Synthesis of new cyano-substituted analogues of Tröger's bases from bromo-derivatives. A stereochemical dependence of long-range ( n J HH , n = 4, 5, and 6) proton-proton and proton-carbon ( n J CH , n = 1, 2, 3, 4, and 5) coupling constants of these compounds

Abstract

A free-catalyst microwave-assisted cyanation of brominated Tröger´s base derivatives (2a-f) is reported. The procedure is simple, efficient and clean affording the nitrile compounds (3a-e, I) in very good yields. Complete assignment of 1 H and 13 C chemical shifts of 2a-f, I and 3a-d, I was achieved using gradient selected 1D NMR techniques (1D zTOCSY, PSYCHE, DPFGSE NOE and DEPT), homonuclear 2D NMR techniques (gCOSY, zTOCSY), and heteronuclear 2D NMR techniques (gHSQCAD/or pure-shift gHSQCAD, gHMBCAD, bsHSQCNOESY, and gHSQCAD-TOCSY) with adiabatic pulses. Determination of the long-range proton-proton coupling constants n JHH (n = 4, 5, 6) was accomplished by simultaneous irradiation of two protons at appropriate power levels. In turn, determined coupling constants were tested by an iterative simulation program by calculating the 1 H NMR spectrum and comparing it to the experimental spectrum. The excitation-sculptured indirect-detection experiment (EXSIDE) and 1 H-15 N CIGARAD-HMBC (constant time inverse-detection gradient accordion rescaled heteronuclear multiple bond correlation) were applied for determination of long-range carbon-proton coupling constants n JCH (n = 2, 3, and 4) and for assignment of 15 N chemical shift at natural abundance, respectively. DFT/B3LYP optimization studies were performed in order to determine the geometry of 2c using 6-31G(d,p), 6-311G(d,p), and 6-311+G(d,p) basis sets. For calculation of 1 H and 13 C chemical shifts, n JHH (n = 2, 3, 4, 5, and 6), and n JCH (n = 1, 2, 3, and 4) coupling constants, the GIAO method was employed at the B3LYP/6-31G(d,p), B3LYP/6-31+G(d,p), B3LYP/6-311+G(d,p), B3LYP/6-311++G(2d,2p), B3LYP/cc-pVTZ), and B3LYP/aug-cc-pVTZ) levels of theory. For the first time, a stereochemical dependence magnitude of the long-range n JHH (n = 4, 5, and 6) and n JCH (n = 1, 2, 3, 4, and 5) have been found in bromo-substituted analogues of Tröger´s bases.