Halide displacement

The reaction of alkenes with nitronium salts proceeds through a nitrocarbocation. The product(s) obtained depends on both the nature of the starting alkene and the conditions used.  

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Halide Displacement. Hahde displacement is a method used to prepare thiols that are not readily available by normal means. It requires a two-phase, water—organic system, that can be quite corrosive. Normally, this type of reaction, a classic Sj 2 type, is undertaken in HasteUoy or glass-lined... 

In the first century of "organic" chemistry much attention was given to the structures of carbogens and their transformations. Reactions were classified according to the types of substrates that underwent the chemical change (for example "aromatic substitution," "carbonyl addition," "halide displacement," "ester condensation"). Chemistry was taught and learned as transformations characteristic of a structural class (e.g. phenol, aldehyde) or structural subunit... 

Mercurydl) halides displace Me3Sn groups from a transition-metal carbonyl derivative ... 

Besides addition reactions, azides or hydrazoic acid can also yield tetrazoles through displacement reactions. Thus, halide displacement in imide chloride (78) yields 1,5-disubstituted tetrazoles (79), and in 2-chloro-pyridine (80), yields tetrazolopyridine (81) (Eq. 16a,b).141 143 Vinylogous... 

The group ofWalborsky probably has described one of the first true anionic/radi-cal domino process in their synthesis of the spirocyclopropyl ether 2-733 starting from the tertiary allylic bromide 2-730 (Scheme 2.161) [369]. The first step is a Michael addition with methoxide which led to the malonate anion 2-731. It follows a displacement of the tertiary bromide and a subsequent ring closure which is thought to involve a SET from the anionic center to the carbon-bromine anti bonding orbital to produce the diradical 2-732 and a bromide anion. An obvious alternative Sn2 halide displacement was excluded due to steric reasons and the ease with which the reaction proceeded. 

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. 

Halide displacement from the carbene ligands of Ru, Os, and Ir halocarbene complexes by N-, O-, and S-based nucleophiles frequently leads to the formation of new heteroatom-substituted carbene complexes. This important class of reactivity will be discussed in more detail in Section V,D, but it is appropriate here to illustrate the scope of this method with several examples ... 

R = Pr) via a bromide displacement process. Halide displacements have been observed previously in the reactions of carbenes with Me3SiI (38). However, this represents the first such reaction with a haloborane. The X-ray crystal structure of 49 was determined and showed that both heterocyclic rings are planar and that the interpla-nar angle is 92.9°. The B-C(carbene) bond distance of 1.580(11) A is comparable to that found in 32 (1.603(3) A). 

Aguilar, A.M. and Daigle, D., Vinyl halide displacement by metallo organo-phosphides. Preparation of fra s-(f-styryldiphenylphosphine oxide and sulfide, /. Org. Chem., 30, 2826, 1965. 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

Oxidative nitration is a one step process from nitroalkane to gem- dinitroalkane, whereas the Ter Meer reaction requires two steps (initial halogenation followed by halide displacement with nitrite anion). 

Thermally insensitive explosives from halide displacement with nitrogen nucleophiles... 

In cm empirical approach (Helferich modification) it has been demonstrated on many occasions that mercuric salts (cyanide or bromide usually) in polar solvents such as nitromethane or acetonitrile favour the formation of a-glucosides, conceivably because under these conditions the mechanism of the halide displacement is rmimolecular and the carbonium ion can be approached from either side o). 

The polymerization of halophenoxides by copper (II) mediated halide displacement is a mechanistically complicated reaction. Elucidation of the structure of the polymers is essential to an understanding of both the polymerization chemistry and the peculiar physical properties of the polymers. The physical tool which has yielded most information on the polymer structure is nmr. The first conclusion which derives from a study of the spectra of poly(dihalophenyleneoxides) is that regioselectivity in halogen displacement is more likely the source of the polymer properties than branching. A more rigorous confirmation of the polymer structures will depend on a detailed analysis of the spectra of model compounds for the chain segments. 

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