title: Deformative transition of the Menschutkin reaction and helical atropisomers in a congested polyheterocyclic system.
abstract: A 4,7-phenanthroline polycyclic 1A designed for probing the limits of the Menschutkin reaction was synthesized in a six-step sequence. The rotational barrier of the phenyl ring nearby the N-methyl group in rac-2A was estimated to be ≫ 18.1 kcal/mol from VT-NMR experiments, making them a new type of helical atropisomer. The methylation rate constants of 9 and 1A with MeI was found to be 2.22 × 10(-4) and 9.62 × 10(-6) s(-1) mol(-1) L, respectively; thus, the formation rate of (P/M)-2A is one of the slowest rates ever reported for a Menschutkin reaction. The N-methyl protons in (P/M)-2A exhibit a significant upfield shift (Δδ 1.0 ppm) in its (1)H NMR, compared to those without a nearby phenyl, indicating a strong CH-π interaction is involved. Conformational flexibility in dipyridylethene 9 is clearly shown by its complexation with BH3 to form helical atropisomers (P,P/M,M)-10. The pKa values of the conjugate acids of 1A and 9 in acetonitrile were determined to be 4.65 and 5.07, respectively, which are much smaller compared to that of pyridine 14a (pKa = 12.33), implying that the basicity, nucleophilicity, and amine alkylation rates of 1A and 9 are markedly decreased by the severe steric hindrance of the flanking phenyl rings in the polyheterocycles.
authors.affiliation: Department of Applied Chemistry, National Chiao-Tung University , Hsinchu 30050, Taiwan-ROC.
authors.forename: Yung-Yu
authors.initials: YY
authors.lastname: Chang
authors.forename: Tse-Lok
authors.initials: TL
authors.lastname: Ho
authors.forename: Wen-Sheng
authors.initials: WS
authors.lastname: Chung
mesh_terms: D000097:Acetonitriles / Q000737:chemistry*; D000478:Alkylation; D018360:Crystallography, X-Ray; D006571:Heterocyclic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008745:Methylation; D008958:Models, Molecular; D010618:Phenanthrolines / Q000138:chemical synthesis* / Q000737:chemistry; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D000097:Acetonitriles; D006571:Heterocyclic Compounds; D010618:Phenanthrolines