arene ring allowed for considerably more interchange between important secondary bonding interactions. During the styrene hydrophosphination reaction the kinetically most favourable protonolysis pathway proceeded from an alkylphosphine fragment that exhibited no secondary stabilising interaction. Such a coordination mode has not been considered previously in previous hydroamination reaction studies. Calculations were also performed to investigate the hydrophosphination reaction mechanism for an experimentally representative catalyst (Chapter 5). A ligand-based proton-assisted mechanism was identified involving a multi-centre transition state. In the ligand-based mechanism the alkylphosphine product was generated alongside the active catalyst in a single step. The single step, ligand-based mechanism was a kinetically more favourable pathway and thermodynamically representative of experiment. To the best of the author’s knowledge, this is the first example of the Ae catalysed heterofunctionalisation reaction, which demonstrates preference for a ligand-based mechanism.