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Tert-butylphosphaacetylene (tBu-CP), also 2,2-Dimethylpropylidynephosphine
The formation of the first stable phosphaalkyne tert-butylphosphaacetylene (t-BuC≡P) was reported by Becker and colleagues in 1981. Before the isolation of stable t-BuC≡P, double bond theory had theorized that elements of Period 3 and higher were unable to share double or triple bonds with lighter main group elements because of their weak orbital overlap. The successful synthesis of t-BuC≡P discredited much of the double bond rule and opened new studies into the formation of unsaturated phosphorus compounds.

The synthesis of t-BuC≡P entails the reaction of pivaloyl chloride and P(SiMe3)3. The reaction proceeds via the intermediacy of a bis(trimethylsilyl)pivaloylphosphine, which undergoes 1,3-silyl shifts to form E or Z phosphoalkene isomers. Carrying out the phosphoalkene reaction at 120-200 °C with small amounts of solid NaOH forms the final t-BuC≡P product. At temperatures above 130°C, the phosphaalkyne undergoes cyclotetramerization.


 * Me3CC(O)Cl +  P(SiMe3)3   →   Me3CC(O)P(SiMe3)2  +  Me3SiCl
 * Me3CC(O)P(SiMe3)2  →   Me3CCP  +  O(SiMe3)2

With their characteristic C-P triple bonds, the phosphorus atoms of phosphaalkynes such as tert-butylphosphaacetylene exhibit reactivities similar to nitriles, despite the significant radii differences between P (1.09Å) and N (0.71Å). The presence of filled inner valence orbitals on phosphorus centers demand that unsaturated phosphorus compounds possess stabilizing, bulky substituents which allow two nonbonding electrons to remain on the phosphorus atoms of phosphaalkynes. Smaller phosphaalkyne substituents cause larger C≡P bond lengths and ionization potentials. 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 1.516Å and 1.532Å, respectively. While t-BuC≡P possesses a C≡P bond length of 1.536Å 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.

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. These isomeric species have never been isolated as stable byproducts. 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.



The reactivity of tert-butylphosphaacetylenes more closely resembles the reactions of alkynes than of nitriles. The higher electronegativity of carbon (2.5) over phosphorus (2.2) leads to polarized Cδ-≡Pδ+ bonds, which induces protonation at its carbon center. Its variety of coordination geometries enable tert-butylphosphaacetylene to participate in several types of reactions, including 1,2-additions of halogenated compounds.

Organolithium products and enophiles can also reaction across 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.