Cyclopropyl Homoconjugation—Experimental Facts and Interpretations

Abstract

Introduction Homoconjugation in Acyclic Systems Monohomoaromatic and Homoantiaromatic Cations Higher Homoaromatic Cations Neutral Homoaromatic Systems Homoaromatic Anions Concluding Remarks Acknowledgements

Triplet state homoaromaticity: concept, computational validation and experimental relevance

“…We take care to compare the neutral, cationic and anionic compounds in S 0 and T 1 within each charge class as in the S 0 state it is known that homoaromaticity is stronger in cationic than neutral or anionic compounds. 14 Finally, we show how homoaromaticity could influence the photochemistry when extending the results to the S 1 excited state.…”

“…2 ). We compared possible T 1 -homoaromatic structures ( 1–11 ) with compounds that are either experimentally established homoaromatics in S 0 ( 12–16 ) 14 , 24 , 25 or those whose homoaromaticity is so far only supported computationally ( 17–18 ). 26 , 27 Although 12 is a transition state and not a stable compound, there have been extensive attempts to make derivatives in which this homoaromatic structure is a true energy minimum.…”

“…Any rigorous assessment of homoaromaticity should include several different aspects. 14 For the geometric aspect, we analyze the degree of the bond length alternation (BLA) and the distance of the though-space conjugative linkages ( r (C···C)). For the electronic aspect, we look at the strength of the through-space and cyclic conjugation through the Wiberg bond indices 29 and the multicenter indices, 30 respectively.…”

“…A low BLA is hallmark of both conventional aromaticity 35 and homoaromaticity. 14 The average and maximum BLAs (BLA avg and BLA max , respectively) are given in Table 1 for the sp 2 -carbon framework of the monocyclic compounds. The BLAs for the T 1 compounds are low and comparable in size to those in the S 0 homoaromatic compounds (homoaromatics).…”

“… 8 , 13 In this study, we focus on homoaromaticity, i.e. , aromaticity due to interaction of π orbitals over a formally saturated center, 14 and show that this concept is applicable also in the excited triplet state. This can actually be anticipated from a simple analysis of the HOMO and LUMO orbitals of the aromatic homotropylium cation with six π-electrons and its larger analogue with eight π-electrons.…”