Simulation chamber studies of the atmospheric degradation of naphthalene, 1-nitronaphthalene and phthaldialdehyde

A detailed series of simulation chamber experiments has been performed on the atmospheric degradation pathways of the primary air pollutant naphthalene and two of its photooxidation products, phthaldialdehyde and 1-nitronaphthalene. The measured yields of secondary organic aerosol (SOA) arising from the photooxidation of naphthalene varied from 6-20%, depending on the concentrations of naphthalene and nitrogen oxides as well as relative humidity. A range of carbonyls, nitro-compounds, phenols and carboxylic acids were identified among the gas- and particle-phase products. On-line analysis of the chemical composition of naphthalene SOA was performed using aerosol time-of-flight mass spectrometry (ATOFMS) for the first time. The results indicate that enhanced formation of carboxylic acids may contribute to the observed increase in SOA yields at higher relative humidity. The photolysis of phthaldialdehyde and 1-nitronaphthalene was investigated using natural light at the European Photoreactor (EUPHORE) in Valencia, Spain. The photolysis rate coefficients were measured directly and used to confirm that photolysis is the major atmospheric loss process for these compounds. For phthaldialdehyde, the main gas-phase products were phthalide and phthalic anhydride. SOA yields in the range 2-11% were observed, with phthalic acid and dihydroxyphthalic acid identified among the particle phase products. The photolysis of 1-nitronaphthalene yielded nitric oxide and a naphthoxy radical which reacted to form several products. SOA yields in the range 57-71% were observed, with 1,4-naphthoquinone, 1-naphthol and 1,4-naphthalenediol identified in the particle phase. On-line analysis of the SOA generated in an indoor chamber using ATOFMS provided evidence for the formation of high-molecular-weight products. Further investigations revealed that these products are oxygenated polycyclic compounds most likely produced from the dimerization of naphthoxy radicals. These results of this work indicate that naphthalene is a potentially large source of SOA in urban areas and should be included in atmospheric models. The kinetic and mechanistic information could be combined with existing literature data to produce an overall degradation mechanism for naphthalene suitable for inclusion in photochemical models that are used to predict the effect of emissions on air quality.

Synthetic and mechanistic aspects of the reactions of α-diazosulfoxides

The research described in this thesis focuses, principally, on synthesis of stable α-diazosulfoxides and investigation of their reactivity under various reaction conditions (transition-metal catalysed, photochemical, thermal and microwave) with a particular emphasis on the reactive intermediates and mechanistic aspects of the reaction pathways involved. In agreement with previous studies carried out on these compounds, the key reaction pathway of α-diazosulfoxides was found to be hetero-Wolff rearrangement to give α-oxosulfine intermediates. However, a competing reaction pathway involving oxygen migration from sulfur to oxygen was also observed. Critically, isomerisation of α-oxosulfine stereoisomers was observed directly by 1H NMR spectroscopy in this work and this observation accounts for the stereochemical outcomes of the various cycloaddition reactions, whether carried out with in situ trapping or with preformed solutions of sulfines. Furthermore, matrix isolation experiments have shown that electrocyclisation of α-oxosulfines to oxathiiranes takes place and this verifies the proposed mechanisms for enol and disulfide formation. The introductory chapter includes a brief literature review of the synthesis and reactivity of α-diazosulfoxides prior to the commencement of research in this field by the Maguire group. The Wolff rearrangement is also discussed and the characteristic reactions of a number of reactive intermediates (sulfines, sulfenes and oxathiiranes) are outlined. The use of microwave-assisted organic synthesis is also examined, specifically, in the context of α-diazocarbonyl compounds as substrates. The second chapter describes the synthesis of stable monocyclic and bicyclic lactone derivatives of α-diazosulfoxides from sulfide precursors according to established experimental procedures. Approaches to precursors of ketone and sulfimide derivatives of α-diazosulfoxides are also described. The third chapter examines the reactivity of α-diazosulfoxides under thermal, microwave, rhodium(II)-catalysed and photochemical conditions. Comparison of the results obtained under thermal and microwave conditions indicates that there was no evidence for any effect, other than thermal, induced by microwave irradiation. The results of catalyst studies involving several rhodium(II) carboxylate and rhodium(II) carboxamidate catalysts are outlined. Under photochemical conditions, sulfur extrusion is a significant reaction pathway while under thermal or transition metal catalysed conditions, oxygen extrusion is observed. One of the most important observations in this work was the direct spectroscopic observation (by 1H NMR) of interconversion of the E and Z-oxosulfines. Trapping of the α-oxosulfine intermediates as cycloadducts by reaction with 2,3-dimethyl-1,3-butadiene proved useful both synthetically and mechanistically. As the stereochemistry of the α-oxosulfine is retained in the cycloadducts, this provided an ideal method for characterisation of this key feature. In the case of one α-oxosulfine, a novel [2+2] cycloaddition was observed. Preliminary experiments to investigate the reactivity of an α-diazosulfone under rhodium(II) catalysis and microwave irradiation are also described. The fourth chapter describes matrix isolation experiments which were carried out in Rühr Universität, Bochum in collaboration with Prof. Wolfram Sander. These experiments provide direct spectroscopic evidence of an α-oxosulfine intermediate formed by hetero-Wolff rearrangement of an α-diazosulfoxide and subsequent cyclisation of the sulfine to an oxathiirane was also observed. Furthermore, it was possible to identify which stereoisomer of the α-oxosulfine was present in the matrix. A preliminary laser flash photolysis experiment is also discussed. The experimental details, including all spectral and analytical data, are reported at the end of each chapter. The structural interpretation of 1H NMR spectra of the cycloadducts, described in Chapter 3, is discussed in Appendix I.

Synthesis and reactivity of α-diazo-β-keto sulfoxides: scope and synthetic potential

The research described in this thesis focuses on the design and synthesis of stable α-diazosulfoxides and investigation of their reactivity under a variety of conditions (transition-metal catalysis, thermal, photochemical and microwave) with a particular emphasis on the synthesis of novel heterocyclic compounds with potential biological activity. The exclusive reaction pathway for these α-diazosulfoxides was found to be hetero-Wolff rearrangement to give α-oxosulfine intermediates. In the first chapter, a literature review of sulfines is presented, including a discussion of naturally occurring sulfines, and an overview of the synthesis and reactivity of sulfines. The potential of sulfines in organic synthesis and recent developments in particular are highlighted. The second chapter discusses the synthesis and reactivity of α-diazosulfoxides, building on earlier results in this research group. The synthesis of lactone-based α-diazosulfoxides and, for the first time, ketone-based benzofused and monocyclic α-diazosulfoxides is described. The reactivity of these α-diazosulfoxides is then explored under a variety of conditions, such as transition-metal catalysis, photochemical and microwave, generating labile α-oxosulfine intermediates, which are trapped using amines and dienes, in addition to the spontaneous reaction pathways which occur with α-oxosulfines in the absence of a trap. A new reaction pathway was explored with the lactone based α-oxosulfines, involving reaction with amines to generate novel 3-aminofuran-2(5H)-ones via carbophilic attack, in very good yields. The reactivity of ketone-based α-diazosulfoxides was explored for the first time, and once again, pseudo-Wolff rearrangement to the α-oxosulfines was the exclusive reaction pathway observed. The intermediacy of the α-oxosulfines was confirmed by trapping as cycloadducts, with the stereochemical features dependant on the reaction conditions. In the absence of a diene trap, a number of reaction fates from the α-oxosulfines were observed, including complete sulfinyl extrusion to give indanones, sulfur extrusion to give indanediones, and, to a lesser extent, dimerisation. The indanediones were characterised by trapping as quinoxalines, to enable full characterisation. One of the overriding outcomes of this thesis was the provision of new insights into the behaviour of α-oxosulfines with different transition metal catalysts, and under thermal, microwave and photolysis conditions. A series of 3-aminofuran-2(5H)-ones and benzofused dihydro-2H-thiopyran S-oxides were submitted for anticancer screening at the U.S. National Cancer Institute. A number of these derivatives were identified as hit compounds, with excellent cell growth inhibition. One 3-aminofuran-2(5H)-one derivative has been chosen for further screening. The third chapter details the full experimental procedures, including spectroscopic and analytical data for the compounds prepared during this research. The data for the crystal structures are contained in the attached CD.

Ab initio calculations of group 4 metallocene reaction mechanisms: atomic layer deposition and bond activation catalysis

Thin film dielectrics based on titanium, zirconium or hafnium oxides are being introduced to increase the permittivity of insulating layers in transistors for micro/nanoelectronics and memory devices. Atomic layer deposition (ALD) is the process of choice for fabricating these films, as it allows for high control of composition and thickness in thin, conformal films which can be deposited on substrates with high aspect-ratio features. The success of this method depends crucially on the chemical properties of the precursor molecules. A successful ALD precursor should be volatile, stable in the gas-phase, but reactive on the substrate and growing surface, leading to inert by-products. In recent years, many different ALD precursors for metal oxides have been developed, but many of them suffer from low thermal stability. Much promise is shown by group 4 metal precursors that contain cyclopentadienyl (Cp = C5H5-xRx) ligands. One of the main advantages of Cp precursors is their thermal stability. In this work ab initio calculations were carried out at the level of density functional theory (DFT) on a range of heteroleptic metallocenes [M(Cp)4-n(L)n], M = Hf/Zr/Ti, L = Me and OMe, in order to find mechanistic reasons for their observed behaviour during ALD. Based on optimized monomer structures, reactivity is analyzed with respect to ligand elimination. The order in which different ligands are eliminated during ALD follows their energetics which was in agreement with experimental measurements. Titanocene-derived precursors, TiCp*(OMe)3, do not yield TiO2 films in atomic layer deposition (ALD) with water, while Ti(OMe)4 does. DFT was used to model the ALD reaction sequence and find the reason for the difference in growth behaviour. Both precursors adsorb initially via hydrogen-bonding. The simulations reveal that the Cp* ligand of TiCp*(OMe)3 lowers the Lewis acidity of the Ti centre and prevents its coordination to surface O (densification) during both of the ALD pulses. Blocking this step hindered further ALD reactions and for that reason no ALD growth is observed from TiCp*(OMe)3 and water. The thermal stability in the gas phase of Ti, Zr and Hf precursors that contain cyclopentadienyl ligands was also considered. The reaction that was found using DFT is an intramolecular α-H transfer that produces an alkylidene complex. The analysis shows that thermal stabilities of complexes of the type MCp2(CH3)2 increase down group 4 (M = Ti, Zr and Hf) due to an increase in the HOMO-LUMO band gap of the reactants, which itself increases with the electrophilicity of the metal. The reverse reaction of α-hydrogen abstraction in ZrCp2Me2 is 1,2-addition reaction of a C-H bond to a Zr=C bond. The same mechanism is investigated to determine if it operates for 1,2 addition of the tBu C-H across Hf=N in a corresponding Hf dimer complex. The aim of this work is to understand orbital interactions, how bonds break and how new bonds form, and in what state hydrogen is transferred during the reaction. Calculations reveal two synchronous and concerted electron transfers within a four-membered cyclic transition state in the plane between the cyclopentadienyl rings, one π(M=X)-to-σ(M-C) involving metal d orbitals and the other σ(C-H)-to-σ(X-H) mediating the transfer of neutral H, where X = C or N. The reaction of the hafnium dimer complex with CO that was studied for the purpose of understanding C-H bond activation has another interesting application, namely the cleavage of an N-N bond and resulting N-C bond formation. Analysis of the orbital plots reveals repulsion between the occupied orbitals on CO and the N-N unit where CO approaches along the N-N axis. The repulsions along the N-N axis are minimized by instead forming an asymmetrical intermediate in which CO first coordinates to one Hf and then to N. This breaks the symmetry of the N-N unit and the resultant mixing of MOs allows σ(NN) to be polarized, localizing electrons on the more distant N. This allowed σ(CO) and π(CO) donation to N and back-donation of π*(Hf2N2) to CO. Improved understanding of the chemistry of metal complexes can be gained from atomic-scale modelling and this provides valuable information for the design of new ALD precursors. The information gained from the model decomposition pathway can be additionally used to understand the chemistry of molecules in the ALD process as well as in catalytic systems.