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Ring-Closure Reactions of 1,2-Diaza-4,5-benzoheptatrienyl Metal Compounds: Experiment and Theory

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posted on 01.04.2011, 00:00 by Tillmann Kleine, Klaus Bergander, Roland Fröhlich, Birgit Wibbeling, Ernst-Ulrich Würthwein
2-Alkenylbenzylidene hydrazones 5am, which are accessible in good to excellent yields in a four-step synthesis, are converted into 1,2-diaza-4,5-benzoheptatrienyl metal compounds 1am by treatment with KO-t-Bu as base. These metal compounds undergo the various types of reactions in good yields and exclusively depending on the nature of substituents R1 and R3. Thus, metal compounds 1ac carrying alkyl substituents R1 and R3 form 3H-benzodiazepines 6ac after electrophilic quench of the intermediate cyclic anion 7 in a 7-endo-trig electrocyclic reaction with a möbius aromatic transition structure 1-TS. Similarly, a benzothienyl derivative 5n is converted into diazepine 6d. Potassium compounds 1d−h, which are N-methyl and aryl substituted at R3, form 1,2-dihydrophthalazines 8ae in a predominantly charge-controlled 6-exo-trig cyclization reaction. In contrast, aryl−aryl-substituted systems 5im did not lead to cyclic products upon deprotonation, but the intermediate open-chain metal compounds 1im were trapped by acid chlorides at N1 to yield the hydrazides 10a−e. We interpret thermodynamics and kinetics of these reactions in the context of the Baldwin rules on the basis of quantum chemical calculations and discuss the transition structures considering the results of NICS and NBO-charge calculations. Examples of the products 6, 8, and 10 could be characterized by X-ray diffraction.