Laponite-supported

Another problem that required solving was the moderate yield obtained in the cyclopropanation reaction when only one equivalent of styrene was used. By increasing the amount of styrene up to its use as the reaction solvent, a noticeable effect on the selectivities was observed when laponite was used as the support [58]. The active role of the clay support was definitely estabhshed when the results in homogeneous and heterogeneous phases were compared (Table 9). These effects involved the reversal of the trans preference in solution to the cis preference with the laponite-supported catalyst in styrene, and also a reversal in the absolute configuration of the major cis enantiomer ob-... 

Fig. 19 Model for surface effects in laponite-supported bis(oxazoline)-copper complexes...

Fraile, Mayoral, and coworkers utilized copper BOX catalysts (115), both homogeneous and immobilized on a laponite support, for their C-H insertion reactions [96], They obtained similar yields and diastereoselectivities to the Rh2(5 -DOSP)4 catalyst (up to about 3 1), but the highest enantioselectivity was 88% ee. Woo [10] and Che [83] chose to use achiral iron porphyrin catalysts. Woo obtained the products in 62-82% yield in about 3.5 1 dr, and Che in 88% yield, but only 2 1 dr. By way of comparison, Perez obtained excellent yields for this insertion reaction with EDA (95-99%) utilizing copper homoscorpionate catalysts 67a and 67b, [36, 39] but the transformations were not asymmetric. 

The first examples of cationic exchange of bis(oxazoline)-metal complexes used clays as supports [49,50]. Cu(II) complexes of ligands ent-6a, 6b, and 6c (Fig. 15) were supported on three different clays laponite (a synthetic clay), bentonite, and montmorillonite KIO. The influence of the copper salt from which the initial complexes were prepared, as well as that of the solvent used in the cationic exchange, was analyzed. 

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

It seems reasonable to believe that this problem could be overcome by studying more coordinating ligands with the same structural features. Very recently, it has been demonstrated [56] that the use of iminobis(oxazolines) (Fig. 18) leads to better enantioselectivities and recoverable catalysts, both with laponite and nalion-silica supports (Table 8). Theoretical calculations are consistent with the stronger coordinating ability of iminobis(oxazolines) being the origin of these results [57]. 

The most significant class of inorganic supports, which is used for the direct ion exchange of positively charged transition-metal complexes, are smectite clays. Pin-navaia has introduced the use of these swelling, layered silicate clays for catalysis. Other clays include montmorillonite, bentonite, and laponite. As shown by Pinna-vaia, cationic transition-metal complexes can be readily exchanged (intercalated) into the solvated interlayers of these silicates (Eq. (1)) [117] ... 

Bis(oxazoline)-copper complexes supported on clays were investigated as heterogeneous catalysts in the cyclopropanation reaction (37, 38). Optimal results were obtained from chloride-derived complexes in nitroethane as reaction medium. Laponite clay was found to provide higher selectivities than montmorillonite or bentonite. In every case, the heterogeneous reaction afforded increased amounts of the cis cyclopropane relative to the homogeneous reaction. 

Balazsik, K., Torok, B., Kiricsi, 1., Dekany, I., Bartok, M. (1999) Characterization of Cinchonidine Doped Montmorillonite Supported Noble Metal Catalysts by Thermoanalytical Methods, J. Therm. Anal. 56, 337-343. Torok, B., Balazsik, K., Dekany, 1., Bartok, M. (2000) Preparation and Characterization of New Chirally Modified Laponites, Mol. Cryst. Liq. Cryst. 341, 339-344. 

As an extension of the previous study, the CATl complex was also anchored through metal axial coordination onto two APTES modified supports, the mesoporous siliea MCM-41 and the Laponite clay, in order to evaluate the effect of the type of support (nature, chemical stability and textural... 

He, F.-A. and Zhang, L.-M. 2006. Using inorganic POSS-modified laponite clay to support a nickel a-diimine catalyst for in situ formation of high performance polyethylene composites. 

The time-resolved luminescence of [Ru(bpy)3] has been deconvoluted into two components, and these have two different emission maxima (83). This observation suggests the existence of two distinct adsorption sites for [Ru(bpy)3] on laponite. At one type of the adsorption zone the water is held rigidly and presents an environment for the [Ru(bpy)3] probe that results in a short lifetime and a blue-shifted emission. The second absorption zone involves a stronger interaction directly with the clay surface such that the photophysics are less influenced by the nature of the surrounding water. This explanation was supported by a direct comparison of photophysical properties of the luminescence probe on the clay and in ice at - 20°C. 

---