Heterobenzenes

The heterobenzenes of the group 15 elements (1-5) comprise a series in which elements of an entire column of the periodic table have been incorporated into aromatic rings. The comparative study of this series has been extremely valuable for evaluating p-p rr-bonding between carbon and the heavier elements.1 However, the heterobenzene series has two important limitations. Only arsabenzene has a well developed aromatic chemistry. Moreover, stibabenzene and particularly bismabenzene are so labile that it has been difficult to obtain derivatives stable enough for study. 

Many of the properties of the group 15 element diheteroferrocences are very similar to ferrocenes and other metallocenes. It seems justified to regard the diheteroferrocenes as perturbed ferrocenes just as we regard the group 15 heterobenzenes as perturbed benzenes. Thus, it is very clear that the elements phosphorus, arsenic, antimony, and bismuth can take part in 7r-bonding in a manner similar to carbon. 

Other congeners of phosphinins—arsenin, antimonin, and bismin—have been shown to be definitely less aromatic than benzene by diverse theoretical treatments that have been reviewed.236 For instance, the Bird aromaticity index for arsenin was found to be 67, compared to 100 for benzene.123 Table 4 summarizes a few parameters used to estimate the aromaticity of heterobenzenes resonance energies... 

Ashe, III, A. J. The Group 5 Heterobenzenes Arsabenzene, Stibabenzene and Bisiriabenzene. 105,... 

The preparation of these materials led to a review by Ashe (78ACR153) of the spectral and chemical properties of the Group V heterobenzenes. He concluded that arsenin, antimonin and bismin were aromatic in character with the degree of aromaticity decreasing with increasing size of the heteroatom. This was supported by NMR, electron diffraction, microwave and UV photoelectron spectral studies and is discussed below. 

Table 5 Comparison of Bond Angles and Bond Lengths for Group V Heterobenzenes...

Arsenin, antimonin and bismin react readily with dienophiles in the Diels-Alder reaction, and the reaction with hexafluoro-2-butyne and other acetylenic dienophiles has been used to trap these heterocycles as barrelenes (115), in particular the unstable bismin (equation 21). The reactivity increases with increasing size of the heteroatom and this may be related to decreased aromatic character of the heterobenzenes with increasing size of the heteroatom. In fact, bismin and antimonin are so reactive that at low temperatures (< -10 °C and < - 50 °C respectively) both exist as Diels-Alder dimers (116 equation 20). 

The dehydrogenation of 30 turned out to be more difficult than the analogous conversion of 27 to 24. The IR spectrum of the matrix-isolated products showed only one absorption at 1273 cm-1, which disappeared upon irradiation with X = 405 nm probably yielding Dewar disilabenzene 32. The UV spectrum proved to be of higher diagnostic value. 1,4-Disilabenzene 31 shows a typical heterobenzene electronic spectrum with absorptions at X = 408, 340 and 275 nm displaying another bathochromic shift compared to silabenzene 24. 

Stibabenzene and bismabenzene have been much less studied due to their greater lability. This higher reactivity coupled with spectroscopic evidence suggests that aromatic character decreases with increasing atomic number. However, the spectra of the group 5 heterobenzenes strongly emphasize similarities in the series. Thus, the entire family of group 5 heterobenzenes is closely related to their benzocyclic cousins. 

Although several comprehensive reviews have appeared on phosphabenzene5-8 i2-i4), Javier heterobenzenes 6,8,12,13 15) have been only modestly covered in the secondary literature. The present work attempts to review arsabenzene, stibabenzene and bismabenzene. Phosphabenzene and pyridine chemistry are only selectively treated for comparison. 

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