C-13 NMR-STUDY OF AROMATIC RING-SUBSTITUTED (E)-3-PHENYLPROPENALS AND (2E,4E)-5-PHENYLPENTA-2,4-DIENALS

13C and 1H chemical shifts together with C,H and H,H coupling constants are presented for (E)‐3‐phenylpropenal, (2E,4E)‐5‐phenylpenta‐2,4‐dienal and their o‐OCH3‐ and o‐, m‐ and p‐NO2‐substituted derivatives. The SCSs calculated for the aldehyde chains show similar effects on the ring carbons, except in the para position where the shorter chain causes a 3.3 ppm deshielding and the longer chain a 1.0 ppm shielding effect. This shift difference is reflected in all the ring‐substituted derivatives of the two series of aldehydes, but not in the one‐bond C,H coupling constants. The effect of aromatic ring substitution on 2J(C,CHO) seems to be mainly inductive in origin. The importance of the structural unit in the study of C,H coupling constants was observed on comparing the 3J(C,H) values in C(sp2)CCH and CCC(2)H pentadienal fragments. In the first type the vicinal coupling varies from 6.5 to 8.8 Hz and in the second from 2.7 to 3.6 Hz. In the latter fragment an s‐cis conformation changes the mutual position of the coupled nuclei, whereas in the former it remains the same as in the s‐trans conformation. Copyright © 1990 John Wiley & Sons, Ltd.