Phospholes Inversion barrier

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... 

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). 

We have used a different approach to compare the aromaticities of phosphole (8) and pyrrole (10) [23, 24], From literature data on derivatives of 8 and 9 it is known that the inversion barrier of phosphole is about 67 kJ mol-1 (70.2 kJ mol-1 at the B3LYP/aug-cc-pVTZ level) [25] while that of tetrahydrophosphole amounts to 163 kJ mol-1. This is explained by the fact that the planar transition state of 8 is highly aromatic. Pyrrole (10) is planar and pyrrolidine has a calculated inversion barrier of 15-17 kJ mol-1. Several aromaticity indices were used in this study, based on different criteria of aromaticity energetic (aromatic stabilization energy, ASE), geometric (harmonic oscillator model of aromaticity, HOMA, and /5), and magnetic (NICS). 

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. 

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. 

Correlated ab initio calculations showed that the 20kcalmoH inversion barrier about phosphorus in a phosphole is reduced by incorporating other phosphorus atoms (as in 1,3-diphosphole) into the ring <1996JPC6194>. 

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... 

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... 

An n.m.r. study of the inversion of phosphole derivatives such as (55) has shown that even in complex spin systems a relatively accurate estimate of the inversion barrier may be obtained without recourse to a complete lineshape analysis. In this study, alkyl and aryl substituents did not... 

Phosphines of entries 8 and 9 are examples of compounds whose configurational stability is inferior due to other effects. Acylphosphines (entry 8) have inferior inversion barriers due to conjugation in the planar transition state." Comparison between entries 7 and 9 shows a dramatic lowering of inversion energies in phospholes compared to phospholanes, probably caused by an increase in the delocalisation of the planar transition state in the phosphole inversion. " In spite of that, more conjugated fused phospholes (entries 10 and 11) are more configurationally stable. 

In fact, calculations have shown that phospholes with a planar P-atom would be more aromatic than pyrrole, due to the good 7i-donor ability of planar-P centers. However, this stabilization is not sufficient to overcome the high planarization barrier of the P-atom (35 kcal mol ), but is responsible for the reduced P-inversion barrier in phosphole (ca. 16 vs. 36 kcal mol for phospholanes) [67-69]. Low barrier inversion is another key property of phospholes since inversion at phosphorus can occur at room temperature. Together, these electronic properties (low aromatic character, a-n hyperconjugation) set phosphole apart from pyrrole and thiophene. In other words, this P-heterole has its own chemistry (synthetic routes, methods of functionalization, etc.) that cannot be predicted by simply extrapolating that of its aromatic S- and N-analogues [1-5, 32 0, 51-53, 70-72]. 

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 crystal and molecular structure of (S)-7-phenyldinaph[2,l-h T,2 -d]arsole (82), which was obtained by spontaneous resolution of the racemate from hot methanol, reveals appreciable bending of the distorted naphthyl residues away from each other (Scheme 4) . The molecule is fluctional in solution on the NMR time scale, however, with similar barriers between the conformational isomers (atropisomers) for the 7-phenyl [AG 59 1 kJ mol" (259 K)] and the 7-methyl [AG 65 1 kJmol" (287 K)] compounds. The analogous phospholes are also unsuitable for resolution because of similarly low barriers to inversion of the atropisomers - Both arsenic ligands, when coordinated to iron(II) in complexes of the type [( -C5H5) l,2-C6H4(PMePh)2 FeL]PFg,... 

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