- Syntheses with Cyclobutadienes, 19. - Ring Opening of Bi- and Tricyclic Adducts of Carbonyl Compounds onto a Kinetically Stabilized Cyclobutadiene
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Benzophenone (6) adds photochemically onto the cyclobutadiene 5b to give the oxabicyclohexene 7 which is in equilibrium with the oxatricyclohexane 8 in acidic medium.The reaction of fluorenone (11) with 5b is comparable (formation of 12 and 13) but the bicyclic compound undergoes spontaneous ring opening reaction to the 1,3-diene 14.The latter one isomerizes photochemically by electrocyclic ring closure to the cyclobutene 15; this process is reversible thermally.The cyclopropenone 16 only reacts slowly at room temperature with 5b; instead of the primary adduct 17 the isomeric trifulvene 18 is isolated.Under the influence of trifluoroacetic acid the oxabicyclohexenes (7->19,21a,c,d->22a,c,d) as well as the oxatricyclohexanes (8->19, 13->14) undergoing ring opening reactions to the corresponding 1,3-dienes.Crystal structure analyses have been performed for 18 and 22a.
- Fink, Juergen,Guembel, Helmut,Eisenbarth, Philipp,Regitz, Manfred
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p. 1027 - 1038
(2007/10/02)
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- Carbenes, 28. tert-Butyl 2,3,4-Tri-tert-butylcyclobutadiene-1-carboxylate
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The cyclopropenyldiazoacetate 3a - available from tri-tert-butylcyclopropenylium bromide (1) and the mercury bis(diazoacetate) 2a - is the starting material for the synthesis of the title compound 5a.The carbene 4a, generated photolytically from 3a, is responsible for the formation of both the cyclobutadiene 5a and the acetylenes 6 and 7a.Acetylene formation is also observed in the flash pyrolysis of 3a (formation of 6, 7a, 8, and 9) and 5a (formation of 6, 8, and 9).The extremely air sensitive cyclobutadiene 5a ( -> 16) shows a pronounced cycloaddition behaviour towards dienophiles.With dimethyl ac etylenedicarboxylate, maleic anhydride, cyclopentadiene, diethyl azodicarboxylate and 4-phenyl-1,2,4-triazoline-3,5-dione cycloadducts were isolated (17, 18, 19, 20, 21).The series of the diazadihydro Dewar benzenes is also entered by the reaction of the cyclopropenyldiazoacetates 3 with the triazolinedione; the betaines 22 and 24 are regarded to be intermediates of this reaction.
- Eisenbarth, Philipp,Regitz, Manfred
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p. 3796 - 3810
(2007/10/02)
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