Spontaneous heterogeneous reactions, oxidic

To describe hypergolic heating, Anderson and Brown (A10) proposed a theoretical model based upon spontaneous exothermic heterogeneous reactions between the reactive oxidizer and a condensed phase at the gas-solid interface. In these studies, the least complex case was considered, i.e., the one in which the solid phase is instantaneously exposed to a stagnant (nonflowing) gaseous oxidizer environment. This situation can be achieved experimentally provided the sample to be tested is suddenly injected into the desired environment in a manner designed to minimize gas flow. 

Our own group is also involved in the development of domino multicomponent reactions for the synthesis of heterocycles of both pharmacologic and synthetic interest [156]. In particular, we recently reported a totally regioselective and metal-free Michael addition-initiated three-component substrate directed route to polysubstituted pyridines from 1,3-dicarbonyls. Thus, the direct condensation of 1,3-diketones, (3-ketoesters, or p-ketoamides with a,p-unsaturated aldehydes or ketones with a synthetic equivalent of ammonia, under heterogeneous catalysis by 4 A molecular sieves, provided the desired heterocycles after in situ oxidation (Scheme 56) [157]. A mechanistic study demonstrated that the first step of the sequence was a molecular sieves-promoted Michael addition between the 1,3-dicarbonyl and the cx,p-unsaturated carbonyl compound. The corresponding 1,5-dicarbonyl adduct then reacts with the ammonia source leading to a DHP derivative, which is spontaneously converted to the aromatized product. 

Metal carbonates are often used as reagents in solid-state reactions because heating them creates finely divided metal oxides as a result of the release of C02(g) from the crystal lattice. These finely divided particles, which are made freshly in situ, are often more reactive in heterogeneous solid-state reactions than are the metal oxides found on the laboratory shelf because they have a higher surface area that is more reactive because of its fresh preparation. Determine the minimum temperature necessary to achieve spontaneous thermal decomposition of BaC03. 

Similarly, radiolytically produced radical cations can be stabilized in zeohtes and related materials. This possibility was exploited by spectroscopists to study the EPR of radical cations and some neutral radicals even before the development of inert matrices such as rare gases and freons for radical cation stabilization. Recently, work in our laboratory has developed the use of inert zeolites as microreactors to control radical cation reactions and to study radiation chemistry in heterogeneous systems. In the case of active catalysts, radiolysis can potentially produce radical cations of products as weU as starting material. Thus, like the spontaneous oxidation process described above, radiolysis combined with EPR permits a method of post-reaction analysis of products by in situ spectroscopy. 

---