Halogenation five-membered rings

Construction of isolated or benzannulated five-membered rings of NHPs can be accomplished by means of various condensation or cycloaddition reactions all of which involve interaction of an electrophilic Pj and a nucleophilic C2N2 building block. Salts containing 1,3,2-diazaphospholide anions or 1,3,2-diazaphospholenium cations can be directly accessed by some of these reactions but the products are in most cases neutral 1,3,2-diazaphospholes or NHP. A particularly concerted effort has been directed toward the synthesis of P-halogen-substituted NHP which are capable of undergoing further reactions via halide displacement or halide abstraction and serve thus as entry points for the preparation of a wide variety of neutral and cationic NHP derivatives. 1,3,2-Diazaphospholide anions are normally accessed by deprotonation of suitable iV-H-substituted precursors. 

Originally conceived as a single review, this work has now been divided into three parts that separate the five-membered rings (Part I), six-membered rings (Part II), and condensed systems (Part III). The section dealing with halogenation methods is introductory to all three parts. 

One of the most frequently studied transition metal catalyzed transformations of azoles and indole is their participation in cross-coupling reactions. Due to the abundance of examples in this field we only present some representative examples of the different reaction classes. In this chapter reactions where a halogenated azole is used to introduce the five membered ring onto the palladium in the oxidative addition and processes,... 

Direct experimental evidence for 1,4-halogen participation comes from the direct observation (by NMR spectroscopy) of five-membered-ring tetramethylenehalonium ions by Olah and Peterson408 and by Olah et al.400... 

Halogenation of heterocycles five-membered rings, 57, 291 fused to other aromatic and heteroaromatic rings, 59, 245 six- and seven-membered rings, 58, 271 of heterocyclic compounds, 7, 1 Hammett equation, applications to... 

Bisftrifluoromethyl] tellurium dichloride and dibromide react with excess halogen in acetonitrile to form trifluoromethyl tellurium trichloride and tribromide2 3. When the dihalidcs were reacted with iodine, trifluoromethyl tellurium triiodide was not detected3. When dibenzotellurophene dichloride and tellurium tetrachloride were refluxed in 1,2-dichlorobenzene, the five-membered ring was opened and 2,2 -bis[trichlorotelluro biphenyl formed4. 

Where halogen or acetate is the counterion, the cyclomet-alated products are halide- or acetate-bridged dimers. Particularly in the case of halides, these dimers are easily cleaved by various ligands, such as pyridine, phosphines, phosphites, and acetylacetone, to give more soluble, monomeric species. Cyclometalation, especially with nitrogen atom donors, inserts into C-H bonds to form five-membered rings. However, formation of six-membered rings can occur (equation 67). 

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