Carbon-13 delta shifts and steric interactions in N-aryl-1-isoindolinones and Isoindoline-1,3-diones

Consistently deshielding 13C δ shifts resulting from the presence of aryl ortho substituents have been measured for the 3‐methylene carbon atoms of some N‐aryl‐1‐isoindolinones (0.12‐3.25 ppm) and for the carbonyl carbon atoms of these compounds and corresponding N‐arylisoindoline‐1,3‐diones (0.04‐1.47 ppm). Low‐temperature (−75 and −150 °C) 1H NMR studies indicate that steric barriers to internal rotation about the aryl CN bonds in the N‐aryl‐1‐isoindolinones are low. The compounds with the bulkiest aryl ortho substituents tend to show the largest δ shifts (particularly for the 3‐methylene carbons). No consistent correlation with the size of aryl ortho‐substituents or their expected electronic properties is evident.