Showing papers on "Cyclobutadiene published in 2014"

Photochemical reactions applied to the synthesis of helicenes and helicene-like compounds

Abstract: Helicenes are composed of ortho annellated benzene moieties. Similar compounds contain heterocyclic or dihydrobenzene rings or smaller rings such as cyclopentadiene or cyclobutadiene (as part of benzocyclobutene units). The present article resumes photochemical reactions used for the preparation of these compounds. A very important and generally applicable reaction used for the synthesis of helicenes and helicene-like compounds is the photocyclization of stilbene subunits followed by oxidation. This reaction is often highly regioselective. The reaction can be conducted in the way that the formation of the helicene structure is favored. This selectivity is caused by the sum of the free valence numbers in the different positions of the stilbene precursor. Very fascinating structures are obtained with the cobalt catalyzed Vollhardt reaction which is photolytically supported. Helicenes are chiral and different methods of asymmetric synthesis were applied to the preparation of these compounds. A very convenient method is optical resolution using HPLC which is now currently used.

Interpretation of Electron Delocalization in Benzene, Cyclobutadiene, and Borazine Based on Visualization of Individual Molecular Orbital Contributions to the Induced Magnetic Field

Abstract: The magnetic response of the valence molecular orbitals (MOs) of benzene, cyclobutadiene, and borazine to an external magnetic field has been visualized by calculating the chemical shielding in two-dimensional grids of points on the molecular plane and on a plane perpendicular to it, using gauge-including atomic orbitals (GIAOs). The visualizations of canonical MO contributions to the induced magnetic field (CMO-IMF) provide a clear view of the spatial extension, the shape, and the magnitude of shielding and deshielding areas within the vicinity of the molecule, originating from the induced currents of each valence orbital. The results are used to investigate the delocalization of each valence MO and to evaluate its contribution to the aromatic character of systems under study. The differentiation of the total magnetic response among the three molecules originates exclusively from π-HOMO orbitals because the magnetic response of the subsets of the remaining MOs is found to be almost identical. Borazine is...

Heavy atom tunneling in the automerization of pentalene and other antiaromatic systems

Abstract: Cyclobutadiene is a well-known system that can automerize (i.e. undergo a π bond-shifting) by a heavy atom tunneling mechanism. To understand the rules that allow this process, a theoretical study has been carried out on the contribution of tunneling to the automerization reactions of several other molecules with antiaromatic π systems: pentalene, heptalene, acepentalene, and substituted pentalenes. The calculations find that automerization of molecules such as pentalene, which have planar structures, are most likely to proceed by rapid carbon tunneling from the lowest vibrational state, since such molecules have relatively low activation energy and narrow barriers. However, if a molecule is not planar (thus formally “non-aromatic”) and/or requires large geometry changes in order to reach the automerization transition state, then the tunneling will be strongly hindered. In some cases, such as heptalene and tri-tert-butylpentalene, the rearrangement of the reactant requires a modest amount of thermal energy, which can be followed by the π bond-shifting through a tunneling mechanism (“thermally activated tunneling”).

Novel Aromatic and Antiaromatic Systems

Abstract: Note from the Editor: August Kekule's dream and his theory of the structure of benzene (1865). Julius Thomsen's logic that benzene contains equivalent electrons between its carbon atoms (ca. 1900). Richard Willstatter's synthesis and observation that 1,3,5,7-cyclooctatetraene is not aromatic (early 1900s). Sir Robert Robinson's proposal of the ‘aromatic sextet’ (1925). Erich Huckel's molecular orbital treatment of benzene and other unsaturated compounds that separated sigma and pi electrons (early 1930s). Tetsuo Nozoe and Michael Dewar's independent proposal of a new aromatic structure for a cycloheptatrieneone (mid-1940s). Ronald Breslow's demonstration that cyclopropenyl cation is aromatic (1958). Breslow's proposal of ‘antiaromaticity’ (1965). I offer several comments. First, in the field of aromaticity, Breslow is in mighty exceptional company. Second, his research has extended well beyond aromaticity. Yes, Breslow is applying the concepts of antiaromaticity toward the development of highly conductive organic materials. But his research has encompassed enzyme mimics, remote functionalization reactions, unnatural DNA analogues and cancer chemotherapy. Indeed, Breslow is the co-discoverer of a highly successful drug, Vorinostat, which is FDA-approved for the treatment of cutaneous T-cell lymphoma. We thank Professor Breslow for joining our project honoring his and our friend, Tetsuo Nozoe. Tetsuo Nozoe would have would have been deeply touched. —Jeffrey I. Seeman Guest Editor University of Richmond Richmond, Virginia 23173, USA E-mail: jseeman@richmond.edu Ronald Breslow with Koji Nakanishi (left) and Tetsuo Nozoe (center), Sendai, 1970. Abstract We contributed to the field of non-benzenoid aromatic compounds by creating the cyclopropenyl cation and various of its derivatives, including cyclopropenone; it was the first aromatic system with other than six pi electrons in a single ring, and the simplest aromatic system. The pioneering work of Tetsuo Nozoe in tropolone chemistry was celebrated with the founding of ISNA, the International Symposium on Non-Benzenoid Aromatic Compounds, where I described our work in the field. It fit the prediction that aromaticity would be found in systems with 4n + 2 pi electrons, where n is an integer. I was also concerned with the properties of monocyclic systems with 4n cyclically conjugated pi electrons. They were expected not to be aromatic, but the interesting question was whether they were actually antiaromatic, especially destabilized by the cyclic conjugation in such 4n species as the cyclopropenyl anion, cyclobutadiene, and cyclopentadienyl cation. The evidence supports antiaromaticity in these cases. We also examined compounds where 4n cyclic pi systems were fused with aromatic systems, and most interestingly systems in which two 4n pi systems were fused. In these cases the periphery of the molecules had 4n + 2 pi electrons, for aromaticity, but the components were antiaromatic. Recently we have studied electrical conductivities in aromatic molecules such as thiophene and saw that aromaticity added resistance to the systems, so non-aromatic compounds are better conductors and antiaromatic compounds are predicted to be the best of all.

Addition of C–C and C–H bonds by pincer-iridium complexes: a combined experimental and computational study

Abstract: We report that pincer-ligated iridium complexes undergo oxidative addition of the strained C–C bond of biphenylene. The sterically crowded species (tBuPCP)Ir (RPCP = κ3-1,3-C6H3(CH2PR2)2) initially reacts with biphenylene to selectively add the C(1)–H bond, to give a relatively stable aryl hydride complex. Upon heating at 125 °C for 24 h, full conversion to the C–C addition product, (tBuPCP)Ir(2,2′-biphenyl), is observed. The much less crowded (iPrPCP)Ir undergoes relatively rapid C–C addition at room temperature. The large difference in the apparent barriers to C–C addition is notable in view of the fact that the addition products are not particularly crowded, since the planar biphenyl unit adopts an orientation perpendicular to the plane of the RPCP ligands. Based on DFT calculations the large difference in the barriers to C–C addition can be explained in terms of a “tilted” transition state. In the transition state the biphenylene cyclobutadiene core is calculated to be strongly tilted (ca. 50°–60°) relative to its orientation in the product in the plane perpendicular to that of the PCP ligand; this tilt results in very short, unfavorable, non-bonding contacts with the t-butyl groups in the case of the tBuPCP ligand. The conclusions of the biphenylene studies are applied to interpret computational results for cleavage of the unstrained C–C bond of biphenyl by (RPCP)Ir.

A tetradentate metalloligand: synthesis and coordination behaviour of a 2-pyridyl-substituted cyclobutadiene iron complex

Abstract: The salt [K([18]crown-6){Cp*Fe(η4-C4py4)}] (K1, py = 2-pyridyl, Cp* = C5Me5) is accessible by the reaction of an iron(0) naphthalene precursor and bis(2-pyridyl)acetylene. Cyclic voltammetry and preparative investigations demonstrate the electron-rich nature of K1, which is reversibly oxidized to neutral [Cp*Fe(η4-C4py4)] (1) at a low potential. The first coordination studies with iron(II) and zinc(II) chloride show that all four 2-pyridyl units may be employed for metal coordination.

Orbital Symmetry and Reaction Mechanism: The OCAMS View

Abstract: I. Preliminary Survey.- 1. The Woodward-Hoffmann Rules in Perspective.- 1.1 Prolegomenon.- 1.2 The Suprafacial-Antarafacial Dichotom.- 1.2.1 Ground-State Reactions.- 1.2.2 Excited State Reactions.- 1.3 Frontier Electrons and Frontier Orbitals.- 1.3.1 HOMO-LUMO Interactio.- 1.3.2 Superjacent and Subjacent Orbitals.- 1.4 Orbital and Configuration Correlation.- 1.4.1 Frontier Electron Energy.- 1.4.2 Correlation of Electron Configurations.- 1.4.2.1 [4+2]-Cycloaddition and Cycloreversion.- 1.4.2.2 [2+2]-Cycloaddition and Cycloreversion.- 1.5 Problems and Prospects.- 1.5.1 Some Unanswered Questions.- 1.5.2 Where Do We Go From Here?.- 1.6 References.- II. Symmetry and Energy.- 2. Atoms and Atomic Orbitals.- 2.1 Is an Isolated Atom Spherically Symmetrical?.- 2.2 Desymmetrization by an External Field.- 2.2.1 p Orbitals in a Magnetic Field.- 2.2.2 p Orbitals in a Quadrupolar Field.- 2.2.3 Digression: Some Elementary Group Theory.- 2.2.4 The Phase of an Orbital.- 2.2.5 Digression: A Bit More Group Theory.- 2.2.6 Hybridization.- 2.2.7 The Formal Expression of Desymmetrization.- 2.2.7.1 I. Desymmetrization to a Subgroup.- 2.2.7.2 II. Desymmetrization by a Perturbation.- 2.3 Something About d Orbitals.- 2.3.1 Splitting d Orbitals by an External Field.- 2.3.2 A Further Excursion into Group Theory.- 2.3.2.1 Classes and Degenerate Representations.- 2.3.2.2 Invariant Subgroups, Kernels and Co-Kernels.- 2.4 References.- 3. Diatomic Molecules and Their Molecular Orbitals.- 3.1 The Hydrogen Molecule Ion.- 3.1.1 The Symmetry of Ht.- 3.1.2 The Molecular Orbitals of Ht.- 3.2 Homonuclear Diatomic Molecules.- 3.2.1 Mulliken' s Orbital Correlation Diagram.- 3.2.2 The Symmetry of Electron Configurations.- 3.2.3 State Symmetry in Dooh and D2h.- 3.3 Heteronuclear Diatomic Molecules.- 3.3.1 The Non-Crossing Rule..- 3.3.2 Configuration and State Correlatio.- 3.4 Symmetry Coordinates.- 3.4.1 Homonuclear Diatomics.- 3.4.2 Heteronuclear Diatomics.- 3.5 References.- 4. Formation and Deformation of Polyatomic Molecules.- 4.1 Triatomic Molecules.- 4.1.1 Molecular Orbitals and Walsh Diagrams.- 4.1.2 Symmetry Coordinates of a Symmetric Triatomic Linear Molecule.- 4.1.3 Symmetry Coordinates of a Symmetric Non-Linear Triatomic.- 4.2 Linear Tetraatomics and Their Deformation.- 4.3 Dimerization of Methylene and Its Reversal.- 4.3.1 The Dimerization of Methylene.- 4.3.1.1 I. Desymmetrization to a Subgroup.- 4.3.1.2 II. Desymmetrization by a Perturbation.- 4.3.1.3 WH-LHA and OCAMS: A Comparison.- 4.3.2 The Fragmentation of Ethylene.- 4.3.2.1 Bader's Analysis of Molecular Fragmentation.- 4.4 Symmetry Coordinates and Normal Modes.- 4.4.1 Non-Degenerate Vibrations.- 4.4.2 Degenerate Vibrations.- 4.4.3 Reducing Reducible Representations.- 4.4.3.1 The CH-Stretching Coordinates of Ethylene.- 4.4.3.2 The NiF -Stretching Coordinates of NiF~.- 4.5 Motion Along the Reaction Coordinate.- 4.5.1 Distortional and Substitutional Desymmetrization Compared.- 4.5.2 At The Transition State.- 4.6 References.- III. The Classical Thermal Reactions.- 5. Electrocyclic Reactions and Related Rearrangements.- 5.1 Rudimentary Analysis of Polyene Cyclization.- 5.1.1 Cyclization of Butadiene to Cyclobutene.- 5.1.2 cis-1 ,3,5-Hexatriene to 1,3-Cyclohexadiene.- 5.2 More Subtle Considerations.- 5.2.1 Correlation vs. Correspondence.- 5.2.2 The Role of IT-Orbitals.- 5.2.3 Substitutional Desymmetrization: Norcaradiene.- 5.2.4 Local vs. Global Symmetry:.- 5.2.4.1 Bisnorcaradiene.- 5.2.4.2 Cyclooctatetraene +--+ Bicyclooctatriene.- 5.3 "Allowedness" and "Forbiddenness".- 5.3.1 Rearrangement of s-cis-Butadiene to Bicyclobutane.- 5.3.2 The Bond-Bisection Requirement: Benzvalene.- 5.3.3 Genuinely Forbidden Valence Isomerizations.- 5.3.4 Quantifying "Allowedness": Cubane +--+ Cyclooctatetraene.- 5.3.4.1 Analysis in Global Symmetry.- 5.3.4.2 Analysis in Local Symmetry.- 5.3.5 The Bottom Line So Far.- 5.4 References.- 6. Cycloadditions and Cycloreversions: I. [2+2]-Cycloaddition.- 6.1 Addition of Singlet Carbenes to Ethylene.- 6.1.1 The Direct Approach.- 6.1.2 Correcting a Geometrically Unreasonable Approach.- 6.1.2.1 Composite Motions.- 6.2 Concerted [1r2+1r2]-Cycloaddition.- 6.2.1 The [1r2s+1r2sl Approach.- 6.2.1.1 Substitutional Desymmetrization: Dimerization of Silaethylene.- 6.2.2 [1r2s+1r2al-Cycloaddition.- 6.3 Cycloaddition via a Tetramethylene Intermediat.- 6.3.1 The Biradical Mechanism.- 6.3.1.1 Stereochemistry of Biradical Cycloaddition.- 6.3.2 The Zwitterionic Mechanism.- 6.4 Ketene Cycloadditions.- 6.4.1 Diversion: Secondary Isotope Effects.- 6.4.2 Reconciling the Evidence.- 6.4.2.1 Product Stereochemistry.- 6.4.2.2 Substituent, Solvent and Isotope Effects.- 6.5 Apologia.- 6.6 References.- 7. Cycloadditions and Cycloreversions: II. Beyond [2+2].- 7.1 [1r4 + 1r2]-Cycloaddition Anasymmetrization.- 7.1.1 The Diels-Alder Reaction.- 7.1.1.1 Anasymmetrization.- 7.1.1.2 Changing the Initial Orientation.- 7.2 Reactions Related to [1r4 + 1r2]-Cycloaddition.- 7.2.1 The Homo-Diels-Alder Reaction.- 7.2.2 n > 4 and/or m >2.- 7.2.3 1,3-Dipolar Cycloaddition.- 7.3 More Complex [1r2 +1r2]-Cycloadditions.- 7.3.1 Dimerization of Cyclobutadiene.- 7.3.1.1 The (Non)-Dimerization of CBD to Cubane.- 7.3.1.2 Dimerization of Cylobutadiene to Tricyclooctatriene.- 7.3.2 [2 + 2]-Cycloreversion of o,o'-Benzene-dimer.- 7.3.2.1 Digression on Entropy of Activation.- 7.3.3 Dimerization of Cyclomonoalkenes.- 7.3.3.1 Cyclopropene.- 7.3.3.2 Dimerization of Silacyclopropenes.- 7.4 References.- 8. Degenerate Rearrangements.- 8.1 Correspondence Between Reactant and/or Product and Transition Structure.- 8.1.1 1,2-Rearrangement of Tetraaryldisilenes.- 8.1.2 Digression: Degenerate X- Ion Substitution in CH3X.- 8.2 The Cope Rearrangement.- 8.2.1 Symmetry Analysis of the Cope Rearrangement.- 8.2.2 Rearrangement of Bridged Hexadienes.- 8.3 [l,j]-Sigmatropic Rearrangements.- 8.3.1 [l,j]-Hydrogen Shifts in Non-Cyclic Molecules.- 8.3.1.1 [1,3]-Sigmatropic Rearrangement of Propylene.- 8.3.1.2 [1,5]-Hydrogen Shift in s-cis-Pentadiene.- 8.3.1.3 A Word About [1,7]-Hydrogen Shifts.- 8.3.2 Circumambulatory Rearrangements.- 8.3.2.1 [1,5]-Hydrogen Shift in Cyclopentadiene.- 8.3.2.2 The "Norcaradiene Walk" Rearrangement.- 8.4 Fluxional Isomerization of Cyclobutadiene.- 8.4.1 Correspondence Between Reactant and Product.- 8.4.2 Correspondence Between Anasymmetrized Reactant and Transition Structure.- 8.5 References.- IV. Spin and Photochemistry.- 9. Electron Spin.- 9.1 Spin and Symmetry.- 9.1.1 The Symmetry of Spinning Electrons.- 9.1.1.1 A Single Spinning Electron.- 9.1.1.2 Two Spinning Electrons.- 9.1.2 Space-, Spin- and Overall Symmetry.- 9.1.2.1 Example 1: Rectangular Cyclobutadiene (D2h).- 9.1.2.2 Example 2: Square-Planar Cyclobutadiene (D4h).- 9.1.2.3 Overall Symmetry.- 9.1.2.4 Geminals.- 9.2 Intersystem Crossing..- 9.2.1 Intersystem Crossing of Carbenes.- 9.2.1.1 Spin-Orbit Coupling.- 9.2.1.2 Spin-Vibronic Coupling.- 9.3 Reactive Intersystem Crossing.- 9.3.1 The Arrhenius Parameters of Spin-Non-Conservative Reactions.- 9.3.2 Thermolysis of Diazomethane.- 9.3.3 Thermolysis of Methylenepyrazoline.- 9.3.3.1 The Zwitterion Cascade Mechanism.- 9.3.3.2 Secondary Isotope Effects.- 9.3.4 "Photochemistry Without Light".- 9.3.4.1 Isomerization of Dewar Benzene to Triplet Benzene.- 9.3.4.2 Fragmentation of 1,2-Dioxetanes.- 9.4 References.- 10. Excited State Reactions.- 10.1 The Basic Photophysical Processes.- 10.1.1 Fluorescence: The Azulene Anomaly.- 10.1.2 Stereoselectivity of Photophysical Processes: Bimanes.- 10.1.3 Chemical Sensitization: Singlet Dioxygen.- 10.2 Photofragmentation.- 10.2.1 Photolysis of Cyclobutadiene.- 10.2.2 Photochemical Decomposition of Formaldehyde.- 10.2.2.1 Pathway I: Sl(H2CO) -+ So(H2CO) -+ H2 + CO.- 10.2.2.2 Pathway II: Sl(H2CO) -+ H + HCO.- 10.2.2.3 Sidelight: Coping with the Limitations.- 10.3 Photoisomerization of Benzene.- 10.3.1 Photoisomerization to Benzvalene.- 10.3.2 Photoisomerization to Dewar Benzene.- lO.4 Spin-Non-Conservative Photoisomerization: Naphthvalene.- 10.5 Rydberg Photochemistry: Photolysis of Methane.- 10.6 References.- 11. Into Inorganic Chemistry.- 11.1 Main-Group Elements.- 11.1.1 Ground-State Isomerization: "Berry Pseudorotation".- 11.1.2 The Allotropy of Phosphorus.- 11.1.3 An Excited-State Reaction: Photoextrusion of Silylene.- 11.2 Transition Metals: Isomerization of NiX4.- 11.3 Afterword.- 11.4 References.- A. Character Tables of the More Common Symmetry Point Groups.- B. Kernels and Co-Kernels of Degenerate Irreducible Representations.- C. Group Correlation Tables.

Tetraazaacenes containing four-membered rings in different oxidation states. Are they aromatic? A computational study.

Abstract: A symmetrical tetraazaacene incorporating a central cyclobutadiene ring was calculated in different oxidation (hydrogenation) states, displaying different tautomers and conformers. Geometries, thermodynamics, and electronic properties were computed, and the aromaticity of all these species was calculated on a per ring basis by NICS-scans and NICS-X-scans. The results unveil unexpected and fascinating insights into the complex aromaticity of those compounds, including a formally aromatic (!) cyclobutadiene ring.

Building complex carbon skeletons with ethynyl[2.2]paracyclophanes

Abstract: Ethynyl[2.2]paracyclophanes are shown to be useful substrates for the preparation of complex, highly unsaturated carbon frameworks. Thus both the pseudo-geminal- 2 and the pseudo-ortho-diethynylcyclophane 4 can be dimerized by Glaser coupling to the respective dimers 9/10 and 11/12. Whereas the former isomer pair could not be separated so far, the latter provided the pure diastereomers after extensive column chromatography/recrystallization. Isomer 11 is chiral and could be separated on a column impregnated with cellulose tris(3,5-dimethylphenyl)carbamate. The bridge-extended cyclophane precursor 18 furnished the ring-enlarged cyclophanes 19 and 20 on Glaser–Hay coupling. Cross-coupling of 4 and the planar building block 1,2-diethynylbenzene (1) yielded the chiral hetero dimer 22 as the main product. An attempt to prepare the biphenylenophane 27 from the triacetylene 24 by CpCo(CO)2-catalyzed cycloisomerization resulted in the formation of the cyclobutadiene Co-complex 26. Besides by their usual spectroscopic and analytical data, the new cyclophanes 11, 12, 19, 20, 22, and 26 were characterized by X-ray structural analysis.

Deformation density and energy decomposition to describe interactions between (η5-C5H5)M and highly reactive molecules C4H4 and (C3H3)−

Abstract: Using DFT calculations, an energy decomposition analysis (EDA) combined with natural orbitals for chemical valence (NOCV), EDA-NOCV approach was used to describe the nature of the interaction between η5-cyclopentadienyl metal complexes (η5-C5H5)M, with M=Co, Rh, and cyclobutadiene (Cb) and cyclopropenyl anion (C3H3)- molecules, which are highly reactive molecules in their free state. EDA-NOCV draws a covalent picture for these interactions. With this interpretation of interactions, the character of aromaticity could be the result of the delocalization of six electrons in π orbitals of the (η5-C5H5)M fragment and Cb/C3H3(-1) ligand. This description of the bonding interaction might also justify the experimental observation that, in complexes of CpM-Cb (M=Co, Rh), the viability of the Friedel-Crafts acylation and other electrophilic substitutions on the four-membered ring is greater than that of the five-membered ring.

On the large σ-hyperconjugation in alkanes and alkenes

Abstract: The conventional view that the σCC and σCH bonds in alkanes and unsaturated hydrocarbons are so highly localized that their non-steric interactions are negligible is scrutinized by the block-localized wavefunction (BLW) method. Even molecules considered conventionally to be “strain free” and “unperturbed” have surprisingly large and quite significant total σ-BLW-delocalization energies (DEs) due to their geminal and vicinal hyperconjugative interactions. Thus, the computed BLW-DEs (in kcal mol−1) for the antiperiplanar conformations of the n-alkanes (CNH2N+2, N = 1-10) range from 11.6 for ethane to 82.2 for n-decane and are 50.9 for cyclohexane and 91.0 for adamantane. Although σ-electron delocalization in unsaturated hydrocarbons usually is ignored, the σ-BLW-DEs (in kcal mol−1) are substantial, as exemplified by D 2h ethylene (9.0), triplet D 2d ethylene (16.4), allene (19.3), butadiene (19.0), hexatriene (28.3), benzene (28.1), and cyclobutadiene (21.1). While each individual geminal and vicinal hyperconjugative interaction between hydrocarbon σ-bonding and σ-antibonding orbitals tends to be smaller than an individual π conjugative interaction (e.g., 10.2 kcal mol−1 in anti-1,3-butadiene, the presence of many σ-hyperconjugative interactions (e.g., a total of 12 in anti-1,3-butadiene, see text), result in substantial total σ-stabilization energies (e.g., 19.0 kcal mol−1 for butadiene), which may surpass those from the π interactions. Although large in magnitude, σ-electron delocalization energies often are obscured by cancellation when two hydrocarbons are compared. Rather than being strain-free, cyclohexane, adamantane, and diamantane suffer from their increasing number of intramolecular 1,4-C…C repulsions resulting in elongated C–C bond lengths and reduced σ-hyperconjugation, compared to the (skew-free) antiperiplanar n-alkane conformers. Instead of being inconsequential, σ-bond interactions are important and merit consideration.

The potential energy surface of singlet cyclobutadiene and substituted analogs: a coupled-cluster study

Abstract: Coupled-cluster investigations (CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ) of singlet cyclobutadiene and fifteen-substituted analogs were conducted. A local minimum with a square frame does not exist on their potential surfaces. The well-known rectangular D2h minimum, the square D4h transition state, and two additional stationary points were found on cyclobutadiene’s potential surface. This included a transition state with a rhombic carbon ring and C2h symmetry, separating two equivalent puckered C2v local minima. The predicted barriers were 19.7 and 19.8 kcal/mol at the CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ levels, respectively. The relative strain energies of rectangular D2h cyclobutadiene and all fifteen-substituted analogs were obtained from isodesmic reactions. Progressive substitution with methyl or BH2 groups continuously lowers ring strain while increasing substitution with fluorines or trifluoromethyl groups steadily increases ring strain. C4(BH2)4 is 16.6 and 13.3 kcal/mol less strained than cyclobutadiene while C4F4 is 17.7 and 21.5 kcal/mol more strained at the levels above. Cyclobutadiene is more strained than both cyclopropene and cyclobutene by 12.2 and 37.0 kcal/mol, respectively. Electron density contours indicate that fluorine substitution raised the electron density especially in the short C=C ring bonds above/below the ring plane (π-electrons) but not in the ring plane (σ-electrons). BH2-substitutions lower the ring π-electron density with little effect in the ring plane. Methyl substituents have little effect on electron densities. All rings retain a strong bond alternation tendency (rectangular) whether substituted with electron-donating or -attracting groups. One-bond coupling constants and the percent p-character in ring C-to-C and C-to-substituent bonds are described.

Photochemical Reactions Applied to the Synthesis of Helicenes and Helicene‐Like Compounds

Abstract: Helicenes are composed of ortho annellated benzene moieties. Similar compounds contain heterocyclic or dihydrobenzene rings or smaller rings such as cyclopentadiene or cyclobutadiene (as part of benzocyclobutene units). The present article resumes photochemical reactions used for the preparation of these compounds. A very important and generally applicable reaction used for the synthesis of helicenes and helicene-like compounds is the photocyclization of stilbene subunits followed by oxidation. This reaction is often highly regioselective. The reaction can be conducted in the way that the formation of the helicene structure is favored. This selectivity is caused by the sum of the free valence numbers in the different positions of the stilbene precursor. Very fascinating structures are obtained with the cobalt catalyzed Vollhardt reaction which is photolytically supported. Helicenes are chiral and different methods of asymmetric synthesis were applied to the preparation of these compounds. A very convenient method is optical resolution using HPLC which is now currently used.

Modern valence-bond description of aromatic annulene ions

Abstract: Spin-coupled theory for ‘N electrons in M orbitals’ active spaces [SC(N,M)], an ab initio valence-bond (VB) approach which uses a compact and easy-to-interpret wave function comparable in quality to a ‘N in M’ complete-activespace self-consistent field [CASSCF(N,M)] construction, is used to obtain modern VB descriptions of the π-electron systems of the most important annulene rings with 4n + 2 π electrons: the cyclopropenium ion, the cyclobutadiene dication and dianion, the cyclopentadienide anion, benzene, the cycloheptatrienyl cation, and the cyclooctatetraene dication and dianion in their highest-symmetry nuclear conformations. The SC wave functions for the cyclopropenium ion, cyclopentadienide anion, cycloheptatrienyl cation, cyclooctatetraene dication and dianion are shown to closely resemble the well-known SC model of the classical example of an aromatic system, benzene. The SC orbitals for the cyclobutadiene dication and dianion are more delocalized and demonstrate the ways in which SC wave functions adjust to electron-deficient and electron-rich environments. The high levels of resonance observed in all annulene ions with 4n + 2 π electrons clearly demonstrate their aromaticity.

Energetics and electronic structures of polymerized cyclobutadiene

Abstract: We investigated electronic and geometric properties of polymerized cyclobutadiene (C4) as a metallic form of a two-dimensional carbon allotrope using first-principles calculations. Our calculations show that the polymerized structure is stable with a total energy that is slightly higher than that of C60 fullerene and close to that of small fullerene polymers. This two-dimensional covalent network with tetragonal symmetry is a metal with linear dispersion bands at the Fermi level due to the inflection point in the cosine bands. We also studied the stable stacking arrangement and electronic structures of layered system comprising the C4 polymer. The calculations show that the AA stacking is a favorable stacking arrangement among four representative stacking structures. Although small but substantial interlayer interaction, electronic structures of the layered system of C4 polymer are almost the same as that of the monolayer of C4 polymer.

Cyclobutadiene automerization and rotation of ethylene: Energetics of the barriers by using Spin-polarized wave functions

Abstract: Financial support by the Spanish MCYT (Grant FIS2009-10325 and FIS2012-35880) and the Universidad de Alicante is gratefully acknowledged.

Über Substituenten-Einflüsse auf die Bindungsverhältnisse der Metall-Komplexe des Cyclobutadiens / The Influence of Substituents on Bonding Properties in Metal Complexes of Cyclobutadiene : p-Substituierte Phenylcyclobutadien-eisentricarbonyle / p-Substituted Phenylcyclobutadiene Irontricarbonyls

Abstract: The syntheses of some p- substituted phenylcyclobutadieneirontricarbonyls are reported. Carbonyl stretching frequencies, ¹³C-H- and H-Η-coupling constants are discussed with respect to bonding properties. There is apparently no conjugative interaction between the π-electron systems of cyclobutadiene and the phenyl ring.

Theoretical Study for the Effect of Hydroxyl Radical on the Electronic Properties of Cyclobutadiene Molecular

Abstract: The present work deals with the electronic properties of organic molecules in form ring, containing semiconductor atoms. Cyclobutadiene is the original ring before replacing the hydrogen atom by hydroxyl radical. Density functional theory with B3LYP/6-21G level has been used to find the electronic structure and electronic properties of the studied molecules. The effect of substitute on cyclebutadiene molecule is discussed on the basis of the calculated electronic properties. It is included total energy, energy gap, ionization potential, electronic affinity and electrophilicity, with comprehensive analysis of the calculated highest-occupied and lowest-unoccupied orbital (HOMO and LUMO respectively) energies. The results in this study show that the calculated electronic properties for cyclebutadiene have been found a good agreement with the previous studies. For other molecules, we have not found a reference data, so this study supplies a new data in this aspect. These calculations have been performed using Gaussian 03 package.

New Cycloaddition/Fragmentation Strategies for Preparing 5‐7‐5 and 5‐7‐6 Fused Tricyclic Ring Systems.

Abstract: Tethering additional functionality to cyclobutadienyl iron tricarbonyl complexes provides new opportunities for the rapid construction of medium-ring-containing polycyclic compounds. Specifically, an intramolecular cycloaddition between cyclobutadiene and a tethered olefin, followed by an intramolecular cyclopropanation of the resulting cyclobutene-containing adduct generates highly strained pentacyclic intermediates. These compounds can then be relaxed thermally to generate 5-7-5 and 5-7-6 fused tricyclic ring systems that are shared with numerous natural products.