Transformations dehydroxylation

Novel aromatic carboxylation reactions have been observed in the anaerobic transformation of phenols to ben2oates (82). A mixed anaerobic microbial consortium apparentiy transforms phenol (33) through an intermediate to ben2oic acid (34) via dehydroxylation. This reaction has not yet been widely exploited for its obvious synthetic value. 

A New Improved Synthesis of Tricycle Thienobenzazepines Apphcation of chemistry recently developed by Knochel" combined with the well-described halogen dance (HD) reaction, allowed preparation of our key intermediate A in only three synthetic transformations (Scheme 6.4). In this respect, treatment of 2-bromo-5-methylthiophene with hthium diisopropylamide followed by dimethylformamide afforded aldehyde 11 in good yield, lodo-magnesium exchange with conunercial 4-iodo-3-nitro anisole followed by reaction with 11 afforded the thiophene catbinol 12. Dehydroxylation of 12 provided our key intermediate A which presented the requisite functionality to examine our approach to the construction of the seven-member ring system. 

Figure 1.9 TG, DTG, and DTA profiles for an amorphous catalyst precursor obtained by coprecipitation of Fe(N03)3 and Mg(N03)2 in solution [65], This precursor is heated at high temperatures to produce a MgFe204 spinel, used for the selective oxidation of styrene. The thermal analysis reported here points to four stages in this transformation, namely, the losses of adsorbed and crystal water at 110 and 220°C, respectively, the decomposition and dehydroxylation of the precursor into a mixed oxide at 390°C, and the formation of the MgFe204 spinel at 640°C. Information such as this is central in the design of preparation procedures for catalysts. (Reproduced with permission from Elsevier.)...

The 7a-dehydroxylation is the most important bacterial transformation of bile acids, rapidly forming secondary from primary bile acids and is seemingly... 

The carbonyl cluster Rh,5(CO)i,5 was initially stable as such on the completely dehydroxylated alumina surface. But as soon as hydroxyl groups were generated (e.g., by adding traces of water) it decomposed to give various surface transformations. First, the cluster structure was dismpted, with breakage of the core cluster frame, into (Al-0-)(Al-0H)Rh (C0)2, Rh > monoatomic species sigma and n-bonded to the oxygens atoms of the alumina surface, with formation of molecular... 

The oxide surface has structural and functional groups (sites) which interact with gaseous and soluble species and also with the surfaces of other oxides and bacterial cells. The number of available sites per unit mass of oxide depends upon the nature of the oxide and its specific surface area. The specific surface area influences the reactivity of the oxide particularly its dissolution and dehydroxylation behaviour, interaction with sorbents, phase transformations and also, thermodynamic stability. In addition, specific surface area and also porosity are crucial factors for determining the activity of iron oxide catalysts. 

With lepidocrocite the dehydroxylation endotherm due to transformation to maghemite is followed by an exotherm indicating transformation of maghemite to hematite. The temperature of the dehydroxylation endotherm was found to increase from 270 to 300 °C as A1 substitution rose from Al/(Fe-tAl) of 0 to 0.12 (Schwertmann Wolska, 1990) and that of the exotherm rose from 500 to 650 °C (Wolska et al., 1992). Synthetic feroxyhyte shows a weak dehydroxylation endotherm at ca. 260 °C (Carlson Schwertmann, 1980). 

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

Thermal dehydroxylation of FeOOH has been studied both in vacuum and under various atmospheres. Kinetic studies of these transformations must be carried out under vacuum (Giovanoli Briitsch, 1974) and at a constant temperature. The temperature at which a phase transformation occurs, however, is determined by increasing the temperature of the sample in a controlled manner, i.e. by using a thermobalance (DTA or TGA method, see Ghap. 7). Mechanical and mechanochemical dehydroxylation of FeOOH at room temperature can also be achieved by grinding. 

The kinetic data for the dehydroxylation process, i. e. the degree of transformation, a, as a function of time could be fitted to both a random nudeation model, i. e. 

The transformation of ferrihydrite to hematite by dry heating involves a combination of dehydration/dehydroxylation and rearrangement processes leading to a gradual structural ordering within the ferrihydrite particles in the direction of the hematite structure. This transformation may or may not be facilitated by the postulated structural relationship between the two phases. EXAFS studies have shown, for example, that some face sharing between FeOg octahedra, characteristic of hematite, also exists in 6-line ferrihydrite (see chap. 2). 

The Sharpless asymmetric dehydroxylation of resin-bound olefins was monitored using 3H, 13C and HMQC HRMAS NMR.63 The authors found 13C HRMAS NMR to be particularly suited to evaluating the progress of this reaction and permitted the enantiomeric excesses of the products to be determined before they were cleaved from the support. Most importantly, they were able to evaluate the types of substrates amenable to this reaction on solid supports, showing the ability of HRMAS NMR to contribute to synthetic questions. Transformation of the unnatural amino acid Lys(NH2) on a poly (ethylene glycol)-dimethylacrylamide (PEGA) resin to 6-hydroxynorleucine was confirmed by application of TOCSY HRMAS experiments.64... 

Reduction of Carboxylic Acids. Esters, and Anhydrides to Aldehydes1215 Hydro-de-hydroxylation or Dehydroxylation (overall transformation)... 

The principal transformations of caffeic acid mediated by microorganisms are reduction of the unsaturated aliphatic side chain, dehydroxylation, and decarboxylation. 

The transformation of kaolinite to metakaolinite is brought about by heating the clay to about 700 °C causing hydroxyl ions to be removed as water. The rate of dehydroxylation as a function of heating has been studied [3.164]. The reaction can either be a batch process with the clay in crucibles in a directly fired kiln, or a continuous process in a tunnel kiln, rotary kiln, or other furnace. 

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