Study of stable primary phosphine oxides and their P−H activation
Final Report Abstract
Compounds containing a phosphorus-hydrogen bond hold a prominent position in modern organophosphorus chemistry, as they are highly reactive precursors and offer unique opportunities for chemical modification. However, the application of primary phosphine oxides (PPOs) in these reactions has been limited due to their chemical lability. This stands in sharp contrast to the extensive use of primary phosphines. This motivated us to explore the strategies for stabilizing PPOs. At the beginning of the project, we focused on studying unusually stable PPOs containing ferrocene. During our systematic investigation, we unexpectedly identified one of the key factor for generally increasing the stability of phosphines against oxidation in air: the presence of a singlet oxygen quencher. We demonstrated that ferrocene is a quencher of singlet oxygen and that antioxidant effect of the ferrocene moiety on the autoxidation of primary phosphines is solventdependent and increases with the electron density on the ferrocene moiety but has no redoxrelated grounds. Consistent with this, the structures of previously published unusually airstable primary phosphines also revealed the presence of singlet oxygen quenchers, thus highlighting the crucial role of singlet oxygen in the oxidation of phosphines. Furthermore, we formulated hypotheses to explain the stabilizing effect of ferrocene in PPOs. These hypotheses include the Lewis acidity of the metal center, quenching of excited states, and radical scavenging. Later, we demonstrated that the presence of ferrocene in solution alone stabilized PPOs. This finding allowed us to develop a new strategy for the synthesis of these compounds. RP(O)H₂, where R = benzyl (Bn), cyclohexyl (Cy), 2-naphthyl (2-Naph), phenyl (Ph), and tert-butyl (t-Bu), was prepared by controlled oxidation of the corresponding primary phosphines in the presence of ferrocene as a stabilizing reagent. The prepared solutions of PPOs are practically stable and storable for months, making them available as reagents and reactive synthetic precursors. Their applicability was demonstrated in twofold phospha-aldol reactions with a set of diketones, providing straightforward access to five-, six-, and sevenmembered heterocyclic tertiary phosphine oxides with a high degree of stereoselectivity, which are otherwise difficult to obtain. Finally, to better understand the stability of isolable PPOs, new non-ferrocene-based members of this family were synthesized. The compounds were characterized using NMR, MS, EA, and X-ray crystallography. The oxides were further investigated in thermal disproportionation, and studied as ligands in half-sandwich Ru(II) complexes bearing an auxiliary η6-p-cymene ligand.
Publications
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A General Strategy for Increasing the Air Stability of Phosphines Including Primary Phosphines. Chemistry – A European Journal, 29(68).
Horký, Filip; Franz, Roman; Bruhn, Clemens & Pietschnig, Rudolf
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Primary phosphine oxides and where to find them.19th European Workshop on Phosphorus Chemistry and 3rd Spanish Workshop on Phosphorus Chemistry, March 28-30, 2023, San Sebastian, Spain, Poster.
F. Horký & R. Pietschnig
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A General Strategy for Increasing the Air-stability of Phosphines Including Primary Phosphines. 20th European Workshop on Phosphorus Chemistry, March 4-6, 2024, Würzburg, Germany, Oral presentation.
F. Horký, R. Franz, C. Bruhn & R. Pietschnig
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A General Strategy for Increasing the Air-stability of Phosphines Including Primary Phosphines. Ferrocene Colloquium, February 15-16, 2024, Düsseldorf, Germany, Oral presentation.
F. Horký, R. Franz, C. Bruhn & R. Pietschnig
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Application of Primary Phosphine Oxides in Domino‐Phospha‐Aldol Reactions of Diketones. Advanced Synthesis & Catalysis, 366(15), 3354-3360.
Horký, Filip; Bruhn, Clemens; Kargin, Denis & Pietschnig, Rudolf
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Exploring the Stability of Primary Phosphine Oxides: Correlation with Primary Phosphine′s Air‐Stability?. European Journal of Inorganic Chemistry, 27(35).
Horký, Filip; Bruhn, Clemens & Pietschnig, Rudolf
