Quantum mechanical assessment of substituent effect of doubly bonded silicon compounds

Abstract

As a landmark discovery in organosilicon chemistry, in 1981, Brook et al synthesised a compound containing C=Sidouble bond and West et al another one, containing Si=Si double bond. The present research titled ‘Quantum Mechanical Assessment of Substituent Effect of Doubly Bonded Silicon Compounds’ encompasses the computational analysis of some typical reactions of the substituted silenes (compounds with C=Si) and cyclotrisilene (compounds with Si=Si) with special emphasis on the substituent effects. The effect of substituents including R = CH3, SiH3, OH, CN and F on the dimerization of H2C=SiH2 is deeply explored and discussed. The study revealed that the dimerization process preferentially adopted a free radical reaction pathway and the substituents affecting the natural polarity of the C=S bond of the silene have a remarkable control over the energetics of the dimerization. Being a highly reactive species, silenes can be made use of for the small molecule activation process. The activity of silenes on NH3, CO and NO is systematically examined and the effect of substituents on the reactive potential of silenes is scrutinised. Our analysis endorsed the competence of silenes in activating the polar bonds in small molecules and identified that the σ- withdrawing and π- donating substituents improve their potential significantly. Silylene is the silicon alternative of carbene. It is an extremely reactive species and can activate even the most stable chemical bonds. We have studied the silene-silylene rearrangement in detail and the effect of the substituents on the energetics and mechanism of the process. Our study revealed that the π-donating substituents that are successful in inducing a polarity reversal of the C=Si bond of silene can bring down the barrier height of the silene-silylene rearrangement to great extends. Cyclotrisilene is the simplest cyclic compound containing a Si=Si double bond. We have conducted a detailed exploration of the reaction pathways and energetics of 1,2,3,3-tetramethyl cyclotrisilene with a few alkynes and aldehydes. Our investigation ascertained that the σ-insertion and π-addition are the spontaneous reaction pathways of the silenes with substrates carrying multiple bonds. Ring opening reactions yielding disilenyl silylene and is also feasible under normal conditions. However, exocyclic σ-insertion reaction involves huge activation energy and is not practicable. The energetics of the reaction significantly depends on the characteristics of the substrate involved. An exactly similar study is carried out with 1,2-bis(trimethylsilyl)-3, 3- dimethylcyclotrisilene instead of 1,2,3,3-tetramethyl cyclotrisilene. This corroborative study manifested the effect of substituents on the cyclotrisilene ring on the four reaction pathways subjected to analysis.