Meisenheimer complexes heterocyclic

Meisenheimer complex formation as separate reactions. Meisenheimer complexes can be considered as the anionic pseudobases derived from neutral aromatic molecules, and in this light it is clear that heterocyclic Meisenheimer complexes are appropriately considered in the current Review. By so doing, it is hoped that attention can be drawn to potentially mutual benefits that may be derived from comparative studies of neutral and anionic pseudobases. Certainly, the spectroscopic techniques applicable to the study of pseudobase and Meisenheimer complex formation are identical. Quantitative studies of substituent effects and structural effects on rates and equilibria for nucleophilic addition should be relevant both to neutral and to anionic e-complex formation. The general rules enunciated by Strauss23,318 and Fendler319 for the prediction of the relative stabilities of Meisenheimer complexes should be directly applicable to analogous pseudobases. Terrier et al.2n have made an important contribution in this area with a detailed comparison of kinetic and thermodynamic parameters for formation of a benzofuroxan Meisenheimer complex and an isoquinoline pseudobase. 

The first examples of heterocyclic Meisenheimer complexes were reported in 1968 for 3,5-dinitropyridines. Fyfe presented320 UV and PMR spectral evidence for C-2 addition of methoxide ion to 3,5-dinitropyridine to give 166. Similar spectroscopic evidence for... 

Quantitative studies of rates and equilibria for formation of heterocyclic Meisenheimer complexes are summarized in Table IX. Association constants are defined by Eq. (42)... 

Rate and Equilibrium Constants for Formation of Heterocyclic Meisenheimer Complexes ... 

Nucleophilic substitution most readily occurs at the 2- and 4-position of the more electron-deficient heterocyclic ring of quinolines. However, SNAr reactions at the carbocyclic ring can occur, mainly at positions 5 and 7. 5,7-Dibromo-8-hydroxyquinoline, 5-bromo-8-hydroxyquinoline, and 7-bromo-8-hydroxy-5-methylquinoline undergo conversion to the corresponding chloroquinolines on treatment with neat pyridine hydrochloride at 220 °C in a process that is postulated to proceed via the formation of stabilized Meisenheimer complexes <1996TL6695> (Equations 20 and 21). 

The formation of -complexes in liquid ammonia solutions has been observed for both heteroaromatic cations and neutral heteroaromatic molecules. These tr-complexes arise via addition of the amide ion to the heterocycle and are exactly analogous to the pseudobases and Meisenheimer complexes of aqueous and alcoholic media. 

A few heterocyclic Jackson-Meisenheimer complexes were recorded, viz. I [29] II [30]. Vickery [31a] rationalized the structure of the complex (HI) obtained in 1883 by Lobry de Bruin [31b] while acting with potassium cyanide on m-dinitrobenzene. 

CH2CHNH2CO2H] The complexes were characterized by i.r., u.v., and n.m.r. spectra. The ground- and excited-state configurations of the electrons of 4-substituted benzothiadiazoles (295 X = S R = H, NH2, or OH) were calculated the 7r-/a-bonds, total energies, and heats of atomization of (295), their protonated forms, and their tautomers were tabulated. The hydroxy-and amino-tautomers are more stable than the 0x0- and imino-tautomers, respectively. Compounds (295) are protonated on N-1. The linear dichroism and m.c.d. spectra of 2,1,3-benzothiadiazole and 2,1,3-benzo-selenadiazole were measured and c.d. spectra reported for the j3-cyclodextrin compound with the heterocycle. The kinetics of formation and equilibrium data have been reported for the Meisenheimer complexes of the benzothia- and benzoselena-diazoles (295 X = S, or Se R = 4-NO2) with MeO" in MeOH/DMSO. ... 

The formation of the Meisenheimer complexes has been modified to take place under electrochemical reduction conditions through a charge-transfer complex between electron-deficient aromatics and electron-donative heterocycles. ... 

If the thiophene ring bears one or more N02 groups, it becomes susceptible to nucleophilic attack by alkoxide ion an anionic cr-complex is thus produced which can be isolated in some cases. This is called a Meisenheimer adduct, and corresponds to the first step in many nucleophilic substitution reactions on activated thiophene substrates. The similarity between these adducts and heterocyclic pseudobases has been pointed out (79AHC(25)l). Kinetic data lead to similar rate equations for both processes both are characterized by negative entropies of activation of similar magnitude. 

A detailed study of the effect of coordination to a [Rh(NH3)5] center of a range of imidazoles and pyrazoles has been reported. The relative o and tt components controlling the pK of the coordinated heterocycle are discussed and compared with related cobalt(III) and ruthenium(III) complexes. The addition of hydroxide to the ligand in a range of 5-nitro-1,10-phenanthroline complexes has been described. It is proposed that attack upon [Ru(bipy) (5-nitrophen)3 ] can occur at the 4- or 7-positions in addition to the expected 6-position of the Meisenheimer adduct. The spectroscopic data supporting this proposal do not appear to be unambiguous. Finally, the displacement of halide from coordinated 4-halo substituted pyridines has been developed into a new synthetic method. 

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