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Abstract
The preparation of highly active manganese, Rieke Manganese (Mn*), by the direct reduction of anhydrous manganese halides using lithium in the presence of a catalytic amount of naphthalene in THF at room temperature is demonstrated. Other electron carriers and solvents tested are also described.
By the simple change of the reaction conditions, functionalized benzyl manganese halides or homocoupling products were obtained from the reaction of active manganese and benzyl halides at room temperature. The resulting benzylic manganese reagents was found to proceed cross-coupling reactions readily with good yields to prepare functionalized diarylmethane in the presence of palladium catalyst. This approach can be also applied for the preparation of bibenzyl lipid derivatives as well as substituted 1,4-dihydropyridines, and/or 4-benzylpyridines.
Functionalized benzyl phosphates were prepared and used for the preparation of organomanganese reagents. Importantly, active manganese readily undergoes oxidative addition to carbon-oxygen bond of benzyl phosphates. The subsequent coupling reactions of the resulting benzylic manganese phosphates with electrophiles gave the corresponding products in good yields at room temperature in the absence of any transition metal catalysts. This provides a new approach to benzylic reagents and obviates the need for the corresponding halide, which may be problematic.
A new class of organomanganese reagents, benzyl manganese phosphates was tested for diastereoselectivity to α-chiral benzaldehyde along with benzyl manganese tosylate and mesylate.
The heteroaryl manganese reagents were prepared via direct oxidative addition and the resulting manganese reagents were reacted with different electrophiles to give the expected coupling products in good yields under mild reaction conditions. It is noteworthy that many functional groups such as ester, cyano are compatible with the organomanganese reagents.
Cobalt-catalyzed cross-coupling reaction between vinyl bromides and functionalized pyridinylmanganese reagents allows for sp2-sp2 C-bond formation with retention of stereochemistry of the double bond. The reactions take place under a mild condition to afford the substituted olefins in moderate yields.
Manganese ester enolates were easily prepared for the Reformatsky reaction with α-haloesters, lactone and alkyl halides with remote ester groups. The resulting reagents demonstrated excellent addition reactions with electrophiles. Importantly, the reaction does not require any Lewis acid or any trapping agent to improve product yields. This approach offers an approach to synthesis of β-hydroxyesters in one-step procedure.
