Metalated 1‐(p‐Methoxybenzyl)pyrazole: A Structural Chameleon

Abstract

When treated with either butyllithium or lithium diisopropylamide, 1‐(p‐methoxybenzyl)pyrazole undergoes metalation at the exocyclic α‐position but mutates to the 5‐lithio species in the course of few minutes or hours. Trapping the intermediates with a rapidly reacting electrophile such as chlorotrimethysilane or carbon dioxide offers selective access to either of the two possible regioisomers. On the other hand, the relatively inert 1‐iodobutane and butyl trifluoromethanesulfonate (“triflate”) inevitably give rise to product mixtures, as the 5‐lithio species is more slowly intercepted than it can revert to the α‐metallomer by a rapid, though intermolecular, dynamic equilibration process. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Mixed AggregAte (MAA): A Single Concept for All Dipolar Organometallic Aggregates. 2. Syntheses and Reactivities of Homo/HeteroMAAs

“…Deprotonative metalation combined with metal hydride elimination, using 1:1 BuLi· t -PentOK (in excess) in hexane, has been used by Clark and Schleyer since 1976 to synthesize resonance-stabilized anions from cyclic olefins, dienes, and aromatics; 1:1 BuLi· t -BuOK (LIC-KOR) is also a suitable reagent for the deprotometalation of resonance-active hydrocarbons . Selective proton abstractions from benzylic positions were achieved for numerous substrates (Table ), ,,, the outcome of the reactions depending on the length of the alkyl chain and the substitution on the aromatic ring . An important role of π-arene/metal bonding in the reactivity of LIC-KOR was advanced in 2005 by Masson and Schlosser …”