Bird indices

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. 

Fig. 1 The aromaticity of five-membered heterocycles characterized by the Bird-Index [32]...

We wished to evaluate the reactivity of triisopropylphenylphosphole (17b), exhibiting a somewhat smaller Bird-index, than the tri-tm-butyl derivative (17d) (40.4 vs. 54.9). It was found that the acylation of the hetero ring took place only to a small extent. Acylation of the trialkylphenyl ring was a concurrent reaction path, but the major product was 2-acyl-5-aryl-bromophosphole (29) shown in Scheme 7 [47],... 

Siloles, germoles, stannoles, and plumboles have been extensively reviewed.180 The aromaticity of these systems is still under scrutiny. In the study, the cyclopentadienyl anion had the highest aromatic character, with a NICS value of —14 and a Bird index of 100. Structures 138 and 139 also showed high and similar aromaticity indices (Scheme 59). For the planar silacyclopentadienyl anion 138, the NICS value is —10.2 and the Bird index 80, while the values for anion 139 are —10.9 and 77, respectively. 

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. 

R.CHY AH° (kcal mol ) HOMA Bird index NICS(-1) HOMA % progress at TS Bird index NICS(-l)... 

Fig. 8 Plot of the Bird index versus IRC for the reaction of 61FI-S with CH2NO.

Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional Bond shortening index Bird index... 

The geometric aromaticity criteria are related to the observations of the equalized bond length of benzene, which is in contrast with that of the conjugated polyenes, having alternating bond lengths (and bond orders). The extent of the equalization can be measured statistically, and perhaps the most widely used method is that proposed by Bird [66]. The Bird index (BI), which can also be applied for heteroatoms (including... 

Table 2 ISE, Bird index (BI) and NICS(O) for hypervalent cf, A.3-phospholes [61]...

Surprisingly, the HOMA index classifies pyrrole as being more aromatic than thiophene but the Bird index gives the order thiophene > pyrrole > furan. Both pyrrole and thiophene are much more aromatic than furan and according to the Bird index indole and isoindole are significantly more aromatic than their oxygen and sulfur analogues. 

Table 4 HF/6-31G calculated geometric parameters (a-c In A sum of the bond angle at P In deg) and aromaticity Indexes (Bird Index)...

Bird index calculated from statistical deviation of bond orders from bond length measurements higher values imply higher aromaticity, on a scale of 0—100 <8571409). 

Rank from 1 (most aromatic highest Bird index) to 3 (least aromatic lowest Bird index). 

An additional problem arises when the Bird index is considered in its unified form Ia- It is biased by the assumption made in the unifying procedure. The Hiickel delocalization energies (DE) of benzene the cyclopentadienyl anion (2.47 ), naphthalene (3.68 ), and indenyl anion (4.17 8), i.e., for the nominal parents of the A, h, and /s.e, were used in the construction of an unified aromaticity index... 

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