Medium-sized rings: conformational preferences in cyclooctanone driven by transannular repulsive interactions

Abstract

Three conformers of cyclooctanone have been identified by broadband rotational spectroscopy. Cyclooctanone shows a strong preference to adopt a boat-chair conformation, driven by minimisation of repulsive non-bonded transannular interactions.

Seven Conformations of the Macrocycle Cyclododecanone Unveiled by Microwave Spectroscopy

“…The r 0 bond lengths are close to the expected values for single C–C bonds, ranging between 1.52–1.54 Å. The bond angles roughly span 111–115°, and they are similar to those determined for cyclooctanone [ 36 ]. The majority of the dihedral angles are close to those of 60°, 180° and −60°, reducing torsional strain.…”

“…In addition, some of the conformers also show HCCH-eclipsed configurations, at distances of 2.31–2.38 Å and dihedral angles of 32–49°. For comparison, eclipsed interactions occurred in all observed conformers of cyclooctanone at slightly shorter distances of 2.27–2.28 Å and dihedral angles of 13–22° [ 36 ]. The effect of eclipsed configurations is therefore expected to be less relevant for cyclododecanone, but their combination with transannular interactions drives conformational stabilities.…”

“…Broadband spectral collection, typically of several GHz, is essential for decoding the complex spectrum of multiconformational molecules [ 31 , 32 , 33 , 34 , 35 ]. Using broadband rotational spectroscopy, we have recently analysed the conformational behaviour of cyclooctanone, a medium-size cyclic ketone [ 36 ]. We identified three conformations: a strongly prevalent boat-chair (analogous to that of the unsubstituted cycloalkane), a twisted boat-chair and another boat-chair conformation.…”

Synthesis of Azocanes from Piperidines via an Azetidinium Intermediate

“…The access to medium‐size rings is challenging due to unfavorable enthalpic and entropic barriers and transannular interactions during the formation of these medium size rings [9,10] . The methods to access eight‐membered N‐heterocyclic systems are rare and most of the methods that have been developed are utilizing metals, such as transition metals which are expensive [11‐19] .…”

Computational study of inversion‐topomerization pathways in 1,3‐dimethylcyclohexane and 1,4‐dimethylcyclohexane: Ab initio conformational analysis

“…The adopted experimental methods briefly include X‐ray diffraction, Raman, infrared, and NMR spectrum detection, [9, 11‐14] and theoretical approaches contain ab initio, molecular force field, and dynamical density functional theory [7, 8, 10, 15]. These previous studies usually cover two mainly aspects: (1) conformational determination of the simple neutral cycloalkane compounds from three‐membered ring to the larger ones, like cyclohexane, cyclooctane, and cyclotridecane [10, 15, 16]; (2) conformational changing investigation of cyclohexane and its substitution derivatives [1, 5, 12, 14, 17‐19]. Since cyclohexane has a fluxional and most symmetrical structure, it serves as the archetype molecule to understanding the energetics and conformational preferences for other related cycloalkanes and yielded derivatives [20, 21].…”