Guanylation/cyclisation of amino acid esters using an imidazolin-2-iminato titanium initiator
We report the catalytic hydroamination of amino acid esters with carbodiimides and isocyanates to furnish corresponding quinazolinone and urea derivatives under mild conditions using two titanium(IV) complexes [(ImRN)2Ti(NMe2)2] (R = tBu, 2a; R = Mes, 2b), which were synthesised by reacting tetrakis(dimethylamido)titanium(IV) [Ti(NMe2)4] with imidazolin-2-imine [ImRNH; R = tert-butyl (tBu) (1a), mesityl (Mes) (1b)] in a 1 : 2 molar ratio in toluene. Furthermore, the reaction of titanium complex 2a with 2,6-diisopropylphenylamine (DippNH2) resulted in the corresponding mixed ligand titanium complex [κ1-(ImtBuN)2Ti(NMe2)(HNDipp)] (3a). In contrast, the reaction of complex 2a with 2,6-dimethyl phenol afforded the mono-imidazolin-2-iminato TiIV phenolate complex [κ1-(ImtBuN)Ti(O-1,6-Me2C6H3)3] (4a). The solid-state structures of complexes 2b, 3a, and 4a, established by single crystal X-ray diffraction analyses, confirmed a very short bond between titanium and imidazolin-2-iminato nitrogen in each case. Titanium complexes 2a and 2b exhibited relatively high conversion, superior selectivity and broad functional group tolerance in both hydroamination reactions under mild conditions. We propose the most plausible mechanism for the guanylation/cyclisation of amino acid esters to carbodiimides on the basis of a number of controlled reactions.