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Mechanistic study on photooxidation of triarylphosphines by time-resolved infrared spectroscopy

Upon laser flash photolysis on a solution of triarylphosphine Ar 3P in acetonitrile or dichloromethane using a YAG laser at 266 nm, transient absorption was observed by time-resolved infrared spectroscopy. Rate constants were determined for the formation step of the absorption. Dependency of the rate on [Ar3P] and atmosphere, along with independent observation in laser flash photolysis/UV-Vis spectroscopy of the Ar3P solutions, suggeststhat electron transfer occurs from Ar3P to the singlet photoexcited state of Ar3P, generating triarylphosphine radical cation Ar3P .+, which reacts with a trace amount of H2O in the solvent to give the final product Ar3P O, which has been observed in steady-state photolysis. Copyright Taylor & Francis Group, LLC.

Reaction of triarylphosphine radical cations generated from photoinduced electron transfer in the presence of oxygen

(Chemical Equation Presented) The 9,10-dicyanoanthracene (DCA)-sensitized photoreaction of triarylphosphines (1) was carried out in acetonitrile under aerobic conditions. Phosphine 1 was oxidized to the corresponding phosphine oxide with no appreciable side reactions. Product analysis and laser flash photolysis experiments suggest that the radical cation of 1 formed by the electron transfer from 1 to DCA in the singlet excited state ( 1DCA*) reacts with O2 to eventually afford the phosphine oxide.

The interactions between the sterically demanding trimesitylphosphine oxide and trimesityphosphine with scandium and selected lanthanide ions

The reactions between lanthanide nitrates, Ln(NO3)3 and scandium and lanthanide trifluoromethane sulfonates, Ln(Tf)3 with trimesitylphosphine oxide, Mes3PO show that coordination to the metal ions does not lead to crystalline complexes. Investigation of the reactions by 31-P NMR spectroscopy shows that weak complexes are formed in solution. The crystal structures of Mes3PO·0.5CH3CN (1) and [Mes3PO]3H3O·2CH3CN·Tf (2), formed in the reaction between ScTf3 and Mes3PO, are reported. Trimesitylphosphine, Mes3P, is protonated by scandium and lanthanide trifluoromethane sulfonates and lanthanide nitrates in CD3CN and the structure of [Mes3PH]Cl·HCl·2H2O (3) is reported.

Photoinduced and Thermal Single-Electron Transfer to Generate Radicals from Frustrated Lewis Pairs

Archetypal phosphine/borane frustrated Lewis pairs (FLPs) are famed for their ability to activate small molecules. The mechanism is generally believed to involve two-electron processes. However, the detection of radical intermediates indicates that single-electron transfer (SET) generating frustrated radical pairs could also play an important role. These highly reactive radical species typically have significantly higher energy than the FLP, which prompted this investigation into their formation. Herein, we provide evidence that the classical phosphine/borane combinations PMes3/B(C6F5)3 and PtBu3/B(C6F5)3 both form an electron donor–acceptor (charge-transfer) complex that undergoes visible-light-induced SET to form the corresponding highly reactive radical-ion pairs. Subsequently, we show that by tuning the properties of the Lewis acid/base pair, the energy required for SET can be reduced to become thermally accessible.

The Trityl-Cation Mediated Phosphine Oxides Reduction

Reduction of phosphine oxides into the corresponding phosphines using PhSiH3 as a reducing agent and Ph3C+[B(C6F5)4]? as an initiator is described. The process is highly efficient, reducing a broad range of secondary and tertiary alkyl and arylphosphines, bearing various functional groups in generally good yields. The reaction is believed to proceed through the generation of a silyl cation, which reaction with the phosphine oxide provides a phosphonium salt, further reduced by the silane to afford the desired phosphine along with siloxanes. (Figure presented.).

Effects of substituents on aryl groups during the reaction of triarylphosphine radical cation and oxygen

In a previous report (S. Yasui, S. Tojo and T. Majima, J. Org. Chem., 2005, 70, 1276), we presented the results from the laser flash photolysis (LFP) and product analysis of the 9,10-dicyanoanthracene (DCA)-photosensitized oxidation of triarylphosphine (Ar3P) in acetonitrile under air, which showed that the photoreaction results in the oxidation of Ar3P to give the corresponding phosphine oxide (Ar3PO) in a nearly quantitative yield, and that the reaction is initiated by the electron transfer (ET) from Ar 3P to DCA in the singlet excited state (1DCA*), producing the triarylphosphine radical cation Ar3P+. This radical cation decays through radical coupling with O2 to afford the peroxy radical cation Ar3P+-O-O, which we proposed to be the intermediate leading to the product Ar3PO. We now examined this photoreaction in more detail using ten kinds of Ar3P with various electronic and steric characteristics. The decay rate of Ar3P + measured by the LFP was only slightly affected by the substituents on the aryl groups of Ar3P. During the photolysis of trimesitylphosphine (Mes3P), the peroxy radical cation intermediate (Mes3P+-O-O) had a lifetime long enough to be spectrophotometrically detected. The quantum yields of Ar3PO increased with either electron-withdrawing or -releasing substituents on the aryl groups, suggesting that a radical center is developed on the phosphorus atom during the step when the quantum yield is determined. In addition, the o-methyl substituents in Ar3P decreased the quantum yield. These results clearly indicated that Ar3P+-O-O undergoes radical attack upon the parent phosphine Ar3P that eventually produces the final product, Ar3PO. The Royal Society of Chemistry 2006.

NON-METAL DERIVATIVES OF THE BULKIEST KNOWN TERTIARY PHOSPHINE, TRIMESITYLPHOSPHINE

Trimesitylphosphine (Pmes3) is the bulkiest known tertiary phosphine.The low yield synthesis of this known compound, its failure to form a carbon disulfide adduct or a selenide derivative, the relatively low values for ν(P-O), ν(P-S) and ν(P-H) in its oxide, sulfide and phosphonium salt respectively, and the high barriers to rotation about the P-Cipso bond in Pmes3, and its derivatives all attest to this fact.Nevertheless, Pmes3, does show chemistry typical of most tertiary phosphines due to its relatively high basicity and the electronic character of even this most bulky phosphine must also be considered in any discussion of its chemistry.Key words: Tertiary phosphine; triaryl phosphine; steric effect; electronic effect.