Nickel complexes aryl phosphites

A useful new method of preparing arylphosphonates (123) involves the reaction of trialkyl phosphites with aryl halides in the presence of a nickel catalyst.The suggested mechanism is via the nickel complex (124), and is non-radical. 

Four main types of antioxidants are commonly used in polypropylene stabilizer systems although many other types of chemical compounds have been suggested. These types include hindered phenolics, thiodi-propionate esters, aryl phosphites, and ultraviolet absorbers such as the hydroxybenzophenones and benzotriazoles. Other chemicals which have been reported include aromatic amines such as p-phenylenediamine, hydrocarbon borates, aminophenols, Zn and other metal dithiocarbamates, thiophosphates, and thiophosphites, mercaptals, chromium salt complexes, tin-sulfur compounds, triazoles, silicone polymers, carbon black, nickel phenolates, thiurams, oxamides, metal stearates, Cu, Zn, Cd, and Pb salts of benzimidazoles, succinic acid anhydride, and others. The polymeric phenolic phosphites described here are another type. 

In 1970, Tavs reported the first nickel-catalyzed arylation of trialkyl phosphites or dialkyl aryl phosphonites with aryl bromides [242] (Scheme 20.72). In this reaction, trialkyl phosphite, a phosphorylating agent, plays a crucial role in the reduction of Ni(II) to active Ni(0) complex [243]. 

Nickel.— Phosphine and phosphite complexes of nickel(0) react with strong acids to produce complexes [NiHL4]+. Reaction with weak acids may proceed further, with attack of the anion at the nickel. Thus the complex Ni(LL)a, where LL = l,4-bis(diphenylphosphino)butane, reacts with hydrogen cyanide to form firstly a hydride, which reacts quickly with cyanide to give the bimolecular intermediate Ni2(CN)2(LL)a. The ultimate products are the nickel(n) monomer Ni(CN)2(LL)2 and dimer [Ni(CN)2-(LL)]2. Ni(PPh3)4 undergoes normal oxidative elimination reactions with aryl halides to produce the new nickel(n) complexes Ni(aryl)(X)-(PPh3)2. ... 

A nickel-catalyzed route to the formation of arylphosphonates starting from aryl halides and secondary phosphites has been reported (Scheme 4.243) [336]. While some of the methods described previously added nickel salts such as NiCl to the reaction mixture along with a ligand to solubilize/stabilize the metal center, the authors opted to use a preformed nickel complex. A significant electronic effect was observed in this... 

Similarly, copper salts (cupric and cuprous) facilitate the reaction of aryl halides with trialkyl phosphites in the formation of dialkyl arylphosphonates under conditions like those found in nickel systems.37-39 Again, the copper salts appear to undergo an initial reaction with the phosphites to form a complex that subsequently undergoes reaction with the aryl halide. The requirement for copper is also similar to that for nickel saltstonly a catalytic amount is needed. Further, a preference among halides on the aromatic ring is noted iodide is replaced preferentially to other halides (Figure 6.10).40... 

Rhodium and nickel have been by far the most common metals aside from palladium employed in Suzuki-Miyaura carbon-carbon bond-forming reactions. Platinum has been used on several occasions, for example Bedford and Hazelwood showed that platinum complexes with n-acidic, ortfto-metalated triaryl phosphite and phosphinite ligands exhibited what they termed unexpectedly good activity in Suzuki biaryl coupling reactions with aryl bromide substrates (Scheme 13.22). Application to aryl chlorides resulted in low conversion to the desired biaryl products. 

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