Benzonitrile reductive cleavage

Ruhland reported a novel solid-phase C2-lithiation of the indole ring [279] using a linker that resembled the MOM-protecting group (Scheme 16). Lithiation of resin-bound indole 73 was accomplished by treatment with tert-butyllithium in toluene followed by quenching with benzonitrile. Reductive cleavage of 74 then gave amine 75 in an overall yield of 2%, proof of principle that this type of transformation is possible. 

The stability of the cyanide ion is important in this reductive cleavage benzonitriles bearing strongly electron-attracting groups may be reduced similarly in basic media.295... 

Because only one stable electroactive product arises from the reduction of /7-chlorobenzonitrile, the interpretation of the cyclic voltammetric behavior is relatively straightforward. The fact that electron attachment to halogen-containing compounds frequently results in the cleavage of the carbon-halogen bond immediately suggests benzonitrile as a possible product. The similarity between the cyclic voltammetric behavior of benzonitrile and that of the p-chlorobenzo-nitrile reduction product supports this prediction (see Fig. 21.1). 

D. The chronoamperometric results can also be used to ascertain the number of electrons involved in the formation of benzonitrile from p-chloro-benzonitrile. In order to translate the chronoamperometric data into a meaningful n value, a compound is selected that has a diffusion coefficient very similar to that of p-chlorobenzonitrile and that gives a stable, known product upon electroreduction. Tolunitrile, which satisfies these criteria, is known to be reduced to its radical anion at a diffusion-controlled rate. Since this one-electron process gives a value of 168 pA s1/2- M x cm 2 for it1/2/CA, the corresponding value of 480 pA s1/2 A/ 1 cm-2 for the reduction of p-chlorobenzonitrile to benzonitrile anion radical must represent an overall three-electron process. When we subtract the one electron that is required to reduce benzonitrile to its radical anion from this total, we immediately conclude that two electrons are involved in cleavage of the carbon-chlorine bond in p-chlorobenzonitrile. A scheme that is consistent with these data is described by Equations 21.1 to 21.6. 

One final example of C-CN cleavage has appeared that involves rhodium instead of nickel [58]. Reduction of [Rh(dippe)Cl]2 with potassium graphite led to a species assigned as [Rh(dippe)]2K2(THF)2. Here the rhodium is formally in the — 1 oxidation state, so the complex is isoelectronic with [Ni(dippe)]. The complex reacts with benzonitrile to give the C-CN cleavage product, just as seen with nickel. Use of labeled substrate Ph CN gives a product that can be readily characterized as K [Rh (dippe)(Ph)( CN)] using H, C, and P NMR spectroscopy. Further study of this reaction was not possible, as sources of protons readily lead to the known complex [(dippe)Rh]2(p-H)(p-N=CHPh) reported by Fryzuk [59]. 

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