Conformational and long-range anisotropy study of ?V? shaped molecules: DFT calculations and NMR assignment

LEANDRO J. TRUPP, ANDREA C. BRUTTOMESSO; SERGIO L. LAURELLA; M. CRISTINA TETTAMANZI

Santiago de Chile

Congreso; 10th Congress of the World Association of Theoretical and Computational Chemists (WATOC 2014); 2014

Building blocks with well defined rigid structures arouse both practical and theoretical interest, as they allow controlling the disposition of different substituents in predictable geometries. Tröger?s base (TB) is one of the most used compounds of this kind, because it adopts a rigid ?V? shaped geometry with the aromatic rings almost perpendicular to each other [1]. In our laboratory, we use TB as a scaffold in the synthesis of compounds with interesting intervalence properties or as ligands in metal complexation. As a result of this study, we obtained N,N'-(4,10-dimethyl-6,12-dihydro-5,11-methano-dibenzo[b,f][1,5]diazocine-2,8-diyl)diformamide 1, and found that it presents special anisotropic properties, as evidenced in Nuclear Magnetic Resonance (NMR) spectra as a complex array of signals. In this work, we present the synthesis and full analysis of NMR spectra, through a computational study of this compound?s conformational behavior. Compound 1 was obtained in three steps, starting from 2-methyl-4-nitroaniline and obtaining first the di-p-nitro-TB derivative, which was later reduced to the diamine to finally obtain the diformamide. 1 was characterized by Mass Spectrometry and NMR spectroscopy. Spectra were obtained with different solvents and temperatures. The molecule under study was subjected to conformational analysis and geometry optimization (without constraints) using the density functional theory (DFT) method and B3LYP/6-31G(d,p) basis set as implemented in Gaussian 03 [2]. Dielectric solvent effects were considered using the Self-Consistent Reaction Field - Polarizable Continuum Model. Finally, the isotropic chemical shifts for 1H and 13C were obtained by gauge-independent atomic orbital calculations with the same methodology and basis set. An extensive conformational analysis was carried out, finding four main conformers in regard with the disposition of each amide bond (E or Z) and the chirality of the nitrogen atoms in the bridge (RS or SR). The used methodology allowed the complete NMR assignment, employing the calculated 1H and 13C chemical shifts, that were obtained within a reasonable accuracy. We also calculated each conformer?s energy in different solvents, and the amide bond conformational barrier in dimethylsulfoxide. The special geometric nature of compound 1, that causes the spatial proximity of substituents far away in the connectivity and their mutual influence, was evidenced in the NMR spectra as long-range anisotropy effects. In this work, we could adequately predict this behavior through DFT calculations including a full conformational study.