The p-substituents of 2,6-dimethyl-N-(2,2,5,5-tetramethylcyclopenty1idene)anilines are modified without interfering reactions at the CN double bond. The resultant series (5-8, lO -19) shows a strong (ca. -4.7 kcal/mol) steric acceleration of (EIZ) diastereotopomerization by front strain along the CN double bond but also the usual electronic substituent dependence, characterized by a Hammett op correlation (e = + 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 rno1-l for 5. The x donor quality of the imino group corresponds to ca. 55 ( f 8)% of pn character as evaluated by spectral (13C NMR of 5 and 13, IR of 13) and reactivity data (of 13).
2,6-Dimethyl-N-(2,2,5,5-tetramethylcyclopentylidene)aniline (4h) is obtained by permethylation; it forms salts (5) by Nprotonation. Its CN double bond is strongly shielded against nucleophilic attack and cannot be hydrolyzed. Nitration and bromination occur smoothly in the aromatic p-position (12,13),showing the directing power of the lone electron pair of the imino function. This x-donor quality is assessed by probing weaker electrophiles and by qualitative competition experiments.The plans to construct sterically strongly hindered Schiff bases arose from projects to develop suitable isoelectronic and isosteric model substances for the non-observable carbanions of vinyllithium compounds. We also hoped that the emerging structural system would allow an unequivocal elucidation of the mechanism of anti/ syn stereomutation[21 for both classes of compounds. Such stereomutation is sterically accelerated[*] in the tetramethyl-2-indanylidene series 1; it was expected that increased front strain along the CN double bond"] should cause a larger acceleration in the system 2. This was indeed observedr41 and explained by increased internal pressure of the 5,s-dimethyl group against the N-aryl moiety, because the 3,4-single bond[31 of t is longer than the C-8/C-9 bond of 1. For easier interpretation of such data, we had chosen the Ncyclopentylidene derivatives 2 rather than the corresponding Ncyclohexylidene analogues in order to avoid stereodynamic complications by chair inversion. Our synthetic route involves permethylation of lithiated imines 3, as described in Section A.
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