Features of the Diels-Alder reaction between 9,10-diphenylanthracene and 4-phenyl-1,2,4-triazoline-3,5-dione

Abstract

The Diels-Alder reaction between substituted anthracenes 1a−1j and 4-phenyl-1,2,4-triazoline-3,5 (2) is studied. In all cases except one, the reaction proceeds on the most active 9,10-atoms of substituted anthracenes. The orthogonality of the two phenyl groups at the 9,10-position of diene 1a is found to shield 9,10-reactive centers. No dienophiles with C=C bonds are shown to participate in the Diels-Alder reaction with 1a; however, the reaction 1a + 2 proceeds with the very active dienophile 2,4-phenyl-1,2,4-triazoline-3,5-dione. It is shown that attachment occurs on the less active but sterically accessible 1,4-reactive center of diene 1a. The structure of adduct 3a is proved by ¹H and ¹³C NMR spectroscopy and X-ray diffraction analysis. The following parameters are obtained for reaction 1a + 2 ⇆ 3a in toluene at 25°C: K _eq = 2120 M⁻¹, ΔH ^≠_f = 58.6 kJ/mol, ΔS ^≠_f = −97 J/(mol K), ΔV ^≠_f = −17.2 cm³/mol, ΔH ^≠_b = 108.8 kJ/mol, ΔS ^≠_b = 7.3 J/(mol K), ΔV ^≠_b = −0.8 cm³/mol, ΔH _r-n = −50.2 kJ/mol, ΔS _r-n = −104.3 J/(mol K), ΔV _r-n = −15.6 cm³/mol. It is concluded that the values of equilibrium constants of the reactions 1a−1j + 2 ⇆ 3a−3j vary within 4 × 10¹−10¹¹ M⁻¹.