Structure-reactivity relationships and intrinsic reaction barriers for nucleophile additions to a quinone methide: A strongly resonance-stabilized carbocation
Structure-reactivity relationships and intrinsic reaction barriers for nucleophile additions to a quinone methide: A strongly resonance-stabilized carbocation
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DOI：
10.1021/ja9937526
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发表时间：
2000-03-01
影响因子：
15
通讯作者：
Crugeiras, J
中科院分区：
文献类型：
--
作者：
Richard, JP;Toteva, MM;Crugeiras, J
测定了一系列亲核试剂在简单醌类化合物9-[双（三氟甲基）亚甲基]环己-2,5-二烯酮(1)上加成得到水中亲核试剂I-Nu的二阶速率常数k(Nu) （M-1 s(-1)）。测定了HBr和HI整体加成为1得到H-1-Nu的平衡常数，并利用这些数据计算了Br-和I-加成为1的平衡常数，以及Cl-和AcO-加成的平衡常数。log k（Nu）的值与Ritchie亲核参数N+呈线性相关，斜率s = 0.92 +/- 0.10，与高共振稳定碳正离子的亲电性无关值1.0基本相同。当I-、Br-、Cl-和AcO-为1时，Marcus本能垒Lambda分别为12.4、13.9、15.4和19.8 kcal/mol。Br-、Cl-和AcO-加成为1的热力学势垒G度和本然势垒λ分别比这些亲核试剂加成到三苯基甲基碳正离子的相应势垒大8.4 +/- 1.0和5.2 +/- 0.2 kcal/mol。根据其化学反应性判断，1表现为高共振稳定的碳正离子。I-、Br-、Cl-和AcO-不形成里奇亲电试剂的稳定加合物，它们的N+值分别为4.0、2、2、1.2和0.60。Ritchie （N+）和Swain-Scott (N)亲核参数线性相关斜率为2.0 (r = 0.98)，表明亲核试剂加成到sp(2)-杂化碳上比加成到sp(3)-杂化碳上时，在过渡态亲核试剂与碳之间的成键要大得多。叠氮离子和亲核试剂（α效应亲核试剂）在亲核位点附近的原子上具有非成键电子Fair(s)，与这种相关性表现出显著的正偏差。
Second-order rate constants k(Nu) (M-1 s(-1)) were determined for addition of a wide range of nucleophiles to the simple quinone methide 9-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) to give the nucleophile adduct I-Nu in water. Equilibrium constants were determined for the overall addition of HBr and HI to 1 to give H-1-Nu, and the data were used to calculate equilibrium constants for the addition of Br- and I- to 1, and to estimate equilibrium constants for the addition of Cl- and AcO-. The values of log k(Nu) show a linear correlation with the Ritchie nucleophilicity parameter N+ with a slops s = 0.92 +/- 0.10 that is essentially the same as the electrophile-independent value of 1.0 for highly resonance-stabilized carbocations. Marcus intrinsic barriers Lambda of 12.4, 13.9, 15.4, and 19.8 kcal/mol are reported for the addition of I-, Br-, Cl-, and AcO- to 1, respectively. The thermodynamic barriers Delta G degrees and intrinsic barriers Lambda for addition of Br-, Cl-, and AcO- to 1 are 8.4 +/- 1.0 and 5.2 +/- 0.2 kcal/mol larger, respectively, than the corresponding barriers for addition of these nucleophiles to the triphenylmethyl carbocation. It is concluded that, by the criterion of its chemical reactivity, 1 behaves as a highly resonance-stabilized carbocation. Values of N+ = 4.0, 2,2, 1.2 and 0.60, respectively, are reported for I-, Br-, Cl-, and AcO-, which do not form stable adducts to Ritchie electrophiles. The slope of 2.0 (r = 0.98) for the linear correlation between Ritchie (N+) and Swain-Scott (n) nucleophilicity parameters shows that there is substantially greater bonding between the nucleophile and carbon at the transition state for nucleophile addition to sp(2)-hybridized carbon than for addition to sp(3)-hybridized carbon. Azide ion and nucleophiles with a nonbonding electron Fair(s) at atoms adjacent to the nucleophilic site (alpha-effect nucleophiles) exhibit significant positive deviations from this correlation.