Catalysts ligand-accelerated

In summary, ligands tend to diminish the affinity of the substrate for the Lewis-acid catalyst as well as the extent of activation by this catalyst, once the ternary complex is formed. Only a few examples of ligand-accelerated catalysis " have been described... 

In summary, for the most active of catalysts, the copper(II) ion, the diamine ligands that were investigated seriously hamper catalysis mainly by decreasing the efficiency of coordination of the dienophile. With exception of the somewhat deviant behaviour of N,N -dimethylethylenediamine, this conclusion also applies to catalysis by Ni" ions. Hence, significant ligand-accelerated catalysis using the diamine ligands appears not to be feasible. 

Likewise, the influence of the ligand catalyst ratio has been investigated. Increase of this ratio up to 1.75 1 resulted in a slight improvement of the enantioselectivity of the copper(L-tryptophan)-catalysed Diels-Alder reaction. Interestingly, reducing the ligand catalyst ratio from 1 1 to 0.5 1 resulted in a drop of the enantiomeric excess from 25 to 18 % instead of the expected 12.5 %. Hence, as anticipated, ligand accelerated catalysis is operative. 

The field of transition metal-catalyzed hydroboration has developed enormously over the last 20 years and is now one of the most powerful techniques for the transformation of C=C and C=C bonds.1-3 While hydroboration is possible in the absence of a metal catalyst, some of the more common borane reagents attached to heteroatom groups (e.g., catecholborane or HBcat, (1)) react only very slowly at room temperature (Scheme 1) addition of a metal catalyst M] accelerates the reaction. In addition, the ability to manipulate [M] through the judicious choice of ligands (both achiral and chiral) allows the regio-, chemo-, and enantioselectivity to be directed. 

As to most chiral atropisomeric ligand, resolution or asymmetric synthesis is requisite. Mikami developed a novel ligand-accelerated catalyst. The chirality of atropos, but achiral triphos ligand-Ru complex, can be controlled by chiral diamines. Using ( -dm-dabn as controller, the single diastereomeric triphos-Ru complex was achieved through isomerization of (i )-triphos-Ru complex in dichloroethane at 80 °G (Scheme l).44... 

There are several Ti-tartrate complexes present in the reaction system. It is believed that the species containing equal moles of Ti and tartrate is the most active catalyst. It promotes the reaction much faster than Ti(IV) tetraalkoxide alone and exhibits selective ligand-accelerated reaction.9... 

As mentioned in Chapter 1, ligand-accelerated catalysis occurs when a more effective chiral catalyst is obtained by replacing an achiral ligand with a chiral one. Mikami et al.89 reported a different phenomenon in which a more active catalyst was formed by combining an achiral pre-catalyst with several chiral ligands. They found that the most active and enantioselective chiral catalyst was formed in preference to other possible ligand combinations (Scheme 8 43). 

It should be noted that the reaction of benzalde-hyde with (Z)-3-trimethylsiloxy-2-pentene in ethanol or dichloromethane in the presence of the chiral catalyst resulted in a much lower yield and selectivity. On the basis of these results, we propose the catalytic cycle shown in Scheme 2. The catalyst A formed from Cu(OTf)2 and a bis(oxazoline) ligand accelerates the aldol reaction to generate the intermediate B. In aqueous solvents, B is rapidly hydrolyzed to produce the aldol product C and regener-... 

Sharpless et al. coined the word ligand-accelerated catalysis (LAC), which means the construction of an active chiral catalyst from an achiral precatalyst via ligand exchange with a chiral ligand. By contrast, a combinatorial library approach in which an achiral pre-catalyst combined with several chiral ligand components (L, L, —) may selectively assemble in the presence of several chiral activators (A, A, —) into the most catalytically active and enantioselective activated catalyst (ML A" ) (Scheme 8.16). ... 

The rapid start could be attributed to the chromium already being alkylated. However, it seems unlikely, at least for the ring compounds, that the ligand starts the first chain. Otherwise there would be no difference between many of the catalysts, which in fact exhibit a great deal of individuality. Instead, the alkylation reaction may be similar to that on the oxide catalysts, only accelerated. In this case also the initiation mechanism is very poorly understood at this point. 

Recently, the enantioselective hydrogenation of ethyl pyruvate catalyzed by cinchona modified Pt/Al203 (ref. 1) was shown to be a ligand accelerated reaction (ref. 2). The rate of reaction for the fully modified system is more than 10 times faster than the racemic hydrogenation using unmodified catalyst. Under certain reaction conditions, this liquid phase hydrogenation exhibits a turn-over frequency of up to 50 s 1 (3.4 mol/kg-cat s). Emphasis until now has been directed at empirically increasing optical yields (ref. 3,4). 

In 1996, Yamamoto and Yanagisawa reported the allylation reaction of aldehydes with allytributyltin in the presence of a chiral silver catalyst.2 They found that the combination of silver and a phosphine ligand accelerates the allylation reaction between aldehydes and allyltributyltin. After this discovery, they screened several chiral phosphine ligands and found that chiral silver-diphosphine catalysts can effect the reaction in an enantioselective fashion (Table 9.1).2 For example, when benz-aldehyde and allyltributyltin were mixed in the presence of 5 mol% of AgOTf and (S)-2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (BINAP), the corresponding homoallyl alcohol was obtained with 96% ee and 88% yield (Table 9.1). Generally, the reaction with aromatic aldehydes afforded the corresponding homoallyl alcohols in excellent... 

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