Tert-Butylphosphaacetylene
Encyclopedia
tert-Butylphosphaacetylene is an organophosphorus compound. Abbreviated t-BuC≡P, 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 successful synthesis of t-BuC≡P discredited much of the double bond rule and opened new studies into the formation of unsaturated phosphorus compounds.
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). At temperatures above 130 °C, the phosphaalkyne undergoes cyclotetramerization. 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.
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.
Phosphaalkyne
In chemistry, phosphaalkynes are organophosphorus compounds that have a phosphorus-carbon triple bond....
. Prior to its synthesis, the double bond rule
Double bond rule
The double bond rule states that chemical elements with a principal quantum numbergreater than 2 do not form multiple bonds with themselves or with other elements...
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 successful synthesis of t-BuC≡P discredited much of the double bond rule and opened new studies into the formation of unsaturated phosphorus compounds.
Synthesis
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 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-BuC≡P product.- Me3CC(O)Cl + P(SiMe3)3 → Me3C(O)P(SiMe3)2 + Me3SiCl
- Me3C(O)P(SiMe3)2 → Me3CP + 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). At temperatures above 130 °C, the phosphaalkyne undergoes cyclotetramerization. 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.
Reactions
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.