The syntheses of pyrazino-containing sultines and their application in Diels-Alder reactions with electron-poor olefins and [60]fullerene

The Diels-Alder reactions of heterocyclic o-quinodimethanes, generated in situ from 6,7-disubstituted quinoxalino[2,3-d]-[1,2λ]oxathiine 2-oxides (6a-c), 2,3-disubstituted-8,9-dihydro-6H-8λ-[1,2]oxathiino[4,5- g]quinoxalin-8-one (7a-c) (sultines), and pyrazinosultine (22), with electron-poor olefins and [60]fullerene are described. The heterocyclic-fused sultines 7a-c and 22 are readily prepared from the corresponding dibromides 9a-c and 24 with the commercially available Rongalite (sodium formaldehyde sulfoxylate). When heated in the presence of electron-poor dienophiles and [60]fullerene, all of the sultines underwent extrusion of SO, and the resulting heterocyclic o-quinodimethanes (3a-d, 4a-c, and 25) were intercepted as the 1:1 adducts in good to excellent yields. The temperature- dependent H NMR spectra of fullerene derivatives 31-38 show a dynamic process for the methylene protons. The activation free energies (ΔG(c)(+)) determined for the boat-to-boat inversion of these pyrazino-containing C compounds (31-34 and 38) are found to be in the range of 14.1-14.8 kcal/mol, but they are in the range of 15.2 to > 17.1 kcal/mol for adducts 35-37. The activation free energies (ΔG(c)(+)) are significantly affected by (1) the orientations and (2) the substituents of the quinoxaline rings and (3) the extended benzannulation in the arenes of C adducts (see Table 2), which implies that both electronic interactions and steric effects between the aromatic addends and C are important. Tautomerization of methylquinoxaline to its enamine is invoked as a rationalization for the lowering of ΔG(c)(+) in some of the fulleroadducts.