¹H, ¹³C, ¹⁷O and ¹⁹F NMR spectroscopic study of isomeric ring‐substituted monofluoro‐(E)‐3‐phenylpropenals

Abstract

Isomeric ring substituted monofluoro‐(E)‐3‐phenylpropenals (cinnamaldehydes) were synthesized in order to provide, with the NMR‐active 19F nucleus, an insight into the influence of substituents and the transmission of electronic effects between the aromatic ring and the side‐chain. Their 1H, 13C and 17O NMR chemical shifts and their nJ(H,H), nJ(H,F), nJ(C,H) and nJ(C,F) values were determined. Generally, the nJ(C,F) coupling constants correlate with the length of the coupling route and thus simplify the assignments of the 13C NMR spectra. Only the side‐chain carbons C‐2 and C‐3 in the ortho‐substituted compounds showed an exception to this rule. Many long‐range nJ(H,F) (n = 4–7) values were easily observed. The 17O NMR chemical shift of the carbonyl oxygen in the o‐ and m‐fluoro derivatives show clear substituent effects, whereas p‐fluorocinnamaldehyde shows a value comparable to that of that parent compound.

1H, 13C and 15N NMR spectral characterization of twenty‐seven 1,2‐diaryl‐(4E)‐arylidene‐2‐imidazolin‐5‐ones

“…In our earlier studies of 3-(o-, m-and p-fluorophenyl)propenals the coupling between fluorine and carbon could be measured even over six bonds and also over the non-protonated aromatic carbon. 18 However, also in these compounds the n J(C,F) values to or over non-protonated carbon were always smaller than to or over the protonated carbons. In ofluorophenylpropenal 3 J(F,C) to a non-protonated aromatic carbon was 8.7 Hz and over non-protonated carbon it varied from 2.3 to 2.8 Hz.…”

“…In ofluorophenylpropenal 3 J(F,C) to a non-protonated aromatic carbon was 8.7 Hz and over non-protonated carbon it varied from 2.3 to 2.8 Hz. 18 However, in m-substituted derivative, the 3 J(F,C) to a protonated aromatic carbon was 8.2 Hz and to the non-protonated carbon it was 7.8 Hz. In the studied compounds we do not, however, have any m-fluorosubstituted aromatic rings.…”

“…18 The 1 Only in compound 3, 3 J(C,F) could be measured between fluorine and the side chain methylene carbon ( CH-), which coupling route includes C-1. That value is 6.9 Hz, smaller than the fluorine coupling to protonated C-4, 8.9 Hz.…”