Studies on the Lithiation, Borylation, and 1,2‐Metalate Rearrangement of <i>O</i>‐Cycloalkyl 2,4,6‐Triisopropylbenzoates
Rory C. Mykura, Pradip Songara, Eugenia Luc, Jack Rogers, Ellie Stammers, Varinder K. Aggarwal
Abstract
A broad range of acyclic primary and secondary 2,4,6-triisopropylbenzoate (TIB) esters have been used in lithiation-borylation reactions, but cyclic TIB esters have not. We have studied the use of cyclic TIB esters in lithiation-borylation reactions and looked at the effect of ring size (3- → 6-membered rings) on the three key steps of the lithiation-borylation protocol: deprotonation, borylation and 1,2-metalate rearrangement. Although all rings sizes could be deprotonated, the cyclohexyl case was impractically slow, and the cyclopentyl example underwent α-elimination faster than deprotonation at -78 °C and so could not be used. Both cyclobutyl and cyclopropyl cases underwent rapid borylation, but only the cyclobutyl substrate underwent 1,2-metalate rearrangement. Thus, the cyclobutyl TIB ester occupies a "Goldilocks zone," being small enough for deprotonation and large enough to enable 1,2-migration. The generality of the reaction was explored with a broad range of boronic esters.