Magnesium-assisted intramolecular demethylation utilizing carborane C–H geometry

A novel type of demethylation reaction was designed in the Pd-catalyzed coupling reaction of iodocarboranes with several Grignard reagents, CH3OPhMgBr. 3-Iodo-o-carborane 1 reacted with 2-CH3OPhMgBr to afford the corresponding phenol compound 4b in 78% yield. However, when compound 1 was reacted with the other Grignard reagents, the corresponding methoxyl compounds 5a and 6a were obtained in excellent yields. 2-Iodo-p-carborane 3 reacted with 2-CH3OPhMgBr to afford the corresponding phenol 8b in 50% yield and the methoxyl compound 8a in 41% yield. The carborane C–H geometry, which can form an intramolecular C–H⋯O hydrogen bonding, seems to be an important factor in the demethylation process. To examine the mechanism of the demethylation, compounds 1 and 4a were treated with CH3MgBr and quenched with D2O. While the two C–Hs of compound 1 were completely deuterated, compound 4a showed a replacement of one C–H with C–D. Therefore, we propose a mechanism involving intramolecular C–Mg⋯O interaction instead of intramolecular C–H⋯O interaction, via the generation of 3-iodo-o-carboranyl(MgBr)2, 11. Since it is also possible to replace the C–Hs with various metals other than Mg, new applications of carboranes in coordination and metal catalyst chemistry can be expected.

Graphical abstractA novel type of demethylation reaction accompanies the Pd-catalyzed coupling reaction of 1 with 2-CH3OPhMgBr. A mechanism involving intramolecular C–Mg⋯O interaction is proposed.Figure optionsDownload full-size imageDownload as PowerPoint slide