Conformational preferences and basicities of monofluorinated cyclopropyl amines in comparison to cyclopropylamine and 2-fluoroethylamine

Abstract

Fluorine substituents in organic molecules do dramatically influence the electronic structure of neighbouring functional groups and the conformation of molecules. Hence the presence of fluorine in a compound changes its chemical reactivity and biological activity. On the basis of MP2 and SCS-MP2 calculations, we discuss the conformational preferences and basicity of the diastereoisomeric 2-fluorocyclopropylamines (cis-2 and trans-2) in comparison to those of cyclopropylamine (1) and 2-fluoroethylamine (3). 1 and 2 are viewed as model compounds for the antidepressant drug tranylcypromine (trans-2-phenylcyclopropylamine, 1′a) and its fluorinated derivatives 2′. The potential energy profile for the rotation of the amino group in cis-2 differs from that of trans-2 and 1 which have very similar rotational curves. For 2 the global minimum conformer is trans-2a and the lowest energy cis-conformer 2c is less stable by 2.57 kcal mol⁻¹. The calculated enthalpy differences between the conformers gauche-1b and s-trans-1a (2.0 kcal mol⁻¹) as well as between gauche-3b and gauche-4a (0.2 kcal mol⁻¹) agree well with the available experimental data of 2.0 kcal mol⁻¹ and 0.1 ± 0.3 kcal mol⁻¹, respectively. The calculated gas phase proton affinities (PA) of 1 (217.6 kcal mol⁻¹), cis-2c (215.6 kcal mol⁻¹), and trans-2a (209.3 kcal mol⁻¹) follow the trends of the pK_a values measured in solution for the diastereomeric 2-phenylcyclopropylamines 1′a and 1′b and their fluorinated derivatives cis-2′ and trans-2′. It is shown that the conformational preferences and basicity of the investigated molecules are due to stereoelectronic effects from hyperconjugative interactions which lead to different local charge distributions and different hybridization of the nitrogen lone-pair. The basicity of gauche-3a (PA = 215.3 kcal mol⁻¹) and anti-3b (PA = 210.1 kcal mol⁻¹) is controlled by the charge of the nitrogen atom, while that of cis-2c and trans-2a is overlap controlled as a result of different hybridization of the nitrogen lone-pair [sp^4.34 (cis-2c), sp^4.07 (trans-2a)].