Phospholes, pyramidalization

The influence of the heteroatom on the structure of a series of dinaphtho-fused five membered potentially aromatic ring systems (317), including the phosphorus and arsenic systems, has been studied by crystallographic techniques. A theoretical study has shown that the incorporation of two o, X -phosphorus atoms into the phosphole ring system decreases the ring strain and significantly lowers the inversion barrier about the phosphole pyramidal phosphorus. Pyramidalisation at phosphorus in phospholes has been shown to increase on... 

The planar cyclic Pj anion isoelectronic with cyclopentadienyl anion has been prepared in the form of M Pj" salts (M = Li, Na) by Handler et al. [49], The penta-phosphole anion Pj" favors planar geometry [50] while the most stable structure of P/ is square-pyramidal [51], The negatively charged pentamers Sb and Bi are planar rings [52, 53],... 

The fifth chapter l-(2,4,6-Trialkylphenyl)-lH-Phospholes with a Flattened P-Pyramid Synthesis and Reactivity presents the interesting chemistry of these compounds including electrophilic substitution and Diels-Alder reactions and sigmatropic rearrangements, making a variety of organophosphorus compounds accessible. 

Trialkylphenyl)-l//-Phospholes with a Flattened P-Pyramid Synthesis and Reactivity... 

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. 

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. 

Contrary to pyrrole, phospholes are not planar, due to the high inversion barrier of the tricoordinate phosphorus (cf. the 6 kcal/mol inversion barrier of ammonia with the 35 kcal/mol inversion barrier of phosphine). As a consequence, unfortunately phospholes are not aromatic (Mathey, F.). Although the aromaticity of phospholes has been disputed in the past, Mislow considered first that phospholes with pyramidal phosphorus are nonaromatic while with planar tricoordinate phosphorus aromatic phospholes could be obtained. It was just recently found that phosphorus can be flattened or even fully planarized (as discussed comprehensively ), resulting in aromatic systems (see section IV.B.l). 

The planar bond configuration of the nitrogen atom in pyrrole is usually explained in terms of aromaticity. The pyramidalization of the phosphorus and arsenic atoms in phosphole (130) and arsole (131) was taken to be the consequence of their much lower aromaticity relative to pyrrole [75JCS(P2)974] (see Table VIII). 

Phospholes can behave as simple two electron donors, in the same way as tertiary phosphines, and most of the transition metals have been complexed to phospholes. For example, ruthenium(II) forms a series of complexes [(Phole)2 Ru(CO)2C12] and [(Phole)3 Ru(CO)C12]. The formation of the tris phosphole complex attests to their small size. Because of the ring structure an unusual isomerism has been observed, with the rings either in the basal plane of the square pyramidal complex or normal to the basal plane (Figure 23). 

The cr ./ -phospholes 3a [7b, 7d] and 3b [7c, 7d] (Scheme 12.1) bearing electron-rich and electron-deficient substituents, respectively, were characterized by x-ray diffraction studies (Figure 12.1). In spite of the different electronic natures of the two 2,5-substituents, compounds 3a and 3b share some important structural features in the solid state. Their molecular crystal structures reveal that the three heterocycles are almost coplanar while the phosphorus atom s environment is strongly pyramidalized [7b-7d]. 

Donor/acceptor-substituted phosphole 22 exhibits classical properties, namely the phosphorus atom has a pyramidal geometry and the aromatic character of the heterole is similar to that of cyclopentadiene <2000JOC2631>. Due to the push-pull substitution pattern, significant delocalization of the endocyclic 7t-electron density over the entire system... 

The heteroatoms in arsole and stibole, like phosphole, are pyramidal, but with reduced planarization energies (inversion barriers) relative to arsines and stibines. The inversion barriers are summarized in Table 12 <1995JMT51>. The steady increase in inversion barriers points to steadily decreasing efficiency of 7t-bonding between carbon and the heteroatom, and probably to greater s character of the lone-pair orbital. 

Phospholes are less stable than azoles because phosphorus prefers a pyramidal geometry rather than a planar one. However, P-mesityl derivatives are forced into planarity. 1,3-Di- and 1,2,4-triphospholes with a P-mesityl group are nearly or fully planar, and so are also 1,2,4-azadiphospholes, 1,2,4-thiadiphospholes and 1,2,4-diazaphosp-holes. Most interestingly, unlike the explosive pentazoles, pentaphosp-holes are thermally stable but dimerize by a [4- -2]-cycloaddition. Pentaphosphole P5H is planar, in contrast to phospholes (96JPC13447). 

All these ligands (I-III) are pyramidal at phosphorus, but with varying degrees of sp hybridization. Due to their reduced phosphorus inversion barrier phospholes are more planar than typical phosphines and their phosphorus lone pair possesses less s-character than does the phosphorus in either I or II. Thus from a frontier orbital point of view since the lone pair is likely the HOMO for all these ligands, we would anticipate that I and II might be poorer donors than III. Likewise, III is considerably less bulky than either I or II. In sum then, if cyclic conjugation is not large in III, its donor ability should approximate those of I and II. 

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-... 

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