Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

¹⁵N and ¹³C NMR spectra have been measured of the compounds type XC(=NOH)Y, where X, Y = COCH₃, CN, COOR, CONHR, and X +Y = C₆H₄(CO)₂. The (E)-(Z) isomerism at the C=NOH bond has been studied by means of ¹⁵N labelling and stereospecific behaviour of the ²J(¹⁵N, ¹³C) coupling constants. The nitrosation of methyl cyanoacetate gives specifically the respective (E)-oxime, whereas that of acetoacetanilide gives the (Z)-isomer. The nitrosation of ethyl acetoacetate gives both (E)- and (Z)-oximes in a ratio of ca 1:10; the (E)-isomer is less stable and is transformed into the (Z)-isomer. The applicability of the ¹J(¹³C, ¹³C) coupling constants and lanthanoid shift reagents to the study of (E)-(Z) isomerism of oximes with geminal carbonyl groups has also been verified. The ¹⁵N NMR chemical shifts of the oximes studied correlate with their respective pK_a values measured in dimethyl sulfoxide according to the equation pK_a = -0.11δ¹⁵N + 13.44.