Stereoelectronic Control in Addition of Nucleophiles to an Amidinium Ion

• Published in: Journal of the American Chemical Society Vol. 123; no. 19; pp. 4451 - 4458
• Main Authors: Perrin, Charles L, Young, David B
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16-05-2001
• Subjects: Amides - chemistry; Catalysis; Ions; Kinetics; Magnetic Resonance Spectroscopy; Stereoisomerism; Thermodynamics
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja004240q

Nucleophilic addition to 1,3-dimethyl-5-phenyl-1,4,5,6-tetrahydropyrimidinium ion provides a quantitative measure of stereoelectronic control. This amidinium ion presents the nucleophile with two distinct paths for attack. Axial attack is favored by interaction between the orbital of the developing bond and antiperiplanar lone pairs on the nitrogens. Reaction of the amidinium salt with diverse nucleophiles (D-, H3C-, n-Bu-, PhCH2 -, allyl-, Ph-, C5F6 -, CH2CH-, HC⋮C-, PhC⋮C-, CN-) produces mixtures of cis and trans stereoisomers. Both kinetic and thermodynamic product distributions were measured by 1H NMR, before and after acid-catalyzed equilibration. The values provide insight into the roles of steric and stereoelectronic forces at the transition state and in products. Stereoelectronic effects on reactivity are found to be weak (ca. 1 kcal/mol).