Chiral Catalysts for Asymmetric Allylation

BINOL-derived phosphoric acid 21.11 was also shown to catalyse addition of allyltrichlorosilane 21.5a to aldehydes. Methyl substituents in 3,3  

Pyridine-type IV-oxides represent another successful class of catalysts for the allylation reaction. They now constitute the most abundant group of nucleophilic activators for silicon reagents. For convenience, the discussion will first focus on catalysts with two or more N-oxide units in the molecule followed by the group of mono-N-oxides. 

Terpene-derived bis-N-oxide 21.22 represent the most recent addition to the successful catalyst series. The catalyst was shown to be particularly efficient in the allylation and crotylation of aromatic and a,p-unsaturated aldehydes ( 99% enantiomeric excess at —60°C), however, with aliphatic aldehydes the selectivity dropped to 50% ee. It is noteworthy that 21.22 was synthesised in four easy steps from inexpensive (I )-myrtenal and the protocol is amendable to scaling up. In contrast, synthesis of enantiopure catalysts 21.19-21.21 requires either resolution of enantiomers or separation of diastereoisomeric mixtures, which hampers their larger-scale application. In this group of polydentate IV-oxides it is also worth mentioning terpyridine N,IV IV -trioxide, the related bis-imidazole Af,M -dioxides and chiral dinitrones,but their efficiency was inferior to the best pyridine-type dioxides, such as 21.20-21.22. 

A family of bipyridine mono-JV-oxides was introduced by Maikov and Kocovsky, who showed the terpene-derived bipyridine derivatives 21.23-21.25 to be even more enantioselective than the structurally related Nakajima s M,M -dioxide 21.17, although the reaction rate dropped, especially with the severely hindered 21.25. The most successful derivative of this series, AT-oxide 21.24 combines the effects of both central and axial chirality. 

The selection of successful N-mono-oxides also includes 21.27 (QUINOX, 98 2 er at 5 mol% loading at -40 °C in dichloromethane) and para-cyclophane 21.28 ( 98 2 er at 1.5 mol% loading at -40°C in acetonitrile). Hoveyda and Snapper reported on the first nonpyridine-type iV-mono-oxide 21.29 derived from proline ( 96 4 er at 10 mol% loading at RT in CH2ClCH2Cl), which later was complimented by piperidine and tetrahydroquinoline analogues. Other pyridine-type N-oxides 

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An effective chiral catalyst for asymmetric allylation of aldehydes or ketones is the complex formed between the axially chiral l,l -bi-2-naphthol (BINOL) 164 and a titanium(IV) salt." Addition of allyltributylstannane to an aldehyde using this Lewis acid gives the product homoallylic alcohol with high optical purity. For example, allylation of iso-butyraldehyde gave the alcohol 165 as a 98 2 ratio of enantiomers (1.155). Use of (5)-BINOL gave the other enantiomer of the product. 

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