Nickel reductive homocoupling

The nickel-catalyzed reductive homocoupling of haloarenes is an excellent method for the synthesis of symmetrical biaryls (Eq. 22). The remarkable ability of [Ni(cod)2] to promote the stoichiometric formation of biaryls from aryl iodides and bromides was originally reported by Semmelhack, Helquist, and Jones [25a]. Unlike the Ullmann reaction [3], the Ni-promoted homocoupling of... 

Metallic nickel was easily prepared by stirring a mixture of nickel halide and lithium metal (2.3 equiv) with naphthalene (0.1 equiv) as an electron carrier at room temperature for 12 h in DME (glyme) (Equation 7.9). The resulting black powders which slowly setded in a colorless solution after stirring was stopped were used for the reductive homocoupling reaction of benzylic halides (Equation 7.10). 

The homocoupling of aryl halides and triflates can be made catalytic in nickel by using zinc as a reductant for in situ regeneration of the active Ni(0) species. 

Homocoupling likely occurs by a related catalytic cycle, but less information is known about this process. A nickel(O) or palladium(O) complex undergoes oxidative addition of an aryl halide. One could imagine that a second aryl halide adds to generate an intermediate in the M(IV) oxidation state, but this step is unlikely to occur with either palladium or nickel catalysts. One can also envision a mechanism involving disproportionation of the arylmetal-halide intermediate to form a biaryl complex and a dihalide complex. Reductive elimination would form the biaryl, and reduction of the dihalide with Zn or other terminal reductant would regenerate the catalyst. 

A very minor amount of homocoupling biaryl is derived during the reduction of a palladium(II) or nickel(II) halide complex with aryl-boronic acid (Eq. 23) or by the metathetic reaction shown in Eq. 48. However, a large number of homocoupling products of arylboronic acids are reported in literature. The mechanism proceeding through oxidative addition of the C-B bond to palladium(O) is recently proposed as the route to homocoupling (Eq. 54). The oxidative addition of the C-B... 

In spite of the usefulness of these complexes, it is generally not possible to cause the satisfactory reaction with transition metals in the metallic state [86] under mild conditions due to their poor reactivity. We have reported that activated metallic nickel, prepared by the reduction of nickel halide with lithium, underwent oxidative addition of benzylic halides to give homocoupled products [45]. We reported that carbonylation of the oxidative adducts of benzylic halides to the nickel proceeded smoothly to afford symmetrical 1,3-diarylpro-pan-2-ones in moderate yields, in which the carbonyl groups of alkyl oxalyl chlorides served as a source of carbon monoxide [43] see Equation 7.5. 

One possible mechanistic sequence for the present reaction is shown in Scheme 7.1. The carbon monoxide insertion into the carbon-metal o bond of alkyltransition metal complexes is well known [88]. Thus, the oxidative addition of benzyl halide to metallic nickel gives benzylnickel (II) halide 4, and the insertion of carbon monoxide, which is formed by decarbonylation of alkyl oxalyl chloride into the benzyl-nickel bond of complex 4, would afford arylacetyl (II) complex 5. The metathesis of complexes 4 and 5 seems to give (arylacetyl)benzylnickel complex 6, which undergoes reductive elimination to yield l,3-diarylpropan-2-one, 3. The formation of 1,2-diarylethane may be explained by the reductive elimination of bisbenzylnickel complex 7 formed by metathesis of benzylnickel complex 4 [89]. It is also possible that the reaction of benzyl halide with complex 4 or 5 gives homocoupled product or ketone, respectively. 

The results of the homocoupling reaction of iodobenzenes by activated nickel powder are summarized in Table 7.11. Metallic nickel worked well for the dehalogenative coupling of unsubstituted and 4-substituted iodobenzenes. For instance, the reaction of 4-iodomethoxybenzene reached completion in 2 h at 80 C to afford 4,4 -dimethoxybiphenyl in 85% yield (via GLC) along with the reduction product anisole (15%). The usual woricup of the reaction mixture gave the biphenyl in 68% isolated yield. Previous workers obtained 4,4 -dimeth-... 

The results of the reaction of benzyl and substituted benzyl halides with metallic nickel powders are summarized in Table 7.13. Benzyl chloride reacted at room temperature with metallic nickel prepared from nickel chloride to give a mixture of the coupled product bibenzyl (40%) and the reduction product toluene (60%). However, the coupled product was found to be formed mainly when the reaction was run at 70°C and when the nickel powders used were prepared from nickel iodide. Under these conditions, a yield of 86% bibenzyl was attained. The fact that iodide ion present in the system facilitates the homocoupling reaction may be ascribed to the chlorine-iodine exchange during the reaction [148]. Higher reaction temperatures such as 70°C may also accelerate the exchange reaction. 

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