Four-membered Carbon Rings Research Articles

Synthesis of Cyclobutanes and Cyclobutenes by Strain-Release-Driven Ring-Opening of Bicyclo[1.1.0]butanes

AbstractCyclobutanes and cyclobutenes exhibit intriguing structures and demonstrate significant biological activities and diverse synthetic applications. This review aims to summarize recent progress in strain-release-driven ring-opening reactions of bicyclo[1.1.0]butanes (BCBs) to synthesize these four-membered carbon rings. It outlines the strategies, regio- and stereoselectivity, the synthetic scope of reactions, and the mechanistic implications of the catalytic ring-opening process, providing a supplementary perspective to existing reviews.1 Introduction2 Thermally Driven Nucleophilic Ring-Opening3 Thermally Driven Rearrangement and Isomerization Reaction4 Light-Driven Ring-Opening5 Transition-Metal/Lewis Acid Catalyzed Ring-Opening6 Conclusion and Outlook

Locally spontaneous dynamic oxygen migration on biphenylene: a DFT study.

The dynamic oxygen migration at the interface of carbon allotropes dominated by the periodic hexagonal rings, including graphene and carbon nanotubes, has opened up a new avenue to realize dynamic covalent materials. However, for the carbon materials with hybrid carbon rings, such as biphenylene, whether the dynamic oxygen migration at its interface can still be found remains unknown. Using both density functional theory calculations and machine-learning-based molecular dynamics (MLMD) simulations, we found that the oxygen migration departing away from the four-membered carbon (C4) ring is hindered, and the oxygen atom prefers to spontaneously migrate toward/around the C4 ring. This locally spontaneous dynamic oxygen migration on the biphenylene is attributed to a high barrier of about 1.5 eV for the former process and a relatively low barrier of about 0.3 eV for the latter one, originating from the enhanced activity of the C-O bond near/around the C4 ring due to the hybrid carbon ring structure. Moreover, the locally spontaneous dynamic oxygen migration is further confirmed by MLMD simulations. This work sheds light on the potential of biphenylene as a catalyst for spatially controlled energy conversion and provides the guidance for realizing the dynamic covalent interface at other carbon-based or two-dimensional materials.

Construction of poly-naphthalocyanine linked by [4]-radialene-like structures on silver surfaces

Cyclic-conjugated linkages between planar-macrocydic molecules contribute to the robustness of the two-dimensional (2D) polymerization and extension of π-interactions. The fabrication of such linkages in 2D polymers remains challenging. Combining scanning tunneling microscope (STM) measurements and density functional theory (DFT) calculations, we demonstrate a linear polymerization of metal-free naphthalocyanine (NPc) molecules with [4]-radialene-like linkages on silver surfaces. Experimentally, by depositing NPc molecules on the Ag(110) surface and subsequent annealing up to 750 K, one-dimensional polymers are constructed along the [ $$1\overline 1 0$$ ] direction. High-resolution STM images show a stem-leaf-like feature. STM simulations based on a linear polymer of NPc molecules linked by four-membered carbon rings, [4]-radialene-like structure, agree well with the experimental observations. DFT calculations reveal that the polymerization process includes detaching two-terminal H atoms of NPc molecules along [ $$1\overline 1 0$$ ] direction, then bonding with a neighboring dehydrogenated NPc molecule by forming a four-membered ring. The dehydrogenation process can be promoted by on-surface impurities such as additional H atoms. Similar polymerizations have been achieved on Ag(111) surfaces in an amorphous way. Moreover, the energy gap of the NPc molecule decreases after linear polymerization, suggesting a red-shift for its optical absorption/scattering spectrum. Our study offers a new route to polymerize conjugated molecules with extended planar π-interactions.

Quantum mechanical tunneling in the automerization of cyclobutadiene.

Cyclobutadiene has a four-membered carbon ring with two double bonds, but this highly strained molecular configuration is almost square and, via a coordinated motion, the nuclei quantum mechanically tunnels through the high-energy square state to a configuration equivalent to the initial configuration under a 90° rotation. This results in a square ground state, comprising a superposition of two molecular configurations, that is driven by quantum tunneling. Using a quantum mechanical model, and an effective nuclear potential from density functional theory, we calculate the vibrational energy spectrum and the accompanying wavefunctions. We use the wavefunctions to identify the motions of the molecule and detail how different motions can enhance or suppress the tunneling rate. This is relevant for kinematics of tunneling-driven reactions, and we discuss these implications. We are also able to provide a qualitative account of how the molecule will respond to an external perturbation and how this may enhance or suppress infra-red-active vibrational transitions.

Slicing through both C-C and C-H bonds

Organic Chemistry A variety of catalysts can cleave the strained bonds in four-membered carbon rings and then link the ends to a reactive partner. Bender et al. found that after prying the ring open, a double-duty ruthenium catalyst could forge bonds to a traditionally unreactive partner. They used

A rhodium catalyst hogs the spotlight

Photochemistry Photochemistry offers an efficient route to constructing four-membered carbon rings, but the product geometry is hard to control precisely. Even though catalysts can selectively orient the reacting partners, it is hard to outpace the unselective background reaction stimulated by catalyst-free light absorption. Huang et al. devised a chiral rhodium catalyst that enhanced the intrinsic blue light absorption of imidazole-substituted enones by more than 100-fold, thereby accelerating their [2+2] reaction with dienes and styrene derivatives. The acceleration was sufficient to achieve 92 to 99% enantiomeric excess in a wide variety of cyclobutane products, several of which bore contiguous quaternary stereocenters. J. Am. Chem. Soc. 10.1021/jacs.7b04363 (2017).

Probing the early stages of solvation of cis-pinate dianions by water, acetonitrile, and methanol: a photoelectron spectroscopy and theoretical study.

cis-Pinic acid is one of the most important oxidation products of α-pinene--a key monoterpene compound in biogenic emission processes. Molecular level understanding of its interaction with water in cluster formation is an important and necessary prerequisite for ascertaining its role in the aerosol formation processes. In this work, we studied the structures and energetics of the solvated clusters of cis-pinate (cis-PA(2-)), the doubly deprotonated dicarboxylate of cis-pinic acid, with H2O, CH3OH, and CH3CN by negative ion photoelectron spectroscopy and ab initio theoretical calculations. We found that cis-PA(2-) prefers being solvated alternately on the two -CO2(-) groups with increase of solvent coverage, a well-known solvation pattern that has been observed in microhydrated linear dicarboxylate dianion (DCn(2-)) clusters. Experiments and calculations further reveal an intriguing feature for the existence of the asymmetric type isomers for cis-PA(2-)(H2O)2 and cis-PA(2-)(CH3OH)2, in which both solvent molecules interact with only one of the -CO2(-) groups, a phenomenon that has not been observed in DCn(2-) water clusters and exhibits that the subtle effect of the rigid four-membered carbon ring brought on the cis-PA(2-) solvation. The dominant interactions between cis-PA(2-) and solvent molecules form bidentate O(-)...H-O H-bonds for H2O, O(-)...H-O and O(-)...H-C H-bonds for CH3OH, and tridentate O(-)...H-C H-bonds for CH3CN. The formation of inter-solvent H-bonds between H2O and CH3CN is found to be favorable in mixed solvent clusters, different from that between H2O and CH3OH. These findings have important implications for understanding the mechanism of cluster growth and the formation of atmospheric organic aerosols, as well as for rationalizing the nature of structure-function relationship of proteins containing carboxylate groups in various solvent environments.

Understanding the Reactivity of Enol Ether Radical Cations: Investigation of Anodic Four-Membered Carbon Ring Formation

The reactivity of enol ether radical cations was investigated in anodic four-membered carbon ring formations, advancing the mechanistic understanding of these reactions. The mono-ring-containing aromatic cations were reduced through inter- or intramolecular electron transfer to give mono- or bis-ring-containing compounds, respectively. Small structural changes in the hydrocarbon linkers tethering two aromatic rings exerted a powerful effect on the efficiency of such electron transfer events.

Ionic conductivity in the Mg intercalated fullerene polymer Mg2C60

We report the identification of the new intercalated fullerene polymer Mg2C60. Mg intercalation was obtained either by solid state reaction between C60 and Mg, or by thermal decomposition of the metallorganic precursor Mg–Anthracene–(THF)3. High resolution powder synchrotron and neutron diffraction data have clearly shown that Mg2C60 is isostructural to the superionic conductor Li4C60, where fullerenes form a two-dimensional network connected either by four-membered carbon rings, or single C–C bonds. Because of its peculiar structural arrangement Mg2C60 behaves as a good ionic conductor by means of uncorrelated ionic hopping across very small energy barriers (ΔE less than 100meV), as found from DC and AC conductivity measurements, thus suggesting its possible use in future Mg-ion batteries.

From Point Defects in Graphene to Two-Dimensional Amorphous Carbon

While crystalline two-dimensional materials have become an experimental reality during the past few years, an amorphous 2D material has not been reported before. Here, using electron irradiation we create an sp2-hybridized one-atom-thick flat carbon membrane with a random arrangement of polygons, including four-membered carbon rings. We show how the transformation occurs step by step by nucleation and growth of low-energy multivacancy structures constructed of rotated hexagons and other polygons. Our observations, along with first-principles calculations, provide new insights to the bonding behavior of carbon and dynamics of defects in graphene. The created domains possess a band gap, which may open new possibilities for engineering graphene-based electronic devices.

Fullerenium Salts: A New Class of C60-Based Compounds

We report here on the preparation and characterization of a fullerenium salt in the solid state, where the fullerene is in the 2+ oxidized state. To succeed in this long-standing challenge, we exploit the oxidizing power of one of the strongest Lewis acids, AsF(5). The weak nucleophilic character of its conjugate base is essential in stabilizing the fullerene dication in a crystal lattice. High-resolution structural analysis of this compound, with the formula C(60)(AsF(6))(2), indicates that the highly reactive C(60)(2+) units are arranged according to a novel 1D "zigzag" polymer structure. The molecules are connected by an alternating sequence of four-membered carbon rings ([2 + 2] cycloaddition) and single C-C bonds. The long awaited high-T(c) superconductivity and magnetism, expected in a hole-doped C(60) compound, are replaced instead by a semiconducting behavior, quite probably originating from the reduced crystal and molecular symmetry upon polymerization. The small value of the energy gap (approximately 70 meV) suggests, nevertheless, the proximity of a metallic phase.

Lewis Acid Catalyzed Reaction of Methylenecyclopropanes with 1,2-Diphenyldiselane or 1,2-Di-p-tolyldisulfane

Catalyzed by Lewis acid, 1,2-diphenyldiselane or 1,2-di-p-tolyldisulfane could add to methylenecyclopropanes smoothly. Compared with the reported free radical additions, the results were quite different. A four-membered carbon ring was constructed to give cyclobutane-1,1-diylbis(phenylselane) derivatives or cyclobutane-1,1-diylbis(p-tolylsulfane) derivatives as products, which are useful intermediates in organic synthesis.

Symmetric and asymmetric ent-kaurane dimers isolated from Isodon japonicus

Bisjaponins A ( 1) and B ( 2), two new dimeric ent-kaurane diterpenoids connected with a rare four-membered carbon ring, which was formed by [2+2] reaction, were isolated from the aerial parts of Isodon japonicus. Their structures were elucidated by the analysis of spectroscopic evidence including extensive 2D NMR and MS data. Both the compounds were inactive for their cytotoxicity against human tumor cell lines, K562 and HepG2.

A DFT study on the dimerization of C62, H2C62, and F2C62

On the basis of calculations using the density functional theory, we show that C(62), a recently synthesized nonclassical fullerene, will presumably undergo dimerization with various isomers at elevated temperatures. This is shown by calculating the dimerization energy and the activation barrier of the dimerization. Eight possible isomers of the dimer were identified, all of which are more stable than the two isolated monomers. The relative stability of various isomers depends upon the kind of C=C bonds within the four-membered carbon ring involved in the dimerization. In addition, similar calculations were performed for the monomers and dimers of H(2)-C(62) and F(2)-C(62). Six isomers were identified for each of the dimers. Although less pronounced than the case of the C(62) dimer, all isomers of the H(2)-C(62) dimer are appreciably more stable than the individual monomers. Although a large steric repulsion due to F atoms significantly reduces the stability of F(2)-C(62) dimer, its two isomers are still more stable than separate monomers.

Conventional Strain Energy in Dimethyl-Substituted Cyclobutane and the gem-Dimethyl Effect

The gem-dimethyl effect is the acceleration of cyclization by substituents in the chain and is often used in organic synthesis as a ring-closing effect. Calculations on cyclobutane, methylcyclobutane, and 1,1-dimethylcyclobutane are performed. 1,1-Dimethylcyclobutane is a four-membered carbon ring with gem-dimethyl substituents. Optimum equilibrium geometries, harmonic vibrational frequencies, and corresponding electronic energies are computed for all pertinent molecular systems using SCF theory, density functional theory (DFT), and second-order perturbation theory (MP2) with two triple-zeta quality basis sets, 6-311G(d,p) and 6-311G+(2df,2pd). Additional single-point calculations are performed using the optimized MP2/6-311G+(2df,2pd) geometries and coupled-cluster theory including single and double excitations and noniterative, linear triple excitations (CCSD(T)). Calculations indicate that 1,1-dimethylcyclobutane is more than 8 kcal mol-1 less strained than cyclobutane, that is, there is at least some thermodynamic component to the gem-dimethyl effect.

Molecular Modeling of Porous Carbons Using the Hybrid Reverse Monte Carlo Method

We apply a simulation protocol based on the reverse Monte Carlo (RMC) method, which incorporates an energy constraint, to model porous carbons. This method is called hybrid reverse Monte Carlo (HRMC), since it combines the features of the Monte Carlo and reverse Monte Carlo methods. The use of the energy constraint term helps alleviate the problem of the presence of unrealistic features (such as three- and four-membered carbon rings), reported in previous RMC studies of carbons, and also correctly describes the local environment of carbon atoms. The HRMC protocol is used to develop molecular models of saccharose-based porous carbons in which hydrogen atoms are taken into account explicitly in addition to the carbon atoms. We find that the model reproduces the experimental pair correlation function with good accuracy. The local structure differs from that obtained with a previous model (Pikunic, J.; Clinard, C.; Cohaut, N.; Gubbins, K. E.; Guet, J. M.; Pellenq, R. J.-M.; Rannou, I.; Rouzaud, J. N. Langmuir 2003, 19 (20), 8565). We study the local structure by calculating the nearest neighbor distribution, bond angle distribution, and ring statistics.

Ab Initio Investigation of Strain in Group 14 Polyhedrane Clusters (MnHn: n= 4, 6, 8, 10, 12, 16, 20, 24)

The results of ab initio calculations on a series of [MH]n polyhedral cages (M = C, Si, Ge, and Sn; n = 4, 6, 8, 10, 12, 16, 20, and 24) are presented. The energies are partitioned into contributions from each ring to obtain an estimate of how strain changes as a function of ring size for each of these elements. Results of these calculations indicate that three-membered rings are significantly strained for all of these elements, and five- and six-membered rings have similar stability. The relative strain in four-membered carbon rings appears to be significantly higher than the strain in four-membered rings of heavier Group 14 elements. These partitioned ring energies can be applied to predict the relative stability of other cages.

Structure and Energetics of Isomers of the Interstellar Molecule C5H

High-level ab initio methods based on the coupled cluster approximation have been used to study properties of several isomers of the C5H radical, a molecule of significant interest in radioastronomy. The three lowest-lying isomers [the well-known linear form (1) as well as two ring-chain structures, HC2C3 (2) and C2C3H (3)] lie within 30 kcal/mol with isomer 2 approximately 5 kcal/mol higher than the lowest-energy isomer 1. The computed rotational constant for the linear isomer is within 0.7% of the value determined in previous experimental analyses. Transition states formed via simple ring-opening mechanisms for the interconversion of the linear and ring-chain isomers have also been located; these lie ca. 27 and 31 kcal/mol above isomers 2 and 3, respectively, indicating reasonable kinetic stability of these structures to isomerization. The computed rotational constants for these isomers should be useful for laboratory and astronomical observation of these species. In addition, four other minimum-energy structures are found to lie somewhat higher in energy. These isomers involve both three- and four-membered carbon rings, as well as a bent-chain structure.

Strukturuntersuchungen an Vierringsystemen, III* Kristall- und Molekülstrukturen von trans-2.4-Bis(trimethylsilyl)-2.4-dihydro- 1.3-dithietan-1.1.3.3-tetroxid, 2.2.4.4-Tetramethyl-3-pyrrolidinocyclobutanon und 2.2.4.4-Tetramethyl-1-phenyl-3-pyrrolidinoeyelobutanol/Structural Investigations on Four-Membered Ring Systems, III* Crystal and Molecular Structures of trans-2,4-Bis(trimethylsilyl)-2,4-dihydro-1,3-dithietane-1,1,3,3-tetroxide, 2,2,4,4-Tetramethyl-3-pyrrolidinocyclobutanone and 2,2,4,4-Tetramethyl-1-phenyl-3-pyrrolidinocyclobutanol

Abstract The molecular structures of the title compounds have been determined by X-ray methods. In the bis(trimethylsilyl) substituted dithietane the (CH3)3Si groups are in trans position; the C2S2-ring is planar. In the cyclobutanone the pyrrolidine group occupies an equatorial position, as does the phenyl group in the cyclobutanol. In the latter the hydroxyl group is trans to the pyrrolidine ring. The four-membered carbon rings are folded.

364. Physical properties and chemical constitution. Part XVIII. Three-membered and four-membered carbon rings

364. Physical properties and chemical constitution. Part XVIII. Three-membered and four-membered carbon rings