Study On The Synthesis Of Triphenylphosphine By Electrochemical Reduction Of Triphenylphosphine Oxide

Catalytic synthesis mediated by organic phosphine TPP caused a large accumulation of phosphine oxide TPPO.The traditional reduction methods is to regenerate TPP by using strong reducing agents such as silane,borane and aluminum(hydride)to obtain the target product.At the same time,it also has obvious disadvantages such as expensive reducing agents and hidden safety hazards in the production process.Electrochemical reduction can be carried out under mild conditions and is environmentally friendly.Many electrochemical reactions have been successfully applied in industry.Compared with the thermodynamically stable P=O bond(?546 kJ/mol)of direct reduction,the dissociation energy of P-Cl bond(H₂P-Cl,315.1 kJ/mol)and the activated P-O bond(H₂P-OH,359.7 kJ/mol)have more advantageous obviously,so that TPP can be obtained more efficiently.The reduction selectivity of electrolysis can be improved by adding Lewis acid(such as AlCl₃)in combination with coordination.The R in ROTf activates the phosphoryl P=O bond of TPPO through O alkylation.In this paper,the electrochemical reduction process and efficiency of TPPO are studied in the above two different activation directions,and the efficient reduction of TPPO is realized..The main contents are as follows:1.In order to improve the selectivity of electroreduction and achieve high reduction yield,we discussed the possibility of electroreduction by chlorination,that is,the electroreduction of TPPCl₂resulted in an Al working electrode with the best reduction performance.To 3 F/mol increases the reduction yield to 47%.Secondly,considering that Al³⁺will form a dense passivation layer on the electrode surface when it is used as an oxygen trap,we use TBACl as a soluble chlorine source to reduce the amount of Al and slow down the formation of the passivation layer;amine ligands are introduced to increase the Al³⁺.The stripping rate,after comparing with DEA,TEA,TPMA,TBAA,PHEN and other ligands,it is concluded that TMED can significantly improve the efficiency of electroreduction.Based on the above research,we can greatly limit the combination of aluminum chloride AlCl₃(0.66 equivalent),tetramethyl ethylenediamine(0.34 equivalent),TBAPF6(0.04 M)and tetrabutylammonium chloride(0.16 M).The stable TPPO electro-reduction voltage,and the reduction yield of 89%was obtained.2.With the addition of alkylation as the entry point,the R of ROTf is used to activate the P=O bond in TPPO using the oxyalkylation mechanism.Firstly,[TPP(OEt)](OTf)was synthesized through a simple pre-reaction step,and it was characterized by NMR,ESI,FTIR,etc.,which differed from triphenylphosphine by nearly 36 ppm displacement and P=O bond stretching Vibration was significantly weakened in FTIR and other results all confirmed the formation of[TPPOEt]⁺,that is,the activation of TPPO by oxyalkylation.Based on this,by adding different Lewis acids as OEt acceptor and OTf acceptor to increase the breaking tendency of the P-O bond during the reduction process,it was confirmed that when 0.6 M Sc³⁺was added,the reduction yield of TPP was 98%,and its Faraday efficiency is close to 60%.The influence of different working electrodes on the reduction yield is also studied in the article.Finally,XPS was used to study the changes in the binding energy and valence state of Sc before and after the reaction.The same binding energy peak position as pure Sc³⁺shows that its valence state has not changed before and after electrolysis,and it maintains the same+3 oxidation state.The computational chemistry results of other researchers made reasonable speculations on the mechanism of the electrochemical reduction process,laying the foundation for the reduction of TPPO to TPP.