The relevant effect of an intramolecular hydrogen bond on the conformational equilibrium of cis-3-methoxycyclohexanol compared to trans-3-methoxycyclohexanol and cis-1,3-dimethoxycyclohexane.
1H NMR data show that an increase in the concentration of cis-3-methoxycyclohexanol (cis-3-MCH) shifts the conformational equilibrium from the 1aa conformer, stabilized by an intramolecular hydrogen bond (IAHB), to the 1ee conformer [X(ee) = 44% (at 0.05 molL(-1)) to 59% (at 0.40 molL(-1)), in CCl4], which forms intermolecular hydrogen bonds (IEHB), as confirmed by IR data. The percentage of 1ee conformer increases with the solvent polarity, from 33% (DeltaG(ee-aa) = 1.72 kJmol(-1)) in cyclohexane (C6D12) to 97% (DeltaG(ee-aa) = -8.41 kJmol(-1)) in DMSO. For trans-3-methoxycyclohexanol (trans-3-MCH), 1ae and 1ea conformers are almost equally present in the studied solvents, 1ae increasing from 41%, in C6D12 (DeltaG(ae-ea) = 0.84 kJmol(-1)), to 49%, in DMSO (DeltaG(ae-ea) = 0.13 kJmol(-1)). A value of 18.4 kJmol(-1) for the strength of IAHB in cis-3-MCH was obtained, from the theoretical data, through the CBS-4M method.
de Oliveira
Paulo R
PR
Physical Organic Chemistry Laboratory, Instituto de Química, UNICAMP, Caixa Postal 6154, 13084-971 Campinas, SP, Brazil.
Rittner
Roberto
R
D003510:Cyclohexanes / Q000737:chemistry; D003511:Cyclohexanols / Q000737:chemistry*; D003514:Cyclohexylamines / Q000737:chemistry*; D006358:Hot Temperature; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008968:Molecular Conformation; D015394:Molecular Structure; D010456:Peptides, Cyclic; D012997:Solvents; D013053:Spectrophotometry; D013816:Thermodynamics
D003510:Cyclohexanes; D003511:Cyclohexanols; D003514:Cyclohexylamines; D010456:Peptides, Cyclic; D012997:Solvents; C503817:cis-1,3-dimethoxycyclohexane; C503816:cis-3-methoxycyclohexanol; D006859:Hydrogen