Diastereoselective amidoboronate formation and transformation from a rac to a different meso amidoboronate via dynamic C–C bonds

Abstract

Amidoboronates, synthesized through the reductive coupling of iminoboronates with cobaltocene in DMSO, form as mixtures of up to three isomers (rac₅, rac₆ and meso₅) and can exhibit up to three types of dynamic covalent bonds (C–C, B–N, B–O). In order to exploit the reductive coupling for the self-assembly of covalent organic macrocycles and cages, control over the diastereoselectivity and reversibility of C–C bond formation is required. We report proof-of-principle in dimeric amidoboronates; diastereoselective rac isomer formation was observed for amidoboronate 5e (based on p-NMe₂-substituted aniline, pyrocatechol and phenylboronate), whereas employing an iminoboronate containing p-OMe-substituted aniline, either pyro- or tetrachlorocatechol and 3-fluorophenylboronate led to rare examples of meso diastereoselectivity (9 : 1 dr for 7d and 8d). The addition of a second iminoboronate to amidoboronate reaction mixtures probed the reversibility of C–C bond formation as evidenced by the formation of new homo- and cross-coupled amidoboronates via exchange. The dynamic covalent C–C bonds were exploited in the unprecedented diastereoselective transformation of rac_5/6-5e to meso₅-7d following addition of excess iminoboronate 3d.