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.

本研究通过Cα位及邻位双锂化的磷腈类化合物与醛、酮的反应，合成了一系列由邻苯甲酰胺（ortho-benzamide, oBA）和N-甲基邻苯甲酰胺（N-methyl ortho-benzamide, MoBA）配体稳定的三取代与四取代螺氧磷杂环丁烷（spiro-oxaphosphetanes）。该系列产物涵盖对映纯化合物，以及首例分离得到的、环C3位带有苯基取代基的氧磷杂环丁烷（oxaphosphetane）。对其热分解过程的动力学研究显示，该反应在电子效应、[1,2]与[1,3]位阻效应及溶剂效应的共同作用下，经极性过渡态（ρ = -0.22）不可逆进行，其中C3/P-[1,2]相互作用对烯化反应的吉布斯自由能活化能ΔG⧧的贡献最大。多例体系中均观察到通过立体异构化实现的磷原子构型翻转现象。密度泛函理论（DFT）计算表明，oBA衍生物通过分离得到的(N,O)(Ph,C6H4,C)型氧磷杂环丁烷（路径A）发生烯化反应；而MoBA类化合物则经由通过MB2重排机制实现磷原子立体异构化所生成的(C6H4,O)(C,N,Ph)型异构体，以更快的速率分解（路径B）。磷原子异构化的能垒低于烯化反应的能垒。裂解过程以协同非同步反应机制进行。氧磷杂环丁烷的热稳定性由强C3/P-[1,2]相互作用决定，该相互作用会使烯化反应的过渡态发生失稳。电荷分布以及C3/C4-[1,2]、C4/P-[1,3]位阻与溶剂相互作用对吉布斯自由能活化能ΔG⧧的影响也得到了考察。