A phosphetene is an unsaturated four-membered organophosphorus heterocycle containing one phosphorus atom. It is a heavier analog of an azetine, or dihydroazete. The first synthesis of a stable, isolable phosphetene was reported in 1985 via ring expansion of a phosphirene-metal carbonyl complex. Other synthesis routes include cyclization of phosphabutadienes, [2 + 2] cycloaddition, intramolecular arrangement, addition, and through organometallic intermediates. The latter is of interest to the application, where organometallic intermediate synthesis led to colored phosphetene compounds suitable for incorporation in OLED devices. Phosphetenes can also participate in reactions involving the lone pair of electrons at the phosphorus ring opening, or ring expansion.
Nomenclature According to the extended Hantzsch-Widman system of naming heterocyclic parent hydrides, the saturated four-membered ring containing one phosphorus atom is called a phosphetane. The presence of one double bond yields a phosphetene. Alternately, it can be called a dihydrophosphete, as a phosphete is the cyclobutadiene-like structure containing two double bonds. Different constitutional isomers are possible, depending on whether or not the double bond is attached to the phosphorus. The phosphorus is assigned position 1 on the ring, and the other structural features are numbered relative to it. They can be distinguished based on where the double bond is in the phosphetene (Δ1-phosphetene vs Δ2-phosphetene) or where the double bond is missing as compared with the phosphete (1,2-dihydrophosphete vs 3,4-dihydrophosphete, the latter also called 2,3-dihydrophosphete).
Synthesis
Via ring expansion Isolation of the first phosphetene was reported by Mathey et al. in 1985 by the thermolysis of phosphirene-metal pentacarbonyl complexes in the presence of carbon monoxide. This resulted in a 34% yield of a solid orange phosphorus analogue of unsaturated β-lactams, where the phosphorus atom was coordinated to the metal pentacarbonyl complex. The team decomplexed the metal pentacarbonyl complex from the phosphetene, but oxidation at the phosphorus takes place spontaneously, resulting in a λ5σ4 1,2-dihydrophosphete oxide solid with a yellow color.
The reaction of phosphatriafulvenes with azides resulted in the ring expansion into 1H-2-iminophosphetes. However, in the presence of excess azide, the Staudinger reaction can take place, which transforms the yellow λ3σ3 1H-2-iminophosphete into a λ5σ4 iminophosphorane product.
A 1,2-dihydro-1-phosphet-2-one complex can be obtained, in a mixture with other compounds, from the reaction of phosphirene-metal complexes with diethylamine.
Phosphabutadiene cyclization Ring formation from phosphabutadienes was observed via 1,2-dihydrophosphetes from the reaction of 1-[bis(trimethylsilyl)amino]phosphadiene with Me3SiN3 or elemental sulfur through a 3-coordinate (methylene)phosphorane intermediate.
Reacting halogenated ylides with AlCl3 forms dihydrophosphetium salts in an intramolecular cyclization reaction. Aromaticity is restored by subsequent reaction with pyridine, then a strong base, sodium bis(trimethylsilyl)amide, to form a neutral λ5-phosphete with a highly polarized P=C bond.
An air-stable, colorless, isolable 1,2-dihydrophosphete intermediate was discovered in 1993 during the synthesis of phosphaalkynes from phosphalkenes.
In 1997 researchers synthesized a η1-3,4-dihydrophosphete ligand complexed to metal pentacarbonyl from η1-2-phosphabutadiene complexes, which was in turn synthesized from the reaction of metal carbene complexes with a chlorophosphane.
[2 + 2] cycloaddition reactions The formation of 1,2-dihydrophosphetes from [2 + 2] cycloaddition reactions involves the reaction of metal phosphaalkane complexes with alkenes or alkynes.
Intramolecular rearrangement Niecke et al. (1994) reacted three equivalents of iminophosphoranes with a diyne, which resulted in a 1,2-dihydrophosphete attached to a diphosphole ring, with the formation of intermediate diphosphetene compounds.
In 1995, 3,4-dihydrophosphetes were obtained from the reaction of 1,3-diphosphetes with CS2, COS, or CO2.
Addition reaction The 2,4-diphosphoniodihydrophosphetide cation is an unusually stable synthetic intermediate, produced from the addition reaction of 1,3-diphosphoniopropenide with chlorophosphines.
Organometallic intermediates The synthesis of 1,2-dihydrophosphetes in 1989 from diphenyltitanacyclobutene (prepared from Tebbe's reagent) and dichlorophenylphosphine was remarkable due to the clean synthesis and workup and the resulting white 1,2-dihydrophosphetes were inert toward oxidation and in high yield (66%). Tebbe's reagent was used to prepare a novel bidentate ligand consisting of phosphorus heterocycles of 1-phosphinine-1,2-dihydrophosphetes.
Majoral et al. (1997) were able to synthesize a 1,2-dihydrophosphete-zirconium complex from intramolecular coupling of dialkynyl phosphane and zirconocene-benzyne. The complex was then treated with an acid to yield a π-extended dihydrophosphetes. This method was applied by Hissler et al. in 2014 to observe the optical and redox properties of π-extended dihydrophosphetes. With the introduction of various electron-rich substituents on the π-backbone, the dihydrophosphetes displayed shifting of color in the visible region varying from blue to green, which was tested in a multilayer OLED device. As of 2023, Cummins et al. have been able to synthesize free, uncomplexed phosphet-2-ones with high yield using a phosphinidene transfer agent derived from anthracene. The reactivity of the phosphet-2-ones with stabilized Wittig reagents at 100 °C led to a high yield of 1,2-dihydrophosphete products with a backbone structure that resembles Hissler's polyunsaturated dihydrophosphetes used in the OLED devices.
Reactivity
Reactivity at the phosphorus atom Phosphetenes with a lone pair at the phosphorus atom behaved similarly to a two-electron P-donor, such as the ability to coordinate with metals. Structural changes in phosphetene metal complexes are consistent with an increased s-character of the phosphorus.
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