Bisphosphine ligand

Jarvo et al. have reported the use of isolated NHCP palladium allyl complexes of the type [(R-2)Pd(i -C3H5)]X (X = C1, OAc, BF ) as catalysts for nucleophihc allylation reactions. They investigated a series of electron-rich monodentate and bidentate ligands (bisphosphines, functionahzed NHCs, monodentate NHCs) for their ability to promote nucleophilic attack of the corresponding palladium allyl complexes on electrophiles in, for example, the conjugate addition of allylboronic esters to a variety of ,p-unsaturated Af-acylpyrroles [15a] as well as the allylation of aldehydes by allylstannanes (Scheme 10.4) [15b]. 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Arylation of alkynes via addition of arylboronic acids to alkynes represents an attractive strategy in organic synthesis. The first addition of arylboronic acids to alkynes in aqueous media catalyzed by rhodium was reported by Hayashi et al.89 They found that rhodium catalysts associated with chelating bisphosphine ligands, such as 1,4-Ws(diphenyl-phosphino)butane (dppb) and 1,1 -/ E(diphenylphospliino)fcrroccnc... 

Sodium iodide can also be used as a bromide replacement, yielding Pd TON of 1195. In this case, bisphosphine ligands are necessary to maintain high regioselectivity (DPC vs. phenylsalicilate) of carbonylation reaction (96 % vs. 72 % for Pd(acac)2). 

The weak Si-Pgq interactions in 9 should be easily broken, giving a chelating bisphosphine ligand to transition metals. With the "phosphophilic" Ni(0) metal center, one diphosphinomethanide ligand is transferred completely. 

The best phosphines found so far are bisphosphines of the general formula R2P(CH2) PR2, where n = 2 or 3 and R = aryl groups. Examples of some very good ligands are 1,2-bis(diphenylphosphino)ethane (32) and 1,2-bis(diphenylphosphino)propane (33) ... 

The number of phosphine ligands on the active catalyst system is also subject to speculation. In Scheme 9 Hata postulated an active complex consists of only one chelating phosphine. However, he (66) and others (70, 71, 83) also observed that 2 moles of the bisphosphine 32 per mole of Co are needed for best selectivity. Sarafidis (55) suggested that a more desired structure might consist of two bisphosphines, with one of the Co—P bonds having the ability to dissociate to provide coordination sites for incoming monomers (see structure 34). 

To demonstrate his point, Sarafidis has shown that the catalyst system containing trisphosphine (71) 35 as ligand is just as effective as the one containing two bisphosphines. 

Chances are, as long as there are two strong Co—P bonds existing at all times, that the requirements for selectivity will be satisfied. The use of two bisphosphines (32) or one trisphosphine (35) is merely to assure the presence of these two Co—P bonds. Thus, with a very good chelating ligand such as the bis(diphenylphosphino)propane 33, Hata (66) showed that only a 1 1 Co/ligand ratio was needed for best selectivity. 

The seven-membered ring containing chiral bisphosphine 121 (n = 1) was made as part of a series of bisphosphines (n = 1-6) to study the influence of ligand dihedral angles on the enantioselectivity of Ru-catalysed asymmetric hydrogenation of p-ketoesters . 

Chan has discovered a completely atropdiasteroselective synthesis of a biaryl diphosphine by asymmetric intramolecular Ullmann coupling or Fe(m)-promoted oxidative coupling. A chiral atropisomeric biaryl bisphosphine ligand 2 was synthesized through this central-to-axial chirality transfer.38 Recently, a xylyl-biaryl bisphosphine ligand, Xyl-TetraPHEMP, was introduced by Moran, and is found to be effective for the Ru-catalyzed hydrogenation of aryl ketone.39... 

Mezzetti91 and Leeuwen/Widhal92 have independently reported P-chiral ferrocenyl bisphosphines 42 and 43. These two ligands have shown excellent enantioselectivity (up to 99% ee) for the asymmetric hydrogenation of... 

A number of chiral phosphine ligands has also been reported (Figure 4). Zhang and co-workers described binaphthalene phosphine 44 with a pyridine moiety to afford the addition product with up to 92% ee.50,50a With chiral bisphosphine 45, Imamoto et al. got only moderate enantioselectivity for the addition of cyclohexenone,... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

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