An Iodabenzene Story

Abstract

We call iodabenzene a cyclic (CH)I molecule. A planar iodabenzene would have 8 π electrons, a situation best avoided by an out-of-plane distortion to a bird-like geometry. The electronic structure and charge distribution of this molecule resemble those of Meisenheimer complexes, derivatives of (CH)CH. A similar substitution strategy, of π-acceptors in ortho and para positions, works in both cases to planarize and stabilize such derivatives. Some 40 kcal/mol (73 kcal/mol for the unsubstituted case) below the bird, a classical 5-iodocyclopentadiene structure awaits, reached through a bicyclic transition state. The calculated activation barrier for the highly exothermic reaction to a classical Lewis structure nevertheless make us optimistic about the chances of detecting and even isolating the bird isomer.

Halogen bonding with the halogenabenzene bird structure, halobenzene, and halocyclopentadiene

“…The maps for trinitro‐ and tricyano‐iodabenzene show a much clearer σ‐hole compared to nonsubstituted iodabenzene. As also found by Rawashdeh et al ., the trinitro‐iodabenzene structure is planar, whereas the tricyano‐iodabenzene structure keeps the bird form.…”

“…Aiming to stabilize the halogenabenzene bird structure, Rawashdeh et al . suggested putting nitro or cyano substituents in the para and ortho positions . The trinitro‐iodabenzene and trinitro‐bromabenzene structures were calculated to be planar, whereas the trinitro‐chlorabenzene, trinitro‐fluorabenzene, and all tricyano‐halogenabenzene structures retained the bird structure.…”

“…Rawashdeh et al dubbed this structure "bird" because of the similarity with a flying bird (the halogen being the bird's head and the closest hydrogens wings stretched upwards). This minimum is by all means not the lowest energy structure on the C 5 H 5 I potential energy surface: a bicyclic structure with one imaginary frequency is 55 kcal/mol more stable than the bird, whereas the corresponding 5-iodocyclopenta-1,3-diene minimum-energy structure is 73 kcal/mol lower than the bird structure, [12] see Figure 1 for their structures. The authors established that there is a 14 kcal/mol barrier from the bird to the 5-iodocyclopenta-1,3-diene minimum.…”

Formation of Aminocyclopentadienes from Silyldihydropyridines: Ring Contractions Driven by Anion Stabilization

“…Fore lectron-withdrawing substituents at the R 5 position the ring-contracting step along Pathway Awas calculated to have free energies of activation for TS1 a-c ranging from 1.2 kcal mol À1 (R 5 = CO 2 Me) to 7.9 kcal mol À1 (R 5 = Ph), with the length of the forming C2 À C6 bond ranging from 1.92 (R 5 = Ph) to 2.01 (R 5 = CO 2 Me). With am ethyl group as the R 5 substituent, the free energy of activation for TS1 d is much higher at 15.3 kcal mol À1 , [15] and the forming C2ÀC6 bond length of 1.79 indicates aconsiderably later transition state.T he formation of 18 was found to be exergonic for all electron-withdrawing R 5 substituents,w ith DG for the conversion of 17 a-c to 18 a-c > ranging from À14.7 kcal mol À1 for R 5 = CO 2 Me to À2.8 kcal mol À1 for R 5 = Ph. In contrast, for 17 d (R 5 = Me), the formation of 18 d is endergonic with a DG of + 11.3 kcal mol À1 .T hese results indicate that both TS1 and 18 are strongly stabilized by an electron-withdrawing R 5 substituent.…”

“…With am ethyl group as the R 5 substituent, the free energy of activation for TS1 d is much higher at 15.3 kcal mol À1 , [15] and the forming C2ÀC6 bond length of 1.79 indicates aconsiderably later transition state.T he formation of 18 was found to be exergonic for all electron-withdrawing R 5 substituents,w ith DG for the conversion of 17 a-c to 18 a-c > ranging from À14.7 kcal mol À1 for R 5 = CO 2 Me to À2.8 kcal mol À1 for R 5 = Ph. With am ethyl group as the R 5 substituent, the free energy of activation for TS1 d is much higher at 15.3 kcal mol À1 , [15] and the forming C2ÀC6 bond length of 1.79 indicates aconsiderably later transition state.T he formation of 18 was found to be exergonic for all electron-withdrawing R 5 substituents,w ith DG for the conversion of 17 a-c to 18 a-c > ranging from À14.7 kcal mol À1 for R 5 = CO 2 Me to À2.8 kcal mol À1 for R 5 = Ph.…”