Electroreduction of Triphenylphosphine Oxide to Triphenylphosphine in the Presence of Chlorotrimethylsilane

Abstract

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...

Highly Chemoselective Metal-Free Reduction of Phosphine Oxides to Phosphines

“…On the other hand, relatively little work is known on the catalytic reduction of related PO bonds, which are thermodynamically highly stable. With a bond energy of around 502 kJ/mol, they are significantly more stable compared to typical organic functional groups, and the general order of bond energy stability is P–O > C–H > C–O > C–C > C–N. , Therefore, it is not surprising that highly chemoselective reductions of phosphine oxides until today represent an unsolved problem, although the resulting organophosphines represent valuable intermediates and ligands for transition metal catalysis (Figure ) …”

Electrochemical Reduction of Quaternary Phosphonium Derivatives: A Systematic Optimization Approach

“…One of the most prominent products of the WOR is the industrial synthesis of Vitamin A (BASF, Rhône-Poulenc, DSM) whose annual production has tripled over the past decade (2012, 2700 t/a; 2020, 7500 t/a). , It has remained indispensable in animal feedstock (∼70–80% of yearly production) due to an ever-increasing demand for meat but is also used as a coloring agent, food stabilizer, emulsifier, and vitamin supplement . The recycling of 2 from 1 has so far been conducted via sacrificial agents (silanes, boranes, and allanes). − ,− Alternatives to conventional reductions have gained increasing interest. ,,,− , In fact, one promising alternative is the electrochemical reduction of 1 to reduce P V to P ΙΙΙ , as shown in Scheme…”

“…E2: counter electrode. A: direct reduction. ,,,, B: reduction of in situ generated derivatives. ,,, C: indirect reduction. ,, …”

“…1−7,9−13 Alternatives to conventional reductions have gained increasing interest. 3,6,7,[9][10][11]13 In fact, one promising alternative is the electrochemical reduction of 1 to reduce P V to P ΙΙΙ , as shown in Scheme 1…”

“…1,3,4,10,13 B: reduction of in situ generated derivatives. 2,3,10,13 C: indirect reduction. 2,3,10 Electrochemical Cell Setup.…”