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Back to basics: A transition-metal-free method developed for the synthesis of indazoles involves an inexpensive catalytic system composed of a diamine and K2CO3. Various (Z)-2-bromoacetophenone tosylhydrazones were converted into indazoles at room temperature in excellent yields (see example; Ts = p-toluenesulfonyl). The yield was improved by photoisomerization with UV light when E/Z isomeric mixtures of the starting material were used.

Unsymmetrical N,N'-bisalkylidene hydrazines 7a,b, 10a-c and 13, which are accessible in good to excellent yields from hydrazones 6, 9, and 12 and commercially available α,β-unsaturated carbonyl compounds, are converted into 3,4-diazaheptatrienyl anions 14a,b, 16a-c, and 18 by treatment with KO-t-Bu as base. These anionic species form pyridines 15a,b, 5,6-dihydrobenzo[h]quinolones 17a-c, and bipyridine 19 in moderate yields. We interpret thermodynamics and kinetics of these reactions by quantum chemical calculations and discuss this multistep anionic rearrangement, based on an electrocyclic ring formation with a Möbius aromatic transition structure 22, the N-N bond fission (25), and the 6-exo-trig cyclization (27) as key steps, considering the results of NICS and NBO-charge calculations. This novel anionic reaction sequence bears an interesting analogy to the mechanism of the (cationic) Fischer indole synthesis. Precursor 10c and product 16c could be characterized by X-ray diffraction.

2-Alkenylbenzylidene hydrazones 5a-m, which are accessible in good to excellent yields in a four-step synthesis, are converted into 1,2-diaza-4,5-benzoheptatrienyl metal compounds 1a-m 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 R(1) and R(3). Thus, metal compounds 1a-c carrying alkyl substituents R(1) and R(3) form 3H-benzodiazepines 6a-c 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 R(3), form 1,2-dihydrophthalazines 8a-e in a predominantly charge-controlled 6-exo-trig cyclization reaction. In contrast, aryl-aryl-substituted systems 5i-m did not lead to cyclic products upon deprotonation, but the intermediate open-chain metal compounds 1i-m 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.

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