Reactions of Phosphoranes

Reactions of Phosphoranes.—Dimethyl(trichloro)phosphorane (94) reacts with 1-acetylsemicarbazides to form the bicyclic phosphoranes (95). . Y-Ray analysis of the bicyclo[3,2,0]heptane derivative reveals a distorted tbp structure for (95). 

Kinetic studies of alkene-phosphorus pentachloride reactions in benzene show the effects of substituents when the double bond is terminal.88 When the alkene is conjugated, the standard work-up conditions (using sulphur dioxide) produce alk-1-enylphosphonyl dichlorides (103), instead of 2-chloroalkylphosphonyl dichlorides (104).87 

The formation of 2-chloroalkylphosphonyl dichlorides (105) from tertiary alcohols and (101) has been ascribed to the intermediate formation of alkenes, as shown.88 Tertiary alcohols are chlorinated by (101), in a mild, efficient procedure which usually occurs with retention of configuration,89 as for (106). 

The reaction between benzyl cyanide (107) and phosphorus pentachloride (101) has been shown to be dependent on both solvent and temperature.90 This allows a rationalization of the century-old result of Claisen,91 who isolated the cw-dichloride 

CN PhCH2CN PhCCl=CaN=PCls — - PhCClJCClJN=PCl3 

Several papers have been devoted to the subject of reactions of phosphoranes with carboxylic acids and their derivatives. Thus triphenylphosphine dibromide (111) in acetonitrile cleaves lactones,93 while the corresponding dichloride (112) converts esters into acid chlorides.94 The reactions of esters with phosphorus pentachloride (101) have been studied further,95 and the influence of structural changes on the yields of products (113) and (114) has resulted in minor modifications to the mechanism previously96 outlined. 

Phosphonyl and phosphinyl fluorides can be obtained from fluorophosphoranes by using the appropriate siloxy compound (6.191,6.192). The hydrolytic stability of fluorophosphoranes increases with the number of P-C bonds present, and generally the sequence is 

In the series of trigonal bipyramidal compounds Me PF5 , the observed apical P-F distances increase with the value of n. The F atom is most loosely bound to P in Me4PF (gas) which most readily changes to the phosphonium form Me4P+ F (solid) [20,21]. A similar scheme of site occupancy occurs if F is replaced by CF3 and Me is simultaneously replaced by Cl. 

Fluorophosphoranes attack silica glass when moisture is present, but can be kept in Teflon or stainless-steel containers. Halophosphoranes react with oxygen donors such as sulphur dioxide to 

Phosphine oxides can be obtained from dihalophosphoranes by reaction with phenol (6.526), and with hydrogen under pressure, phosphines are obtainable (6.527).  

Halophosphoranes of type R4PX and (RO)4PX appear to exist mostly in ionised forms. Covalent and ionic solid forms have been isolated for phenyl chloro derivatives, for example, (6.528). Recently 

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Reactions of Phosphoranes.—The reactions of phosphorus pentachloride (101) with simple organic molecules continue to attract attention, notably in the Russian literature. For example, the preparative uses of alkene-addition reactions of (101) have been examined for a-methylstyrene (102), as outlined in Scheme 7.85... 

The disproportionation of III into IV and V can be rationalized on the basis of the known reaction of phosphoranes with halogen sources giving a-haloalkyl-phosphonium salts. 

Before proceeding to the preparation and reactions of phosphoranes, it is important to recall some of the fundamental properties of the pentacoordinated P atom, but since this... 

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