Allylic Silicon Reagents

A method using dual activation has been developed in which a Lewis acid activates the aldehyde with concomitant nucleophilic activation of the allylic silicon reagent with fluoride anion. Thus, by using a BINOL-based titanium... 

Carbonyl allylation represents an important class of reactions that allows the construction of polypropionate motifs found in many bioactive natural products. Boron Lewis acids have been employed primarily to activate aldehyde and ketone substrates toward allylic tin and allylic silicon reagents. 

Because allyltrimethylsilane 82 or benzyltrimethylsilane 83 can be regarded as combinations of the hard trimethylsilyl cation and the soff allyl or benzyl anions, pyridine N-oxide 860 reacts with excess 82 or 83 in the presence of catalytic amounts of tetrabutylammonium fluoride di- or trihydrate in THF to give 2-allyl-or 2-benzylpyridines 948 and 950 [60]. The general reaction of silicon reagents such as 82 and 83 or of trimethylsilyl cyanide 18 with fluoride to generate allyl or... 

The report suggests that the actual reagent may be a pentacoordinate allyl-siliconate, such as CH,CH=CHCH2Si F4Cs+, which reacts with an aldehyde via a six-membered cyclic transition state. 

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

The Wittig reagent is made by alkylation of the simplest ylid with the same silicon reagent. Notice that the leaving group (iodide) is on the carbon next to silicon, not on the silicon itself. Anion formation occurs next to phosphorus because PhsF is much more anion-stabilizing than MesSi. The ylid reacts with carbonyl compounds such as cyclohexanone in the usual way to produce the allyl silane with no ambiguity over which end of the allyl system is silylated. 

This chapter is focused on the activation of silicon reagents of general type RSiCls, where R is a nucleophilic fragment (allyl, enol. Cl), by Lewis bases. It covers the work published until the end of 2014 but selected examples of the early 2015 literature are also included. 

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. 

Kennedy JWJ, Hall DG (2003) Recent advances in the activation of boron and silicon reagents for stereocontroUed allylation reactions. Angew Chem Int Ed 42 4732-4739... 

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