Mechanistic Insights Into the [3+1] Cycloadditions of Me3SiN3 on Bis-Silylene and the Formation of Pseudo-Silaazatriene

Abstract

The reaction mechanism for the formation of the pseudo-silaazatriene 1 (LN(SiMe₃)₂−Si−N−Si(L)NSiMe₃; L=PhC−(N^tBu)₂) by the reaction of 1 equivalent of bis-silylene (LSi−SiL; L=PhC(N^tBu)₂) with 3 equivalents of azide Me₃SiN₃ has been explored at M06/def2-TZVPP//BP86-D3(BJ)/def2-TZVPP level of theory. The reaction mechanism is guided by the high Lewis basicity of the silicon lone pair and the high Lewis acidic character of Si−N bonds, as well as by the high tendency to eliminate N₂ from Me₃SiN₃. The first step of the reaction is the formation of [3+1] cycloaddition product (I1) by the synergetic interactions of the lone pair on the silylene silicon with the π* molecular orbital of Me₃SiN₃ which is majorly localized on the terminal nitrogen atom, and the donation of the σ-type lone pair on the nitrogen atom connected to the SiMe₃ group with the Si−N σ* orbital on silylene. The elimination of N₂ from I1 results in the formation of pseudo-silaimine intermediate I2 having a dicoordinated, monovalent nitrogen atom. The most favourable pathway involves the second addition of Me₃SiN₃ followed by N₂ elimination resulting bis-silaimine intermediate I4-P2. The highly reactive lone pair on the dicoordinated, monovalent nitrogen atom can be coordinated to the Si−N σ* MO of the second silylene moiety resulting in Si−N bond formation and Si−Si bond cleavage. This step is similar to the S_N2′ reaction where the incoming nucleophile and the leaving group are on the same side resulting in the formation of silaimine I5. Further, the [3+1] cycloaddition of one more molecule of Me₃SiN₃ followed by N₂ elimination results in the formation of silaimine intermediate I7. I7 undergoes 1,3-silyl migration leading to the final product 1. The lone pairs on di- and tricoordinated nitrogen centres of all the intermediates are stabilized by hyperconjugative interactions. The delocalized hyperconjugative interaction stabilizes the lone pairs in 1.