89703-52-6Relevant academic research and scientific papers
Cope rearrangements versus retro diels-alder reactions
Beck, Karin,Hoffman, Petra,Huenig, Siegfried
, p. 1588 - 1899 (2007/10/03)
The two isomeric [4+2] cyclo-adducts from two different 1,3-dienes may result from direct cycloadditions as well as from Cope rearrangements (Scheme 1). This general question is tackled by employing two energetically different types of dienes, protonated pyrazolines (1H+, 2H+) or dihydropyridazines (3H+), prepared in situ from their trimers and alicyclic (4-6) or aliphatic (7-9) 1,3-dienes. Depending on structural features and conditions (amount of acid, reaction time), various ratios of the two isomeric [4+2] cycloadducts A and B are obtained; A and B are azo compounds 10, 14,16, 20, 22, 24, 27, 32, 34, 36-39, 41, 42, pyrazolines endo-11, endo-13, endo-15, endo-endo-17, endo-18, endo-19, 21, 23, 25, 26, 28, and hydropyridazines 31, endo-33, endo-35, 40 and 43 (Schemes 3, 4). These results were backed by others from acid-catalyzed isomerizations, trapping experiments, and calculations of the equilibria (ΔΔH) between the isomers (by analogy with the corresponding olefins). A critical discussion reveals: a) Azo compounds 20, 22, 24, 27, 34, 38, and 42 must result from a [4+ + 2] cycloaddition with inverse electron demand, whereas hydropyridazines endo-33, endo-35, 40, and 43 originate from a [4 + 2+] cycloaddition with normal electron demand. b) All isomerizations occur by a [3,3] sigmatropic rearrangement; [4 + 2] cycloreversion is energetically disfavored. c) A clear-cut distinction between the [4+ +2] or [4+2+] cycloaddition reaction routes to the energetically well-balanced systems 10? endo-11 and 12? endo-13 is not possible. d) The two cycloadditions may well favor a nonconcerted reaction through an allylic cationic intermediate which also governs the [3,3] rearrangements (Scheme 8).
Azo Bridges from Azines, II. Synthesis of 2,3-Diazabicyclooctenes from 4,5-Dihydropyridazines as Electron Deficient Dienes
Huenig, Siegfried,Prokschy, Frank
, p. 534 - 553 (2007/10/02)
According to the general principle of + + 2> Diels-Alder reactions with inverted electron demand, 4,5-dihydropyridazine (2) and its derivatives 4, 6, 8, and 10 with annelated bicycles are used as electron deficient dienes.These azines are trimeric (Tr").Trifluoroacetic acid produces the protonated monomers, which cycloadd to cyclopentadiene, norbornene, norbornadiene, and to some extent even to cyclopentene.A number of so far unknown polycycles are obtained, all derivatives of 2,3-diazabicyclooct-2-ene (C).Among these, the derivatives 27, 28, 30, 33, 35, and 39 contain neighbouring parallel C = C/N = N bonds, while in 32 the azo bridge is flanked by two C = C bonds.Spectroscopic data, especially of the last mentioned products, are discussed.
