No description available.

No description available.

Thesis (M. Phil.)--University of Hong Kong, 1983.

published_or_final_version / Chemistry / Master / Master of Philosophy

It has been shown that photolysis of 4-azidopyridine N-oxide yields the singlet nitrene, which undergoes intersystem crossing at room temperature to generate triplet 4- nitrenopyridine N-oxide. The room temperature photochemistry is dominated by triplet nitrene chemistry leading to the formation of the azo-dimer. This unusual behavior is a result of selective stabilization of the lowest singlet state of the nitrene by the N-oxide group. In this study, we wish to investigate the effect of complexation of the N-oxide group with a metal cation on the kinetics and reactivity of 4-nitrenopyridine N-oxide and related compounds. It is envisaged that complexation will alter the polarity of the N-oxide bond making it less capable of spin delocalization in the nitrene. Complexation may be achieved through two different methods: complexation with cations in aqueous salt solutions and complexation of cations inside crown ethers. Crown ethers provide useful models due to the selectivity of complexation with different ions based on ring size and slower diffusion of cations away from the N-oxide group. Progress toward the multi-step synthesis of crown ethers containing the 4- azidopyridine N-oxide substructure is described herein. / Department of Chemistry

We have investigated the chemistry of 4-azidopyridine-N-oxide. One aspect of this thesis involved the photolysis of this azido heteroaromatic-N-oxide, to generate highly reactive nitrene intermediates. We have studied these intermediates and determined that the intermediates vary significantly in structure and reactivity from those of simpler phenyl and pyridyl systems. This study yields new insight into the structure, bonding, and energetics of nitrene species.The nitrene intermediate originating from this azide has features which could in the future help in synthetic practices and photoaffinity studies. It may aid in photoaffinity studies due to the fact that the N-oxide group imparts water solubility.This thesis investigates the premise that a change in the electronic nature of the aromatic ring results in a change in the systems reactivity. We are interested in whether changes in the nature of the ring in the azides induce large changes in the chemistry, and if so, whether they do so in a predictable manner.Another focus of the research involved 1,3-dipolar cycloaddition reactions with an azide, where a carbon-carbon double or triple bond will attach to an aromatic azide, 4-azidopyridine-N-oxide in this case, at the azido group (-N3) to generate new ring compounds. 1,3-dipolar cycloadditions are fascinating because the mechanism which takes place in the attachment phase is still under debate. These reactions were investigated by reacting 4-azidopyridine-N-oxide with an unsaturated compound (alkene or alkyne) and then analyzing the product mixtures by HPLC methods. / Department of Chemistry

Thesis (Ph. D.)--Brandeis University, 2000. / "UMI:9967593." MICROFILM COPY ALSO AVAILABLE IN THE UNIVERSITY ARCHIVES. Includes bibliographical references.

The studies presented in this thesis are in two areas. An investigation of aromatic nitrene molecules using EPR spectroscopy was undertaken. This revealed a general tendency for these molecules to exhibit two forms when trapped in a crystalline lattice. This is qualitatively interpreted as a perturbing effect of nitrogen evolved during generation from the azide. Single crystal experiments were made on 2,4,6-tribromophenyl nitrene, which exhibited an unusually large anisotropy in the zero field splitting, implying a bent structure. Temperature variation of several aromatic nitrene zero field splittings were made. The variation was interpreted as thermal averaging amongst states possessing different zero field parameters. The nature of these states however, could not be inferred. The variation is compared with similar studies. A computer program to least squares fit EPR angular data to a spin Hamiltonian has been written. This programme was used to obtain a description of the angularly dependent EPR data from an aromatic nitrene, and also for a radical species with a large hyperfine interaction. / Science, Faculty of / Chemistry, Department of / Graduate

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Transition metal catalyzed selective nitrene insertion into C-H bonds, which allows direct incorporation of nitrogen functionality into hydrocarbons, represents an appealing methodology for C-N bond formation, a type of bond formation of great importance in organic synthesis due to the prevalence of amino groups in biologically active natural products and pharmaceuticals. Organic azides are atom-economic and an environment-benign nitrene source. This dissertation reports the use of organic azides as a nitrene source to develop a series of protocols for C-H bond functionalization by metal-catalyzed nitrene insertion, including the diimination of indoles, the phosphoramidation of aldehydes and the amination of hydrocarbons catalyzed by ruthenium porphyrins. Carbonylruthenium(II) porphyrin complex Ru(TTP)(CO) (TTP = meso-tetrakis(p-tolyl)porphyrinato dianion) is an effective catalyst for nitrene transfer to sp2 C-H bonds of indoles using aryl azides (ArN3) as a nitrene source. This “Ru(TTP)(CO) + ArN3” protocol selectively results in the diimination of indoles without the corresponding monoimination products being detected. In the presence of a catalyst Ru(TTP)(CO), the reactions of N-methylindole with ArN3 (Ar = 4-nitrophenyl; 3,5-bis(trifluoromethyl)phenyl), and reactions of a variety of N-substituted indoles with 4-nitrophenylazide, afford 2,3-diiminoindoles in good to excellent yields (up to 90%). This unique type of 2,3-diimination products was characterized by NMR spectroscopy, mass spectrometry and single crystal X-ray crystallography. The catalytic diimination product from N-methylindole and ArN3 (Ar = 3,5-bis(trifluoromethyl)phenyl) can also be obtained through stoichiometric reaction of N-methylindole with the corresponding bis(arylimido)ruthenium(VI) porphyrin, suggesting the possible involvement of RuVI(TTP)(NAr)2 intermediates in the Ru(TTP)(CO)-catalyzed diimination reactions. Dichlororuthenium(IV) porphyrin complex Ru(TTP)Cl2 efficiently catalyzes the phosphoramidation of aldehydes with phosphoryl azides (RO)2P(O)N3 via a nitrene insertion into sp2 C-H bonds of aldehydes. This represents the first study on the catalytic activity of a ruthenium(IV) porphyrin towards nitrene insertion into C-H bonds. The “Ru(TTP)Cl2 + (RO)2P(O)N3” protocol exhibits high chemoselectivity and functional group tolerability. Good to excellent product yields (up to 99%) have been obtained for the Ru(TTP)Cl2-catalyzed phosphoramidation of a wide variety of aldehydes with commercially available (PhO)2P(O)N3 (DPPA) and phosphoramidation of p-tolualdehyde with various (RO)2P(O)N3 (R = Me, Et, CCl3CH2, 4-nitrophenyl). The reaction can be scaled up by adding phosphoryl azide dropwise. The use of commercially available DPPA in this protocol offers a convenient and practical method for the synthesis of N-acylphosphoramidates. “Ru(TDCPP)Cl2 + (CCl3CH2O)2P(O)N3” (TDCPP = meso-tetrakis(2,6-dichlorophenyl)porphyrinato dianion) serves as an effective protocol for intermolecular nitrene insertion into sp3 C-H bonds of hydrocarbons. Using this protocol, a variety of hydrocarbons including cycloalkanes (such as cyclohexane) and ethylbenzenes undergo sp3 C-H amination in moderate to high yields (up to 86%). Compared with ruthenium(II) porphyrins such as Ru(TDCPP)(CO) and dirhodium carboxylates such as Rh2(OAc)4, Ru(TDCPP)Cl2 displays a markedly higher catalytic activity towards the nitrene sp3 C-H insertion with (CCl3CH2O)2P(O)N3. In addition, intramolecular nitrene insertion into sp3 C-H bond can also take place in good yields with Ru(TDCPP)Cl2 as the catalyst. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy