Protic thermal stability

The isomeric adamantane-spirothiadiazolines (145 and 146) (Scheme 8.34) exhibit different thermal stability [145 Xi/2 = 33 min at 45 °C 146 Xi/2 = 25.6 min at 110 °C (206a)]. Elimination of N2 from 145 generates thiocarbonyl ylide 147 that was trapped not only with the dipolarophiles mentioned above for 140, but also with aldehydes and imines (206a) (147 —> 149). Without a trapping reagent, thiirane 151 was formed from both 145 (at 80 °C) and 146. In the latter case, the extrusion probably proceeds via intermediate 148 and is accompanied by homo-adamantanethione 152 and a trace of methyleneadamantane. When 145 was decomposed at 45°C rather than at 80 °C, dimer 150 was also obtained. The isolation of 150 suggests that ylide 147 is also able to act as a base toward its precursor 145 (213). In fact, 147 can also be trapped with other protic nucleophiles. 

The high Lewis acidity, thermal stability and resistance to protic B-C bond cleavage conspire to make B(C6F5)3 an extremely effective activator of organometallic precatalysts for olefin polymerization and other reactions. As a result, other related boranes incorporating perfluoro biphenyl (Chart 1, III,93 IV94), naphthyl (V95) or fluorinated 9-borafluorene (VI96) frameworks have been prepared in an effort to increase the Lewis acidity of the boron center and studied in the context of olefin polymerization. 

The thermal and chemical stability of C—M bonds (M = Si, Ge, Sn, Pb), and therefore of all organic compounds of the silicon subgroup elements, decreases, both in homolytic and in heterolytic processes, when the atomic number of the M element increases. For example, the thermal stability of tetraalkyl derivatives R4M diminishes essentially when M is changed consecutively from Si to Pb151 l52. The ease of oxidation of R4M compounds and the ease of cleavage of C—M bonds by halogens, protic and aprotic acids, etc.,... 

With respect to ILs as cosolvents, the interest was centred on two types of ILs aUcylammonium and 1,3-dialkylimidazolium-based ionic liquids. For the first type, we selected ethylammonium nitrate, considering that it is a protic ionic liquid (PIL) and that it can act as a potential acid catalyst. With respect to the second type of ILs, we selected those based on l-methyl-3-n-butylimidazolium cation, characterised by a slighter HBD acidity than that of the alkylammonium type (Figs. 13.2, 13.3). These ILs exhibit a wide spectrum of physicochemical properties. It was demonstrated that their water content, density, viscosity, surface tension, melting point and thermal stability are affected by the length of the alkyl chain and the nature of the anion. Several anions were incorporated in this class of ILs. 

Artemisinin and its antimalarial derivatives belong to the chemical class of unusual 1,2,4-trioxanes. Artemisinin is poorly soluble in water and decomposes in other protic solvents, probably by opening of the lactone ring. It is soluble in most aprotic solvents and is unaffected by them at temperatures up to 150 °C and shows a remarkable thermal stability. This section will focus on biological and pharmaceutical aspects synthetic routes to improve antimalarial activity and to synthesize artemisinin derivatives with differ-... 

Intrinsically, an acid-base equilibrium exists in PILs that are prepared by proton transfer reactions from Bronsted acids to Bronsted bases. The thermal stabihty of PILs is dominated by the amount of neutral species (i.e., free acids and bases) because neutral species evaporate more easily than ionic species. Angell et al. [12] suggested that the difference between the pK values (ApJCJ of an add and a base is a good indicator of the equilibrium. Dai et al. [13] reported that protic ILs based on phosphazene or bicyclic guanidine superbases exhibit high thermal stability comparable to that of aprotic ILs. Ishiguro et al. [14] explored the... 

Some very recent stodies on protic ammonium species solely composed of C, O, N and H are very promising in preventing wear and giving ultra-low friction even in water [42,65,66], although the thermal stability of these species is not so high as that of the halogen-containing imidazolium salts. 

Moreover, sulfonated polyimides (SPIs) have been used to prepare polymer electrolyte manbranes for nonhumidified fuel cells by the addition of a protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate [demaJCCFjSOs] (See Fig. 9.6) [78, 79]. The uniform, tough, and transparent membranes have good thermal stability (>300 °C) and ionic conductivity (>10 S cm with an ionic liquid content higher than 67 %) under anhydrous conditions. A current density higher than 240 mA cm is achieved with a maximum power density of 100 mW cm at 80 °C when using a membrane in the H2/O2 fuel cell operation under dry conditions. 

Moreover, we could demonstrate that biodiesel can act as oil phase in high temperature microemulsions highlighting a way towards the formulation of biocompatible microemulsions (Zech et al., 2010 c). These model systems can be extended to other ILs, with [bmim][BF4] instead of EAN as polar phase, where a remarkable thermal stability can be achieved as well (Zech 2010). The different microemulsions containing protic ionic liquids, the methods used to characterize them and the corresponding references are summarized in Table 2. 

Due to some stability concerns with the N-Cbz group of 8 at high temperatures, compound 25 was used as a model substrate for the reaction. Substrate 25 was irradiated for 2 min (internal temperature reached 185 °C) in a variety of solvents and all thermal reactions reached >95% conversion (Table 6.1). Both aprotic polar solvents (entries 6 and 9) and protic polar solvent (entry 7) gave poor assay yields of product 26. With nonpolar solvents (entry 10) such as o-xylene and xylenes, the rearrangement reaction provided the highest assay yield and proved to be the best solvent choice [9e],... 

Due to its thermal and protic stability, B(C6F5)3 forms stable, isolable adducts with many Lewis bases [Eq. (4)], including weak Lewis bases often activated by Lewis acids for selective organic transformations. Because of the high LA strength of the borane,... 

---