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 (pK_a: 7.83, 6.73, 5.15; 298 K) because of steric inhibition to solvation. As a result of intramolecular hydrogen-bonding and stabilising σ_C–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 S_N2 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.