Electrochemical and Chemical Reduction of Phosphine Oxides to the Corresponding Phosphines

Phosphines (R₃P) have been used as reductants in Wittig, Staudinger, Appel, and Mitsunobu reactions as well as ligands in various transition metal catalysts.  Though phosphines can be synthesized by reduction of the corresponding phosphine oxides (R₃P=O) with hydrosilanes, P=O bond is so strong that the reduction proceeds only under harsh conditions, and C-P bonds would be cleaved.  We reported that Ph₃P was obtained from Ph₃P=O via Ph₃PCl₂.¹  Since Ph₃PCl₂ is moisture-sensitive, Ph₃PCl₂ is hard to be stored and should be prepared just before the reduction.  We investigated direct reduction of R₃P=O to R₃P under mild conditions.  In this paper, we describe that the reduction proceeds electrochemically   and chemically under 45 ºC in the presence of an efficient oxophile, Me₃SiCl (Fig. 1).

Fig. 1.  Reduction of Phosphine Oxides 1

A typical procedure for electrochemical reduction (ECR) is as follows.  In an undivided cell fitted with Zn anode (1.0 x 1.5 cm²) and Cu cathode (1.0 x 1.5 cm²) was placed an acetonitrile (5 mL) solution of 1 (2 mmol), Me₃SiCl (6 mmol),² and Bu₄NBr (2 mmol).  The electroreduction was carried out under constant current conditions (100 mA, 4 F/mol-1) at 45 ºC.  Usual work-up and purification by silica gel column chromatography gave 2(Table 1, ECR).  

Table 1.  Me₃SiCl-Assisted Electrochemical (ECR) and Chemical Reduction (CR) of Phosphine Oxides 1

The reduction of triarylphosphine oxides 1a and 1c-1e proceeded smoothly to give the corresponding triarylphosphines 2a and 2c-2e in 80-96% yields (Table 1, Runs 1, 3-5, ECR), whereas tris(o-tolyl)phosphine oxide 1b was reduced less effectively to give 2b in 22% yield, presumably due to steric hindrance of the o-substituents (Run 2).  Electroreduction of alkyl-diarylphosphine oxides 1f and 1g was performed in a similar manner to give the corresponding phosphine 2f and 2g in 82% and 46% yields, respectively (Runs 6, 7).  It is worthy to note that no appreciable peaks except for 1 and 2 were observed in ³¹P NMR of the crude products. Electroreduction of triphenyl phosphate 1hwas not successful (Run 8).  .

Similar reduction of 1 was performed chemically (CR) by using Mg as an electron source.  A typical procedure is as follows:  A mixture of 1 (2 mmol), magnesium powder (8 mmol),³ and Me₃SiCl (6.0 mmol)² in 1,3-dimethylimidazolidinone (8 mL) was stirred for 2 hours at room temperature under Ar atmosphere.  Usual work-up followed by silica gel column chromatography gave 2  (Table 1, CR).⁴  Proper choice of the solvent is essential:  the reduction of 1 proceeded smoothly only in DMI, and 2was obtained in less than 10% yields in MeCN and DMF, and no reduction occurred in THF. 

Chemical shift of Ph₃P=O (1a, ³¹P NMR: d 27.3 ppm) did not shift by the addition of Me₃SiCl.  CV curve of 1a showed a single, irreversible wave at -3.0 V versus Ag/Ag⁺, whereas Me₃SiCl did not show any reduction wave at range of 0 - -3.5 V.  Addition of Me₃SiCl caused significant increase of the reduction current without change of the reduction potential of 1a.  From these facts, we propose a plausible mechanism of the reduction of phosphine oxide 1 (Fig. 2).  In the initial stage of the reduction of 1, one-electron reduction would occur to form an anion radical [R₃P·-O]^- 3, which would be trapped with Me₃SiCl to give radical [R₃P·-OSiMe₃] 4.  Further one-electron reduction of 4, leading to anion 5, and subsequent P-O bond cleavage followed by silylation with Me₃SiCl would give 2 together with (Me₃Si)₂O. 

Fig. 2.  A Plausible Mechanism

In conclusion, R₃P=O is reduced to the corresponding R₃P electrochemically and chemically in the presence of Me₃SiCl under mild conditions.

 

¹Yano, T.; Hoshino, M.; Kuroboshi, M.; Tanaka, H.  Synlett  2010, 801-803.

²Though the reduction proceeded with ^tBuMe₂SiCl and ⁱPr₃SiCl similarly, no 2 was obtained with MgCl₂ and SiCl₄, probably because 2 is easily adsorbed on MgCl₂ and SiO₂ derived from SiCl₄, to prevent electron transfer from Mg.

³Mg turnings gave 2in lower yield, whereas zinc powder and aluminum chips did not proceed the reduction at all.

⁴Though CuCl activated Mg to give 2 quantitatively, 2 formed complex with Cu species.