The importance of fluorine in a wide array of different areas within chemistry and biochemistry has been demonstrated time and time again. Fluorine-containing products range from essential substituents in pharmaceuticals and relatively long-lived tracers for PET imagining, to fluoropolymers with outstanding properties, to essential components in most of the strongest acids available to chemists today. Fluorine’s extreme electronegativity makes it a truly unique element, but its acute toxicity in its elemental F2 form makes it difficult to handle, prompting researchers to explore different options for incorporating this important element into a variety of different molecular scaffolds. Due to the remarkable thermodynamic and kinetic stabilities of C-F bonds, methods for forming and breaking these cleanly, and under relatively mild conditions, are in high demand. Fortunately, transition metals have greatly aided in this process. However, fluoroorganometallic chemistry is much less developed than transition metal chemistry involving hydrocarbons, and certainly less understood. One of the primary reasons for this relative dearth of fluoroorganometallic complexes is the difficulty associated with their synthesis. In this work, important steps towards perfluoroalkyl chain-growth within the coordination sphere of a transition metal will be presented, stemming in part from the synthesis and characterization of novel cobalt fluoride and bis(perfluoroalkyl) complexes. As important electrophiles, fluoroalkenes have primarily been used as monomers for the formation of important fluoropolymers. However, their direct reactivity with organics remains rare and is usually difficult to control, with limited substrate scopes. Herein, the formation of stable N-heterocyclic fluoroalkene adducts as versatile synthons for the incorporation of fluoroalkene fragments into various chemical environments will be introduced. By forming these adducts, the inconvenience of manipulating fluorinated gases in further reactions can be avoided, and the N-heterocyclic fragment is shown to aid in directing substitutions involving polyfluoroalkenyl imidazolium salts and organic nucleophiles to form a variety of C-E (E = C, N, O, S) and C-M bonds (M = Mn, Mo). The ease with which C-F bonds are manipulated in these systems is quite remarkable, as the substitution reactions occur cleanly and efficiently at room temperature, to form a variety of new bonds without the need for a transition metal. By expanding on the fundamental reactivity between N-heterocyclic carbenes and fluoroalkenes, attempts were made to correlate the observed reactivity with certain electronic and steric parameters unique to the utilized carbenes. Although a correlation has not yet been established, the effects of atypical steric constraints in a cyclic (alkyl)(amino)carbene were demonstrated, wherein the initial point of attack by the carbene on the fluoroalkene was modified. It is hoped that this work will eventually lead to new roles for organocatalysts in fluoroalkene transformations.

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The first chapter is an introduction to the f-elements, with a focus on the synthesis and chemistry of tetravalent cerium complexes. The synthesis, characterisation and reactivity of carbenes, particularly N-heterocyclic carbenes (NHCs), and anionic-functionalised NHC ligands is discussed. The synthesis and reactivity of s-block, Group three and fblock NHC complexes is reviewed. The synthesis of the alcohol-functionalised unsaturated imidazolium proligand, [H2L]I [H2L = HOCMe2CH2(1-CH{NCHCHNiPr})], is extended to saturated imidazolinium analogues, [H2LR]X, [HOCMe2CH2(1-CH{NCH2CH2NR})]X (R = iPr, abbreviated to P; R = Mes, abbreviated to M; R = Dipp, abbreviated to D, X = Cl, I). Mono-deprotonation results in the isolation of bicyclic imidazolidines HLR, which can be ring-opened and silylated by treatment with Me3SiI, to afford [HLR, OSiMe3]I, R = iPr and Mes. Further deprotonation of HLR with MN"2 (M = Mg, Zn; N" = N(SiMe3)2) affords LRMN" (M = Mg and Zn) and ZnLR 2. These complexes proved active for the polymerisation of raclactide at room temperature without the need for an initiator. The third chapter focuses on cerium chemistry. Repetition of the literature synthesis of CeIV tert-butoxide compounds affords the unsolvated Ce(OtBu)4 and also the cluster Ce3(OtBu)11. Treatment of Ce(OtBu)4 with HL did not yield a CeIV-NHC complex, but one bearing a tethered imidazolium group, (OtBu)3Ce( -OtBu)2( -HL)Ce(OtBu)3. The synthesis of a CeIII-NHC complex, CeL3, and the oxidation of this forms an unprecedented CeIV-NHC complex, CeL4, via ligand redistribution; the complex contains two bidentate ligands and two alkoxide-tethered free NHC groups. Functionalisation of the free NHCs with boranes affords the adducts Ce(L)2(L-B)2, where B = BH3 or 9-BBN (9-Borabicyclo[3.3.1]nonane). A number of cerium complexes of the saturated-backbone NHC, LR, LRCeN"2 and LR 2CeN", have been synthesised and their oxidation chemistry and reactivity investigated. The final chapter contains an NMR study of the synthesis of a series of yttrium LR adducts, LP xYN"(3-x) (x = 1, 2 or 3), and of the synthesis of uranium complexes LRUN"2, R = Mes or Dipp, including a range of small molecule reaction chemistry. The uranyl NHC complexes, UO2LR 2, R = Mes or Dipp, have also been synthesised and characterised; these possess very high frequency carbene carbon chemical shifts.

(1)Flash vaccum pyrolysis of furo[3,2-c]pyran-4-one gave starting material, but in photolytic system we gave a isomer: furo[2,3-c]pyran-5-one. (2)Flash vaccum pyrolysis of benzoic 1,2-dimethyl-3-indoly anhydride,via a ketene intermediate,gave a dimmer.

(1) The pyrolysis of benzoic 5-benzyl-2-furoic anhydride by using SS-FVP method can obtain the ring-opened product naphthalene, and no ring-contracted product and its further derivatives. (2) Flash vacuum pyrolysis (FVP) of 2-(2-azidoethyl)thiophene, via a nitrene intermediate gave 1,2-di(2-thienyl)ethane, 3,5-di(2-thienyl)pyridine, and 2-methylthiophene as the main products.

(1) Flash vacuum pyrolysis (FVP) of (5-methyl-2-selenophenyl)methyl benzoate gave 4-methylene-4H-selenopyran. The structure was confirmed by trapping experiment. The mechanism for the 4-methylene-4H-selenopyran will be discussed. (2) Flash vacuum pyrolysis of 2-Azidomethylselenophene, via a nitrene intermediate, gave a trimer (N,N¡¦-di-2-selenophenylmethylidene-2-selenophenylmethylidene diamine).

Please read the abstract in the dissertation / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Chemistry / Unrestricted

Cross-coupling, which relies on the use of transition metals, is among the most utilized chemical means of establishing carbon-carbon or carbon-heteteroatom bonds between appropriately functionalized sp, sp2, or sp3 centres. However, most cross-coupling reactions rely on the use of palladium to synthesize valuable synthetic targets. This is problematic for the chemical industry as palladium is limited in supply and expensive. Chemists have therefore sought to replace palladium with first-row transition metals (e.g., iron, cobalt and nickel) and recent reports on cobalt and nickel catalyzed cross-coupling reactions indicate that these metals can be used in this capacity. Unfortunately, protocols developed (so far) for these metals are unsuitable for the synthesis of targets with base-sensitive functional groups as they involve strongly basic reaction conditions. Research in this thesis aims to develop both cobalt and nickel pre-formed catalysts that will display high catalytic activity in mildly basic reaction conditions. Current methodologies for cobalt and nickel cross-coupling reactions use either phosphine ligands or NHC ligands of moderate steric bulk (IMes or IPr). Recent advancements in the development of Pd-PEPPSI catalysts have demonstrated that both pre-forming the catalyst and using larger NHC ligands (IPent, IPentCl, or IHept) are required for high catalytic activity in weakly basic conditions. Thus, it was hypothesized that the development of pre-formed cobalt and nickel NHC complexes analogous to their Pd counterparts would improve reactivity in Negishi, Suzuki-Miyaura, and Buchwald-Hartwig amination cross-coupling reactions. Co(IPent)Cl2(Pyr), Co(IPentCl)Cl2(Pyr), and Co2IPr2(OAc)4 were prepared, identified by X-ray crystallography, and evaluated in preliminary Negishi cross-coupling reactions. These complexes were found to be ineffective, but Co2IPr2(OAc)4 was found to be effective for Suzuki-Miyaura cross-coupling. A base screen was performed to enable the use of weak bases, however, the reaction only worked by pre-forming the boronate with n-BuLi, rendering the reaction conditions intolerant of base-sensitive functional groups. [Ni(IPr)]2(µ-Cl)2 , Ni(IPr)Cl(allyl), and Ni(IPent)Cl(allyl) complexes were synthesized and evaluated in Buchwald-Hartwig aminations. Several bases were examined for these reactions but only sodium tert-butoxide was found to be effective. The presence of TEMPO and BHT in these transformations proved deleterious suggesting the involvement of radical intermediates. Finally, stoichiometric reactions were performed to isolate intermediates in the catalytic cycle, supporting the formation of nickel(0).

The reactions of iodomethane, diiodomethane, and formaldehyde over stoichiometric, O-terminated, and Cl-terminated α-Cr₂O₃ surfaces, were studied by thermal desorption spectroscopy. Adsorbed formaldehyde forms dioxymethylene species on the nearly-stoichiometric surface that react primarily above 600 K. Dioxymethylene decomposes via a Cannizzaro-type process with dehydrogenation to formate, and hydrogenation to methoxide. Methoxide hydrogenation produces methane and methanol near 670 K. Formate decomposition occurs at 720 K, producing acetylene, carbon monoxide, carbon dioxide and formic acid. The site requirements for these reactions are a cation/anion site pair. When the surface cations are capped with Cl, the reactivity associated with dioxymethylene intermediates above 600 K is not observed. At higher coverages, polymerization to paraformaldehyde is observed on both surfaces, and decomposition to formaldehyde is observed at 400 K in thermal desorption. Iodomethane and diiodomethane are used as sources of methyl and methylene surface species, respectively. Methyl fragments on the stoichiometric surface dehydrogenate to surface methylene and hydrogen as a rate limiting step to produce ethylene and methane at 505 K. On the oxygen-terminated surface, the methyl fragments undergo dehydrogenation and coupling to ethylene at 425 K, undergo oxygen insertion to formaldehyde at 425 K, and produce carbon dioxide, formic acid, and water above 700 K from the dehydrogenation of formate. Methylene fragments on the stoichiometric surface undergo diffusion limited coupling to ethylene at 390 to 490 K and produce methane at 520 K from dehydrogenation. On the oxygen-terminated surface, methylene undergoes oxygen insertion to produce formaldehyde at 450 K, produce carbon monoxide, formaldehyde, and water at 695 K from dioxymethylene dehydrogenation, and produce carbon dioxide, formic acid, and water above 700 K from the dehydrogenation of formate. / Ph. D.