Substituted systems with temperature

The Friedlander annulation is one of the most straightforward approaches towards poly-substituted quinolines. Thus, a 22-membered library of quinolines was synthesized in a TsOH-catalyzed cyclocondensation-dehydration of 2-aminoaryl ketones and 2-aminoarylaldehydes with ketones in a household microwave oven (with power control) under solvent-free conditions [112]. It was observed that the Friedlander reaction occurred readily also in an oil-bath (at 100 °C). To compare the conventional and dielectric heating conditions precisely, a purpose-built monomode microwave system with temperature control was employed instead of the household oven. The experiments at 100 °C under otherwise identical conditions demonstrated that the dielectric heating protocol was only slightly faster. Products were isolated by a simple precipitation-neutralization sequence (in the case of solid products) or neutralization-extraction for oily or low melting point products (Scheme 43). 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

After the discovery of the Al6Mn i-QC [1], development of QCs were limited for almost a decade to ternary systems with a major A1 constituent, such as Al-(Pd,Mn)-Si, Al-Zn-(Li,Mg), Al-Cu-TM (TM = Fe, Ru, Os), Al-Pd-(Mn,Re) [2,25,26], (This may be the reason why jargon such as Al-based QCs was coined.) After all, most QC discoveries were achieved by chemical additions to, or substitutions in, known compounds. From the mid-1990s to about 2000, QCs were also found in Zn-Mg-R (R = rare-earth-metal), Cd-Mg-R, and (Yb,Ca)-Cd systems, the last being the first stable binary i-QC at room temperature. Experience and insight are worth a lot — Tsai and coworkers produced 90% of these i-QCs [27],... 

Impurity substitution that is effectively neutral, that is, neither donor nor acceptor, can also lead to significant changes in properties that are utilized in NTC thermistors. For example, the replacement of Ga3+ in the spinel MgGa204 by Mn3+ involves no apparent donor or acceptor action. The conductivity in the system MgGa2 Jt.MnJt.04 evolves from insulating (conductivity about 10-9 0 1m 1) for the parent phase with x = 0, to a conductivity approximately equal to that of germanium (10 10-1 m-1) in the compound MgGaMn04, in which x = 1. The resistivity decreases markedly with temperature and the compounds display typical NCT behavior. 

Azolium systems with C-H bonds can be deprotonated to give rise to an internal azolium ylide or an isoelectronic stable azole carbene system, whereas fully substituted heterocycles such as 142 can evolve in a more complex manner under basic conditions or with nucleophiles. Thus, in the presence of cyanide ion, the formation of adduct 143 could be evidenced by NMR spectroscopy. This intermediate, when stirred at ambient temperature for 48 h in the presence of dimethylacetylene azodicarboxylate (DMAD), led to the adduct 146 (28%) along with products 148 (40%) and 149 (12%) (Scheme 14) <1997JP12919>. 

In this section we are concerned with the properties of intrinsic Schottky and Frenkel disorder in pure ionic conducting crystals and with the same systems doped with aliovalent cations. As already remarked in Section I, the properties of uni-univalent crystals, e.g. sodium choride and silver bromide which contain Schottky and cationic Frenkel disorder respectively, doped with divalent cation impurities are of particular interest. At low concentrations the impurity is incorporated substitutionally together with an additional cation vacancy to preserve electrical neutrality. At sufficiently low temperatures the concentration of intrinsic defects in a doped crystal is negligible compared with the concentration of added defects. We shall first mention briefly the theoretical methods used for such systems and then review the use of the cluster formalism. 

Control of the pH and temperature of the precipitating solution is important to provide optimised conditions for stoichiometric, homogeneous, fluorhydroxyapatite formation. Similar conditions and set-up can be used for the synthesis of fluoride-substituted apatite crystals with varying size, crystallinity and morphology depending on the preparation temperature [124] a purge of the synthesis system with nitrogen gas ensures the preparation of carbonate-free fluorhydroxyapatite at ambient temperature [125]. 

This work has been extended from aryl and alkyl substituted systems (42) (R = aryl, alkyl) to analogues where R is an amino group, so giving access to synthetic equivalents of the nonstabilized amino nitrile ylides (45). Adducts were obtained in good-to-moderate yield with A-methyhnaleimide (NMMA), DMAD, electron-deficient alkenes and aromatic aldehydes (27,28), and with sulfonylimines and diethyl azodicarboxylate (29). Similarly the A-[(trimethylsilyl)methyl]-thiocarbamates (46) undergo selective S-methylation with methyl triflate and subsequent fluorodesilylation in a one-pot process at room temperature to generate the azomethine ylides 47. 

In many spinel systems, substitution for iron may result in a distribution of the solute on both A and B sites that varies strongly with temperature. In this case it is possible to vary the physical properties by quenching from different annealing temperatures. In each of the examples chosen to illustrate this effect, there is a point of additional interest to be introduced. 

No limits on the content of transition metal ion were found for Fe-substituted systems. As mentioned above, and in line with crystallographic literature,45 a monophasic sample with magnetoplumbite structure was obtained for the completely substituted BaFe O. This sample showed excellent CH4 combustion activity upon calcination at 700 °C, but it markedly deactivated either upon increasing the calcination temperature or upon treatment at high tempera-... 

Note that, with the minimized rate constant in hand, a generalized activation free energy can be defined as the difference between the free energy of the reactants and that for the point. s mm- Note also that for the computation of isotope effects, VTST proceeds exactly like conventional TST, except that there is no requirement at a given temperature that the value of. y that minimizes the rate constant for the light-atom-substituted system will be the same value of. y that minimizes the rate constant for the heavy-atom-substituted system. Each must be determined separately, at which point the ratio of rate constants for that temperature may be expressed. 

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