TY - JOUR A1 - Kleinpeter, Erich A1 - Heydenreich, Matthias A1 - Shainyan, Bagrat A. T1 - At the experimental limit of the NMR conformational analysis BT - Si-29 and C-13 NMR study of the conformational equilibrium of 1-phenyl-1-tert-butylsilacyclohexane JF - Organic letters N2 - The low temperature (95 K) NMR study of 1-Ph-1-t-Bu-silacyclohexane (1) showed the conformational equilibrium to be extremely one-sided toward thePh(ax),t-Bueq conformer. The barrier to interconversion has been measured (4.2-4.6 kcal/mol) and the conformational equilibrium [Delta nu = 1990.64 ppm (Si-29), 618.9 ppm (C-13), 1-Ph-ax:1-Pheq = (95.6-96.6%):(3.4-4.4%), K = 25 +/- 3, Delta G degrees = -RT ln K = 0.58-0.63 kcal/mol] analyzed. The assignment and quantification of the NMR signals is supported by MP2 and DFT calculations. Y1 - 2021 U6 - https://doi.org/10.1021/acs.orglett.0c03878 SN - 1523-7060 SN - 1523-7052 VL - 23 IS - 2 SP - 405 EP - 409 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Belasri, Khadija A1 - Topal, Leila A1 - Heydenreich, Matthias A1 - Koch, Andreas A1 - Kleinpeter, Erich A1 - Fulop, Ferenc A1 - Szatmari, Istvan T1 - Synthesis and conformational analysis of naphthoxazine-fused phenanthrene derivatives JF - Molecules N2 - The synthesis of new phenanthr[9,10-e][1,3]oxazines was achieved by the direct coupling of 9-phenanthrol with cyclic imines in the modified aza-Friedel-Crafts reaction followed by the ring closure of the resulting bifunctional aminophenanthrols with formaldehyde. Aminophenanthrol-type Mannich bases were synthesised and transformed to phenanthr[9,10-e][1,3]oxazines via [4 + 2] cycloaddition. Detailed NMR structural analyses of the new polyheterocycles as well as conformational studies including Density Functional Theory (DFT) modelling were performed. The relative stability of ortho-quinone methides (o-QMs) was calculated, the geometries obtained were compared with the experimentally determined NMR structures, and thereby, the regioselectivity of the reactions has been assigned. KW - modified Mannich reaction KW - cyclic imines KW - [4+2] cycloaddition KW - NMR KW - spectroscopy KW - conformational analysis KW - DFT calculations Y1 - 2020 U6 - https://doi.org/10.3390/molecules25112524 SN - 1420-3049 VL - 25 IS - 11 PB - MDPI CY - Basel ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - Dative or coordinative carbon-boron bond in boron trapped N-heterocyclic carbenes (NHCs)? BT - an answer given on the magnetic criterion JF - Tetrahedron : the international journal for the rapid publication of full original research papers and critical reviews in organic chemistry N2 - The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of isolated as well as B-C bond length varied model compounds (BR3 trapped NHCs) have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values (actually the anisotropy effects measurable in H-1 NMR spectroscopy) are employed to qualify and quantify the present dative vs. coordinative bond character of the boron-carbon bond in the trapped NHCs. Results are confirmed by bond lengths and B-11/C-13 chemical shift variations in the BR3 trapped NHCs. KW - NHCs KW - Dative vs. coordinative NHC -> BR3 bond KW - Through-space NMR KW - shieldings (TSNMRS) KW - NICS KW - Anisotropy effect Y1 - 2021 U6 - https://doi.org/10.1016/j.tet.2020.131787 SN - 0040-4020 SN - 1464-5416 VL - 80 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - Quantification of sigma-acceptor and pi-donor stabilization in O, S and Hal analogues of N-heterocyclic carbenes (NHCs) on the magnetic criterion JF - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory N2 - The spatial magnetic properties, through-space NMR shieldings (TSNMRSs), of stable O, S and Hal analogues of N-heterocyclic carbenes (NHCs) have been calculated using the GIAO perturbation method employing the nucleus-independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSSs) of various sizes and directions. The TSNMRS values (actually the anisotropy effects measurable in H-1 NMR spectroscopy) are employed to qualify and quantify the position of the present mesomeric equilibria (carbenes <-> ylides). The results are confirmed by geometry (bond angles and bond lengths), IR spectra, UV spectra, and C-13 chemical shifts of the electron-deficient carbon centers. Y1 - 2021 U6 - https://doi.org/10.1021/acs.jpca.1c05257 SN - 1089-5639 SN - 1520-5215 VL - 125 IS - 33 SP - 7235 EP - 7245 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - Intramolecular carbene stabilization via 3c,2e bonding on basis of the magnetic criterion JF - Tetrahedron : the international journal for the rapid publication of full original research papers and critical reviews in organic chemistry N2 - The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of bent cyclobutylcarbene 8, 1,2-diboretane-3-ylidene 9, and some carbene analogues of boron 14-18 as most intriguing examples of carbenes, which can be stabilized as homoaromatic systems with 3c,2e bonding, have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values (actually, ring current effect/anisotropy effects as measurable in H-1 NMR spectroscopy) are employed to qualify and quantify the degree of present 3c,2e-homoaromaticity. Results are confirmed by geometry (bond angles and bond lengths) and spectroscopic data, the delta(B-11)/ppm data and the C-13 chemical shifts of the carbene electron-deficient centre. KW - Cyclobutylcarbene KW - 1,2-diboretane-3-ylidene KW - 3c,2e-bonding KW - Through-space NMR shieldings (TSNMRS) KW - NICS Y1 - 2021 U6 - https://doi.org/10.1016/j.tet.2021.132357 SN - 0040-4020 SN - 1464-5416 VL - 95 PB - Elsevier Science CY - Amsterdam ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - Bent Allenes or Di-1,3-betaines-An Answer Given on the Magnetic Criterion JF - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory N2 - The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of bent allene 1, the corresponding C-extended 1,3-butadiene derivative 2, and a number of related compounds 3 -20 have been calculated using the gauge-independent atomic orbital perturbation method, employing the nucleus-independent chemical shift concept and visualized as isochemical shielding surfaces of various sizes and directions. Prior to that, both structures and C-13 chemical shifts were calculated and compared with available experimental bond lengths and delta(C-13)/ppm values (also, as a quality criterion for the computed structures). Bond lengths, the delta(C-13)/ppm, and the TSNMRS values are employed to qualify and quantify the electronic structure of the studied compounds in terms of dative or classical electron-sharing bonds. Y1 - 2020 U6 - https://doi.org/10.1021/acs.jpca.0c01392 SN - 1089-5639 SN - 1520-5215 VL - 124 IS - 16 SP - 3180 EP - 3190 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - The 13 C chemical shift and the anisotropy effect of the carbene electron-deficient centre BT - simple means to characterize the electron distribution of carbenes JF - Magnetic resonance in chemistry N2 - Both the C-13 chemical shift and the calculated anisotropy effect (spatial magnetic properties) of the electron-deficient centre of stable, crystalline, and structurally characterized carbenes have been employed to unequivocally characterize potential resonance contributors to the present mesomerism (carbene, ylide, betaine, and zwitter ion) and to determine quantitatively the electron deficiency of the corresponding carbene carbon atom. Prior to that, both structures and C-13 chemical shifts were calculated and compared with the experimental delta(C-13)/ppm values and geometry parameters (as a quality criterion for obtained structures). KW - C-13 chemical shift KW - carbenes KW - zwitterions KW - carbene electron deficiency KW - nucleus-independent chemical shifts (NICS) KW - through space NMR shieldings KW - (TSNMRS) Y1 - 2019 U6 - https://doi.org/10.1002/mrc.4979 SN - 0749-1581 SN - 1097-458X VL - 58 IS - 3 SP - 280 EP - 292 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - Cyclazines-structure and aromaticity or antiaromaticity on the magnetic criterion JF - European journal of organic chemistry N2 - Structure and spatial magnetic properties, through-space NMR shieldings (TSNMRSs), of all ten cycl[2.2.2]azine to cycl[4.4.4]azine, hetero-analogues and the corresponding hydrocarbons have been calculated at the B3LYP/6-311G(d,p) theory level using the GIAO perturbation method and employing the nucleus independent chemical shift (NICS) concept. The TSNMRS values (actually, the ring current effect as measurable in H-1 NMR spectroscopy) are visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction, and employed to readily qualify and quantify the degree of (anti)aromaticity. Results are confirmed by NMR [delta(H-1)/ppm, delta(N-15)/ppm] and geometry (planar, twisted, bow-shaped) data. The cyclazines N[2.2.2](-) up to N[2.4.4](-) are planar or at most slightly bowl-shaped and, due to coherent peripheral ring currents (except in N[2.3.3](-), N[2.3.4], N[3.3.4](+) and N[2.4.4](+)), develop aromaticity or anti-aromaticity of the whole molecules dependent on the number of peripheral conjugated pi electrons. The cyclazines N[2.3.3](-), N[2.3.4], N[3.3.4](+) and N[2.4.4](+) develop two ring currents of different direction within the same molecule, in which the dominating ring current proves to be paratropic (in N[3.3.4](+) diatropic) including the nodal N p(z) lone pair into the conjugation. The residual cyclazines N[3.4.4], N[4.4.4](-) and N[4.4.4](+) are heavily twisted and, therefore, are not developing peripheral or diverse ring currents. The TSNMRS information about cyclazines and the parent tricyclic annulene analogues is congruent subject to structure and number of peripheral or internal conjugated pi electrons, the corresponding (anti)aromaticity is in unequivocal accordance with Huckel's rule. KW - Aromaticity KW - Cyclazines KW - NMR spectroscopy KW - Peripheral ring current Y1 - 2022 U6 - https://doi.org/10.1002/ejoc.202101362 SN - 1434-193X SN - 1099-0690 VL - 2022 IS - 8 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Shainyan, Bagrat A. A1 - Belyakov, Alexander V. A1 - Sigolaev, Yurii F. A1 - Khramov, Alexander N. A1 - Kleinpeter, Erich T1 - Molecular Structure and Conformational Analysis of 1-Phenyl-1-X-1-Silacyclohexanes (X = F, Cl) by Electron Diffraction, Low-Temperature NMR, and Quantum Chemical Calculations JF - The journal of organic chemistry N2 - The molecular structure and conformational preferences of 1-phenyl-1-X-1-silacyclohexanes C5H10Si(Ph,X) (X = F (3), Cl (4)) were studied by gas-phase electron diffraction, low-temperature NMR spectroscopy, and high-level quantum chemical calculations. In the gas phase only three (3) and two (4) stable conformers differing in the axial or equatorial location of the phenyl group and the angle of rotation about the Si-C-ph bond (axi and axo denote the Ph group lying in or out of the X-Si-C-ph plane) contribute to the equilibrium. In 3 the ratio Ph-eq:Ph-axo:Ph-axi is 40(12):55(24):5 and 64:20:16 by experiment and theory, respectively. In 4 the ratio Ph-eq:Ph-axo is 79(15):21(15) and 71:29 by experiment and theory (M06-2X calculations), respectively. The gas-phase electron diffraction parameters are in good agreement with those obtained from theory at the M06-2X/aug-ccPVTZ and MP2/aug-cc-pVTZ levels. Unlike the case for M06-2X, MP2 calculations indicate that 3-Ph-eq conformer lies 0.5 kcal/mol higher than the 3-Ph-axo, conformer. As follows from QTAIM analysis, the phenyl group is more stable when it is located in the axial position but produces destabilization of the silacyclohexane ring: By low temperature NMR spectroscopy the six-membered ring interconversion could be frozen, at 103 K and the present conformational equilibria of 3 and 4 could be determined. The ratio of the conformers is 3-Ph-eq:3-Ph-ax = (75-77):(23-25) and 4-Ph-eq:4-Ph-ax = 82:18. Y1 - 2017 U6 - https://doi.org/10.1021/acs.joc.6b02538 SN - 0022-3263 VL - 82 IS - 1 SP - 461 EP - 470 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - Paramagnetic ring current effects in anti-aromatic structures subject to substitution/annelation quantified by spatial magnetic properties (TSNMRS) JF - Tetrahedron N2 - The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of the typically anti-aromatic cyclopentadienyl cation, cyclobutadiene, pentalene, s-indacene and of substituted/annelated analogues of the latter structures have been calculated using the CIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to visualize and quantify the dia(para) magnetic ring current effects in the studied compounds. The interplay of dia(para)magnetic ring current effects due to substitution/annelation caused by heavy exo-cyclic n,pi-electron delocalization can be qualified. KW - Aromaticity KW - Anti-aromaticity KW - Through-space NMR shieldings (TSNMRS) KW - GIAO KW - NICS KW - Annelation effect Y1 - 2018 U6 - https://doi.org/10.1016/j.tet.2017.12.020 SN - 0040-4020 VL - 74 IS - 7 SP - 700 EP - 710 PB - Elsevier CY - Oxford ER -