No description available.

In an attempt to expand the scope of titanacyclobutenes, malonate-derived ,-alkylpropargyl dibromides and ,-bis(bromopropargyl) malonates were prepared and examined for their reactivity with titanocene reagents. Unfortunately, all of the ,-alkylpropargyl dibromomalonates failed, presumably from chelation of the carbonyl oxygen followed by radical or nucleophilic attack. The ,-bis(bromopropargyl) malonate substrates allowed the successful synthesis of ester-functionalized allenyl or alkynyl-substituted bicyclic titanacyclobutene complexes. Allenyl-substitution is favoured when the being formed ring is small. When ring strain is minimal, cycloalkynes are obtained. An impressive example of a twelve-membered macrobicyclic titanacyclobutene was achieved, supported by extensive NMR spectroscopy and X-ray crystallography. 3-Propargyltitanium(III) complexes were synthesized and displayed equilibrium behaviour between the monomeric propargyl and dimeric di(titanacyclobutene) forms. Both steric and electronic effects are believed to be contributing factors for dimerization. Bimetallic titanacyclobutenes are obtained from the reaction of an epoxide and titanocene monochloride in the presence of a 3-propargyltitanium(III) complex.

In organometallic chemistry, metal complexes bearing unsaturated hydrocarbon ligands are of extensive interest, especially the C3H3-M system which includes propargyl (HC¡ÝCCH2-M), allenyl (H2C=C=CH-M), and acetylide (H3CC¡ÝC-M) forms. To study the chemistry of these species on metal surfaces, we used proprargyl bromide (HC¡ÝCCH2-Br) as precursor to produce C3H3(ad) on Ag(111) under ultrahigh vacuum (UHV) conditions. The thermal reactions pathway was investigated by Temperature-Programmed Desorption (TPD), and Reflection-Absorption Infrared Spectroscopy (RAIRS). In addition, density functional theory (DFT) calculations were conducted to obtain the optimized geometry for the adsorbates, and the computed IR spectra facilitated the vibrational mode assignments. TPD spectra showed that hydrogenation products C3H4 evolved at 310 K and 475 K. However, the desorption peak at 310 K was broad, indicating that more than one species were encompassed. Besides the hydrogenation product, a coupling product C6H6 (2,4-hexadiyne) was also unveiled as part of the desorption feature at 475 K. The identity of the possible C3H4 hydrogenation products (propyne and/or allene) was not discriminable by the mass spectrometry. The problem was circumvented by using £\,£\-dimethyl-substituted propargyl chloride because this dimethyl-substituted species also resulted in hydrogenatioin products around 310 K and 475 K, respectively; and the corresponding allenic and acetylenic end-products are distinguishable by the mass spectrometry. The results indicated that the broad feature at 310 K, in fact, contained both allene (lower temperature) and propyne (higher temperature), whereas the hydrogenation product at 475 K was propyne. The RAIR spectrum at 200 K showed that all C3H3(ad) on Ag(111) readily took on the allenyl form after the C-Br bond scission. It is thus obvious that allene at 310 K was generated by adding one hydrogen to the £\-carbon of the surface allenyl. RAIR spectroscopy revealed a drastic change after annealing the surface to 250 K, where the spectrum was almost identical to that obtained from using propynyl iodide (H3C-C¡ÝC-I) as a direct source for methylacetylide (H3C-C¡ÝC-Ag). Consequently, the products of propyne and 2,4-hexadiyne could be reasoned out.

No description available.

The recovery of the selected biological material in affinity-based separations relies on reversing the biological interaction responsible for the binding. General elution methods which are independent of the bioaffinity interaction have attracted increasing attention. The first three chapters of this thesis describe the development of a novel “click” functionalised azobenzene-based linker for affinity-independent elution protocols and the preliminary affinity studies using this linker. Ligands functionalised with a bioorthogonal propargyl label were readily attached to the terminal azide of the linker using the copper(I) catalysed Huisgen cycloaddition (or "click" reaction). Following separation, the linker was cleaved under mild non-denaturing conditions using Na2S2O4. In the last three chapters a novel approach towards the synthesis of the 4-methyl proline fragment of the cytotoxic natural product bisebromoamide (a potential affinity target) is proposed. For the pyrrolidine ring construction an enamide-olefin ring-closing metathesis (RCM) approach has been attempted. The installation of the required absolute stereochemistry has been achieved using a phase-transfer catalyst for the enantioselective alkylation of Schiff bases derived from glycine esters.

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<p> Cobalt-clusters are versatile reagents in organometallic chemistry. Their ability to protect an alkyne allows one to selectively manipulate a ligand without undergoing a competitive reaction from the alkyne. Cobalt-clusters geometrically modify linear alkynes to 136-145° degrees, thereby allowing for some non-traditional alkynyl chemistry to occur. In particular, the focus of this dissertation lies upon the chemistry of cobalt-complexed propargyl alkynols, the ability of cobalt to stabilize neighbouring cations generated from these alcohols, and the chemistry that can be accomplished by altering the steric and electronic effects. We have chosen to study the possibility of inducing migration of various substituents from one terminus of the cobalt-complexed alkyne to the alcoholic site of the propargyl group via protonation of the desired complex. While examining various silanes, and altering the propargyl alcohol itself, we have considered both steric and electronic effects, thereby determining the idealized conditions for such transfers to occur. Furthermore, in our attempts to successfully apply these migrations to several systems, we have acquired a diverse synthetic knowledge of propargyl cobalt-clusters and their intricate reactivity.</p> <p> An examination of the potential for allyl migrations in norbornyl derivatives revealed several fascinating transformations. Upon protonation with HBF4, [(2-endo-allyldimethylsilyl)ethynylborneol]Co2(CO)6, 63, suffers elimination of water or propene, to yield [(2-allyldimethylsilyl)ethynylborn-2-ene]Co2(CO)6, 68, [(2-endo-dimethylfluorosilyl)ethynylborneol]Co2(CO)6, 69, respectively, and, surprisingly, the tricobalt complex (2-norbornylidene)CHCCo3(CO)9, 70, In contrast, protonation of the terminal alkyne (2-endo-ethynylborneol)Co2(CO)6, 76, an anticipated precursor to 70, led instead to (2-ethynyl-2-bornene)Co2(CO)6, 78, and the ring-opened species (2-ethynyl-4-isopropyl-1-methylcyclohexa-1,3-diene)Co2(CO)6, 79. However, conversion of 76 to 70 was achievable upon prolonged heating at reflux in acetone, thereby also affording the corresponding alcohol, [2-(2-hydroxybornyl)]CH2CCo3(CO)9, 77. A mechanistic rationale is offered for the formation of RCH2CCo3(CO)9 clusters upon protonation of alkyne complexes of the type (RC≡CH)Co2(CO)6.</p> <p> Our interest in acid-promoted rearrangements in cobalt-clusters led us to novel propargyl radical chemistry induced by using particular solvents. The protonation of (1,1-diphenyl-2-propyn-1-ol)Co2(CO)6, 108, with HBF4 in dichloromethane generates the expected metal-stabilized propargyl cation, and also rearranges to give the tricobalt cluster Ph2C=CH-CCo3(CO)9, 33. In contrast, use of THF as solvent affords the radical (Co2(CO)6)[HC≡C-CPh2·], which dimerizes at the methyne position; subsequent cyclization and carbonylation yields 2,5-bis-(diphenylmethylene)cyclopent-3-en-1-one, 112.</p> <p> Furthermore, use of a fluorenyl substituent, instead of the diphenyl analogue, has uncovered a route to transition-metal peroxides of general synthetic potential. Treatment of benzyl- or vinyl-dimethylsilylethynylfluoren-9-ol[Co2(CO)6], 53 and 54, respectively, with HBF4 in diethyl ether or THF has afforded the very first known bimetallic transition metal peroxides, 124 and 125.</p> <p> Finally, the ability of cobalt-clusters to alter the geometry of cycloalkanes has been investigated. Treatment of 1-[axial]-(trimethylsilylethynyl)cyclohexan-1-ol, 129, with dicobalt octacarbonyl results in a conformational ring flip such that the bulky dicobalt-alkyne cluster moiety now occupies the favored equatorial site. However, when a 4-tert-butyl substituent is present, the coordinated alkynyl group retains its original axial or equatorial position.</p> <p> Complexation of trans-[diaxial]-1,4-bis(triphenylsilylethynyl)cyclohexan-1,4-diol, 142, brings about a chair-to-chair conformational inversion such that both cluster fragments now occupy equatorial sites. In contrast, cis-1,4 bis(triphenylsilylethynyl)cyclohexan-1,4-diol, 143, reacts with Co2(CO)8 to yield the twist-boat conformer, 145, in which the two axial hydroxy substituents exhibit intra-molecular hydrogen bonding. Likewise, the corresponding reaction of cis-1 ,4bis(trimethylsilylethynyl)cyclohexan-1,4-diol, 147, with Co2(CO)8 leads to a twist-boat, 149, but, in this case, the molecules are linked through intermolecular hydrogen bonds. The importance of X-ray crystallography in the unambiguous determination of molecular conformations has been emphasized.</p> / Thesis / Doctor of Philosophy (PhD)

Weak intermolecular interactions have very strong impact on the structures and properties of life giving molecules like H2O, DNA, RNA etc. These interactions are responsible for many biological phenomena. The directional preference of some of these interactions is used for designing different synthetic approaches in the supramolecular chemistry. The work reported in this Thesis comprises of investigations of weak intermolecular interactions in gas phase using home-built Pulsed Nozzle Fourier Transform Microwave (PN-FTMW) spectrometer as an experimental tool and ab-initio and Atoms in Molecules (AIM) theory as theoretical tools. The spectrometer which is coupled with a pulsed nozzle is used to record pure rotational spectra of the molecular clusters in a jet cooled molecular beam. In the molecular beam molecules/complexes are free from interactions with other molecules/complexes and thus, spectroscopy in the molecular beams provides information about the 'isolated' molecule/complex under investigation. The rotational spectra of the molecules/complexes in the molecular beam provide their geometry in the ground vibrational states. These experimental geometries can be used to test the performance and accuracy of theoretical models like ab-initio theory, when applied to the weakly bound complexes. Further the AIM theory can be used to gain insights into the nature and strength of the intermolecular interactions present in the system under investigation. Chapter I of this Thesis gives a brief introduction of intermolecular interactions. Other than hydrogen bonding, which is considered as the most important intermolecular interaction, many other intermolecular interactions involving different atoms have been observed in past few decades. The chapter summarizes all these interactions. The chapter also gives a brief introduction to the experimental and theoretical methods used to probe these interactions. In Chapter II, the experimental and theoretical methods used in this work are summarized. Details of our home-built PN-FTMW spectrometer are given in this chapter. The chapter also discusses briefly the theoretical methods like ab-initio, AIM and Natural bond orbital (NBO) analysis. We have made few changes in the mode of control of one of our delay generators which have also been described. Chapter III and Chapter V of this Thesis are dedicated to the propargyl alcohol complexes. Propargyl alcohol (PA) is a molecule of astrophysical interest. It is also important in combustion chemistry since propargyl radical is considered as the precursor in soot formation. Moreover, PA is a multifunctional molecule, having a hydroxyl (-OH) and an acetylenic (-C≡C-H) group. Both of the groups can individually act as hydrogen bond acceptor as well as donor and thus PA provides an exciting possibility of studying many different types of weak interactions. Due to internal motion of -OH group, PA monomer can exist in gauche as well as trans form. However, rotational spectra of PA-monomer show the presence of only gauche conformer. In Chapter III, rotational spectra of Ar•••PA complex are discussed. The pure rotational spectra of the parent Ar•••PA complex and its two deuterated isotopologues, Ar•••PA-D (OD species) and Ar•••PA-D (CD species), could be observed and fitted within experimental uncertainty. The structural fitting confirmed a structure in which PA is present as gauche conformer and argon interacts with both the O-H group and the acetylenic group leading to Ar•••H-O and Ar•••π interactions respectively. Presence of these interactions was further confirmed by AIM theoretical analysis. In all the three isotopologues c-type rotational transitions showed significant splitting. Splitting patterns in the three isotopologues suggest that it originates mainly due to the large amplitude motion of the hydroxyl group and the motion is weakly coupled with the carbon chain bending motion. No evidence for the complex with trans conformer of PA was found. Although, we could not observe Ar•••trans-PA complex experimentally, we decided to perform ab-initio and AIM theoretical calculations on this complex as well. AIM calculations suggested the presence of Ar•••H-O and a unique Ar•••C interaction in this complex which was later found to be present in the Ar•••methanol complex as well. This prompted us to explore different possible interactions in methanol, other than the well known O-H•••O hydrogen bonding interactions, and eventually led us to an interesting interaction which we termed as carbon bond. Chapter IV discusses carbon bonding interaction in different complexes. Electrostatic potential (ESP) calculations show that tetrahedral face of methane is electron-rich and thus can act as hydrogen/halogen bond acceptor. This has already been observed in many complexes, e.g. CH4•••H2O/HF/HCl/ClF etc., both experimentally and theoretically. However, substitution of one of the hydrogens of methane with -OH leads to complete reversal of the properties of the CH3 tetrahedral face and this face in methanol is electron-deficient. We found that CH3 face in methanol interacts with electron rich sites of HnY molecules and leads to the formation of complexes stabilized by Y•••C-X interactions. This interaction was also found to be present in the complexes of many different CH3X (X=OH/F/Cl/Br/NO2/NF2 etc.) molecules. AIM, NBO and C-X frequency shift analyses suggest that this interaction could be termed as "carbon bond". The carbon bonding interactions could be important in understanding hydrophobic interactions and thus could play an important role in biological phenomena like protein folding. The carbon bonding interaction could also play a significant role in the stabilization of the transition state in SN2 reactions. In Chapter V of this Thesis rotational spectra of propargyl alcohol dimer are discussed. Rotational spectra of the parent dimer and its three deuterated (O-D) isotopologues (two mono-substituted and one bi-substituted) could be recorded and fitted within experimental uncertainty. The fitted rotational constants are close to one of the ab-initio predicted structure. In the dimer also propargyl alcohol exists in the gauche form. Atoms in molecules analysis suggests that the experimentally observed dimer is bound by O-H•••O, O-H•••π and C-H•••π interactions. Chapter VI of the thesis explores the 'electrophore concept'. To observe the rotational spectra of any species and determine its rotational constant by microwave spectroscopy, the species should have a permanent dipole moment. Can we obtain rotational constants of a species having no dipole moment via microwave spectroscopy? Electrophore concept can be used for this purpose. An electrophore is an atom or molecule which could combine with another molecule having no dipole moment thereby forming a complex with a dipole moment, e.g. Argon atom is an electrophore in Ar•••C6H6 complex. The microwave spectra of Ar•••13CC5H6 and Ar•••C6H5D complexes were recorded and fitted. The A rotational constant of these complexes was found to be equal to the C rotational constant of 13CC5H6 and C6H5D molecules respectively and thus we could determine the C rotational constant of microwave 'inactive' 13CC5H6. This concept could be used to obtain the rotational spectra of parallel displaced benzene-dimer if it exists. We recently showed that the square pyramidal Fe(CO)5 can act as hydrogen bond acceptor. Appendix I summarizes the extension of this work and discusses interactions of trigonal bipyramidal Fe(CO)5 with HF, HCl, HBr and ClF. Our initial attempts on generating a chirped pulse to be used in a new broadband spectrometer are summarized in Appendix II. Preliminary investigations on the propargyl•••water complex are summarized in Appendix III.

Rhodium-catalysed hydroacylation provides a highly atom economic synthesis of ketone products from the combination of aldehydes and multiple bond systems by C-H bond activation. This work evaluates the combination of intermolecular hydroacylation for the synthesis of classical heterocycle precursors and their dehydrative cyclisation to give rise to a range of substituted heterocyclic compounds. Chapter 1 outlines recent developments in the chemistry of hydroacylation. Particular attention is paid to the various chelation strategies employed in intermolecular hydroacylation. Chapter 2 discusses some relevant and recent developments in the field of pyridine and pyrrole synthesis. Having established that β-sulphur chelation controlled hydroacylation can be used to synthesise pyridines in Chapter 3; attention was turned to hydroacylation of propargyl amines in Chapter 4. The methodology was expanded to provide a synthesis of γ-amino enones. The hydroacylation reaction and cyclisation is combined in a procedure that utilises thermal Boc-deprotection and cyclisation to give a range of highly-substituted pyrroles. The regioselectivity of the hydroacylation of propargyl amines is investigated in Chapter 5 by application of statistical Design of Experiments methodology. Optimised conditions were identified with minor improvements in the selectivity of the reaction.

No description available.