Preparation and Reactivity of Organomanganese Compounds

Abstract

This chapter deals with the preparation of organomanganese reagents from organolithium or organomagnesium compounds (transmetallation) as well as from organic halides and manganese metal (oxidative addition). The influence of various parameters on the stability of organomanganese reagents is then discussed (nature of the reagent RMnX, R 2 Mn, R 3 MnLI or R 3 MnMgX; effect of ligands and solvents…). The second part of the chapter is devoted to the reactivity of these reagents and to their uses in organic synthesis. They are especially interesting for their chemoselectivity, as illustracted by the addition to aldehydes and ketones, the preparation of ketones by acylation or the copper‐catalyzed conjugate addition to enones, enals and enoates. Organomanganese reagents can also be used to perform various iron, palladium or nickel‐catalyzed cross‐coupling reactions. Manganese salts are efficient catalyst to achieve various homo‐ and heterocoupling reactions using Grignard reagents. They are also used to catalyze the addition of Grignard reagents to acetylenes, 1,3‐dienes and allenes. Finally, various radical cyclization reactions promoted by organomanganates are presented.

Direct Amidation of Carboxylic Acids with Nitroarenes

“…Nitrosoarene is then further reduced to form N -aryl hydroxylamine, and hydrogen iodide (NMP-H + I – ) generated from the activation of carboxylic acid likely behaves as the proton source to facilitate this process. Finally, nitrosoarene presumably reacts with acyloxyphosphonium iodide, acyl iodide, and acyl chloride in the presence of Mn and TMSCl/TMSI to form an N -(trimethylsilyl)­oxy amide species A , presumably via the oxidative addition of acyloxyphosphonium iodide and acyl halides to Mn to form acyl-Mn species [Figure , (iv)]. N -Aryl hydroxylamine also presumably reacts with acyloxyphosphonium iodide and acyl halides to form N -hydroxy amide species B [Figure , (iv)].…”

Optimization of Manganese Coupling Reaction for Kilogram-Scale Preparation of Two Aryl-1,3-dione Building Blocks

“…Therefore, MnCl 2 is involved in the formation of both the desired product and the reduction byproduct. Reduction byproduct 26 may be formed via β-hydrogen elimination of ethyl-manganese species and aryl bromide 25 reduction according the mechanism described by Cahiez . A vacant coordination site on manganese next to the ethyl substituent is needed for β-hydrogen elimination, and so the addition of coordinating reagents (such as ethylene, solvent molecule, or methyl Grignard) is presumably improving the reaction selectivity by blocking this site.…”

“…It was oxidized using selenium oxide in ethanol to give the corresponding dione 11. 4 Ring expansion using diazonium acetate, followed by decarboxylation, gave 2,2,6,6-tetramethyltetrahydropyrane-3,5-dione (12) in 50−60% yield. 5 Unfortunately, direct arylation of building block 12 using palladium catalyzed conditions 6 proved only moderately successful, probably due to high steric hindrance of both dione and aryl bromide coupling partners.…”

“…11 Furthermore, electron rich substrates such as 4-bromo-anisole were also shown to undergo the same transformation albeit with reduction predominating in reactions performed with Grignard reagents possessing β-hydrogens (compound 22 to 23 and 24, Scheme 4). 12 In an initial test reaction, addition of an excess of ethyl magnesium bromide to 25 without manganese catalyst gave only a trace amount of the desired product 18 (Table 1, Entry 1). The same reaction in the presence of anhydrous MnCl 2 (10 mol %) gave a clean reaction providing a 4:1−5:1 mixture of the desired coupling product 18 and byproduct 26 (Table 1, Entry 2).…”

“…Reduction byproduct 26 may be formed via β-hydrogen elimination of ethyl-manganese species and aryl bromide 25 reduction according the mechanism described by Cahiez. 12 A vacant coordination site on manganese next to the ethyl substituent is needed for β-hydrogen elimination, and so the addition of coordinating reagents (such as ethylene, solvent molecule, or methyl Grignard) is presumably improving the reaction selectivity by blocking this site. Despite significantly improving the reaction selectivity, we could not improve it further and challenges in separating compounds 18 and 26 led us to consider an alternative approach.…”