Enamines I. Vinyl Amine, a Theoretical Study of its Structure, electrostatic potential, and proton affinity

Abstract

SummaryThe structure of vinyl amine and its reactivity towards a proton is studied by the PRDDO SCF MO method. The equilibrium structure is found to be non-planar and barriers to inversion-and rotation-dominated processes are calculated. Proton addition to vinyl amine, as a model of enamine protonation, is examined by means of electrostatic molecular potentials and C-versus N-proton affinities.1. Introduction. -Enamines are readily accessible, highly reactive, and thus synthetically useful intermediates [ 11 [2]. They represent ambident conjugated systems with high nucleophilicity at both the nitrogen and the a-carbon atom [3]. Attempts to arrive at a detailed understanding of their chemical reactivity are faced with many intriguing problems. First, the relative site preference for electrophilic attack depends not only on the nature of the enamine but also on the electrophile and the solvent. Second, careful differentiation between kinetic and thermodynamic control is necessary but often quite difficult. Third, little is known about the structure of enamines, particularly the degree of intrinsic nonplanarity of the enamine unit and its influence on nucleophilic reactivity. This last point is fundamental in discussions of the stereochemistry of electrophilic additions to enamines and is the subject of considerable experimental activity [4].As a first step in a theoretical study of the structure and reactivity of enamines, the case of vinyl amine and its protonation is examined. Vinyl amine is prototypical of aliphatic enamines. Furthermore, it is of interest in view of recent semi-empirical calculations [5] [6] and a microwave spectroscopic analysis [7], which suggest this molecule to be non-planar [7] [8]. However, from the rather limited set of experimental data [7] no detailed structural information is available. Quantum chemical calculations on the structure and internal motions of vinyl amine thus provide a valuable basis for a more quantitative interpretation of its microwave spectrum [9].

High level ab initio, DFT, and RRKM calculations for the unimolecular decomposition reaction of ethylamine

“…The three-fold rotational potential is found to entail a barrier of 11.1 kJ/mol with a rotational constant at 271.5 GHz. These values are in excellent agreement with previously reported theoretical predictions [35].…”

“…The experimental results suggest a pyramidal nitrogen for EA. Earlier results [34,35], and a microwave spectroscopic analysis [12] suggested that EA is non-planar. Radom et al [28] and Manocha et al [23] predicted that the gauche conformer is more stable than the trans conformer.…”

“…They proposed three alkene elimination reaction channels. Several studies have discussed the structure of vinylamine by microwave rotational spectra [12] and ab initio calculations [35]. Vinylamine is a model of enamine protonation which represents ambident-conjugated systems with high nucleophilicity at both the nitrogen and β-carbon atom.…”

The High-Resolution FTIR Far-Infrared Spectrum of Vinylamine

“…Müller and Brown (8), using a semi-empirical method PRDDO (partial retention of diatomic differential overlap), calculated a nonplanar geometry and investigated some aspects of the inversion and torsion motion of vinylamine. Meyer (7), using a flexible molecular model and the results of Müller and Brown (8), examined several possible pathways of the inversion motion.…”

Computational study for the second-stage cracking of the pyrolysis of ethylamine: Decomposition of methanimine, ethenamine, and ethanimine

“…They characterized the microwave and infrared spectra of VA and EI two isomers, E-and Z-ethyleneimine in a temperature range from 150 to 900 °C and suggested a non-planar structure of vinylamine (CH 2 =CHNH 2 ) [1,2]. Moreover, theoretical studies on VA structure produced the same result [16,18,20,[56][57][58][59][60]. Traeger et.…”