Dimetalation: The Acidity of Monometalated Arenes Towards Superbasic Reagents

Twofold hydrogen/metal interconversions ("dimetalations") can be accomplished with “spiny” arenes (tert-butylbenzene, 1,4-di-tert-butylbenzene, 1,1,3,3-tetramethylindane and congeners) and N,N-crowded anilines (2,2,6,6-tetramethyl-1-phenylpiperidine and N,N-bis[triethylsilyl]aniline), albeit incompletely (average and optimum yields of trapping products 25% and 41%, respectively). All the evidence points at an aggregate phenomenon. Dimetalation occurs only with an excess of a highly concentrated butyllithium/potassium tert-butoxide mixture in hydrocarbon media. In tetrahydrofuran, no trace of a dimetalated species is found, although such species, once generated in a different manner, are moderately stable in this solvent at −75 °C. 1,4-Dimetalloarenes are favored over the 1,3-isomers; 1,2-isomers do not form at all. As found by competition kinetics, 2,2,6,6-tetramethyl-1-phenylpiperidine and N,N-bis(triethylsilyl)aniline undergo monometalation at roughly the same rate as tert-butylbenzene and much more slowly than benzene does.