Phosphaalkyne

In chemistry, a phosphaalkyne (IUPAC name: alkylidynephosphane) is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general formula R-C≡P.

[2] Phosphaalkynes are the heavier congeners of nitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent of alkynes.

Condensation of the gaseous products in a –196 °C (–321 °F) trap revealed that the reaction had produced acetylene, ethylene, phosphaethyne, which was identified by infrared spectroscopy.

[5] 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.

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] 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 (–121 °C).

[18] 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.

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.

[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.

Molecular structure of triphenylmethylphosphaacetylene, a phosphaalkyne. [ 1 ]
A scheme showing the conversion of phosphine gas to HCP, acetylene, and ethylene, following passage through an electric arc produced by carbon electrodes.
Gier's 1961 synthesis of phosphaethyne from low-pressure phosphine via electric discharge by carbon electrodes.
Scheme showing the flash pyrolysis of a generically substituted dichloromethylphospine to yield a substituted phosphaalkyne.
Synthesis of substituted phosphaalkynes by flash pyrolysis of substituted dichloromethylphosphines. Here, R=CH 3 , CH=CH 2 , Cl, or F.
Figure showing the reaction of a phosphine or lithium phosphide with an acyl chloride yielding an acyl phosphane which rapidly undergoes a [1,3]-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.
Synthesis of substituted phosphaalkynes via the intermediate silylated phosphaalkene. Heating these phosphaalkenes results in the formation of a phosphaalkyne and the expulsion of hexamethyldisiloxane (HMDSO).
Synthesis of phosphaalkynes from an anthracene based phosphine chloride and a Wittig reagent, as demonstrated by Cummins and coworkers. Here, R=H, Me, Et, i Pr, or s Bu. [ 19 ]
One of two degenerate pi-bonds in various phosphaalkyne species showing the interactions between C-P pi-bonds and substituent sigma bonds in Me-C≡P and (Me) 3 C-C≡P, but not in the cyaphide anion or in H-C≡P. Surfaces were calculated at the B3LYP level of theory using the def2-tzvpp basis set in ORCA. [ 25 ] Molecules shown are (from left to right) the cyaphide anion, H-C≡P, Me-C≡P, and (Me) 3 C-C≡P. Geometries utilized in creating this figure are those reported by Lucas and coworkers. [ 24 ]
A graphic showing some prototypical reactivity espoused by the phosphaalkyne functional group, including 1,2-additions, [2+1] cycloadditions, [2+3] cycloadditions, and [2+4] cycloadditions. The phosphaalkyne core is shown in orange throughout the graphic.
Synthesis of a cuboidal phosphaalkyne tetramer by heating a kinetically stable phosphaalkyne. [ 34 ]
Some of the reported phosphaalkyne oligomers generated upon treatment of a phosphaalkyne (usually t Bu-C≡P) with a transition metal or main group metal complex. Note that several of these species are unstable in their free forms, and instead exist stably only when bound to a transition metal. In this figure, the • symbols individually represent one C-R unit, and are utilized for clarity. [ 35 ]