Use of Vanadium Complexes as Catalysts in the Synthesis of Guanidines: New Experimental Data and DFT Analysis of the Carbodiimide Interaction with the Catalyst

Abstract

The activity of several imido vanadium complexes as catalysts of the guanylation reaction was studied. Complex V(N-2,6-iPr2C6H3)Cl3, 1, was found to be an efficient precatalyst for the reaction between carbodiimides and both primary and secondary aryl amines to give the corresponding guanidines 2−6. Two possible pathways for the reaction were considered for a DFT study: (i) [2+2] carbodiimide addition to the vanadium−imido bond, a mechanism previously ascertained by Richeson and co-workers for the synthesis of guanidines catalyzed by titanium imido compounds (J. Am. Chem. Soc. 2003, 125, 8100); and, alternatively, (ii) carbodiimide insertion into the vanadium−amido bond, which would be formed in situ by the interaction of the amine reagent with 1. First, the titanium-catalyzed reaction, using {(Me2N)C(NMe)2}2Ti(NMe) (A) as a model complex, was studied. Second, two different vanadium model complexes, V(NMe)Cl3 (B) and V(NMe)(NMe2)Cl2 (C), were considered for the guanylation. Noticeably, whereas for model A the [2+2] carbodiimide addition to the metal−imido bond was an exergonic process, for model B the same pathway was not exergonic and gave a much higher activation barrier than that computed for A. Finally, the two pathways were investigated with model C, containing both imido and amido functionalities. The results show that for vanadium-catalyzed guanylation reactions the carbodiimide insertion into the metal−amido bond is favorable with respect to the carbodiimide addition to the metal−imido bond, which is the mechanism operative for titanium.