Halides dehalogenation

Pyrolysis of carboxylic acids Dehydrohalogenation of acyl halides Dehalogenation of a-halo acyl halides Rearrangement of diazo ketones (Wolff)... 

Predict the major and minor products of dehydrohalogenation of alkyl halides, dehalogenation of dibromides, and dehydration of alcohols. 

Dehydrohalogenation of Alkyl Halides Dehalogenation of 12-Dihalides Dehydration of Alcohols Alkenes from Ethers... 

Dehydrohalogenation cf Vinylic or Allylic Halides Dehalogenation of23-Dihalopropenes Dehydration ef Allylic Alcohols Dehalogenation of gem-Dibromocyclopropanes... 

Ni(Gl)Gp(carbene) complexes with the carbenes l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro imidazol-2-ylidene, l,3-bis(2,6-diisopropylphenyl)-4,5-dihydorimidazol-2-ylidene, and l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene have also been prepared, structurally characterized, and shown to have catalytic activities in the aryl amination and aryl halide dehalogenation reactions. Structurally similar carbene complexes have also been reported with the novel carbene formed by aryl cyclohexadienyl-ylidene rearrangement, as shown in Equation (11) the solid-state structure of this complex has shown an Ni-Gcarben bond of ca. 189 pm. 

Formate is an excellent hydride source for the hydrogenolysis of aryl halides[682]. Ammonium or triethylammonium formate[683] and sodium formate are mostly used[684,685]. Dechlorination of the chloroarene 806 is carried out with ammonium formate using Pd charcoal as a catalyst[686]. By the treatment of 2,4,6-trichloroamline with formate, the chlorine atom at the /iiara-position is preferentially removed[687]. The dehalogenation of 2,4-diha-loestrogene is achieved with formic acid, KI, and ascorbic acid[688]. 

The most common leaving groups are sulfonate esters and halides. For the sake of convenience, the discussion of certain dehalogenation reactions is also included in this section even though they may not involve 8 2 type displacement. Benzylic alcohols are also known to be displaced by hydrides or deuterides, but there is no evidence for the application of these reactions to the steroid field. 

Mercury can be used to dehalogenate acyl halides to ketenes when a halogen IS present next to the COCl group [67] (equation 35)... 

In the presence of a precious metal catalyst, aryl halides can undergo dehalo-dimerization to give biaryl products, with varying degrees of selectivity. The major byproduct of this reaction is usually the dehalogenated aryl compound. This type of chemistry is currently one of the very few viable means for the large scale preparation of biaryl compounds. 

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

Ketenes can be prepared by dehalogenation of a-halo acyl halides with zinc or with... 

Compounds of the type E(IXE(III)R2] that are analogous to the group-IA-group-IVB compounds, e.g., LiSnPh3, should be given by dehalogenation of diorganometal halides ... 

Cr(II) has been used to bring about dehalogenation of alkyl halides involving the production of alkyl radicals, and details have been provided in a substantive review (Castro 1998). The ease of reduction is generally iodides > bromides > chlorides, while tertiary halides are the most reactive and primary halides the least (Castro and Kray 1963, 1966). 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

As a consequence of facile homolytic cleavages, sulfonyl halides (I > Br > Cl F unsuitable) are able to add to unsaturated C—C systems. To prevent (or reduce) competing polymerizations, the additions of sulfonyl chlorides have been recommended to be carried out in the presence of copper(I/II) salts (Asscher-Vofsi reaction ). Comprehensive surveys have been published on the resulting j8-halogeno sulfones (or their vinyloguous compounds) as well as on their dehalogenation products (vinyl sulfones, 1-sulfonyl-l, 3-dienes, etc.). Table 5 reviews a series of sulfonyl halide additions and facile hydrogen halide eliminations. 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

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