硅苯和锗苯与2,3-二甲基丁二烯杂Diels-Alder反应的理论研究

采用密度泛函理论(DFT)方法在B3LYP/6-311G(d,p)水平上研究了硅苯和锗苯与2,3-二甲基丁二烯的两类杂Diels-Alder反应的微观机理、势能剖面、取代基效应和溶剂效应.计算结果表明,所研究反应均以协同非同步的方式进行,且C—Si或C—Ge键总是先于C—C键形成.在硅苯或锗苯分子作为杂亲二烯体的[2+4]反应中,endo进攻方式的非同步性比exo进攻稍大一些,而后者比前者一般要稍稍有利一些.在硅苯或锗苯分子作为杂二烯烃的[4+2]反应中,反应非同步性的大小与产物中不对称的亲二烯体上的取代基与硅或锗原子之间的相对位置有关,且在动力学上总是非同步性较大的反应更容易进行一些.硅或锗原子上的CCl3或NH2取代基在热力学和动力学上一般有利于反应的进行,而C(CH3)3取代基的影响则相反.[2+4]反应在热力学和动力学上均远比相应的[4+2]反应容易进行,这与实验完全一致.苯和甲醇溶剂对所研究反应的势能剖面影响较小.

Theoretical Studies of Hetero-Diels-Alder Reactions of Silabenzenes and Germabenzenes with 2,3-Dimethyl-butadiene

Density functional theory (DFT), at the B3LYP/6-311G(d,p) level of theory, has been employed to study the mechanism, potential energy surface, substituent and solvent effects of the titled reactions. The obtained results indicate that all of these reactions are concerted but nonsynchronous processes, in which the formation of C—Si or C—Ge bond always keeps ahead the formation of C—C bond. For [2＋4] reactions with silabenzenes or germabenzenes as heterodienophiles, the exo approach is less asynchronous and is generally slightly favored. While for [4＋2] reactions with silabenzenes or germabenzenes as heterodienes, the asynchronicity in the bond-formation process is related to the relative position of Si or Ge atom with respect to the substituent at C atom of dienophile in products, and the reaction path with more asynchronicity is kinetically favored. The CCl₃ or NH₂ groups at Si or Ge atom of silabenzene or germabenzene molecules are in favor of both [2＋4] and [4＋2] processes not only thermodynamically but also kinetically, while C(CH₃)₃ as a substituent has opposite effect generally. Compared with corresponding [4＋2] reactions, [2＋4] reactions are decidedly preferred, both thermodynamically and kinetically, which is in full agreement with experimental observations. Benzene or methanol as a solvent has only trivial influence on the potential energy surfaces of the studied reactions.