Phosphetanes are a well understood class of molecules that have found broad applications as chemical building blocks, reagents for organic/inorganic synthesis, and ligands in coordination chemistry.

The general scheme for phosphetane synthesis from mono-enes is given below: In the case of electrophilic addition by a diene,[5][6] carbocation rearrangement first occurs via cation-π cyclization.

The synthesis of phosphetanes from P(III) alkylation and subsequent cyclization usually proceeds through sequential phosphanide/phosphine displacement of 1,3-alkyl dihalides or sulfonate esters (OTf, OTs, OMs, etc.).

Although relieving the cyclopropane ring strain is of great assistance in the initial C-P bond, exhaustive alkyl substitution to stabilize the formed carbocation is often required.

[14] An example of each, and the mechanism, are seen below: Experimental and crystallographic data exists for many of the phosphetane types listed below, however, all of the geometric and electronic (HOMO and LUMO) information below was determined theoretically with the B3LYP functional[15][16][17][18] and DEF2-SVP basis set[19] using ORCA (5.0.3)[20] for the parent molecule at each coordination number to provide a general and consistent trend as an introduction to the subject.

[21] Though rarely reported in the literature, if at all, dicoordinate phosphetanes of phosphenium, phosphanide, and phosphorus radical archetypes are theoretically possible as transient reactive intermediates.

In this ion, there is significantly more pucker within the phosphetane ring, along with widening of the C-P-C angle, but the HOMO and HOMO-1 are similar to the radical case, now both being doubly occupied.

Conformational isomerism is introduced in these tricoordinate molecules, albeit with a very low kinetic barrier (~2.45 kcal/mol for the given example),[22] in which the hydrogen can be pseudo-axial (as shown), or pseudo-equatorial.

The acidity of the α-carbon hydrogens is significantly increased due to the charge neutralization driving force; this is reflected in the C-H σ-antibonding contributions to the LUMO.

[23] As one may expect of a covalently bound oxide, the HOMO is an oxygen lone pair and the LUMO is largely contributed to by the P-O π-antibonding interaction.

A result of this geometric perturbation is the emergence of a P-H σ-antibonding that is represented prominently in the LUMO, accounting for the characteristic Lewis acidity of square pyramidal phosphoranes.

Hexacoordinate, anionic phosphates are mainly known in the literature as counterions (hexafluorophosphate), but are theoretically possible as reactive intermediates for associative mechanisms at phosphorus centers.

Similar to the dicoordinate case, these optimized physical and electronic geometries are presented mainly as a means of comparison to the more commonly observed tri, tetra, and pentacoordinate phosphetanes.Phoshetanes display a broad range of reactivity and appear in the literature in many different facets of a chemical reaction.

[27] One intentional and constructive method of ring-opening has been outlined in the literature and features a phosphetane ylide that undergoes Wittig reactivity with aldehydes to form γ-unsaturated phosphine oxides.

Insertion of oxygen into the P-C bond of a phosphetane oxide is done with mCPBA and proceeds via a currently unknown mechanism with unusually high regioselectivity for the less substituted carbon.

All of these processes include the in-situ formation and decomposition of oxaphosphetane intermediates through metathesis-type pathways to form alkenes or alkynes from aldehydes and a phosphorus reagent.

The uncharacteristic biphilic nature of these phosphines, and other non-trigonal pnictogen compounds, is a result of molecular symmetry perturbation,[51] in this case, imposed by the ring strain inherent to phospetanes.

Most of these transformations are probed based on stoichiometric reactivity of the phosphetane, illustrating their utility as catalysts or reagents in the event there is substrate incompatibility with the hydride.