Guanylation/Cyclisation of Amino Acid Esters using Imidazolin-2-iminato Titanium Initiator

Panda, Tarun K and Das, Suman and Bhattacharjee, J (2019) Guanylation/Cyclisation of Amino Acid Esters using Imidazolin-2-iminato Titanium Initiator. Dalton Transactions. ISSN 1477-9226 (Submitted)

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Abstract

We report the catalytic guanylation/cyclisation 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. Further, the reaction of titanium complex 2a with 2,6-diisopropylphenylamine (DippNH2) resulted in the corresponding mixed ligand titanium complex [1-(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 [1-(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 guanylation and cyclisation rections under mild conditions. We propose the most plasubile mechanism for guanylation/cyclisation of amino acid esters to carbodiimides and isocyanates on the basis of a number of controlled reactions.

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