Dehalogenation reagents

In the presence of AIBN, tributyltin hydride is an excellent dehalogenating reagent for generating radicals. The bromoalkylcyclobutanone 206 undergoes reductive ring expansion to give, via the annealed alkoxy radical, the cis-fiised bicycle 207 stereospecifically as the major product [113], (Scheme 81)... 

Dialkyldichlorostannanes undergo reduction to dialkylstannanes (100), as shown in reaction 26. Product 100 is a good dehalogenation reagent for more complex stannyl chlorides, such as compound 32 (see Table 5) yielding compound 19 (see Table 2)134. 

Dehalogenation of 2 with an equimolar amount of /BusSiNa in THF at -78°C leads to the isolable monosodium compound 6 in quantitative yield. A subsequent silylation of 6 by MesSiCl under formation of 7 is possible (Eq. 3). This reactions show that tBusSiNa is a suitable dehalogenation reagent for (rBu3Si)2Si2H4 Br . 

Recently, Yamamoto et al. presented a new route to PPP using the zero-valent nickel complex itself (Scheme 6.15) as a dehalogenation reagent [59]. This reaction proceeds under mild conditions and can be applied to a wide range of aromatic compounds including those with carbonyl and cyano groups. 

Zero-Valent Nickel Complexes. A new polycondensation reaction uses the zero-valent nickel complex itself as the dehalogenation reagent [271,570,571] ... 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Ultraviolet light with a hydrogen source can also be a selective reagent for dehalogenation [36, 37, 3S] (equations 27-29)... 

The 2-pentafluoropropenylzinc reagent has been prepared via a novel one-pot dehalogenation-insertion reaction [103] (equation 82)... 

The synthetic route represents a classical ladder polymer synthesis a suitably substituted, open-chain precursor polymer is cyclized to a band structure in a polymer-analogous fashion. The first step here, formation of the polymeric, open-chain precursor structure, is AA-type coupling of a 2,5-dibromo-1,4-dibenzoyl-benzene derivative, by a Yamamoto-type aryl-aryl coupling. The reagent employed for dehalogenation, the nickel(0)/l,5-cyclooctadiene complex (Ni(COD)2), was used in stoichiometric amounts with co-reagents (2,2 -bipyridine and 1,5-cyclooctadiene), in dimethylacetamide or dimethylformamide as solvent. 

The Reformatsky reaction is a classical reaction in which metallic zinc, an a-haloester, and a carbonyl compound react to give a (i-hydroxyester.162 The zinc and a-haloester react to form an organozinc reagent. Because the carboxylate group can stabilize the carbanionic center, the product is essentially the zinc enolate of the dehalogenated ester.163 The enolate effects nucleophilic attack on the carbonyl group. 

Aromatic dehalogenation suffers from the disadvantage that only 50% of the tritium is incorporated, the rest appearing as waste. This situation is even more marked for borotritide reductions but the problem can be overcome by using some of the new tritide reagents that have recently become available as a result of the synthesis of carrier-free lithium tritide (Scheme 13.1) [22], Their reactivity can be fine-tuned through the elements (e. g. B, Al, Sn) to which the tritium is attached and by the electronic and steric nature of the substituents at the central atom. 

Dehalogenation.1 DMBI effects dehalogenation of a-halo carbonyl compounds in a variety of ethereal solvents with formation of DMBI+X in generally high yield. The order of relative reactivity is Br > Cl > F (halides) and primary > secondary > tertiary (for the a-substituted position). In combination with HO Ac (1 equiv.) the reagent also reduces acyl chlorides to aldehydes (70-90% yield). 

Reviews. The recent use of this reagent in synthesis has been reviewed (187 references).1 The review includes not only radical dehalogenation but also reductive... 

Some investigations directed to the dehalogenation of polyhalogenated organic derivatives by means of tellurium reagents have been reported. 

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