Alkylphosphines as Ligands

COi is another molecule which reacts with conjugated dienes[10,95,96], COt undergoes cyclization with butadiene to give the five- and six-membered lactones 101. 102. and 103, accompanied by the carboxylic esters 104 and 105[97.98], Alkylphosphines such as tricyclohcxyl- and triisopropylphosphine are recommended as ligands. MeCN is a good solvent[99],... 

Sterically demanding, water-soluble alkylphosphines 6.10 and 6.11 as ligands have been found to have a high activity for the Suzuki coupling of aryl bromides in aqueous solvents (Eq. 6.35).115 Turnover numbers up to 734,000 mmol/mmol Pd have been achieved under such conditions. Glucosamine-based phosphines were found to be efficient ligands for Suzuki cross-coupling reactions in water.116... 

The previously problematic second substitution after the /ram-sclcclivc monoalkynylation can now be achieved selectively (>98% selective with retention) and in high yields. The use of alkylzincs and most critically alkylphosphines such as (f-Bu)3P, as ligands, was the key to the development of the satisfactory procedure (Scheme 47)85. Conventional arylphosphines, such as PPh3, I I P and DPEphos, as well as the use of Ni catalysts in conjunction with alkylmetals containing Mg106 and Zn led to low yields and/or extensive and synthetically unattractive stereoisomerization85. 

Commonly used metal salts and palladium precursors include Pd(OAc)2, Pd (acac)2 (acac = acetylacetonato) and Pd2(dba)3 or Pd(dba)2 (dba = dibenzylide-neacetone). If a Pd(II) salt is used as pre-catalyst, reduction by base or by excess phosphine ligand is required. The exact nature of the reducing agent is somewhat contended, but it is often assumed that the phosphine takes up this role, for which evidence has been reported [26]. A typical catalytic system consists of a palladium source and an aryl- or alkylphosphine (typically PPh3), in at least 2 eq., as ligand. The addition of bases, typically amines and alkoxides, is often found to be beneficial for activity, which is also reflected in the patent literature [27-29]. The bases are thought to facilitate in the attack of the nucleophile in the rate-determining step and, in the case of the amines, in the reduction of Pd(II) to Pd(0). 

The palladium-catalyzed a-arylation of ketones has become a useful and general synthetic method [106]. Initial studies required preformed zinc [378] or tin enolates [379]. On the other hand, Ni-mediated [380] or Ni-catalyzed couplings are also known. A great development of the reaction has occurred since 1997 based on the use of new catalysts with electron-rich alkylphosphines and NHCs as ligands [173, 381]. The reactions resemble cross-couphng processes in which the enolates behave as the nucleophilic organometaUic reagents (Scheme 1.47). 

Yoshida and Otsuka found that platinum(O) complexes [PtLj] (26) (a L = PEtj b L = P Prj) and rhodium hydrido complexes such as [RhHLj] (L = P Prs 33, PEts), [RhiHidr-NJlPCyslJ (34), tram-[RhH(N2)(PPh Bu2)J, and [RhH(P Bu3)J, all of which carry electron-donating alkylphosphine ligands, can catalyze the water gas shift reaction under fairly mild conditions (100-150°C CO 20 kg/cm ) (Eq. 6.32) [23, 60]. Among these complexes, [RhH(P Pr3)3] (33) was the most active catalyst precursor. Several complexes were isolated from or detected in the reaction mixture... 

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