Clays cleaving

Cleavage of thioacetals. Fe(NO,)j or CulNO,) supported on the clay cleaves thioacetals to the carboniyl compound in essentially quantitative yield. They are particularly efficient for cleavage of dithianes and dithiolancs. 

Allyl groups are subject to oxidative deprotection with Chromiapillared Montmorillonite Clay, -BuOOH, CH2CI2, isooctane, 85% yield. Allylamines are cleaved in 84—90% yield, and allyl phenyl ethers are cleaved in 80% yield. 

Chromium-pillared clay, -BuOOH, CH2CI2, 10 h, 80% yield. Simple allyl ethers are cleaved to give ketones, and allylamines are also depro-tected (84-90% yield). 

A number of other reagents have been used to cleave C=N bonds, especially those not easily hydrolyzable with acidic or basic catalysts or which contain other functional groups that are attacked under these conditions. In particular, oximes have been converted to the corresponding aldehyde or ketone by treatment with, among other reagents, microwave irradiation on clay (Clayan), on wet sodium periodate... 

Hydrolysis reactions occur by nucleophilic attack at a carbon single bond, involving either the water molecule directly or the hydronium or hydroxyl ion. The most favorable conditions for hydrolysis, e.g. acidic or alkaline solutions, depend on the nature of the bond which is to be cleaved. Mineral surfaces that have Bronsted acidity have been shown to catalyze hydrolysis reactions. Examples of hydrolysis reactions which may be catalyzed by the surfaces of minerals in soils include peptide bond formation by amino acids which are adsorbed on clay mineral surfaces and the degradation of pesticides (see Chapter 22). 

Kaolinite is easily cleaved perpendicular to the c direction, since the interactions between the aluminosilicate layers are much weaker than the intralayer interactions. Therefore as a part of this work the structure and surface energy, of the resulting 001 surface are considered. The surface energy may be evaluated from the energy of a single layer surface block of clay in a vacuum, U, and the energy of a portion of the bulk clay, U, containing the same number of atoms as the surface block. 

Scheme 4.307],528 If water is an objectionable component or low solubility a problem, THP removal can be accomplished by an acetal exchange process using methanol or ethanol as solvent Scheme 4.308 illustrates the deprotection of an acid-sensitive allylic alcohol using camphor sulfonic add in methanol.448 Pyridi-nium p-toluenesulfonate in methanol or ethanol (pH 3.0) can also be used, but higher temperatures (45-55 C) are required [Scheme 4.309].558 559 Ion exchange resins in methanol cleave THP ethers at room temperature.5601561 Other THP deprotection conditions to beware or exploit are CAN in the presence of pH 8 buffer,562 DDQ in aqueous acetonitrile,563 and Montmorilonite clay in methanol.564... 

Cleavage of N,N-dimethylhydrazones. These hydrazones are cleaved to ketones by clay-supported Fe(NO,)i in 70-90% yield. 

Ether cleavage. 4-Methoxybenzyl ethers are cleaved with clay-supported ammonium nitrate on microwave irradiation (13 examples, 70-88%). No solvent is used. 

K-10 clay, toluene, 1,4-dimethoxybenzene or anisole, heat, 87-98% yield. Microwave heating was also effective. Cinnamyl esters were cleaved similarly. 

The cinnamyl ester, which is somewhat more stable to nucleophiles, can be prepared from an activated carboxylic acid derivative and cinnamyl alcohol or by transesterification with cinnamyl alcohol in the presence of the H-Beta zeolite (toluene, reflux, 8h, 59-96% yield) or DMAP (CH3CN, heat). It is cleaved under nearly neutral conditions [Hg(OAc)2, MeOH, 23°C, 2-4 h KSCN, HjO, 23°C, 12-16 h, 90% yield] by treatment with sulfated Sn02, toluene, anisole, reflux or with K-10 clay and microwave heating. The latter conditions will also cleave crotyl and prenyl esters. Pd catalysis may also be used to induce cleavage either with a nucleophile or reductively with TEA/HC02H. ... 

Montmorillonite KIO clay, CICH2CH2CI, reflux, 64-98% yield. This method selectively cleaves the BOC group from aromatic amines. ... 

Shales are fine grained, clay-like rock structures that cleave readily into thin layers. Many shales are coloured black on account of the bitumen that they contain. There are also gas shales in which considerable quantities of natural... 

Like the montmorillonites, talc cleaves easily along one plane it is soft and has a characteristic soapy feel. In the massive block form it can be readily machined, and fired to produce a strong body. Unlike the montmorillonites it is not plastic and so clay is sometimes added as a binder. 

Anisotropic crystals. Some important materials have crystal structures that when cleave can result in the producrion of both positively and negatively charged surfaces (49). This is the case of the aluminosilicate layers in clays. 

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