Pyridine heterobenzene

Together with the parent compound, pyridine, the group 5 heterobenzenes comprise a unique series in which elements of an entire column of the periodic table have been incorporated into aromatic rings. The comparative study of this series seems particularly interesting and should clarify the concepts of aromaticity and of element-carbon Ti-bonding. 

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. 

M.O. calculations have predicted that all of the heterobenzenes have the same direction of polarization with negative end of the dipole towards the heteroatom67-70). This has been confirmed by substituent effects from pyridine, phos-phabenzene and arsabenzene 27). In each case the 4-methyl derivatives have dipole moments which exceed the parent compound since the electron donating methyl group reinforces the ring dipole. 

Benzene and all of the group 5 heterobenzenes except pyridine behave as dienes in Diels-Alder reactions n). Qualitatively reactivity increases with the atomic number of the heteroatom. For example, benzene reacts with hexafluorobutyne at 180° 80), while phosphabenzene gives adduct 44b at 100°. Arsabenzene reacts at 25° while stibabenzene and bismabenzene react at 0°. 

Pyridine is attacked by alkyllithium reagents at its most electropositive atom (C2) to give the charge delocalized anion 87112). In a similar manner the heavier heterobenzenes are attacked at their most electropositive atoms (the heteroatoms) to give heterocyclohexadienides 88 113). 

The NICS values for pyridine have been compared with other pnictogen derivatives (P, As, Sb, Bi). Reported NICS values for phosphinine 105 vary from -6.4 to -11.4, but the values are consistently negative and large, and consistent with aromatic character . Table 3 compares NICS values for five pnicogen heterobenzenes and on magnetic criteria they are all aromatic. It should be noted, however, that on thermodynamic criteria the aromaticity decreases with stibinine 107 being very unstable and bismuthine 108 unknown <2004JMT (674)125>. NICS values are not, therefore, an indication of the thermodynamic stability associated with aromatic compounds. 

The heterobenzene ( -C5H5P)2Cr can be directly prepared by the metal-ligand vapor cocondensation technique. This heterocycle is aromatic and prefers ( -coordination to -coordination. For pyridine, cr-coordination via a lone pair is highly preferred over bonding that utihzes the tt-electron system. Therefore the direct synthesis of jj -pyridine metal complexes requires the N-atom to be blocked by means of substitution in the 2,6-positions ... 

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