[26521-33-5] · C19H37BrNiP2 · Bromo(o-tolyl)bis(triethylphosphine)nickel(II) · (MW 466.06)
Physical Data: mp 102-103 °C.
Solubility: sol benzene, diethyl ether, THF, DMF, ethanol, DMSO, acetone, cyclohexane, cumene, nitromethane, dichloromethane.
Form Supplied in: brown, air stable, crystalline solid.
Purification: recrystallization from ethanol.
Handling, Storage, and Precautions: although no special precautions are reported, this complex is likely to be toxic, as with many nickel reagents.6
As part of a study examining the syntheses of biaryls under nickel catalysis,1 the title reagent (1) and other nickel(II) complexes were found to react with aryl halides to give biaryls via a radical chain process and were thus proposed as intermediates in nickel(0)-catalyzed Ullmann coupling reactions. The reactions of (1) are most efficiently conducted in a sealed, evacuated tube at 70 °C with a nonpolar organic solvent such as benzene or toluene. In a similar study (1) was found, upon treatment with Methyllithium and heating in hexanes, di-n-butyl ether, or 1,2-dimethoxyethane, to provide o-xylene, suggesting that nickel(II) species are intermediates in the cross coupling of Grignard reagents and aryl halides with nickel(0) reagents.2
A number of nickel complexes have been found to catalyze the substitution of aryl halides with nucleophiles such as amines, phosphines, and cyanide.7 Reagent (1) is well suited for the exchange of aryl and vinyl halides with more electronegative halides (eqs 1-4). These reactions are most frequently carried out in refluxing benzene or THF in the presence of an appropriate ammonium halide salt.1,3 In cases where cis- or trans-vinyl halides are used (eqs 3 and 4), no detectable isomerization is observed.
The cyclodimerization of butadiene to provide 2-methylenevinylcyclopentane (eq 5) is carried out quite efficiently with (1) and other nickel catalysts.4 This process is best carried out at an elevated temperature in benzene in the presence of (1) and methanol. The use of protic reagents other than methanol leads to a diminished yield of 2-methylenevinylcyclopentane and the formation of other cyclodimerized products.
Edmund L. Ellsworth
Parke-Davis Pharmaceutical Research, Ann Arbor, MI, USA