Compounds with intermediate thermal stability

The following simplified reaction scheme represents the formation of compound X via the reaction of HCHO with the thermally stabilized entity of the Criegee intermediate CH200 [123] ... 

The temperatures at which these various reaction steps occur are reported in table 9 they depend upon the exact experimental conditions. The 12C4 complexes are stable up to about 30(TC and then decompose according to reaction IV in a seemingly one-step exothermic reaction. The 15C5 coronates of the Ughter lanthanides (R = La-Eu) decompose at lower temperature. From Gd on, the thermolysis oceurs according to reactions I, II, and III, that is, intermediate 4 3 compounds form with a thermal stability interval ranging between 55 and... 

Esters of a-diazoalkylphosphonic acids (95) show considerable thermal stability but react with acids, dienophiles, and triphenylphosphine to give the expected products. With olefinic compounds in the presence of copper they give cyclopropane derivatives (96), but with no such compounds present vinylphosphonic esters are formed by 1,2-hydrogen shift, or, when this route is not available, products such as (97) or (98) are formed, resulting from insertion of a carbenoid intermediate into C—C or C—H bonds. The related phosphonyl (and phosphoryl) azides (99) add to electron-rich alkynes to give 1,2,3-triazoles, from which the phosphoryl group is readily removed by hydrolysis. 

The most common route to alkyl or aryl complexes of the type [AuRL] is by the treatment of a halide complex with an alkyl- or aryllithium reagent. The first reactions of this type were performed (15) in 1959 [Eq. (5)], and the methyl and phenyl compounds were found to have chemical and thermal stabilities intermediate between those of the previously known organopalladium and -platinum complexes. 

Catalytic reduction of aromatic nitro compounds to the amines is highly exothermic (AH = —548 12 kJ/mol) and has high potential for hazard in the event of cooling- or other process-failure. The total reaction proceeds via nitroso and hydroxylamino intermediates, both of which are reactive and may undergo undesired condensation or disproportionation reactions, and the thermochemistry of all these possibilities was investigated. The reduction or disproportionation of the hydroxylamino intermediate (which is of low thermal stability) is identified as the fastest and most exothermic step (despite which it can frequently be concentrated or trapped) implications for process safety are considered in detail and verified by experiment with typical compounds and intermediates [1]. A calorimetric study of the hazards inherent in hydrogenation of nitroaromatics was made, using nitrobenzene as model compound [2]. Individual incidents of this type are ... 

Generally the azolidine derivatives of structures 14-16 are less studied than other classes of (oxa/thia)-2-azoles, either because of their thermal instability or because of the difficulty in their preparation. Some of them are isolable compounds, others are nonisolable intermediates or reversibly dissociate to their starting precursors. Several examples dealing with thermal stability of certain 1,4,2-dioxazolidines, 3-imino-l,4,2-dioxazolidines, as well as 1,4,2-oxathizo-lidine derivatives, have been reported <1996CHEC-II(4)519>. Some new data are given below. 

Many carbene complexes, especially those bearing one or two hydrogen substituents, are thermally unstable with respect to the formation of alkenes or their complexes (Figure 5.13), usually via bimolecular intermediates. For this reason the thermal stability of carbene complexes can normally be enhanced by inclusion of sterically demanding co-ligands. Often within a triad the stability of carbene complexes increases in the order 4d < 3d < 5d when analogous compounds can be obtained and compared. 

The reaction of perfluoroalkylcopper compounds, prepared in situ from the perfluoroalkyl iodide and copper, with ip -hybridized organic halides [Eq. (73)] in refluxing pyridine utilizes the thermal stability of the intermediate copper compounds and their failure to react with sp -hybridized halides (29, 30). 

Intramolecular rDA processes have been used to produce a variety of cyclic compounds. Thermolysis of azoalkane (223) followed by tautomerization gives 4-(2 -styryl)pyrazole (224) as shown in equation (100). Apparently the C-1—C-6 and C-4—C-5 bonds are sufficiently weakened compared with the C-1—N-2 and N-3—C-4 bonds in strained (223) that the rDA process is preferred over expected molecular nitrogen extrusion. An investigation of the thermal stability of (225) led to production of 2-vinylindene (227) via an intramolecular cycloreversion to intermediate (226), which underwent a l,S-hydrogen shift to yield the more stable aromatized product (equation 101). ... 

Titanathietane complexes are obtained as brown crystals of high thermal stability when the vinylidene intermediate species reacts with isothiocyanates RNGS by a [2 + 2]-cycloaddition process (Scheme 529). Complexes with the sulfur atom in cr-position bonded to titanium are formed exclusively. The structure for the cyclohexyl derivative has been confirmed by X-ray diffraction. Heating of these titanathietane complexes in the presence of pyridine at 80 °C for 20 min results in isomerization, with the formation of a new titanacyclobutane compound (Scheme 529). The regioselectivity of these reactions was discussed on the basis of Hartree-Fock ab initio calculations.1365... 

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