Phospholes review

The fourth chapter of this volume comprises the second part of an ongoing series by Professor A. P. Sadimenko (Fort Hare University, South Africa) dealing with organometallic compounds of pyrrole, indole, carbazole, phospholes, siloles, and boroles. This follows the review in Volume 78 of Advances covering organometallic compounds of thiophene and furan. The enormous recent advances in this area are summarized and classified according to the nature of the heterocycle and of the metals. 

The phosphacyclopentadienes with phosphine function are called 1//-phospholes and they form an important part of contemporary P-heterocyclic chemistry. Phosphole chemistry has undergone an intensive development, which is well demonstrated by the fact that while in the first edition of Comprehensive Heterocyclic Chemistry (1984), only several pages were devoted to phospholes [1], the second edition (1996) discusses this topic in a lengthy chapter [2], Besides these, exhaustive monographs have been published in P-heterocyclic chemistry also incorporating the new developments of phosphole chemistry [3, 4], The first review in the subject was written by Mathey [5],... 

The co-ordination chemistry of heterocyclic phosphines, including various phospholes, has been reviewed.188... 

Both arsoles and stiboles can act as ligands in reactions with metal carbonyls. The arsoles have been studied in depth and are reviewed by Abel (79MI11801). They are particularly versatile, forming a wide range of derivatives. In this respect their chemistry closely parallels that of the phospholes. This is illustrated by Scheme 13. Reaction of either l-chIoro-2,3,4,5-pentaphenylarsole or the corresponding alkali metal compound with a transition metal... 

Up to 1994 the majority of publications on five-membered monoheterocycles were on phospholes. Since 1995, thiophene has become the ligand of the year. The organotransition metal chemistry of thiophene has been the subject of a substantial number of reviews on its pure and applied chemical... 

As discussed in Section 3.15.2.1 (Scheme 3), lf/-phospholes can be transformed into 2/f-phosphoies upon heating. To date, very few stable 2f/-phospholes are known <2004ACR954>. However, a great variety of 2f/-phospholes are accessible using the lf/-phosphole/2/f-phosphole equilibrium, and their heterodiene behavior makes them powerful intermediates for the synthesis of P-heterocycles. Only the results that have appeared since 1996 are presented in this section for a general overview of the chemistry of 27f-phospholes, an excellent review by Mathey is available <2004ACR954>. 

As already mentioned, the chemistry of reduced phospholes is extremely vast and the description of all the synthetic methods leading to these species is beyond the scope of this section. The reader should refer to a number of excellent reviews to give an overview of the syntheses and applications of reduced phospholes <1994CRV1375, 2002CRV201, 2003CRV3029>. In this section, selected examples illustrating new trends are presented. 

This review shows that the growing interest aroused by phospholes in coordination chemistry can be attributed to three main reasons 1. The influence of the partial delocalization of the phosphorus lone pair upon the complexing ability of phospholes. 2. The discovery of phosphametallocenes. 3. The tremendous variety of complexing modes found for the phosphole nucleus. For a given phosphole, no less than ten different types of complexes can be obtained. 

Numerous recent reviews describe the properties of the phosphole ring. Our aim here is just to summarize the salient points which are useful for understanding the behaviour of phospholes towards transition metal derivatives. 

From all the data summarized in this review it clearly appears that the potential of phospholes in coordination chemistry is enormous. As 2-electron ligands their behavior is already original. Contrary to what could be expected on the basis of their potentially aromatic structure, metal-phosphole bonds are strong due to three contributing factors ... 

Phospholes bearing additional heteroatoms other than phosphorus have also been the subject of further study. Cycloaddition reactions of heterophospholes have received a theoretical treatment and these reactions have also been reviewed. Improvements in the synthesis of fused 1,3-azaphospholes via the reactions of N-alkyl-isoquinolinium salts with phosphorus trichloride have been described and their reactivity towards cycloaddition studied. New fused... 

Interest has continued in the extent to which factors affecting the planarity of the trivalent phosphorus atom have a bearing on the aromaticity and other properties of the phosphole ring system, and a review has appeared. A study of the coordination chemistry of phospholes bearing a sterically bulky substituent at phosphorus has shown that coordination to platinum results in increased pyramidality at phosphorus. In the same vein, an ab initio theoretical study of the triphosphole (357) has shown that the steric interac-... 

D. The calculations confirm that the negative end of the dipole is towards phosphorus. - The calculated dipole moments of phosphole (1.9 D) and pyrrole (2.0 D) are similar, and, unlike furan, the positive ends of the dipoles are towards the heteroatoms. Dipole moments have been used, in combination with results from other methods of study, to estimate the preferred conformations of the dichloride (179), of the phosphites (180), and of triarylphosphine oxides. The use of dipole moments to aid stereochemical studies of compounds has been reviewed. Additive polarizability parameters should not be used in the calculations, and it has been recommended that data should be obtained from model compounds containing identical environments for the phosphorus atoms. The sensitivity of bond moments to structural changes has been studied perfluoroalkyl groups lower the phosphoryl bond moment, and the P—N bond moment is very sensitive to the valence state of the phosphorus atom. The conformational analyses of phospho-nates, phosphonamides, silyl phosphates, and a number of dioxaphosphori-nans (181) - have been reported. The P—Se bond moment has been estimated to be 1.24 D. The zwitterionic structure (182) was identified by its high dipole... 

Phosphoies and other reiated heterocycies are an important class of main group compounds. The chemistry of phosphoies and their preparation has been reviewed extensiveiy by Mathey.3 We provide details here for a simple, one-pot procedure for the preparation of 1-phenyl-2,3,4,5-tetramethylphosphole applying zirconocene chemistry.4 The procedure involves reduction of (ri-C5H5)2ZrCl2 with butyllithium in the presence of 2-butyne which (as reported initially by Negishi, et al. ) forms a zirconium metallacycle. Addition of dichiorophenylphosphine to this reaction mixture produces the phosphole. One other procedure for the preparation of 1-phenyl-... 

---