Chiral alkyldihaloboranes

Chiral alkyldihaloboranes are among the most powerful of chiral Lewis acids. The Diels-Alder reaction of cyclopentadiene with methyl acrylate proceeded smoothly at — 78 °C in the presence of 530 or 531 to give the endo-adduct with >99% ee (Equation (250)).1046-1052... 

Chiral alkyldihaloboranes are among the most powerful chiral Lewis acids. In general, however, because alkyldihaloboranes readily decompose to alkanes or alkenes as a result of protonolysis or /3-hydride elimination, it is difficult to recover them quantitatively as alkylboronic acids. Aryldichloroborane is relatively more stable and can be reused as the corresponding boronic acid. We have developed chiral aryldichlorobor-anes 23 bearing binaphthyl skeletons with axial chirality as asymmetric catalysts for the Diels-Alder reaction of dienes and a,/3-unsaturated esters (see, e.g., Eq. 37) [36]. 

Applications in Diels-Alder Chemistry. Alkyldihaloboranes have also been used as chiral catalysts for the stereocontrolled Diels-Alder Chemistry. High ewdo-selectivity is observed for this reaction although the ee s are only in the range of 5-50% (eq 20). 

One of the limitations of (a-chloroalkyl)boronic ester chemistry has been that the chiral directors are difficult to cleave from boron, and boronic esters are inert in some of the useful transformations of trialkylboranes and alkyldihaloboranes. The vigorous conditions that will remove pinanediol from any pinanediol boronic ester, treatment with boron trichloride [12], leave the pinanediol as tarry ruins and with it any sensitive functionality on the boronic ester. The much milder cleavage by transfer of pinanediol or other chiral diol to phenylboronic acid in a two-phase system works well if the boronic acid to be isolated is water soluble [26]. Other cleavage methods include reduction of pinanediol boronic esters with lithium aluminum hydride or alkylation to borinic ester intermediates [73]. 

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