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Aggregation of some low-molecular weight compounds in organic liquids is known to result in the formation of rod-shaped colloidal particles. Beyond the overlap concentration of the aggregates, the systems usually exhibit remarkable viscoelastic features which account for their description as gel-like materials. In particular, their typical consistency results from the development of a three-dimensional network made of interconnected colloidal aggregates. The sensitivity of the aggregation process to parameters such as temperature, concentration, and impurities, the fragility of the resulting network, and its thermal reversibility characterize such systems whose structural study raises specific problems. By contrast to the commonly used electron microscopy technique, the small-angle scattering techniques enable a nondestructive investigation of these materials swollen by their solvents. Furthermore, comparative X-ray and neutron studies can provide complementary information concerning the internal structure of the aggregates, such as their cross-sectional heterogeneity or contrast profile.

2014 Des gels formés à partir de solutions diluées dans le cyclohexane de stéroïdes modifiés, sont étudiés par diffusion de neutrons aux petits angles. La présence d'objets très longs associés avec le réseau du gel est mise clairement en évidence. Des experiences de variation de contraste montrent que ces objets sont essentiellement composés par des molécules stéroïdes. Des mesures d'intensités absolues et l'ajustement des courbes de diffusion à des modèles simples suggèrent que ces objets sont chiraux, probablement des doubles hélices symétriques de diamètre d'environ 100 Å. On suggère que les « points de contacts » entre objets sont formés en torsadant deux ou plusieurs de ces longs objets sur des distances relativement grandes par rapport au diamètre. L'influence de ce recouvrement des objets sur la forme des courbes de diffusion est discutée.

Luminescent organogels formed by the small molecules cholesteryl 4-(2anthryloxy) butanoate (CAB) and 2,3-bis-n-decyloxyanthracene (DDOA) in decane and alcohol solvents have been studied by scattering techniques. The results are compared with previous studies of hydrocarbon-based gels of an androstanol derivative (STNH) analogue. Rheology experiments confirm that the materials, obtained through a sharp transition from a solution to a soft solid, have all of the characteristics of gels. For instance, the thermoreversible DDOA-octan-1-01 gel exhibits a yield stress (a* x 550 Pa at C = 1 wt.%), and its elasticity (G' x lo4 Pa) suggests the existence of numerous structural results have been confirmed by electron microscopy measurements. Structural correlations derived from studies of this family of gelators may bear upon the gelation process.

The design of chiral porphyrins as asymmetric catalysts 1 and selective receptors of optically active natural products 2a-g or synthetic ligands 2h is of increasing interest in the development of enantioselective synthesis, and of molecular and chiral recognition. 3 Our approach to C 2 -symmetric metalloporphyrin catalysts, based on Pfaltz's analysis, 4 is to attach bulky substituents on chiral centers near the plane of the tetrapyrrolic ring, as close as possible to the metal atom to maximize steric interaction with incoming substrate. We have shown recently 5 that a chiroporphyrin catalyst possessing these topological features can be easily constructed from a chiral cyclopropanecarbaldehyde derivative. We report here that the chiroporphyrin unit also possesses multipoint binding properties with potentially important applications in chiral recognition of aliphatic alcohols. We find that tetramethylchiroporphyrin H 2 -TMCP, 1, is obtained from (1R)-cis-caronaldehydic acid methyl ester 6,7 and pyrrole as the desired D 2 -symmetric R R atropisomer exclusively. 8 Its (carbonyl)ruthenium(II) complex was obtained by gentle metal insertion with dodecacarbonyltriruthenium in refluxing toluene. Recrystallization from dichloromethane-ethanol gave X-ray quality crystals of the sixcoordinate complex Ru(CO)(EtOH)TMCP, 9a 2. The 1 H and 13 C NMR spectra of 2 are consistent with the expected C 2 symmetry axis along the Ru-C-O fragment. 9b,cThe stereochemistry of Ru(CO)(EtOH)TMCP, 2, is confirmed by its X-ray structure 10 (Figure 1). The cyclopropyl substituents of the ruffled porphyrin are oriented alternatively toward either face of the macrocycle. The cis configuration of the ester and porphyrin groups on each cyclopropane constrains the methyl (1) (a) Collman, J. P.; Zhang, X.; Lee, V. J.; Uffelman, E. S.; Brauman, Kato, Y.; Higashioji, T.; Hasegawa, J.; Kawanami, S.; Takahashi, M.; Shiraishi, N.; Tanabe, K.; Ogoshi, H. Bakshi, D.; Mahidroo, V. K.; Soman, R.; Dev, S. Tetrahedron 1989, 45, 767. (7) This compound was obtained in 71% overall yield and >99% diastereomeric purity from its precursor (1R)-cis-hemicaronaldehyde ("biocartol") by an improved procedure described in the Supporting Information. (8) Yield: 4% of 1. (9) (a) Analytical data for 2 are supplied in the Supporting Information. (b) Le Maux, P.; Bahri, H.; Simonneaux, G. Tetrahedron 1993, 49, 1401. (c) Le Maux, P.; Bahri, H.; Simonneaux, G.; Toupet, L. Inorg. Chem. 1995, 34, 4691. (10) C 51H58N4O10Ru, tetragonal, space group P43212, a ) 13.324(4) Å, c) 26.165(4) Å, Z ) 4, T ) 127 K. Initial coordinates of the porphyrin ligand were taken from the isomorphous structure of the corresponding chloromanganese(III) complex, the structure of which was solved by Patterson methods. Atoms of the axial ligands and ruthenium were found in a difference Fourier map. All non-hydrogen atoms were refined anisotropically on F 2 with SHELX 11 using the both Friedel reflection pairs (FAST area detector data). Hydrogen atoms were included as fixed, idealized contributors. The absolu...

COMMUNICATIONS ence experiments, which indicate the proximity of the marked CH, groups ( Fig. 1) to the pyridine protons, and by the small chemical shift of the pyridine protons which reflect the anisotropy of the pyridine ring.

High levels of stereoinduction are obtained in the epoxidation of 1,2-dihydronaphthalene catalyzed by sterically crowded manganese chiroporphyrin catalysts, and asymmetric induction is correlated to the degree of nonplanar distortion of the porphyrin free base in solution.