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 (oBA) and N-methyl ortho-benzamide (MoBA) ligands have been synthesized by the reaction of Cα,Cortho-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 ΔG⧧ of olefination. Inversion
of the phosphorus configuration through stereomutation has been observed
in a number of cases. DFT calculations showed that oBA derivatives olefinated through the isolated (N, O)­(Ph, C6H4, C) oxaphosphetanes (Channel A), whereas MoBA compounds decomposed faster via the isomer (C6H4, O)­(C, N, Ph) formed by P-stereomutation involving a MB2 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 ΔG⧧ was also evaluated.