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    Regioselectivity Patterns in Radical Cyclization of Diosphenol Derivatives with Different Ring Size: A Combined Experimental and DFT Study
    (Wiley-V C H Verlag Gmbh, 2021) Boz, Esra; ozcan, Hafize; Zaim, Omer; Tuzun, Nurcan S.
    Radical cyclization reactions are still a challenging field in synthetic organic chemistry. Herein, the radical reactions of 6- and 7-membered diosphenol derivatives (tropolone) with a significant difference in reactivity, ring size, aromaticity and stability were investigated. While the former produces a mixture of products including oxabicycloalkanones, the experimental work performed herein resulted with a single 2-(3-hydroxypropyl)cyclohepta-2,4,6-trienone product, regioselectively. DFT calculations (UM05/6-311++G(d,p)) were performed following the experimental studies in order to describe the outcomes correctly. Therefore, all possible reaction pathways were investigated for both diosphenol and tropolone. The reaction barriers and intrinsic barriers from the Marcus theory were calculated to investigate the thermodynamic and intrinsic electronic effects to the experimental product distribution. Both experiment and theory show that the aforementioned systems have different selectivity routes governed by several factors. As a result of this comparative study, the charge distribution at the reactive atoms, entropy factors for cyclization vs termination and aromatization of the ring effects were found to be the driving factors for the observed regioselectivity.
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    Synthesis, reactions and DFT study of tropolone N,N-dimethylthiocarbamate
    (Pergamon-Elsevier Science Ltd, 2015) Zaim, Omer; Tuzun, Nurcan S.; Cevik, Busra; Ozcan, Hafize; Boz, Esra
    A new tropolone derivative namely O-(7-oxocyclohepta-1,3,5-trienyl) dimethylthiocarbamate was synthesized from tropolone and N,N-dimethylthiocarbamoyl chloride and its reactions with lithium halides in acidic media were examined. The product of each lithium halide treatment was identical and identified as S-(3-oxocyclohepta-1,4,6-trienyl) dimethylcarbamothioate. Eventually it was converted to 3-mercapto 2,4,6-cycloheptatrien-1-one on treatment with base. DFT calculations at the B3LYP/6-311++G** level were performed on the key elimination step of the mechanism, which was proposed by Ponaras et al. on analogous five-membered ring systems. The experimentally observed regioselectivity was correlated to the calculated activation barriers on five and seven-membered systems in the elimination step such that in both cases, experimentally observed product was favored over the alternative one. Thus, quantum mechanical calculations have provided insights into the synthetic pathway and regioselectivity of the reaction in this study, in comparison to the analogous five-membered system. (C) 2015 Elsevier Ltd. All rights reserved.