The compounds [CH₂(6-t-Bu-4-Me-C₆H₂O)₂]PCl (1), (OCH₂CMe₂CH₂O)-PCl (2), and [ClPN(t-Bu)]₂ (3) have been utilized as precursors in the synthesis of (i) new pentacoordinate phosphorus compounds [e.g. CH₂(6-t-Bu-4-Me-C₆H₂O)₂ P(NRR′)(O₂C₆C1₄), CH₂(6-t-Bu-4-Me-C₆H₂O)₂PX[OC(O-i-Pr)N=N(C(O)O-i-Pr)],(ii) cyclic phosphates and their complexes [e.g. imidazolium⁺CH₂(6-t-Bu-4-Me-C₆H₂O)₂PO₂^-.MeOH], (iii) new cycloaddition products [e.g. CH₂(6-t-Bu-4-Me-C₆H₂O)₂PC(CO₂Me)C(CO₂Me)C(O)N, (iv) macrocyclic compounds [e.g. [(t-BuN)P]₂[-OCH₂CMe₂CH₂O-]_h2] and (v) phosphonates [e.g. (OCH₂CMe₂CH₂O)P (O)CH₂C(CN)=CHC₅H₄FeC₅H₅]. The synthetic and structural aspects of these new products are discussed.

The diversity of products in the reaction of diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD) and activated acetylenes with P^III compounds bearing oxygen or nitrogen substituents is discussed. New findings that are useful in understanding the nature of intermediates involved in the Mitsunobu reaction are highlighted. X-ray structures of two new compounds (2-t-Bu-4-MeC₆H₃O)P (μ-N-t-Bu)₂P⁺[(NH-t-Bu)N[(CO₂]-i-Pr)(HNCO₂-i-Pr)]](Cl^-)(2-t-Bu-4-MeC₆H₃OH)(23)and [CH₂(6-t-Bu-4-Me-C₆H₂O)₂P(O)C(CO₂Me)C-(CO₂Me)CClNC(O)Cl] (33) are also reported. The structure of23 is close to one of the intermediates proposed in the Mitsunobu reaction.

Synthesis, structures and stability (thermal and air) of allenylphosphonates of the type (OCH₂CMe₂CH₂O)P(O)C(R)=C=CR'R" and [R"R'C=C=C(R)P(O)(OCH₂)₂]₂C are discussed. Thermally activated dimerization (cycloaddition) of (OCH₂CMe₂CH₂O)P(O)C(H)=C=CMe₂ leads to the phosphonocyclobutane [(OCH₂CMe₂CH₂O)P(O)C(H)=]₂[C-CMe₂]₂ (18). Many of these allenes undergo addition of diethylamine to lead to enaminophosphonates that are readily hydrolysed by water to lead to 𝛽-ketophosphonates. The latter compounds are useful as Horner-Wadsworth-Emmons (HWE) reagents. Molecular structures of (OCH₂CMe₂CH₂O)P(O)C(CH₂OH)=C=CH₂ (6), [H₂C=C=C(H)P(O)(OCH₂)₂]₂C (9), (OCH₂CMe₂CH₂O)P(O)CH=C=CH₂ (12), [(OCH₂CMe₂CH₂O)P(O)C(H)=]₂[C-CMe₂]₂ (18), and the 𝛽-ketophosphonate (OCH₂CMe₂CH₂O)P(O)CH₂)-C(O)CHMe₂ (24) have been determined. In compound 6, intermolecular hydrogen bonding between the phosphoryl oxygen and the hydroxyl group leading to an infinite chain is observed. In 6, there is a significant deviation (ca 7°) from the orthogonality expected between the planes containing 4 atoms of (a) H₂C=C and (b) C=C(H)P in the allene part. In 9, weak C-H…O interaction between the phosphoryl oxygen atom and a CH₂ proton of the six-membered ring is present.