Inchemistry, aphosphaalkyne (IUPAC name:alkylidynephosphane) is anorganophosphorus compound containing atriple bond betweenphosphorus andcarbon with the generalchemical formulaR−C≡P.^[2] Phosphaalkynes are the heavier congeners ofnitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent ofalkynes.^[3] Due to their high reactivity, phosphaalkynes are not found naturally on earth, but the simplest phosphaalkyne,phosphaethyne (H−C≡P) has been observed in the interstellar medium.^[4]

The first of preparation of a phosphaalkyne was achieved in 1961 when Thurman Gier produced phosphaethyne by passingphosphine gas at low pressure over an electric arc produced between two carbon electrodes. Condensation of the gaseous products in a −196 °C (−321 °F) trap revealed that the reaction had produced acetylene, ethylene, andphosphaethyne, which were identified byinfrared spectroscopy.^[5]

Following the initial synthesis of phosphaethyne, it was realized that the same compound can be prepared more expeditiously via the flashpyrolysis ofmethyldichlorophosphine (CH₃PCl₂), resulting in the loss of two equivalents ofhydrogen chloride. This methodology has been utilized to synthesize numerous substituted phosphaalkynes, including the methyl,^[6] vinyl,^[7] chloride,^[2] and fluoride^[8] derivatives. Fluoromethylidynephosphane (F−C≡P) can also be prepared via the potassium hydroxide promoted dehydrofluorination of trifluoromethylphosphine (CF₃PH₂). It is speculated that these reactions generally proceed via an intermediate phosphaethylene with general structure RClC=PH. This hypothesis has found experimental support in the observation ofF₂C=PH by³¹PNMR spectroscopy during the synthesis ofF−C≡P.^[9]

The high strength ofsilicon–halogen bonds can be leveraged toward the synthesis of phosphaalkynes. Heatingbis-trimethylsilylated methyldichlorophosphines (((CH₃)₃Si)₂C(R)−PCl₂) under vacuum results in the expulsion of two equivalents ofchlorotrimethylsilane and the ultimate formation of a new phosphaalkyne. This synthetic strategy has been applied in the synthesis of 2-phenylphosphaacetylene^[10] and 2-trimethylsilylphosphaacetylene.^[11] As in the case of synthetic routes reliant upon the elimination of ahydrogen halide, this route is suspected to involve an intermediate phosphaethylene species containing a C=P double bond, though such a species has not yet been observed.^[2]

Like the preceding method, the most popular method for synthesizing phosphaalkynes is reliant upon the expulsion of products containing strong silicon-element bonds. Specifically, it is possible to synthesize phosphaalkynes via the elimination ofhexamethyldisiloxane (HMDSO) from certain silylatedphosphaalkenes with the general structureRO−(Me₃Si)C=P−SiMe₃. These phosphaalkenes are formed rapidly following the synthesis of the appropriate acylbis-trimethylsilylphosphine, which undergoes a rapid [1,3]-silyl shift to produce the relevant phosphaalkene. This synthetic strategy is particularly appealing because the precursors (an acyl chloride andtris-trimethylsilylphosphine orbis-trimethylsilylphosphide) are either readily available or simple to synthesize.^[2]

This method has been utilized to produce a variety of kinetically stable phosphaalkynes, including aryl,^[2][12][13] tertiary alkyl,^[14] secondary alkyl,^[2] and even primary alkyl^[15] phosphaalkynes in good yields.

Dihalophospaalkenes of the general formR−P=CX₂, where X is Cl, Br, or I, undergo lithium-halogen exchange withorganolithium reagents to yield intermediates of the formR−P=CXLi. These species then eject the corresponding lithium halide salt, LiX, to putatively give a phospha-isocyanide, which can rearrange, much in the same way as an isocyanide,^[16] to yield the corresponding phosphaalkyne.^[17]

Simulation suggests that simple isophosphiles are not triple-bonded between P and C; instead both atoms bear a lone pair.^[18]

It has been demonstrated by Cummins and coworkers thatthermolysis of compounds of the general formC₁₄H₁₀PC(=PPh₃)R leads to the extrusion ofC₁₄H₁₀ (anthracene), triphenylphosphine, and the corresponding substituted phosphaacetylene:R−C≡P. Unlike the previous method, which derives the phosphaalkyne substituent from anacyl chloride, this method derives the substituent from aWittig reagent.^[19]

The carbon-phosphorus triple bond in phosphaalkynes represents an exception to the so-called "double bond rule", which would suggest that phosphorus tends not to form multiple bonds to carbon, and the nature of bonding within phosphaalkynes has therefore attracted much interest from synthetic and theoretical chemists. For simple phosphaalkynes such asH−C≡P andMe−C≡P, the carbon-phosphorus bond length is known bymicrowave spectroscopy, and for certain more complex phosphaalkynes, these bond lengths are known from single-crystalX-ray diffraction experiments. These bond lengths can be compared to the theoretical bond length for a carbon-phosphorus triple bond predicted byPekka Pyykkö of 1.54 Å.^[20] By bond length metrics, most structurally characterized alkyl and aryl substituted phosphaalkynes contain triple bonds between carbon and phosphorus, as their bond lengths are either equal to or less than the theoretical bond distance.

The carbon-phosphorus bond order in phosphaalkynes has also been the subject of computational inquiry, where quantum chemical calculations have been utilized to determine the nature of bonding in these molecules from first principles. In this context,natural bond orbital (NBO) theory has provided valuable insight into the bonding within these molecules. Lucas and coworkers have investigated the electronic structure of various substituted phosphaalkynes, including thecyaphide anion (⁻C≡P), using NBO, natural resonance theory (NRT), andquantum theory of atoms in molecules (QTAIM) in an attempt to better describe the bonding in these molecules. For the simplest systems,⁻C≡P andH−C≡P, NBO analysis suggests that the only relevantresonance structure is that in which there is a triple bond between carbon and phosphorus. For more complex molecules, such asMe−C≡P andMe₃C−C≡P, the triple bonded resonance structure is still the most relevant, but accounts for only some of the overall electron density within the molecule (81.5% and 72.1%, respectively). This is due to interactions between the two carbon-phosphoruspi-bonds and the C-H or C-Csigma-bonds of the substituents, which can be visualized by inspecting the C-P pi-bonding molecular orbitals in these molecules.^[24]

Phosphaalkynes possess diverse reactivity profiles, and can be utilized in the synthesis of various phosphorus-containing saturated of unsaturatedheterocyclic compounds.

One of the most developed areas of phosphaalkyne chemistry is that ofcycloadditions. Like other multiply bonded molecular fragments, phosphaalkynes undergo myriad reactions such as [1+2] cycloadditions,^[26][27][28] [3+2] cycloadditions,^[29][30] and [4+2] cycloadditions.^[2][31] This reactivity is summarized in graphical format below, which includes some examples of 1,2-addition reactivity^[32][33] (which is not a form of cycloaddition).

The pi-bonds of phosphaalkynes are weaker than most carbon-phosphorus sigma bonds, rendering phosphaalkynes reactive with respect to the formation ofoligomeric species containing more sigma bonds. These oligomerization reactions are triggered thermally, or can be catalyzed bytransition ormain-group metals.

Phosphaalkynes with small substituents (H,F,Me,Ph, etc.) undergo decomposition at or below room temperature by way of polymerization/oligimerization to yield mixtures of products which are challenging to characterize. The same is largely true of kinetically stable phosphaalkynes, which undergo oligomerization reactions at elevated temperature.^[35] In spite of the challenges associated with isolating and identifying the products of these oligimerizations, however, cuboidal tetramers oftert-butylphosphaalkyne andtert-pentylphosphaalkyne have been isolated (albeit in low yield) and identified following heating of the respective phosphaalkyne.^[36]

Computational chemistry has proved a valuable tool for studying these synthetically complex reactions, and it has been shown that while the formation of phosphaalkyne dimers is thermodynamically favorable, the formation of trimers, tetramers, and higher order oligomeric species tends to be more favorable, accounting for the generation of intractable mixtures upon inducing oligomerization of phosphaalkynes experimentally.^[37][38]

Unlike thermally initiated phosphaalkyne oligomerization reactions, transition metals and main group metals are capable of oligomerizing phosphaalkynes in a controlled manner, and have led to the isolation of phosphaalkyne dimers, trimers, tetramers, pentamers, and even hexamers.^[35] A nickel complex is capable of catalytically homocoupling^tBu-C≡P to yield a diphosphatetrahedrane.^[39]

• Arsaalkyne
• Cyaphide

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