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

Reactions of the N-aryl­(diphenylphosphanyl)­aminosilane Ph2PN­(Ar)­SiCl3–nMen (Ar = 2,4,6-Me3C6H2, n = 0 (1a), 1 (2a), 2 (3a), 3 (4a); Ar = 2,6-iPr2C6H3, n = 0 (1b), 1 (2b), 2 (3b), 3 (4b)) with methyl propiolate and dimethyl acetylenedicarboxylate (DMAD) give two types of products, the zwitterionic heterocycles [Ph2PN­(2,4,6-Me3C6H2)­SiCl3]­(HCCCO2Me) (5c) and [Ph2PN­(Ar)­SiCl3–nMen]­(MeO2CCCCO2Me) (Ar = 2,4,6-Me3C6H2, n = 0 (5a), 1 (6a), 2 (7a); Ar = 2,6-iPr2C6H3, n = 0 (5b), 1 (6b), 2 (7b)) and (Z)-silyliminophosphoranylalkene ArNP­(Ph2)­C­(CO2Me)C­(CO2Me)­SiMe3 (Ar = 2,4,6-Me3C6H2 (8a), 2,6-iPr2C6H3 (8b)). The reaction of Ph2PN­(SiMe3)2 with DMAD gives only the acyclic alkene Me3SiNP­(Ph2)­(MeO2C)­CC­(CO2Me)­SiMe3 (9), which is similar to 8a,b. In these reactions, compounds 1a–4a and 1b–4b 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 (4a,b), the heterocyclic intermediates undergo ring opening by Si–N bond cleavage and concomitant NP bond formation resulting in 8a,b. The formation of the acyclic (Z)-alkene (8a,b and 9) can be considered as a stepwise SN2 reaction at the silicon center.