Trialkylphosphine ligands

From a steric viewpoint, the considerably bulkier trialkylphosphine ligand should, and does, favor formation of linear products. While linear to branched ratios of only 2-3 1 are typically found for HCo(CO)4, higher regioselectivities of 7-8 1 occur for HCo(CO)3(PR3). There is a phosphine Cone Angle cutoff at about 132°, after which the steric effects of the phosphine ligand do not increase the product linear regioselectivity any further. 

The presence of sufficiently powerful a-donor ligands can make a metal a proton acceptor. The trialkylphosphine ligands in Pt(PEt3)3, and h -C5HjCo(PMe3)2 make the metals sufficiently basic to deprotonate EtOH and H O ... 

Other reactions are alkane formation by hydrogenation, ketone formation (especially with ethylene ), ester formation through hydrogen transfer and formate ester synthesis. An improved catalyst system in which one CO ligand of CoH(CO)4 is substituted with a trialkylphosphine ligand , was disclosed by Shell workers in the early 1960s. With this catalyst, which is more thermally stable than the unsubstituted cobalt carbonyl, reaction proceeds at 140-190 C with 3-7 MPa of CO and Hj. Additionally, mostly linear aldehydes are obtained from linear terminal and internal olefins. This remarkable result arises from the high preference for the terminal addition to an a-olefin, and the isomerization of the olefinic position which occurs simultaneously with hydroformyiation. 

In a second report, the Bergman and Ellman groups improved on their original procedure for the arylation of heterocycles with aryl halides [103]. The key improvements over the previous method were three-fold (1) the use of the hindered tertiary amine base Pr.NBu in replacement of Et3N (2) microwave irradiation for rapid (<1 h), high temperature (250 °C) reactions in 1,2-dichlorobenzene (3) the use of bulky bicyclic trialkylphosphine ligands of the phobane family in replacement of PCy3 (Scheme 15). Under typical reaction conditions, the heterocycle... 

Homogenous ether or hydrocarbon solutions of silver chloride are prepared by using excess trialkylphosphine ligands to solubilize the metal salt. At room tern-... 

Octa-l,7-diyne treated with CO2 (initial pressure 50 kgcm - at room temp.) in THF at 100° in the presence of 10 mol.% of Ni(0)-catalyst (prepared from Ni(cod)2 2 eqs. l-(2 -pyridyl)-2-(di-A -butylphosphino)ethane) for 20 h - 5,6,7,8-tetrahydro-3/f-2-benzopyran-3-one. Y 68%. Such functionalized tert. phosphines with hard ligand components owe their effectiveness to intramolecular coordination of the pyridyl group to the Ni-atom in a 6-membered chelate. Trialkylphosphine ligands were not effective for terminally unsubst. acetylenes. F.e. and effect on regioselectivity s. T. Tsuda et al., J. Chem. Soc. Chem. Commun. 1989, 9-10 review of syntheses based on pyridine coordination s. P. Tomasik, L. Brzozka, Wiadom. Chem. 41, 741-57 (1987). 

The oxidative addition of disilanes occurs to palladium complexes of isonitrile ligands and platinum complexes of trialkylphosphine ligands as part of tiie catalytic silylation of alkynes and aryl halides. The addition of stannylboranes to Pd(0) complexes has also been reported,and the addition of diboron compounds to many metal systems, such as Pt(0) complexes (Equation 6.67), is now common. These reactions all occur with metal complexes that do not undergo intermolecular reactions with alkane C-H bonds, let alone C-C bonds. Thus, the Lewis acidic character of these reagents must accelerate the coordination of substrate and cleavage of the E-E bonds. 

Weakly donating and sterically hindered ligands dissociate most readily. As outlined in Scheme 13.18, pyridine and arylphosphine ligands dissociate more readily from these carbene complexes than do N-heterocyclic carbene or trialkylphosphine ligands. 

Figure 5.2 Bicyclic trialkylphosphine ligands for cobalt-catalyzed hydroformylation.

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