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Synthesis, Structure, and Hydroamination Kinetics of (2,2‘-Diaryldipyrrolylmethane)- and Bis(2-arylpyrrolyl)titanium Complexes

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journal contribution
posted on 18.12.2006, 00:00 by Douglas L. Swartz, Aaron L. Odom
The trifluoroacetic acid catalyzed reaction of acetone and 2-arylpyrroles results in the formation of 2,9-diaryl-5,5-dimethyldipyrrolylmethanes. Ligands bearing 3,5-(CF3)2C6H3 (H2dmpm3,5-CF3) and mesityl (H2dmpmmes) arenes were prepared in 72% and 68% yields using this procedure. The new dipyrrolylmethanes react with Ti(NMe2)4 to form Ti(NMe2)2(dmpm3,5-CF3) and Ti(NMe2)2(dmpmmes) in 92% and 30% yields, respectively. The solid-state structures of these complexes are quite similar to that of the sterically smaller 5,5-dimethyldipyrrolylmethane complex Ti(NMe2)2(dmpm) with one η5- and one η1-pyrrolyl; however, the substituted derivatives display much lower barriers to pyrrolyl conformational exchange, as judged by VT 1H NMR spectroscopy. Also synthesized were bis(pyrrolyl) complexes without the methylene connector. Reactions of 2-arylpyrroles where the aryl group was 3,5-(CF3)2C6H3, mesityl, 4-(CF3)C6H4, and p-tolyl generated Ti(NMe2)2(pyrr3,5-CF3)2, Ti(NMe2)2(pyrrmes)2, Ti(NMe2)2(pyrr4-CF3)2, and Ti(NMe2)2(pyrrtol)2 in 38%, 74%, 45%, and 37% purified yields, respectively. Hydroamination catalysis rates under pseudo-first-order conditions were tested using aniline and 1-phenylpropyne at 75 °C. On the dmpm framework, 2-substitution in Ti(NMe2)2(dmpm3,5-CF3) and Ti(NMe2)2(dmpmmes) results in reduced pseudo-first-order rate constants of (780 ± 30) × 10-7 and (403 ± 80) × 10-7 s-1 relative to Ti(NMe2)2(dmpm) at (1976 ± 130) × 10-7 s-1, which is attributed to steric constraints near the substrate-binding pocket. Testing catalysis rates on the approximately isosteric complexes Ti(NMe2)2(pyrr4-CF3)2 and Ti(NMe2)2(pyrrtol)2 provided rate constants of (1255 ± 193) × 10-7 and (880 ± 20) × 10-7 s-1, demonstrating that electron-withdrawing groups increase catalytic activity in these systems. Ti(NMe2)2(dmpm3,5-CF3), Ti(NMe2)2(dmpmmes), Ti(NMe2)2(pyrr3,5-CF3)2, and Ti(NMe2)2(pyrrmes)2 were studied by single-crystal X-ray diffraction.