Transition-metal Lewis acids

Nishiyama H., Motoyama Y. Other Transition Metal Reagents Chiral Transition-Metal Lewis Acid Catalysis for Asymmetric Organic Synthesis in Lewis Acid Reagents 1999 225, Ed Yamamoto H., Pb. Oxford Univ. Press, Oxford Keywords asymmetric Diels-Alder reactions, chiral transition metal Lewis-acid catalysis, asymmetric synthesis... 

In the above cases, an optically active reducing agent or catalyst interacts with a prochiral substrate. Asymmetric reduction of ketones has also been achieved with an achiral reducing agent, if the ketone is complexed to an optically active transition metal Lewis acid. ... 

Bonnesen PV, Puckett CL, Honeychuck RV, Hersh WH (1989) Catalysis of Diels-Alder reactions by low oxidation state transition-metal Lewis acids fact and fiction. J Am Chem Soc 111 6070-6081... 

Scheme 5. First nucleophilic enantioselective epoxide opening with enantiopure transition metal Lewis acid 29.

Synthesis of Chiral Lewis Acids. BIPHOP-F is used in the synthesis of chiral transition metal Lewis acids. Because of its electronic properties, it enhances the acidity of the metal. Coordination of the bidentate ligand to the metal is accomplished by CO substitution (eq 1 and 2 ). The cationic ruthenium or iron complexes are obtained after one or two additional steps (L is a labile ligand and X the counter anion). 

Lewis acids play key roles in a large number of reactions, and their use in organic synthesis continues to see rapid development, particularly in the field of asymmetric catalysis [1], Late transition metal Lewis acid catalysts have emerged as a new class of compounds within this area. They offer neutral and mild conditions that are of interest for the needs of modern chemistry and its focus on economically and ecologically friendly methods. 

In comparison with classic Lewis acids derived from main group halides (e.g., B, Al, Sn), f-elements, and early transition metal halides, late transition metal Lewis acids often are more inert to ubiquitous impurities such as water, offer higher stability, tunable properties by ligand modification, and a well-defined structure and coordination chemistry, thus allowing detailed studies of reaction mechanisms, and a rational basis for catalyst optimization. Among this new class of late transition metal Lewis acids, ruthenium complexes - the subject of this chapter - display remarkable properties... 

Transition Metal Lewis Acids From Vanadium to Platinum... 

Transition metal Lewis acids can be used to effect deacetalization under mild conditions. Thus, 1,3-dioxolanes are hydrolyzed by PdCl2(MeCN)2 in wet acetonitrile but a better procedure is transacetalization of the dioxolanes by the same complex (1-5 %, room temperature in acetone) [6]. A good example of this method is the clean deprotection of an oxolane of a /3-hydroxyketone that is susceptible to elimination (Table 2, entry 1) [20]. TBDPS ethers and epoxides are tolerated [21]. 

Extensive studies on the Lewis acid [( 7 " -C5H5)Re(NO)(PPh3)] have yielded detailed insight into transition metal Lewis acid-Lewis base interactions and the question of a or coordination of carbonyl groups [100]. The Re Lewis acid binds aldehydes via the TT-system whereas ketones coordinate to form M-O cr-complexes. Both react with Et4N CN to give Re-bound cyanohydrins. Diastereoselectivity ranges from 53 to 89 % de for additions to aldehydes and 71 to 99 % de for additions to ketones (Sch. 19) [101]. Diastereoselective reductions to primary and secondary alcohols have also been reported [102]. 

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