A range of air- and moisture-stable phosphonium salts was prepared. Compounds were isolated in high yield and fully characterized. The properties of these compounds and the nature of their formation were explored. The phosphonium salts react with base to give phosphino-aldehydes which are important building blocks in the synthesis of PNNP ligands. The condensation reaction between phosphino-aldehydes and a diamine usually employed for the preparation of PNNP ligands was not applicable to the phosphino-aldehydes derived from these phosphonium salts as a result of the high reactivity of the nucleophilic phosphorus causing uncontrollable side-reaction.
In order to resolve this problem, a template reaction with iron(II) Lewis acid was used to suppress the reactivity of the phosphorus via coordination. The reaction was successful and gave rise to bis-tridentate complexes with PNN ligands ([Fe(Ph2PCH2CH=N---NH2)2][BPh4]2, where N---NH2 depends on diamine used) as the kinetic product and to desired tetradentate complexes with PNNP ligands (trans-[Fe(Ph2PCH2CH=N---N=CHCH2PPh2)(CH3CN)2][BPh4]2, where N---N depends on diamine used) as a thermodynamic product of the reaction. The reaction appeared to be very general; complexes
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with various diamines incorporated in the ligand backbone were prepared in high yield and fully characterized.
Mono-carbonylation reaction of the complexes containing tetradentate PNNP ligands resulted in the formation of the precatalysts with a general formula (trans-[Fe(Ph2PCH2CH=N---N=CHCH2PPh2)(CO)(Br)][BPh4]. These precatalysts give active (TOF up to 28000 h-1) and enantioselective (up to 95 % ee) catalytic systems for the ATH of ketones when activated with base in a solution of 2-propanol as the reducing agent.
On the basis of a kinetic study and other evidence, we propose a mechanism of activation and operation of the catalytic system involving the precatalyst trans-[Fe(CO)(Br)(Ph2CH2CH=N-((S,S)-C(Ph)H-C(Ph)H)-N=CHCH2PPh2)][BPh4] and acetophenone as a model substrate. We determined that the activation of the precatalyst to the active species involves the stereoselective reduction of one imine group of the ligand, since when the active species are quenched with acid, the complex trans-[Fe(CO)(Cl)(Ph2CH2CH-(H)N-((S,S)-C(Ph)H-C(Ph)H)-N=CHCH2PPh2)][BPh4] containing amine and imine functionalities in the backbone is produced.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/36292 |
| Date | 16 August 2013 |
| Creators | Mikhailine, Alexandre |
| Contributors | Morris, Robert |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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