Intermolecular Insertion of an N,N-Heterocyclic Carbene into a Nonacidic CH Bond: Kinetics, Mechanism and Catalysis by (K-HMDS)2 (HMDS=Hexamethyldisilazide)
作者:Guy C. Lloyd-Jones、Roger W. Alder、Gareth J. J. Owen-Smith
DOI:10.1002/chem.200600266
日期:2006.7.5
association constant (Ka) for rapid equilibrium-complexation of 2 with (K-HMDS)2 to form [2(K-HMDS)2] is extracted by nonlinear regression of the 13C NMR shift of C2 in [13C1]-2 versus [(K-HMDS)2] yielding: Ka = 62(+/-7) M(-1); delta(C(2)) in 2=237.0 ppm; delta(C(2)) in [2(K-HMDS)2] = 226.8 ppm. It is thus concluded that there is discrete, albeit inefficient, molecular catalysis through the 1:1 carbene/(K-HMDS)2
2- [13C] -1-乙基-3-异丙基-3,4,5,6-四氢嘧啶-1-六氟磷酸盐([13C1] -1-PF6)与稍微过量(1.03当量)的二聚体反应甲苯中的六甲基二硅叠氮化钾(“(K-HMDS)2”)产生2- [13C] -3-乙基-1-异丙基-3,4,5,6-四氢嘧啶-2-亚烷基([13C1] -2)。如此生成的受阻亚稳态N,N-杂环卡宾[13C1] -2与甲苯(溶剂)进行缓慢但定量的反应,生成氨基2- [13C] -2-苄基-3-乙基-1-异丙基六氢嘧啶([13C1] -14)是通过在甲苯甲基上正式CH插入C2(“卡宾碳”)来实现的。尽管存在明显的pKa错配(在DMSO中Delta pKa 1+和甲苯估计约为16),该反应仍显示出去质子化机理的所有特征,反应速率在很大程度上取决于甲苯对位取代基(rho = 4.8(+/- 0.3)),并显示出大量的限速伯(k(H)/ k(D)= 4.2(+/-