Mineral support

Catalyst type Coal minerals Coal minerals Coal minerals Supported aKZO-aO-60 aKZO-aO-60... 

Some apparently specific effects can, however, arise from the supports. Mineral supports are usually poor heat conductors, i. e. significant temperature gradients can develop inside the vessels under the action of conventional heating, whereas they behave as efficient absorbers of microwave energy with consequently more temperature homogeneity (Scheme 3.2). 

In chemical syntheses under the action of microwave irradiation the most successful applications are necessarily found to be the use of solvent-free systems [6], In these systems, microwaves interact directly with the reagents and can, therefore, drive chemical reactions more efficiently. The possible acceleration of such reactions might be optimum, because they are not moderated or impeded by solvents. Reactions on solid mineral supports and, in turn, the interaction of microwaves with the reagents on the solid phase boundary, which can substantially increase the rate of the reactions, are of particular interest [7]. 

The kinetics of the acid-catalyzed esterification reaction of 2,4,6-trimethylbenzoic acid in i-PrOH under microwave irradiation have been investigated [84], A simple and practical technique for MW-assisted synthesis of esters has been reported wherein the reactions are conducted either on solid mineral supports or by using a phase transfer catalyst (PTC) in the absence of organic solvents [85], The esterification of enols with acetic anhydride and iodine has also been recorded [86],... 

Heterocyclic chemistry has benefited substantially from MW-expedited processes developed over the last decade. An exhaustive overview is provided in adjoining chapters [87, 88] with limited solvent-free chemistry utilizing mineral supported reagents being covered in this section. 

Loupy and Soufiaoui described a comparative study of the reactivity of diphenylnitri-limine 200 with several dipolarophiles under microwave irradiation in the absence of solvent using a solid mineral support or phase-transfer catalysis (PTC) conditions (Scheme 9.62) [30b]. The results showed that the best yields of adducts were achieved upon impregnating KF-alumina with a mixture of the hydrazynoyl chloride 199 and the dipolarophile followed by irradiation of the mixture in a focused oven. Reaction of this mixture under solid-liquid PTC conditions with KF-Aliquat under microwaves afforded lower yields of cycloadducts, perhaps owing to the partial decomposition of Aliquat at the reaction temperature (140 °C). In all cases, worse yields were obtained by classical heating under comparable reaction conditions (time and temperature). 

The preparation of 1,3-azoles (benzoxazoles, benzimidazoles and benzothiazoles) from oximes using oxidants on mineral supports such as Ca(0Cl)2/Al203 or Mu02/Si02 or by fusion in dry media has been described. Eor instance, benzoxazoles 232 can be obtained by reaction of o-aminophenols (R, R = H, N02,C1) with substituted beuzaldehyde oximes 231 in the presence of Ca(0Cl)2/Al203 under microwave irradiation (equation 100). ... 

The natural tendency of polypeptide chains to grow homochirally may suggest an alternative mechanism for the breaking of symmetry, based on macromolecules instead of monomers. The argument is that it should be easier to separate enantiomeric homochiral chains, rather than racemic low-molecular-weight monomers, from each other. It has been shown for example that when the NCA-polycondensation is performed on mineral support, the oligomeric product remained absorbed on the surface. The lower oligomers are, however, easily... 

Ukrainczyk, L., Chibwe, M., Pinnavaia, T.J. Boyd, S. A. (1995). Reductive dechlorination of carbon tetrachloride in water catalyzed by mineral-supported biomimetic cobalt macrocycles. Environmental Science Technology, 29, 439-45. 

Pillai, U.R., Sahle-Demessie, E. and Varma, R.S., Environmentally friendlier organic transformations on mineral supports under non-traditional conditions, J. Mater. Chem., 2002, 12, 3199-3207. 

Microwave heating has proven to be of benefit particularly for reactions under dry media (e.g., solvent-free conditions) in open vessel systems (i.e., in the absence of a solvent, on solid support with or without catalysts) [4]. Reactions under dry conditions were originally developed in the late 1980 s [51], but solventless systems under microwave conditions offer several additional advantages. The absence of solvent reduces the risk of explosions when the reaction takes place in a closed vessel. Moreover, aprotic dipolar solvents with high boiling points are expensive and difficult to remove from the reaction mixtures. During microwave induction of reactions under dry conditions, the reactants adsorbed on the surface of alumina, silica gel, clay, and other mineral supports absorb microwaves whereas the support does not, and transmission of microwaves is not restricted. Moreover, microwaves can interact directly with reagents and, therefore, can more efficiently drive chemical reactions. The possible accelerations of such reactions are expected... 

Aluminas, silicas, clays, zeolites or graphite can be selected as acidic, basic or neutral supports, depending on the type of reactions. Alumina alone can act as a base towards a acid compounds but if a strong base is necessary, KF on alumina can ionize carbon acids up to pKa = 35. On the other hand, montmorillonites (clays) such as K10 or KSF offer acidities very comparable to strong mineral acids (e.g., nitric and sulfuric acids, respectively). Reactions are simply performed by mixing reagents and a mineral support in the absence of any organic solvent followed by microwave irradiation [52]. 

Reactions of reagents supported on mineral supports 7.2.1. Diphenylbutadiyne... 

Heterocyclic chemistry was facilitated by microwave-expedited solvent-free chemistry utilizing mineral supported reagents [65]. The scope now includes parallel synthesis [66]. A representative multi-component condensation to create a library of imidazo[l,2-a]pyridines, imidazo[l,2-a]pyrazines and imidazo[ 1,2-a]pyrimidines is depicted in Scheme 12 [67]. 

Almost all solid mineral supports are oxides. They behave like ion exchangers when their surface bears electric charges. Two categories may be distinguished (i) natural exchangers and (ii) amphoteric oxides. 

Only few reports deal with the use of mineral supports for immobilising chiral auxiliaries able to. activate dialkylzinc addition to aldehyde (Scheme 10). 

Mineral Support ff-aminoalcohol Enantiomeric excess Heterogeneous Homogeneous Ref. ... 

Some degree of success in supported enantioselective catalysis was accomplished by using functionalisation of mineral support. Due to their unique textural and surface properties, mesoporous micelle-templated silicas are able to bring new interesting properties for the preparation of optically active solids. Many successfully examples have been reported for enantioselective hydrogenation, epoxidation and alkylation. However, the stability of the immobilised catalysts still deserves efforts to allow industrial development of such attractive materials. 

Bougrin et al. (1995) reported the first practical utilization of microwave irradiation with nitrile imine as 1,3-dipole using solvent-free conditions. In this work, a comparative study of the reactivity of diphenylnitrilimine (DPNI) (1) with some dipola-rophiles was made in dry media using microwave irradiation. The good yields were obtained on mineral support in a monomode reactor. 

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