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]-[1,2 lambda(4)]oxathiine 2-oxides (6a-c), 2,3-disubstituted-8,9-dihydro-6H-8 lambda(4)-[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 SO2, 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-1 NMR spectra of fullerene derivatives 31-38 show a dynamic process for the methylene protons. The activation free energies (Delta G(e)double dagger) determined for the boat-to-boat inversion of these pyrazino-containing C-60 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 35-87. The activation free energies (Delta G(c)double dagger) 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-60 adducts (see Table 2), which implies that both electronic interactions and steric effects between the aromatic addends and C-60 are important. Tautomerization of methylquinoxaline to its enamine is invoked as a rationalization for the lowering of Delta G(c)double dagger in some of the fulleroadducts.