Novel tridentate diamino organomanganese(II) complexes as homogeneous catalysts in manganese(II)/copper(I) catalyzed carbon-carbon bond forming reactions

The new, paramagnetic arylmanganese(II) complex Li[MnCl₂(NCN)] (2, NCN [C₆H₃(CH₂NMe₂)₂-2,6] ^-) has been obtained in high yield from the reaction of MnCl₂ and [Li(NCN)]₂ in a 2:1 molar ratio. In THF solution, 2 is likely an ionic species [Li(THF)_n] [MnCl₂(NCN)] (molecular weight determination and conductivity measurements), while magnetic measurements indicate that a high spin d⁵ manganese(II) center is present. Subsequent reaction of 2 with RLi afforded [MnR(NCN)] (R=Me (3a), n-Bu (3b)). Complex 2, using CuCl as a co-catalyst, is an effective catalyst system for cross-coupling of Grignard reagents with alkyl bromides and the 1,4-addition of organomagnesium halides to α,β-unsaturated ketones. No further additives or co-solvents are necessary. For both reactions a dramatic decrease in reaction times is observed when compared to standard manganese/copper systems. Alkyl bromides with unsaturated or heteroatom functionalities can be cross-coupled. Also, excellent reactivity towards normally unreactive β,β-disubstituted ketones has been observed in the 1,4-addition reaction.