Carbodiphosphorane-Stabilized Parent Dioxophosphorane: A Valuable Synthetic HO2P Source

Introducing a small phosphorus-based fragment into other molecular entities via, for example, phosphorylation/phosphonylation is an important process in synthetic chemistry. One of the approaches to achieve this is by trapping and subsequently releasing extremely reactive phosphorus-based molecules such as dioxophosphoranes. In this work, electron-rich hexaphenylcarbodiphosphorane (CDP) was used to stabilize the least thermodynamically favorable isomer of HO₂P to yield monomeric CDP·PHO₂. The title compound was observed to be a quite versatile phosphonylating agent; that is, it showed a great ability to transfer, for the first time, the HPO₂ fragment to a number of substrates such as alcohols, amines, carboxylic acids, and water. Several phosphorous-based compounds that were generated using this synthetic approach were also isolated and characterized for the first time. According to the initial computational studies, the addition-elimination pathway was significantly more favorable than the corresponding elimination-addition route for "delivering" the HO₂P unit in these reactions.

Probing the Structural and Electronic Effects on the Origin of π-Facial Stereoselectivity in 1-Methylphosphole 1-Oxide Cycloadditions and Cyclodimerization

We have examined the π-facial stereoselectivity in the Diels-Alder reactions of phosphole oxides computationally. The experimentally observed syn-cycloadditions have been rationalized with the Cieplak model and distortion-interaction model. The natural bond orbital analysis suggests that the hyperconjugative interactions are energetically preferred between the antiperiplanar methyl group present in the -P=O unit and the developing incipient (-C-C-) bond in syn-adducts in accordance with the Cieplak model. The distortion-interaction analysis carried out for syn and anti transition states of Diels-Alder reactions of 1-substituted phosphole 1-oxide with different dienophiles reveals that the syn selectivity is favored by distortions and interaction energies compared with the anti selectivity. The formation of a syn adduct is also stabilized by the π_CC-σ*_PO orbital interaction, and the repulsive n-π interaction destabilizes the anti adduct that leads to the 7.0 kcal/mol thermodynamic preference for the former adduct. Furthermore, the distortion-interaction model rationalizes the formation of stereospecific products in these Diels-Alder reactions, which however is not explicable with the much-debated Cieplak model.