Dehalogenation of organic halides

The dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

Electrochemical methods are available for the direct dehalogenation of organic halides to a limited extent fluorides and monochlorides are generally not reducible [1], In the presence of transition-metal complexes as mediators (Med), however, the electrolysis of halocarbons (RX) can be performed more effectively and selectively under various conditions [155-158]. Mediated electroreduction is most efficient when the electron transfer step E° (Med/Med -) is more negative than E° (RX/RX -) [157] (cf. Section 18.4.1). 

Magdesieva, T.V., Graczyk, M., Vallat, A., Nikitin, O.M., Demyanov, P.I., Butin, K.P. and Vorotyntsev, M.A. (2006) Electrochemically reduced titanocene dichloride as a catalyst of reductive dehalogenation of organic halides. Electrochim. Acta 52, 1265-1280. 

The reduction of saturated alkyl halides to alkanes, as represented in equation (1), is the most fundamental reaction of reductive dehalogenations of organic halides. The importance of these reductions has stimulated considerable investigation, and a number of successful approaches have been reported hitherto. Numerous reducing agents or reagent systems are available and many of them have been applied to practical organic synthesis with notable success. 

The order of ease of reductive dehalogenation of organic halides in the same type of structural environment is I > Br > Cl F. This order is parallel with the dissociation energy of carbon-halogen bonds (HsC—I 234 kJ mol- H3C—Br 293 kJ mol" H3C—Cl 351 kJ mol- H3C—F 452 kJ moL ) and is generally observed in the reduction of alkyl halides. Consequently, selective reduction of di- or polyhalides containing different halogen atoms is possible. Fluorides are often removed only with difficulty and examples of such reductions are comparatively limited. 

Radical reactions with organotin hydrides have been valuable for dehalogenation of organic halides. Bu3SnH/AIBN dehalogenation of a bicyclic carbamate furnished the ring-contracted product by carbamate rearrangement (Scheme 12.126) [229]. 

Hydrogenolysis of the C-halogen bond is an important reaction both from preparative and from environmental points of view. [ HCo(CN)5]3 was studied in detail as a catalyst for reductive dehalogenations of organic halides, which proceed according to Eqs. (38) and (39). The results of the early experiments are summarized in [12]. 

Shiraishi Y, Takeda Y, Sugano Y, Ichikawa S, Tanaka S, Hirai T (2011) Highly efficient photocatalytic dehalogenation of organic halides on Ti02 loaded with bimetallic Pd-Pt alloy nanoparticles. Chtan Commnn 47 7863-7865... 

The hydrides can react with alkenes and alkynes. A more active system, organolanthanide hydride/NaH, is able not only to reduce specifically the terminal carbon-carbon double bond but also to catalyse dehalogenation of organic halides. 

Pletcher and associates [155, 159, 160] have studied the electrochemical reduction of alkyl bromides in the presence of a wide variety of macrocyclic Ni(II) complexes. Depending on the substrate, the mediator, and the reaction conditions, mixtures of the dimer and the disproportionation products of the alkyl radical intermediate were formed (cf. Section 18.4.1). The same group [161] reported that traces of metal ions (e.g., Cu2+) in the catholyte improved the current density and selectivity for several cathodic processes, and thus the conversion of trichloroacetic acid to chloroacetic acid. Electrochemical reductive coupling of organic halides was accompanied several times by hydrodehalogena-tion, especially when Ni complexes were used as mediators. In many of the reactions examined, dehalogenation of the substrate predominated over coupling [162-165]. 

The applications of polyoxometalates in catalytic dehalogenation of halocar-bons have been succinctly reviewed by Hill and coworkers [188]. This reaction involves the photocatalytic transformation of organic halides coupled with the oxidation of sacrificial organic reductants (secondary alcohols or tertiary amides) (Eq. (9)) [189, 190] ... 

In 1980, Finder presented an excellent review dealing with the hydrogenolysis of organic halides. Many other reviews are also available, although they are not devoted to reductive dehalogenation but deal with it as part of the broader topic of reduction. 

Hydrogenolysis of the carbon-halogen bond is an important reaction, both from synthetic and from environmental points of view. The results of the early experiments with [HCo(CN)5] as catalyst are summarized in Ref [4], A variety of organic halides could be effectively dehalogenated with aqueous sodium formate with [PdCl2L2] (L= various sulfonated phosphines) [57], [RuCl2(TPPMS)2]2 and [Ru(H20)3(PTA)3] + [63], and [Ir(bipy)(Cp )(H20)] [64] catalysts. 

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