Phosphole aromaticity

Concerning the question of phosphole aromaticity there are some indications of 6-rr-delocalization from NMR spectroscopy and other physical methods. The formation of metal complexes with derivatives of Mn, Fe and other metals, however, gives the best chemical proof of such 6ir- delocalization but there are still some doubts arising from the considerable differences in chemical behaviour compared with pyrroles, furans and thiophenes. This once more points to the very different properties of phosphorus, compared to nitrogen in particular. Therefore the problem is still open and much work, both practical and theoretical, remains. 

The lower barrier in phospholes relative to the acyclic phosphines is attributed to stabilization in the planar transition state. This stabilization is assigned to cyclic (3p-2p)7r delocalization in P and (4p-2p)7r de-localization for arsenic. The increased barrier upon annelation in the phosphorus system may be attributed to a disruption of the phosphole aromaticity. Similar effects have been noted in indoles. 

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... 

Phospholes can readily be prepared on a large scale and are known with a vast range of substituants [6, 16b,c]. However, the synthesis of oligo(phosphole)s analogous to (C) (Fig. 1) is a real synthetic challenge since the low aromatic character of phosphole prevents the functionalisation of the P-Ca,a carbon atoms via electrophilic substitution and inhibits their preparation using electropolymerisation. As a consequence, no poly(phosphole)s have yet been reported, although bi- and tetra-phospholes have been prepared by stepwise routes (Scheme 2). 

One may consider phospholes to belong to the family of five-membered P-heterocycles pyrrole, furan, and thiophene. A significant difference, however, is that the phospholes described in the literature display only a slight extent of aromaticity. This is well demonstrated by the comparison of the Bird-indexes [32] of benzylphosphole [33], furan, pyrrole, and thiophene (Fig. 1). The Bird-index is an indicator of aromaticity based on the bond-equalizaton. It is the maximum (100) for benzene. 

Beside the bigger size of the phosphorus atom, as compared to that of nitrogen, the lack of aromaticity is due to the P-pyramide the criterion of coplanarity is not fulfilled and so the lone electron pair of the phosphorus cannot overlap with the pz orbitals of the sp2 carbon atoms (Fig. 2). While in the case of pyrrole, the aromatic stabilization covers the energy requirement of planarization, in the case of phospholes, there is a bigger barrier for the inversion. 

Fig. 3 Stereostructure and electron-delocalization in hypothetical aromatic phospholes...

Nyulaszi summarized the theoretical background of the aromaticity of phospholes and the possibilities of establishing aromatic phospholes [34],... 

Aromaticity of 2,4,6-tri-fert-butylphenylphosphole (17d) was also revealed in chemical reactions phosphole 17d could undergo aromatic electrophilic substitution. In reaction with acetyl chloride, a mixture of 2-, 4-, and 5-acetyl phospholes (23a, 24a, and 25a, respectively), as well as a diacetyl derivative (26a) were formed (Scheme 6) [39], Interestingly, the most crowded 2-acetyl derivative (23a) was the main product of the Friedel-Crafts reaction. A similar situation was observed for 3-methylpyrrol [46],... 

Preliminary studies showed that phospholes without any aromaticity could also be involved in reaction with phosphorus tribromide, although these reactions were not too efficient. This means that the above substitution protocol has not much to do with the heteroaromaticity. 

We observed that the slightly aromatic triisopropylphenyl-1 W-phosphole (17b) underwent a sigmatropic rearrangement at 150°C to afford the corresponding... 

The new family of phospholes with 2,4,6-trialkylphenyl substituent on the phosphorus atom show, in many respects, a special reactivity. Due to the flattening of the P-pyramid, the arylphospholes exhibit aromaticity and hence underwent Friedel-Crafts reactions. The regioselective functionalization through reaction with phosphorus tribromide gave a variety of phospholes with an exocyclic P-moiety. Novel phosphole platinum and rhodium complexes were prepared and a part of them was tested in hydroformylation reactions. 

Tricoordinate phosphorus is essentially nonplanar in nature and due to the significant s character of the lone pair in a compound such as phosphole, effective overlap with the carbon p orbitals is inhibited and the compound is nonaromatic. Geometry optimizations and aromaticity analyses performed by Glukhovtsev et al. <1996JPC13447> have shown both pentaphosphole 1 and the bicyclic octaphosphane P8 to be both planar and aromatic in character. 

Modification of DIOP by incorporation of a methyl group at the 3-position (meta) of each aromatic ring increased the optical yield to 90% from 80% for a DOPA precursor (224) inclusion of methyl at the 2-position or at the 2- and 5-positions, or using a phosphole derivative 13 with R =... 

Phosphorus. Early reports on the aromaticity of phospholes (Scheme 52) were controversial.152 X-ray crystallographic data show that 1,2,5-triphenylphos-phole has a nonplanar phosphole ring, while NMR, chemical, and thermodynamical data account for delocalization. Low inversion barriers of the phosphorus atom compared to the saturated congeners suggest nljz conjugation.153 154... 

Photoelectron.—n-Orbital energies appear to be unsuitable to discern the aromatic nature of phospholes (120) due to combined nn and ttn interactions. It is concluded that n conjugative and P-Cw hyperconjugative interactions stabilize the phos-phole system relative to the interrupted cw-butadiene and phosphorus subunits, and that the p.e. spectrum can be interpreted in favour of an aromatic phosphole ring.143 Other workers have discussed144 and reviewed145 this aromaticity problem, and there has been a quantum-chemical study of the aromatic nature of phosphorus hetero-cycles.144 The p.e. spectrum of the ylide (121) contains peaks at 6.19, 8.32, and... 

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