Tröger′s bases have been attracting great interest due to their potential applications in nanoelectronics, supramolecular chemistry, molecular recognition, biological activity and auxiliaries for asymmetric synthesis. However, there is still no detailed step by step proposal for the mechanism leading to the production of these compounds available. A set of five model syntheses of symmetric and asymmetric Tröger′s base derivatives starting from substituted anilines and formaldehyde was studied and envisaged as representative for understanding [a] Scheme 3. Calculated mechanistic pathways for the reaction 1 + 1 → 5z. Relative free energies (ΔG 0 ) in kcal/mol. The colours of the arrows correspond to: path I (blue), path II (green), path III (purple), path IV (yellow) and path V (red).

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 1H and 13C chemical shifts of 2a‐f, I and 3a‐d, I was achieved using gradient selected 1D nuclear magnetic resonance (NMR) techniques (1D zTOCSY, PSYCHE, DPFGSE NOE, and DEPT), homonuclear 2D NMR techniques (gCOSY and 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 nJHH (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 1H NMR spectrum and comparing it to the experimental spectrum. The excitation‐sculptured indirect‐detection experiment (EXSIDE) and 1H‐15N CIGARAD‐HMBC (constant time inverse‐detection gradient accordion rescaled heteronuclear multiple bond correlation) were applied for determination of long‐range carbon–proton coupling constants nJCH (n = 2, 3, and 4) and for assignment of 15N 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 1H and 13C chemical shifts, nJHH (n = 2, 3, 4, 5, and 6), and nJCH (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 nJHH (n = 4, 5, and 6) and nJCH (n = 1, 2, 3, 4, and 5) have been found in bromo‐substituted analogues of Tröger's bases.

Several studies have shown that pyrolysis conditions and feedstocks are the key factors influencing biochar chemical and physical properties. The information on the nature of biochar is quite important, especially when this carbonaceous material is intended to be used as a potential soil amendment. In this study, we investigated the formation and characterisation of biochars produced from vacuum pyrolysis of sunflower seed shells (SSS), peanut shells (PS) and Spirulina algae (Sp) at 280 °C (for SSS, PS and Sp) and 350 °C (for PS). As a proxy to test the potential of each biochar as soil amendment, we assessed the germination and growth effects of the biochar water-extractable substances (BWES) at different concentrations (10; 7.5; 5; and 2.5% w/v) on Lactuca sativa. Results showed that the biochar from pyrolysis of PS at 280 °C would be the most suitable soil amendment, since its BWES did not affect germination and exhibited a remarkable growth-promoting effect (50-100%) on roots and stems of L. sativa. In contrast, BWES from SSS, Sp and certain concentrations of PS produced at 350 °C inhibited growth of Lactuca sativa, and particularly BWES of Spirulina dramatically reduced germination, posing a risk for direct application as soil amendment. The presence of carbonyl derivatives in the BWES from PS may be linked to the stimulatory effects of this extract. Aromatics could be responsible for the germination and growth inhibition in the BWES of SSS, while nitrogen organic compounds would enhance the inhibitory effect in BWES from Sp.

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