Cyclic aromatic systems with hypervalent centers

Strong attractive intramolecular X<--Y(S, NR') interactions of 1,5-type occurring between terminal electron-rich main-group 15-17 centers (X = pnictogen, chalcogen, or halogen) and carbonyl, thione, or imine groups (Y = O, S, NR') incorporated into the conjugated -X-CH=CH-CH=Y fragments result in significant stabilization of cis-s-cis structure of these molecules, which may be viewed as the five-membered pseudo-heterocycles with the hypervalent arrangements across the pnictogen, chalcogen, or halogen centers. In derivatives of beta-chalcovinylaldehydes and isoelectronic chloronium cations, one of two lone electron pairs at X = S, Se, Te, Cl+ possesses pure p character and is involved in conjugation with the pi-system of the rest of the molecule, which leads to an appreciable contribution of the aromatic stabilization of the 6 pi-electron ring closed by the X<--Y bond. The aromaticity of these structures evaluated through calculation of the homodesmotic stabilization energies (HSE) increases in parallel with an increase in the strength of the intramolecular coordination bonds, i.e., in the order X = S, Se, Te and with an increase in electronegativity of a substituent attached to the chalcogen atom. Clear manifestation of the aromatic character of the cis-s-cis isomers of beta-chalcovinylaldehydes, their imines, as well as congeneric thiones and chloronium cations comes from the theoretical and experimental evidence for the pronounced equalization (as compared with the open trans-s-trans isomers) of the CC bond lengths. The cooperative effects of the hypervalent bonding and aromaticity are most distinct in the bicyclic structures of 1,6,6a lambda 4-trichalcapentalenes, their 1,6-dioxa(aza) analogues, and 1,6-dioxa-3a-aza-6a lambda 4-pnictapentalenes, in which two conjugated -X-CH=CH-CH=Y fragments are fused together via the symmetric -Y-X-Y- triad. The assessments based on the HSE values indicate that the aromaticity of these 10 pi-electron compounds is estimated as about 30-50% of the aromatic character of the most aromatic bicyclic structure of naphthalene. As shown by extensive ab initio calculations, substantial hypervalent bonding effects also operate in the case of beta-pnictovinylaldehydes formed by second and lower row pnictogen atoms (X = P, As, Sb, Bi), providing for the enhanced stability of their cis-s-cis configuration with respect to the free-of-strain trans-s-trans structures. The bicyclic 1,6,6a lambda 5-dioxapnictapentalene structure is also the most energy favorable form of these compounds. However, by contrast with the chalcogen and halonium analogues, the pseudo-cyclic structure of beta-pnictovinylaldehydes and the bicyclic structure of 1,6,6a lambda 5-dioxapnictapentalenes are maintained through primarily the occurrence of the hypervalent bonding across the pnictogen centers, whereas pi-delocalization is not (or weakly) operative in these compounds. A possible explanation for this finding is the low p character of the single lone electron pair at the pnictogen and orientation of the axis of its orbital, which is unfavorable for the optimal overlap with the pi-system of the rest of a molecule. As first proposed by Arduengo and Dixon, configurational isomerizations at the second and lower row tricoordinate pnictogen centers may proceed through the T-shaped hypervalent structures. Such a structure is strongly stabilized in the aromatic 1,6-dioxa-3a-aza-6a lambda 4-pnictapentalenes. It was found by calculations that the aromatic stabilization of the T-shaped hypervalent arrangement at the tricoordinate pnictogen center in 2H-1,3,2-dioxaphosphole and arsole arising due to withdrawal of two electrons from the pi-system of the ring onto the hypervalent electron-excessive center facilitates the rearrangement and directs it along the reaction path with the aromatic transition-state structure. Investigation of the influence of cooperative effects of hypervalent bonding and aromaticity on stability and mechanisms of reactions of main-group 14-17 compounds not considered in this review is the subject of our continuing research.