A model of porphyrin ring current effect

A new simple model of porphyrin ring current effect was proposed based on a line current approximation. It can reproduce the porphyrin-induced shifts for some Sn(IV)(tpp) dicarboxylate complexes quite satisfactorily.     

Nonlinear dielectric effect (NDE) and molecular orbital study of the conformational equilibrium in 1,4-dimethoxybenzene in benzene solution

The conformational equilibrium in 1,4-dimethoxybenzene (1,4-DMB) in benzene solutions has been studied. On the basis of experimental values of the nonlinear dielectric effect (NDE) parameter, electric permittivity and density, determined in this work, and applying the general statistical theory of NDE, the contributions of the syn-anti and syn-syn conformers and the electric dipole moment of the polar syn-syn conformer were calculated. The molecular orbital method (PM3) has also been applied for calculation of the dipole moments and energies of particular conformers. The results of the NDE study and PM3 calculations are consistent and they reveal the existence of two conformers (syn-anti and syn-syn) of comparable energy values, but different values of dipole moments, and the predominance of the polar form (syn-syn) of the mole fraction in benzene. Moreover, the energies of intermolecular interactions have been determined from the concentration dependence of linear and nonlinear polarisability.     

Synthesis and conformational analysis of naphth[1′,2′:5,6][1,3]oxazino[3,2-c][1,3]benzoxazine and naphth[1′,2′:5,6][1,3]oxazino[3,4-c][1,3]benzoxazine derivatives

A new functional group, the hydroxy group, was inserted into a Betti base by reaction with salicylaldehyde, and the naphthoxazine derivatives thus obtained were converted by ring-closure reactions with formaldehyde, acetaldehyde, propionaldehyde or phosgene to the corresponding naphth[1′2′:5,6][1,3]oxazino[3,2-c][1,3]benzoxazine derivatives. Further, the conformational analysis of these polycyclic compounds by NMR spectroscopy and an accompanying molecular modelling are reported; especially, both quantitative anisotropic ring current effects of the aromatic moieties in these compounds and steric substituent effects were employed to determine the stereochemistry of the naphthoxazinobenzoxazine derivatives.     

Gas phase and water solution conformational analysis of the herbicide diuron (DCMU): an ab initio study

In the present work, the conformational equilibrium for the herbicide diuron (DCMU) has been investigated using high level ab initio calculations. The solvent effect was included through two different continuum models: (1) the real cavity IPCM method and (2) the standard dipole Onsager model SCRF. The effect due to solute-solvent hydrogen-bond interactions was analyzed considering a hybrid discreet-continuum model. At the Hartree-Fock level, the gas phase results showed that only the trans forms (A and B) are present in the equilibrium mixture, with the relative concentrations found to be 33% (A) and 67% (B) (HF/6-311+G**//6-31G**). When the electronic correlation effect is included (MP2/6-31G*//HF/6-31G*), a relative stabilization of the cis forms was observed, with the conformational distribution calculated as 38% (A), 50% (B), 6% (C) and 6% (D). The trans conformations were found to be completely planar, which has been considered to be a prerequisite for the herbicide binding. In water solution, the trans conformation A should be the most abundant conformer, the IPCM and SCRF values being ca. 100% and ca. 85% respectively. The IPCM calculations with the isodensity level set to 0.0005 present a conformational distribution close to that obtained from the hybrid model [92% (A) and 8% (B)], which has been considered our best solvent approach. Regarding the biological action of urea-type herbicides, the results presented here are important, because some QSAR studies have suggested that the partition coefficient is related to the herbicide activity, so the conformational equilibrium may play a role in the biological action. Received: 23 February 1998 / Accepted: 28 May 1998 / Published online: 19 August 1998     

Extreme projection of a proton into the pi-cloud of an aromatic ring: record shielding of an aromatic proton in trans-10b-methyl-10c-(1-naphthyl)-10b,10c-dihydropyrene

A synthetic sequence involving dithiametacyclophane --> metacyclophanediene --> dihydropyrene was employed to prepare trans-10b-methyl-10c-(2-naphthyl)- and trans-10b-methyl-10c-(1-naphthyl)-10b,10c-dihydropyrene 5 and 6, respectively. Both exhibit a strong diamagnetic ring current despite the introduction of an internal bulky substituent within the pi-electron cloud. Their electronic spectra suggest interaction between the two near-perpendicular naphthyl and dihydropyrenyl pi systems, resulting in red shift and band broadening. All naphthyl protons are well resolved in their 1H NMR spectra due to a strong shielding effect of the dihydropyrene ring. The most shielded protons in 5 and 6 are H1' and H2' at delta 2.47 and 1.42, respectively, being 5.25 and 5.95 ppm shifted from those of reference protons. There is evidence for free rotation on the NMR time scale of the 2-naphthyl ring in 5 with a preference for a particular conformer, whereas the 1-naphthyl ring in 6 is conformationally rigid with its H2' projecting deeply into the pi-cloud, thus accounting for the most shielded aromatic proton (H2' in 6) reported to date.     

DNMR, DFT and preparative study on the conformation of (Z)-4,5,6,7-tetrahydropyrazolo[1,5-e]benzo[g][1,5]diazonin-8-ones and (Z)-4,5-dihydropyrazolo[1,5-d]benzo[f][1,4]diazocin-7(6H)-ones

By means of base-catalyzed ring enlargement of triazaindenoindenes and pentalenoindenes obtained from anhydride-induced ring transformation of 3,4-dihydro-2H-pyrimido- and 2,3-dihydroimidazo[2,1-a]phthalazinium-olates, respectively, a series of pyrazolo[1,5-e]benzo[g][1,5]diazonin-8-ones and pyrazolo[1,5-d]benzo[f][1,4]diazocin-7(6H)-ones were obtained. Alternative pathways and energetics for the ring inversion of symmetrically substituted medium-size ring systems were determined by combined use of DNMR measurements and B3LYP/6-31G(d,p) calculations using the IEFPCM solvent model. One pyrazolobenzodiazonine carrying hydrogen at the C1 position was found to undergo facile ring inversion by a two-step mechanism, while 1-Me and 1-Ph substituents rendered complete rigidity to this ring system. A three-step mechanism was revealed for the ring inversion of the two studied pyrazolobenzodiazocines with energetics practically invariant to the investigated C1-substituents (H and Me). The attempted RCM of the N,O-diallyl derivative of a selected rigid model effected by Grubbs II catalyst led to deallylation and olefin isomerization avoiding the formation of bridged products with enhanced skeletal strain. A tolerable degree of ring strain associated with negligible skeletal distortion could be introduced into the same benzodiazonine by N,O-dialkylation with 1,3-bis(bromomethyl)benzene.     

Synthesis of 4-(3-hydroxyalkyl)pyrimidines by ring transformation reactions of 2-alkylidenetetrahydrofurans with amidines

Domino reactions of amidines with 2-alkylidenetetrahydrofurans, prepared by cyclization of 1,3-dicarbonyl dianions or 1,3-bis-silyl enol ethers with various dielectrophiles, provided an efficient access to 4-(3-hydroxyalkyl)pyrimidines.     

Synthesis and solvatochromism study of some new 4-(benzyloxy)-2-((4-aryl)diazenyl)phenols

A series of novel azo disperse dyes were synthesized by coupling reaction of 4-benzyloxyphenol with diazotized p-substituted aniline derivatives as diazo components.These compounds were characterized by spectroscopic techniques and elemental analysis. Ionization constants,pK_a,for these dyes were determined using electronic spectroscopic method.     

Scandium(III) triflate mediated intramolecular ring expansion of aziridines: a direct access to 4-aryltetrahydroisoquinolines

A novel, high yielding facile synthesis of 4-substituted tetrahydroisoquinolines has been developed by employing scandium(III) triflate mediated intramolecular ring expansion of aziridines. The meta-substituted electron-donating group on the benzene ring facilitates trapping of an in situ generated benzyl carbenium ion cation leading to the formation of tetrahydroisoquinolines.     

Formation of 3-[amino(aryl)-methylidene]-1,3-dihydro-2H-indol-2-ones involving ring transformation of 2-aryl-5-(2-aminophenyl)-4-hydroxy-1,3-thiazoles

The reaction of 3-bromooxindole with substituted (hetero)aromatic thioamides in acetonitrile was studied. At room temperature the reaction preferably gives products of ring transformation i.e. 2-aryl-5-(2-aminophenyl)-4-hydroxy-1,3-thiazoles (3b-f,h) whereas at elevated temperature products of an Eschenmoser coupling reaction, i.e. 3-[amino(aryl)-methylidene]-1,3-dihydro-2H-indol-2-ones (2b-f), are formed exclusively. There exist only two exceptions (4-methoxy and 2-pyridinthioamide) in which the Eschenmoser coupling reaction always takes place giving 2a and 2g. Also N-methylation of the starting 3-bromooxindole completely prevents formation of thiazoles. The prepared thiazoles 3b-f are unstable in solution and they undergo slow ring transformation to 2b-f. The rate limiting step of this rearrangement involves cleavage of an intermediary thiirane ring, which is slowed down by electron-withdrawing substituents on the thioamide (ρ = ?1.15).     

Thermodynamic functions of thermotropic polyethers based on the semiflexible mesogen 1-(4-hydroxyphenyl)-2-(2-methyl-4-hydroxyphenyl)ethane

Heat capacities of a series of thermotropic polyethers consisting of a semiflexible mesogen [1-(4-hydroxyphenyl) - 2 - (2 - methyl - 4 - hydroxyphenyl)ethane], with n methylene flexible spacers (n = 4–12) (MBPE-n) have been measured by DSC and fitted at low temperature to an approximate frequency spectrum, as well as at high temperature to a general equation for the liquid MBPE-n. The latter equation is: C_p = n(17.33 + 0.04551T) + (280.9 + 0.3839T) where n is the number of methylenes in the polyether spacer. The calculated vibration-only heat capacities start to show deviations from the measured heat capacities below the melting temperature, reflecting contributions from conformational disorder and motion in methylene spacers. It is suggested that part of this increase in heat capacity can be looked upon as a glass transition of the partially conformationally disordered crystals. Solid state ¹³C–NMR studies showed similarly that over the range of temperature some of the C? C bonds in the spacer are in a rotational state similar to that in the melt. The equilibrium heats of fusion (ΔH) and the changes of heat capacity (ΔC_p) for the amorphous polymer at the glass transition temperature, T_g, were determined by WAXD and DSC. Based on the discrepancy of ΔC_p it is concluded that these phenylene containing polyethers have a certain amount of rigid amorphous polymer. Thermodynamic functions H, S and G for all of the polyethers have been established.     

Synthesis and conformational study of 1,3,2-oxazaphosphorino[4, 3-a]isoquinolines: a new ring system

A set of 1,3,2-oxazaphosphorino[4,3-a]isoquinolines 6a,b-9a,b, a new ring system, was synthesized, and their stereochemical and conformational analyses were performed by (1)H, (13)C, and (31)P NMR methods. X-ray measurements were also carried out to confirm the stereochemical assignments and conformational results obtained by means of NMR. Intermediate coupling constants (3)J(P,H) were found for compounds 7 and 9; these do not relate to equilibria between previously reported conformers, but are indicative of new distorted conformational states in solution. The connecting isoquinoline and the steric interaction between the aromatic moiety and the Me-1 substituent can block the oxazaphosphorinane ring. The conformational behavior of compounds 6 and 8 was characterized by the usual chair-twist equilibrium.     

Electronic structure and conformational properties of (carboxy-alkenyl)-phosphonic acids

The general conformational properties and electronic structure of (carboxy-alkenyl)-phosphonic derivatives were determined at RHF/STO-3G^* level. In all the series, low rotation barriers were found for the two C=C/P=O conformers. In the compounds in which the interactions between the carboxylic and phosphonic moieties are smaller, the most stable conformers are the C=C/P=O s-cis ones. In most of the conformers, the C=C/C=O system presents the disposition s-cis. The Z-(2-carboxy-vinyl) and Z-(2-carboxy-propenyl) phosphonic acids present intramolecular hydrogen bonds, existing in at least four conformer with internal hydrogen bonds. These last compounds were more rigorously studied at RHF/3-21G^* and RHF/6-31G^** levels. The most stable conformer shows a trans structure for the C=C/P=O angle, with an intramolecular hydrogen bond located between the hydroxylic hydrogen of phosphonic group and the carbonyl oxygen of carboxylic moiety. A secondary conformer is found with a double intramolecular hydrogen bond between two hydroxylic hydrogens of the phosphonic moiety and the oxygen of carboxylic bond. Another secondary conformer appears with an intramolecular hydrogen bond between the oxygen of the phosphoryl bond and the hydroxylic hydrogen of the carboxylic group. A study of the topology of charge densities is carried out. This analysis reveals bonds with an ionic participation. A very weak π conjugation, variable with the conformers, is found in the C=C/P=O system, as well as a strongly polarized P=O partial triple bond. The intramolecular hydrogen bonds give rise to cyclic structures.     

A stereoselective synthesis of (R)-5-hydroxy-3-(4-methoxyphenethyl)cyclohex-2-enone,towards total synthesis of Prelunularin

A first chiral strategy for synthesis of (R)-5-hydroxy-3-(4-methoxyphenethyl) cyclohex-2-enone reported. Key transformation includes Barbier allylation, ionic hydrogenation, Jeffery reaction, Katsuki Sharpless asymmetric epoxidation, reductive opening of epoxide and ring closing metathesis.     

Properties of 2-, 3-, and 4-acetylpyridine substituted ruthenium(II) bis(bipyridine) complexes: substituent effect on the electronic structure,spectra, and photochemistry of the complex

Bis(2,2′-bipyridine) complexes of ruthenium(II) with 2-, 3-, and 4-acetylpyridine derivatives were synthesized and structurally characterized. The effect of changing the location of the pyridine's acetyl substituent was studied experimentally and theoretically to clarify the effect of substituent position on the chemical behavior and photochemical properties of the complex. The substituent position on the heterocyclic-pyridine was found to strongly affect the chemical and photochemical properties of the complex. Variation of the position of the substituent, and thus ligand modification brought by as a consequence of this variation, offers possibilities to design complexes of desired structural and photochemical properties.     

Synthesis and conformational analysis of N-aryl-N-(6-azulenyl)acetamides

We synthesized a series of N-(6-azulenyl)-N-arylacetamides, and investigated their conformational preferences in solution and in the crystalline state by means of NMR and X-ray analyses. The results indicated that the conformational preferences of these amides are dependent on both the relative π-electron densities of the N-aromatic parts and the three-dimensional relationship of the carbonyl group to the aryl groups. The N-(6-azulenyl) group has a very different effect from the previously reported N-(2-azulenyl) group on the conformational preferences of aromatic amides.