Phosphorus halides reduction

Secondary phosphines can be prepared by three main routes. The first method, reduction of a phosphinous halide, R2PCI with UAIH4 (Scheme 12) requires an efficient synthesis of the starting phosphorus halide, as both the starting material and the product may be difficult to purify. 

Lithium tetrahydroaluminate reduction of phosphorus halides is a route to PHj and primary and secondary phosphines ". Phosphorus trichloride at — 115°C in Me O is reduced to PHj in 79% yield ... 

Alkali-metal tetrahydraborate reduction of phosphorus halides is of limited utility . Lithium tetrahydroborate reaction with PClj or PClj in Et20 at — 80°C produces PHj, but yields are low" ... 

Electrochemical reduction of l-methyl-2-oxo-5,6-diphenyl-l, 2-dihydropyrazine has been discussed in Section 6F (1096, 1097). Direct amination of the oxo group appears to be unexplored, and little is known of the action of phosphorus halides except that a number of 1-alkyl-3,5,6-trichloro-2-oxo-l,2-dihydropyrazines has been prepared from 1,4-dialkylpiperazine-2,5-diones and phosphorus pentachloride (853) Section 9A(4)]. 

In related Pd(Xantphos)-catalyzed cross-couplings of benzyl halides, reductive elimination was proposed as the slow step, since the overall rate increased with large bite angle ligands [98], P-C bond formation with isolated diastereomers of dinucleoside H-phosphonates was stereospecific, likely with retention of configuration at phosphorus, as above (Scheme 58). 

The reduced phospholes are never made directly by hydrogenation of phospholes most commonly, the ring is created in the desired state of reduction while phosphorus is in the oxide or sulfide form, and the process is completed by removal of these atoms. For the synthesis of 2- and 3-phospholenes, the McCormack cycloaddition of trivalent phosphorus halides with dienes, followed by hydrolysis, is the method of choice for obtaining the requisite P-oxides. This process, first described in US patents <53USP2663737,53USP2663738) by W. B. McCormack of the duPont Co., has... 

In a similar vein, the keto bridge in 5 can be replaced by oxygen with retention of activity. Reduction of acetophenone derivative 19 by means of sodium borohydride leads to the corresponding alcohol (20). Reaction with phosphorus tribromide with cyanide gives... 

Detailed investigations of the chemical reactivity of the diketiminato-stabilized phosphenium cations like 28 (Scheme 17) are to date rare and include only two reports dealing with the substitution and reduction of P-halogen-derivatives. Thus, reaction of 28 (X=Br) with sodium hydroxide in toluene was reported to proceed with displacement of the halide substituent at phosphorus and conservation of the heterocyclic ring to give a mixture of bromide and triflate salts containing a P-hydroxy-substituted cation, both of which were isolated in small yields [89], The products are remarkable as they represent one of very few examples of a stable phosphinous acid which does not rearrange to the tautomeric secondary phosphine oxide. Potassium reduction of the P-chloro-substituted derivative 34 produced the... 

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