Magnesium Amide Bases and Amido-Grignards. 1. Ortho Magnesiation. (Reannouncement with New Availability Information).

摘要

Hauser bases (R2NMgBr) and magnesium diamides ((R2N)2Mg) are known, but their uses in organic synthesis have barely been explored. With this communication we introduce the first of our work with these bases and demonstrate their exceptional utility and bright future. Ortho lithiation of substituted aromatics has been developed elegantly into one of the major tools of organic synthesis. It has been extended to include lithiation of activated vinyl, allylic, and strained saturated systems, and its utility broadened by the introduction of transmetalation processes in which other metals, e.g., mercury, magnesium, and zinc, can be substituted for lithium after the initial lithiation step. To our knowledge, it has always been assumed that the initial metalation depended crucially, on the properties of lithium. To the contrary, as we show here, direct ortho magnesiation can be accomplished readily. 2,2,6,6-Tetramethylpiperidine (TMPH) on reaction in refluxing tetrahydrofuran with 1 equiv of dibutylmagnesium gave, respectively, clear solutions of 2,2,6,6-tetramethylp iperidinomagnesium bromide, TMPMgBr, or bis(2,2,6,6-tetramethyl piperidino)magnesium, (TMP)2Mg. For our purpose we prepared half-molar solutions. The reactions were complete within a few hours, as judged by the complete lack of propanoic acid or pentanoic acid in aliquots quenched with carbon dioxide followed by aqueous acid.28.