Formation of dilithiated bis-(1H-pyrazol-1-yl)alkanes and their application in the synthesis of diboronic acids

Bis-(1H-pyrazol-1-yl)alkanes were deprotonated at the pyrazole 5-positions on treatment with LDA in THF at low temperature. These dianions reacted with tert-butylisocyanate as the electrophile to install a tert-butylamide group at the pyrazole 5-position. The obtained amides were next converted into the respective diboronic acids by Br–Li exchange with t-BuLi in THF at low temperature, followed by the use of triethyl borate as the electrophile. The X-ray analysis of the obtained diboronic acids revealed the presence of a variety of structural motifs, which stabilize the structure by hydrogen bond formation. The stabilization pattern differs greatly with a minor modification of the linker connecting the pyrazole rings.