This thesis describes the results obtained in four completely unrelated projects within the area of "Organic Reactive Intermediate" chemistry. Each project is discussed in a separate chapter.
Chapter one describes the matrix photolysis, the stepwise nature and final products of the pyrolysis, as well as the nature of the azidetetrazole isomerism, of seven trifluoromethylated derivatives of tetrazolo[l,5-a]pyridine (1.13-1.19). Overall, it showed the existence and importance of ring expanded cyclic carbodiimides (1,3-diazacyclohepta- 1,2,4,6-tetraenes) (1.37 (from 1.13 and 1.14), 1.43 (from 1.15 and 1.16), 1.49 (from 1.17 and 1.18), 1.54 (from 1.19)) as intermediates in the photochemical and thermal decomposition and subsequent rearrangements of the tetrazolo[l,5-a]pyridines. These cyclic carbodiimides were directly observed in argon matrices (ca. 12 K, generated photochemically) and as thin films (-180°C, generated thermally) but polymerised at ca. -65°C. The pyrolysis of the trifluoromethylated tetrazolo[l,5-a)/azidopyridines was a stepwise process proceeding from ring closed tetrazolo[l,5- ajpyridines to ring opened 2-azidopyridiries (ca. 200°C), with nitrogen loss and ring expansion to cyclic carbodiimides (ca. 350-400°C) to ring contracted cyanopyrroles (500+ °C) with successively higher temperatures required to make the reaction proceed further along the sequence. NMR spectroscopic analysis identified the trifluoromethylated cyanopyrroles and trifluoromethylpent-2-enedinitriles as the final pyrolysis products. The product distributions from the different precursors were consistent with the intermediacy of the cyclic carbodiimides in the pyrolysis reaction.
In the neat state, 2-azido-6-trifluoromethylpyridine (1.15) and 2- azido-4,6-bis(trifluoromethyl)pyridine (1.17) existed in the azide form as colourless oils, whereas the other five compounds (1.13, 1.14, 1.16, 1.18, 1.19) existed in the tetrazole form. In solution (CDCl3, DMSO-d6 and TFA-d), the greater the polarity of the solvent, the more the tetrazole isomer was favoured in the equilibrium. Variable temperature 1H NMR spectroscopy found that the ∆1H°i80m of the azide-tetrazole equilibrium was of the order ca. -3 to -4 kcal/mol. Thus, the tetrazole isomer was of lower energy than the azide isomer. Entropy effects were found to be significant in the equilibrium. In two cases (1.15 and 1.17, DMSO-d6) the entropy factor (-T∆S°i80m) governed the equilibrium. ..................................................