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The Mechanism of 1,2‐Addition of Disilene and Silene: Hydrogen Halide Addition
Author(s) -
Hajgató Balázs,
Takahashi Masae,
Kira Mitsuo,
Veszprémi Tamás
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020503)8:9<2126::aid-chem2126>3.0.co;2-2
Subject(s) - chemistry , exothermic reaction , electrophile , transition state , computational chemistry , halide , halogen , ab initio , dissociation (chemistry) , ionic bonding , photochemistry , oxidative addition , reagent , reaction mechanism , ion , inorganic chemistry , organic chemistry , alkyl , catalysis
The mechanism of 1,2‐addition reactions of HF and HCl to Si=Si, Si=C, and C=C bonds has been investigated by ab initio quantum chemical methods. Geometries and relative energies of the stationary points and all the transition states were determined by using the MP2/6‐311++G(d,p), B3LYP/6‐311++G(d,p), and CBS‐Q levels of theory. The investigated reactions can be characterized by two main thermodynamic profiles. The type in which the reagent molecule attacks a carbon atom is moderately exothermic with a high activation barrier. The second type in which a hydrogen halide attacks a silicon is strongly exothermic with a low activation energy. At the early stage of all the reactions a weakly bonded initial complex is found which indicates that the initial step of all the reactions is an electrophilic attack of hydrogen halide. The geometry and charge distribution of the transition state of the reactions indicate two main types of mechanism. If silicon is attacked, the halogen–silicon bond formation precedes the H−Y bond breaking. If, however, carbon is attacked, the first step is always an ionic dissociation of the hydrogen halide and a carbenium ion formation, which is stabilized by the C−Y bond formation in the final step of the reaction. The reaction diagrams and proposed mechanisms explain the experimentally found regioselectivity well.