Ohta Brian K, Scupp Thomas M, Dudley Timothy J
Department of Chemistry, Villanova University, Villanova, Pennsylvania 19085, USA.
J Org Chem. 2008 Sep 19;73(18):7052-9. doi: 10.1021/jo800517b. Epub 2008 Aug 20.
The nucleophilic addition of SO2, SO2ClF, and SO2F2 to carbenium ions and the nucleophilic addition of SO2 to 1,2-bridged halonium ions are theoretically investigated by using B3LYP/6-311+G(d,p). On the basis of geometric changes in ion-solvent complexes compared to isolated molecules, the theoretical data for the addition of solvent to carbenium ions uniformly agree with experiments by Olah and Donovan. The relative reactivity of carbenium ions (methyl > ethyl > iso-propyl > tert-butyl) follows the familiar trend based on electron demand at the carbenium center. The theoretical data for the addition of SO2 to 1,2-bridged halonium ions with use of similar methods indicate that this addition exhibits a reversed trend on the basis of the electron demand; SO2 adds to 2,2-dimethylethylene chloronium and bromonium ions but does not add to the fluoronium analogue. Furthermore, the addition depends on the stereochemistry of the approaching SO2. When SO2 approaches syn to the halogen atom on the halonium ion, addition is observed. When SO2 is anti, addition is not observed. The reversed reactivity and stereochemistry of the addition of SO2 to halonium ions can be explained by electron donation from the halogen atom to the sulfur atom of the approaching SO2. This sulfur-halogen interaction activates the nucleophilicity of the approaching SO2 and makes a normally unreactive tertiary carbenium carbon susceptible to addition. The theoretical evidence for covalently bound halonium ion-SO2 complexes is discussed in the context of previously reported experimental evidence for the existence of equilibria involving beta-halocarbenium ions.
采用B3LYP/6 - 311 + G(d,p)理论方法研究了二氧化硫（SO₂）、二氯二氟甲烷（SO₂ClF）和二氟二氧化硫（SO₂F₂）对碳正离子的亲核加成反应，以及二氧化硫对1,2 - 桥连卤鎓离子的亲核加成反应。基于离子 - 溶剂络合物与孤立分子相比的几何变化，二氧化硫对碳正离子加成反应的理论数据与奥拉（Olah）和多诺万（Donovan）的实验结果一致。碳正离子的相对反应活性（甲基＞乙基＞异丙基＞叔丁基）遵循基于碳正离子中心电子需求的常见趋势。使用类似方法对二氧化硫加成到1,2 - 桥连卤鎓离子的理论数据表明，这种加成反应基于电子需求呈现出相反的趋势；二氧化硫能加成到2,2 - 二甲基乙烯氯鎓离子和溴鎓离子上，但不能加成到氟鎓类似物上。此外，加成反应取决于接近的二氧化硫的立体化学。当二氧化硫与卤鎓离子上的卤素原子以顺式接近时，会发生加成反应。当二氧化硫以反式接近时，则不会发生加成反应。二氧化硫对卤鎓离子加成反应的反应活性和立体化学的反转可以通过卤素原子向接近的二氧化硫的硫原子供电子来解释。这种硫 - 卤相互作用激活了接近的二氧化硫的亲核性，并使通常不反应的叔碳正离子碳易于发生加成反应。在先前报道的涉及β - 卤代碳正离子平衡存在的实验证据的背景下，讨论了共价结合的卤鎓离子 - 二氧化硫络合物的理论证据。
