Suzuki bulky phosphine ligands

Buchwald et al. have reported a number of bulky phosphine ligand PR 2R which give high activity in Suzuki— Miyaura catalysis for the synthesis of sterically hindered biaryls.The conditions used in the catalysis involved the in situ preparation of the Pd(0) catalyst from Pd2(dba)3 and the phosphine. Two ligands which proved superior to the others had in common the same polyaromatic R (R = Cy, Ph) as a difference from the rest. A Pd(0) complex could be prepared by mixing Pd2(dba)s and PR 2R in toluene, which has the X-ray structure 40 sketched in the upper reaction of Scheme 25. The key structural feature of the complex is the short distance of Pd to one double bond of the phenanthrene moiety (distances to the two carbons are 2.298 and 2.323 A) supporting an 77 -phenanthrene... 

Various bulky phosphines and AT-heterocydic carbene ligands have proved to be effective in a number of palladium-catalyzed reactions of aryl chlorides and bromides [28-30], e.g., Mizoroki-Heck reaction, Suzuki-Miyaura reaction, Migita-Kosugi-Stille reaction, amination, and alkoxylation, as well as the reaction with various carbon nucleophiles as described below. The ligands are considered to enhance both the initial oxidative addition of aryl halides and the reductive elimination of products. The identity of bases is also an important factor to obtain satisfactory results commonly in the arylation of carbon nucleophiles NaH,NaO-t-Bu, K3PO4,M2CO3 (M=K, Cs), and MN(SiMe3)2 (M= Li, Na, K) are typical choices. 

The Suzuki cross-coupling methods described in Sect. 2.1 establish that palladium complexes that bear electron-rich trialkylphosphines of the appropriate size can serve as effective catalysts for reactions of alkyl electrophiles. Naturally, it was of interest to determine if non-phosphine ligands that are bulky and electron-rich can also furnish active catalysts. 

Several other research groups have also explored the use of bulky, electron-rich phosphine ligands for Suzuki coupling reactions. Ligands 23... 

With the aim of designing new and more efficient catalysts for Suzuki reaction, the bulky and electron-rich dialkylbiaryl phosphine ligands (Fig. 3.9) were found to dramatically improve the efficiency and selectivity of this type of reactions [76]. In particular, Buchwald s group reported that the use of 2-(2, 6 -dimethoxybi-phenyl)-dicyclohexylphosphine, SPhos (Fig. 3.9) in the palladium-catalyzed Suzuki reaction exhibited unprecedented scope, reaction rate, and stability [77]. More specifically, the use of this ligand conferred to the catalyst a unique activity allowing the coupling of boronic acids with aryl bromides and chlorides with low... 

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