Phospholes—

A further report of the synthesis of phospholes from the conjugated-diene-dihalogenophosphine adduct and DBU has appeared.  

Reduction of 1-benzyl-3,4-dibromophospholan oxide (125) with tri-chlorosilane, followed by debromination, gave 1-benzylphosphole. Determination of the molecular structure by X-ray analysis showed slight puckering of the ring with retention of pyramidal configuration at phosphorus.  

I-Phenyl-A -phospholen-l-oxide (126) and /rani -l,4-diacetoxybutadiene gave (127), which could be converted to the phosphole oxide. Reduction 

Mathey, R. Mankowski-Favelier, and R. Maillet, Bull. Soc. chim. France, 1970, 4433. 

A simplified route to penta-arylphosphoranes aryl-lithiums on the iminophosphoranes (1). 

The aromatic properties of phosphinines and phospholes have been extensively reviewed and much discussed in relation to their physical and chemical properties. 

Data from NMR studies does not appear to have been of great help in this connection. It is now believed that the replacement of C by P in a tt system does not alter the course of many reactions [43]. 

Phosphinine (phosphorin or phosphabenzene), C5H5P, is a colourless volatile liquid which is very reactive, air-sensitive and characteristically odorous. It was first prepared by Ashe in 1970 [39] using reaction (6.860). Arsenin can be prepared by a similar reaction using arsenic trichloride. 

Published data on the properties and reactions of phosphinine is still somewhat limited. A few years earlier the first phosphinine derivative, 2,4,6 triphenylphosphinine was synthesised by Markl from pyrilium fluoroborate (6.861). Electron diffraction and crystal structure analyses have confirmed the presence of planar rings with equivalent P-C bond lengths in both these compounds (6.862). The intense UV spectra, ring-vibration frequencies and low-field NMR shifts of the ring protons are all consistent with 3pji-2pjr aromatic delocalisation in this class of compound. Addition readily occurs (6.863). 

Derivatives of phosphole (6.864) were first synthesised by Markl [50] using the method (6.865). Another synthetic method, due to Quin, [51] is from 3-phospholenes (6.866). 

Interest in the controversial issue of the aromaticity of phosphole continues. The photoelectron spectroscopic (p.e.s.) data of Schafer et al (noted in the previous Report), which were interpreted as indicating that the phospholes (164) are aromatic systems which do not involve appreciable delocalization of the lone pair on phosphorus with the diene jr-system, have now been re-evaluated, using a different theoretical approach. It has been concluded that there is a significant interaction between the lone pair on phosphorus and the diene n -system, leading to a traditional aromatic stabilization of the phosphole system in both planar and pyramidal geometries, consistent with earlier assessments of the aromaticity of phosphole that were based on studies of n.m.r. spectra, structure, and reactivity. The non-planarity 

An interesting new development in aromatic phosphole chemistry has been the preparation and subsequent study of the electrophilic substitution reactions of transition-metal complexes of phospholyl anions, e.g. the phosphaferrocenes (165), which are obtained by the reaction of the appropriate P-phenylphosphole with transition-metal cyclopentadienyls or carbonyls, - and which undergo acetylation at 

Metallation of the phosphole sulphides (167) with t-butyl-lithium, followed by treatment with an ester, gives the functionalized phosphole sulphides (168), which can be desulphurized by heating with, e.g., tri-n-butylphosphine to form the functionalized phospholes (169).  

Phospholes bearing a functionalized exocyclic P-substituent have been prepared from the bromophospholen (170) by quaternization followed by dehydrohalogena- 

An attempted synthesis of the betaine (171) from the reaction of 1,2,5-triphenyl-phosphole and o-bromophenol at 250 °C leads to the formation of bromobenzene, diphenyl phenylphosphonate, hydrogen bromide, and 1,4-diphenylnaphthalene. The latter does not arise by addition of benzyne (from a possible mode of decomposition 

A general method of synthesis of phosphole derivatives is illustrated by the preparation of l-phenoxy-3,4-diphenylphosphole 1-oxide (112) by a bromination-debromination sequence as shown.  

Further reports on the preparation of phospholes, e.g. (113), by the addition of phosphines to diacetylenes have appeared. Braye and coworkers found that the reaction was best catalysed by concentrated potassium hydroxide or by means of cuprous or mercury salts. Contrary to previous reports the free radical reaction, catalysed by AIBN, also gave good yields. A full account has now been produced of the low inversion barrier of phospholes (114). The energy barriers to inversion of phos-phindoles (115) and dibenzophospholes (116) are significantly higher, results interpreted in terms of disruption of stabilization due to phosphole aromaticity in the planar transition state. The site of protonation of 

Oehling, W. Schafer, and Armin Schweig, Angew. Chem. Internat. Edn., 1971, 10, 656. 

5-triphenylphosphole is the phosphorus atom. Some phospholium salts (117) are remarkably stable. 

5-Phenyl-dibenzophosphole (164) results from the reaction of tetraphenyl-phosphonium chloride with certain lithium dialkylamides a free-radical mechanism 

Photolysis of the phosphine (166) gives the benzophosphole (167) in good yield.154 The dilithium aldimine (168) reacts with phenyldichlorophosphine to form the benzophosphazole (169).155 

The products of the reaction of the benzophosphole (170) with dimethyl acetylene-dicarboxylate include the ylide (171) and the benzodihydrophosphonin (172).15  

The chlorodiazaphospholine (173) readily loses hydrogen chloride, via the phosphanylium ion (174), to form the diazophosphole (175).167 

Ronman, Ph.D. Thesis, Florida State University, 1975 (Univ. Microfilms, Order No. 75-17948). 

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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 sigmatropic rearrangement of 1//-phospholes to the 2H derivatives gives a new entry to novel 1-phosphanorbomadienes after trapping with tolane. 

CN 4 1//-phospholes make up the largest class of structurally characterized phospholes and encompasses their metal complexes, and (r ,A -derivatives (Table 11). The metric parameters associated with these ring systems... 

The phospholide ions are also used in the synthesis of 1//-phospholes. However, in a paper that is of great importance in phosphole chemistry, 1//-phospholes were found to be quite unstable, undergoing a [1,5] sigmatropic hydrogen shift to form 2//-phospholes (e.g. (108)) which then dimerize <82TL511> by a [4 + 2] cycloaddition (e.g. to (109)). This reaction can be so rapid that the parent 1//-phosphole and some substituted derivatives can only be observed at low temperatures, and dimerization is complete at room temperature (Scheme 14) <83PS(l8)5i>. 

The discovery that 1//-phospholes are unstable relative to 2//-phospholes is of theoretical significance, and represents a unique difference from pyrrole chemistry. The subject has received comment in Section 2.15.7.1. 

Phosphole A Weakly Aromatic Heterole With a Reactive Heteroatom... 

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