Michael addition zinc catalysts

In another study Feringa et al. [20] reported a catalytic enantioselective three-component tandem conjugate addition-aldol reaction of dialkyl zincs. Here, zinc enolates were generated in situ via catalytic enantioselective Michael addition of dialkylzinc compounds to cydohexenone in the presence of a chiral Cu catalyst. Their diastereoselective reaction with an aldehyde then gave trans-2,3-disubstituted cyclohexanones in up to 92% yields and up to >99% ees (Scheme 9.11). 

Alkoxy-l-siloxycyclopropane, e.g. 30, on treatment with zinc(II) chloride gives a stable zinc homoenolate, that can, for example, be reacted via an intermediate cuprate with enones to afford the Michael addition product.Addition of a palladium catalyst to the zinc species allows for arylation and for vinylation of the homoenolate in good yield. 

The. V-alkylation of ephedrine is a convenient method for obtaining tertiary amines which are useful as catalysts, e.g., for enantioselective addition of zinc alkyls to carbonyl compounds (Section D. 1.3.1.4.), and as molybdenum complexes for enantioselective epoxidation of allylic alcohols (Section D.4.5.2.2.). As the lithium salts, they are used as chiral bases, and in the free form for the enantioselective protonation of enolates (Section D.2.I.). As auxiliaries, such tertiary amines were used for electrophilic amination (Section D.7.I.), and carbanionic reactions, e.g., Michael additions (Sections D. 1.5.2.1. and D.1.5.2.4.). For the introduction of simple jV-substituents (CH3, F.t, I-Pr, Pretc.), reductive amination of the corresponding carbonyl compounds with Raney nickel is the method of choice13. For /V-substituents containing further functional groups, e.g., 6 and 7, direct alkylations of ephedrine and pseudoephedrine have both been applied14,15. 

The asymmetric Michael addition of nonstabilised ketone enolates has proved difficult, with most success achieved using 1,3-dicarbonyls as donors. However, Shibasaki and coworkers have achieved high ees in the addition of a-hydroxyketones with both aromatic Michael acceptors such as (11.32) and also cyclic enones and alkyl vinyl ketones, using bifiinctional zinc catalysts prepared from linked BINOL (11.33). These catalysts are also effective in the asymmetric aldol reaction (see Section 7.1) and incorporate two zinc atoms, one of which activates the acceptor carbonyl group and the other forms a zinc enolate with the donor. In addition, catalysts of this type have been used to good effect in the addition of P-ketoesters to cyclic enones. 

Zare, A., Hasaninejad, A., Khalafi-Nezhad, A., Zare, A.R.M., Parhami, A. and Nejabat, G.R. 2007. A green solventless protocol for Michael addition of phthalimide and saccharin to acrylic acid esters in the presence of zinc oxide as a heterogeneous and reusable catalyst. Arkivoc 2007(i) 58-69. 

Zare A, Hasaninejad A, Zare ARM, Parhami A, Sharghi H, Khalafi-Nezhad A (2007) Zinc oxide as a new, highly efficient, green, and reusable catalyst for microwave-assisted Michael addition of sulfonamides to a,P-unsaturated esters in ionic liquids. Can J Chem 85 438-444... 

During the last decade, use of oxazaborolidines and dioxaborolidines in enantioselective catalysis has gained importance. [1, 2] One of the earliest examples of oxazaborolidines as an enantioselective catalyst in the reduction of ketones/ketoxime ethers to secondary alco-hols/amines was reported by Itsuno et al. [3] in which (5 )-valinol was used as a chiral ligand. Since then, a number of other oxazaborolidines and dioxaborolidines have been investigated as enantioselective catalysts in a number of organic transformations viz a) reduction of ketones to alcohols, b) addition of dialkyl zinc to aldehydes, c) asymmetric allylation of aldehydes, d) Diels-Alder cycloaddition reactions, e) Mukaiyama Michael type of aldol condensations, f) cyclopropana-tion reaction of olefins. 

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