Molecular Tailoring: Reaction Path Control with Bulky Substituents

Steric groups are often regarded in reactions as chemically irrelevant, inert parts of the molecules, which have no influence on the structure of the forming reactive center of the product but rather on the reaction rate; therefore, they are usually not taken into account in theoretical work. However, in some cases, e.g. in the general reaction scheme of reductive dehalogenation of halosilanes, bulky substituents can cause major structural changes in the product simply by their presence. Our calculations using real substituents suggest that the use of proper substituents can prefer and stabilize only one structure on the potential energy surface (PES), eliminating all other relevant minima, not just increasing activation barriers as chemical intuition dictates. Since the preparation of these compounds are generally unpredictably slow process, the theoretical design may bring fundamental breakthroughs in the field of the synthesis of hitherto unknown reactive compounds. With the help of this concept, one can easily design proper substituents for the synthesis of a specific structure, since the mapping of the reaction routes can be spared and only a few calculations are needed. To illustrate the concept in practice, we suggest substituents, asymmetric R-Ind and terpenyl groups, for the synthesis of hexasilabenzene, which is one of the most desired silicon compounds.