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Tert-Butylphosphaacetylene

Chemical compound From Wikipedia, the free encyclopedia

Tert-Butylphosphaacetylene
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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.

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Synthesis and reactions

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 in diglyme at 20 °C in the presence of catalytic amounts of solid NaOH forms the final t-BuCP product.[1]

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

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.

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Reactions

Summarize
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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]

References

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