Sterically hindered boronic esters of (R,R)-1,2-dicyclohexyl-1,2-ethanediol or pinanediol react with thionyl chloride and excess imidazole in acetonitrile on a borosilicate glass surface (but not on silica or sodalime glass) to form the corresponding cyclic sulfite of the diol and the easily separable organoborane derivative containing one boron-bound chloride and two imidazole groups, which may cross link with additional organoborane moieties to form oligomeric species. Other heterocyclic amines react similarly but usually less efficiently. With excess pyridine, the 1:1 complex PhBCl 2 (py) rapidly forms the 2:1 complex, PhBCl(py) 2 + Cl -. Hydrolysis of the amino boron chlorides to boronic acids in near neutral aqueous solution provides a mild process for the net hydrolysis of hindered boronic esters, many of which cannot be hydrolyzed by ordinary means, though yields were only 54-82%. The diol sulfites are stable toward water or aqueous acid but rapidly hydrolyzed by base to the diol and inorganic sulfite.
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