Computational Investigation on the Role of Disilene Substituents Toward N<sub>2</sub>O Activation

2016-12-07T00:00:00Z (GMT) by Bholanath Maity Debasis Koley
The effect of substituents in disilene mediated N<sub>2</sub>O activation was studied at the M06-2X/QZVP//ωB97xD/TZVP level of theory. The relationship between structural diversity and the corresponding reactivity of six disilenes (<b>I</b><sub><b>A–F</b></sub><sup><b>t</b></sup>) in the presence of four different substituents (−NMe<sub>2</sub>, −Cl, −Me, −SiMe<sub>3</sub>) is addressed in this investigation. We primarily propose two plausible mechanistic routes: Pathway I featuring disilene → silylene decomposition followed by N<sub>2</sub>O coordination and Pathway II constituting the N<sub>2</sub>O attack without Si–Si bond cleavage. Depending on the fashion of N<sub>2</sub>O approach the latter route was further differentiated into Pathway IIa and Pathway IIb detailing the “end-on” and “side-on” attack to the disilene scaffold. Interestingly, the lone pair containing substituents (−NMe<sub>2</sub>, −Cl,) facilitates disilene → silylene dissociation; on the contrary it reduces the electrophilicity at Si center in silylene, a feature manifested with higher activation barrier during N<sub>2</sub>O attack. In the absence of any lone-pair influence from substituents (−Me, −SiMe<sub>3</sub>), the decomposition of disilenes is considerably endothermic. Therefore, Pathway I appears to be the less preferred route for both types of substituents. In Pathway IIa, the N<sub>2</sub>O moiety uniformly approaches via O-end to both the silicon centers in disilenes. However, the calculations reveal that Pathway IIa, although not operational for all disilenes, is unlikely to be a viable route due to the predominantly higher transition barrier (<i>ca</i>. 36 kcal/mol). The most feasible route in this current study accompanying moderately low activation barriers (∼19–26 kcal/mol) is Pathway IIb, which involves successive addition of two N<sub>2</sub>O units proceeding via terminal N, O toward the Si centers and is applicable for all disilenes. The reactivity of substituted disilenes can be estimated in terms of the first activation barrier of N<sub>2</sub>O attack. Surprisingly, in Pathway IIb, the initial activation barrier and hence the reactivity shows negligible correlation with Si–Si bond strength, indicating toward the versatility of the reaction route.