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To What Extent Can Nine‐Membered Monocycles Be Aromatic?
Author(s) -
von Ragué Schleyer Paul,
Nyulászi László,
Kárpáti Tamás
Publication year - 2003
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200200422
Subject(s) - planarity testing , antiaromaticity , chemistry , aromaticity , delocalized electron , heteroatom , ring (chemistry) , substituent , hyperconjugation , crystallography , annulation , conjugated system , mindo , cyclobutadiene , ab initio , computational chemistry , stereochemistry , molecule , organic chemistry , polymer , catalysis
Ab initio and density functional computations have been used to characterize the heteronins. New targets for synthesis are suggested on the basis of the results. The aromaticity of the heteronins was evaluated via numerous criteria such as magnetic properties (NICS and δ H ), geometric indices (Bird, BDSHRT and HOMA), aromatic stabilization energies (ASE), and the barriers to planarity. Along with the experimentally characterized cyclononatetraenyl ( 1CH − ) and azonide ( 1N − ) anions, the phosphonide ion ( 1P − , which has not been reported before), favors planar ( C 2 v ) symmetry. All the other heteronins are nonplanar in order to alleviate ring strain. Comparisons of the neutral planar heteronins (constrained to C 2 v symmetry) show the extent to which heteroatoms like P and S, as well as N and O (in oxonin, 1O ), can participate in the cyclic electron delocalization. Due to hyperconjugation, the C 2 v ‐symmetric 9,9‐distannylcyclononatetraene 1C(SnH 3 ) 2 is moderately aromatic, in contrast to 1CH 2 , which is a nonaromatic, conjugated system. The planar C 2 v ‐symmetric 1BH and 1AlH and 1CF 2 are antiaromatic, like the planar form of 1CH + . Annelation with three‐membered rings resulted in planar heteronins. The sole exception was phosphonin ( 8PH ), but the barrier to planarity decreased to 1.8 kcal/mol in 8PR , due to the bulky R : 2,6‐di‐ tert ‐butylphenyl substituent at the phosphorus atom. Among the 8π heteronins, only the previously reported 1CH + is Möbius‐aromatic; 1BH , 1AlH and 1CF 2 also have C 2 structures but are nonaromatic due to large torsional angles which preclude cyclic delocalization. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)