Phosphine oxides are phosphorus compounds with the formula OPX3. When X = alkyl or aryl, these are organophosphine oxides. Triphenylphosphine oxide is an example. An inorganic phosphine oxide is phosphoryl chloride (POCl3).[1] The parent phosphine oxide (H3PO) remains rare and obscure.
Tertiary phosphine oxides are the commonly encountered phosphine oxides. With the formula R3PO, they are tetrahedral compounds.
They are usually prepared by oxidation of tertiary phosphines. The P-O bond is short and polar. According to molecular orbital theory, the short P–O bond is attributed to the donation of the lone pair electrons from oxygen p-orbitals to the antibonding phosphorus-carbon bonds.[2] The nature of the P–O bond was once hotly debated. Some discussions invoked a role for phosphorus-centered d-orbitals in bonding, but this analysis is not supported by computational analyses. In terms of simple Lewis structure, the bond is more accurately represented as a dative bond, as is currently used to depict an amine oxide.[3][4]
Preparation and occurrence
Phosphine oxide are typically produced by oxidation of organophosphines. The oxygen in air is often sufficiently oxidizing to fully convert trialkylphosphines to their oxides at room temperature:
R3P + 1/2 O2 → R3PO
Oxidation of less basic phosphines, such as methyldiphenylphosphine can be achieved using hydrogen peroxide:[5]
Another route to phosphine oxides is the thermolysis of phosphonium hydroxides:
[PPh4]Cl + NaOH → Ph3PO + NaCl + PhH
The hydrolysis of phosphorus(V) dihalides also affords the oxide:[6]
R3PCl2 + H2O → R3PO + 2 HCl
Secondary phosphine oxides
Secondary phosphine oxides (SPOs), formally derived from secondary phosphines (R2PH), are again tetrahedral at phosphorus.[7] One commercially available example of a secondary phosphine oxide is diphenylphosphine oxide. SPOs are used in the formulation of catalysts for cross coupling reactions.[8]
Unlike tertiary phosphine oxides, SPOs often undergo further oxidation:
R2P(O)H + H2O2 → R2P(O)OH + H2O
These reactions are preceded by tautomerization to the phosphinous acid (R2POH):
R 2P(O)H → R 2POH
R 2POH + H 2O 2 → R 2PO 2H + H 2O
Syntheses
A nonoxidative route is applicable secondary phosphine oxides, which arise by the hydrolysis of the chlorophosphine. An example is the hydrolysis of chlorodiphenylphosphine to give diphenylphosphine oxide:
Ph2PCl + H2O → Ph2P(O)H + HCl
P-chiral phosphine oxides[9][10] are valuable intermediates in the synthesis of P-chiral phosphines[11] and phosphates, important as ligands in catalysis[12] and in the synthesis of oligonucleotide drugs.[13]
Primary phosphine oxides
Primary phosphine oxides, formally oxidized derivatives of primary phosphines, are again tetrahedral at phosphorus. With four different substituents (O, OH, H, R), they are chiral. The primary phosphine oxides subject to tautomerization, which leads to racemization. Like SPO's they are susceptible to further oxidation. Primary phosphine oxides disproportionate to the phosphinic acid and the primary phosphine:[14]
Some phosphine oxides are well-known photoinitiators in photopolymer chemistry. UV/LED exposure induces a type I Norrish fission to free radicals, which then polymerize in a radical chain. An example is 2,4,6‑trimethylbenzoyldiphenylphosphine oxide, which absorbs around 380-410nm (near UV).[15]
Deoxygenation
Phosphine oxide deoxygenation has been extensively developed because some useful reactions convert stoichiometric tertiary phosphines to the corresponding oxides. Regenerating the tertiary phosphines requires strongly oxophilic reagents,[16] and can retain or invert chirality at P, depending on the reductant.[17]
Industrial deoxygenation usually begins with treatment with phosgene or equivalents. The resulting chlorotriphenylphosphonium chloride is then reduced.[18]
In the laboratory, phosphine oxides are usually reduced with silicon derivatives,[16] typically inexpensive trichlorosilane.
Trichlorosilane and triethylamine reduce phosphine oxides with inversion, whereas the reaction proceeds with retention absent the base:Cite error: Closing </ref> missing for <ref> tag
In coordination chemistry, they are known to have labilizing effects to CO ligands cis to it in organometallic reactions. The cis effect describes this process.
↑Gilheany, Declan G. (1994). "No d Orbitals but Walsh Diagrams and Maybe Banana Bonds: Chemical Bonding in Phosphines, Phosphine Oxides, and Phosphonium Ylides". Chemical Reviews94 (5): 1339–1374. doi:10.1021/cr00029a008. PMID27704785.
↑Denniston, Michael L.; Martin, Donald R. (1977). "Methyldiphenylphosphine Oxide and Dimethylphenylphosphine Oxide". Inorganic Syntheses. 17. pp. 183–185. doi:10.1002/9780470132487.ch50. ISBN9780470132487.
↑Horký, Filip; Císařová, Ivana; Štěpnička, Petr (2021). "A Stable Primary Phosphane Oxide and Its Heavier Congeners". Chemistry – A European Journal27 (4): 1282–1285. doi:10.1002/chem.202003702. PMID32846012.
