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Syntheses with Cyclobutadienes, 18. - Dewar Benzenes from Tri-tert-butylcyclobutadienecarboxylates and Monoacceptor-Substituted Acetylenes - Thermal and Photochemical Secundary Reactions
Wingert, Horst,Irngartinger, Hermann,Kallfass, Dietmar,Regitz, Manfred
, p. 825 - 838 (2007/10/02)
The methyl cyclobutadienecarboxylate 1a adds methyl propiolate (2a) to the regioisomeric Dewar benzenes 3a (73percent) and 4a (19percent).In contrast, the corresponding tert-butyl carboxylate 1b reacts with the same dienophile as well as the acceptor-substitueted olefines 2b and c exclusively by formation of the Dewar benzenes 3b-d (68 - 82percent). - Both Dewar benzenes 3a and b are smoothly transformed into the benzenes 5a and b by thermal means.Photolysis of both aromatic compounds do not lead back to 3a and b but produce the Dewar isomers 7a and b, which finally undergo intramolecular cycloaddition to give the prismanes 9a and b.A fundamentally comparable reaction sequence can be achieved with 4a (--> 6a --> 8a --> 9a).Reaction branching is observed on thermolysis of 9b: on the one hand the benzene 5b (54percent) accompanied with 10percent of the Dewar benzene 7b, on the other hand a 1:1 mixture of the aromatic compounds 6b and c is obtained. - Photochemically the Dewar benzenes 3a and b can directly be converted into the prismanes 10a and b (97 and 86percent, respectively).Prismane/prismane ismerization (10 --> 16) succeeds as a consequence of a thermal (10a,b --> 12a,b) as well as of two light-induced reaction steps (12a,b --> 14a,b and 14a,b --> 16a,b).In an analogous way, the prismane 11 is received from 4a and then transformed into the isomer 16 via 13 and 15.As a special feature of the thermolysis of 11 the formation of 12a has to be noted, a reaction which is explained by the benzvalene intermediate 25. - The crystal structure analysis of 10b shows an evident influence of only one of both carbonyl groups (methyl ester) onto the prismane skeleton.This is manifested by two long, vicinal and a short distal three-membered ring bonds .Furthermore, the C1-C4 bond is shortened relative to the C2-C3 distance , whereas the C5-C6 bond is prolonged by the strong repulsion of both tBu groups.
