Laboratorium für Organische Chemie, ETH Zürich, Vladimir-Prelog-Weg 3, 8093 Zürich, Switzerland.
Laboratorium für Physikalische Chemie, ETH Zürich, Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland.
J Am Chem Soc. 2020 Jul 29;142(30):12954-12965. doi: 10.1021/jacs.9b13768. Epub 2020 Jul 17.
The tris- and tetra-adducts of MN@C metallofullerenes were synthesized and characterized for the first time. The 1,3-dipolar cycloaddition (Prato reaction) of YN@-C and GdN@-C with an excess of -ethylglycine and formaldehyde provided tris- and tetra-fulleropyrrolidine adducts in a regioselective manner. Purification by HPLC and analyses of the isolated peaks by NMR, MS, and vis-NIR spectra revealed that the major products were four tris- and one tetra-isomers for both YN@-C and GdN@-C. Considering the large number of possible isomers (e.g., at least 1140 isomers for the tris-adduct), the limited number of isomers obtained indicated that the reactions proceeded with high regioselectivity. NMR analyses of the YN@-C adducts found that the tris-adducts were all-[6,6]- or [6,6][6,6][5,6]-isomers and that some showed mutual isomerization or remained intact at room temperature. The tetra-adduct obtained as a major product was all-[6,6] and stable. For the structural elucidation of GdN@-C tris- and tetra-adducts, density functional theory (DFT) calculations were performed to estimate the relative stabilities of tris- and tetra-adducts formed upon Prato functionalization of the most pyramidalized regions of the fullerene structure. The most stable structures corresponded to additions on the most pyramidalized (i.e., strained) bonds. Taking together the experimental vis-NIR spectra, NMR assignments, and the computed relative DFT stabilities of the potential tris- and tetra-adducts, the structures of the isolated adducts were elucidated. Electron resonance (ESR) measurements measurements of pristine, bis-, and tris-adducts of GdN@C suggested that the rotation of the endohedral metal cluster slowed upon increase of the addition numbers to C cage, which is favored for accommodating the Gd atoms of the relatively large GdN cluster inner space at the sp addition sites. This is presumably related to the high regioselectivity in the Prato addition reaction driven by the strain release of the GdN@C fullerene structure.
首次合成并表征了 MN@C 金属富勒烯的三-和四-加合物。YN@-C 和 GdN@-C 与过量的 -乙基甘氨酸和甲醛的 1,3-偶极环加成（Prato 反应）以区域选择性方式提供三-和四-全富勒吡咯烷加合物。通过 HPLC 进行纯化，并通过 NMR、MS 和可见-近红外光谱对分离峰进行分析，结果表明对于 YN@-C 和 GdN@-C，主要产物都是四种三-和一种四-异构体。考虑到可能的异构体数量众多（例如，三-加合物至少有 1140 种异构体），获得的异构体数量有限表明反应具有很高的区域选择性。YN@-C 加合物的 NMR 分析发现，三-加合物都是全-[6,6]-或 [6,6][6,6][5,6]-异构体，并且一些在室温下表现出相互异构化或保持完整。作为主要产物获得的四-加合物是全-[6,6]并且稳定。对于 GdN@-C 三-和四-加合物的结构阐明，进行了密度泛函理论（DFT）计算，以估计富勒烯结构最尖顶化区域的 Prato 官能化形成的三-和四-加合物的相对稳定性。最稳定的结构对应于在最尖顶化（即应变）键上的加成。综合实验可见-近红外光谱、NMR 分配和计算的潜在三-和四-加合物的相对 DFT 稳定性，阐明了分离加合物的结构。GdN@C 的原始、双-和三-加合物的电子共振（ESR）测量表明，随着向 C 笼的加成数增加，内笼金属簇的旋转速度减慢，这有利于在 sp 加成位点容纳相对较大的 GdN 簇内空间的 Gd 原子。这可能与由 GdN@C 富勒烯结构的应变释放驱动的 Prato 加成反应的高区域选择性有关。
