Mechanism of Lithium Diisopropylamide-Mediated Substitution of 2,6-Difluoropyridine
摘要:
Treatment of 2,6-difluoropyridine with lithium diisopropylamide in THF solution at -78 degrees C effects ortholithiation quantitatively. Warming the solution to 0 degrees C converts the aryllithium to 2-fluoro-6-(diisopropylamino)pyridine. Rate studies reveal evidence of a reversal of the ortholithiation and a subsequent 1,2-addition via two monomer-based pathways of stoichiometries [ArH center dot i-Pr2NLi(THF)]double dagger and [ArH center dot iPr(2)NLi(THF)(3)]double dagger. Computational studies fill in the structural details and provide evidence of a direct substitution without the intermediacy of a Meisenheimer complex.
作者:Lekha Gupta、Alexander C. Hoepker、Kanwal J. Singh、David B. Collum
DOI:10.1021/jo802713y
日期:2009.3.6
Ortholithiations of a range of arenes mediated by lithium dilsopropylamide (LDA) in THF at -78 degrees C reveal substantial accelerations by as little as 0.5 mol % of LiCl (relative to LDA). Substrate dependencies suggest a specific range of reactivity within which the LiCl catalysis is optimal. Standard protocols with unpurified commercial samples of n-butyllithium to prepare LDA or commercially available LDA show marked batch-dependent rates-up to 100-fold-that could prove significant to the unwary practitioner. Other lithium salts elicit more modest accelerations. The mechanism is not discussed.