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

Unique Insertion Mechanisms of Bis-dehydro-β-diketiminato Silylene

The unique insertion reactions of the first, stable six-membered-ring silylene ({HC[CMeN(R)]₂}Si, R = 2,6-diisopropylphenyl) with eight reactants were investigated by the B3LYP/cc-pVTZ method. The initial step (<b>IS</b>) of all the reactions is the formation of an intermediate 1,4-adduct, <b>IM</b>, which will be then the starting point toward the different final states (<b>FS</b>). In this study three different mechanisms were found and studied to the 1,4-adduct and six reaction paths from the 1,4-adduct to the final products. On the basis of the results, the different reaction paths, the experimental insertion products, and the special reactivity of the six-membered-ring silylene have been explained

Trimethylaluminum and Borane Complexes of Primary Amines

Trimethylaluminum (TMA) complexes of methyl-, <i>n</i>-propyl-, cyclopropyl-, allyl-, and propargylamine were synthesized and their experimental properties and theoretical characteristics were compared with the respective amine–borane analogues. The amine ligand of an amine–TMA Lewis acid–base complex can be easily changed by another amine through a 2:1 amine–TMA intermediate in pentane at room temperature. The exchange of the same ligands in the case of amine–boranes requires remarkably more time in line with the calculated relative energy of the respective transition state. The ¹H and ¹³C NMR experiments examining the addition of one or more equivalent of amine to the respective Lewis acid–base complex conclude in the fast exchange of the amine ligand in the NMR time scale only in the cases of amine–TMA complexes, which could also be caused by similar 2:1 complexes. However, in gas phase, only 1:1 amine–TMA complexes are present as evidenced by ultraviolet photoelectron spectroscopy (UPS). The observed UP spectra, which are the first recorded photoelectron spectra of primary amine–TMA compounds, indicate that the stabilization effect of the lone electron pair of nitrogen atom in amines during the borane complexation is stronger than that of the TMA complexation. In line with this observation, the destabilization of the σ_Al–C orbitals is lower than that of σ_B–H orbitals during the formation of amine–TMA and amine–borane complexes, respectively. As showed by theoretical calculations, the CH₄ elimination of the studied amine–TMA complexes is exothermic, indicating the possibility of using these compounds in metal organic chemical vapor deposition techniques (MOCVD). On the other hand, our experimental conditions avoid this methane elimination and constitutes the first procedure employing distillation to isolate primary amine–TMA complexes