Die thermische Umlagerung von 2‐(Buta‐1′,3′‐dienyl)‐phenolen in 2‐Alkyl‐2H‐chromene; Beispiele für aromatische [1,7a]‐ und [1,5s]sigmatropische Wasserstoffverschiebungen

Abstract

2‐(1′‐cis,3′‐cis‐)‐ and 2‐(1′‐cis,3′‐trans‐Penta‐1′,3′‐dienyl)‐phenol (cis, cis‐4 and cis, trans‐4, cf. scheme 1) rearrange thermally at 85–110° via [1,7 a] hydrogen shifts to yield the o‐quinomethide 2 (R  CH3) which rapidly cyclises to give 2‐ethyl‐2H‐chromene (7). The trans formation of cis, cis‐ and cis, trans‐4 into 7 is accompanied by a thermal cis, trans isomerisation of the 3′ double bond in 4. The isomerisation indicates that [1,7 a] hydrogen shifts in 2 compete with the electrocyclic ring closure of 2. The isomeric phenols, trans, trans‐ and trans, cis‐4, are stable at 85–110° but at 190° rearrange also to form 7. This rearrangement is induced by a thermal cis, trans isomerisation of the 1′ double bond which occurs via [1, 5s] hydrogen shifts. Deuterium labelling experiments show that the chromene 7 is in equilibrium with the o‐quinomethide 2 (R  CH3), at 210°. Thus, when 2‐benzyl‐2H‐chromene (9) or 2‐(1′‐trans,3′‐trans,‐4′‐phenyl‐buta1′,3′‐dienyl)‐phenol (trans, trans‐6) is heated in diglyme solution at >200°, an equilibrium mixture of both compounds (∼ 55% 9 and 45% 6) is obtained.

Palladium-Catalyzed Hydrofunctionalization of Vinyl Phenol Derivatives with Heteroaromatics

“…Additionally, a reaction of the vinyl phenol with 1,2-dimethyl indole in the absence of any catalyst was performed to achieve a similar yield (Table , entry 10). It should be noted that such 1,5-sigmatropic rearrangements are not commonly used for o -QM formation and functionalization and often require more forcing conditions . While these control experiments suggest that alternative mechanistic pathways are operational for product formation (vide infra), the presence of the Pd catalyst and the alkyl chloride significantly improves the reaction outcome.…”

Aromatische sigmatropische Wasserstoffverschiebungen in 2‐Vinyl‐ und 2‐Allylphenolen

“…Wasserstoffverschiebungen gegen D-Atome 7, Die starkere H-Brucke bei den (2)-Phenolen scheint, wenigstens zum Teil, fur die wesentlich kleineren gas-chromatographischen Retentionszeiten der (2)-Phenole im Vergleich zu ihren (0-Formen an Glaskapillarkolonnen (beladen mit XE60 oder Emulphor) verantwortlich zu sein (vgl. auch [38]). Die aufgrund sterischer Wechselwirkungen starkere Storung der Konjugation zwischen der Propenylgruppe und dem Phenylring bei den (2)-Formen spielt aber sicher ebenfalls eine Rolle (vgl.…”