Flash vacuum pyrolysis (FVP) of cyclopenta-2,4-dienylidenemethylbenzene gave acenaphthylene¡Bacenaphthene and dimer¡G5a,5b,11b,11c-tetrahydrocyclobuta[1",2":3,4;4",3":3',4']dicyclopenta[1,2-a:1',2'-a']diindene¡CPyrolysis of 1-bromo-4-cyclopenta-2,4-dienylidenemethylbenzene gave acenaphthene¡Bdimer¡B5-bromoacenaphthylene and 5-bromoacenaphthene¡CPyrolysis of 2-cyclopenta-2,4-dienylidenemethyl-3-methylthiophene gave 5H-1-thia-s-indacene and 7H-1-thia-s-indacene¡Aand naphthalene¡CPyrolysis of 2-inden-1-ylidene-methyl-3-methylthiophene gave 5H-1-thiacyclopenta[b]fluorene and unidentified products¡C

Flash vacuum pyrolysis (FVP) of 1-(2-(cyclopenta-2,4-dienylidene)ethyl)benzene gave 1H-benzo[e]indene, 3H-benzo[e]indene, and fluorene. Pyrolysis of 2-cyclopentadienylidenemethyl-3-methylbenzofuran gave 1H-benzofurano[2,3-d]indene and 3H-benzofurano[2,3-d]indene. Pyrolysis of 2-cyclopentadienylidenemethylbenzofuran gave 1H-benzo[e]indene, 3H-benzo[e]indene, fluorene, 2-phenylbenzofuran, and unidentified products

The main focus of this thesis is on the aromaticity of the ground state and electronically excited states of π-conjugated molecules and polymers, as well as how aromaticity is connected to their properties. The electronic structures of polybenzenoid hydrocarbons (PBHs) were explored through density functional theory (DFT) calculations and the π-component of the electron localization function (ELFπ). The study revealed how the π-electronic structure is influenced by the fusion of double bonds or benzene rings to the PBHs. We also demonstrated that the π-electrons of benzene extend to accommodate as much aromaticity as possible when bond length distorted.   The aromatic chameleon property displayed by fulvenes, isobenzofulvenes, fulvalenes, bis(fulvene)s, and polyfulvenes were investigated using DFT calculations. The tria-, penta-, and heptafulvenes were shown to possess ionization energies and electron affinities which can be tuned extensively by substitution, some of which even outperform TTF and TCNQ, the prototypical electron donor and acceptor, respectively. The singlet-triplet energy gap of pentafulvenes can be tuned extensively by substitution to the point that the triplet state is lower than the singlet state and thus becomes the ground state. The ELFπ of isobenzofulvene shows that the benzene ring in an electronically excited state can be more aromatic than the corresponding ring in the ground state. We have shown that the 6-ring of [5.6.7]quinarene is influenced by a Hückel aromatic resonance structure with 4n+2 π-electrons in the excited quintet state. The bis(fulvene)s which are composed of a donor type heptafulvene and an acceptor type pentafulvene, retain the basic donor-acceptor properties of the two fragments and could function as compact donor-acceptor dyads. A few of the designed polyfulvenes were found to have band gaps below 1 eV at the PBC-B3LYP/6-31G(d) level. Various 2,7-disubstituted fluorenones and dibenzofulvenes were synthesized and their excited state properties were investigated by absorption spectroscopy and time-dependent DFT calculations. It was found that the 1A → 1B transition of ππ* character can be tuned by substitution in the 2,7-positions. The 2,7-bis(N,N-dimethyl) derivatives of fluorenone and dibenzofulvene displayed low energy transitions at 2.18 and 1.61 eV, respectively, in toluene.

Organometallic pyridazines and compounds derived from them have been of interest in polymer research due to their atypical environmental stability (as compared to other non-aromatic organic semiconductors) as well as their conductivity. The off-metal synthesis and characterization of several pyridazyl thallium, manganese, and rhenium complexes, beginning with fulvenes 1,2- C5H3(COHR)(COR), are reported here. The diacyl fulvenes were treated with hydrazine hydrate to ring close to pyridazines. Next, the pyridazines were converted to their respective thallium salts through treatment with thallium (I) ethoxide. Lastly, the salts were transmetallated into the respective rhenium or manganese complexes through treatment with the respective metal bound to five carbonyl groups and one bromide (MnCO5Br, ReCO5Br). Our research focused on the synthesis of a variety of 5,6-fused ring pyridazines that will act as a model for homeland security research in polymer studies and medicinal research. Synthesis and characterization of several aryl-substituted 5,6-fused ring pyridazines have been completed. The fulvenes, pyridazines, and thallium salts are important compounds for research due to their reactivity and stability in moisture and air. The complexes synthesized were confirmed using mass spectrometry, infrared spectroscopy, nuclear magnetic resonance spectroscopy, and elemental analysis. Single crystal X-ray diffraction confirmed the structure of x compound 1A. As evidenced by related previous papers published by the research group, the synthesized complexes displayed stability to air, moisture, and temperature.18, 19, 23, 25

The thesis entitled "Structure and reactivity in bridged polycyclic systems: cis-trans enantiomerism, fulvene cycloadditions and crystallographic studies of bridgehead β-ketoacids " consists of two parts. Part I contains 3 chapters, and deals with cycloaddition reactions of 6-arylfulvenes with maleic anhydride and nitrones (The products in the case of maleic anhydride display cis-trans enantiomerism). Part II contains 2 chapters, and deals with resolution of racemic primary amines, racemic amino acids and the relative decarboxylation propensities of bicyclic β-ketoacids in solid state. Part I Chapter 1: A new case of the uncommon cis-trans enantiomerism is presented in the Diels-Alder cycloadducts (3 & 4) of 6-arylfulvenes (1) with maleic anhydride (2).1 The resolution of the cis-trans enantiomers were accomplished via the formation of diastereomeric imides 6 and 7 with (1S)-(naphth-1-yl)ethylamine (5), and their subsequent hydrolysis and recyclisation (Scheme 1). The enantiomers 3 and 4 were characterized spectrally, polarimetrically (including CD) and by chiral HPLC. The chiral anhydrides were also stereospecifically converted to the corresponding imides by treatment with aq. ammonia in excellent yields. The crystal structure of one of the diastereomeric imides (derived from 6-phenylfulvene) was determined, and based on the known S configuration of the naphthylethylamine moiety, the configurations of the original anhydride adducts could be assigned.2 Scheme 1 Chapter 2: In this chapter tricyclic imides (8a-c) were prepared by Diels-Alder reaction of 6-arylfulvenes (1a-c) and maleic anhydride (2),2 followed by treatment with aq. NH3. The exo isomers were found to exist as conglomerates when the aryl group was p-tolyl or p-anisyl (although not phenyl). Triage of the p-tolyl racemate (Scheme 2), followed by reaction with p-toluenesulphonyl chloride in CH2Cl2/Et3N, led to the crystalline enantiopure N-tosylimides 9 (These were also found to be conglomerates). X-ray diffraction analysis of the N-tosylimides (9) via the anomalous dispersion technique led to the assignment of the absolute configurations (as either E or Z).3, 4 The original p-tolyl imide enantiomers were found to racemise upon UV irradiation in CHCl3. Based on this, a possible second order asymmetric transformation under photochemical conditions was attempted, and indeed led to the isolation of crystalline imide with a small ee (~15%).5 Scheme 2 Chapter 3: This chapter deals with the fulvene-nitrone cycloadditions. The possibility of discovering examples of the rare (6π + 4π) cycloaddition prompted an exploration of the reaction between electron-rich nitrones and pentafulvenes. In previous reports of such cycloadditions, diazomethane or benzonitrile oxide was used as 4π component.6 Building on previous work from this laboratory,7 the reaction between a set of substituted fulvenes and electron rich nitrones were studied. Theoretical calculations indicate that the (6π + 4π) mode would be favored when the fulvene-nitrone cycloaddition is controlled by the LUMO (fulvene) – HOMO (nitrones) interaction.8 Electron withdrawing groups on the fulvene would lower the LUMO and facilitate the above orbital interaction. Therefore the reaction between electron poor fulvenes and nitrones was taken up for further study. In particular, fulvene (10) was reacted with nitrones (11). However, only a (2π + 4π) mode was observed, involving one of the endocyclic double bond of the fulvene, in moderate yields (Scheme 3). Structures of these adducts were assigned based on NMR and X-ray crystal structure determination. The failure to observe the (6π + 4π) mode (14) is intriguing, and it is not clear whether this is due to electronic or steric reason. Scheme 3 Part II Chapter 1 describes the resolution of racemic primary amines and racemic amino acids (16) via the formation of diastereomeric imides. For this purpose D-camphoric anhydride (15) was chosen as the chiral auxiliary for the following reasons: it is of low-molecular weight with a rigid backbone, and is also easily prepared and purified.9 Primary amine (16) was treated with D-camphoric anhydride (15) in presence of CHCl3/DCC to form the corresponding diastereomeric imides 17 and 18. (In the case of amino acids, the corresponding methyl esters were treated with D-camphoric anhydride (15) in presence of triethylamine in chloroform). The resulting diastereomeric imides 17 and 18 were separated by silica gel column chromatography (Scheme 4), and hydrolyzed to the chiral amines (or amino acids). (The by-produced camphoric acid could be reconverted to D-camphoric anhydride (15). Scheme 4 Chapter 2: The relative ease with which β-ketoacids tend to lose CO2 is intriguing and has been the focus of numerous mechanistic studies.10-12 It is generally believed that the decarboxylation of β-ketoacids occurs via a six-centered hydrogen bonded transition state (19), which leads to the formation of the enol tautomer (20) of the final ketone product (Scheme 5). Scheme 5 Scheme 6 The initial formation of the enol is apparently supported by the high thermal stability of bicyclic β-ketoacids, in which the carboxylic acid functionality is at bridgehead. In these the formation of the enol would be disfavored by Bredt’s rule, which forbids the formation of a double bond at the bridgehead (particularly in the smaller bicyclic compounds). Also, it may be expected that these trends would be manifested in the ground state. This is because there would be a stereoelectronic requirement for the decarboxylation reaction, by which the bond to the carboxylic group would need to be parallel to the C=O π bond of the keto group. Therefore, it was of interest to study the crystal structures of suitable β-ketoacids in the hope of evidencing the above structural trends (Structure for the analogs 21-23 have been reported previously (Scheme 6)).13-15 In fact, the approach pioneered by Dunitz was of particular interest in this regard. 16 In this approach crystal structures of a series of analogs were studied; these analogs possess varying degrees of strain that could be considered as leading to the transition state of a certain reaction. The bond length and related data are then employed to ‘map’ the reaction dynamics. Compound Bond* lengths (Å) Increase in the bond length compared to ketopinic acid (%) Decarboxylation temp.17 * fine bond at the bridgehead to the COOH group. In the case of the decarboxylation of β-ketoacids, a correlation between the lengthening of the bond to the COOH group and the ease of decarboxylation was sought. Therefore the set of analogs 24-26 were prepared (Scheme 6) and their crystal structures determined by X-ray diffraction (at 100K). In the case of 26, an increase of 2.47% relative to 21 in the Cα-COOH bond length was observed. However, no evidence for an intramolecular O=C-O-H…O=C H-bonding, was observed in the crystal structures of 24-26. Instead, the COOH moieties were seen to participate in intermolecular O-H…O hydrogen bonding via the well known carboxylic acid dimer motif. The β-ketoacids were also converted into their corresponding S-benzylisothiouronium salts (Scheme 6), to study the effect of destroying the COOH dimer motif. The salts 27 and 28 could be obtained in a form suitable for single crystal X-ray diffraction. The crystal structures revealed an increase in the Cα-COO- bond length to an extent of 1.97% in case of 28 relative to 27. Also, there is an increase in the relevant bond length of ~0.8% on going from 24 (m.p. 145 °C) to 26 (m.p. 132 °C). Note also that these compounds melts with decompositions. Therefore, it appears that the ease of decarboxylation of these analogs is reflected in the relative lengthening of the bond to the COOH group. Thus, this study represents an application of the Dunitz crystallographic approach to reaction dynamics,16 to the case of the decarboxylation of β-ketoacids.(For structural formula pl see the pdf file)

The cyclopentadienone-fused dihydropyrenes 46 and 47 were synthesized. The internal methyl resonances, the coupling constants, NICS calculations and X-ray results confirmed that the cyclopentadienone displays antiaromatic character resulting in bond localization in the annulene ring consistent with a 4n-pi fused system. The ring current of the dihydropyrene fragment is reduced by fusion of the antiaromatic system by about 80% of that caused by benzene. The syntheses of the methylfulvene fused dihydropyrene 56 and the phenylfulvene fused dihydropyrene 58 have been accomplished. The calculated and experimental NMR data and NICS calculations all demonstrated that the fulvenes had weak diatropic ring currents and caused bond localizations in the DHP rings, in which phenyl fulvene has a larger effect than that of methyl fulvene. A number of bis-dihydropyrene systems, bis-dihydropyrene ketone 117, bis-benzo[e]dihydropyrene ketone 119, benzo[e]dihydropyrene dihydropyrene ketone 122, bis-benzo[e]dihydropyrene methylene 124 and the benzo[e]dihydropyrene- dihydropyrene acetylene 130, have been synthesized, in which 117, 119 and 124 are homo-systems and 122 and 130 are hetero-systems. The multiple photoswitching properties study found that all of these systems except 130 showed multi-states during the photo opening and photo closing processes, which means that each end of the DHP units photo opens or closes separately rather than synchronously. In the homo switches 117, 119 and 124, the two DHP units act independently, but the relative differentiation is not very significant. On the other hand, the hetero-switch 122 showed fully differentiated photo opening process and almost a pure open-closed intermediate 122’ could be achieved. This is the first example which clearly showed four states in the UV closing process. The relative photo opening and closing rates compared to benzoDHP 36 have also been studied. It was found that while the carbonyl linker largely increased the relative photo opening rate (117, 119 and 122), the methylene linker only increased it slightly (124). The photo closing processes were always fast as usual. The studies of the thermal return reactions of these systems showed that while the carbonyl linker substantially slowed down the thermal return reactions of the DHP units (117, 119 and 122), the methylene linker speeded it up slightly (124). The mono-iron tricarbonyl benzo[e]dihydropyrene complex 152, the bis-iron tricarbonyl benzo[e]dihydropyrene complex 153 and the iron tetracarbonyl dihydropyrene complex 151 were synthesized. The structures of 152 and 153 were determined by X-ray crystallography. The coordinations of iron tricarbonyl moieties to the DHP rings caused a distortion of ca. 30 degree away from the central DHP plane. Coordination also increased bond alternation and reduced ring currents in the DHP rings. 1H-NMR and X-ray studies showed that 152 showed a weak paratropic ring current in the DHP ring. Iron coordination of the DHP completely stopped the photochromic properties of the dihydropyrenes.