Between Oxirane and Phosphirane: The Spring‐loaded Oxaphosphirane Ring

Abstract

Dedicated to Professor Christoph Janiak on the occasion of his 60th birthday. Due to the potential interest of oxaphosphiranes in ringopening polymerizations, accurate ring strain energies (RSEs) of a wide variety of oxaphosphirane derivatives was computed, after validation of the optimization method by comparison with reported X-ray structures. The parent oxaphosphirane exhibits a moderate RSE (22.44 kcal/mol) that is significantly enhanced by k-P-complexation (especially with boranes), the introduction of P=O or P +-Me groups as well as exocyclic double bonds at the ring carbon, such as 3-methylene, 3-imino and 3-oxo functionalities. However, C3 alkyl substitution or pentacoordination at P in σ 5 λ 5-oxaphosphirane does not lead to significant variation of RSE. Bicyclic spiro-oxaphosphirane derivatives show an RSE decrease when the size of the spiro ring increases. A moderate linear correlation between RSE and G(r)/1(r) values calculated at the ring critical points and also with the relaxed force constant (k 0) for the PÀ C bond is observed for most oxaphosphiranes. The possibility of ring-opening polymerization by using better (anionic) nucleophiles in the initiation step can be foreseen from the exergonicity and relatively low barrier of the endocyclic CÀ O cleavage by nucleophilic attack of methanol, thus underlining the effect of the RSE as driving force compared to acyclic analogs.

Accurate Ring Strain Energies of Unsaturated Three-Membered Heterocycles with One Group 13–16 Element

“…Ring strain is one of the most remarkable aspects of small rings as it is closely linked to chemical reactivity and has been found to affect properties such as the stereochemical stability of σ 3 λ 3 -pnictogen ring atoms . It frames the driving force for a ring to be transformed into open-chain products . RSEs were calculated using homodesmotic reactions (reaction class 4 or “RC4”), which constitute the penultimate type in a hierarchy of increasingly accurate processes, due to the conservation of larger fragments, according to a recent classification and redefinition of reaction types used in thermochemistry .…”

“…RSEs were calculated using homodesmotic reactions (reaction class 4 or “RC4”), which constitute the penultimate type in a hierarchy of increasingly accurate processes, due to the conservation of larger fragments, according to a recent classification and redefinition of reaction types used in thermochemistry . Good estimates for RSE concerning saturated ,, and unsaturated , three-membered heterocycles were obtained according to this type of thermochemical reaction schemes. The highest ranked hyperhomodesmotic reactions (reaction class 5 or “RC5”) were skipped because they have been shown to give rise to very similar RSE data to RC4, but being somewhat problematic in some cases by introducing undesired noncompensated interactions due to the presence of longer chains.…”

“…30 It frames the driving force for a ring to be transformed into openchain products. 31 RSEs were calculated using homodesmotic reactions (reaction class 4 or "RC4"), which constitute the penultimate type in a hierarchy of increasingly accurate processes, due to the conservation of larger fragments, according to a recent classification and redefinition of reaction types used in thermochemistry. 32 Good estimates for RSE concerning saturated 25,31,33 and unsaturated 12,34 three-membered heterocycles were obtained according to this type of thermochemical reaction schemes.…”

Stability of Carbocyclic Phosphinyl Radicals: Effect of Ring Size, Delocalization, and Sterics

“…As an exception, in the three-membered radical 22 , practically no delocalization energy can be obtained. It is known that the exocyclic CC bond results in a significant destabilization of the three-membered heterocycles . Thus, the deminishing Δ E deloc for radical 22 is most likely the result of the opposing effects of stabilizing π-delocalization (as supported by an NBO analysis) and destabilizing strain.…”

“…It is known that the exocyclic C�C bond results in a significant destabilization of the threemembered heterocycles. 61 Thus, the deminishing ΔE deloc for radical 22 is most likely the result of the opposing effects of stabilizing π-delocalization (as supported by an NBO analysis) and destabilizing strain. Compared to radical 2, the delocalization energy is remarkably larger for the four-, five-, six-, and seven-membered exocyclic radicals with saturated bridges (23−26).…”