Phosphorus pentaphenyl

Only in the 1960s did and compounds become established. The comparatively rare 2-connected phosphanes are represented by h d phosphenium cations (6.5a) and phosphide anions (6.5b), while and compounds are represented by phosphorus pentaphenyl (6.5c) and the hexaphosphoride anion (6.5d). 

Phosphonium salts are produced by acidification of an appropriate ylid (6.422) or by adding halophosphines or alkyl halides to ylids (6.377, 6.378). Phosphonium cations are obtained by reacting phosphorus pentachloride with phosphorus pentaphenyl (6.477) or with lithio biphenyl (480). Oxiranes react with phosphines to produce betaines which, in the presence of acids, give hydroxy-alkyl phosphonium salts (3.114). 

That the carbonyl olefination by phosphorus ylides was in fact discovered rather incidentally in the course of experiments aimed at the study of pentaphenyl-phosphorane and the syntheses of new derivatives thereof is clearly demonstrated in the title of the original publication by Wittig and Geissler Zur Reaktionsweise des Pentaphenylphosphors und einiger Derivate 33). 

As pointed out in the introduction (Section 3.16.1) neutral 1,3-diphospholes and 1,3-benzodiphos-pholes contain a pyramidal phosphorus atom and are not fully conjugated their anions, however, are. Synthesis is therefore generally aimed at these anions, i.e. at alkali metal 1,3-diphospholides (11) and 1,3-benzodiphospholides (3). In some cases they can be converted to neutral diphospholes by protonation, alkylation, silylation, acylation, or oxidation (see Section 3.16.3.5.1). 1,3-Diphos-pholes and 1,3-benzodiphospholes have also been prepared by direct synthesis. 1,1,1,3,3-Pentaphenyl-lA 3A -benzodiphospholes have also been reported <95AG2186>. 

This has led to interesting phosphorus-boron complexes. Pentaphenyl phosphorus will donate a phenyl anion to triphenylborane ... 

This was the first approach used for the synthesis of a monocyclic phosphole, the pentaphenyl derivative, and stands as a landmark in heterocyclic phosphorus chemistry. In 1959, two laboratories reported the synthesis of this compound. In one synthesis, the dilithio derivative of tetra-phenylbutadiene was reacted with phenylphosphonous dichloride (Scheme 61) the reaction proceeded in excellent yield (84% <59JA3163,60JA5099 . Since the starting diene is easily prepared from diphenylethyne and lithium, it remains the best way to prepare this derivative, which has found use in many subsequent studies. In addition to the P-phenyl derivative, the P-benzyl compound was also reported (38% <6UA4406 . 

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