Phosphines bulky ligand condition

In summary, then, in the rhodium-catalyzed industrial process for propylene hydroformylation a high phosphine-to-Rh molar ratio is used. Under these conditions the use of a moderately bulky ligand such as triphenyl phosphine ensures that the catalysis takes place by the topmost cycles in Fig. 5.5, and n-butyraldehyde with high selectivity is produced. 

The success of the new method is due to selection of bases which are stable under the reaction conditions and also the use of bulky phosphine ligands. Later, reductive elimination was fovmd to be crucial in the C—N bond formation, which is accelerated by bulky ligands. As bases, MN(TMS)2 (M = Li, Na, K) and r-BuONa give good results. Alkoxides such as MeONa, EtONa, and n-BuONa seem to be unsuitable, because oxidation of alcohols and reduction of aryl halides occur via facile jS-H elimination of the palladium alkoxide 5. 

Gyclometallation reactions are less common for Ni than for Pd or Pt. However, intramolecular C-H activations can be observed under favorable conditions. For instance, cyclometallation can be favored by very bulky ligands (Equation (58)) or by the presence of acidic C-H bonds in the complex (Equation (59)). The reaction of phosphine ligands containing aldehyde functionalities with a dimethylnickel complex gives cyclometallated products featuring an internal acyl functionality (Equation (60)). ... 

Buchwald et al. have reported a number of bulky phosphine ligand PR 2R which give high activity in Suzuki— Miyaura catalysis for the synthesis of sterically hindered biaryls.The conditions used in the catalysis involved the in situ preparation of the Pd(0) catalyst from Pd2(dba)3 and the phosphine. Two ligands which proved superior to the others had in common the same polyaromatic R (R = Cy, Ph) as a difference from the rest. A Pd(0) complex could be prepared by mixing Pd2(dba)s and PR 2R in toluene, which has the X-ray structure 40 sketched in the upper reaction of Scheme 25. The key structural feature of the complex is the short distance of Pd to one double bond of the phenanthrene moiety (distances to the two carbons are 2.298 and 2.323 A) supporting an 77 -phenanthrene... 

The hydrosilylation of alkynes has also been studied using as catalysts Pt, Rh, Ir and Ni complexes. The improvement of the regioselectivity of the catalyst and the understanding of stereoelectronic factors that control it have been major incentives for the ongoing research. From numerous studies involving non-NHC catalysts, it has been established that there is a complex dependence of the product ratio on the type of metal, the aUcyne, the metal coordination sphere, the charge (cationic versus neutral) of the catalytic complex and the reaction conditions. In the Speier s and Karstedt s systems, mixtures of the thermodynamically more stable a- and -E-isomers are observed. Bulky phosphine ligands have been used on many occasions in order to obtain selectively P-f -isomers. 

Helmchen and co-worker investigated the use of phosphinooxazolines as ligands for copper(II) catalyzed Diels-Alder reactions (Scheme 19) (214). Optimal selectivities are found for a-naphthyl-substituted phosphinooxazoline (299). These catalysts require 2.5 h to induce complete conversion to cycloadduct, compared to 18 h using the triflate complex 269c under identical conditions. Helmchen invokes a square-planar metal geometry to explain the stereochemistry of the adducts, similar to the model proposed by Evans. He suggests that the bulky phosphine substituents are required to orient binding of the dienophile in such a way as to place the olefin directly below the terf-butyl substituent on the oxazoline. 

Under these conditions, a variety of linear aliphatic terminal alkynes were transformed into aldehydes with good selectivity. The efficiency, regioselec-tivity of the addition and substituent tolerance were improved by using RuCl(Cp)(phosphine)2, where Cp is cyclopentadienyl, or RuCl(Cp)(diphos-phine) as catalyst precursors [30]. The best results were obtained with diphenylphosphinomethane as a ligand, which made possible the preparation of aldehydes from bulky aliphatic alkynes (tert-BuC=CH), aromatic alkynes (PhC=CH), diynes [HC=C(CH2)6C=CH] and functional terminal alkynes [NC(CH2)3C=CH, PhCH20(CH2)2C=CH,...]. The mechanism of this reaction was investigated in details by isolation of intermediates, deuterium-... 

Palladium catalysts with simple monodentate phosphine ligands (e.g. PPh3) can catalyze the methoxycarbonylation of ethylene. However, the Lucite process employs a bulky diphosphine, 1,2-( Bu2PCH2)2CgH4, and is highly active and selective under quite mild conditions (10 bar/80°C). Two alternative catalytic cycles are possible, based either upon a palladium hydride or a palladium methoxide complex (Figure 9), and mechanistic and spectroscopic studies indicate that the hydride cycle is dominant. The alkene and CO insertion steps are the same as those in the Pd-catalyzed co-polymerisation of CO and alkenes to polyketones (Section 4.4). 

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