Nickel reduction dihalides

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

Organic Electroreductive Coupling Reactions using Transition Metal Complexes as Catalysts Table 2. Reductive electropolymerisation of aryl dihalides using nickel catalysts... 

Geminal dihalides undergo partial or total reduction. The latter can be achieved by catalytic hydrogenation over platinum oxide [512], palladium [512] or Raney nickel [63, 512], Both partial and total reduction can be accomplished with lithium aluminum hydride [513], with sodium bis(2-meth-oxyethoxy)aluminum hydride [514], with tributylstannane [503, 514], electro-lytically [515], with sodium in alcohol [516] and with chromous sulfate [193, 197]. For partial reduction only, sodium arsenite [220] or sodium sulfite [254] are used. 

At the cathode, a chemoselective elimination depending on the degree of alkylation of the v/c-dihalide is possible [73]. Chemically these reductions are more limited in scope they can be conducted with 1 in DMF, with Zn, Mg, or Cr " [74]. The cleavage of 1,2-diols can be inexpensively achieved at the nickel hydroxide electrode [13], while chemically the more expensive reagents Pb(OAc)4 or IO4 must be used. 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

Corey and Wat17 found that the method of forming cycloolefins from allylic dihalides and nickel carbonyl provides an unusually efficient route for the formation of large rings. Because it leads to cyclic 1,5-dienes, it makes available a variety of cyclic structures not obtainable in a practical way via the acyloin synthesis. Diacetylenic diols (10) were converted by selective reduction into the corresponding cis.cis- and owu,/ra -ethylenic diols followed by reaction with PBr3 to form the diallylic dibromides (12), which were then cyclized with nickel carbonyl. 

The 1,1-dihalides are rarely isolated but instead are hydrolysed to the 1-oxides,40-45 the position of the double bond being apparently unaltered thereby.39 The 1-sulfides can be obtained from (1) or (2) by means of H2S,46 and the phospholenes themselves by reduction with Mg or LiAlH4.47 Hydrogenation of the phospholene 1-oxides over Raney nickel leads to the saturated phospholane 1-oxides.48... 

Budzelaar and coworkers [50] have reported the two-electron reduction chemistry of the corresponding bis(imino)pyridine nickel dihalide derivative. Stirring (iPrpDi)NiBr2 2 equiv ofNaH inTHF under a dinitrogen atmosphere furnished... 

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. 

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