The development of alkali-metal zincate chemistry for application in synthesis and catalysis

Abstract

The work reported within this thesis focuses on two main projects. With these projects involving the chemistry of the metal zinc. The first of these projects was to investigate the chemistry an unprecedented tetrazincated ferrocene complex (4) and the second project was to investigate the primarily interactions in the zincate [{K(HMDS)2ZnBn}∞]. The first project is a continuation of a previous study that resulted in the formation of a tetrazincated ferrocene complex (4), with the intention of using this as an intermediate for the synthesis of tetrasubstituted ferrocenes.;These functionalizations were attempted using three different electrophilic reagents, specifically I2, D2O and CO2. The results from this were often confusing and inconclusive. However, when using D2O for a quench of the tetrazincated ferrocene complex, this did in fact seem to produce the desired D4-ferrocene product. Furthermore, the reactivity of the tetrazincated ferrocene complex towards aromatic substrates was probed to provide understanding of how this species behaves as a base.;Previous studies had shown that when pyridine is added to compound 4, a polymeric γ-metallated pyridine species (6) is formed. However, this was only acquired in an isolated yield of 5%. Here, the reaction was optimized, improving the crystalline yield to 29%. This study has also revealed that this process is more complicated than originally thought, via electrophilic quenching reactions with I2, as it seems to also metallate at the β-position of pyridine and not exclusively the γ-position as previously thought.;During the course of this project, an alternative zinc reagent to tBu2Zn was desired in order to avoid safety issues. A synthesis of isomeric iBu2Zn has been successfully developed and was found to be reproducible to a yield of 78%. This compound was then tested in a series of reactions in order to form a comparison with tBu2Zn and its related bimetallic 'ate' bases.;The second of these two projects, focuses on a recent publication by Guan et al. which has shown that the polymeric potassium zincate [{K(HMDS)2ZnBn}∞] (8) can be used for the direct catalytic functionalization of the benzylic C-H bond of diarylmethanes. One of the main aims of this project was to achieve the isolation and characterisation of the proposed intermediate in the investigation performed by Guan et al., which was the metallated diphenylmethyl system [{KZn(HMDS)2(CHPh2)}∞]. Despite there being promising signs of achieving this, due to time constraints, this ultimately was not achieved.;The second main aim of this section was to investigate how the potassium atom in [{K(HMDS)2ZnBn}∞] primarily interacts with the zincate component by disrupting its polymeric nature. This was achieved through the use of the highly effective monomerizing agent Me6TREN which revealed that the monomeric unit should be considered as (HMDS)2ZnBnK rather than K(HMDS)2ZnBn as suggested by Guan. This can be rationalized due to the π-philicity of the heavier alkali metal which preferentially coordinates to the delocalised charge within the aryl ring rather than the localised charge of the amido nitrogen centres of the HMDS ligands when the hemisolvating Me6TREN ligand forces it to choose only one.The work reported within this thesis focuses on two main projects. With these projects involving the chemistry of the metal zinc. The first of these projects was to investigate the chemistry an unprecedented tetrazincated ferrocene complex (4) and the second project was to investigate the primarily interactions in the zincate [{K(HMDS)2ZnBn}∞]. The first project is a continuation of a previous study that resulted in the formation of a tetrazincated ferrocene complex (4), with the intention of using this as an intermediate for the synthesis of tetrasubstituted ferrocenes.;These functionalizations were attempted using three different electrophilic reagents, specifically I2, D2O and CO2. The results from this were often confusing and inconclusive. However, when using D2O for a quench of the tetrazincated ferrocene complex, this did in fact seem to produce the desired D4-ferrocene product. Furthermore, the reactivity of the tetrazincated ferrocene complex towards aromatic substrates was probed to provide understanding of how this species behaves as a base.;Previous studies had shown that when pyridine is added to compound 4, a polymeric γ-metallated pyridine species (6) is formed. However, this was only acquired in an isolated yield of 5%. Here, the reaction was optimized, improving the crystalline yield to 29%. This study has also revealed that this process is more complicated than originally thought, via electrophilic quenching reactions with I2, as it seems to also metallate at the β-position of pyridine and not exclusively the γ-position as previously thought.;During the course of this project, an alternative zinc reagent to tBu2Zn was desired in order to avoid safety issues. A synthesis of isomeric iBu2Zn has been successfully developed and was found to be reproducible to a yield of 78%. This compound was then tested in a series of reactions in order to form a comparison with tBu2Zn and its related bimetallic 'ate' bases.;The second of these two projects, focuses on a recent publication by Guan et al. which has shown that the polymeric potassium zincate [{K(HMDS)2ZnBn}∞] (8) can be used for the direct catalytic functionalization of the benzylic C-H bond of diarylmethanes. One of the main aims of this project was to achieve the isolation and characterisation of the proposed intermediate in the investigation performed by Guan et al., which was the metallated diphenylmethyl system [{KZn(HMDS)2(CHPh2)}∞]. Despite there being promising signs of achieving this, due to time constraints, this ultimately was not achieved.;The second main aim of this section was to investigate how the potassium atom in [{K(HMDS)2ZnBn}∞] primarily interacts with the zincate component by disrupting its polymeric nature. This was achieved through the use of the highly effective monomerizing agent Me6TREN which revealed that the monomeric unit should be considered as (HMDS)2ZnBnK rather than K(HMDS)2ZnBn as suggested by Guan. This can be rationalized due to the π-philicity of the heavier alkali metal which preferentially coordinates to the delocalised charge within the aryl ring rather than the localised charge of the amido nitrogen centres of the HMDS ligands when the hemisolvating Me6TREN ligand forces it to choose only one