Ring Inversion Research Articles

Formation of phenanthrenyl radicals via the reaction of acenaphthyl with acetylene

Ab initio G3(MP2,CC)/B3LYP/6–31G** calculations of the potential energy surface for the reaction of 3-acenaphthyl radical with acetylene were combined with Rice-Ramsperger-Kassel-Marcus Master Equation calculations of the temperature- and pressure-dependent reaction rate constants and product branching ratios with the goal to examine the transformation of an edge five-membered ring into a six-membered one. The reaction mechanism has been shown to contain two stages: (1) the formation of 3-ethynylacenaphthylene + H in the 3-acenaphthyl + C2H2 reaction via acetylene addition to the radical site and an H loss, (2) the formation of the 1-phenanthrenyl radical in the 3-ethynylacenaphthylene + H reaction via H addition to the five-membered ring followed by the ring flip. The results of kinetic modeling show this pathway being faster than the pathway proposed earlier featuring the CCH chemisorption on the six-membered ring without involvement of the five-membered ring. A kinetic test demonstrated the dominance of the former pathway for the formation of pyrene from naphthalene. It is expected that the new mechanism plays a major role in the transformation of five-membered rings on edges of PAH structures.

Janus Face All-cis 1,2,4,5-tetrakis(trifluoromethyl)- and All-cis 1,2,3,4,5,6-hexakis(trifluoromethyl)- Cyclohexanes.

We report the synthesis of all‐cis 1,2,4,5‐tetrakis (trifluoromethyl)‐ and all‐cis 1,2,3,4,5,6‐hexakis (trifluoromethyl)‐ cyclohexanes by direct hydrogenation of precursor tetrakis‐ or hexakis‐ (trifluoromethyl)benzenes. The resultant cyclohexanes have a stereochemistry such that all the CF3 groups are on the same face of the cyclohexyl ring. All‐cis 1,2,3,4,5,6‐hexakis(trifluoromethyl)cyclohexane is the most sterically demanding of the all‐cis hexakis substituted cyclohexanes prepared to date, with a barrier (ΔG) to ring inversion calculated at 27 kcal mol−1. The X‐ray structure of all‐cis 1,2,3,4,5,6‐hexakis(trifluoromethyl)cyclohexane displays a flattened chair conformation and the electrostatic profile of this compound reveals a large diffuse negative density on the fluorine face and a focused positive density on the hydrogen face. The electropositive hydrogen face can co‐ordinate chloride (K≈103) and to a lesser extent fluoride and iodide ions. Dehydrofluorination promoted decomposition occurs with fluoride ion acting as a base.

Janus Face All‐cis 1,2,4,5‐tetrakis(trifluoromethyl)‐ and All‐cis 1,2,3,4,5,6‐hexakis(trifluoromethyl)‐ Cyclohexanes

AbstractWe report the synthesis of all‐cis 1,2,4,5‐tetrakis (trifluoromethyl)‐ and all‐cis 1,2,3,4,5,6‐hexakis (trifluoromethyl)‐ cyclohexanes by direct hydrogenation of precursor tetrakis‐ or hexakis‐ (trifluoromethyl)benzenes. The resultant cyclohexanes have a stereochemistry such that all the CF3 groups are on the same face of the cyclohexyl ring. All‐cis 1,2,3,4,5,6‐hexakis(trifluoromethyl)cyclohexane is the most sterically demanding of the all‐cis hexakis substituted cyclohexanes prepared to date, with a barrier (ΔG) to ring inversion calculated at 27 kcal mol−1. The X‐ray structure of all‐cis 1,2,3,4,5,6‐hexakis(trifluoromethyl)cyclohexane displays a flattened chair conformation and the electrostatic profile of this compound reveals a large diffuse negative density on the fluorine face and a focused positive density on the hydrogen face. The electropositive hydrogen face can co‐ordinate chloride (K≈103) and to a lesser extent fluoride and iodide ions. Dehydrofluorination promoted decomposition occurs with fluoride ion acting as a base.

Beyond the ring flip: A molecular signature of the glass–rubber transition in tetrafunctional epoxy resins

The glass–rubber structural transition of epoxy resins is a critical quantity in the design of resins for thermally demanding applications. Currently, many high Tg epoxy resins exhibit reduced mechanical performance and are not suitable for structural applications. A present lack of understanding regarding the molecular origins of the glass–rubber transition, together with the cost of resin synthesis, has limited progress in the targeted development of novel, mechanically strong, high-Tg resins. Here, molecular dynamics simulations are used to predict the molecular-level structure and thermo-mechanical properties of three tetrafunctional epoxy resins. Via introduction of a conceptual framework of monitoring local molecular motions during molecular dynamics simulations, a characteristic molecular signature of segmental dynamics, correlating with the glass transition temperature, is identified. This computational framework provides a cost-effective strategy for rapidly assessing the comparative thermal performance of novel epoxy resins. Additionally, the impact of ring substitution on the thermo-mechanical properties of the naphthalene-based resins reveals promising directions for future design to yield desirable mechanical and thermal properties.

Dual C–Cl isotope analysis for characterizing the anaerobic transformation of α, β, γ, and δ-hexachlorocyclohexane in contaminated aquifers

Hexachlorocyclohexanes (HCHs) are widespread and persistent environmental pollutants, which cause heavy contamination in soil, sediment and groundwater. An anaerobic consortium, which was enriched on β-HCH using a soil sample from a contaminated area of a former pesticide factory, was capable to transform α, β, γ, and δ-HCH via tetrachlorocyclohexene isomers stoichiometrically to benzene and chlorobenzene. The carbon and chlorine isotope enrichment factors (εC and εCl) of the dehalogenation of the four isomers ranged from −1.9 ± 0.3 to −6.4 ± 0.7‰ and from −1.6 ± 0.2 to −3.2 ± 0.6‰, respectively, and the correlation of δ37Cl and δ13C (Λ values) of the four isomers ranged from 1.1 ± 0.1 to 2.4 ± 0.2. The evaluation of Λ and the apparent kinetic isotope effects (AKIE) for carbon and chlorine may lead to the hypothesis that the two eliminated chlorine atoms of α- and γ-HCH were in axial positions, the same as for the β-HCH conformer which has six chlorine atoms in axial positions after ring flip. The dichloroelimination of δ-HCH resulted in distinct AKIE and Λ values as one chlorine atom is in axial whereas the other chlorine atoms are in the equatorial positions. Significant chlorine and carbon isotope fractionations of HCH isomers were observed in the samples from a contaminated aquifer (Bitterfeld, Germany). The 37Cl/35Cl and 13C/12C isotope fractionation patterns of HCH isomers from laboratory experiments were used diagnostically in a model to characterize microbial dichloroelimination in the field study. The comparison of isotope fractionation patterns indicates that the transformation of HCH isomers at the field was mainly governed by microbial dichloroelimination transformation.

Dibenzoylbenzodipyrroles: Key Precursors for the Synthesis of Fused meso-Aryl Sapphyrins.

A simple strategy has been developed for the synthesis of α,α'-dibenzoylbenzodipyrroles, which are key synthons for the synthesis of fused porphyrinoids. α,α'-Dibenzoylbenzodipyrroles were characterized by high-resolution mass spectrometry (HRMS), NMR, and X-ray crystallography. To show the use of α,α'-dibenzoylbenzodipyrrole, we synthesized five different fused meso-aryl sapphyrins under acid-catalyzed reaction conditions. α,α'-Dibenzoylbenzodipyrrole was reduced to diol and condensed with five different tripyrranes such as aza, oxa, thia, selena, and telluratripyrranes under mild acid-catalyzed conditions to afford fused meso-aryl sapphyrins in 15-18% yields. One-dimensional (1D) and two-dimensional (2D) NMR studies revealed that in fused sapphyrins, the furan ring in oxabenzosapphyrin and the pyrrole ring in benzosapphyrin, which are present opposite to the benzodipyrrole moiety, attained ring inversion (inverted sapphyrins), whereas the selenophene ring in selenabenzosapphyrin and the tellurophene ring in tellurabenzosapphyrin did not show ring inversion (normal sapphyrins). However, thiabenzosapphyrin exhibits both normal and inverted conformations in different ratios. All fused sapphyrins showed typical aromatic absorption features; however, the absorption features of normal fused sapphyrins are different from the inverted fused sapphyrins. Redox studies indicate that normal fused sapphyrins are difficult to oxidize but easier to reduce compared to inverted fused sapphyrins. Density functional theory (DFT) studies support the experimental observations.

Synthesis, Characterization, Stereochemistry and Antimicrobial Evaluation of N-Acyltetrahydrobenzodiazepin-2-ones

A few N5-substituted (1) and N1,N5-disubstituted tetrahydro-1,5-benzodiazepin-2-ones (2-8) viz. N5-chloroacetyl-, N5-formyl-, N5-dichloro-acetyl-, N1,N5-diethoxycarbonyl-, N1,N5-bischloroacetyl-, N5-piperazinoacetyl- and N5-morpholinoacetyltetrahydro-1,5-benzodiazepin-2-ones have been synthesized. The characterization and conformational analysis of these compounds 2-8 have been carried out using IR and 1H, 13C, DEPT & 2D (COSY & HSQC) NMR spectral techniques. The coupling constants for compounds 3-6 were determined by irradiating the C4-methyl doublet. The appearance of major and minor conformers has been found in the case of benzodiazepin-2-ones (3 and 6) and the spectral data confirm the equilibrium due to ring inversion over the N-C=O rotation. The spectral data and the extracted coupling constant values revealed that the substituted tetrahydro-1,5-benzodiazepin-2-ones (2-8) prefer to adopt boat conformation. The antimicrobial activity of compounds 1-8 has also been evaluated.

Systematic description of molecular deformations with Cremer-Pople puckering and deformation coordinates utilizing analytic derivatives: Applied to cycloheptane, cyclooctane, and cyclo[18]carbon.

The conformational properties of ring compounds such as cycloalkanes determine to a large extent their stability and reactivity. Therefore, the investigation of conformational processes such as ring inversion and/or ring pseudorotation has attracted a lot of attention over the past decades. An in-depth conformational analysis of ring compounds requires mapping the relevant parts of the conformational energy surface at stationary and also at non-stationary points. However, the latter is not feasible by a description of the ring with Cartesian or internal coordinates. We provide in this work, a solution to this problem by introducing a new coordinate system based on the Cremer-Pople puckering and deformation coordinates. Furthermore, analytic first- and second-order derivatives of puckering and deformation coordinates, i.e., B-matrices and D-tensors, were developed simplifying geometry optimization and frequency calculations. The new coordinate system is applied to map the potential energy surfaces and reaction paths of cycloheptane (C7H14), cyclooctane (C8H16), and cyclo[18]carbon (C18) at the quantum chemical level and to determine for the first time all stationary points of these ring compounds in a systematic way.

Core-EP inverse in rings with involution

Core-EP inverse in rings with involution

Characterizations and maximal classes of elements related to pseudo core inverses

We present some new characterizations for the pseudo core inverse in rings with involution. Firstly, we characterize the pseudo core inverse by a unique projection. Secondly, we prove that the pseudo core inverse is EP and obtain another expression of the pseudo core inverse. Furthermore, we establish the relation between the EP element and the pseudo core inverse. Finally, maximal classes of elements in rings determining the pseudo core inverse are investigated.

Inclusion properties of generalized inverses in semiprime rings

Let [Formula: see text] be a semiprime ring, not necessarily with unity, with extended centroid [Formula: see text]. For [Formula: see text], let [Formula: see text] (respectively [Formula: see text], [Formula: see text]) denote the set of all outer (respectively inner, reflexive) inverses of [Formula: see text] in [Formula: see text]. In the paper, we study the inclusion properties of [Formula: see text], [Formula: see text] and [Formula: see text]. Among other results, we prove that for [Formula: see text] with [Formula: see text] von Neumann regular, [Formula: see text] (respectively [Formula: see text]) if and only if [Formula: see text] (respectively [Formula: see text]). Here, [Formula: see text] is the smallest idempotent in [Formula: see text] such that [Formula: see text]. This gives a common generalization of several known results.

Conformational analysis of selected [2.2]heterophanes: A comparison of stereochemical switching via computational chemistry

Conformational switching of selected [2.2]heterophanes was investigated by computational chemistry. Analyses were carried out by various methods, including Conformational Search in HyperChem, and forced conformational transformations with Energy Profile in Spartan. Stable anti and syn conformers arising from flipping of aromatic rings within the molecules were observed. The activation energies of the ring flipping process, as well as dipole moments and directions, were obtained by molecular mechanics. The present work shows that simple computational techniques can be employed to screen certain compounds as potential candidates for molecular machinery. Thus, heterophanes demonstrated reversible shifts between two or more configurations which are energetically stable and have different electronic properties, constituting a basic requirement for possible applications as molecular switches.

Slow ring flips in aromatic cluster of GB1 studied by aromatic 13C relaxation dispersion methods

Ring flips of phenylalanine and tyrosine are a hallmark of protein dynamics. They report on transient breathing motions of proteins. In addition, flip rates also depend on stabilizing interactions in the ground state, like aromatic stacking or cation–π interaction. So far, experimental studies of ring flips have almost exclusively been performed on aromatic rings without stabilizing interactions. Here we investigate ring flip dynamics of Phe and Tyr in the aromatic cluster in GB1. We found that all four residues of the cluster, Y3, F30, Y45 and F52, display slow ring flips. Interestingly, F52, the central residue of the cluster, which makes aromatic contacts with all three others, is flipping significantly faster, while the other rings are flipping with the same rates within margin of error. Determined activation enthalpies and activation volumes of these processes are in the same range of other reported ring flips of single aromatic rings. There is no correlation of the number of aromatic stacking interactions to the activation enthalpy, and no correlation of the ring’s extent of burying to the activation volume. Because of these findings, we speculate that F52 is undergoing concerted ring flips with each of the other rings.

P-Hirano inverses in rings

We introduce and study a new class of generalized inverses in rings. An element a in a ring R has p-Hirano inverse if there exists b ? R such that bab = b,b ? comm2(a),(a2-ab)k ? J(R) for some k ? N. We prove that a ? R has p-Hirano inverse if and only if there exists p = p2 ? comm2(a) such that (a2-p)k ? J(R) for some k ? N. Multiplicative and additive properties for such generalized inverses are thereby obtained. We then completely determine when a 2 x 2 matrix over local rings has p-Hirano inverse.

Idempotents generated by weighted generalized inverses in rings with involution

Let R be an associate ring with unity 1 and involution. In this paper, we investigate several properties and characterizations of idempotents generated by weighted Moore-penrose inverses and weighted pseudo core inverses in R. Moreover, several new characterizations about weighted EP elements and existence criteria of weighted pseudo core inverses are also given.

One-sided (b,c)-inverses in rings

In this paper we introduce left and right annihilator (b,c)-inverses and we investigate some of theirs properties. Furthermore, here we study some properties of left and right (b,c)-inverses.

Which isomer is it, 1,2,5,6- or 1,4,5,8-tetrasubstituted cycloocta-1,3,5,7-tetraene? Synthesis of symmetrically tetrasubstituted cycloocta-1,3,5,7-tetraene derivatives.

Ni(0) catalyzed cyclotetramerization [(2 + 2 + 2 + 2) cycloaddition] of propargyl alcohol gave D2 symmetric 1,4,5,8-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene (1) as the major product contrary to what was reported earlier to be the corresponding C2v symmetric isomer, namely 1,2,5,6-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene. The structure of 1 has been unambiguously assigned on the basis of single crystal XRD data which showed a very interesting "densely" intermolecularly hydrogen bonded solid state supramolecular structure. Tetrabromide (3) prepared from tetra-alcohol (1) reacted with a variety of nucleophiles, namely carbon, nitrogen, sulfur and phosphorus nucleophiles, to give the corresponding derivatives in good yields. The tetraphosphonate derivative (9) underwent the Wittig-Horner reaction with aromatic aldehydes to give the corresponding tetra-styryl derivatives (10a-i). The reaction of tetrabromide with aniline, benzyl amine and R(+)-α-methylbenzyl amine gave the corresponding C2v symmetric pyrroline fused 1,2,5,6-tetra-substituted cycloocta-1,3,5,7-tetraene derivatives, while all the other derivatives belonged to D2 symmetric 1,4,5,8-tetra-substituted cycloocta-1,3,5,7-tetraene. Quantum mechanical calculations support the above conclusion based on the relative stabilities of the two isomers. The fusion of a 5-membered pyrroline ring with the tub-shaped cyclooctatetraene (COT) ring leads to flattening of the tub (shallow tub) making it easier for the ring inversion of COT through a planar intermediate to take place readily resulting in the formation of C2v symmetric pyrroline fused COT derivatives.

Template-Free Synthesis of a Macrocyclic Bis(pyridine-dienamine) Proligand and Metal Complexes of Its Bis(pyridine-diimine) and Bis(pyridine-dienamido) Forms.

We describe the template-free synthesis of the bis(pyridine-dienamine) proligand [4,5-(m-xylylenediamine)NH-C═(CH)(9-butyl-octahydroacridine)]2 (2'), a variant of Burrows's macrocyclic bis(pyridine-diimine) (bis-PDI) ligand [2,6-(m-xylylenediamine)N═C(py)]2 (A), using octahydroacridine as the ligand backbone. The octahydroacridine backbone favors macrocyclization by constraining the PDI units in the (s-cis)2 conformation. The template-free synthesis of 2' enables facile access to a wide array of bis-PDI and bis(pyridine-dienamido) (bis-PDE) metal complexes. Five-coordinate binuclear bis-PDI (2)M2Cl4 complexes {2 = [4,5-(m-xylylenediamine)N═C(9-butyl-octahydroacridine)]2; M = Zn, Co, or Fe} and a four-coordinate bis-PDI [(2)Pd2Br2][B(3,5-(CF3)2-Ph)4]2 complex were synthesized and characterized. (2)Zn2Cl4 undergoes macrocyclic ring inversion on the nuclear magnetic resonance (NMR) time scale with a free energy barrier ΔG⧧ of 15.5(3) kcal/mol at 295 K. In contrast, (2)Fe2Cl4 and (2)Co2Cl4 undergo slow ring inversion on the NMR chemical shift time scale at 295 K. The amine elimination reaction of 2' with Zr(NMe2)4 yields the bis-PDE complex (2'-4H)Zr2(NMe2)4, which was alkylated with AlMe3 and Al(CH2SiMe3)3 to generate (2'-4H)Zr2Me4 and (2'-4H)Zr2(CH2SiMe3)2(NMe2)2, respectively.

A note on inner and reflexive inverses in semiprime rings

Let [Formula: see text] be a semiprime ring, not necessarily with unity, and [Formula: see text]. Let [Formula: see text] (respectively, [Formula: see text]) denote the set of inner (respectively, reflexive) inverses of [Formula: see text] in [Formula: see text]. It is proved that if [Formula: see text], then [Formula: see text] if and only if [Formula: see text] for all [Formula: see text]. As an immediate consequence, if [Formula: see text], then [Formula: see text] (see Theorem 7 in [A. Alahmadi, S. K. Jain and A. Leroy, Regular elements determined by generalized inverses, J. Algebra Appl. 18(7) (2019) 1950128] for rings with unity). We also give a generalization of Theorem 10 in [A. Alahmadi, S. K. Jain and A. Leroy, Regular elements determined by generalized inverses, J. Algebra Appl. 18(7) (2019) 1950128] by proving that if [Formula: see text] then [Formula: see text].

Ring inversion in cyclohexane: a textbook example

ABSTRACT The ring inversion of cyclohexane is a classic textbook example in organic chemistry of large-scale intramolecular interconversion. Early work by Luz using deuterated cyclohexane has shown how the interconversion rate can be studied. Here the complex proton NMR spectrum of cyclohexane is reported. We apply liquid-crystal 1H NMR spectroscopy as a function of temperature to study the interconversion rate process. For low temperatures interconversion is slow, at higher temperatures the fast interconversion limit is approached. For intermediate temperatures significant line broadening is observed as the signature of the interconversion process.