The study of silicates and refractory materials at high pressures and temperatures

To better understand high pressure behavior of solids, both silicates and oxides have been investigated to clarify the high pressure melting, phase transformations and thermal parameters as well as their size dependences, both theoretically and experimentally. ^ To judge the precision of data determined experimentally, the reliabilities of different high pressure techniques have been discussed. A thermodynamic model has been developed and demonstrated to be able to closely reproduce the melting of solids by comparison between results calculated and data obtained experimentally, including metals (Al, Ni and Pt), Silicates (Mg3Al 2Si3O12 and CaMgSi2O6), Halides (NaCl, CsCl and LiF) and Oxides (MgO, FeO and Al2O3). The melting data obtained have been discussed to address the dynamics of the Earth's interior. ^ Results obtained with Raman spectroscopy and x-ray diffraction show that solids including silicates (andradite and pyrope) and oxides (CeO2 and TiO2) undergo a series of pressure-induced phase transformations. The effects of particle size under high pressures have been investigated. The results obtained indicate that the reduction of particle size leads to the enhancement of the bulk modulus and a significant decrease of transition pressure in TiO2 (rutile) and CeO2. The pressure-induced amorphization in anatase also results from the size effects. ^ Combining the data obtained with global seismic tomography, the physics and chemistry of the Earth's mantle and the dynamics of the core-mantle interaction have been discussed. The high pressure phases of Al3+- and Fe3+-bearing minerals play important roles in the dynamics of the lower mantle. ^

Investigation of Mantle Dynamics from Platinum Group Elements and Rhenium-Osmium Isotope Systematics of Mantle Xenoliths from Oahu, Hawaii

Intraplate volcanism that has created the Hawaiian-Emperor seamount chain is generally thought to be formed by a deep-seated mantle plume. While the idea of a Hawaiian plume has not met with substantial opposition, whether or not the Hawaiian plume shows any geochemical signal of receiving materials from the Earth’s Outer Core and how the plume may or may not be reacting with the overriding lithosphere remain debatable issues. In an effort to understand how the Hawaiian plume works I report on the first in-situ sulfides and bulk rock Platinum Group Element (PGE) concentrations, together with Os isotope ratios on well-characterized garnet pyroxenite xenoliths from the island of Oahu in Hawaii. The sulfides are Fe-Ni Monosulfide Solid Solution and show fractionated PGE patterns. Based on the major elements, Platinum Group Elements and experimental data I interpret the Hawaiian sulfides as an immiscible melt that separated from a melt similar to the Honolulu Volcanics (HV) alkali lavas at a pressure-temperature condition of 1530 ± 100OC and 3.1±0.6 GPa., i.e. near the base or slightly below the Pacific lithosphere. The 187Os/188Os ratios of the bulk rock vary from subchondritic to suprachondritic (0.123-0.164); and the 187Os/188Os ratio strongly correlates with major element, High Field Strength Element (HFSE), Rare Earth Element (REE) and PGE abundances. These correlations strongly suggest that PGE concentrations and Os isotope ratios reflect primary mantle processes. I interpret these correlations as the result of melt-mantle reaction at the base of the lithosphere: I suggest that the parental melt that crystallized the pyroxenites selectively picked up radiogenic Os from the grain boundary sulfides, while percolating through the Pacific lithosphere. Thus the sampled pyroxenites essentially represent crystallized melts from different stages of this melt-mantle reaction process at the base of the lithosphere. I further show that the relatively low Pt/Re ratios of the Hawaiian sulfides and the bulk rock pyroxenites suggest that, upon ageing, such pyroxenites plus their sulfides cannot generate the coupled 186Os- 187Os isotope enrichments observed in Hawaiian lavas. Therefore, recycling of mantle sulfides of pyroxenitic parentage is unlikely to explain the enriched Pt-Re-Os isotope systematics of plume-derived lavas.

Stereoselective synthesis of vinyl germanes and silanes. Applications of tris(trimethyl)germanes and silanes in Pd-catalyzed cross -coupling reactions

Although group 14 organometallic compounds (Si, Sn) have been well developed as transmetallation reagents in cross-coupling reactions, the application of organogermanium compounds as cross-coupling reagents is still a relatively new area with few papers published. This study aimed to develop methods for the synthesis of new classes of vinyl germane and vinyl silane compounds, mainly Z and E tris(trimethylsilyl)germanes and silanes, which were then applied to Pd-catalyzed cross-couplings with aryl and alkenyl halides. The stereoselective radical-mediated desulfonylation of vinyl sulfones with tris(trimethyl)germanium or silane hydrides provided access to the synthesis of trans vinyl germanes or silanes. Alternatively hydrogermylation or hydrosilylation of terminal alkynes gave cis vinyl germanes or silanes. The application of these new classes of organometallic compounds in cross-coupling reactions with various aryl and alkenyl halides under aqueous [NaOH/H2O2/Pd(PPh 3)4] and anhydrous [KH/t-BuOOH/Pd(PPh 3)4] oxidative conditions were investigated. ^ It was found that the vinyl tris(trimethylsilyl)germanes successfully underwent Pd-catalyzed cross-couplings with aryl and alkenyl halides and aryl triflates under aqueous and anhydrous oxidative conditions. These procedures provided examples of "ligand-free" Pd-catalyzed coupling of organogermanes with aryl and alkenyl halides. Interestingly, couplings with fluorinated vinyl germanes appeared to occur more easily than with the corresponding (α-fluoro)vinyl stannanes and silanes since neither addition of an extra ligand nor activation with fluoride was necessary. The vinyl tris(trimethyl)silanes were found to be alternative substrates for the Hiyama reaction. The coupling of TTMS-silanes with various aryl, heteroaryl as well as alkenyl halides proceeded smoothly upon treatment with hydrogen peroxide in the presence of sodium hydroxide and fluoride ion. ^

Novel approaches for the synthesis of C-5 modified pyrimidine nucleosides

The antiviral or anticancer activities of C-5 modified pyrimidine nucleoside analogues validate the need for the development of their syntheses. In the first half of this dissertation, I explore the Pd-catalyzed cross-coupling reaction of allylphenylgermanes with aryl halides in the presence of SbF 5/TBAF to give various biaryls by transferring multiple phenyl groups, which has also been applied to the 5-halo pyrimidine nucleosides for the synthesis of 5-aryl derivatives. To avoid the use of organometallic reagents, I developed Pd-catalyzed direct arylation of 5-halo pyrimidine nucleosides. It was discovered that 5-aryl pyrimidine nucleosides could be synthesized by Pd-catalyzed direct arylation of N3-free 5-halo uracil and uracil nucleosides with simple arenes or heteroaromatics in the presence of TBAF within 1 h. Both N3-protected and N3-free uracil and uracil nucleosides could undergo base-promoted Pd-catalyzed direct arylation, but only with electron rich heteroaromatics. ^ In the second half of this dissertation, 5-acetylenic uracil and uracil nucleosides have been employed to investigate the hydrogermylation, hydrosulfonylation as well as hydroazidation for the synthesis of various functionalized 5-vinyl pyrimidine nucleosides. Hydrogermylation of 5-alkynyl uracil analogues with trialkylgermane or tris(trimethylsilyl)germane hydride gave the corresponding vinyl trialkylgermane, or tris(trimethylsilyl)germane uracil derivatives. During the hydrogermylation with triphenylgermane, besides the vinyl triphenylgermane uracil derivatives, 5-[2-(triphenylgermyl)acetyl]uracil was also isolated and characterized and the origin of the acetyl oxygen was clarified. Tris(trimethylsilyl)germane uracil derivatives were coupled to aryl halides but with decent yield. Iron-mediated regio- and stereoselective hydrosulfonylation of the 5-ethynyl pyrimidine analogues with sulfonyl chloride or sulfonyl hydrazine to give 5-(1-halo-2-tosyl)vinyluracil nucleoside derivatives has been developed. Nucleophilic substitution of the 5-(β-halovinyl)sulfonyl nucleosides with various nucleophiles have been performed to give highly functionalized 5-vinyl pyrimidine nucleosides via the addition-elimination mechanism. The 5-(β-keto)sulfonyluracil derivative has also been synthesized via the aerobic difunctionalization of 5-ethynyluracil analogue with sulfinic acid in the presence of catalytic amount of pyridine. Silver catalyzed hydroazidation of protected 2'-deoxy-5-ethynyluridine with TMSN3 in the presence of catalytic amount of water to give 5-(α-azidovinyl)uracil nucleoside derivatives was developed. Strain promoted Click reaction of the 5-(α-azidovinyl)uracil with cyclooctyne provide the corresponding fully conjugated triazole product.^

Novel Approaches for the Synthesis of C-5 Modified Pyrimidine Nucleosides

The antiviral or anticancer activities of C-5 modified pyrimidine nucleoside analogues validate the need for the development of their syntheses. In the first half of this dissertation, I explore the Pd-catalyzed cross-coupling reaction of allylphenylgermanes with aryl halides in the presence of SbF5/TBAF to give various biaryls by transferring multiple phenyl groups, which has also been applied to the 5-halo pyrimidine nucleosides for the synthesis of 5-aryl derivatives. To avoid the use of organometallic reagents, I developed Pd-catalyzed direct arylation of 5-halo pyrimidine nucleosides. It was discovered that 5-aryl pyrimidine nucleosides could be synthesized by Pd-catalyzed direct arylation of N3-free 5-halo uracil and uracil nucleosides with simple arenes or heteroaromatics in the presence of TBAF within 1 h. Both N3-protected and N3-free uracil and uracil nucleosides could undergo base-promoted Pd-catalyzed direct arylation, but only with electron rich heteroaromatics. In the second half of this dissertation, 5-acetylenic uracil and uracil nucleosides have been employed to investigate the hydrogermylation, hydrosulfonylation as well as hydroazidation for the synthesis of various functionalized 5-vinyl pyrimidine nucleosides. Hydrogermylation of 5-alkynyl uracil analogues with trialkylgermane or tris(trimethylsilyl)germane hydride gave the corresponding vinyl trialkylgermane, or tris(trimethylsilyl)germane uracil derivatives. During the hydrogermylation with triphenylgermane, besides the vinyl triphenylgermane uracil derivatives, 5-[2-(triphenylgermyl)acetyl]uracil was also isolated and characterized and the origin of the acetyl oxygen was clarified. Tris(trimethylsilyl)germane uracil derivatives were coupled to aryl halides but with decent yield. Iron-mediated regio- and stereoselective hydrosulfonylation of the 5-ethynyl pyrimidine analogues with sulfonyl chloride or sulfonyl hydrazine to give 5-(1-halo-2-tosyl)vinyluracil nucleoside derivatives has been developed. Nucleophilic substitution of the 5-(β-halovinyl)sulfonyl nucleosides with various nucleophiles have been performed to give highly functionalized 5-vinyl pyrimidine nucleosides via the addition-elimination mechanism. The 5-(β-keto)sulfonyluracil derivative has also been synthesized via the aerobic difunctionalization of 5-ethynyluracil analogue with sulfinic acid in the presence of catalytic amount of pyridine. Silver catalyzed hydroazidation of protected 2'-deoxy-5-ethynyluridine with TMSN3 in the presence of catalytic amount of water to give 5-(α-azidovinyl)uracil nucleoside derivatives was developed. Strain promoted Click reaction of the 5-(α-azidovinyl)uracil with cyclooctyne provide the corresponding fully conjugated triazole product.