Arylboronic acids tricyclohexylphosphine

Recently, the groups of Fu and Buchwald have coupled aryl chlorides with arylboronic acids [34, 35]. The methodology may be amenable to large-scale synthesis because organic chlorides are less expensive and more readily available than other organic halides. Under conventional Suzuki conditions, chlorobenzene is virtually inert because of its reluctance to oxidatively add to Pd(0). However, in the presence of sterically hindered, electron-rich phosphine ligands [e.g., P(f-Bu)3 or tricyclohexylphosphine], enhanced reactivity is acquired presumably because the oxidative addition of an aryl chloride is more facile with a more electron-rich palladium complex. For... 

Among the nickel-based catalysts, beside the nickel on charcoal [43], mainly the various nickel(0)-phosphine complexes were employed [12,13,44,45]. Several common bidentate phosphine ligands such as dppf, dppe, dppp, dppb and tricyclohexylphosphine (PCys) are the most popular in the nickel-based SM crosscoupling reactions of less reactive aryl chlorides with arylboronic acids [44], While the nickel complexes of electron-rich trialkylphosphines react readily with aryl chlorides. 

The method X affects the SM reactions catalysed by Ni (Cy3)4, which is in situ generated from Ni(PCy3)2Cl2 and tricyclohexylphosphine (2 eq.), to give para- and /neta-substituted biaryls in good to excellent yields [44]. Alternative phosphines such as dppf [48] can be used where previous reduction of the nickel(II)-precatalyst is not necessary since the arylboronic acid acts as an effective reductant. The method is sensitive to steric hindrances in both reactants, and resulted in significantly lower yields of biaryls, as can be seen from the reaction of phenylboronic acid (260) and tosylate 293, derived from methyl salicylate, where methyl biphenyl-2-carboxylate (294) was isolated in 47% yield [44], Scheme 31. 

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