Phospholes, stability

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 mechanism may involve a nucleophilic attack of the double-bond of the phosphole (17) on the phosphorus atom of phosphorus tribromide to provide intermediates that are stabilized by the loss of proton, pseudorotation, and finally the departure of a bromide anion [48, 49],... 

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... 

The planar form of phosphole is a first-order saddle point on the potential energy surface, 16—24 kcal/ mol above the minimum (at different levels of the theory). ° (The calculated barriers are the highest at the HF level, which underestimates aromatic stabilization of the planar saddle point, while the MP2 results are at the low end.) It has been demonstrated by calculation of the NMR properties, structural parameters, ° and geometric aromaticity indices as the Bird index ° and the BDSHRT, ° as well as the stabilization energies (with planarized phosphorus in the reference structures) ° and NIGS values ° that the planar form of phosphole has an even larger aromaticity than pyrrole or thiophene. 

An extensive theoretical investigation does not exist for the siloles, but PM3 calculations of formation enthalpies of 2 and its tautomers have indicated that the l//-silole is the most thermodynamically stable species200. The activation barrier for 11 — 2 isomerization was calculated to be 96 kJ moC1, comparable to that for cyclopentadiene2d 116. The (1H + 1H) dimer 1019 is isolated rather than the (2H + 1H) dimer as in the case of phosphole. This directly confirms the thermodynamic stability and the Diels-Alder kinetic instability of 2. The marked difference in the stability of the parent silole and phosphole was explained3 by the relative stabilities of the a bonds in silanes and phosphines (Si > P) and of the ji bonds in silenes and phosphenes (P > Si)117. 

This is indeed the case, and the ASE for 42 (reaction 8, which was termed homo-molecular homodesmotic reaction [170]) dropped from 5.9 kcal mol 1 stabilization in phosphole to 5.1 kcal mol 1 destabilization in 42 (R H, R2 H) and 12.3 kcal mol 1 destabilization in 42 R2 F) [170], This behavior led the authors to conclude... 

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). 

Another theoretical criterion applied to estimation of aromaticity of homo- and heteroaromatic ring system is aromatic stabilization energy (ASE). Based on this approach, the aromatic sequence of five-membered ring systems (ASE in kcal mol-1) is pyrrole (20.6) > thiophene (18.6) > selenophene (16.7) > phosphole (3.2) [29], According to geometric criterion HOMA, based on the harmonic oscillator model [30-33], thiophene is more aromatic than pyrrole and the decreasing order of aromaticity is thiophene (0.891) > pyrrole (0.879) > selenophene (0.877) > furan (0.298) > phosphole (0.236) [29],... 

There remain no X-ray crystallographically determined structures of 2H- and J //-phospholes as a result of their low stability. A number of structures of 2,3- and 2,5-dihydrophospholes have been determined since 1995 and are collected in Table 14, with most possessing a tetrahedral four-coordinate phosphorus center. 

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