Das Energieprofil deso‐Chinodimethan‐Benzocyclobuten‐Gleichgewichtes

Abstract

Durch Verbrennungskalorimetrie wird die Bildungsenthalpie des Benzocyclobutens (2)zu AH: (g) = 199.4 kJ mol-' (47.7 kcal mol-') bestimmt. Aus der Kinetik der Reaktion von 2 mit Maleinsaureanhydrid (MSA) ergeben sich fur die o-Chinodimethanbildung 2 + 1 Aktivierungsparameter E, = 166.9 kJ mol-' (39.9 kcal mol-'), A = 2.8 x IOl4 s-', Die entsprechenden Werte der Ruckreaktion 1 + 2 werden aus der Dimerisierungsgeschwindigkeit des Benzocyclobutens sowie der durch Blitzlichtphotolyse von 5,6-Dimethylenbicyclo[2.2.l]hept-2-en-7-on (5) ermittelten Geschwindigkeitskonstanten der o-Chinodimethandimerisierung zu E, = 122.7 kJ mol-' (29.3 kcal mol-'), A = 2.2 x IOl3 s-', bestimmt. Energy Profile of the o-Quinodimethane-Benzocyclobutene EquilibriumThe enthalpy of formation of benzocyclobutene (2) is measured by combustion calorimetry to be AHp(g) = 199.4 kJ mol-' (47.7 kcal mol-'). From the kinetics of the reaction of 2 with maleic anhydride the activation parameters for the o-quinodimethane formation 2 + 1 are obtained: E, = 166.9 kJ mol-' (39.9 kcal mol-'), A = 2.8 x I O l 4 s-'. The corresponding values for the back reaction 1 + 2 are derived from the dimerisation rate of o-quinodimethane, obtained by flash photolysis of 5,6-dimethylenebicyclo[2.2.l]hept-2-en-7-one (S), and from the dimerisation of benzocyclobutene yielding E, = 122.7 kJ mol-' (29.3 kcal mol-I),o-Chinodimethan (1) kann, wie seine Resonanzstruktur 1 b anzeigt, als hoch stabilisiertes Diradikal aufgefal3t werden. Im Hinblick auf die Diskussion uber die Lage von Diradikalen auf der Reaktionskoordinate ') ist es von Interesse, das Energieprofil der Cyclisierung des o-Chinodimethans (1) zu Benzocyclobuten (2) zu ermitteln. Dieses wird durch ') L. Salem und C. Rowland, Angew. Chem. 84, 86 (1972); Angew. Chem., Int. Ed. Engl. 11, 92 (1972).

Thermochemistry and photochemistry of spiroketals derived from indan-2-one: Stepwise processes versus coarctate fragmentations

“…The latter can then either undergo ring closure to form indan-2-one ( IN ), or decarbonylate to give o -xylylene ( XY ). The equilibrium of XY and benzocyclobutene ( BC ) is established in the literature [28], as well as the formation of styrene ST from BC [29]. An alternative mechanism, the chelotropic elimination of 1,3-dioxolan-2-ylidene is not likely.…”

On the ring-opening of substituted cyclobutene to benzocyclobutene: analysis of π delocalization, hyperconjugation, and ring strain

“…The so-obtained homodesmotic reactions give access to the 6-membered ring strains and are then connected by the two cyclobutene ring-opening reactions. The resulting Hess cycles for (11) and (12) are presented in Fig. 3 and 4.…”

“…Quite interestingly, only in the case of the aromatic benzocyclobutenes, closed forms become more stable: the benzocyclobutene is about 13 kcal mol À1 more stable than the o-xylylene (experimental observations). 11,12 Several theoretical studies have been conducted on the ring-opening of cyclobutene-like systems. Nevertheless, the comparison between open and closed forms was rarely the main issue, [13][14][15] since many studies have been focusing on substitution effects on barrier heights or torquoselectivity.…”

Polymerization of Thin Film Polymers

“…al. The ring opening has a 10 times higher reaction rate constant [9] [10]. The polymerization reaction could be described by a pseudo first-order reaction equation, because the Diels-Alder reaction is the reaction rate limited factor.…”