Ab initio Study on Formation Mechanism of Spiro-Si-Heterocyclic Ring Compound Involving Ge from H2Ge=Si: and Formaldehyde

H₂Ge=Si: and its derivatives (X₂Ge=Si:, X=H, Me, F, Cl, Br, Ph, Ar,…) are new species. Its cycloaddition reactions are new area for the study of silylene chemistry. The cycloaddition reaction mechanism of singlet H₂Ge=Si: and formaldehyde has been investigated with the MP2/aug-cc-pVDZ method. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rule is that two reactants firstly form a four-membered Ge-heterocyclic ring silylene through the [2+2] cycloaddition reaction. Because of the 3p unoccupied orbital of Si: atom in the four-membered Ge-heterocyclic ring silylene and the π orbital of formaldehyde forming a π→p donor-acceptor bond, the four-membered Ge-heterocyclic ring silylene further combines with formaldehyde to form an intermediate. Because the Si: atom in the intermediate undergoes sp³ hybridization after transition state, then the intermediate isomerizes to a spiro-Si-heterocyclic ring compound involving Ge via a transition state. The result indicates the laws of cycloaddition reaction between H₂Ge=Si: or its derivatives (X₂Ge=Si:, X=H, Me, F, Cl, Br, Ph, Ar,…) and asymmetric π-bonded compounds are significant for the synthesis of small-ring involving Si and Ge and spiro-Si-heterocyclic ring compounds involving Ge.     

1,5,-Electrocyclizations—An Important Principle of Heterocyclic Chemistry

The principle of orbital control of pericyclic reactions has deepened our understanding of reaction phenomena and provided an excellent classification of these one-step processes. The electrocyclic reaction of the pentadienyl anion ? cyclopentenyl anion type is relatively unimportant in all-carbon systems and has not even been verified in the case of the parent compound. In the heterocyclic series, however, where up to five C-atoms of the pentadienyl anion are replaced by heteroatoms, a multitude of ring closures and ring openings find their ordering principle in the mentioned electrocyclic reaction. The replacement of the carbon atoms by heteroatoms can take place isoinically, i. e., with retention of the anionic character, or isoelectronically. An isoelectronic replacement of CR₂ in position 1 by NR₂ OR, and or CR in position 3 by NR or O leads to a charge-free resonance structure for the open-chain species; the migration of the charge during the electrocyclization results in a correlation with a cyclic zwitterion. Conversely, isoelectronic exchange of CR in position 2 by NR or O produces a conjugated 1,3-dipole, which cyclizes to a charge-free unsaturated five-membered ring. Twofold isoelectronic exchange allows the whole process to take place in a cation. Selected examples are to shed light on the classification and the thermodynamics of this electrocyclic reaction.     

Interaction between peroxide ion and phenol group which are coordinated to the same iron(III) ion

We have prepared several new iron(III) complexes with ligands which contain a phenol group; these are tetradentate [(X-phpy)H, X and H(phpy) represent the substituents on the phenol ring and N,N-bis(2-pyridylmethyl)-N-(2-hydroxybenzyl)amine, respectively] and pentadentate ligands [(R-enph-X)H; R=ethyl(Et) or methyl(Me) derivative and H(Me-enph) denotes N,N-bis(2-pyridylmethyl)-N″-methyl-N″-(2″-hydroxyl-benzylamine)ethylenediamine] and have determined the crystal structures of Fe(phpy)Cl₂, Fe(5-NO₂-phpy)Cl₂, and Fe(Me-enph)ClPF₆, which are of a mononuclear six-coordinate iron(III) complex with coordination of one or two chloride ion(s). These compounds are highly colored (dark violet) due to the coordination of phenol group to an iron(III) atom. When hydrogen peroxide was added to the solution of the iron(III) complex, a color change occurs with bleaching of the violet color, indicating that oxidative degradation of the phenol moiety occurred in the ligand system. The bleaching of the violet color was also observed by the addition of t-butylhydroperoxide. The rate of the disappearance of the violet color is highly dependent on the substituent on the phenol ring; introduction of an electron-withdrawing group in the phenol ring decreases the rate of bleaching, suggesting that disappearance of the violet band should be due to a chemical reaction between the phenol group and a peroxide adduct of the iron(III) species with an η¹-coordination mode and that in this reaction the peroxide adduct acts as an electrophile towards phenol ring. The intramolecular interaction between the phenol moiety and an iron(III)-peroxide adduct may induce activation of the peroxide ion, and this was supported by several facts that the solution containing an iron(III) complex and hydrogen peroxide exhibits high activities for degradation of nucleosides and albumin.     

Ab initio study on the mechanism of cycloaddition reaction between H<Subscript>2</Subscript>Si=Si: and formaldehyde

The mechanism of the cycloaddition reaction between singlet H₂Si=Si: and formaldehyde has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it could be predicted that the reaction has three competitive dominant reaction pathways. The reaction rules presented is that the 3p unoccupied orbital of the Si: atom in H₂Si=Si: inserts the π orbital of formaldehyde from the oxygen side, resulting in the formation of an intermediate. Isomerization of the intermediate further generates a four-membered ring silylene (the H₂Si–O in the opposite position). In addition, the [2+2] cycloaddition reaction of the two π-bonds in H₂Si=Si: and formaldehyde also generates another four-membered ring silylene (the H₂Si–O in the syn-position). Because of the unsaturated property of the Si: atom in the two four-membered ring silylenes, the two four-membered ring silylenes could further react with formaldehyde, generating two silicic bis-heterocyclic compounds. Simultaneously, the ring strain of the four-membered ring silylene (the H₂Si–O in the syn-position) makes it isomerize to a twisted four-membered ring product.     

Sensitive, selective spectrophotometric determination of phenols with periodic acid

A new spectrophotometric determination of phenols with periodic acid has been developed. The colored product of phenol periodic acid reaction has a λ_max 380 nm. The determination of phenol, pyrocatechol, α-naphthol, β-naphthol, quinol, p-cresol, m-cresol, 8-quinolinol, resorcinol, phloroglucinol, and gallic acid has been done. The effects of possible variables e.g., temperature, reagent, pH buffers have been studied. A study of interferences is made. Mechanism of the reaction is discussed.     

Metal complexes with macrocyclic ligands. XIII. The complexation of Cu2+ with triazacycloalkanes

The potentiometric study of the complexation of Cu²⁺ with 1,4,7-triazacyclononane ( 1 ), 1,4,8-triazacyclodecane ( 2 ) 1,5,9-triazacyclododecane ( 3 ) has shown that CuL, CuL₂ and (CuLOH)₂ are the main species present in solution. Their stabilities (Table 1) and their absorption spectra (Table 2) indicate facial coordination of the cyclic triamines in a distorted octahedral geometry. The formation and dissociation kinetics have been measured by stopped-flow techniques. The formation in acetate buffer can be described by the reaction of Cu²⁺ and CuAcO⁺ with the monoprotonated species of the ligand. The bimolecular rate constants for these complexations (Table 3) decrease when the ring size increases. In contrast the dissociation induced by acid is only little affected by the ring size. Thus for these complexes the rate of formation and not that of the dissociation determines the overall stability.     

Ab initio study of mechanism of forming bis-heterocyclic compound with Si and Ge between dimethylsilylene germylidene (Me2Si=Ge:) and ethene

The mechanism of the cycloaddition reaction between singlet dimethylsilylene germylidene (Me₂Si=Ge:) and ethene has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rules presented is that the two reactants firstly form a Si-heterocyclic four-membered ring germylene through the [2+2] cycloaddition reaction. Due to the sp ³ hybridization of the Ge: atom in Si-heterocyclic four-membered ring germylene, the Si-heterocyclic four-membered ring germylene further combined with ethene to form a bis-heterocyclic product with Si and Ge (P2).     

A study of the planarity of the pyrrolone fragment in 2‐isopropyl‐2,3‐dihydro‐1H‐isoindol‐1‐one

Orthophthalaldehyde (o‐phthalaldehyde, OPA) is an aromatic dialdehyde bearing two electron‐withdrawing carbonyl groups. The reactions of OPA with primary amines are broadly applied for the synthesis of important heterocyclic compounds with biological relevance. A number of such reactions have been investigated recently and several structures of condensation products have been reported, however, the complex reaction mechanism is still not fully understood and comprises concurrent as well as consecutive reactions. The reaction products depend on the primary amine which reacts with OPA, the reaction environment (solvent) and the proportion of the reactants. The title molecule, C₁₁H₁₃NO, the product of the reaction of OPA with isopropylamine, contains a five‐membered pyrrole C₄N ring with a carbonyl substituent, which forms part of the isoindolinone unit. Though this pyrrole ring contains one C atom in the sp³‐hybridized state, it is fairly planar. The title molecule has been compared with similar structures retrieved from the Cambridge Structural Database in order to study this phenomenon. The planarity of this fragment has been explained by the presence of partially delocalized C—C, C—N and C—O bonds, and by an inner angle in the planar pentagonal ring (∼108°), which is close to the ideal tetrahedral value for the sp³‐hybridized state of the constituent C atom. Due to this propitious angle, this C atom can be present in states intermediate between sp³‐ and sp²‐hybridized in different structures, while still maintaining the planarity of the ring. There are only weak intermolecular C—H…O hydrogen bonds and C—H…π‐electron ring interactions in the structure. In particular, it is the pyrrole ring which is involved in these interactions.     

The unusual formation of methyl α-(5,6-dimethoxycarbonyl-2,3-dimethoxyazepin-7-ylidene)-α-[5-methoxycarbonyl-2,3-dimethoxypyrid-6-yl)acetate during the pyrolysis of “Azido-meta-hemipinate”: First example of a reaction involving a concomitant ring expansion and ring extrusion

The ortho methoxycarbonyl substituent constitutes a sole exception in the ring closure reactions of ortho substituted aryl azides, as it provides no rate acceleration to this reaction. Pyrolysis of “azido-meta-hemipinate”, an aryl azide containing such a substituent, led us to the title compound, a new azepinylidene-pyridylacetic ester, whose structure has been established unambiguously by a single crystal X-ray diffraction study. This is the first report of a reaction involving both a ring expansion to an azaheptafulvalene and a ring extrusion to a pyridyl ring residue.     

Reactivity of titanium isopropoxide,zirconium propoxide and niobium ethoxide in the system of 2-methoxyethanol, 2,4-pentanedione and water

Titanium isopropoxide, zirconium propoxide and niobium ethoxide have been studied in reaction with 2-methoxyethanol (moe), acetylacetone (acac) and water. Structural information has been obtained using FTIR and ¹H, ¹³C FT NMR spectroscopies. The number of interchanged alkoxy groups increased with increasing excess of 2-methoxyethanol and reaction temperature. The exchange reaction between Nb(OEt)₅ and 2-methoxyethanol was the least effective. Reaction with acetylacetone proved to be very efficient, but dilution with an excess of solvent quenched this reaction. A solid phase of Zr(acac)₄ was formed for more than 2 modes of acac added. The appearance of multiplets in NMR spectra due to CH, CO centers of acac ring in Zr monoacetylacetonate indicated non-symmetric bonding of the bidentate acac group and more than one magnetically non-equivalent species. Conversely, a modified Nb(OEt)₄ (acac) precursor proved to be monomeric with a symmetrically bound acac group. During the interchange reaction the symmetry of the Nb(OEt) _x (Omoe)_4−x (acac) species was reduced. Alkoxy groups were first hydrolyzed followed by very slow removal of the acac ring. Zirconium and niobium modified precursors, respectively, were the least and the most susceptible to gelation. Addition of strong acid further increased the stability of acetylacetonates against condensation.     

Studies in ring-opening polymerization. II. Cyclohexane spiro-5-1,3,2-dioxathiolan-4-one 2-oxide

The effect of spiro cyclohexane substitution on the polymerizability of the 1,3,2-dioxathiolan-4-one-2-oxide ring in various solvents has been examined. The steric hindrance of the cyclohexane ring inhibits the bimolecular chain propagation reaction which involves direct attack by a terminal hydroxyl group on the ring and which has been shown to occur in simpler dioxathiolan systems. The conjoined cyclohexane ring does not, however, markedly affect the “thermal” polymerization which occurs in nonhydroxylic solvents and in which chain propagation is thought to involve a reactive α-lactone intermediate. The rate-determining step in the sequence of reaction leading to polymer formation is a ring-scission process in which sulfur dioxide is evolved and the α-lactone intermediate formed. The values of the activation energy (25–30 kcal/mole) and frequency factor (10¹¹–10¹³sec^?1) associated with this reaction are, therefore, those which govern the the overall polymerization, since the subsequent steps are sensibly instantaneous. In the presence of adventitious traces of water the resultant polymer, poly(1-hydroxycyclohexanecarboxylic acid) has one carboxyl and one hydroxyl endgroup per chain. Polymers having M?_n ～ 15,000 are readily obtained; these are amorphous materials, in contrast to the analogous poly-β-ester and dialkyl-substituted poly-α-esters which are crystalline. At temperatures in excess of 120°C a competitive first-order fragmentation reaction leading to the formation of cyclohexanone, carbon monoxide, and sulfur dioxide was observed. Kinetic studies demonstrated that this reaction, which is characterized by an activation energy of ～40 kcal/mole is unimportant, in the sense that it does not interfere with polymer formation at temperatures below 100°C.     

Bare Histidine–Serine Models: Implication and Impact of Hydrogen Bonding on Nucleophilicity

A new family of 2‐hydroxyalk(en/yn)ylimidazoles has been evaluated as serine–histidine bare dyad models for the ring‐opening reaction of L ‐lacOCA, a cyclic O‐carboxyanhydride. These models were selected to unravel the implication of intramolecular hydrogen bonding and to substantiate its influence on the nucleophilicity of the alcohol moiety, as it is suspected to occur in enzyme active sites. Although designed to exclusively facilitate the preliminary step of proton transfer during the studied ring‐opening reaction, these minimalistic models depicted a measureable increase in reactivity relative to the isolated fragments. A couple of reliable experimental and theoretical methods have been developed to readily monitor the strength of the intramolecular hydrogen bond in dilute solution. Results show that the folded conformers are the most nucleophilic species because of the intramolecular hydrogen bond.     

Development and Validation of Spectrophotometric Methods for the Determination of Cefetamet in Pharmaceutical Dosage Forms

Two simple and sensitive spectrophotometric methods for the determination of cefetamet in either pure form or in its pharmaceutical formulations were described. The method Ⅰ is based on the interaction of 3-methylbenzo[d]- thiazolin-2-one hydrazone （MBTH） with cefetamet in the presence of freshly prepared ferric chloride in a neutral medium. The resulting blue colored product has λmax at 628 nm. The method Ⅱ describes the reduction of ferric ion by the drug to ferrous ion followed by a complex formation reaction with 1,10-phenanthroline （1,10-phen） to form an orange red colored chromogen exhibiting 2max at 510 nm. The products are stable for more than 5 and 8 h respectively. Common excipients used as additives in pharmaceutical preparations do not interfere in the proposed methods. Both methods are highly reproducible and have been applied to a wide variety of pharmaceutical preparations and the results are comparable with those of official methods.     

Photoelimination reactions of N-heteroaromatic compounds

N-Heteroaromatic compounds substituted with a side chain next to the nitrogen and bearing a hydrogen gamma to the ring, undergo a photoelimination reaction to yield the methyl-substituted heterocycle and an alkene (if the gamma hydrogen is on a side chain carbon) or an aldehyde (if the gamma hydrogen is on a side chain oxygen). The reaction has been shown to occur with substituted isoquinolines, plienanthridines, pyridines and pyrazines.     

Electron-impact-induced fragmentation of morellin and related compounds

The mass spectra of morellin and other related natural products, isolated from the various Garcinia species, have been recorded and the data has been rationalised. The characteristic cleavage is the opening of the bicyclo(2.2.2) octenone ring system by a retro-Diels-Alder reaction. The γ-pyrone ring also undergoes the retro-Diels-Alder fragmentation. These two modes of cleavage indicate the different substituents on the main skeleton of morellin. The hydrogenated derivatives show slight variations in their fragmentation modes. The mass spectral data assists considerably in the structural elucidation of similar complex molecules, if isolated in minute quantities.     

Theoretical investigation on geometries and aromaticity of heterocyclic platinabenzenes

The electronic structure and properties of the heterocyclic platinabenzenes isomers have been investigated using the hybrid density functional B3LYP theory. Basic measures of aromatic character were derived from the structure and nucleus-independent chemical shift (NICS). The energetic criterion suggests that the ortho-isomer enjoy conspicuous stabilization where heteroatom is P or As. But the meta-isomer is most stable isomer, where heteroatom is N. The NICS values calculated at several points above the ring center to gave the result consistent with that cased on the relative energy. The atoms in molecules analysis indicates a correlation between NICS(1.0) and the electron density of ring critical point in all species. The natural bond orbital analysis has been used to study the bond characterizations in all species.     

Ab initio study of mechanism of forming a silicic <Emphasis Type="Italic">bis</Emphasis>-heterocyclic compound between (CH<Subscript>3</Subscript>)<Subscript>2</Subscript>Si=Si: and ethane

The mechanism of the cycloaddition reaction of forming a silicic bis-heterocyclic compound between singlet state (CH₃)₂Si=Si: and ethene has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has one dominant reaction pathway. The presented rule of this dominant reaction pathway that the 3p unoccupied orbital of Si: in (CH₃)₂Si=Si: and the π orbital of ethane forming a π → p donor-acceptor bond, resulting in the formation of a three-membered ring intermediate (INT1); Then, INT1 isomerizes to a four-membered ring silylene (P1), which driven by ring-enlargement effect; Due to sp ³ hybridization of Si: atom in the four-membered ring silylene (P1), P1 further combines with ethene to form a silicic bis-heterocyclic compound (P2).     

Bromenium‐Catalysed Tandem Ring Opening/Cyclisation of Vinylcyclopropanes and Vinylcyclobutanes: A Metal‐Free [3+2+1]/[4+2+1] Cascade for the Synthesis of Chiral Amidines and Computational Investigation

We present a detailed study of a [3+2+1] cascade cyclisation of vinylcyclopropanes (VCP) catalysed by a bromenium species (Br^δ+? X^δ?) generated in situ, which results in the synthesis of chiral bicyclic amidines in a tandem one‐pot operation. The formation of amidines involves the ring‐opening of VCPs with Br? X, followed by a Ritter‐type reaction with chloramine‐T and a tandem cyclisation. The reaction has been further extended to vinylcyclobutane systems and involves a [4+2+1] cascade cyclisation with the same reagents. The versatility of the methodology has been demonstrated by careful choice of VCPs and VCBs to yield bicyclo[4.3.0]‐, ‐[4.3.1]‐ and ‐[4.4.0]amidines in enantiomerically pure form. On the basis of the experimental observations and DFT calculations, a reasonable mechanism has been put forth to account for the formation of the products and the observed stereoselectivity. We propose the existence of a π‐stabilised homoallylic carbocation at the cyclopropane carbon as the reason for high stereoselectivity. DFT studies at B3LYP/6‐311+G** and M06‐2X/6‐31+G* levels of theory in gas‐phase calculations suggest the ring‐opening of VCP is initiated at the π‐complex stage (between the double bond and Br? X). This can be clearly perceived from the solution‐phase (acetonitrile) calculations using the polarisable continuum model (PCM) solvation model, from which the extent of the ring opening of VCP was found to be noticeably high. Studies also show that the formation of zero‐bridge bicyclic amidines is favoured over other bridged bicyclic amidines. The energetics of competing reaction pathways is compared to explain the product selectivity.     

Synthesis and Characterization of Novel 7-6-7 Ring-Based PCP Pincer Rhodium Complexes

Abstract

A series of Rh-PCP pincer complexes (^iPrPCP)Rh(L) bearing a novel alkyl–aryl mixed “7-6-7” ring skeleton has been synthesized and fully characterized. The 7-6-7 ring skeleton in the hydrido-chloro and carbonyl species was found to assume the cis and trans conformations, respectively, in solid state as determined by single crystal X-ray diffraction analysis. This suggested a flexibility of the backbone compared with the similar but strictly fixed anthracene backbone. Electron density on the central rhodium atom was investigated by IR experiments and by DFT computations. NMR monitoring of the reaction of the hydrido-chloro complex with base followed by application of H₂ showed the presence of what are likely to be the active 14e^? species and the dihydride derivative. Especially, noteworthy is the former that was found to be relatively stable, in contrast to the corresponding iridium complex.

[Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfer, and Silicon for the following free supplemental files: Additional text, tables, and figures.]     

Heat-gradient analytical technique for the simultaneous determination of some aromatic amines

A new sensitive analytical technique has been developed for the simultaneous determination of aromatic amines (aniline, p-nitroaniline, m-nitroaniline, o-nitroaniline, 1,3-phenylenediamine, and 1,4-phenylenediamine). It is based on the differential migration of colored derivatives formed by the reaction of diazotized amines with 8-hydroxy quinoline 5-sulphonic acid (FERRON) on a silica gel plate. Quantitative evaluation of amines is made by visual comparison of the intensities of color by spectrophotometry. The Beer’s law, molar absorptivity, and Sandell’s sensitivity have been determined. The effect of the analytical parameters on the migration and analysis have been evaluated. The method is highly reproducible and has been applied to the determination of amines in environmental samples. The text was submitted by the authors in English.