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Cheletropic Decomposition of Cyclic Nitrosoamines Revisited:  The Nature of the Transition States and a Critical Role of the Ring Strain

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journal contribution
posted on 16.05.2000, 00:00 by Gennady V. Shustov, Arvi Rauk
The cheletropic decompositions of 1-nitrosoaziridine (1), 1-nitroso-Δ3-pyrroline (2), 7-nitroso-7-azabicyclo[2.2.1]hepta-2,5-diene (3), and 6-nitroso-6-azabicyclo[2.1.1]hexa-4-ene (4) have been studied theoretically using high level ab initio computations. Activation parameters of the decomposition of nitrosoaziridine 1 were obtained experimentally in heptane (ΔH298 = 18.6 kcal mol-1, ΔS298 = −7.6 cal mol-1 K-1) and methanol (20.3 kcal mol-1, 0.3 cal mol-1 K-1). Among employed theoretical methods (B3LYP, MP2, CCD, CCSD(T)//CCD), the B3LYP method in conjunction with 6-31+G*, 6-311+G**, and 6-311++G(3df,2pd) basis sets gives the best agreement with experimental data. It was found that typical N-nitrosoheterocycles 24 which have high N−N bond rotation barriers (>16 kcal mol-1) extrude nitrous oxide via a highly asynchronous transition state with a planar ring nitrogen atom. Nitrosoaziridine 1, with a low rotation barrier (<9 kcal mol-1) represents a special case. This compound can eliminate N2O via a low energy linear synperiplanar transition state (ΔH298 = 20.6 kcal mol-1, ΔS298 = 2.5 cal mol-1 K-1). Two higher energy transition states are also available. The B3LYP activation barriers of the cheletropic fragmentation of nitrosoheterocycles 24 decrease in the series: 2 (58 kcal mol-1) ≫ 3 (18 kcal mol-1) > 4 (12) kcal mol-1. The relative strain energies increase in the same order: 2 (0 kcal mol-1) ≪ 3 (39 kcal mol-1) < 4 (52 kcal mol-1). Comparison of the relative energies of 24 and their transition states on a common scale where the energy of nitrosopyrroline 2 is assumed as reference indicates that the thermal stability of the cyclic nitrosoamines toward cheletropic decomposition is almost entirely determined by the ring strain.