Use of Magnesium Anthracene·3THF in Synthesis: Generation of Grignard Compounds and Other Reactions with Organic Halides

Abstract

The course (a), (b), (c) (Scheme 1) of the reaction of magnesium anthracene·3THF (1) with organic halides (RX) is dependent on the nature of RX. With alkyl halides in THF 1 reacts as a nucleophile, whereby primary as well as secondary alkyl halides produce dialkyldihydroanthracenes (4–4′′) and tertiary alkyl halides yield primarily monoalkyl‐substituted dihydroanthracenes (2, 2′). With bromo‐ and iodobenzene in THF 1 reacts predominantly as a radical with H atom abstraction from the solvent affording benzene and 9. The formation of Grignard compounds (5) and anthracene (6), originating from primary and secondary alkyl and aryl halides and 1 in toluene or ether at elevated temperatures, is not caused by the reaction of 1 but by the “active magnesium” (Mg*) formed by decomposition of 1 in these solvents. In contrast, allyl, propargyl, and benzyl halides react with 1 independently of the solvent under mild conditions to produce 5 and 6. Allyl‐ and the difficultly accessible allenylmagnesium chloride can be prepared in THF at –78 and 0°C, respectively, from the corresponding halides and ordinary Mg powder via catalytic amounts of 1.