Phosphine-Based Ligands

The stabilization induced by the bridging halogens is probably less significant than that provided by an additional ligand and, thus, leads to a faster initiation that should be borne in mind when testing new reactions. 

The main driving forces behind the development of new tertiary phosphine palladium complexes for C(sp )—C(sp) couplings have been (i) a reduction or elimination of side reactions, such as Glaser-type homocouplings (ii) the development of environmentally friendly reaction protocols, such as copper-free reactions in benign solvents (iii) the improvement of catalyst stabihty and activity [higher turnover number (TON) and turnover frequency (TOP)] and (iv) a cost reduction by using less-expensive aryl bromides, or even aryl chlorides under mild reaction conditions, for example, at ambient temperature. 

A convenient catalyst system, consisting of [Pd(OAc)2] and PPhs in dimethyl-sulfoxide (DMSO) as solvent and K3PO4 as base was recently reported [14], and found to be effective for the coupling of aryl halides with terminal alkynes. Among various ligands tested, an iminophosphine ligand gave (in some cases) better conversions than when PPhs was employed. 

2 Palladium-Catalyzed Reactions Ligands and Reaction Protocols 1187 [(AllylPdCOd, P(f-Bu)3 

The excellent performance of P(t-Bu)3 as ligand in cross-coupling reactions is attributed to its electron-rich nature and impressive steric demand [17]. Thus, the oxidative addition step in cross-coupling reactions is facihtated by its electronic-donating properties, and it is therefore understandable why challenging substrates can be activated at mild reaction temperatures using P(t-Bu)3. 

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Balch, A.L. (1994) Construdion Of Small Polynudear Complexes With Trifimctional Phosphine-Based Ligands As Backbones, in Progress In Inorganic Chemistry, vol. 41 (ed. S.J. Lippard), John Wiley Sons, Inc., New York, pp. 239-329. 

Balch, Alan L., Construction of Small Polynuclear Complexes with Trifunctional Phosphine-Based Ligands as Backbones. 41 239... 

In 1994, Sen and Jiang reported aqueous ethylene/CO copolymerization by cationic palladium(II) catalysts with water-soluble bidentate sulfonated nitrogen- or phosphine-based ligands, such as 1. At 50 °C and 35 bar each of efhylene and CO, moderate activities of up to ca. 10 TO h were observed (with a catalyst prepared in situ from [Pd (NC(d I. ) i (l b )2/bidentalc water-soluble ligand). 

The major drawback of phosphine-based ligands is their instability toward air oxidation, which is suspected to be the major cause for catalyst deactivation and metal leaching from the support. For triarylphosphines, such as Ph2PCgH4CH2-PS, the problem is not as pronounced as their alkyldiaryl, dialkylaryl, or trialkyl analogs. Attachment of these hg-ands to a polymeric matrix does seem to attenuate their susceptibility toward air oxidation considerably, and the polymeric ligands can be handled in air briefly for most practical purposes. Moreover, commercial availability of polystyrene-bound triarylphosphines from several sources will undoubtedly stimulate the use of this class of polymeric ligands in the area. 

Further work has been reported on the synthesis of complex phosphine-based ligand systems via the coordination-promoted self-assembly of simpler phosphines bearing appropriate functional groups. A series of ligands bearing one to four mem-substituted diphosphinoarene groups attached to a central core has been prepared and self-assembled via coordination to silver. Also reported is a route to the pyrene-appended phosphine (122), which, when coordinated to rhodium in a square-planar complex via the P and S centres, forms a self-assembled tweezer system in which two nearby pyrenyl... 

The iron(II) complexes other than the phosphine-based ligand have also been intensively studied for controlling the polymerization. In 1997, Matyjaszewski a reported the... 

Phosphine-based ligands are weU known to coordinate to palladium and form inorganic compounds. However, their use as stabUizers for Pd nanoparhcles has not been extensively uhUzed. In 2003, Hyeon s group inveshgated the use of phosphine ligands as stabilizers for Pd nanoparhcles [49] by preparing a... 

Scheme 7.19 Fluorous phosphine-based ligand and related reactions.

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