1,4-Dioxane Adducts of Grignard Reagents: Synthesis, Ether Fragmentation Reactions, and Structural Diversity of Grignard Reagent/1,4-Dioxane Complexes

The 1,4-dioxane precipitation method was employed to obtain solutions of R₂Mg from Grignard reagents via precipitation of polymeric magnesium dihalides as dioxane adducts. The addition of a slight excess of dioxane resulted in partial cleavage of the initially formed [(μ-diox)MgR₂]_∞ polymer into smaller fragments of the type [(diox)_n+1(MgR₂)_n]. In addition to the polymeric compounds [(μ-diox)MgR₂]_∞, as found for [(μ-diox-O,O’)Mg(cyclo-C₆H₁₁)₂]_∞ (1), other oligomers such as [(diox)₃(Mg{CH₂Ph}₂)₂] (2) can be obtained from such solutions. In the system MesMgBr/1,4-dioxane the compounds [(μ-diox)MgBr₂]_∞ and [(μ-diox)Mg(Mes)₂]_∞ are both sparingly soluble, offering the opportunity to remove most of these compounds from the reaction mixture and leaving the products of ether cleavage reactions as the major species in solution. Using this procedure, the new ether degradation products [(diox)Mg(Mes)(μ-OEt)]₂ (3) and [(EtOCH(Me)CH(Me)OEt)Mg(Mes)₂] (4) were isolated. Excess dioxane yields mononuclear [(diox)₂Mg(Mes)₂] (5). Exchange of the thf coligands in closely related [(thf)₂Mg(Mes)₂] for other Lewis bases such as 2,2′-bipyridine (bpy) and 2,4,6-trimethylphenyl (Mes) anions allowed the preparation of [(bpy)Mg(Mes)₂] (6) and [(thf)₄Li]⁺[Mg(Mes)₃]⁻ (7). Molecular structures of all new compounds 2−7 are reported.