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Title: The regioselective hydrogenation of acrylic acids using rhodium-mixed anhydride catalyst precursors
Author: Fairfax, Neil Robert
Awarding Body: University of St Andrews
Current Institution: University of St Andrews
Date of Award: 1993
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Complexes of general composition [RhCl(PPh3)n(Ph2P02CCR = CR'R'')] (n=2 R = H, R' =R" =Me; R = R' =H, R'' =CH = CHMe; R = Me, R' =H, R'' =Ph; n=l, R = R'=H, R'' =Me or CH = CHMe) have been used as precursors for the catalytic hydrogenation of various substituted acrylic acids, [HO2CCR = CR' R'']. The reactions of [RhCl(PPh3)(Ph2P02CCH = CHMe)] and '[02CCH = CHMe] and [RhCl(PPh3)2(Ph2P02CCMe = CHPh)] and '[02CCIVIe = CHPh] give [Rh(02CCH = CHMe)(PPh3)(Ph2P02CCH = CHMe)] and [Rh(02CCMe = CHPh)(PPh2)(Ph2P02CCMe = CHPh)] accordingly and these complexes are the respective active species in the catalytic hydrogenation of but-2- enoic acid and 2-methyl-3-phenylpropenoic acid. The crossed reactions between [RhCl(PPh3)(Ph2P02CCH = CHMe)] and [02CCMe = CHPh], [RhCl(PPh3)2(Ph2P02CCMe = CHPh)] and [02CCH = CHMe], [RhCl(PPh3)(Ph2P02CCH = CHMe)] and '[09CCH = CMe2] and [RhCl(PPh3)2(Ph2P02CCH = CMe2)] and '[02CCH = CHMe] predominantly yield products in which the chelate mixed anhydride ligand is but-2-enoic acid derived, on account of its lesser substituted carbon-carbon double bond. Attempts at isolating hydride intermediates in the catalytic cycle have proved unsuccessful. However, investigations of the step by which the product carboxylic acid anion is released from the metal complex (transesterification reaction) have involved the synthesis of [RhCl(PPh2)2(Ph2P07CCH2CH2Me)] (2 isomers) from [RhCl(PPh3)3] and [Ph2PO2CCH2CH2Me] and [Rh(PPh3)2(Ph2P02CCH2CH2Me)]+ from [RhCl(PPh3)2(Ph2P02CCH2CH2Me)] and TlPFg. The reactions of both these products with '[02CCH = CHMe] yield the active species [Rh(02CCH = CHMe)(PPh3)(Ph2P02CCH = CHMe)]. Prior to transesterification the most likely intermediate in the catalytic cycle is [Rh(02CCH2CH2Me)(PPh3)n(Ph2P02CCH2CH2Me)] (n = l or 2) in which both the mixed anhydride and the coordinated anion have been hydrogenated. The reaction of [RhCl(PPh3)2(Ph2P02CCH2CH9CH3)] with K[02CCH2CH2CH3)] affords the fluxional, square-planar species, [Rh(02CCH2CH2CH3)(PPh3)2(Ph2P02CCH2CH2CH3)]. The hydrogenation of hexa-4-dienoic acid, [H02CCH = CHCH = CHMe], using rhodium-mixed anhydride complexes yields both hexanoic and hex-4-enoic acids via two concurrent mechanisms, since the active species, [Rh(02CCH = CHCH = CHMe)(PPh3)(Ph2P02CCH = CHCH = CHMe)] exists in two forms; one containing a chelate mixed anhydride, the other a chelate hexa-4- dieonate ligand. The production of hexanoic acid is brought about by the active involvement of the chelate hexa-4-dienoate ligand in the catalytic cycle. Attempts to prevent this involvement have centred around the replacement of the chloride ligand in the catalyst precursor with non-labile fluoride and trifluoromethyl anions. [RhF(PPh2)3l reacts with mixed anhydride ligands to give 5-coordinate species, [RhF(PPh2)2(Ph2P02CCR = CR' R' ' )], in which the mixed anhydride is bound via phosphorus and carbonyl oxygen, however these precursors show no increased regioselectivity. The reaction of [RhCl(PPh3)2] with [Hg(CF3)2] does not yield [RhCF2(PPh2)3] but [RhCl(PF2)(PPh3)2], and in the presence of excess PPh3 yields [RhCl(CF3)2(PPh3)2]. Other attempts to prevent the involvement of the bidentate hexa-4-dienoate ligand in the catalytic cycle have involved the addition of PPh3 or CH3CO2H to the catalytic solution, and to a certain degree increased regioselectivity towards the production of hex-4-enoic acid has been achieved. NOTE: All 31P NMR spectra presented, have their chemical shifts quoted relative to H3PO4, whilst all 19F NMR shifts are relative to CCI3F.
Supervisor: Cole-Hamilton, David John Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD341.A2F2 ; Acids