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Abstract
Meisenheimer complexes are important intermediates in Nucleophilic Aromatic Substitution Reactions (SNAr). They are formed by the addition of electron rich species to polynitro aromatic compounds or aromatic compounds with strong electron withdrawing groups. It is possible to distinguish two types of Meisenheimer or σ-complexes, the σH-complex or σX-complex (also named ipso), depending on the aromatic ring position attacked by the nucleophile (a non-substituted or substituted one, respectively). Special examples of σX- or ipso-complexes are formed through intermediate spiro adducts, via intramolecular SNAr. Some of these spirocyclic Meisenheimer complexes, a type of σX-complex, are exceptionally stable in solution and/or as solids. They can be isolated and characterized using X-ray, and various spectroscopic techniques such as NMR, UV-Vis, IR, and fluorescence. A few of these stable spirocyclic Meisenheimer complexes are zwitterionic and exhibit interesting photophysical and redox properties. We will review recent advances, synthesis and potential applications of these stable spirocyclic Meisenheimer complexes.
Highlights
Nucleophilic aromatic substitution (SNAr) is an important reaction which takes place when electron rich species add to an aromatic ring containing electron withdrawing groups [1]
We have surveyed the different types of spirocyclic Meisenheimer complexes (SMCs) and Zwitterionic Spirocyclic Meisenheimer Complexes (ZSMCs) and their synthesis
In general SMCs are more stable than ZSMCs and more versatile
Summary
Nucleophilic aromatic substitution (SNAr) is an important reaction which takes place when electron rich species (nucleophiles) add to an aromatic ring containing electron withdrawing groups [1]. Further treatment of compound (7) with sodium methoxide deprotonated the OH group on compound (7) and afforded SMC (8) as a stable sodium salt (Scheme 6) This is an example of an asymmetric SMC formed by inter- followed by intramolecular attack of two different nucleophiles on the same ipso position of the aromatic ring. An interesting type of SMCs do not contain an independent cation, instead the anionic σcomplex is balanced out by a cation that is chemically attached to the SMC These interesting compounds, named Zwitterionic Spirocyclic Meisenheimer Complexes (ZSMCs), are only stable in a buffered solution; they could rarely be isolated. A wide variety of stable ZSMCs were synthesized by adding a nucleophile, attached to an ammonium cation (Compound 59), to the electron deficient aromatic ring These ZSMCs are zwitterionic in nature but differ only in that the metal cation is replaced with an organic cation. Since the ZSMC compounds were stable, it was possible to definitively confirm the zwitterionic nature of (60), (61) and (62) [61] by determination of their X-ray structure
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