Regiochemistry of twofold additions to [6,6] bonds in C-60: Influence of the addend-independent cage distortion in 1,2-monoadducts

Three series of regioisomeric bisadducts of C-60, namely, C-62(anisyl)(4) and the mixed systems C-62(anisyl)(2)(COOEt)(2) and C-61(COOEt)(2)(NCOOEt), were synthesized starting from the 1,2-monoadducts C-61(COOEt)(2) (1), C-61(anisyl)(2) (2), and C-60(NCOOEt) (4) by using the Bingel and Bamford-Stevens reactions, and nitrene additions. In the case of C-61(COOEt)(2)(NCOOEt) the complete series of nine possible regioisomers were isolated for the first time. For steric reasons the cis-1 isomers of C-62(anisyl)(4) and C-62(anisyl)(2)(COOEt)(2) were not formed. The transannular [6,6] bonds in the cis-1 isomer 42 of C-61(COOEt)(2)(NCOOEt) are closed. The properties and regioselectivities of formation of these bisadducts and their monoadduct precursors were compared with those of the series C-62(COOEt)(4) and C-60(NCOOEt)(2), which we synthesized previously. In the additions to 1, 2, and 4 the preferred positions of attack are e and trans-3 for sterically demanding addends (e.g., combinations of C(anisyl)(2) and C(COOEt)(2)) and cis-1, e, and trans-3 for sterically less demanding addends (e.g., combinations of N(COOEt) and C(COOEt)(2)). A detailed analysis of the MO structures, the experimental and calculated geometries of monoadduct precursors, and the stabilities of reaction products leads to the conclusion that the addend-independent cage distortion itself is responsible for the observed regioselectivities of bisadduct formations.