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General syntheses have been developed for meso-substituted porphyrins with one or two substituents in the 5,10- positions and no beta substituents. 5-Substituted porphyrins with only one meso substituent are easily prepared by an acid-catalyzed condensation of dipyrromethane, pyrrole-2-carbaldehyde. and an appropriate aldehyde using a "[2+1+1]" approach. Similarly, 5,10-disubstituted porphyrins are accessible by simple condensation of unsubstituted tripyrrane with pyrrole and various aldehydes using a "[3+1]" approach. The yields for these reactions are low to moderate and additional formation of either di- or mono-substituted porphyrins due to scrambling of the intermediates is observed. However, the reactions can be performed quite easily and the desired target compounds are easily removed due to large differences in solubility. A complementary and more selective synthesis involves the use of organolithium reagents for SNAr reactions. Reaction of in situ generated porphyrin (porphine) with 1.1-8 equivalents of RLi gave the monosubstituted porphyrins, while reaction with 3-6 equivalents of RLi gave the 5,10-disubstituted porphyrins in yields ranging from 43 to 90%. These hitherto almost inaccessible compounds complete the series of different homologues of A-, 5,15-A(2)-, 5,10-A(2)-, A(3)-, and A(4)-type porphyrin's and allow an investigation of the gradual influence of type, number, and regiochemical arrangement of substituents on the properties of meso-substituted porphyrins. They also present important starting materials for the synthesis of ABCD porphyrins and are potential synthons for supramolecular materials requiring specific substituent orientations
Self-diffusion measurements in microemulsion systems composed of a naturally occurring soybean lecithin mixture, an aqueous phase, either water or a 1% aqueous PDADMAC solution, and isooctane were accomplished by pulsed field gradient (PFG) (HNMR)-H-1 spectroscopy at oil dilution lines of low and intermediate water/lecithin ratios. The concentration-dependent diffusion data reveal water-in-oil (W/O) reverse micellar aggregates with dimensions on the nanometer scale being slightly smaller at low water content. With increasing micellar volume fractions, both hydrodynamic as well as direct interactions between particles significantly slow aggregate diffusion. The surfactant mean square displacements (msd's) in dilute and concentrated polymer-free systems studied as a function of diffusion time (20-1000 ms) are characterized by a crossover from Gaussian diffusion, due to slow aggregate motion, to anomalously enhanced diffusion, due to fast surface-bulk surfactant exchange at intermediate times revealing weak, barrier-controlled adsorption behavior. Upon addition of the polycation PDADMAC, the diffusion characteristics change to exclusively superdiffusive behavior with surfactant msd scaling with time as t(3/2) over the entire time range studied. This is caused by surfactant molecules performing Levy walks along the surface of reverse micelles mediated by the dilute bulk. The bulk-mediated surface diffusion is a consequence of the diffusion-controlled micelle-bulk exchange dynamics induced by interactions of PDADMAC with surfactant headgroups
The conformational equilibria of the cis/trans isomers of some 1,4-di-substituted cyclohexanes (X = OH, OMe, Me, OCOCH3, OCOC(CH3)(3), OCOCCl3, OCOCF3) were calculated at several levels of theory; the best correlation between calculated and experimentally available Delta G(0)s refers to the MP2/6-311 +G*//MP2/6-311G* results. In addition, the hyperconjugative effect of the substituents was studied with the NBO options included in the GAUSSIAN-98 package; a number of interactions between filled NBOs and antibonding orbitals could be considered as most representative for delocalization along the molecules studied. The effect of the substituents on the molecular geometry of the substituted cyclohexanesas well as the partitioning of both hyperconjugative and steric substituent effects on the present conformational equilibria is critically evaluated. Our model [E. Kleinpeter, F. Taddei, J. Mol. Struct. (THEOCHEM) 683 (2004) 29] for interpreting the relative stability of conformers of substituted cyclohexanes could be further verified and its reliability assessed. (c) 2005 Elsevier B.V. All rights reserved
Interpretation of conformational effects on 2-endo-norborneol by natural chemical shielding analysis
(2005)
This paper represents an extension of our work on the H-1 and C-13 NMR chemical shifts of norbornane and 2-endo- norborneol. NCS-NBO analysis was employed to probe contributions of bond orbitals and orbitals of lone pairs to nuclear shielding in conformers of the alcohol generated by rotation of the C-O bond. Variations in H-1 and C-13 chemical shifts with the dihedral angle are discussed in terms of Lewis and non-Lewis partitioning and their respective importance is evaluated. In addition to hyperconjugation of the lone pair in a p orbital of oxygen that was previously reported, a sizable participation of the lone pair which is in an sp orbital is also observed and their combined effect dominates the carbon chemical shifts of the C-1-C-2-OH and C-3-C-2-OH fragments. Both lone pairs on oxygen also contribute to localized, though-space effects on nuclei in the vicinity, these effects answering for the largest deviations in hydrogen chemical shifts on rotation around the C-O bond. On the other hand, for conformers in which nonbonded repulsions lead to distortions in the molecular framework, variations in chemical shifts may be attributed to angular effects
Conformational equilibria for a number of methyl substituted 1,3-dioxanes 1, 1,3-oxathianes 2 and 1,3-dithianes 3 were calculated at the HF and DFT levels of theory. In addition to the chair conformers also the energetically adjacent twist conformers were considered and the positions of the corresponding conformational equilibria estimated. On the basis of the global energy minima of conformers, participating in the conformational equilibria, the 1J(C,Hax,equ) coupling constants were calculated using the GIAO method and compared with the experimental values obtained from C-13, H- 1 coupled C-13 NMR spectra. The Perlin effect, the influence of the solvent and the suitability of this NMR parameter for assigning the conformational equilibria present are critically discussed. (c) 2005 Elsevier Ltd. All rights reserved
The quotient of the occupation numbers of pi bonding and pi* antibonding orbitals of the central C=C partial double bond, pi*(C=C)/pi(C=C), proved to be a useful parameter to quantify the push-pull effect completely for the first time in substituted alkenes by examination of a comprehensive set of compounds. (c) 2005 Elsevier Ltd. All rights reserved