On the mechanism of arene-catalyzed lithiation: the role of arene dianions--naphthalene radical anion versus naphthalene dianion.
The use of lithium and a catalytic amount of an arene is a well-established methodology for the preparation of organolithium reagents that manifest greater reactivity than the classical lithium-arene solutions. In order to rationalize this conduct, the participation of a highly reduced species, the dianion, is proposed and its reactivity explored. Studies of kinetics and of distribution of products reveal that the electron-transfer (ET) reactivity profile of dilithium naphthalenide in its reaction with organic chlorides excludes alternative mechanisms of halogen-lithium exchange. The process generates organolithium compounds. The dianion thus emerges along with the radical anion as a suitable candidate for catalytic cycles in certain processes. Endowed with a higher redox potential than its radical anion counterpart, dilithium naphthalene displays a broader spectrum of reactivity and so increases the range of substrates suitable for lithiation. The reaction of dilithium naphthalene with THF is one example of the divergent reactivity of the radical anion and the dianion, which has been the source of apparent misinterpretation of results in the past and has now been appropriately addressed.
Yus
Miguel
M
Departamento de Química Orgánica Facultad de Ciencias Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain. yus@ua.es
Herrera
Raquel P
RP
Guijarro
Albert
A