tert-Butylphosphaacetylene is an organophosphorus compound. Abbreviated t-BuCP, it was the first example of an isolable phosphaalkyne. Prior to its synthesis, the double bond rule had suggested that elements of Period 3 and higher were unable to form double or triple bonds with lighter main group elements because of weak orbital overlap. The synthesis of t-BuCP discredited much of the double bond rule and opened new studies into the formation of unsaturated phosphorus compounds.
The synthesis of t-BuCP entails the reaction of pivaloyl chloride and P(SiMe3)3. The reaction proceeds via the intermediacy of a bis(trimethylsilyl)pivaloylphosphine, which undergoes a 1,3-silyl shift to form E- or Z-phosphoalkene isomers. Carrying out the phosphoalkene reaction at 120-200 °C in the presence of catalytic amounts of solid NaOH forms the final t-BuCP product.[1]
Phosphaalkynes possessing a C≡P bonded to bulky aryl groups are also known, e.g. Mes*C≡P and P≡C(Tript)C≡P are known to possess C≡P bond lengths of 1.516 and 1.532 Å, respectively (see below).[2][3][4] While t-BuCP possesses a carbon-phosphorus bond length of 1.536 Å and a first ionization potential (π MO) of 9.70eV, H-C≡P possesses a C≡P bond length of 1.5421Å and a first ionization potential (π MO) of 10.79eV.[5]
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These physical properties produce characteristic reactivity differences between the two species: tert-butylphosphaacetylene is a stable volatile liquid (b.p. 61 °C), and phosphaacetylene readily reacts to form elemental phosphorus.
It has been proposed that isophosphaalkynes (R-P≡C) are produced as intermediates during the syntheses of phosphaalkynes. Such isomeric species have never been isolated.
With their characteristic C-P triple bonds, the phosphorus atoms of phosphaalkynes such as tert-butylphosphaacetylene exhibit reactivities similar to nitriles, despite the significant differences between the radii of P (1.09 Å) and N (0.71 Å). At temperatures above 130 °C, the phosphaalkyne undergoes cyclotetramerization. To some extent its reactivity more closely resembles the reactions of alkynes.
tert-Butylphosphaacetylene can bind to metals via various coordination modes to give inorganic and organometallic complexes. These complexes utilize either the triple bond or the nonbonding electrons on P.
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The higher electronegativity of carbon (2.5) over phosphorus (2.2) leads to polarized Cδ−≡Pδ+ bonds, which induces protonation at its carbon center.[6] Its variety of coordination geometries enable tert-butylphosphaacetylene to participate in several types of reactions, including 1,2-additions of halogenated compounds.
Organolithium compounds and enophiles can also react with C-P triple bonds, along with [2+1], [2+2], [2+3], and [2+4] cycloadditions. tert-Butylphosphaacetylene also undergoes a homo Diels-Alder cycloaddition reaction.[7][8]
^Becker, Gerd; Gresser, Gudrun; Uhl, Werner. "2,2-Dimethylpropylidinphosphan, eine stabile Verbindung mit einem Phosphoratom der Koordinationszahl 1." Zeitschrift für Naturforschung, Teil B:Anorganische Chemie, Organische Chemie 1981, 36, 16.
^Maerkl, Gottfried; Sejpka, Hans. "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." Tetrahedron Lett. 1986, 27, 171. doi:10.1016/S0040-4039(00)83969-6.
^Arif, Atta M.; Barron, Andrew R.; Cowley, Alan H.; Hall, Stephen W. "Reaction of the phospha-alkyne ArCP (Ar = 2,4,6-But 3C6H2) with nucleophiles: a new approach to 1,3-diphosphabutadiene synthesis." J. Chem. Soc., Chem. Commun. 1988, 3, 171. doi:10.1039/c39880000171.
^Brym, Markus.; Jones, Cameron. "Synthesis, characterisation and reactivity of the first diphosphaalkyne." Dalton Trans. 2003, 19, 3665. doi:10.1039/b309061b.
^Oberhammer, Heinz; Berker, Gerd; Gresser, Gudrun. "Molecular structures of phosphorus compounds : Part IX. Gas-phase structure of 2,2-dimethylpropylidynephosphine." Journal of Molecular Structure 1981, 75, 283-289. doi:10.1016/0022-2860(81)85242-8.
^Laali, Kenneth K.; Geissler, Bernhard; Regitz, Manfred; Houser, John J. 『C-Protonation of Adamantylphosphaacetylene (1-AdC≡P) and tert-Butylphosphaacetylene (tBuC≡P) in Superacids: Phosphavinyl Cation Generation and Trapping To Form Phosphaalkenes, Formation of Isomeric Boron-Containing Spirocyclic Betaines by Reaction of 1-AdC≡P with B(OTf)3, and Theoretical Studies on Protonation of MeC≡P.』 J. Org. Chem. 1995, 60, 6362.
^Nixon, John F. "Coordination chemistry of compounds containing phosphorus-carbon multiple bonds." Chem. Rev. 1988, 88, 1327. doi:10.1021/cr00089a015.
^Regitz, Manfred. "Phosphaalkynes: New Building Blocks in Synthetic Chemistry." Chem. Rev. 1990, 90, 191. doi:10.1021/cr00099a007.
