Development of the method of alkylation of C_<60> by way of alkylfullerenyl cation

我们发现，当在氯烷（RCl）中加入大量的AlCl_3时，富勒烯C_<60>发生亲电加成反应，得到RC_<60>-Cl的加合物产率较高。在CF_3SO_3H和H_2SO_4等强酸中溶解富勒烯醇（RC_<60>OH），可以生成长态离子，中间产物烷基富勒烯基阳离子（RC_<60>OH）。这些阳离子通过核磁共振和可见光/近红外光谱被观察到是长寿命的物种。^<13>C核磁共振谱表明，正电荷主要位于C-2上，结构具有C_s对称。阳离子中心的化学位移（175 ~ 180 ppm）表明R族氯原子与阳离子中心的配位较弱。用AlCl_3从氯化物RC_<60-> Cl中提取一个氯离子，也可以原位生成RC_<60>^+阳离子。在苯甲醚中进行反应时，溶剂分子与RC_<60>^+发生亲核加成，得到相应的加合物RC_<60>-C_6H_4OCH_3-p。AgBF_4也可以萃取氯离子。用该试剂可在醇（R' oh）中进行反应，得到相应的醚RC_<60>-OR'，收率高。氯化物RC_<60>-Cl （R=氯烷基）在茴香醚-三氟乙醇混合溶剂（1:9 v/v）中溶解得到S_N1反应产物RC_<60>-Y（Y=对茴香基和三氟乙氧基）。活性自由能（25℃时24 ~ 25 kcal/mol）仅略低于叔丁基氯，表明RC_<60>^+阳离子的稳定性与叔丁基阳离子相近。根据相应的富勒烯醇的电离平衡常数，确定了RC_<60>^+阳离子的pK_R^+值。所得值与叔丁基阳离子的pK_R+相当。这一结果也支持了RC_<60>^+与叔丁基阳离子在稳定性上的相似性。

We found that fullerene C_<60> undergoes electrophilic addition when treated with a large excess of AlCl_3 in chloroalkanes (RCl), giving adducts RC_<60>-Cl in good yields. The intermediate, alkylfullerenyl cations (RC_<60>^+), could be generated as long-likved ions by dissolving fullerenols (RC_<60>OH) in strong acids such as CF_3SO_3H and H_2SO_4. These cations were observed by NMR and vis/NIR spectra as long-lived species. The ^<13>C NMR spectra indicated that the positive charge is located mainly on C-2, and the structure has C_s symmetry. The chemical shift of the cationic center (175-180 ppm) suggested a weak coordination of chlorine atom(s) in the group R to the cationic center.Cations RC_<60>^+ could be also generated in situ by the abstraction of a chloride ion from chlorides RC_<60-> Cl with AlCl_3. When the reactions were carried out in anisole, nucleophilic addition of a solvent molecule to RC_<60>^+ occurred, giving the corresponding adducts RC_<60>-C_6H_4OCH_3-p. The chloride ion can be also abstracted by AgBF_4. With this reagent the reaction could be carried out in alcohols (R'OH), giving the corresponding ethers RC_<60>-OR' in good yiels.Chlorides RC_<60>-Cl (R=chloroalkyl) were found to undergo solvolysis in anisole-trifluoroethanol mixed solvent (1:9 v/v) to give S_N1 reaction products, RC_<60>-Y(Y=p-anisyl and trifluoroethoxy). The free energies of activation (24-25 kcal/mol at 25℃) were only slightly lower than that for the tert-butyl chloride, indicating that cations RC_<60>^+ have stabilities close to that of the tert-butyl cation.The pK_R^+ values of cations RC_<60>^+ were also determined from the equilibrium constants for the ionization of the corresponding fullerenols. The obtained values are comparable to the pK_R+ of the tert-butyl cation. Thsi result also support the similarity in stability between RC_<60>^+ and the tert-butyl cation.