Fine Tuning of the Substituents on the N‑Geminal Phosphorus/Silicon-Based Lewis Pairs for the Synthesis of <i>Z</i>‑Type Silyliminophosphoranylalkenes

Reactions of the <i>N</i>-aryl­(diphenylphosphanyl)­aminosilane Ph₂PN­(Ar)­SiCl_3–<i>n</i>Me_<i>n</i> (Ar = 2,4,6-Me₃C₆H₂, <i>n</i> = 0 (<b>1a</b>), 1 (<b>2a</b>), 2 (<b>3a</b>), 3 (<b>4a</b>); Ar = 2,6-<i>i</i>Pr₂C₆H₃, <i>n</i> = 0 (1<b>b</b>), 1 (<b>2b</b>), 2 (<b>3b</b>), 3 (<b>4b</b>)) with methyl propiolate and dimethyl acetylenedicarboxylate (DMAD) give two types of products, the zwitterionic heterocycles [Ph₂PN­(2,4,6-Me₃C₆H₂)­SiCl₃]­(HCCCO₂Me) (<b>5c</b>) and [Ph₂PN­(Ar)­SiCl_3–<i>n</i>Me_<i>n</i>]­(MeO₂CCCCO₂Me) (Ar = 2,4,6-Me₃C₆H₂, <i>n</i> = 0 (<b>5a</b>), 1 (<b>6a</b>), 2 (<b>7a</b>); Ar = 2,6-<i>i</i>Pr₂C₆H₃, <i>n</i> = 0 (<b>5b</b>), 1 (<b>6b</b>), 2 (<b>7b</b>)) and (<i>Z</i>)-silyliminophosphoranylalkene ArNP­(Ph₂)­C­(CO₂Me)C­(CO₂Me)­SiMe₃ (Ar = 2,4,6-Me₃C₆H₂ (<b>8a</b>), 2,6-<i>i</i>Pr₂C₆H₃ (<b>8b</b>)). The reaction of Ph₂PN­(SiMe₃)₂ with DMAD gives only the acyclic alkene Me₃SiNP­(Ph₂)­(MeO₂C)­CC­(CO₂Me)­SiMe₃ (<b>9</b>), which is similar to <b>8a</b>,<b>b</b>. In these reactions, compounds <b>1a</b>–<b>4a</b> and <b>1b</b>–<b>4b</b> behave as N-geminal P/Si-based Lewis pairs, which undergo a dipolar cycloaddition reaction with the alkyne. The theoretical calculations indicate that the reactions proceed through a concerted cycloaddition reaction mechanism. The stability of these heterocycles decreases as the number of the Me substituent on the pentacoordinated Si atom increases. When the Si center is substituted with three Me groups (<b>4a</b>,<b>b</b>), the heterocyclic intermediates undergo ring opening by Si–N bond cleavage and concomitant NP bond formation resulting in <b>8a</b>,<b>b</b>. The formation of the acyclic (<i>Z</i>)-alkene (<b>8a</b>,<b>b</b> and <b>9</b>) can be considered as a stepwise S_N2 reaction at the silicon center