Phosphine-bound aryls

Various combinations of catalysts, bases, and solvents allow the biaryl coupling of arylboronic acids with aryl halides and triflates if there are no side-reactions such as the hydrolytic B-C bond cleavage or the participation of phosphine-bound aryls [4] (Scheme 29). The representative conditions are summarized in the scheme and the effects of catalysts and bases on yields and side reactions are discussed in sections 3.1 and 3.2. 

The by-products observed are the coupling products of phosphine-bound aryls and dimers of arylboronic acids. The oxidative addition of ArX to palladium(O) to afford 3, followed by a sequence of transmetal-lation-reductive elimination yields a cross-coupling product 4. Triaryl-phosphines are an excellent ligand for stabilizing the palladium(0) species. However, an undesirable side reaction of the aryl-exchange between palladium- and phosphine-bound aryls (3 —> 5) leads to the coupling product 6 of phosphine-bound aryls (Eq. 51). 

Aryl exchange occurs before transmetallation. Thus, the transmetal-lation rate constant which is low for steric and electronic reasons results in increasing the coupling product of phosphine-bound aryls (Eq. 51). Transmetallation is slowed down when electron-rich haloarenes and weak bases are used and accelerated with electron-deficient haloarenes because the transmetallation shown in Eqs. 16 and 17 involves nucleophilic substitution of Pd-X. The reported equilibrium ratio of 3/5 is 4/96 at 60 °C when Ar is p-methoxyphenyl. Thus, it is quite reasonable that the reaction accompanies a large amount of 6 when the rate constant of transmetallation ifej is lower than ky A strong base, polar solvent, and a sterically less hindered bidentate ligand, such as dppf, increase k. The formation of yields of 9 in p-iodoanisole higher than the bromo derivative... 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

Ruhland et al. used PdCl2(dppf)-NEt3 Heck conditions to add to a resin-bound aryl iodide, thereby generating supported 4-styryl (3-lactams, as shown in Scheme 17.45 This catalyst system, also found to be effective for the Suzuki reaction (see Section 2.4), is unusual for the Heck reaction and had only previously been used for an intramolecular cyclization.46 The more usual conditions of Pd(OAc)2/phosphine/NEt3 or K2C03 were found to be ineffective. 

The stability of the alkyl and aryl derivatives increases with the number of phosphines bound to nickel(II). The bis-phosphine derivatives can be prepared according to equations (144)-(146) and using organomagnesium halides or organolithium compounds.1224-1231 A tetrakis phosphine complex was obtained similarly (equation 147). 

Tosylate is displaced by weak oxyanions with little elimination in aprotic solvents, providing alternative routes to polymer-bound esters and aryl ethers. Alkoxides, unfortunately, give significant functional yields of (vinyl)polystyrene under the same conditions. Phosphines and sulfides can also be prepared from the appropriate anions (57), the latter lipophilic enough for phase-transfer catalysis free from poisonning by released tosylate. 

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