Phospholide, phosphonio-substituted

Transformation of a phosphonio-substituted phospholide derivative under conservation of the r-electron system has been demonstrated in a number of cases which include substitution of a ring-hydrogen atom, replacement of a PRs -moiety by a hydride, or reductive de-arylation of a PhsP " - to a phosphinyl substituent PPh2. Subsequent re-quaternisation of the latter allows the formal substitution of a PhsP " - by a modified Ph2(R)P -moiety in a two-step reaction. The application of this scheme is not only perfectly suitable for the synthesis of bis-phosphonio-benzophospholides with different phosphonio-moieties but, since the last reaction stage tolerates a variety of functional groups in the electrophile, offers as well a convenient pathway for the synthesis of side-chain functionalised phosphonio-phospholide derivatives from more simple substituted precursors. 

Table 1 Average values of selected bond distances in phosphonio-substituted phospholide derivatives (data from [16, 20-24, 27, 33, 34]). Endocyclic bonds refer exclusively to those in the phosphole rings if annulated ring systems are present...

The phosphonio-substituted phospholide derivatives known to date were characterised routinely by multinuclear NMR and UV-VIS spectroscopy, and mass spectrometry. Beside serving the purpose of compound identification, the spectroscopic data allowed some important conclusions concerning trends in electronic structures of different molecules. 

In spite of the general ambiphilicity of phosphonio-substituted phosphoHde derivatives, the aromaticity of the phosphoHde ring [10, 11] tends to reduce their electrophilicity while the intramolecular compensation of the negative charge by the phosphonio-substituents lowers at the same time their nucle-ophilicity [15, 16]. Bis-phosphonio-benzophospholides and -1,2,4-diaza-phospholides are therefore less reactive towards electrophiles and nucleophiles than other types of phosphorus containing multiple-bond systems and lack the notorious hydrolytic instabihty of many of these species [15, 16, 24]. Reactions are observed, however, with sufficiently strong electrophiles such as triflic acid or methyl triflate, or nucleophiles such as OH" or lithium alkyls, respectively. 

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