7. Synthesis and Characterization of Mono- and Bis-methano[60]fullerenyl Amino Acid Derivatives and Their Reductive Ring-Opening Retro-Bingel Reactions

The addition of N-(diphenylmethylene)glycinate esters (Ph₂C_NCH₂CO₂R) 3−6 to [60]fullerene under Bingel conditions gives, respectively, the methano[60]fullerenyl iminoesters 7−10. Upon treatment of 7−9 with sodium cyanoborohydride, in the presence of a protic or a Lewis acid, a novel reductive ring-opening reaction occurred to give the corresponding 1,2-dihydro[60]fullerenyl glycine derivatives 11−13. Using tethered bis-N-(diphenylmethylene)glycinate esters 33 and 34 derived from m- and p-benzenedimethanol scaffolds, the corresponding bis-methano[60]fullerenyl iminoesters 35−38 were synthesized under double Bingel reaction conditions. The m-benzenedimethanol derivative 33 gave the trans-4 (35) and cis-3 (36) regioisomeric bisadducts in a ratio of 80:20. The analogous para-tethered derivative 34 afforded the trans-3 (37) and trans-4 (38) regioisomers in a 80:20 ratio. The regiochemistry of the major bisadducts 35 and 37 (via the trans-esterified 39) were unequivocally determined using 2D INADEQUATE and C−C TOCSY NMR experiments. The regiochemistry of these bis-additions were unexpected on the basis of literature precedents. These results unequivocally show that the regiochemistry of tethered bis-additions is not solely dependent on the nature of the tether. A mixture of the trans-4 and cis-3 nonsymmetrical bisadducts 45 and 46 was obtained from the double-Bingel cyclopropanation of a bis-N-(diphenylmethylene)glycinate tether based on a 1,3-naphthyldimethanol scaffold. The regiochemistry of these compounds (45 and 46) was identified by correlation with the diethyl esters 40 and 47, prepared by trans-esterification of 35/45 and 36/46, respectively. The INADEQUATE and molecular modeling experiments allowed topological mapping of the fullerene surfaces of the bis-methano[60]fullerenes 38 and 42. Reductive ring-opening reactions on the tethered bis-methano[60]fullerenes 35−37, 45, and 46 gave none of the expected bis-fullerenylglycinates rather the reductive ring-opening-retro-Bingel products, the 1,2-dihydro[60]fullerenylglycinates 48, 49, 52, and 53. These compounds resulted from the reductive ring-opening of one methanoimino ester moiety and a retro-Bingel reaction of the other. Under analogous reductive ring-opening-retro-Bingel conditions, the nontethered bis-methano[60]fullerene 40 afforded the 1,2-dihydro[60]fullerenylglycinate 12. Thus, it was concluded that the tether was not the driving force for the reductive elimination of one of the methano groups.