Stoichiometric halogenating reagents

Numerous reagents may react with alkenes to yield mixed 1,2-dihalogenated compounds. Interhalogens prepared on mixing two different halides or their stoichiometric equivalents are used in mixed halogenations. 

Elemental chlorine, bromine, and iodine, sulfuryl chloride and thionyl chloride convert diorgano ditellurium compounds to organo tellurium trihalides (p. 314). The reactions are carried out in inert organic solvents with stoichiometrically required amounts of reagents. Dialkyl ditellurium compounds may lose alkyl halides if the halogenolysis is not performed under mild conditions at low temperatures. When equimolar amounts of halogens and diaryl ditellurium derivatives are combined in appropriate solvents, aryl tellurium halides are formed (p. 239). The formation of aryl tellurium halides is facilitated by stabilizing substituents in the orf/to-position to the tellurium atom or by the presence of thioureas or selenoureas in the reaction mixture. 

In this work a reagent, an aryl iodide dichloride, was directly attached to the substrate through a tether, but it had disadvantages. With such a scheme only stoichiometric amounts of the reagent could be used, and the reactions led to some recovered starting material. Thus we considered whether a new process was possible in which we would use both a tether and a template to direct halogenations. We decided to try to invent what we called a radical relay process. 

Alkylated oxazaborolidines such as (3.123) and (3.124) are more stable to air and moisture than (3.115) and exhibit better selectivities in some cases. BHs-THF is not the only borane carrier used and both BHs-DMS and catecholborane can be used as the stoichiometric reductant. As the latter reagent is effective at low temperature and is less reactive than BH3 it can be used to effect reduction of the ketone group of a,(3-enones such as (3.125) with no hydrobora-tion of the alkene moiety. Catecholborane can also be used as a stoichiometric reductant in the asymmetric reduction of halogenated ketones (3.126) and (3.127). 

Muniz and co-workers prepared a series of substituted indoles (e.g., 76) using a modified Koser reagent that was made from iodosobenzene and 2,4,5-tris-isopropylbenzene sulfonic acid (77, TIPBSA). The hypervalent iodine reagent was used either stoichiometrically or in catalytic amounts with mCPBA as the terminal oxidant. A variety of N-protecting groups were tolerated and substituents on the aryl ring of 75 include halogens, carbonyls (aldehydes, ketones, esters), alkynes, and nitriles (HAG(I)7349). 

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