Photochemistry studies of phenyl substituted-1,2,4-thiadiazoles have revealed that 5-phenyl-1,2,4-thiadiazoles 31, 90, 98, 54 and 47 undergo a variety of photochemical reactions including photofragmentation, phototransposition, and photo-ring expansion while irradiation of 3-phenyl-1,2,4-thiadiazoles 46, 105 and 106 leads mainly to the formation of photofragmentation products. The formation of the phototransposition products has been suggested to arise from a mechanism involving electrocyclic ring closure and sigmatropic sulfur migration via a bicyclic intermediate: phenyl-1,3-diaza-5-thiabicyclo[2.1.0]pentene (BC). 15N-Labeling experiments confirm that sulfur undergoes sigmatropic shifts around all four sides of the diazetine ring. Thus, irradiation of 31-4-15N or 54-4-15N leads to the formation of 31-2-15N or 54-2-15N and to an equimolar mixture of 46-2-15N and 46-4-15N or 57-2-15N and 57-4-15N. Work in this laboratory on 15N-labeling of 46-2-15N also shows that 46 does not undergo electrocyclic ring closure but reacts exclusively by photofragmentation of the thiadiazole ring. 15N-Scrambling in the photofragmentation products observed after irradiation of 31-4-15N or 54-4-15N is greater than 15N-scrambling in the starting thiadiazoles suggesting that these products cannot arise only from direct fragmentation of the thiadiazole rings. An additional pathway for the formation of these products is required. The formation of phenyltriazines, the photo-ring expansion products 39 and 40 or 65 and 66 from photolysis of 31 or 54 is proposed to arise via phenyldiazacyclobutadienes (CB), generated from elimination of atomic sulfur from the bicyclic intermediates. It is suggested that phenyldiazacyclobutadienes then undergo [4+2] cycloaddition self-coupling resulting in the formation of unstable tricyclic intermediates which finally cleave to give phenyltriazines and nitriles. The observed 15N distribution in the phenyltriazine photoproducts formed after photolysis of 31-4-15N or 54-4-15N and the formation triazine 72 after irradiation of a mixture of 31+54 are consistent with this mechanism. The formation of nitriles by this pathway would account for the additional 15N-scrambling in the photofragmentation products. The photochemically generated phenyl-1,3-diaza-5-thiabicyclo[2.1.0]pentenes are the key intermediates in this suggested mechanism. In the presence of furan, these intermediates are expected to be trapped as Diels-Alder adducts. Irradiation of phenylthiadiazoles 31, 54 and 47 in furan solvent lead to increased consumption of these thiadiazoles, to quenching of the known photoproducts, and to the formation of new products suggested to result from furan trapping of the thiadiazoles followed by elimination of sulfur. Irradiation of 46 in furan solvent leads only to the formation of the photofragmentation product; no furan trapping adduct is observed. This result is consistent with the 15N-labeling experiment indicating that 46 does not undergo electrocyclic ring closure after irradiation. The photoreactivity of these phenylthiadiazoles in acetonitrile is substantially decreased when the phenyl ring at position 4 is substituted with an electron donating or withdrawing group. However, they are more photoreactive in cyclohexane solvent than in acetonitrile. The fluorescence emission spectra of these (4¢-substituted)phenyl-1,2,4-thiadiazoles exhibit moderate - large Stokes' shifts in acetonitrile. The magnitudes of these Stokes' shifts decrease in cyclohexane. This suggests a charge transfer character associated with the excited states of these thiadiazoles. In acetonitrile, these charge transfer excited states would be stabilized and become the lowest energy excited state. These charge transfer excited states would not be photoreactive and, thus, fluorescence emission becomes an effective deactivation process. In cyclohexane solvent, the charge transfer excited states would be less stabilized and, thus, the relaxed S1 would, then, become the lowest excited state. The relaxed S1 would be the state from which the observed photoproducts originate and the observed fluorescence with the smaller Stokes' shifts compared with the Stokes' shifts observed in acetonitrile.

The synthesis of fused nitrogen heterocycles has been investigated. Chapter 1 describes the synthesis of quinolones through a copper catalyzed amide coupling to ortho acetyl aryl halide followed by an aldol condensation that produces the quinolone in modest to good yields. The reaction works well with lactam partners and leads to tricyclic ring systems. Chapter 2 describes the synthesis of 8 membered enamide rings through a [3,3] sigmatropic rearrangement ring expansion of N-vinyl β-lactams. The N-vinyl β-lactams also undergo a 6π electrocyclization following the [3,3] sigmatropic rearrangement to furnish amino cyclobutanes. Previously reported amino cyclobutanes have been made using [2+2] photocycloaddition and resulted in a mixtures of diastereoisomers. Our method produces the amino cyclobutanes in good to high yields and with excellent levels of diastereoselectivity. Chapter 3 describes the synthesis and applications of reduced hydantoins from aziridine aldehydes and isocyanates. The resulting heterocycles contain an aziridine ring fused to a 5 membered urea. We also demonstrate that the reduced portion of the hydantoin, namely the hydroxyl group, can undergo further displacement with β-ketoesters similar to iminium ion chemistry.

The synthesis of fused nitrogen heterocycles has been investigated. Chapter 1 describes the synthesis of quinolones through a copper catalyzed amide coupling to ortho acetyl aryl halide followed by an aldol condensation that produces the quinolone in modest to good yields. The reaction works well with lactam partners and leads to tricyclic ring systems. Chapter 2 describes the synthesis of 8 membered enamide rings through a [3,3] sigmatropic rearrangement ring expansion of N-vinyl β-lactams. The N-vinyl β-lactams also undergo a 6π electrocyclization following the [3,3] sigmatropic rearrangement to furnish amino cyclobutanes. Previously reported amino cyclobutanes have been made using [2+2] photocycloaddition and resulted in a mixtures of diastereoisomers. Our method produces the amino cyclobutanes in good to high yields and with excellent levels of diastereoselectivity. Chapter 3 describes the synthesis and applications of reduced hydantoins from aziridine aldehydes and isocyanates. The resulting heterocycles contain an aziridine ring fused to a 5 membered urea. We also demonstrate that the reduced portion of the hydantoin, namely the hydroxyl group, can undergo further displacement with β-ketoesters similar to iminium ion chemistry.

This Thesis describes the systematic theoretical study aimed at prediction of the essential properties for the functional organic molecules that belong to diarylethene (DA) family of compounds. Diarylethenes present the distinct ability to change color under the influence of light, known as photochromism. This change is due to ultrafast chemical transition from open to closed ring isomers (photocyclization). It can be used for optical data storage, photoswitching, and other photonic applications. In this work we apply Density Functional Theory methods to predict 6 of the related properties: (i) molecular geometry; (ii) resonant wavelength; (iii) thermal stability; (iv) fatigue resistance; (v) quantum yield and (vi) nanoscale organization of the material. In order to study sensitivity at diode laser wavelengths, we optimized geometry and calculated vertical absorption spectra for a benchmark set of 28 diarylethenes. Bond length alternation (BLA) parameters and maximum absorption wavelengths (λmax) are compared to the data presently available from X-ray diffraction and spectroscopy experiments. We conclude that TD-M05/6-31G*/PCM//M05-2X/6-31G*/PCM level of theory gives the best agreement for both the parameters. For our predictions the root mean square deviation (RMSD) are below 0.014 Å for the BLAs and 25 nm for λmax. The polarization functions in the basis set and solvent effects are both important for this agreement. Next we consider thermal stability. Our results suggest that UB3LYP and UM05-2X functionals predict the activation barrier for the cycloreversion reaction within 3-4 kcal/mol from experimental value for a set of 7 photochromic compounds. We also study thermal fatigue, defined as the rate of undesirable photochemical side reactions. In order to predict the kinetics of photochemical fatigue, we investigate the mechanism of by-product formation. It has been established experimentally that the by-product is formed from the closed isomer; however the mechanism was not known. We found that the thermal by-product pathway involves the bicyclohexane (BCH) ring formation as a stable intermediate, while the photochemical by-product formation pathway may involve the methylcyclopentene diradical (MCPD) intermediate. At UM05-2X/6-31G* level, the calculated barrier between the closed form and the BCH intermediate is 51.2 kcal/mol and the barrier between the BCH intermediate and the by-product 16.2 kcal/mol. Next we investigate two theoretical approaches to the prediction of quantum yield (QY) for a set of 14 diarylethene derivatives at the validated M05-2X/6-31G* theory level. These include population of ground-state conformers and location of the pericycylic minimum on the potential energy surface 2-A state. Finally, we investigate the possibility of nanoscale organization of the photochromic material based on DNA template, as an alternative to the amorphous polymer matrix. Here we demonstrate that Molecular Dynamic methods are capable to describe the intercalation of π-conjugated systems between DNA base pairs and accurately reproduced the available photophysical properties of these nanocomposites. In summary, our results are in good agreement with the experimental data for the benchmark set of molecules we conclude that Density Functional Theory methods could be successfully used as an important component of material design strategy in prediction of accurate molecular geometry, absorption spectra, thermal stability of isomers, fatigue resistance, quantum yield of photocyclization and photophysical properties of nanocomposites.

In this work, the [3,3] sigmatropic rearrangement of different substituted propargyl thiocyanates and double [3,3] sigmatropic rearrangement of enynyl isothiocyanates either by flash vacuum pyrolysis or by thermolysis in solution are studied. Additionally, the intramolecular reactions of the resulting allenyl isothiocyanates are studied, and the reaction mechanisms for the generation of the final products, such as [1,5] sigmatropic migrations or electrocyclic ring closures, are explained. These highly reactive allenyl isothiocyanates are used as appropriate electrophilic precursors for the preparation of novel examples of thiazoles substituted at C-2 position using different types of nucleophiles. For the formation of these substituted thiazoles, the necessary nucleophilicity as well as the regioselectivity, the stereoselectivity, and the reaction mechanisms are investigated. / In der vorliegenden Arbeit sind die [3,3]-sigmatrope Umlagerung von verschiedenen Propargylthiocyanaten und die doppelte [3,3]-sigmatrope Umlagerung von Eninylisothiocyanaten entweder durch Blitzvakuumthermolyse oder Thermolyse in Lösung untersucht worden. Zusätzlich wurden die intramolekularen Reaktionen der resultierten Allenylisothiocyanate studiert. Außerdem sind die Reaktionsmechanismen zur Bildung der Thermolyseprodukte wie z.B. die [1,5]-sigmatropen Umlagerungen und die elektrocyclischen Ringschlüsse erklärt. Die hochreaktiven Allenylisothiocyanate sind als geeignet elektrophile Vorläufer zur Synthese von neuen Thiazolen verwendbar, die an der C-2 Position substituiert sind. Dabei kommen verschiedene Nucleophile zum Ansatz. Für die Bildung dieser substituierten Thiazole sind die Regioselektivität, Stereoselektivität, Reaktionsmechanismen und der Bereich der einsetzbaren Nucleophile untersucht worden.

In this work, the [3,3] sigmatropic rearrangement of different substituted propargyl thiocyanates and double [3,3] sigmatropic rearrangement of enynyl isothiocyanates either by flash vacuum pyrolysis or by thermolysis in solution are studied. Additionally, the intramolecular reactions of the resulting allenyl isothiocyanates are studied, and the reaction mechanisms for the generation of the final products, such as [1,5] sigmatropic migrations or electrocyclic ring closures, are explained. These highly reactive allenyl isothiocyanates are used as appropriate electrophilic precursors for the preparation of novel examples of thiazoles substituted at C-2 position using different types of nucleophiles. For the formation of these substituted thiazoles, the necessary nucleophilicity as well as the regioselectivity, the stereoselectivity, and the reaction mechanisms are investigated. / In der vorliegenden Arbeit sind die [3,3]-sigmatrope Umlagerung von verschiedenen Propargylthiocyanaten und die doppelte [3,3]-sigmatrope Umlagerung von Eninylisothiocyanaten entweder durch Blitzvakuumthermolyse oder Thermolyse in Lösung untersucht worden. Zusätzlich wurden die intramolekularen Reaktionen der resultierten Allenylisothiocyanate studiert. Außerdem sind die Reaktionsmechanismen zur Bildung der Thermolyseprodukte wie z.B. die [1,5]-sigmatropen Umlagerungen und die elektrocyclischen Ringschlüsse erklärt. Die hochreaktiven Allenylisothiocyanate sind als geeignet elektrophile Vorläufer zur Synthese von neuen Thiazolen verwendbar, die an der C-2 Position substituiert sind. Dabei kommen verschiedene Nucleophile zum Ansatz. Für die Bildung dieser substituierten Thiazole sind die Regioselektivität, Stereoselektivität, Reaktionsmechanismen und der Bereich der einsetzbaren Nucleophile untersucht worden.