Transformation thermally-induced

This transformation can also be carried out under solvent-free conditions in a domestic oven using acidic alumina and ammoniiun acetate, with or without a primary amine, to give 2,4,5-trisubstituted or 1,2,4,5-tetrasubstituted imidazoles, respectively (Scheme 15A) [69]. The automated microwave-assisted synthesis of a library of 2,4,5-triarylimidazoles from the corresponding keto-oxime has been carried out by irradiation at 200 ° C in acetic acid in the presence of ammonium acetate (Scheme 15B) [70]. Under these conditions, thermally induced in situ N - O reduction occurs upon microwave irradiation, to give a diverse set of trisubstituted imidazoles in moderate yield. Parallel synthesis of a 24-membered library of substituted 4(5)-sulfanyl-lff-imidazoles 40 has been achieved by adding an alkyl bromide and base to the reaction of a 2-oxo-thioacetamide, aldehyde and ammonium acetate (Scheme 15C) [71]. Under microwave-assisted conditions, library generation time was dramatically re-... 

Transformations through 1,2-addition to a formal PN double bond within the delocalized rc-electron system have been reported for the benzo-l,3,2-diazaphospholes 5 which are readily produced by thermally induced depolymerization of tetramers 6 [13] (Scheme 2). The monomers react further with mono- or difunctional acyl chlorides to give 2-chloro-l,3,2-diazaphospholenes with exocyclic amide functionalities at one nitrogen atom [34], Similar reactions of 6 with methyl triflate were found to proceed even at room temperature to give l-methyl-3-alkyl-benzo-l,3,2-diazaphospholenium triflates [35, 36], The reported butyl halide elimination from NHP precursor 13 to generate 1,3,2-diazaphosphole 14 upon heating to 250°C and the subsequent amine addition to furnish 15 (Scheme 5) illustrates another example of the reversibility of addition-elimination reactions [37],... 

The thermally induced rearrangements in the furoxan series have also been found. In particular, the transformation of 3-R-substituted 4-(3-ethoxycarbonylthioureido)-l,2,5-oxadiazole 2-oxides into derivatives of 5-amino-3-(a-nitroalkyl)-l,2,4-thiadiazole and into (5-amino-l,2,4-thiadiazol-3-yl)nitroformaldehyde arylhydrazones has been reported (Equation 8) <2003MC188>. 

In another study, thermodiffractometry was used to study phase transformations in mannitol and paracetamol, as well as the desolvation of lactose monohydrate and the dioxane solvatomorph of paracetamol [56]. The authors noted that in order to obtain the best data, the heating cycle must be sufficiently slow to permit the thermally induced reactions to reach completion. At the same time, the use of overly long cycle times can yield sample decomposition. In addition, the sample conditions are bound to differ relative to the conditions used for a differential scanning calorimetry analysis, so one should expect some differences in thermal profiles when comparing data from analogous studies. 

A case of a thermally induced phase change involving ring inversion was recently described by Kaftory (31). He found that a crystal of the exo isomer of the adduct, 7a, of ll-cyano-l,6-methano[10]annulene with 4-methyl-1,2,4-triazoline-3,5-dione is transformed to the endo isomer 7b on heating to 175°C. The process involves nucleation and growth of the product phase, but maintains... 

I. Miroshnyk, L. Khriachtchev, S. Mirza, J. Rantanen, J. Heinamaki and J. Yliruusi, Insight into thermally induced phase transformations of erythromycin A dihydrate, Cryst. Growth Des., 6, 369-374 (2006). 

A unique example is a thermally induced cyclotrimerization catalyzed by Si2Cl6.538 In this case, terminal alkynes are transformed to isomeric mixtures. 

Oxa-l -silabicyclo[ . 1,0 alkanes (n = 3 111 n = 4 113) were the only products isolated from the photochemical, thermal or transition-metal catalyzed decomposition of (alkenyloxysilyl)diazoacetates 110 and 112, respectively (equation 28)62. The results indicate that intramolecular cyclopropanation is possible via both a carbene and a carbenoid pathway. The efficiency of this transformation depends on the particular system and on the mode of decomposition, but the copper triflate catalyzed reaction is always more efficient than the photochemical route. For the thermally induced cyclopropanation 112 —> 113, a two-step noncarbene pathway at the high reaction temperature appears as an alternative, namely intramolecular cycloaddition of the diazo dipole to the olefinic bond followed by extrusion of N2 from the pyrazoline intermediate. A direct hint to this reaction mode is the formation of 3-methoxycarbonyl-4-methyl-l-oxa-2-sila-3-cyclopentenes instead of cyclopropanes 111 in the thermolysis of 110. 

Not only must precursor fibers be self-supporting as extruded, they must also remain intact (e.g. not melt or creep) during pyrolytic transformation to ceramic fibers. Thus, precursor fibers (especially melt spun fibers) must retain some chemical reactivity so that the fibers can be rendered infusible before or during pyrolysis. Infusibility is commonly obtained through reactions that provide extensive crosslinking. These include free radical, condensation, oxidatively or thermally induced molecular rearrangements. 

After a brief discussion of the notion of molecular topology and the analogy principle as related to topology/reactivity relationships more recent developments in the field of reactivity indices for polynuclear benzenoid hydrocarbons are reviewed. Reaction mechanisms and correlations of reactivity indices with rates of electrophilic substitution and Diels-Alder reactions, thermally induced polymerization, and biochemical transformations of benzenoid hydrocarbons are discussed. 

Thermally induced deactivation of catalysts is a particularly difficult problem in high-temperature catalytic reactions. Thermal deactivation may result from one or a combination of the following (i) loss of catalytic surface area due to crystallite growth of the catalytic phase, (ii) loss of support area due to support collapse, (iii) reactions/transformations of catalytic phases to noncatalytic phases, and/or (iv) loss of active material by vaporization or volatilization. The first two processes are typically referred to as "sintering." Sintering, solid-state reactions, and vaporization processes generally take place at high reaction temperatures (e.g. > 500°C), and their rates depend upon temperature, reaction atmosphere, and catalyst formulation. While one of these processes may dominate under specific conditions in specified catalyst systems, more often than not, they occur simultaneously and are coupled processes. 

In conclusion, the present study of the kinetics of the lamellar to hexagonal transformation of mesoporous zirconia shows that a loss of surfactant molecules accompanies the transformation. Transformation to the cubic form seems to require that all the starting material be in the hexagonal form. The thermally induced lamellar— hexagonal transformation is associated with an activation energy comparable to the hydrogen bond energy. 

The Br0nsted basicity of a surface is related to its deprotonation ability, which can be probed by investigating the dissociative adsorption of protic molecules (Bailly et al., 2005a Chizallet et al., 2006). The 0Lc2 0Lc H transformation thus induced can be followed by PL, which is one of the few techniques able to simultaneously characterize oxide ions and their protonated forms. The same kind of equilibrium is also involved when a hydroxylated surface is undergoing thermal pretreatment (Section 2.1). PL is thus an interesting tool to evaluate the surface basic properties of alkaline earth oxides. 

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