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DOI: 10.1055/sos-SD-009-00155

Regitz, M.; Bergsträßer, U.Science of Synthesis, (20019137.

Irradiation of 3-azido-1,2,3-tri-tert-butylcyclopropene (2) in an argon matrix results in the formation of di-tert-butylacetylene and 2,2-dimethylpropanenitrile, despite the ideal conditions for the formation of a kinetically stabilized azete.[‌33‌] 2,3,4-Tri-tert-butylazete (3), isolated from other reactions, is photochemically stable and therefore can be discounted as an intermediate in this process. The formation of these products can be explained in terms of a cheletropic fragmentation of the triplet nitrene generated as an intermediate by the cleavage of nitrogen.[‌15‌] Utilizing experience from the chemistry of cyclobutadiene,[‌34‌] 2,3,4-tri-tert-butylazete (3) is successfully prepared by thermolysis of neat 3-azido-1,2,3-tri-tert-butylcyclopropene (2) at 125°C giving 3 in 65% yield together with the 1,2,3-triazine 4 (5%) (Scheme 2).[‌15‌] In this process the formation of a triplet nitrene is avoided if it is assumed that nitrogen cleavage and ring expansion by a 1,2-shift proceed as concerted steps. However, the scope of this method is limited: 3-Azido-1,2-di-tert-butyl-3-mesitylcyclopropene (5) gives 2,3-di-tert-butyl-4-mesitylazete (6) in almost quantitative yield, while thermal decomposition of 3-(1-adamantyl)-3-azido-1,2-di-tert-butylcyclopropene (7) results in a mixture of the regioisomeric azetes 8 and 9 in a ratio of 3:1 and 90% yield. Flash chromatography of the mixture of 8 and 9 on silica gel at 70°C results in the isolation of 8 in 20% yield.[‌18‌]

Scheme 2 Synthesis of Azetes by Ring Enlargement of Azidocyclopropenes[‌15‌,‌18‌]

Experimental Procedure

2,3,4-Tri-tert-butylazete (3):[‌15‌]

3-Azido-1,2,3-tri-tert-butylcyclopropene (2; 1.25g, 5mmol) was heated under argon for 90min in a pressure Schlenk tube at 125°C (oil-bath temperature). The thermolysis product turned dark-brown; bulb-to-bulb distillation (60°C/0.05Torr) afforded a dark-brown oil, which immediately solidified. Recrystallization (pentane) gave reddish-brown needles of 3; yield: 0.72g (65%); mp 37°C.