Two‐stage [2π+2σ+2σ]‐cycloaddition of quadricyclane (2) with 2,3‐dicyano‐1,4‐benzoquinone (1) with a huge difference in the activity of two reaction centers has been studied. In the first stage (kinetic control), the cycloaddition of 2 takes place on the activated С2=С3 bond of 1 to form the monoadduct 3, and in the second stage the cycloaddition of 2 on the С5=С6 bond of the monoadduct 3 occurs by 6 orders of magnitude lower with the formation of bisadduct 4. The structures of adducts 3 and 4 have been proved by NMR data and the X‐Ray method, respectively. The kinetics of the first and second stages, the enthalpy of dissolution of 1 in the π‐donor solvents, and the enthalpy of the reaction 1+2→3 have been measured.
The data on temperature, solvent, and high hydrostatic pressure influence on the rate of the ene reactions of 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione (1) with 2‐carene (2), and β‐pinene (4) have been obtained. Ene reactions 1+2 and 1+4 have high heat effects: ∆Hr‐n (1+2) −158.4, ∆Hr‐n(1+4) −159.2 kJ mol−1, 25°C, 1,2‐dichloroethane. The comparison of the activation volume (∆V≠(1+2) −29.9 cm3 mol−1, toluene; ∆V≠(1+4) −36.0 cm3 mol−1, ethyl acetate) and reaction volume values (∆Vr‐n(1+2) −24.0 cm3 mol−1, toluene; ∆Vr‐n(1+4) −30.4 cm3 mol−1, ethyl acetate) reveals more compact cyclic transition states in comparison with the acyclic reaction products 3 and 5. In the series of nine solvents, the reaction rate of 1+2 increases 260‐fold and 1+4 increases 200‐fold, respectively, but not due to the solvent polarity.
The kinetic parameters and enthalpies of the Diels–Alder reactions between cyclopentadiene and 2,3‐dicyano‐1,4‐benzoquinone leading to the formation of two different monoadducts and bisadduct were determined. The stability of adducts is compared. Monoadduct appears to be thermodynamically more stable than the bisadduct. Comparison with the other Diels–Alder reactions studied previously allows us to conclude that the heat effects upon formation of the considered Diels–Alder adducts are the lowest in comparison with all the studied dienophiles.
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