Chair/Twist-Boat Energy Gap in Monocyclic, Conformationally Unconstrained Polyalkylcyclohexanes

Abstract

A conformational search procedure (HUNTER), in combination with the MM3(92) program, was used for the exploration of the conformational hypersurface of alkyl-substituted cyclohexanes and for the calculation of their chair/twist-boat (TB) energy gap. The systems studied were conformationally unconstrained polyalkylcyclohexanes (alkyl = methyl, ethyl, isopropyl, and tert-butyl) possessing either geminal and/or vicinal arrangements of the alkyl groups, but differing in the number of alkyl substituents and in their relative disposition (i.e., cis or trans). The calculations indicate that in 1,1,3,3,5,5-hexaisopropylcyclohexane the TB is the lowest energy form. Modification of the cis,trans relationship of vicinal alkyl groups changes the chair/TB energy gap, and in the minimum energy conformation of cis,trans,trans-1,2,3,4-tetraisopropylcyclohexane (23c) and cis,syn,cis-1,2,4,5-tetraisopropylcyclohexane (31c) the cyclohexyl ring adopts a TB conformation. The tetrasubstituted systems cis,syn,cis-1,2-diisopropyl-3,4-dimethylcyclohexane (46), cis,syn,cis-1,4-diisopropyl-2,5-dimethyl-cyclohexane (47), and cis,trans,trans-1,2-diisopropyl-3,4-dimethylcyclohexane (41) are the least crowded monocyclic unconstrained cyclohexanes found in which the TB conformation is of lower energy than the chair form. The present study indicates that two methyls and two isopropyl substituents are sufficient for stabilizing the TB form of a cyclohexyl ring relative to the chair form.

Adsorption of Cyclohexane in Pure Silica Zeolites: High‐Throughput Computational Screening Validated by Experimental Data

“…The presence of different conformers of the molecule of cyclohexane in gas phase is possible due to chair-chair inversion of the ring (Figure 1). According to dynamical DFT calculations of Ionescu et al [37] this process undergoes through two half-chair (or sofa) and one t-boat conformations, with energy barrier of 41.4 kJ mol À1 (9.9 kcal mol À1 ), which is in agreement with experimental data. [38] The main objective of this work is to study the adsorption mechanisms of cyclohexane in numerous zeolitic frameworks with the final aim of finding a relation between the host structures and the preferential configuration adopted by the molecules.…”

“…The lowest energy conformation of cyclohexane is chair form, but this molecule also exhibits local energy minima for twisted boat (t‐boat) conformation . The difference between enthalpies of conformations of the molecule of cyclohexane is presented in Figure ,,. Considering only chair and t‐boat conformers, Boltzmann distribution indicates that for gaseous cyclohexane at 298 K only ca 0.01 % of the molecules remain in the t‐boat conformation.…”

HUNTER: A conformational search program for acyclic to polycyclic molecules with special emphasis on stereochemistry

“…17m y -␤-caryophyllene 36 molecules , an investigation of the chairrtwist energy gap in polyalkyl-Ž . 40 WEISER, HOLTHAUSEN, AND FITJER manuscript. This work was supported by the Fonds der Chemischen Industrie and the Niedersachsen Fund.…”

Conformational analysis of calixarene derivatives substituted at the methylene bridges

“…10 A systematic MM3 study revealed that the minimal steric requirements for the relative stabilization of the twist conformation of cyclohexane are two methyls and two isopropyl groups arranged in either a cis-trans-trans-1,2,3,4-or all-cis-1,2,4,5-substitution pattern. 19 In principle, if the steric destabilization ensuing from the presence of a bulky axial substituent in a tetrahydroxycalix[4]arene is sufficiently large, it could be expected that the intrinsic conformation preferences of the calix skeleton for the cone conformation may be altered. However, analysis of the NMR spectra of the trans alkyl-substituted derivatives 7a-d indicated that, in all cases, the preferred conformation is still the cone.…”