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.