Phosphaalkyne
In chemistry, a phosphaalkyne is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general formula R-C≡P. Phosphaalkynes are the heavier congeners of nitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent of alkynes. Due to their high reactivity, phosphaalkynes are not found naturally on earth, but the simplest phosphaalkyne, phosphaethyne has been observed in the interstellar medium.
Synthesis
From phosphine gas
The first of preparation of a phosphaalkyne was achieved in 1961 when Thurman Gier produced phosphaethyne by passing phosphine gas at low pressure over an electric arc produced between two carbon electrodes. Condensation of the gaseous products in a –196 °C trap revealed that the reaction had produced acetylene, ethylene, phosphaethyne, which was identified by infrared spectroscopy.By elimination reactions
Elimination of hydrogen halides
Following the initial synthesis of phosphaethyne, it was realized that the same compound can be prepared more expeditiously via the flash pyrolysis of methyldichlorophosphine, resulting in the loss of two equivalents of hydrogen chloride. This methodology has been utilized to synthesize numerous substituted phosphaalkynes, including the methyl, vinyl, chloride, and fluoride derivatives. Fluoromethylidynephosphane can also be prepared via the potassium hydroxide promoted dehydrofluorination of trifluoromethylphosphine. 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 of F2C=PH by 31P NMR spectroscopy during the synthesis of F-C≡P.Elimination of chlorotrimethylsilane
The high strength of silicon-halogen bonds can be leveraged toward the synthesis of phosphaalkynes. Heating bis-trimethylsilylated methyldichlorophosphines under vacuum results in the expulsion of two equivalents of chlorotrimethylsilane and the ultimate formation of a new phosphaalkyne. This synthetic strategy has been applied in the synthesis of 2-phenylphosphaacetylene and 2-trimethylsilylphosphaacetylene. As in the case of synthetic routes reliant upon the elimination of a hydrogen 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.Elimination of hexamethyldisiloxane
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 of hexamethyldisiloxane from certain silylated phosphaalkenes with the general structure ROC=PSiMe3. These phosphaalkenes are formed rapidly following the synthesis of the appropriate acyl bis-trimethylsilylphosphine, which undergoes a rapid -silyl shift to produce the relevant phosphaalkene. This synthetic strategy is particularly appealing because the precursors are either readily available or simple to synthesize.File:RCP by elimination of HMDSO.gif|alt=Figure showing the reaction of a phosphine or lithium phosphide with an acyl chloride yielding an acyl phosphane which rapidly undergoes a -silyl shift to yield either the E or Z isomer of a phosphaalkene. These species can then be heated to produce a phosphaalkyne with concomitant expulsion of HMDSO.|thumb|center|813x813px|Synthesis of substituted phosphaalkynes via the intermediate silylated phosphaalkene. Heating these phosphaalkenes results in the formation of a phosphaalkyne and the expulsion of hexamethyldisiloxane.
This method has been utilized to produce a variety of kinetically stable phosphaalkynes, including aryl, tertiary alkyl, secondary alkyl, and even primary alkyl phosphaalkynes in good yields.
Via rearrangement of a putative phospha-isonitrile
Dihalophospaalkenes of the general form R-P=CX2, where X is Cl, Br, or I undergo lithium-halogen exchange with organolithium reagents to yield intermediates of the form R-P=CXLi. These species then eject the corresponding lithium halide salt, LiX, to putatively give a phospha-isonitrile, which can rearrange, much in the same way as an isonitrile, to yield the corresponding phosphaalkyne. This rearrangement has been evaluated using the tools of computational chemistry, which has shown that this isomerization process should proceed very rapidly, in line with current experimental evidence showing that phosphaisonitriles are unobservable intermediates, even at –85 °C.Other methods
It has been demonstrated by Cummins and coworkers that thermolysis of compounds of the general form C14H10PCR leads to the extrusion of C14H10, triphenylphosphine, and the corresponding substituted phosphaacetylene: R-C≡P. Unlike the previous method, which derives the phosphaalkyne substituent from an acyl chloride, this method derives the substituent from a Wittig reagent.Structure and bonding
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 as H-C≡P and Me-C≡P, the carbon-phosphorus bond length is known by microwave spectroscopy, and for certain more complex phosphaalkynes, these bond lengths are known from single-crystal X-ray diffraction experiments. These bond lengths can be compared to the theoretical bond length for a carbon-phosphorus triple bond predicted by Pyykkö of 1.54 Å. 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.| R | Bond Length |
| H | 1.5442 |
| Me | 1.544 |
| tert-butyl | 1.542 |
| triphenylmethyl | 1.538 |
| 2,4,6-triphenyl | 1.533 |
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 theory has provided valuable insight into the bonding within these molecules. Lucas and coworkers have investigated the electronic structure of various substituted phosphaalkynes, including the cyaphide anion, using NBO, natural resonance theory, and quantum theory of atoms in molecules in an attempt to better describe the bonding in these molecules. For the simplest systems, C≡P– and H-C≡P, NBO analysis suggests that the only relevant resonance structure is that in which there is a triple bond between carbon and phosphorus. For more complex molecules, such as Me-C≡P and 3C-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. This is due to interactions between the two carbon-phosphorus pi-bonds and the C-H or C-C sigma-bonds of the substituents, which can be visualized by inspecting the C-P pi-bonding molecular orbitals in these molecules.
level of theory using the def2-tzvpp basis set in ORCA. Molecules shown are the cyaphide anion, H-C≡P, Me-C≡P, and 3C-C≡P. Geometries utilized in creating this figure are those reported by Lucas and coworkers.
Reactivity
Phosphaalkynes possess diverse reactivity profiles, and can be utilized in the synthesis of various phosphorus-containing saturated of unsaturated heterocyclic compounds.Cycloaddition reactivity
One of the most developed areas of phosphaalkyne chemistry is that of cycloadditions. Like other multiply bonded molecular fragments, phosphaalkynes undergo myriad reactions such as cycloadditions, cycloadditions, and cycloadditions. This reactivity is summarized in graphical format below, which includes some examples of 1,2-addition reactivity.File:Phosphaalkyne Reactivity pinwheel.tif|center|thumb|790x790px|A graphic showing some prototypical reactivity espoused by the phosphaalkyne functional group, including 1,2-additions, cycloadditions, cycloadditions, and cycloadditions. The phosphaalkyne core is shown in orange throughout the graphic.
Oligomerization
The pi-bonds of phosphaalkynes are weaker than most carbon-phosphorus sigma bonds, rendering phosphaalkynes reactive with respect to the formation of oligomeric species containing more sigma bonds. These oligomerization reactions are be triggered thermally, or can be catalyzed by transition or main group metals.Uncatalyzed
Phosphaalkynes with small substituents 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 oligimerization reactions at elevated temperature. In spite of the challenges associated with isolating and identifying the products of these oligimerizations, however, cuboidal tetramers of tert-butylphosphaalkyne and tert-pentylphosphaalkyne have been isolated and identified following heating of the respective phosphaalkyne.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.