Sterically Congested Molecules, 6. Lone Electron Pair Donor Quality of the Imino Function: Increased Front Strain and Electronic Substituent Effects on Sterically Accelerated Nitrogen Inversion in Iminocyclopentanes

The p-substituents of 2,6-dimethyl-N-(2,2,5,5-tetramethylcyclopentylidene)anilines are modified without interfering reactions at the CN double bond. The resultant series (5 – 8, 10 – 19) shows a strong (ca. —4.7 kcal/mol) steric acceleration of (E/Z) diastereotopomerization by front strain along the CN double bond but also the usual electronic substituent dependence, characterized by a Hammett sp– correlation (Q = +2.7). Conversely, the substituent constant for lithium at the p-position of 7 may be estimated. The volume of activation is 1.5(8) cm3 mol–1 for 5. The π donor quality of the imino group corresponds to ca. 55 (±8)% of Pπ character as evaluated by spectral (13C NMR of 5 and 13, IR of 13) and reactivity data (of 13).