Dinitrogen thermal formation

The effect of (internal) coordination on the thermal stability of Cp TiR was not worked out in detail, but some qualitative results can be mentioned here. Coordination is more difficult than for Cp2TiR as was concluded from the insensitivity of Cp TiR spectra to solvent (pentane vs. THE) and also from the observation that dinitrogen complex formation, (CpWiR)2N2, does not take place in the temperature range (-120 - 20°C) studied. 

As an overall conclusion two routes seem to operate in the thermal degradation of 1,2,3,4-thiatriazoles a major route leading to nitrile (28) and dinitrogen sulfide (29) (Equation (1)), and another leading to isothiocyanate (27) most likely via thioacyl azide (24) from the Z-form (32) and presumably accompanied by formation of nitrile (28) from the -form (31). 

On the other hand, the reaction of 1,2,3-selenadiazole 166 with 1 equiv of Pt(PPh3)4 in toluene at 140 °C (3 h) led to the formation of new selenoplatinum complex 52 in 35% yield (Equation 13) <2005TL1001>. This reaction may involve the insertion of di(triphenylphosphino)platinum into the selenadiazole ring, followed by 1,3-dipolar addition of an intermediate formed in situ by thermal elimination of dinitrogen with the elimination of triphenylphosphine, similar to the reaction with [Pd2(dba)3] and trialkylphosphine described above. The structure of complex 52 was established by X-ray analysis. Complex 52 is a selective catalyst for the hydrosilylation of terminal alkynes. 

Scheme 1.2. Regioselective formation of chiral hexakis- and octakis-adducts of C6o by repeated addition of diazomethane to a C2V-symmetric pentakis-adduct, followed by thermal dinitrogen extrusion.

Azidobenziodoxoles The noncyclic azido X -iodanes, for example, PhI(N3)OAc or PhI(N3)2, in general lack stability and rapidly decompose at -25 to 0 °C with the formation of iodobenzene and dinitrogen (Section 2.1.12.1). The incorporation of hypervalent iodine atom into a five-membered heterocycle leads to a significant stabilization of the azidoiodane. Stable azidobenziodoxoles 122-124 can be prepared by the reaction of hydroxybenziodoxoles 121 with trimethylsilyl azide in acetonitrile [251, 292], or by treatment of acetoxybenziodoxoles 125 with trimethylsilyl azide in dichloromethane in the presence of catalytic trimethylsilyl triflate (Scheme 2.43) [259]. All three azides 122-124 were isolated as thermally stable, non-explosive, microcrystalline solids that can be stored indefinitely in a refrigerator. 

Not only azirines but also nitriles were obtained in thermal reactions of vinyl azides (see, for example. Schemes 5.15 and 5.17). Hassner and coworkers summarized some rules, ° which allow to expect the former or latter products after loss of dinitrogen from enazides. The 1-azidocyclopentadiene 202 tnmed ont to be highly unstable because fragmentation to furnish the nitrile 203 was observed already at room temperature (Scheme 5.24). On the other hand, the isomer 204 was isolated as a relative stable lemon-yellow solid, which was converted to the indole 205 by reflnxing in toluene or irradiation in chloroform. In some cases, the formation of cyano compounds from vinyl azides was... 

Compared to the cycloadditions of nitroalkenes, tandem cycloadditions of 1,3,4-oxadiazoles consist not of two, but of three elementary reactions. In the first step, a 1,3,4-oxadiazole 428 serves as an electron-poor heterodiene (4jt-component) and reacts thermally with an alkene dienophile (2jt-componait) by a [4-1-2] cycloaddition reaction. The preferred dienophile is electron-rich, unhindered, and strained. At the elevated temperatures required for the first step [169], the immediate product of this reaction, 430, is unstable and undergoes extrusion of dinitrogen by a [3 + 2] cycloreversion to form dipole 431 [170]. At the high temperatures required for its formation, 431 cannot be directly observed either and reacts by a [3 + 2] cycloaddition with an available dipolarophUe, which can be the same 27i-component that served as the dienophile or something else. Because none of the intermediates is isolated and no change in reaction conditions is required, tandem [4-f 2]/[3- -2] cycloadditions of 1,3,4-oxadiazoles are tandem cascade processes as defined earlier in this chapter. 

Dinitrogen-fused heterocycles have been formed in high yield by thermal 3-1-2-cycloadditions of two types of azomethine imines with allenoates. Rhodium-catalysed formal 3 -l- 2-cycloadditions of racemic butadiene monoxide with imines in the presence of a chiral sulfur-alkene hybrid ligand have furnished spirooxindole oxazolidines and 1,3-oxazolidines stereoselectively. ° Formation of 1,2-disubstimted benzimidazoles on reaction of o-phenylenediamine with aldehydes is promoted by fluorous alcohols that enable initial bisimine formation through electrophilic activation of the aldehyde. 

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