Phosphaalkyne

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, phosphaethyne, which was identified by infrared spectroscopy.{{cite journal |last=Gier |first=T. E. |date=1961-04-01 |title=HCP, A Unique Phosphorus Compound |journal=Journal of the American Chemical Society |volume=83 |issue=7 |pages=1769–1770 |issn=0002-7863 |doi=10.1021/ja01468a058}}

File:HCP From Phosphine2.tif

File:RCP by HX elimination.gif

Following the initial synthesis of phosphaethyne, it was realized that the same compound can be prepared more expeditiously via the flash pyrolysis of methyldichlorophosphine ({{chem2|CH3PCl2}}), resulting in the loss of two equivalents of hydrogen chloride. This methodology has been utilized to synthesize numerous substituted phosphaalkynes, including the methyl,{{cite journal |last1=Kroto |first1=H. W |last2=Nixon |first2=J. F |last3=Simmons |first3=N. P. C |date=1979-08-01 |title=The microwave spectrum of 1-phosphapropyne, CH3CP: Molecular structure, dipole moment, and vibration-rotation analysis |journal=Journal of Molecular Spectroscopy |volume=77 |issue=2 |pages=270–285 |issn=0022-2852 |bibcode=1979JMoSp..77..270K |doi=10.1016/0022-2852(79)90108-5}} vinyl,{{cite journal |last1=Ohno |first1=K |last2=Kroto |first2=H. W |last3=Nixon |first3=J. F |date=1981-12-01 |title=The microwave spectrum of 1-phosphabut-3-ene-1-yne, CH2=CHCP |journal=Journal of Molecular Spectroscopy |volume=90 |issue=2 |pages=507–511 |issn=0022-2852 |doi=10.1016/0022-2852(81)90142-9}} chloride, and fluoride{{cite journal |last1=Kroto |first1=H. W. |last2=Nixon |first2=J. F. |last3=Simmons |first3=N. P. C. |last4=Westwood |first4=N. P. C. |date=1978-01-01 |title=FC.tplbond.P,C-fluorophosphaethyne: preparation and detection by photoelectron and microwave spectroscopy |journal=Journal of the American Chemical Society |volume=100 |issue=2 |pages=446–448 |issn=0002-7863 |doi=10.1021/ja00470a013}} derivatives. Fluoromethylidynephosphane ({{chem2|F\sC\tP}}) can also be prepared via the potassium hydroxide promoted dehydrofluorination of trifluoromethylphosphine ({{chem2|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 {{chem2|F2C\dPH}} by ³¹P NMR spectroscopy during the synthesis of {{chem2|F\sC\tP}}.{{cite journal |last1=Eshtiagh-Hosseini |first1=Hossein |last2=Kroto |first2=Harold W. |last3=Nixon |first3=John F. |last4=Brownstein |first4=Sydney |last5=Morton |first5=John R. |last6=Preston |first6=Keith F. |date=1979 |title=19F and 31P n.m.r. characterisation of phospha-alkene and phospha-alkyne intermediates in the alkaline hydrolysis of trifluoromethylphlphosphine |journal=Journal of the Chemical Society, Chemical Communications |language=en |issue=15 |pages=653–654 |issn=0022-4936 |doi=10.1039/c39790000653 |url=https://pubs.rsc.org/en/content/articlelanding/1979/c3/c39790000653|url-access=subscription }}

The high strength of silicon–halogen bonds can be leveraged toward the synthesis of phosphaalkynes. Heating bis-trimethylsilylated methyldichlorophosphines ({{chem2|((CH3)3Si)2C(R)\sPCl2}}) 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{{cite journal |last1=Appel |first1=Rolf |last2=Maier |first2=Günther |last3=Reisenauer |first3=Hans Peter |last4=Westerhaus |first4=Axel |date=1981-02-01 |title=Elimination and Addition at the Phosphorus-Carbon pπ-pπ Bond |journal=Angewandte Chemie International Edition in English |language=en |volume=20 |issue=2 |pages=197 |issn=1521-3773 |doi=10.1002/anie.198101971}} and 2-trimethylsilylphosphaacetylene.{{cite journal |last1=Appel |first1=Rolf |last2=Westerhaus |first2=Axel |date=1981-01-01 |title=(CH3)3SiCP, ein silylfunktionelles phospha-alkin |journal=Tetrahedron Letters |volume=22 |issue=23 |pages=2159–2160 |issn=0040-4039 |doi=10.1016/S0040-4039(01)90486-1}} 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.

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 {{chem2|RO\s(Me3Si)C\dP\sSiMe3}}. 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.

File:RCP by elimination of HMDSO.gif

This method has been utilized to produce a variety of kinetically stable phosphaalkynes, including aryl,{{cite journal |last1=Märkl |first1=Gottfried |last2=Sejpka |first2=Hans |date=1986-01-01 |title=2-(2,4,6-tri-tert-butylphenyl)-1-phosphaethin, 1,4-bis-(trimethylsiloxy)-1,4-bis-(2,4,6-tri-tert-butylphenyl)-2, 3-diphosphabutadien |journal=Tetrahedron Letters |volume=27 |issue=2 |pages=171–174 |issn=0040-4039 |doi=10.1016/S0040-4039(00)83969-6}}{{cite journal |last1=Jones |first1=Cameron |last2=Waugh |first2=Mark |date=2007-10-15 |title=Synthesis and structural characterization of a terphenyl substituted phosphaalkyne, PC{C6H3(C6H2Me3-2,4,6)2-2,6} |journal=Journal of Organometallic Chemistry |volume=692 |issue=22 |pages=5086–5090 |issn=0022-328X |doi=10.1016/j.jorganchem.2007.07.021}} tertiary alkyl,{{cite journal |last1=Allspach |first1=T. |last2=Regitz |first2=M. |last3=Becker |first3=G. |last4=Becker |first4=W. |date=1986 |title=Unusually Coordinated Phosphorus Compounds; 7 1. Adamant-1-ylmethylidynephosphine, A New, Stable Phosphaalkyne |journal=Synthesis |language=en |volume=1986 |issue=1 |pages=31–36 |issn=0039-7881 |doi=10.1055/s-1986-31467}} secondary alkyl, and even primary alkyl{{cite journal |last1=Rösch |first1=Wolfgang |last2=Vogelbacher |first2=Uwe |last3=Allspach |first3=Thomas |last4=Regitz |first4=Manfred |date=1986-05-20 |title=Ungewöhnlich koordinierte phosphoverbindungen: VII.Neue phosphaalkinen und deren cycloadditionsverhalten gegenüben 1,3-dipolen |journal=Journal of Organometallic Chemistry |volume=306 |issue=1 |pages=39–53 |issn=0022-328X |doi=10.1016/S0022-328X(00)98932-0}} phosphaalkynes in good yields.

Dihalophospaalkenes of the general form {{chem2|R\sP\dCX2}}, where X is Cl, Br, or I, undergo lithium-halogen exchange with organolithium reagents to yield intermediates of the form {{chem2|R\sP\dCXLi}}. 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,{{cite journal |last1=Meier |first1=Michael |last2=Mueller |first2=Barbara |last3=Ruechardt |first3=Christoph |date=February 1987 |title=The isonitrile-nitrile rearrangement. A reaction without a structure-reactivity relationship |journal=The Journal of Organic Chemistry |volume=52 |issue=4 |pages=648–652 |issn=0022-3263 |doi=10.1021/jo00380a028}} to yield the corresponding phosphaalkyne.{{cite journal |last1=Goede |first1=Simon J. |last2=Bickelhaupt |first2=Friedrich |date=1991 |title=Synthesis and Reactions of P-Supermesityl-C-halophosphaalkenes |journal=Chemische Berichte |language=en |volume=124 |issue=12 |pages=2677–2684 |issn=1099-0682 |doi=10.1002/cber.19911241207}} 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).{{cite journal |last1=Nguyen |first1=Minh Tho |last2=Ha |first2=Tae-Kyu |date=1986-08-01 |title=Ab initio study of structures and relative stabilities of RCP (R=H, F) and their energetically higher-lying isomers RPC |journal=Journal of Molecular Structure: THEOCHEM |volume=139 |issue=1 |pages=145–152 |issn=0166-1280 |doi=10.1016/0166-1280(86)80114-2}}

It has been demonstrated by Cummins and coworkers that thermolysis of compounds of the general form {{chem2|C14H10PC(\dPPh3)R}} leads to the extrusion of {{chem2|C14H10}} (anthracene), triphenylphosphine, and the corresponding substituted phosphaacetylene: {{chem2|R\sC\tP}}. Unlike the previous method, which derives the phosphaalkyne substituent from an acyl chloride, this method derives the substituent from a Wittig reagent.{{cite journal |last1=Transue |first1=Wesley J. |last2=Yang |first2=Junyu |last3=Nava |first3=Matthew |last4=Sergeyev |first4=Ivan V. |last5=Barnum |first5=Timothy J. |last6=McCarthy |first6=Michael C. |last7=Cummins |first7=Christopher C. |date=2018-12-26 |title=Synthetic and Spectroscopic Investigations Enabled by Modular Synthesis of Molecular Phosphaalkyne Precursors |journal=Journal of the American Chemical Society |volume=140 |issue=51 |pages=17985–17991 |pmid=30485736 |s2cid=54145482 |issn=0002-7863 |doi=10.1021/jacs.8b09845}}

File:Cummins phosphaalkyne synthesis.tif, Me, Et, ⁱPr, or ^sBu.]]

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 {{chem2|H\sC\tP}} and {{chem2|Me\sC\tP}}, 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.{{cite journal |last=Pyykkö |first=Pekka |date=2015-03-19 |title=Additive Covalent Radii for Single-, Double-, and Triple-Bonded Molecules and Tetrahedrally Bonded Crystals: A Summary |journal=The Journal of Physical Chemistry A |volume=119 |issue=11 |pages=2326–2337 |pmid=25162610 |issn=1089-5639 |bibcode=2015JPCA..119.2326P |doi=10.1021/jp5065819}} 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 the cyaphide anion ({{chem2|–C\tP}}), using NBO, natural resonance theory (NRT), and quantum theory of atoms in molecules (QTAIM) in an attempt to better describe the bonding in these molecules. For the simplest systems, {{chem2|–C\tP}} and {{chem2|H\sC\tP}}, 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 {{chem2|Me\sC\tP}} and {{chem2|Me3C\sC\tP}}, 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-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.{{cite journal |last1=Lucas |first1=Maria F. |last2=Michelini |first2=Maria C. |last3=Russo |first3=Nino |last4=Sicilia |first4=Emilia |date=2008-03-01 |title=On the Nature of the CP Bond in Phosphaalkynes |journal=Journal of Chemical Theory and Computation |volume=4 |issue=3 |pages=397–403 |pmid=26620780 |issn=1549-9618 |doi=10.1021/ct700277w}}

File:Phosphaalkyne series bonds.tif level of theory using the def2-tzvpp basis set in ORCA.{{cite journal |last=Neese |first=Frank |date=2012 |title=The ORCA program system |journal=Wiley Interdisciplinary Reviews: Computational Molecular Science |language=en |volume=2 |issue=1 |pages=73–78 |s2cid=62137389 |issn=1759-0884 |doi=10.1002/wcms.81}} Molecules shown are (from left to right) the cyaphide anion, {{chem2|H\sC\tP}}, {{chem2|Me\sC\tP}}, and {{chem2|Me3C\sC\tP}}. Geometries utilized in creating this figure are those reported by Lucas and coworkers.]]

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

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,{{cite journal |last1=Wagner |first1=Oliver |last2=Ehle |first2=Michael |last3=Regitz |first3=Manfred |date=1989 |title=2-Chloro-2H-phosphirene/1-Chloro-1H-phosphirene Isomerization by [1,3]-Chlorine Shift |journal=Angewandte Chemie International Edition in English |volume=28 |issue=2 |pages=225–226 |issn=1521-3773 |doi=10.1002/anie.198902251}}{{cite journal |last1=Schäfer |first1=Annemarie |last2=Weidenbruch |first2=Manfred |last3=Saak |first3=Wolfgang |last4=Pohl |first4=Siegfried |date=1987 |title=Phosphasilirene, dreigliedrige Ringe mit PC-Doppelbindung |journal=Angewandte Chemie |volume=99 |issue=8 |pages=806–807 |bibcode=1987AngCh..99..806S |issn=1521-3757 |doi=10.1002/ange.19870990823}}{{cite journal |last1=Cowley |first1=Alan H. |last2=Hall |first2=Stephen W. |last3=Nunn |first3=Christine M. |last4=Power |first4=John M. |date=1988 |title=Synthesis and structure of a phosphagermirene |journal=Journal of the Chemical Society, Chemical Communications |language=en |issue=11 |pages=753–754 |issn=0022-4936 |doi=10.1039/c39880000753 |url=http://xlink.rsc.org/?DOI=c39880000753|url-access=subscription }} [3+2] cycloadditions,{{cite journal |last1=Rösch |first1=Wolfgang |last2=Hees |first2=Udo |last3=Regitz |first3=Manfred |date=1987 |title=Phosphorverbindungen ungewöhnlicher Koordination, 191) 1,2,4-Diazaphosphole durch [3+2]-Cycloaddition von Diazoverbindungen an ein stabiles Phosphaalkin |journal=Chemische Berichte |language=de |volume=120 |issue=10 |pages=1645–1652 |issn=1099-0682 |doi=10.1002/cber.19871201007}}{{cite journal |last1=Rösch |first1=Wolfgang |last2=Facklam |first2=Thomas |last3=Regitz |first3=Manfred |date=1987-01-01 |title=Phosphorus compounds with unusual coordination - 201. 1,2,3,4- triazaphospholes by [3+2]-cycloaddition of azides to a stable phosphaalkyne |journal=Tetrahedron |volume=43 |issue=14 |pages=3247–3256 |issn=0040-4020 |doi=10.1016/S0040-4020(01)90292-3}} and [4+2] cycloadditions.{{cite journal |last1=Rösch |first1=Wolfgang |last2=Regitz |first2=Manfred |date=March 17, 1986 |title=Phosphorverbindungen ungewöhnlicher Koordination, 12 [ 1 ] Diels-Alder-Reaktionen mit 'Bu—C=P — ein ergiebiger Weg zu A3-Phosphininen |journal=Zeitschrift für Naturforschung B |volume=41 |pages=931 |via= |s2cid=96323757 |doi=10.1515/znb-1986-0723 |url=http://www.znaturforsch.com/ab/v41b/41b0931.pdf}} This reactivity is summarized in graphical format below, which includes some examples of 1,2-addition reactivity{{cite journal |last1=Appel |first1=Rolf |last2=Maier |first2=Günther |last3=Reisenauer |first3=Hans Peter |last4=Westerhaus |first4=Axel |date=1981 |title=Elimination and Addition at the Phosphorus-Carbon pπ-pπ Bond |journal=Angewandte Chemie International Edition in English |volume=20 |issue=2 |pages=197 |issn=1521-3773 |doi=10.1002/anie.198101971}}{{cite journal |last1=Arif |first1=Atta M. |last2=Barron |first2=Andrew R. |last3=Cowley |first3=Alan H. |last4=Hall |first4=Stephen W. |date=1988 |title=Reaction of the phospha-alkyne ArCP (Ar=2,4,6-Bu t 3 C 6 H 2 ) with nucleophiles: a new approach to 1,3-diphosphabutadiene synthesis |journal=J. Chem. Soc., Chem. Commun. |language=en |issue=3 |pages=171–172 |issn=0022-4936 |doi=10.1039/C39880000171 |url=http://xlink.rsc.org/?DOI=C39880000171|url-access=subscription }} (which is not a form of cycloaddition).

File:Phosphaalkyne Reactivity pinwheel.tif

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.

File:Phosphaalkyne tetramer thermolysis.tif

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.{{cite journal |last1=Chirila |first1=Andrei |last2=Wolf |first2=Robert |last3=Chris Slootweg |first3=J. |last4=Lammertsma |first4=Koop |date=2014-07-01 |title=Main group and transition metal-mediated phosphaalkyne oligomerizations |journal=Coordination Chemistry Reviews |series=Frontiers in Organometallic Chemistry: 2014 |volume=270-271 |pages=57–74 |issn=0010-8545 |doi=10.1016/j.ccr.2013.10.005}} 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.{{cite journal |last1=Wettling |first1=Thomas |last2=Schneider |first2=Jürgen |last3=Wagner |first3=Oliver |last4=Kreiter |first4=Cornelius G. |last5=Regitz |first5=Manfred |date=1989 |title=Tetra-tert-butyltetraphosphacubane: The First Thermal Cyclooligomerization of a Phosphaalkyne |journal=Angewandte Chemie International Edition in English |volume=28 |issue=8 |pages=1013–1014 |issn=1521-3773 |doi=10.1002/anie.198910131 |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.198910131|url-access=subscription }}

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.{{cite journal |last1=Caliman |first1=Vicinus |last2=Hitchcock |first2=Peter B. |last3=Nixon |first3=John F. |last4=Hofmann |first4=Matthias |last5=Schleyer |first5=Paul Von Ragué |date=1994 |title=Ein neues Hexamer von tBuCP: Synthese, Struktur und theoretische Untersuchungen |journal=Angewandte Chemie |volume=106 |issue=21 |pages=2284–2286 |bibcode=1994AngCh.106.2284C |issn=1521-3757 |doi=10.1002/ange.19941062118}}{{cite journal |last1=Fiedler |first1=Wolfgang |last2=Löber |first2=Oliver |last3=Bergsträßer |first3=Uwe |last4=Regitz |first4=Manfred |date=1999-02-18 |title=On the Unusual Dienophilicity of Trimethylsilylphosphaacetylene |journal=European Journal of Organic Chemistry |language=en |volume=1999 |issue=2 |pages=363–371 |issn=1434-193X |doi=10.1002/(SICI)1099-0690(199902)1999:2<363::AID-EJOC363>3.0.CO;2-2 |url=https://onlinelibrary.wiley.com/doi/10.1002/(SICI)1099-0690(199902)1999:23.0.CO;2-2|url-access=subscription }}

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. A nickel complex is capable of catalytically homocoupling ^tBu-C≡P to yield a diphosphatetrahedrane.{{cite journal |last1=Hierlmeier |first1=Gabriele |last2=Coburger |first2=Peter |last3=Bodensteiner |first3=Michael |last4=Wolf |first4=Robert |date=2019-10-24 |title=Di- tert -butyldiphosphatetrahedrane: Catalytic Synthesis of the Elusive Phosphaalkyne Dimer |journal=Angewandte Chemie International Edition |volume=58 |issue=47 |pages=16918–16922 |language=en |pmid=31591760 |pmc=6899750 |doi=10.1002/anie.201910505 |doi-access=free}}

File:Phosphaalkyne oligomers2.tif

• Arsaalkyne
• Cyaphide

{{Commons category|Phosphaalkynes}}

{{Reflist}}

Category:Functional groups

Category:Organophosphanes