Synthesis, Structures, and Characterizations of [ArTi(O-<i>i</i>-Pr)<sub>3</sub>]<sub>2</sub> and Efficient Room-Temperature Aryl−Aryl Coupling of Aryl Bromides with [ArTi(O-<i>i</i>-Pr)<sub>3</sub>]<sub>2</sub> Catalyzed by the Economic Pd(OAc)<sub>2</sub>/PCy<sub>3</sub> System
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The series of aryltris(2-propoxo)titanium reagents [ArTi(O-i-Pr)3]2 (Ar = Ph (1a), 2-MeC6H4 (1b), 4-MeC6H4 (1c), 4-ClC6H4 (1d), 4-TMSC6H4 (1e), 4-CF3C6H4 (1f), 3,5-Me2C6H3 (1g)) was synthesized from reactions of the in situ prepared ClTi(O-i-Pr)3 with ArMgBr in THF. All compounds were characterized by NMR spectroscopy and elemental analyses. A variable-temperature 1H NMR study of 1c suggested that compounds 1 are dimeric species in solution. The structures of complexes 1b,g were further determined by single-crystal X-ray analyses. X-ray diffraction studies confirmed that the aryltris(2-propoxo)titanium complexes have a dimeric structure with a Ti2O2 core bridging through the oxygen atom of two 2-propoxide groups. The application of aryltris(2-propoxo)titanium reagents to cross-coupling with aryl bromides catalyzed by the catalytic system of 1 mol % Pd(OAc)2 and 2 mol % PCy3 was studied. Results show that aryltris(2-propoxo)titanium compounds are excellent reagents for aryl−aryl coupling reactions at room temperature and that the system works well for a wide range of aryl bromides regardless of the electronic or steric nature of the substituents on the aryl bromides. Couplings with pyridyl bromides under the mild reaction condition of room temperature were also demonstrated to afford products in high yields over 2−3 h. It is worth noting that couplings with 3,3′-dibromo-2,2′-dimethoxy-1,1′-binaphthylene proceeded smoothly also at room temperature over 3−5 h, affording products in excellent yields. The coupling reactions demonstrated in this study are highly efficient compared to the typical synthesis of 3,3′-diaryl-2,2′-dimethoxy-1,1′-binaphthylenes via Suzuki couplings, which required higher catalyst loading under reaction conditions of longer reaction times (18−24 h in general) at elevated temperatures.
