Mild Reduction of Phosphine Oxides with Phosphites To Access Phosphines

Abstract

Anew method for the iodine-catalyzed reduction of phosphine oxides with phosphites at room temperature is reported. The mild reaction conditions,scalability,and simple purification requirements render it amethod of choice for the large-scale production and facile regeneration of av ariety of phosphines.M echanistic studies,s upported by DFT calculations of the oxygen transfer between the starting phosphine oxide and the phosphite reagent, are also presented. Such transmutations of phosphorus species were previously unknown.Phosphines and their derivatives represent an important class of compounds in the life sciences and the chemical industry.T rivalent phosphines are used in numerous classical organic transformations,s uch as the Mitsunobu, [1] Staudinger, [2] Wittig, [3] Rauhut-Currier, [4] and Appel reactions. [5] Furthermore,i no rganometallic chemistry,t hey represent prime ligands to control most transition-metal-catalyzed reactions owings to their excellent metal ligation properties. [6] Numerous methods for preparing tertiary phosphines have been reported, including the alkylation of halogenated phosphorus compounds (e.g., PCl 3 ,R 2 PCl) with organometallic reagents, [7] nucleophilic and free radical addition of R 2 PH to alkenes, [8] and the coupling of R 2 PH or R 2 PCl with aryl triflates in the presence of NiCl 2 (dppe). [9] Ther eduction of phosphine oxides to phosphines is also important for the recycling of phosphorus resources.I np articular from an industrial standpoint, phosphine oxides are stoichiometric byproducts of the above-mentioned organic transformations, and are generally discarded as intractable waste in large quantities.T he reduction of phosphine oxides remains amethod of choice for the synthesis of P-chiral phosphines. [10] Chemical reduction of phosphine oxides to phosphines has drawn alot of attention and has been performed with several reductants such as metal hydrides (e.g., hydrosilanes,LiAlH 4 , alane), low-valent metal reductants (e.g., SmI 2 /hexamethyl-phosphoramide,Cp 2 TiCl 2 /Mg,Sipowder), and organic reductants (e.g., hydrocarbon/activated carbon, hexaethylphosphorus triamide/POCl 3 ). These procedures,h owever,r equire harsh reaction conditions and highly reactive and/or explosive reducing agents.C ÀPb ond cleavage also occurs as as ide reaction. [11] Such reaction conditions prevent scale-up and generate even more waste,m aking them not suitable for industrial use.In 2011, Laven and Kullberg patented an iodine-catalyzed reduction of aromatic phosphine oxides to phosphines at room temperature. [12] In essence,s acrificial electron-rich tributylphosphine is used for the capture of the oxygen atom from electron-deficient phosphine oxides.A side from the hazard, high cost, and instability of tributylphosphine, difficulties were encountered in separating it from some of the phosphine products.A ctually,t his very interesting reaction does not solve the industrial issue of waste treatment as anew phosphine oxide waste is created.Theu se of sacrificial triphenylphosphine...

A Mild One-Pot Reduction of Phosphine(V) Oxides Affording Phosphines(III) and Their Metal Catalysts

“…Thus, much attention has been focused on the conversion of P (V) =O to P (III) 15 , 16 ( Scheme 1 a), including the use of silanes and siloxanes such as HSiCl 3 , 22 − 25 HSiCl 3 /Ph 3 P, 26 Si 2 Cl 6 , 24 , 27 Si 2 Me 6 with CsF/TBAF, 28 HSi(OEt) 3 /Ti(O- i -Pr) 4 , 29 PhSiH 3 , 30 − 32 1,1,3,3-tetramethyldisiloxane (TMDS) with CuX 2 , 33 polymethylhydrosiloxane (PMHS), 34 , 35 1,3-diphenyldisiloxane (DPDS), 36 and (EtO) 2 MeSiH/(RO) 2 P(O)OH; 37 aluminum hydrides such as LiAlH 4 , 38 , 39 LiAlH 4 /CeCl 3 , 40 AlH 3 , 41 and HAl( i -Bu) 2 ; 42 low-valent metals such as SmI 2 /HMPA (hexamethylphosphoramide) 43 or Cp 2 TiCl 2 /Mg; 44 hydrocarbon/activated carbon; 45 and electrochemical reduction. 46 − 48 A mild iodine-catalyzed reduction of phosphine(V) oxides employing a sacrificial electron-rich phosphine was developed by Laven and Kullberg, 49 while Li et al 50 employed less expensive phosphite, although in both cases P (V) =O-containing contaminants must be removed from the final products. Thus, disadvantages of these procedures include harsh reaction conditions, toxic and/or highly reactive, potentially explosive reducing agents, narrow scope or undesirable side reactions, e.g.…”