Alkyl halides radical dehalogenation

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). 

Mechanism of the radical chain dehalogenation of alkyl halides by tributyltin hydride. The various termination steps are not shown. 

Radical Dehalogenation of an Alkyl Halide with Tributyltin Hydride Section 21.7 Figure 21.3... 

In Chapter 4, we saw that Sml2 mediates the dehalogenation of a range of alkyl halides. These reactions proceed by reduction to alkyl radicals that are then... 

For a way of overcoming this problem, let s go back to the reaction we looked at a few pages ago, the dehalogenation of alkyl halides by Bu3SnH. The mechanism involves formation of an alkyl (carbon-centred) radical by abstraction of Br by Bu3Sn. This alkyl radical then just abstracted H from... 

Since it is unlike that hydride anions are involved under acidic conditions, the most reasonable mechanism for the debromination is the C-Br homolytic bond breaking followed by hydrogen abstraction from the medium of the resulting radical. Related precedents for a radical mechanism for the dehalogenation of alkyl halides are well documented in the chemical literature [13],... 

The halogen atom of an alkyl halide can be replaced by the hydrogen atom bonded to tin in tributyltin hydride (BUsSnH). The process, called dehalogenation, is a radical reaction, and it can be initiated by AIBN (2,2 -azobisisobutyronitrile). AIBN decomposes to form nitrogen gas and two isobutyronitrile radicals, which initiate the reaction. Write a mechanism for the reaction. 

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]. 

In 2007 the scope of the trialkylborane/water system was extended to the dehalogenation of alkyl iodides and the chemoselective deoxygenation of secondary alcohols in the presence of alkyl and aryl halides [86]. The rate constants for the hydrogen-atom transfer from this reagent to secondary radicals (Scheme 37) are substantially lower than those of the Ti(III) aqua-complex [78, 87]. 

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