Asymmetric addition to aldehyde

Asymmetric addition to aldehydes. In the presence of 1, lithium trimethylsilyl-acctylide reacts with aliphatic aldehydes to form predominately (R)-alkynyl alcohols. One of these products was converted into optically active 5-octyl-2(5//)-luranonc (2) as shown in equation (I).1... 

The direct hydrozincation of olefins14 is possible using Et2Zn in the presence of catalytic amount of Ni(acac)2 and 1,5-cyclooctadiene (COD). The resulting diorganozincs can be used for asymmetric additions to aldehydes. The best hydrozincation reactions are obtained with allylic alcohols or amines. In these cases the raction is driven to completion by the formation of a zinc-heterocycle of type 7 (Scheme 5.10). 

Reagents of this type are suitable for performing catalytic asymmetric additions to aldehydes. For example, an enantiomerically pure Lewis acid is generated in situ from Ti(OiPr)4 and the enantiomerically pure bis(sulfonamide) C. It catalyzes the enantioselective addition of functionalized (or unfunctionalized) dialkylzinc compounds to widely variable aldehydes. There is no detailed, substantiated rationalization of the underlying addition mechanism in this case. 

Diorganozincs can also be prepared by a nickel-catalyzed hydrozincation. The reaction of Et2Zn with Ni(acac)2 may produce a nickel hydride that adds to an alkene leading after transmetallation with Et2Zn to a diakylzinc (Scheme 9). This reaction proceeds in the absence of solvent and at temperatures of 50-60 °C. A number of functionalized olefins like allylic alcohols or amines can be directly used. This method is especially well suited for the preparation of functionalized diorganozincs for the asymmetric addition to aldehydes (Equation (45)).108,50... 

Scheme 4.5 Brown allylation using a chiral reagent for asymmetric addition to aldehydes...

Langer, R, Schwink, L., Devasagayaraj, A., Chavant, P. Y. and Knochel, P. 1996. Preparation of functionalized dialkylzincs via a horon-zinc exchange Reactivity and catalytic asymmetric addition to aldehydes. J. Org. Chem. 61 8229-8243. 

Full details are now available of the application of the L-proline-derived pyrrolidine (22) as ligand in the asymmetric addition to aldehydes of alkyl-lithiums (45—95% optical yields) and dialkylmagnesiums (22—92% optical yields). Modification of lithium (and sodium) tetra-alkylaluminates with the chiral amino-alcohols (-)-N-methylephedrine (13), quinine, and cinchonine (Scheme 10) produces reagents (23) that have been shown to give asymmetric alkyl transfer... 

In comparison with the extensive developments in catalytic asymmetric additions to aldehydes and ketones (Chapters 2, 4, and 5), the corresponding catalytic enantioselective additions to imines are less well developed. The lower electrophilicity of the imine carbon in relation to carbonyls, together... 

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