Conformational Analysis of Cyclohexane-1,2-diol Derivatives and MM3 Parameter Improvement

1996 ◽  
Vol 49 (3) ◽  
pp. 379 ◽  
Author(s):  
GD Rockwell ◽  
TB Grindley
Keyword(s):  
Hydrogen Bonding ◽  
Conformational Analysis ◽  
Ab Initio ◽  
Gas Phase ◽  
Marked Improvement ◽  
Solvent Polarity ◽  
Intramolecular Hydrogen Bonding ◽  
Experimental Results ◽  
Intramolecular Hydrogen

The positions of the equilibria between the diequatorial and diaxial conformers of trans-1,2-dimethoxycyclohexane (2) and trans-2-methoxycyclohexanol (3) have been measured accurately by 13C n.m.r. spectroscopy at -80°C in a series of solvents ranging from non-polar pentane to highly polar methanol. The equilibria favour the diequatorial conformers under all conditions but the extent increases with solvent polarity and is greater for (3). Improved parameters for the OCCO torsional term in MM3 (94) have been developed (V1, V2, and V3, 3.0, -2.5, 1.25, respectively) by comparison with conformational data for (2) and 1,2-dimethoxyethane (1). Application of the modified parameters to a number of examples demonstrates marked improvement for ethers. These examples include 1,2-dimethoxypropane, for which n.m.r. spectra in (D12) cyclohexane have been analysed and those from the gas phase reevaluated. Lesser improvement was achieved for systems having intramolecular hydrogen bonding. Ab initio results for rotation about the C 5-C 6 bond in pyranosides are satisfactorily reproduced but experimental results favour the gg and tg rotamers over the tg rotamer considerably more than calculated.

scholarly journals Conformational analysis and intramolecular interactions in monosubstituted phenylboranes and phenylboronic acids

2013 ◽  
Vol 9 ◽  
pp. 1127-1134 ◽  
Author(s):  
Josué M Silla ◽  
Rodrigo A Cormanich ◽  
Roberto Rittner ◽  
Matheus P Freitas
Keyword(s):  
Hydrogen Bond ◽  
Hydrogen Bonding ◽  
Conformational Analysis ◽  
Conformational Equilibrium ◽  
Intramolecular Hydrogen Bonding ◽  
Intramolecular Interactions ◽  
Coupling Constant ◽  
Intramolecular Hydrogen ◽  
Phenylboronic Acids ◽  
Electron Donation

A 1 TS J F,H(O) coupling pathway, dictated by a hydrogen bond, in some 2-fluorobenzoic acids has been observed, while such an interaction does not occur in 2-fluorophenol. Thus, this work reports the conformational analysis of 2-fluorophenylboronic acid (1), in order to evaluate a possible intramolecular OH∙∙∙F hydrogen bond in comparison to an nF→pB interaction, which mimics the quantum nF→σ*OH hydrogen bond that would be expected in 2-fluorophenol. 2-Fluorophenylborane (3), which does not experience hydrogen bonding, was used to verify whether nF→pB interaction governs the conformational equilibrium in 1 due to a predominant OH∙∙∙F hydrogen bond or to other effects. A series of 2-X-phenylboranes (X = Cl, Br, NH2, PH2, OH and SH) were further computationally analyzed to search for electron donors to boron, capable of influencing the conformational equilibrium. Overall, the intramolecular OH∙∙∙F hydrogen bond in 1 is quite stabilizing and dictates the 1 h J F,H(O) coupling constant. Moreover, electron donation to the empty p orbital of boron (for noncoplanar BH2 moiety relative to the phenyl ring) is also significantly stabilizing for the NH2 and PH2 derivatives, but not enough to make the corresponding conformers appreciably populated, because of steric effects and the loss of πCC→pB resonance. Thus, the results found earlier for 2-fluorophenol about the lack of intramolecular hydrogen bonding are now corroborated.

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