No abstract

No abstract

Direct reductive transformation of triphenylphosphine oxide to triphenylphosphine was performed successfully by electrolysis with TMSCl in an acetonitrile/Bu 4 NBr/(Zn anode)-(Pt cathode)/undivided cell/constant current electrolysis system. A plausible ECEC mechanism involving the formation of silylated phosphorus radical is proposed.

Electroreduction of triphenylphosphine oxide to triphenylphosphine in an acetonitrile solution of tetrabutylammonium bromide in the presence of chlorotrimethylsilane was performed successfully in an undivided cell fitted with a zinc anode and a platinum cathode under constant current. A plausible mechanism involving, (1) one-electron reduction of triphenylphosphine oxide generating the corresponding anion radical [Ph 3 P · -O -], (2) subsequent reaction with chlorotrimethylsilane affording the (trimethylsiloxy)triphenylphosphorus radical [Ph 3 P · -OSiMe 3 ], and (3) further one-electron reduction followed by P-O bond fission leading to triphenylphosphine is proposed. In a similar manner, electroreduction of some triarylphosphine oxides and alkyldiarylphosphine oxides was executed to give the corresponding phosphine derivatives in good to moderate yields.In modern organic synthesis, triphenylphosphine (1a) is an important reagent for various organic reactions, such as the Wittig reaction, 1 Mitsunobu reaction, 2 MukaiyamaCorey lactonization, 3 Appel reaction, 4 Staudinger reaction, 5 and so on. In these reactions, however, triphenylphosphine (1a) is turned into triphenylphosphine oxide (2a), which is a stable and flame-resistant chemical; as a result, a significant amount of 2a has been stored as a troublesome waste. Moreover, phosphorus is an exhaustible resource. 6 Therefore, facile methods for the reduction of 2a to 1a have been in great demand from the view points of waste treatment and the recycling of phosphorus resources (Scheme 1). 7 Scheme 1 Recycling of triphenylphosphine (1a)Reduction of triphenylphosphine oxide (2a) to triphenylphosphine (1a) (Scheme 1, path A) has been intensively investigated, and performed successfully with various reductants, such as metal hydrides (hydrosilanes, 8 aluminum hydrides 9 ), low-valent metals (SmI 2 /HMPA, 10 TiCp 2 Cl 2 /Mg 11 ), and organic reductants [hydrocarbon/activated carbon, 12 (Et 2 N) 3 P/POCl 3 13 ]. The P=O bond of 2a is sufficiently strong (ca. 500 kJ/mol) that highly reactive reductants are required to cleave it efficiently. The reported procedures to date are not practical, since they always required more than a stoichiometric amount of the reductant, and this generally expensive, explosive, and/or not easy to handle.As an alternative access to triphenylphosphine (1a), chlorination of triphenylphosphine oxide (2a) leading to triphenylphosphorus dichloride (3a) and subsequent reduction of 3a to 1a has been investigated thus far (Scheme 1, path B). Phosphorus dichloride 3a is easily prepared by treatment of 2a with chlorinating reagents such as phosgene and oxalyl chloride. 14,15 It is reasonable to assume that reduction of the P-Cl bond of 3a proceeds more smoothly than that of the P=O bond of 2a, since the P-Cl bond (ca. 310 kJ/mol) 16 is far weaker than the P=O bond. Indeed, reduction of 3a to 1a was performed by hydrogenation with transition-metal catalysts (Pt, Rh, and Pd), 17,18 and reduction with several metals such as sodium, 19 aluminum, 20 sili...

One-pot transformation of triphenylphosphine oxide to triphenylphosphine was achieved by the reaction of triphenylphosphine oxide with oxalyl chloride, which led to the formation of triphenylphosphine dichloride, and subsequent reduction of triphenylphosphine dichloride with a combination of aluminum-catalytic metal salt.