Boronic thermodynamic hydrolysis

Hydrolysis of 1,3,2-dioxaborolanes is thermodynamically disfavored, no doubt as a result of the same kinds of entropic factors that favor formation of cyclic acetals in preference to acyclic acetals. Hydrolysis of a 1,3,2-dioxaborolane (1) converts three molecules to two, but hydrolysis of a boronic acid dimethyl ester keeps the total number of molecules at three (Scheme 5). (It m be noted that hydrolysis of a cyclic acetal with one molecule of water keeps the number of reactant and product molecules equal at two, and hydrolysis of an acyclic acetal converts two molecules to three.) Adding base does almost nothing to the equilibrium, since hydroxide ion coordinates to the boronic ester 1 as well as to the boronic acid product. Furthermore, it spears that the trans R° substituents in 1 further stabilize the stmcture. Chiral boronic esters of this series are harder to hydrolyze than pinacol boronic esters, and treatment of pinacol boronic esters with DICHED results in liberation of the pinacol and formation of the DICHED boronic ester. 

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