Alkyleisen‐ und Alkylcobalt‐Reagenzien, III. Nichtstabilisierte Eisenalkyle: Bildung, Nachweis und Chemoselektivität

Abstract

Alkyliron and Alkylcobalt Reagents, III. — Nonstabilized Ironalkyls: Formation, Detection, and Chemoselectivity The crystalline ate complex Me 4 FeLi 2 (Et 2 O) 2 ( 1 d ) described in the literature as well as the complexes MeFeCl ( 1a ), Me 2 Fe ( 1b ), Me 3 FeLi ( 1c ), n Bu 2 Fe ( 2 a ), n Bu 4 FeLi 2 ( 2b ), n Oct 2 Fe ( 3a ), n Oct 3 FeLi ( 3b ), and n Oct 4 FeLi 2 ( 3c ) are prepared in situ by transmetallation of MeLi, n BuLi, and n OctLi with FeCl 2 (readily available by in situ reduction of FeCl 3 with MeLi). All these complexes have been subjected to reaction with organic substrates for the first time. Evidence for this transmetallation has been furnished by a novel test (referred to as “β‐bromostyrene‐ketone test”). In this test, involving addition of β‐bromostyrene ( 4 ) and 4‐methylpentan‐2‐one ( 5 ), the alkyllithium compounds alkylate selectively only the ketone and the alkyliron compounds almost exclusively the β‐bromostyrene. The high preference of the alkyliron reagents for the alkylation of the β‐bromostyryl residue has been utilized for regioselective butylations and octylations of 1‐{4‐[(Z)‐2‐bromoethenyl]phenyl}‐ethanone ( 6 ). Moreover, in competition reactions with benzaldehyde/4‐methylpentan‐2‐one the alkyliron reagents have been found to react usually specifically with aldehydes. The decomposition temperatures of the reagents 1a,b,c and 2b in THF have been determined to be 0, −10, 25, and −20°C, respectively.