Imidazolin-2-iminato Ligand-Supported Titanium Complexes as Catalysts for the Synthesis of Urea Derivatives
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posted on 2016-02-01, 00:00 authored by Kishor Naktode, Suman Das, Jayeeta Bhattacharjee, Hari Pada Nayek, Tarun K. PandaThe
reactions of tetrakis(dimethylamido)titanium(IV) [Ti(NMe<sub>2</sub>)<sub>4</sub>] with three different imidazolin-2-imines (Im<sup>R</sup>NH; R = <i>tert</i>-butyl (<i>t</i>Bu), mesityl
(Mes), and 2,6-diisopropylphenyl (Dipp)) afforded the corresponding
titanium imidazolin-2-iminato complexes [(Im<sup>R</sup>N)Ti(NMe<sub>2</sub>)<sub>3</sub>] (R = <i>t</i>Bu, <b>1a</b>;
R = Mes, <b>1b</b>; R = Dipp, <b>1c</b>). Treatment of
complex <b>1a</b> with two different carbodiimides [R′NCNR′;
R′ = cyclohexyl (Cy) and isopropyl (<i>i</i>Pr)]
resulted in the formation of imidazolin-2-iminato titanium mono(guanidinate)
complex of the type [(Im<sup>R</sup>N)Ti(R′NC(NMe<sub>2</sub>)NR′) (NMe<sub>2</sub>)<sub>2</sub> (R′ = <i>i</i>Pr; R = <i>t</i>Bu (<b>2a</b>), R = Dipp
(<b>2c</b>); R′ = Cy, R = <i>t</i>Bu (<b>3a</b>)], as yellow solid in 94% yield. However, a similar reaction
of <b>1b</b> and <b>1c</b> with 2 equiv of phenyl isocyanates
at ambient temperature resulted in the formation of corresponding
titanium bis(ureate) complexes [(Im<sup>R</sup>N)Ti{κ<sup>2</sup>-OC(NMe<sub>2</sub>)NPh}<sub>2</sub>(NMe<sub>2</sub>)] (R = Mes, <b>4b</b> and R = Dipp, <b>4c</b>). Three equivalents of phenyl
isothiocyanate reacted with complex <b>1c</b> to afford respective
titanium tris(thioureate) complex [(Im<sup>Dipp</sup>N)Ti{κ<sup>2</sup>-SC(NMe<sub>2</sub>)NPh}<sub>2</sub>{κ<sup>1</sup>-SC(NMe<sub>2</sub>)NPh}] (<b>6c</b>). The molecular
structures of <b>1a</b>–<b>c</b>, <b>2a</b>, <b>2c</b>, <b>3a</b>, <b>4c</b>, and <b>6c</b> were established by X-ray diffraction analyses, and, from the solid-state
structures of <b>1a</b>–<b>c</b>, <b>2a</b>, <b>2c</b>, <b>3a</b>, <b>4c</b>, and <b>6c</b>, it was confirmed that the imidazolin-2-iminato titanium bond in
each case is very short and possesses a multiple-bonding character.
The imidazolin-2-iminato titanium complex <b>1c</b> was utilized
as a precatalyst for the addition of amine N–H bond to phenyl
isocyanate. High yields of the corresponding urea derivatives were
achieved under mild conditions. The mechanistic study of the aforementioned
catalytic reaction was performed, and the active catalyst complex <b>7b</b> was isolated using 2 equiv of iminopyrrole [2-(2,6-<i>i</i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>NCH)C<sub>4</sub>H<sub>3</sub>NH] and the complex <b>4b</b>. The molecular
structure of <b>7b</b> was thereafter established.
