Outeirino, Jorge Gonzalez; (2002) Eclipsed and other conformations of simple organic compounds. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In the last decade, it has been reported that certain simple saturated bonds prefer to adopt an eclipsed conformation. The saturated C-O bond, for example, prefers to adopt the eclipsed conformation if the central framework in a molecule, R1-C-C(-OR)-C-R2, adopts an extended antiperiplanar arrangement and R1or R2 are space-demanding enough. We have found that silyl ethers and alkyl to oxygen bonds in esters (R= --SiR3 or R = --C(=O)R in the structure above) may be potentially included in this set of compounds exhibiting eclipsed conformations (according with the statistical analysis of 1521 torsional angles ψ(H-C-O-C), found in crystal structures, from Cambridge Crystallographic Database. However, esters not only show one conformational possibility, but the acyl to oxygen bonds, ψ(C-O-C=O), can also show different conformations (Cis-Trans). This situation has also been investigated and the results about the equilibrium between two different conformations has been reported. In carbohydrate chemistry uncertain situation has arisen, since Matsuda et al reported the hypothesis that the silyl protecting groups are able to change the conformation of pyranoside rings, and as a result, stereoisomeric products may be synthesised depending on the protecting groups employed. Trans-1,2-disilyloxycyclohexane derivatives have been chosen as model compounds, and the chair-chair equilibrium has been investigated. In solution, a wide range of population conformation ratios have been reported, starting from almost entirely diequatorial to almost entirely diaxial depending on the substituents attached to silicon atom. Dipole-dipole and steric (repulsive and attractive) interactions have been investigated as potential reasons for the aforementioned change in the position of the Chair-Chair equilibrium. Furthermore, trans-1,4-disilyloxycyclohexane has been investigated to explain the additional relative stability of the trans diaxial conformation with respect to the axial conformation of monosilyloxycyclohexane. The role of the steric attractive interactions for trans-1,4-derivatives have been investigated, since the analysis of compounds with substituents in neighbouring position (the trans-1,2-derivatives) may be ambiguous. The extent of the occurrence of eclipsed conformations has not yet been completely defined, however, an unexpected connection between the two parts of this dissertation, chair-chair cyclohexane equilibrium, and the eclipsed theory, have been found and reported on the basis of crystal structure determinations. Various techniques have been used to detect and characterise the predominant conformation of the molecules in the solid, liquid and gaseous states. Solid state has been studied with the information retrieved from the Cambridge Structural Database, and by X-ray diffraction. Gas phase was studied with the help of the MM3 and MM2 programs, semi-empirical PM3 calculations and ab initio calculations (HF, MP2 and DFT calculations). The solution state has been studied from the application of different 1D and 2D-NMR techniques, and since the spectra showed dependence on the temperature, variable temperature NMR techniques were applied to determine the thermodynamic properties involved in the equillibria.

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