The Diels-Alder reactions of heterocyclic o-quinodimethanes, generated in situ from 6,7-disubstituted quinoxalino[2,3-d]-[1,2λ4]oxathiine 2-oxides (6a-c), 2,3-disubstituted-8,9-dihydro-6H-8λ4-[1,2]oxathiino[4,5- g]quinoxalin-8-one (7a-c) (sultines), and pyrazinosultine (22), with electron-poor olefins and 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 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 1H 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 C60 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 C60 adducts (see Table 2), which implies that both electronic interactions and steric effects between the aromatic addends and C60 are important. Tautomerization of methylquinoxaline to its enamine is invoked as a rationalization for the lowering of ΔG(c)(+) in some of the fulleroadducts.
All Science Journal Classification (ASJC) codes
- Organic Chemistry