Chemistry of organomanganese(II) compounds.

Organomanganese(II) reagents. XV: Conjugate addition of organomanganese reagents to alkylidenemalonic esters and related compounds

A comprehensive experimental and computational mechanistic study of the highly enantioselective hydroboration of ketones catalyzed by a manganese(II) alkyl boxmi pincer complex is reported. The catalyst operates at low catalyst loadings (down to 0.01 mol %) under very mild conditions (typically −40 °C) and facilitates the reduction of both aryl alkyl and dialkyl ketones with excellent selectivity (up to >95%ee). Catalyst activation pathways were investigated, demonstrating that a manganese(II) hydride and a manganese(II) alkoxide species are part of the catalytic cycle and can be generated via σ-bond metathesis of the alkyl precursor with the borane or by alcoholysis. Extensive kinetic experiments based on a “one-pot sequential kinetic profiling” approach under various conditions in combination with kinetic simulations reveal that two catalytic cycles are effective with this earth-abundant base metal catalyst: (i) a minor MnH/borane-mediated insertion cycle, in which the subsequent, product-releasing meta...

One-Pot Sequential Kinetic Profiling of a Highly Reactive Manganese Catalyst for Ketone Hydroboration: Leveraging σ-Bond Metathesis via Alkoxide Exchange Steps.

Ligand effects in selective carbonyl addition reactions of organomanganese and cerium reagents

Reaction of {Li(THF)Ar′MnI2}2 (Ar′ = C6H3-2,6-(C6H2-2,6-iPr3)2) with LiAr′, LiCCR (R = tBu or Ph), or (C6H2-2,4,6-iPr3)MgBr(THF)2 afforded the diaryl MnAr′2 (1), the alkynyl salts Ar′Mn(CCtBu)4{Li(THF)}3 (2) and Ar′Mn(CCPh)3Li3(THF)(Et2O)2(μ3-I) (3), and the manganate salt {Li(THF)}Ar′Mn(μ-I)(C6H2-2,4,6-iPr3) (4), respectively. Complex 4 reacted with one equivalent of (C6H2-2,4,6-iPr3)MgBr(THF)2 to afford the homoleptic dimer {Mn(C6H2-2,4,6-iPr3)(μ-C6H2-2,4,6-iPr3)}2 (5), which resulted from the displacement of the bulkier Ar′ ligand in preference to the halogen. The reaction of the more crowded {Li(THF)Ar*MnI2}2 (Ar* = C6H3-2,6-(C6H2-2,4,6-iPr3)2) with LitBu gave complex Ar*MntBu (6). Complex 1 is a rare monomeric homoleptic two-coordinate diaryl Mn(II) complex; while 6 displays no tendency to eliminate β-hydrogens from the tBu group because of the stabilization supplied by Ar*. Compounds 2 and 3 have cubane frameworks, which are constructed from a manganese, three carbons from three acetylide ligands, three lithiums, each coordinated by a donor, plus either a carbon from a further acetylide ligand (2) or an iodide (3). The Mn(II) atom in 4 has an unusual distorted T-shaped geometry while the dimeric 5 features trigonal planar manganese coordination. The chloride substituted complex Li2(THF)3{Ar′MnCl2}2 (7), which has a structure very similar to that of {Li(THF)Ar′MnI2}2, was also prepared for use as a possible starting material. However, its generally lower solubility rendered it less useful than the iodo salt. Complexes 1–7 were characterized by X-ray crystallography and UV-vis spectroscopy. Magnetic studies of 2–4 and 6 showed that they have 3d5 high-spin configurations.

B. Figadere

Papers

Organomanganese (II) reagents XIII: Highly selective addition of organomanganese halides to aldehydes in the presence of ketones☆