Structural studies by nuclear magnetic resonance—XV : Conformations and configurations of oximes

Abstract

Conformations and configurations were assigned to oximes of several aldehydes and ketones from analysis of their 60-Mc. NMR spectra. Interpretation of the spin-spin coupling data from the cis isomers (hydroxy cis to hydrogen) of the aldoximes in terms of rotamers I and II, whereby a single bond eclipses the double bond, led to the following: For-α-monosubstituted acetaldoximes, when R was changed from methyl to t-butyl, ΔH° for I ⇄ II varied from + 500 to +4,500 cal/mol. The α,α-disubstituted acetaldoximes behaved similarly. Interpretation of the analogous data from the trans isomers led to the conclusion that, whereas the eclipsing conformation XII is the most stable conformation of the α,ga-disubstituted acetaldoximes, both eclipsing XIII and bisecting XIV conformations, or other conformations in-between the two, are important. From the chemical shift data, the s-cis conformation about the NO bond is excluded as the equilibrium conformation of the oximes.