Formation Of Benzene Rings Research Articles

A Versatile Thioesterase Involved in Dimerization during Cinnamoyl Lipid Biosynthesis

AbstractThe cinnamoyl lipid compound youssoufene A1 (1), featuring a unique dearomatic carbon‐bridged dimeric skeleton, exhibits increased inhibition against multidrug resistant Enterococcus faecalis as compared to monomeric youssoufenes. However, the formation process of this intriguing dearomatization/dimerization remains unknown. In this study, an unusual “gene‐within‐gene” thioesterase (TE) gene ysfF was functionally characterized. The gene was found to naturally encodes two proteins, an entire YsfF with α/β‐hydrolase and 4‐hydroxybenzoyl‐CoA thioesterase (4‐HBT)‐like enzyme domains, and a nested YsfFHBT (4‐HBT‐like enzyme). Using an intracellular tagged carrier‐protein tracking (ITCT) strategy, in vitro reconstitution and in vivo experiments, we found that: i) both domains of YsfF displayed thioesterase activities; ii) YsfF/YsfFHBT could accomplish the 6π‐electrocyclic ring closure for benzene ring formation; and iii) YsfF and cyclase YsfX together were responsible for the ACP‐tethered dearomatization/dimerization process, possibly through an unprecedented Michael‐type addition reaction. Moreover, site‐directed mutagenesis experiments demonstrated that N301, E483 and H566 of YsfF are critical residues for both the 6π‐electrocyclization and dimerization processes. This study enhances our understanding of the multifunctionality of the TE protein family.

A Versatile Thioesterase Involved in Dimerization during Cinnamoyl Lipid Biosynthesis.

The cinnamoyl lipid compound youssoufene A1 (1), featuring a unique dearomatic carbon-bridged dimeric skeleton, exhibits increased inhibition against multidrug resistant Enterococcus faecalis as compared to monomeric youssoufenes. However, the formation process of this intriguing dearomatization/dimerization remains unknown. In this study, an unusual "gene-within-gene" thioesterase (TE) gene ysfF was functionally characterized. The gene was found to naturally encodes two proteins, an entire YsfF with α/β-hydrolase and 4-hydroxybenzoyl-CoA thioesterase (4-HBT)-like enzyme domains, and a nested YsfFHBT (4-HBT-like enzyme). Using an intracellular tagged carrier-protein tracking (ITCT) strategy, in vitro reconstitution and in vivo experiments, we found that: i) both domains of YsfF displayed thioesterase activities; ii) YsfF/YsfFHBT could accomplish the 6π-electrocyclic ring closure for benzene ring formation; and iii) YsfF and cyclase YsfX together were responsible for the ACP-tethered dearomatization/dimerization process, possibly through an unprecedented Michael-type addition reaction. Moreover, site-directed mutagenesis experiments demonstrated that N301, E483 and H566 of YsfF are critical residues for both the 6π-electrocyclization and dimerization processes. This study enhances our understanding of the multifunctionality of the TE protein family.

Pyrolytic decomposition of methanol, ethanol, and butanol at various temperatures and residence times in a high-temperature flow reactor

The potential use of low-carbon renewable fuel molecules, such as short chain alcohols, as a substitute or extender for fossil fuels, offers an opportunity to address pollutant emissions from transport and the sustainability of energy sources. Moreover, it may help reduce pollutants detrimental to air quality and human health, of which particulate matter (PM) is a significant contributor. To this end, the pyrolysis of three renewable alcohol fuels, methanol, ethanol, and butanol, has been studied in a laminar flow reactor at a sample inlet temperature range of 869 0C to 1120 0C. All the tested fuels were injected into a nitrogen carrier gas stream at a fixed concentration of 10,000 ppm on a carbon atom basis. A range of pyrolysis temperatures and residence times were selected so as to gain insight into the pyrolytic decomposition of short chain alcohol fuels to intermediate species important to the subsequent formation of benzene rings, larger polycyclic aromatic hydrocarbons (PAH), and particulate matter. In this paper, the presence of ten intermediary species that may aid in the formation of benzene rings and the growth of PAHs were quantified, with an emphasis on probable routes of first ring formation from the alcohols of varying chain length. Gaseous samples were collected from the pyrolyser at different temperatures and residence times using a novel high-temperature air-cooled ceramic sampling probe. The identification and quantification of intermediate gaseous species were undertaken by Gas Chromatography-Flame Ionization Detection (GC-FID). During methanol pyrolysis, only a tenth of the supplied carbon was detected as hydrocarbon gaseous species (mainly methane), with no benzene found, highlighting that methanol pyrolysis does not readily result in polyaromatic hydrocarbons and soot. Ethanol and butanol pyrolysis produced substantial amounts of ethylene and acetylene, suggesting a role in benzene and soot formation. Uniquely, pyrolysis of the longest chain alcohol, butanol, produced appreciable levels of C3 and C4 hydrocarbons, in addition to C1 and C2 hydrocarbons, suggesting additional reaction pathways for benzene formation and growth in comparison with ethanol.

Furan and benzene ring formation in cellulose char: the roles of 5-HMF and reducing ends.

The authors previously proposed that 5-hydroxymethylfurfural (5-HMF) can be produced from the reducing ends of cellulose as a key intermediate during carbonization. The present work investigated the mechanisms by which furan and benzene rings are formed in cellulose char based on carbonization at 280 °C using 13C-labeled 5-HMF together with 13C-labeled glucose (as a model for the cellulose reducing ends). Glycerol was added to the 5-HMF to prevent the formation of stable glassy polymers. The resulting char was subjected to pyrolysis gas chromatography/mass spectrometry (764 °C, 5 s hold time) and the incorporation of 13C in furan-, phenol-, benzofuran- and benzene-type fragments was assessed. The apparent formation mechanisms include a direct rearrangement of the six carbons of 5-HMF to phenols, Diels-Alder reactions of furan rings with double bonds to give benzofurans and a more random process involving reactive fragments producing benzene rings. On the basis of these results, the roles of 5-HMF and reducing ends during cellulose carbonization are discussed herein.

Sampling of Gas-Phase Intermediate Pyrolytic Species at Various Temperatures and Residence Times during Pyrolysis of Methane, Ethane, and Butane in a High-Temperature Flow Reactor

Air pollution in many major cities is endangering public health and is causing deterioration of the environment. Particulate emissions (PM) contribute to air pollution as they carry toxic polyaromatic hydrocarbons (PAHs) on their surface. Abatement of PM requires continuous strict emission regulation and, in parallel, the development of fuels with reduced formation of PM. Key processes in the formation of PM are the decomposition of hydrocarbon fuels and the synthesis of potential precursors that lead to the formation of benzene rings and thereafter growth to PAHs and eventually PM. Methane, ethane and butane are important components of natural gas and liquefied petroleum gas, and are also widely used in transportation, industrial processes and power generation. This paper reports on a quantitative investigation of the intermediate gaseous species present during pyrolysis of methane, ethane and butane in a laminar flow reactor. The investigation aimed to further the understanding of the decomposition process of these fuels and the subsequent formation of aromatic rings. The pyrolysis of methane, ethane and butane were carried out in a tube reactor under laminar flow conditions and within a temperature range of 869–1213 °C. The fuels were premixed in nitrogen carrier gas at a fixed carbon atom concentration of 10,000 ppm, and were pyrolysed under oxygen-free conditions. Intermediate gaseous species were collected from within the tube reactor at different residence times using a specially designed high-temperature ceramic sampling probe with arrangements to quench and freeze the reactions at entry to the probe. Identification and quantification of intermediate species were carried out using a gas chromatography-flame ionization detector (GC-FID). During methane pyrolysis, it was observed that as the concentration of acetylene increased, the concentration of benzene also increased, suggesting that the benzene ring is formed via the cyclo trimerisation of acetylene. With all three fuels, all intermediate species disappeared at higher temperatures and residence times, suggesting that those species converted into species higher than benzene, for example naphthalene. It was observed that increasing carbon chain length lowered the temperature at which fuel breakdown occurred and also affected the relative abundance of intermediate species.

Cytochrome P450 Catalyzes Benzene Ring Formation in the Biosynthesis of Trialkyl-Substituted Aromatic Polyketides.

Lorneic acid and related natural products are characterized by a trialkyl-substituted benzene ring. The formation of the aromatic core in the middle of the polyketide chain is unusual. We characterized a cytochrome P450 enzyme that can catalyze the hallmark benzene ring formation from an acyclic polyene substrate through genetic and biochemical analysis. Using this P450 as a beacon for genome mining, we obtained 12 homologous type I polyketide synthase (PKS) gene clusters, among which two gene clusters are activated and able to produce trialkyl-substituted aromatic polyketides. Quantum chemical calculations were performed to elucidate the plausible mechanism for P450-catalyzed benzene ring formation. Our work expands our knowledge of the catalytic diversity of cytochrome P450.

Cytochrome P450 Catalyzes Benzene Ring Formation in the Biosynthesis of Trialkyl‐Substituted Aromatic Polyketides

AbstractLorneic acid and related natural products are characterized by a trialkyl‐substituted benzene ring. The formation of the aromatic core in the middle of the polyketide chain is unusual. We characterized a cytochrome P450 enzyme that can catalyze the hallmark benzene ring formation from an acyclic polyene substrate through genetic and biochemical analysis. Using this P450 as a beacon for genome mining, we obtained 12 homologous type I polyketide synthase (PKS) gene clusters, among which two gene clusters are activated and able to produce trialkyl‐substituted aromatic polyketides. Quantum chemical calculations were performed to elucidate the plausible mechanism for P450‐catalyzed benzene ring formation. Our work expands our knowledge of the catalytic diversity of cytochrome P450.

Effects of prenol and isoprenol addition on soot and PAH formation in ethylene/methane counterflow diffusion flames

Prenol has attracted much attention due to its high heating value and novel octane hyperboosting phenomenon; while its soot formation behavior is not studied yet. In this study, experimental and numerical analysis of the effects of prenol and isoprenol addition on polycyclic aromatic hydrocarbon (PAH) and soot formation in non-premixed ethylene/methane flames is performed. Distribution of soot volume fraction and PAHs in the flames is measured via laser induced incandescence and laser induced fluorescence, respectively. The results show that both prenol and isoprenol addition enhances soot and PAH formation in ethylene/methane flames, and the prenol shows a stronger enhancing effect. Modeling of the sooting behavior is performed with a constructed gas-phase chemistry and a sectional method. The numerical results well capture the experimental observations and reveal that the increased concentrations of C2H2, C3H3 or C4H6 after prenol or isoprenol blending chemically promote benzene ring formation, subsequent PAH growth and soot formation processes. ROP (rate of production) analysis is performed to investigate the effect of prenol and isoprenol addition on rate-limiting ring formation process. In methane and ethylene flame, prenol or isoprenol blending promotes benzene formation in different ways, due to the discrepancies on the radical pool in two flames. Moreover, prenol also shows stronger effect on both of soot inception rate and surface growth rate than isoprenol, which is due to more PAHs and C2H2 formed in the prenol-doping flames. Reaction pathway analysis shows that decomposition of prenol generates C2H2 and C3H3 while isoprenol mainly generate C3H3 and almost no C2H2, the distinctive decomposition pathways of prenol results in its higher formation propensity of PAHs and soot.

In Vitro Reconstitution of Cinnamoyl Moiety Reveals Two Distinct Cyclases for Benzene Ring Formation.

Cinnamoyl-containing natural products (CCNPs) are a small class of bacterial metabolites with notable bioactivities. The biosynthesis of cinnamoyl moiety has been proposed to be assembled by an unusual highly reducing (HR) type II polyketide synthases (PKS). However, the biosynthetic route, especially the cyclization step for the benzene ring formation, remains unclear. In this work, we successfully reconstituted the pathway of cinnamoyl moiety in kitacinnamycin biosynthesis through a step-wise approach in vitro and demonstrated that a three-protein complex, Kcn17-Kcn18-Kcn19, can catalyze 6π-electrocyclization followed by dehydrogenation to form the benzene ring. We found that the three-protein homologues were widely distributed among 207 HR type II PKS biosynthetic gene clusters including five known CCNPs. In contrast, in the biosynthesis of youssoufene, a cinnamoyl-containing polyene, we identified that the benzene ring formation was accomplished by a distinct orphan protein. Thus, our work resolved the long-standing mystery in cinnamoyl biosynthesis and revealed two distinct enzymes that can synthesize benzene rings via polyene precursors.

Effects of NH3 addition on polycyclic aromatic hydrocarbon and soot formation in C2H4 co-flow diffusion flames

As a promising carbon-free alternative fuel, ammonia (NH3) cofiring with hydrocarbon fuels can improve its own combustion properties and suppress the soot emission. In this work, the chemical effects of NH3 on soot volume fraction (SVF) and polycyclic aromatic hydrocarbons (PAHs) spatial distributions and the soot morphological evolution were experimentally investigated in five C2H4 diffusion flames with and without NH3/Ar addition using nonintrusive and intrusive methods (planar laser induced incandescence, PLII, planar laser induced fluorescence, PLIF and thermophoretic sampling particle diagnostic method combined with transmission electron microscope, TSPD-TEM). In order to highlight the chemical effects of ammonia on soot and PAHs formation, taking argon-doping flame as reference, the chemical effects are defined as the disparity of soot reduction between the flames with doping Ar and NH3. Meanwhile, a numerical study was carried out to reveal the chemical effects of NH3 on the PAHs formation and its main pathways. Both the experimental and numerical results showed that NH3 delays and suppresses the formation of PAHs and NH3 has greater reduction influence than Ar. In addition, NH3 and Ar decrease the flame temperature at the early stage of flame and increase the temperature at the post stage. The soot distribution in pure C2H4 and C2H4NH3 flames revealed that as NH3 content increases, the region of maximum SVF is transformed from the flame wings to flame centerline. Moreover, with NH3 addition, the soot loading zone is shrunk and shifted to the downstream due to the lift-off of flame height and decrease of flame temperature. Sensitivity and reaction pathways analysis of A1 formation indicate that NH3 addition provides new pathways for C2H4 and NH3 through the nitrogen-hydrocarbon interactions. The existence of NH3 suppresses the key formation intermediates (C2H2, C3H3, PC3H4 and C4H4) responsible for the first aromatic benzene ring formation and larger PAH growth. In conclusion, the chemical effects of ammonia not only reduce the formation of soot and PAHs precursors, but significantly affect the evolution of soot morphology. The underlying mechanism of NH3 on the whole soot generation and oxidation process will be further revealed in the future work.

Synthesis of Triphenylene Derivatives: Introducing the Formation of Benzene Rings into the Experimental Teaching of Organic Chemistry

Synthesis of Triphenylene Derivatives: Introducing the Formation of Benzene Rings into the Experimental Teaching of Organic Chemistry

Pyrolysis of 2-methylfuran/jet fuel surrogate blends: An experimental and kinetic modeling study

Owing to significant advantages over traditional biofuels, 2-methylfuran (MF) has arisen extensive interest as next generation biofuels. Atmospheric pressure pyrolysis of a 1:1 blend of MF with a three-component (3C) surrogate jet fuel was conducted in a flow tube using synchrotronphotoionization and molecular beam mass spectrometry techniques within a temperature range of 898–1223 K. Therein, the 3C surrogate fuel consists of 66.2% n-dodecane (NC12H26), 18.0% 1,3,5-trimethylcyclohexane (T135MCH) and 15.8% n-propylbenzene (NPB, A1C3H7). The detailed kinetic model including 579 species and 3474 reactions was developed via validating against both measured and literature data with reasonable agreement. MF exhibits no obvious influence on the 3C components pyrolysis. However, 3C mixtures play an important roles in MF decomposition: lower initial temperature and different decomposition pathways. In the meantime, n-propylbenzene (NPB) pyrolysis curve in 3C and 3C+MF significantly transfers to low temperature zone after 1000 K in contrast to pure NPB, while smaller change was found in the decomposition curve of n-dodecane. Compared with 3C pyrolysis, 3C+MF blends have less formation of aromatic species and light hydrocarbons except for vinylacetylene (C4H4) and acetylene (C2H2). Allyl radical (C3H3) has significant roles in benzene ring formation. The combination of allyl radical with propargyl radical producing benzene and H radical (aC3H5 + C3H3 = A1 + 2H) has the highest contribution to the benzene formation in both 3C and 3C+MF. Moreover, most of naphthalene (A2) formation is also related to C3H3 radical via reaction: A1CH2 + C3H3 = A2 + H, rather than via HACA (hydrogen abstraction carbon addition) reaction. Interaction of fuel blends was mainly presented through the common and important sensitive reactions which are much to do with CH3 and H radicals. The overview of product distribution was investigated by providing comparisons of carbon conversion rates during the pyrolysis of 3C and 3C+MF. These results will be helpful to better understand the pyrolysis properties of MF/jet fuel surrogate blends and could contribute to developing new generation biofuels for aviation.

Palladium Catalyzed Intramolecular Cyclization and C-H Activation Reaction Interesting Tool for the Synthesis of Some Exquisite Molecules

We have successfully investigated the palladium catalyzed cyclization in diene system to the formation of three unusual products. By varying the reaction conditions, we observed the formation of fused cyclopentane-cyclopropane rings, fused cyclopentane-cyclopentane rings or benzene ring formation from similar types of diene substrates. The most unexpected and unusual step, the formation of cyclopropane, was studied using density functional theory based calculations.

Thermogravimetric and mass spectrometry analyses of cellulose pyrolysis under the synergistic effect of CaO and K2HPO4∙3H2O

ABSTRACTThe cellulose pyrolysis processes under the individual and synergistic effects of CaO and K2HPO4∙3H2O were studied by the thermogravimetric-mass spectrometric analysis (TG-MS) technique, mainly focusing on the changes of some important products such as CH4 during the pyrolysis process under different conditions. The experimental results showed that after the addition of CaO, the amounts of CO, CH4, and other species decreased. CaO could fix the “CO2-like reactive intermediate” produced during the cellulose pyrolysis process, thus reducing the evolution of CO2 in the temperature range of 300–440°C and continuing the CO2 evolution in the temperature range of 520–710°C. When K2HPO4∙3H2O was added, the amounts of CO, CH4, CO2, and other gaseous species increased, indicating that K2HPO4∙3H2O could significantly change the pyrolysis reaction process, such as promoting the benzene ring formation reactions involving small molecules such as toluene. When the mixture of CaO and K2HPO4∙3H2O was added, the weight loss at the initial stage of pyrolysis was relatively moderate compared to that adding K2HPO4∙3H2O alone. At the same time, an obvious weight loss appears in the pyrolysis process in the temperature range of 600–770°C, and the amounts of various species generated increased compared to those generated in the single catalytic pyrolysis. Therefore, CaO and K2HPO4∙3H2O exhibited a strong synergistic effect in the pyrolysis process.

On the opposing effects of methanol and ethanol addition on PAH and soot formation in ethylene counterflow diffusion flames

An experimental and numerical analysis of the effects of methanol and ethanol addition on polycyclic aromatic hydrocarbon (PAH) and soot formation in non-premixed ethylene flames is reported here. Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to measure soot volume fractions and relative PAH concentrations in counterflow diffusion flames, respectively. A comprehensive chemical kinetic analysis was performed by modeling soot with detailed gas-phase chemistry and a sectional method. The results showed that although both methanol and ethanol are typically regarded as clean fuels, their presence in ethylene diffusion flames had the opposite effects on PAH and soot formation. The LIF and LII signals decreased significantly as methanol fraction increased, suggesting a soot-inhibiting role for methanol. Apart from the fact that methanol addition reduced the carbon supply for soot thus having a fuel-dilution effect (methanol converted primarily to CO), the increased H2 concentration from methanol decomposition was seen to chemically suppress incipient benzene ring formation and subsequent PAH and soot growth processes. In contrast, a small amount of ethanol addition enhanced soot formation, which was well captured by the numerical model. Reaction pathway analysis showed that ethanol decomposition produced a relatively large amount of methyl radicals, enhancing the chemical interaction between CH3 and C2 species and, thereby promoting the formation of propargyl and C4 species. As a result, benzene formation was promoted through reactions between C2H2 and C4 species and via C3H3 recombination reaction, leading sequentially to the enhancement of PAH growth and soot formation.

Connecting a carbonyl and a π-conjugated group through a p-phenylene linker by (5+1) benzene ring formation.

A benzene ring was formed to connect a carbonyl group of various methyl ketones with a π-conjugated group through a p-phenylene linker. Methyl ketones and streptocyanines were used as the C1 and C5 sources, respectively, in the (5+1) annulation, which could form donor-π-acceptor molecules.

The Morphine and Codeine Equianalgesic’s

In Equianalgesic’s treatment, Morphine and Codeine are the strongest metabolism inhibitors of Opioids, the Phenanthrenes display symptoms of none, as their counterparts of the Opioid class, Methadone of the Diphenylheptanes, Meperidine of the Phenylpiperidines, and related tolerant building drugs in that nomenclature. With acute to chronic conditions, the withdrawal effects of the symptomatic opioids are of concern, and forth, solution may be the use of Phenanthrenes as opioid treatment. In addition, prevention of side-effects in postmedication and withdrawal effects in recovery. The Phenanthrenes compose chemical make-ups thought of no irritation due to the benzene ring formation, regarding the circulatory movement of the organic compound in the bodily fluid, and considering soluble properties comprised of. The dissolution of compounds create irritation of no such to the circulatory system, however, in contrast, mixture causes irritation to the folded tissue with issue in diffusion through the epithelial cell, due to the structure and the size of the opposing. Surprisingly, in comparison, soluble reactions with water allow diffusion to take place and acidification later in the kidney, which causes drainage of the waste product, hence, prevention of hepatic impairment. Insofar, renal impairments are avoided as well, due to the ionic gradient formed hormonally, and dissolving of Phenanthrenes to ions, thus allowing complete diffusion through the liver. To conclude, hepatic impairments and renal complications are prevented in transit of treatment due to chemical properties of the organic and inorganic compounds and the characteristics realized. Treatment of no symptoms and withdrawals of none after-effect should be experienced with future recovery of no tolerance and utter completeness.

Effects of DME mixing on number density and size properties of soot particles in counterflow non-premixed ethylene flames

In order to investigate the effect of DME mixing on the number density and size of soot particles, DME was mixed in a counter flow non-premixed ethylene flame with mixture ratios of 5%, 14% and 30%. A laser extinction/scattering technique has been adopted to measure the volume fraction, number density, and mean size of soot particles. The experimental results showed that the highest soot concentrations were observed for flames with mixture ratios of 5% and 14%; however, for a mixture ratio of 30% the soot concentration decreased. Numerical results showed that the concentrations of propargyl radicals (C3H3) at the 5% and 14% ratios were higher than those measured in the ethylene-based flame, and the production of benzene (C6H6) in the 5% and 14% DME mixture flames was also increased. This indicates the crucial role of propargyl in benzene ring formation. These reactions generally become stronger with increased DME mixing, except for A1- + H2 → A1 + H (-R554) and n-C4H5 + C2H2 → A1 + H (R542). Therefore, it is indicated that adding DME to ethylene flames promotes benzene ring formation. Note that although the maximum C6H6 concentration is largest in the 30% DME mixing flame, the soot volume fraction is smaller than those for the 5% and 14% mixture ratios. This is because the local C6H6 concentration decreases in the relatively low temperature region in the fuel side where soot growth occurs.

ChemInform Abstract: Palladium/Copper‐Catalyzed Aerobic Intermolecular Cyclization of Enediyne Compounds and Alkynes: Interrupting Cycloaromatization for (4 + 2) Cross‐Benzannulation.

AbstractThe process leads to formation of benzene rings and regioselective incorporation of 2 oxygen atoms at positions 2 and 6 from molecular oxygen and H2O, resp., to produce diketone systems.

Molecular Mechanisms in the Pyrolysis of Unsaturated Chlorinated Hydrocarbons: Formation of Benzene Rings. 2. Experimental and Kinetic Modeling Studies

The mechanism of formation of benzene rings during the pyrolysis of dichloro- and trichloroethylenes has been investigated by the method of laser powered homogeneous pyrolysis coupled with product analysis by gas chromatography. Additionally, selected (co)pyrolyses between the chlorinated ethylenes, CH2Cl2, C4Cl4, C4Cl6, and C2H2 have been performed to explicitly probe the roles of 2C3 and C4/C2 reaction pairs in aromatic growth. The presence of odd-carbon products in neat C4Cl6 pyrolyses indicates that 2C3 processes are operative in these systems; however, comparison with product yields from C2HCl3 suggests that C4/C2 processes dominate most other systems. This is further evidenced by an absence of C3 and other odd-carbon species in (co)pyrolyses with dichloromethane which should seed C3-based growth. The reactions of perchlorinated C4 species C4Cl5, C4Cl3, and C4Cl4 with C2Cl2 were subsequently explored through extensive kinetic simulations of the possible reaction pathways based on previous kinetic models and the exhaustive quantum chemical investigations of our preceding work. The experimental and theoretical results strongly suggest that, at moderate temperatures, aromatic ring formation from chlorinated ethylenes normally follows a Diels-Alder coupling of C4 and C2 molecular units followed by internal shifts; the one exception is the C4Cl4 + C2Cl2 system, where steric factors lead to the formation of nonaromatic products. There is little evidence for radical-based routes in these systems.