The outcomes of manganese-mediated coupling reactions depend on both the structure of the substrate(s) and the reaction conditions. This section describes the scope and limitations of inter- and intramolecular manganese-mediated radical coupling reactions and is organized according to the carbonyl compound employed as the substrate.
Intermolecular reactions between ketones/aldehydes and alkenes tend to result in low yields. In the absence of copper(II) acetate, hydrogen atom abstraction occurs, yielding saturated ketones or aldehydes.
When Cu(OAc) is present, further oxidation to carbocations followed by elimination takes place, leading to the formation of β,γ-unsaturated carbonyl compounds in moderate yields.
Aromatic compounds are also useful radical acceptors in manganese(III)-mediated coupling reactions. Furan reacts selectively at the α position to afford substituted products in high yield.
Lactonization of alkenes in the presence of acetic acid and acetate salts is a synthetically useful method for the synthesis of γ-lactones. Selectivity is high for the radical addition that leads to the more stable adduct radical, and trans lactones are selectively formed from either cis or trans acyclic alkenes.
β-Dicarbonyl compounds are useful substrates for the formation of dihydrofurans. Copper(II) acetate is not necessary in this case because of the high resonance stabilization of the intermediate diphenylmethyl radical.
When alkenes or carbonyl compounds containing pendant unsaturated moiety are treated with manganese(III) acetate, tandem intramolecular cyclization reactions may occur. Generally, exo cyclization of terminal double bonds is favored, as shown in equation (10). A variety of substitution patterns may be employed for this transformation, and yields are generally higher than intermolecular coupling reactions.
The stereochemical course of tandem reactions can be understood in some cases by invoking a chairlike transition state with as many substituents as possible in pseudoequatorial positions; however, a number of examples exhibiting unpredictable stereochemistry are known.
Nitriles are useful as radical acceptors in tandem cyclizations. Hydrolysis of the resulting imine leads to polycyclic ketones in moderate yields with good stereoselectivity.