Preparation of Polyfunctional Arylmagnesium, Arylzinc, and Benzylic Zinc Reagents by Using Magnesium in the Presence of LiCl

• Published in: Chemistry : a European journal Vol. 15; no. 29; pp. 7192 - 7202
• Main Authors: Piller, Fabian M., Metzger, Albrecht, Schade, Matthias A., Haag, Benjamin A., Gavryushin, Andrei, Knochel, Paul
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 20-07-2009; WILEY‐VCH Verlag
• Subjects: Grignard reaction; heterocycles; insertion; regioselectivity; zinc
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200900575

The presence of LiCl considerably facilitates the insertion of magnesium into various aromatic and heterocyclic bromides. Several functional groups, such as ‐OBoc, ‐OTs, ‐Cl, ‐F, ‐CF3, ‐OMe, ‐NMe2, and ‐N2NR2, are well tolerated. The presence of a cyano group leads in some cases to competitive reduction of the organic halide to the corresponding ArH compound. The presence of sensitive groups such as methyl or ethyl ester is tolerated upon in situ trapping of the intermediate magnesium reagent with ZnCl2. This method can also be applied to the preparation of functionalized benzylic zinc reagents from benzylic chlorides. In the case of di‐ or tribromoaryl derivatives, directing groups such as ‐OPiv, ‐OTs, ‐N2NR2, or ‐OAc orient the zinc insertion (Zn/LiCl) to the ortho‐position, while the reaction with Mg/LiCl or Mg/LiCl/ZnCl2 leads to regioselective insertion into the para‐carbon–bromine bond. Large‐scale experiments (20–100 mmol) for all of the metalation procedures are described.
Metal‐driven: The preparation of functionalized zinc and magnesium reagents is reported. LiCl catalyzes the insertion of Mg and Zn into various aromatic and heterocyclic bromides (see scheme). Several functional groups, such as ‐OBoc, ‐OTs, ‐Cl, ‐F, ‐CF3, ‐OMe, ‐OPiv, ‐OAc, ‐NMe2, and ‐N2NR2, are well tolerated under these new mild conditions. Zn/LiCl and Mg/LiCl lead to orthogonal regioselectivities for insertions into polybrominated arenes. Large‐scale reactions have been performed.