A Masked Boryl-Substituted Oxo-Bridged Bis-Silylene: Synthesis and Reductive-Elimination and Synergistic Oxidative-Addition Reactivity

Controlled oxidation of NHB-stabilized disilyne (NHB)Si ≡ Si(NHB) (1, NHB = [ArN(CMe)₂NAr]B, Ar = 2,6-iPr₂C₆H₃) with one equivalent of trimethylamine N-oxide (Me₃N⁺─O^-) in dry n-hexane gave oxo-bridged bis-silepin 2 in high yields. DFT calculations disclosed that silepin 2 is only more stable by 13.4 kcal/mol than the corresponding oxo-bridged bis-silylene intermediate 2' (NHB)Si(μ-O)Si(NHB), and 2 was very likely to be formed by the insertion of the two divalent Si atoms into the pendant aryl rings in bis-silylene intermediate 2'. The two silicon atoms in bis-silepin 2 could undergo formal reductive-elimination of the aryl rings and sequential oxidative-insertion reactions with small molecules and organic substrates. Treatment of 2 with H₂O, S₈, and P₄ at 60 °C yielded compounds 3-5 via reductive-elimination of the aryl rings, followed by the sequential oxidative-addition of these molecules at the two Si(II) centers. Similarly, reactions of 2 with PhSiH₃, a diphenylalkyne, pyridines, 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe₄), Ph₂CO, and thiophene yielded the corresponding polycyclic bis-silanes 6-12 via reductive-elimination and oxidative-addition of C-H, Si-H, C≡C, and aromatic C═C, C-S, and C═N bonds at the two Si atoms. These novel reactions indicated the pronounced bis-silylene reactivity of bis-silepin 2, consistent with the low-energy barrier for the interconversion between 2 and 2', as disclosed by DFT calculations.