[105870-26-6] · C3H9LiMn · Lithium Trimethylmanganate · (MW 107.00)
Solubility: sol THF; sol ether if prepared from MnI2.
Preparative Methods: can be prepared as an ether solution from MnI2 and Methyllithium, or as a THF solution from Manganese(II) Chloride or Li2MnCl4 and MeLi. For instance, to prepare Me3MnLi in THF from MeLi, charge a dry, N2-purged round-bottom flask equipped with magnetic stirring bar, N2 inlet, and septum stopper with 1.0 equiv of high purity, (>99%) dried (2 h at 180 °C) MnCl2 and add sufficient dry, O2-free THF to form a ca. 0.2 M slurry. Cool the slurry to -10 to 0 °C and add 3.0 equiv of a solution of MeLi dropwise. After the addition is complete, stir the reaction at room temperature for 30 min. A brownish-red to clear yellow solution results and can be used directly.7
Handling, Storage, and Precautions: best results are obtained with high purity, dry, O2-free solvents and manganese salts. MeLi is pyrophoric;8 due care must be exercised in its handling. The reagent has greater thermodynamic stability when prepared in THF than in ether.
Originally prepared from MnI2 and MeLi in ether,9 the formulation of the reagent as Me3MnLi is a convenience only and this may not be the reactive species in solution. There is considerable evidence that, over a wide range of reagent ratios, the reaction equilibrium favors formation of Li2[MnMe3]4; [MnMe3]- species form only as intermediates.10
Trialkylmanganates are sufficiently nucleophilic to react with good electrophiles, such as phenyl isocyanate, in good yield (eq 1).3 Conjugate additions to 2-alkenones are more problematic: relatively rare b-reductive dimerization5 (eq 2) is sometimes observed in addition to b-alkylation11 (eq 2). Dimerization is, however, substrate specific;7 it may be possible to improve yields using catalytic or stoichiometric amounts of CuI salts.12
Although cuprate reagents generally are found to be superior in nucleophilic substitution reactions with haloalkanes, trialkylmanganates provide the same products, usually in good yields.6 Primary iodoalkanes, allylic halides (eq 3), and vinylic halides are satisfactory substrates; secondary and tertiary haloalkanes are not. Cross-coupling reactions between trialkylmanganates and enol phosphates are catalyzed by Tetrakis(triphenylphosphine)palladium(0); Li2MnCl4 catalyzes similar coupling between Grignard reagents and enol triflates, presumably via formation of an R3MnLi species.13
Creighton University, Omaha, NE, USA