Dimetallation of Aromatic Compounds

Trimitsis and co-workers [1] reported that the formation of a,tx -disodium compounds of 1,2-, 1,3-, 1,6-, and 1,8-dimethylnaphthalene and of o- and m-xylene proceeds with quantitative yields, when the hydrocarbons are treated with butylsodium in the presence of TMEDA. 1,4-Dimethylnaphthalene and p-xylene could not be converted into the disodium compounds. Our attempts [2] to dimetallate o- and m-xylene with the much more easily available 1 1 1 molar mixture (2 equivalents) of BuLi, r-BuONa, and TMEDA in hexane gave mainly the mono-metallated derivatives. Using a 30mol% excess of the more strongly basic reagent BuLi t-BuOK TMEDA we obtained a 80 20 mixture of a,a -dimetallated and mono-a-metallated compound from m-xylene. 

The paper of Bates and Ogle [3] mentions the successful formation of the p-xylene dianion from p-xylene and the Lochmann-Schlosser base, BuLi r-BuOK, 

BuLi- -BuOK TMEDA, hexane BuLi-TMEDA, hexane 

Klein and co-workers [4] needed a large excess of BuLi-TMEDA for the a,a -dilithiation of m-xylene. In their experiments they obtained comparable amounts of mono-(a)-metallated product. Attempts to lithiate both CH3-groups in p-xylene failed the main products were the mono-lithiated and geminally di-lithiated derivatives. Treatment of o-xylene and mesitylene with an excess of BuLi-TMEDA was reported to give product mixtures, containing mono- and geminally di-lithiated compounds in addition to the desired a,a - or a,a, a -polylithiated derivatives. 

Summarizing, one may conclude that there is no simple and preparatively attractive method available for the generation of a,a -di-alkalimetal compounds from the isomeric xylenes and mesitylene. 

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