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Towards transition metal-catalyzed hydration of olefins, aquo ions, and pyridylphosphine-platinum and palladium complexes

University of British Columbia

This thesis work resulted from an on-going project in this laboratory focusing on the hydration of olefins, using transition metal complexes as catalysts, with the ultimate aim of achieving catalytic asymmetric hydration, for example: (HO₂C)CH=CH(CO₂H) → (HO₂C)CH₂-CH(OH)(CO₂H) (C = chiral carbon atom). Initially, the hydration of maleic to malic acid, catalyzed by Cr(H₂O)₆³⁺ at 100°C in aqueous solution was studied, including the kinetic dependences on Cr³⁺, maleic acid and pH. A proposed mechanism involving 1:1 complexes of Cr³⁺ with the maleato and malato monoanions is consistent qualitatively with the kinetic data. This Cr system was, however, ineffective for hydration of prochiral olefins, and the work became a minor component of the thesis and is described in the last chapter. Emphasis was switched to the study of water-soluble phosphine systems based on Pd and Pt. The major part of this thesis describes the synthesis and characterization, principally by ¹H, ³¹P{¹H} and ¹⁹⁵Pt{¹H} NMR spectroscopies, of: square-planar complexes of the type MX₂(PPh₃₋npyn)₂ (M = Pd, Pt; X = halides; n = 1, 2, 3); the binuclear species M₂X₂(µ-PPh₃₋npyn)₂ (head-to-tail, HT) and Pt₂I₂ (µ-PPh₃₋pyn)₂ (head-to-head, HH; n = 1,10a, n = 2, 10b and n = 3, 10c); and the Pt(PPh₂py)₃,27a, and Pt(Ppy₃)₃, 26c, complexes. The reactivities of the binuclear complexes toward acetylenes, and the Pt(0) species toward O₂, olefins, HCl and MeI, are also described. With use of PPhpy₂ within the binuclear phosphine-bridged species, the P atom incidentally becomes chiral. The diastereomers of 10b were isolated and characterized by ³¹P{¹H} NMR spectral data. All the isolated binuclear complexes react in CH₂Cl₂ with dimethylacetylene-dicarboxylate, DMAD, to form an A-frame insertion product. The HH or HT configuration of the precursor is maintained in every case except for 10b and 10c which form initially an HH-DMAD adduct that slowly isomerizes to the corresponding HT-DMAD adduct. Detailed ³¹P{¹H} NMR spectroscopic studies show that the presence of a properly positioned pyridyl group promotes the isomerization by forming a detectable chelated P-N intermediate, and that insertion of DMAD precedes chelation. The reactions of Pt₂l₂ (u-PPh₃₋npyn) ₂ (HH) (n = 1, 2, 3) with DMAD in CH₂CI₂ are kinetically first-order in both [Pt₂] and [DMAD] for the insertion step, and first-order in [Pt₂] and zero-order in [DMAD] for the isomerization step. The activation parameters for the insertion step are consistent with oxidative addition to a binuclear system. A proposed mechanism is fully supported by ³¹P{¹H) and ¹⁹⁵Pt{¹H] NMR spectral data. Complex 26c, reacts in CH₂CI₂ or CDCI₃ with limited oxygen to give Pt(Ppy₃)₃(O₂), which may contain an end-on superoxo structure as judged by an IR band at 1114 cm⁻¹. Complex 26c, under 1 atm O2, forms the 'expected' peroxo species Pt(Ppy₃)₂O₂. Complexes 26c and 27a, react with the olefins (maleic anhydride, acrylonitrile, methacrylonitrile and crotonitrile) to give the square-planar species Pt(PPh₃₋npyn) ₂(ɳ²-olefin). The square-planar geometry infers strong Π-back donation from metal to olefin, a state which is probably undesirable for the purpose of olefin activation toward hydration. Indeed, complex Pt(PPh₂py) ₂(Π²-maleic anhydride), 47a, shows no olefin hydration product when heated at 80°C in aqueous NaOH solution. Trans-Pt(H)Cl(PPh₂py) ₂, 50a, was prepared from 27a and gaseous HCl in THF; 50a in acetone-d6, reacts with acrylonitrile to give cis-PtCl(CH₂CH₂CN)(PPh₂py)₂, but in the presence of aqueous NaOH at 80°C, 50a was inactive for hydration of acrylonitrile to either β-cyanoethanol or acrylamide.

Text

2011-02-03

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http://hdl.handle.net/2429/31034

Chemistry

University of British Columbia

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