The addition mechanism of TMSCN into N-acetylpyridinum to give 2-cyanoacetylpyridinum: The roles of TpW(NO)(PMe3) and DABCO

The addition of trimethylsilylnitrile (TMSCN) into N-acetylpyridinum (NAP) to give 2-cyanoacetylpyridinum (NAP-CN) was investigated using density functional theory (DFT). Comparative studies on four computational models (Model I: with both TpW(NO)(PMe3) ([W]) and DABCO; Model II: with TpW(NO)(PMe3) only; Model III: with DABCO only; and Model IV: without TpW(NO)(PMe3) and DABCO) indicate that this reaction is most possible to undergo the coordination of TpW(NO)(PMe3) into NAP to form TpW(NO)(PMe3)(η2-NAP) (NAP-[W]), the nucleophilic attack of TMSCN into NAP-[W] to give TMSCN-NAP-[W] 3a, the dissociation of trimethylsilyl (TMS) from 3a to give [W]-NAP-NC 5a, the isomerization of 5a into [W]-NAP-CN 7, and the release of TpW(NO)(PMe3) from 7 to give the final product NAP-CN (1 → NAP-[W] → TS2a → 3a→ MI-4a → 5a → 7 → 8). The presence of TpW(NO)(PMe3) promotes the nucleophilic attack of TMSCN into NAP via enhancing positive electricity of C2 atom of NAP, while the presence of DABCO restrains this nucleophilic process via competitively reacting with NAP. In the dissociation step of TMS, TpW(NO)(PMe3) plays a restrainer role, whereas DABCO improves this step via neutralizing and stabilizing TMS. These calculated results are in good agreement with experimental findings.

The HOMO of N1 atom in TMSCN is dumbbell-like with two different phases, while HOMO of C1 atom in TMSNC is spherical with one same phase. When TMSCN or TMSNC is vertically close to C2 atom of acetylpyridinum, the HOMO of TMSCN with two different phases will overlap with the LUMO of acetylpyridinum to form a bonding orbital and an antibonding orbital, and it is difficult to form a σ bond; the HOMO of TMSNC with a same phase is easier to form a σ bond with C2 atom of acetylpyridinum in the way of head-to-head overlap.Figure optionsDownload as PowerPoint slideHighlights
► The addition mechanism of TMSCN into acetylpyridinum (NAP) was outlined.
► The attack of TMSCN into NAP is promoted by TpW(NO)(PMe3) and restrained by DABCO.
► The dissociation of TMS is restrained by TpW(NO)(PMe3) and improved by DABCO.