The synthesis of α-alkoxy and α-aminostannanes as precursors to Novel Chromium Fischer Carbenes
- Authors: Meyer, Annalene
- Date: 2012
- Subjects: Alkoxides , Organometallic compounds , Carbenes (Methylene compounds) , Chromium , Molybdenum , Tungsten , Organolithium compounds
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4371 , http://hdl.handle.net/10962/d1005036 , Alkoxides , Organometallic compounds , Carbenes (Methylene compounds) , Chromium , Molybdenum , Tungsten , Organolithium compounds
- Description: The present study involves the use of main group organometallics: organostannanes and organolithiums as precursors to chromium Fischer carbene complexes. Fischer carbenes are well stabilized by the π‐donor substituents such as alkoxy and amino groups and low oxidation state metals such as Group 6 (Chromium, Molybdenum or Tungsten). Carbenes are an important intermediate in the synthesis of a range of compounds through cyclopropanations, insertions, coupling and photochemical reactions. Synthesis and successful characterisation of three α‐alkoxystannanes was achieved via nucleophilic addition of tributylstannyllithium to the respective aldehydes, followed by an immediate MOM protection of the resulting alcohol. Six α‐aminostanannes were synthesised, consisting of N‐BOC, N‐acetyl and N‐ethyl derivatives of pyrrolidine and piperidine, via α‐lithiation and subsequent tinlithium transmetallation in the presence of TMEDA. The ¹³C NMR analysis highlighted an interesting phenomenon of tin‐carbon coupling that revealed unique structural information of both types of stannanes. DFT analysis was completed on the series of stannanes; a predicted frequency analysis was obtained which complemented the experimental Infra‐red data in elucidation of the compounds. The α‐alkoxy and α‐aminostannanes provided stable precursors to the organolithiums required for the synthesis of the novel Fischer chromium carbenes. The organolithiums were obtained via tinlithium exchange at low temperatures, followed by the addition of chromium hexacarbonyl to form the acylpentacarbonyl‐chromate salt. Alkylation of this intermediate using a Meerwein salt, Me₃OBF₄, gave rise to the novel Fischer chromium carbene complexes. Fischer chromium carbenes derived from the two isomeric butyl and isobutyl stannanes and the two N‐ethyl α‐aminostannanes were successfully synthesised. The difficulty encountered in the purification of the Fischer carbene complexes hindered the full characterisation, due to the presence of a by‐product, tetrabutyltin.
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- Date Issued: 2012
Reduction of tungsten oxides with carbon and hydrogen
- Authors: Venables, Dean Stuart
- Date: 1996
- Subjects: Oxidation-reduction reaction , Tungsten , Hydrogen , Carbon
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4342 , http://hdl.handle.net/10962/d1005004 , Oxidation-reduction reaction , Tungsten , Hydrogen , Carbon
- Description: The reductions of WO₃ with hydrogen, with CO, and with carbon, as well as the reduction of WO₃/graphite mixtures with hydrogen, were studied using thermogravimetry, evolved gas analysis, X-ray powder diffraction, and scanning electron microscopy. The intermediate phases W₂₀O₅₈, W₁₈O₄₉ and WO₂, were observed in the reductions. The final product of the reductions with hydrogen and carbon was tungsten, and we was formed in the reduction with CO. The reaction paths in the overall processes were determined. The reactant/product gas ratio had a considerable influence on which reactions took place. The morphology of the sample was characterised at different stages of the reduction. The shape of the WO₃ particles was retained during the reduction. Particle growth was observed in the reduction with hydrogen and was attributed to the formation of WO₂(OH)₂(g). The kinetics of the reductions were investigated , and the reaction mechanisms determined. The reduction of WO₃ with CO was studied from 650 to 900°C, and occurred at a phase boundary with an activation energy of 40 kJ mol⁻¹ . The reduction of WO₂, was studied under the same conditions. The reaction also occurred at a phase boundary and had an activation energy of 62 kJ mol⁻¹. The reduction of WO₃ with carbon was studied from 935 to 1100°C and took place via CO and CO₂. Two stages were observed in the reduction . The first stage, which corresponded approximately to the formation of WO₂ had an activation energy of 66 kJ mol⁻¹ and was limited by diffusion through the porous reacting particles. The second stage was first order and had an activation energy of 40 kJ mol⁻¹. The reduction of WO₃ and WO₃ graphite mixtures with hydrogen were studied from 575 to 975 °C. The reactions were controlled by mass-transfer under the conditions investigated. The addition of carbon increased the rate of the reduction process , but did not affect the phases formed in the system. CO₂ was evolved mainly at the start, and CO mainly at the end of the process.
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- Date Issued: 1996