Syntheses of extreme sterically hindered 4-methoxyboronic acids

4-Iodoanisoles 3a,b, 3d and 4-bromoanisoles 4a-d were readily obtained. An extreme steric hindrance precluded obtaining 3c. Catalytic borylation of 3a,b, 3d followed by hydrolysis of boronic ester 26a,b, 26d easily provided the boronic acids 5a,b, 5d. Compounds 5a and 5d were also synthesised, starting from 4a and 4d, by halogen/metal exchange. Because of a too important steric hindrance, this last reaction failed with 4c and 4b led to the unexpected but stable boronic ester 6. The final obtaining of 5b required a strongly basic hydrolysis with heating.

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