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(diphenylphospha:nyl)aminosilane Ph2PN(Ar)SiCl3-nMen (Ar = 2,4,6-Me3C6H2, n = 0 (la), 1 (2a), 2 (3a), 3 (4a); Ar = 2,6-iPr(2)C(6)H(3), n = 0 (lb), 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) (Sc) and [Ph2PN(Ar)SiCl3-nMen](MeO2CC=CCO2Me) (Ar = 2,4,6-Me3C6H2, n = 0 (5a), 1 (6a), 2 (7a); Ar = 2,6-iPr(2)C(6)H(3), n = 0 (5b), 1 (6b), 2 (7b)) and (Z)-silyliminophosphoranylalkene ArN=P(Ph-2)C(CO2Me)=C(CO2Me)SiMe3 (Ar = 2,4,6-Me3C6H2 (8a), 2,6-iPr(2)C(6)H(3) (8b)). The reaction of Ph2PN(SiMe3)(2) with DMAD gives only the acyclic alkene Me3SiN=P(Ph-2)(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 cydoaddition reaction with the alkyne. The theoretical calculations indicate that the reactions proceed through a concerted cydoaddition 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 S(N)2 reaction at the silicon center.