Spiro[1,2]oxaphosphetanes of Nonstabilized and Semistabilized Phosphorus Ylide Derivatives: Synthesis and Kinetic and Computational Study of Their Thermolysis.

A series of tri- and tetrasubstituted spiro-oxaphosphetanes stabilized by ortho -benzamide ( o BA) and N -methyl ortho -benzamide (M o BA) ligands have been synthesized by the reaction of C _α ,C _ortho -dilithiated phosphazenes with aldehydes and ketones. They include enantiopure products and the first example of an isolated oxaphosphetane having a phenyl substituent at C3 of the ring. Kinetic studies of their thermal decomposition showed that the process takes place irreversibly through a polar transition state (ρ = -0.22) under the influence of electronic, [1,2], [1,3] steric, and solvent effects, with C3/P-[1,2] interactions as the largest contribution to Δ G ^⧧ of olefination. Inversion of the phosphorus configuration through stereomutation has been observed in a number of cases. DFT calculations showed that o BA derivatives olefinated through the isolated (N, O)(Ph, C ₆ H ₄ , C) oxaphosphetanes (Channel A), whereas M o BA compounds decomposed faster via the isomer (C ₆ H ₄ , O)(C, N, Ph) formed by P-stereomutation involving a M _B2 permutational mechanism (Channel B). The energy barrier of P-isomerization is lower than that of olefination. Fragmentation takes place in a concerted asynchronous reaction. The thermal stability of oxaphosphetanes is determined by strong C3/P-[1,2] interactions destabilizing the transition state of olefination. The effect of charge distribution and C3/C4-[1,2] and C4/P-[1,3] steric and solvent interactions on Δ G ^⧧ was also evaluated.