Phosphaalkyne
In
Synthesis
From phosphine gas
The first of preparation of a phosphaalkyne was achieved in 1961 when Thurman Gier produced phosphaethyne by passing
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,[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 of F2C=PH by 31P NMR spectroscopy during the synthesis of F-C≡P.[9]
Elimination of chlorotrimethylsilane
The high strength of
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(SiMe3)C=PSiMe3. These phosphaalkenes are formed rapidly following the synthesis of the appropriate acyl bis-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 and tris-trimethylsilylphosphine or bis-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.
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,[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]
Other methods
It has been demonstrated by Cummins and coworkers that
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.[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.
R | Bond Length (Å) |
---|---|
H[21] | 1.5442 |
Me[6] | 1.544(4) |
tert-butyl[22] | 1.542(2) |
triphenylmethyl[1] | 1.538(2) |
2,4,6-tri(tert-butyl)phenyl[23] | 1.533(3) |
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,
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 [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).
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 triggered thermally, or can be catalyzed by transition or main-group metals.
Uncatalyzed
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 of tert-butylphosphaalkyne and tert-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]
Metal-mediated
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]
See also
References
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