Chiral Boronate Lewis Adds

Chelation of prochiral dienophiles by chiral boron, aluminum and titanium Lewis adds... 

The efficiency of haloboranes as Lewis add catalysts and the sensitivity of the boron atom environment to steric hindrance (see Chapter 6) make boron compounds useful chiral catalysts. Hawkins and Loren [779] prepared enantiopure chloroborane (17 ,2I )-3.6, and they showed that the complex 3.6 with methyl crotonate lies under the s-trans conformation 9.40 both in the solid state and in solution. This conformation is also the reactive one, and [4+2]-cycloadditions of acrylates with cyclopentadiene or cydohexadiene catalyzed by (17 ,2i )-3.6 lead highly selectively to the product resulting from the attack of the diene on the unshielded face of complex 9.40 (Figure 9.15). 

Acetylenic cobalt complexes greatly facilitate the heterolytic cleavage of adjacent alcohols or ethers. On treatment with Lewis acids, these complexes afford cobalt stabilized carbenium ions, which can be captured by nucleophiles such as enolates. Jacobi and Zheng have employed chiral boron enolates of Evans s oxa-zolidinone 6.91 (R = i-Pr). After removal of the chiral auxiliary, they obtained anti adds 11.43 with a high selectivity [1677] (Figure 11.9). The reaction can be extended to the boron enolates of related oxazolidinones and to a-branched propargyl derivatives. This reaction has been applied to the synthesis of P-aminoacids after Curtius rearrangement and oxidation of the triple bond [1677]. 

Hetero Diels-Alder cycloaddition of a,p-unsaturated carbonyl compounds and dioxygenated alkenes in the presence of a chiral bisoxazoline-Cu(OTf)2 complex as Lewis add catalyst furnished hexopyranose precursors in good yields and high enantomeric excess. In the synthesis of the precursor 6 of ethyl tetra-0-acetyl-P-D-mannopyranose outlined in Scheme 1, for example, a 69% overall yield and 99% ee were achieved. A new route to hex-2-uloses involving boron-or, preferably, lithium-enolates is exemplified in Scheme 2. Only 3,4-trans-... 

High enantioselectivities are obtained using tartaric acid-derived boronate ester 31 in combination with lithium borohydride or sodium borohydride for asymmetric reduction of alkyl or aryl ketones. The chiral Lewis add is easUy prepared in one hour, and the resulting alcohols are obtained in enantiomeric excesses of 88-99% (Equation 46) [44]. 

In Section 12.7.B, the Stille coupling reaction reacted a vinyl tin compound with a vinyl triflate, in the presence of palladium(O). Tetravalent tin complexes add to aldehydes and ketones, in the presence of a Lewis acid. Allyltin complexes are, by far, the most widely used of these compounds.297 a typical example is taken from the work of Keck, in which a chiral aldehyde (455) was treated with allyltributyltin, in the presence of various Lewis acids. S As shown in Table 12.19, a mixture of syn (456) and anti (457) products was obtained. The ratio of 456/457 was dependent on the structure of the R group in 455, the solvent and the Lewis acid.The anti product (457) was obtained by using the tert-butyldimethylsilyloxy derivative (sec. 7.3.A.i) of 455 with 2 equivalents of boron trifluoride in dichloromethane. The syn product is obtained preferentially when the benzyloxy derivative of 455 is used with titanium tetrachloride in dichloromethane.298... 

Boronic acids and their derivatives are very popular as components of chiral Lewis acids and promoters for various reaction processes [481]. Indeed, the chiral acyloxyb-oranes and the oxazaborolidines (Section 1.2.3.5) described in Chapter 11 made a mark in organic synthesis. Recently, Ryu and Corey extended the apphcation of chiral oxaborolidinium catalysts to the cyanosilylation of aldehydes [482]. Chiral diaz-aborohdine salts were evaluated in the enantioselective protonation of enol ethers [145]. Likewise, a tartramide-derived dioxaborolane is key as a chiral promoter in the asymmetric cyclopropanation of allyhc alcohols [483]. More examples and details on the applications of boronic add derivatives as reaction promoters and catalysts are provided in Chapter 10. 

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