Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C60 with α‑Trimethylsilyl Group-Containing N‑Alkylglycinates Yielding Aminomethyl-1,2-dihydrofullerenes or Fulleropyrrolidines

Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that are utilized to produce complex organic substances. In the current study, single-electron transfer (SET)-promoted photoaddition reactions of fullerene C60 with both trimethylsilyl and various alkyl group-containing glycinates and ethyl N-alkyl-N-((trimethylsilyl)­methyl)­glycinates were explored to evaluate how the nature of N-alkyl substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction pathways followed. The results showed that photoreactions of C60 with glycinates, performed in deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway involving the addition of α-amino radical intermediates that are generated by sequential SET-solvent-assisted desilylation of glycinate substrates to C60. Under oxygenated conditions, photoreactions of glycinate substrates, except N-benzyl-substituted analogues, did not take place efficiently owing to quenching of 3C60* by oxygen. Interestingly, N-benzyl-substituted glycinates did react under these conditions to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of in situ formed azomethine ylides to C60. The ylide intermediates were formed by regioselective H-atom transfer from glycinates by singlet oxygen. Furthermore, methylene blue (MB)-photosensitized reactions of C60 with glycinates under oxygenated conditions took place efficiently to produce fulleropyrrolidines independent of the nature of N-alkyl substituents of glycinates.

调控化学选择性的相关认知，对于构建用于合成复杂有机物质的反应体系至关重要。在本研究中，我们针对富勒烯C₆₀与同时含三甲基硅基（trimethylsilyl）及多种烷基的甘氨酸酯，以及N-烷基-N-((三甲基硅基)甲基)甘氨酸乙酯之间的单电子转移（single-electron transfer, SET）促进的光加成反应展开探究，以评估甘氨酸酯底物的N-烷基取代基性质与反应条件如何调控反应路径的化学选择性。实验结果表明，在脱氧条件下进行的C₆₀与甘氨酸酯的光反应，可通过以下路径高效生成氨甲基-1,2-二氢富勒烯：甘氨酸酯底物经连续SET-溶剂辅助脱硅基反应生成α-氨基自由基中间体，该中间体随后与C₆₀发生加成反应。在有氧条件下，除N-苄基取代类似物外，其余甘氨酸酯底物的光反应均难以高效进行，这是因为单线态氧（singlet oxygen）会淬灭激发三线态富勒烯³C₆₀*。值得注意的是，N-苄基取代的甘氨酸酯在该条件下仍可发生反应，通过原位生成的甲亚胺叶立德与C₆₀发生1,3-偶极环加成反应，生成富勒烯吡咯烷类产物。该叶立德中间体由单线态氧对甘氨酸酯进行区域选择性氢原子转移后形成。此外，在有氧条件下，通过亚甲基蓝（methylene blue, MB）光敏化引发的C₆₀与甘氨酸酯的反应可高效进行，且无论甘氨酸酯的N-烷基取代基性质如何，均能生成富勒烯吡咯烷类产物。