Diisopinocampheyl boron derivatives

SCHEME 10.40. Application of diisopinocampheyl boron derivatives in synthesis of... 

A 100-mL flask protected with a positive pressure of nitrogen is charged with 11 mmol of a 0.25 M pentane solution of iodo(diisopinocampheyl)borane [derived from ( +)-a-pinene]. The solution is cooled to — 100 C and treated dropwise with a solution of 980 mg (10 mmol) of 1,2-epoxycyclohexane in 2 mL of pentane. After stirring the solution for 0.5 h at —100 CC, the mixture is quenched by the addition of 1.8 mL (excess) of butanal. The mixture is allowed to warm gradually to r.t. and stirred for an additional 1 h. By that time "B NMR indicates clean formation of boronate [R OB(OEt)Ipc, d = 31], The mixture is diluted with 50 mL of pentane and treated with a 4 M THF solution of 12 mmol of 2.2 -diethanolamine. After the solution is stirred for 0.5 h, the precipitated boronate 2,2 -diethanolamine complex is filtered off and the filtrate is washed with water and brine and dried with anhyd Na2S04. The solution is concentrated under reduced pressure and the residue is purified by column chromatography followed by crystallization from pentane to provide the title compound in 89% yield mp 44 - 45°C [a] 3 —31.6 (c = 5, CHClj). 

By a similar method, the (Z)-crotylborate is synthesized from cA-2-butene in 70-75% yield with a 98% isomeric purity. The tartrate esters of allylboronic acids are an excellent reagent for asymmetric allylboration of carbonyl compounds. Allyl(diisopinocampheyl)borane [51] and the allylic boron derivatives of ester and amide, such as camphordiol [52], pinanediol [53], 1,2-diphenyl-1,2-ethylenediamine [54], have also been successfully used for asymmetric allylboration of carbonyls. 

In 1978, Hoffmann reported enantioselective allylation reactions of aldehydes with a camphor-derived chiral boronate 13 (Equation 1) [55]. Three classes of chiral boron reagents that furnish allylation products with very high levels of enantioselectivity have emerged Brown s diisopinocampheyl or diisocaranyl allyl boranes [35-41], Roush s tartrate-derived boronates [33, 42-48], and Hoffmann s "a-chiral boronates [56-61],... 

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