72443-16-4Relevant academic research and scientific papers
Retention of Configuration in Two Photochemical Reactions: Formation of Cyclopropanimines by Extrusion of Molecular Nitrogen from Tetraalkyl-4-imino-1-pyrazolines and Cycloreversion of Cyclopropanimines to Isocyanides and Alkenes
Quast, Helmut,Fuss, Andreas,Heublein, Alfred,Jakobi, Harald,Seiferling, Bernhard
, p. 2545 - 2554 (2007/10/02)
The 1-pyrazolin-4-ones 7 and 9 and the pyrazolidin-4-one 13 are condensed with alkanamines 8 to produce the imines 3, 10 and 12 in high yields.Direct irradiation of 3 with 350-nm light at 90 deg C in deuterated hydrocarbon solvents affords the cyclopropanimines 4 in almost quantitative yields besides molecular nitrogen and small amounts of the imines 12 as a result of photoreduction.In acetone, the cyclopropanimine 4b isomerizes in part of the α,β-unsaturated imine 14.Direct irradiation of 3 with the unfiltered light of the high-pressure mercury lamp results in quantitative cycloreversion of the primary photoproducts 4 into the alkene 6 and an isocyanide 5.At low temperature (10 deg C), photolysis of 3 occurs much more slowly giving rise to photoextrusion of nitrogen (-> 4) and photoreduction (-> 12) to about the same extent. - Photolysis of the stereochemically labelled iminopyrazolines cis- and trans-10 (d. e. 99percent) at 90 deg C produces the cyclopropanimines cis- and trans-16 (d. e. 94percent) with high stereospecifity.The configuration of cis- and trans-16 is established by a comparison with the corresponding methylenecyclopropanes cis- and trans-19 and the quantitative and completely stereospecfic cycloreversion into methyl isocyanide (5a) and the 3,4-dimethyl-3-hexenes (Z)- and (E)-17 on irradiation with the unfiltered light of the mercury arc.The necessity of thermal activation for efficient nitrogen extrusion from the 1(n, ?*) state of 3 and 10 is indicative of a considerable energy barrier towards the transition into a dissociative state.At low temperature, hydrogen abstraction from the solvent or other molecules becomes important for the deactivation of the 1(n, ?*) state, in addition to decay and fluorescence.The stereospecific formation of cis- and trans-16 is interpreted in terms of diastereomeric bis-orthogonal azatrimethylenemethane diradicals as intermediates which retain the configuration on cyclization.The minor non-stereospecific path may involve mono-orthogonal azatrimethylenemethane diradicals.Thus, mechanisms that involve the same types of diradical intermediates can rationalize the photolysis of the iminopyrazolines 3, 10 and of the methylenetriazoline 1 as well.The cycloreversion of cis- and trans-16 into the alkenes (Z)- and (E)-17 and methyl isocyanide (5a) demonstrates for the first time that such photoreactions can be entirely stereospecific. Key Words: 4H-pyrazoles, 3,3,5,5-tetraalkyl-4-imino-3,5-dihydro- / cyclopropanes, 2,2,3,3-tetraalkyl-1-(alkylimino)- / alkanamines, 2,2,3,3-tetraalkylcyclopropylidene- / photolysis / extrusion of molecular nitrogen / photoreduction / configuration, retention of / azatrimethylenemethanes / cycloreversion, stereospecific, photochemical
