Conformational analysis, Part 31.¹ A theoretical and lanthanide induced shift (LIS) investigation of the conformations of some epoxides

Abstract

An improved LIS technique, using Yb(fod)₃ to obtain the paramagnetic induced shifts of all the spin ½ nuclei in the molecule, together with complexation shifts obtained by the use of Lu(fod)₃, has been used to investigate the conformations of a group of epoxides. These are cis (1) and trans (2) stilbene oxide, cyclopentene oxide (3), cyclohexene oxide (4), cycloheptene oxide (5), propene oxide (6) and styrene oxide (7).

The LIRAS3 complexation model involving two symmetric lone pairs on the oxygen atom was used for the symmetric compounds but, for the unsymmetric compounds, a more complex unsymmetric complexation model (HARDER) was found to be necessary. The calculated LIS for styrene oxide and cis- and trans-stilbene oxide were in excellent agreement with the observed data for both the molecular mechanics (MM) and the ab initio geometries with the phenyl ring dihedral angles optimised.

In styrene oxide and trans-stilbene oxide the phenyl rings are approximately perpendicular to the oxirane ring, in agreement with the conformation in the solid state and with the theoretical calculations. In cis-stilbene oxide steric repulsions between the phenyl rings splay them apart so that they are now exo to the oxirane ring. Again the LIS analysis is in good agreement with the theoretical calculations.

Both the LIS data and the modelling studies agree that cyclopentene oxide is in a boat conformation with an angle of pucker of ca. 30° and that cyclohexene oxide is in a half-chair conformation with C4 and C5 displaced from the ring plane.

The LIS analysis of cycloheptene oxide gave good agreement for two equilibrating chair conformations with an endo/exo ratio of 70∶30, in excellent agreement with low temperature NMR data.

The accurate reproduction of the LIS data provides an unambiguous method of assigning the proton chemical shifts of the individual methylene protons in the cyclic epoxides, which are not easily available by any other technique.

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