Catalysts Bearing Chelating Ligands

Another methodology to achieve latent chelated olefin metathesis catalysts was adopted by Grubbs et al. [28]. They reported that exchange of the anionic ligands from chlorides to iodides caused a decrease in activity of the catalyst. While in most cases this change is detrimental, it opened another pathway to achieve latent ROMP initiators. Thus, iodide bound complexes 41 and 42, more sterically hindered for olefin association [29], were synthesized and studied for 

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While it is beyond the scope of this chapter to cover the asymmetric hydrosilylation of ketones and imines in any detail, a number of the more catalytically active ML combinations will be mentioned here. A full review of the area has recently appeared.138 Asymmetric hydrosilylation of carbonyl groups is usually performed with rhodium or titanium catalysts bearing chelating N- or P-based ligands. Representative results for some of the most active Rh/L combinations (Scheme 32) for addition of Si H to acetophenone are given in Table 11. 

Complexes containing P(t-Bu)3 as ligand will also catalyze the reaction of unactivated aryl chlorides with diarylamines. Thus far, the reaction of aryl triflates with diarylamines has only been reported with catalysts bearing chelating phosphines. A few representative examples of the reactions of diarylamines with aryl triflates do occur in nearly quantitative yield when using DPPF-ligated palladium as catalyst. Reactions of diarylamines with aryl nonaflates as part of the synthesis of discrete oligomeric triarylamines has also been reported. ... 

Fig. 1 A prototype and modified bifunctional catalysts bearing chelating amine ligands...

Despite the importance of organozinc catalysts bearing /Taminoalkoxides in asymmetric catalysis, there has been no comprehensive computational screening of suitable monoprotic and aprotic bidentate chelating ligands for this... 

Catalysts based on nickel that dimerize or oligomerize a-olefins have been known for many years and are commercially valuable. The Shell higher olefin process (SHOP), for example, uses Ni(II) catalysts developed by Keim and coworkers such as 1.1 and 1.2 bearing P-O chelating ligands to oligomerize ethylene into higher olefins in the manufacture of surfactants, lubricants, and fine chemicals (Fig. 1) [9-11]. Late transition metals are more suited for the polymerization of... 

Numerous modifications to the bis(aryl)-a-diimine ligand motif have been reported, especially in the patent literature. The review by Ittel includes an index of diimine and related ligands in proprietary publications up to the year 2000 [26]. Instead, this discussion will focus on academic reports of Ni(II) and Pd(II) polymerization catalysts bearing a-diimine or other closely related chelating neutral nitrogen ligands. 

Acyclic aromatic A-arylimines, Ar1-C(Me)=N-Ar2, have been reduced to the corresponding amine with up to 99% ee, using 1 atm of hydrogen and an iridium(I) catalyst bearing a chiral diphosphinoethane chelating ligand.52... 

Similarly, ra 5-cyclopropanes were obtained from alkenes, such as styrene and 2,5-dimethyl-hexa-2,4-diene, with relative yields > 90% when a diazoacetate bearing a bulky ester group was decomposed by a copper catalyst with bulky salicylaldimato ligands. Several metal complexes with bulky Cj-symmetrlc chiral chelating ligands are also suitable for this purpose, e.g. (metal/ligand type) copper/bis(4,5-dihydro-l,3-oxazol-2-yl)methane copper/ethyl-enediamine ruthenium(II)/l,6-bis(4,5-dihydro-l, 3-oxazol-2-yl)pyridine cobalt(III)/ salen. The same catalysts are also suited for enantioselective reactions vide infra). For the anti selectivity obtained with an osmium-porphyrin complex, see Section 1.2.1.2.4.2.6.3.1. 

The nickel catalysts described in the preceding section are neutral in character, in our hands cationic nickel catalysts such as those described by Brookhart et al. [20] are ineffective in the copolymerization of norbornene and ethylene. Indeed norbornene has been described as a strong catalyst poison (for both nickel and palladium) by the same workers. Surprisingly we found that cationic palladium catalysts, with a wide variety of chelate ligands, are very effective in the copolymerization of ethylene and norbornenes [89]. These catalysts possess the same ability as the above-described neutral nickel catalysts to incorporate norbornenes bearing functional groups, but overcome two of the major limitations ... 

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