Biaryl phosphine ligands

Buchwald also reported details of the arylation of malonates using monodentate biaryl phosphine ligands [220], Here, with Pd(OAc)2, t-Bu-DavePhos and the inexpensive base K3PO4 diethyl malonate could be arylated successfully. Later, Buchwald also reported on the arylation of malonates employing the attractive aryl benzene sulfonates [221] when, by using Pd(OAc)2 and X-Phos as the catalytic system, together with CS2CO3 as base in toluene, diethyl malonate could be successfully arylated (Scheme 3.15). 

Tri-t-butyl phosphine can be more effective in the coupling of reluctant aryl bromides (Scheme 2.118). Bulky electron-rich biaryl phosphine ligands, such as 2385, can also promote the Sonogashira reaction of difficult aryl chlorides (Scheme 2.119). ° Interestingly, in this case, the presence of copper(I) salts inhibited the reaction, and the copper-free version provided better yields. Aryl tosylates, easily prepared from phenols, also couple under these conditions (Scheme 2.120). 

Fig. 4 Hemilabile biaryl phosphine ligands for Pd-catalyzed N-arylation reactions...

Fagnou and co-workers reported on the use of a palladium source in the presence of different phosphine ligands for the intramolecular direct arylation reaction of arenes with bromides [56]. Later, they discovered that new conditions employing palladium complex 27 promoted the direct arylation of a broad range of aryl chlorides to form six- and five-membered ring biaryls including different functionalities as ether, amine, amide and alkyl (Scheme 7.11) [57]. 

In terms of methodologies for the preparation of A-arylindoles 140, Buchwald reported improved conditions for the palladium-catalyzed coupling of aryl chlorides, bromides, iodides and triflates 138 with a variety of 2-, 7- and polysubstituted indoles 139 utilizing novel electron-rich biaryl(dialkyl)phosphine ligands in combination with Pd2(dba)3 <00OL1403>. Alternatively, Watanabe reports similar A-arylations of pyrrole, indole and carbazoles with aryl bromides and chlorides using Pd(OAc)2/P(f-Bu)3 in xylene at 120°C <00TL481>. 

Milne and Buchwald used phosphine ligand 91 (Figure 19) in combination with Pd2(dba)3 to prepare tri- and tetra-6 r/i -substituted biaryls.Excellent yields were obtained even at low catalyst loadings (0.1-1 mol% Pd), with a good tolerance for group functionalities. 

Reactions of cycloaurated complexes with phosphine ligands that lead to reductive coupling and the formation of biaryls are discussed separately in Section IV.K. 

Nishimura, M. Ueda, M. Miyaura, N. Palladium-catalyzed biaryl-coupling reaction of arylboronic adds in water using hydrophilic phosphine ligands. Tetrahedron 2002, 58, 5779-5787. 

Similarly superior results in comparison to the application of sulfonated phosphine ligands were achieved in the Pd-catalyzed biaryl coupling of arylboronic acids in aqueous media by use of the ligand glcaPHOS 11 [16]. The phosphine was prepared in a single step by the condensation of D-glucono-1,5-lactone with p-diphenylphosphinobenzylamine. 

Due to the great importance of phosphines and phosphinites as chiral ligands for asymmetric catalysis, several hundred compounds of this type have been prepared. In this section, only those compounds which have been mentioned in this Houben-Weyl volume will be discussed. Because of their close relation to the specific topics, biaryl phosphines have already been mentioned in Section 6 and ferrocene derivatives in Section 7.1. All other phosphorus compounds are treated here. An excellent review on the synthesis of enantiomerically enriched compounds where phosphorus is a stereogenic center has recently been published73. 

It is important to note that there are successful examples of biaryl coupling using electron-rich and sterically-hindered phosphine ligands on palladium(O). ... 

The reactivity of aryl triflates [24,32,36,49] and other sulfonates [45,48] can be slightly different than is presented here, what depends on the catalyst and the reaction conditions used. For example, the SM reactions of aryl chlorides in the presence of aryl triflates catalysed by Pd2(dba)3 / 2Pt-Bu3 have given exclusively the biaryl derived from the chloride, whereas the use of Pd(OAc)2 / 2PCy3 catalytic system leads to predominant SM reaction of aryl triflate. The latter results indicate the fact, that the (phosphine) ligand(s), not the palladium (nickel) source, controls chloride / triflate selectivity, as well as other closely reactive substrates. 

Methods M and N are based on different phosphine ligands which activate the resulting palladium-complex providing higher electron-density at palladium-metallic centre, more reactive catalyst, and very mild reaction conditions [32,35], The excellent-yielding SM reactions of aryl iodides, bromides, as well as chlorides catalysed with Pd2(dba)3 / Pt-Bus (1 2), in the presence of potassium fluoride as the base, smoothly proceed at room temperature [35], Scheme 24. All three isomeric 4-halo-acetophenones, 73, 284, 285, in reaction with 2-tolylboronic acid (286) gave appropriate biaryl 287 in almost quantitative yields. 

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