Phosphaalkyne

In chemistry, a phosphaalkyne (IUPAC name: alkylidynephosphane) is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general chemical 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 (H−C≡P) 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 (−321 °F) trap revealed that the reaction had produced acetylene, ethylene, and phosphaethyne, which were 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 (CH3PCl2), 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 (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 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 (((CH3)3Si)2C(R)−PCl2) 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 (HMDSO) from certain silylated phosphaalkenes with the general structure RO−(Me3Si)C=P−SiMe3. These phosphaalkenes are formed rapidly following the synthesis of the appropriate acyl bis(trimethylsilyl)phosphine, 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 and tris(trimethylsilyl)phosphine or bis(trimethylsilyl)phosphide) are either readily available or simple to synthesize.

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.

By rearrangement of a putative phospha-isocyanide 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-isocyanide, which can rearrange, much in the same way as an isocyanide, to yield the corresponding phosphaalkyne. Simulation suggests that simple isophosphiles are not triple-bonded between P and C; instead both atoms bear a lone pair.

Other methods It has been demonstrated by Cummins and coworkers that thermolysis of compounds of the general form C14H10PC(=PPh3)R leads to the extrusion of C14H10 (anthracene), 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 Pekka 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.

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