Other Aspects of a-Chloroalkyl boronic Ester Chemistry

Other Aspects of (a-Chloroalkyl)boronic Ester Chemistry 8.4.1 

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]. 

Mild methods were known in some other specific instances. The (R,J )-2,3-butane-diol ester of (l-chloro-2-methylpropyl)boronic acid with diethanolamine yielded the crystalline diethanolamine ester [13]. DICHED (a-benzyloxyalkyl)boronates have been cleaved with TAPS buffer [a water-soluble gem-tris(hydroxymethyl) compound] and excess base [74]. However, these methods were inefficient and not general. 

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