Cope Rearrangements versus Retro Diels‐Alder Reactions

AbstractThe 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).