Reaction chemistry of tri-substituted mesitylene derivatives and the synthesis of sterically buttressed 1,3,5-triaminocyclohexyl ligands

Abstract

The sterically buttressed triamine cis,cis-2,4,6-trimethyl-1,3,5-triaminocyclohexane is weakly basic in aqueous solution (pKa: 7.83, 6.73, 5.15; 298 K) because of steric inhibition to solvation. As a result of intramolecular hydrogen-bonding and stabilising σC-H-σ*C-N interactions, the free amine and its monoprotonated salt adopt a conformation in which the three amino groups are axially disposed. It possesses a reduced reactivity in SN2 reactions and may be converted via selective alkylation or condensation to polydentate ligands incorporating carboxylate, hydroxyphenyl or phosphinate groups. An intramolecular hydride transfer mechanism accounts for the formation of a stable bicyclic amidine formed during acidic hydrolysis of a tricyclic bis-aminal derived from the parent triamine by a phosphinoxymethylation. The crystal structures of two of the mesitylene derivatives have been determined.