Pentacarbonylmethylmanganese1,2

(CO)5MnMe

[13601-24-6]  · C6H3MnO5  · Pentacarbonylmethylmanganese  · (MW 210.03)

(prototypical reagent for migratory insertion of CO into metal-alkyl bonds;3,4 can acylate alkenes and alkynes;2 can cyclometalate keto- and iminoarenes1,5)

Physical Data: colorless crystals; mp 95 °C; sublimes at 25-40 °C/0.1 mmHg.

Solubility: sol organic solvents.

Form Supplied in: not commercially available.

Analysis of Reagent Purity: IR.

Preparative Methods: (a) alkylation of Sodium Pentacarbonylmanganate under N2 atmosphere (eq 1).1,6,7 (b) decarbonylation of pentacarbonylacetylmanganese; not used on a preparative scale, but often the method of choice for synthesis of analogous alkyl- and arylmanganese complexes.

Purification: sublimation (25-40 °C/0.1 mmHg).

Handling, Storage, and Precautions: slightly light sensitive; may be handled in air, but decomposes upon prolonged exposure to oxygen. Indefinitely stable when stored as a solid in the absence of air at -20 °C. While no information concerning this compound is available, one should assume that metal carbonyls are highly toxic; since this is a volatile solid (vapor pressure ~2 mmHg at 20 °C),6 extra care should be taken to avoid exposure. Use in a fume hood.

Migratory Insertion Reactions: Alkyl Carbonylation.

(CO)5MnMe was employed in seminal studies elucidating the mechanism of migratory insertion reactions of alkyl ligands to coordinated CO.3,4,8-12 Stereochemical and labeling experiments demonstrated (a) that the methyl group migrates to an adjacent CO, opening a cis coordination site, and (b) that the coordinatively unsaturated intermediate is a configurationally stable square pyramid in the absence of a coordinating solvent.4,13 The process is faster in polar solvents than nonpolar ones, consistent with the formation of solvent-bound species,14,15 although recent flash photolysis experiments suggest the intermediacy of an h2-coordinated acyl moiety.16 The migratory insertion step is accelerated by Lewis17-20 or Brønsted21 acids. Thermochemical measurements have been reported,20 and the reaction has been subjected to theoretical analysis.19,22-26 The Mn-Me bond dissociation energy is 44.7 ± 1 kcal mol-1.27

Much of the synthetic chemistry of pentacarbonylmanganese alkyl compounds derives from the trapping of the coordinatively unsaturated acyl intermediate (CO)4Mn(COR) produced from migratory insertion. Addition of simple donor ligands such as phosphine or CO provide good yields of acyl complexes (eq 2). In addition to the processes outlined below, the following miscellaneous trapping agents have been reported (product type in parentheses): a W&tbond;C triple bond (bridging dinuclear acyl-carbyne complex),28 various metal hydrides (metal-metal bound dinuclear complexes,29 including those bearing an h1-coordinated aldehyde resulting from formylation of the starting alkyl);30 Ph2PSiMe3 (metallacyclic a-silyloxymanganese phosphide complex);31,32 and chiral and achiral germyl anions, R3GeLi, or Mn(CO)5- (cis-tetracarbonylacyl complexes bearing Mn-Ge or Mn-Mn bonds, leading to Fischer carbene compounds).33-35

Hydroacylation of Alkenes and Alkynes: Sequential CO/Alkene or CO/Alkyne Insertion.

Electron-deficient or strained alkenes are metalloacylated by exposure to (CO)5Mn-alkyl complexes under high pressure (2-10 kbar; eq 3).2,36-40 Alkynes undergo a similar reaction, sometimes at ambient pressure (eq 4).39,41,42 Advantages include: (1) no b-elimination is observed; (2) multiple insertion of alkenes and alkynes does not occur; (3) addition is regiospecific for terminal (R2 = H) or trimethylsilyl-substituted (R2 = SiMe3) alkynes;41 and (4) demetalation is accomplished under mild oxidative (replacing Mn with H), acidic (Mn -> H), or reductive conditions.42,43 Yields are low for sterically hindered alkenes. 55Mn NMR chemical shifts have been used to characterize the products and predict reactivity.40,42-44 The alkene insertion methodology has been used in the synthesis of spiroacetal systems present in natural products39,40 and in the elaboration of glycosyl halides.45

Cyclometalation.

Aromatic imines and ketones, when treated with (CO)5MnMe, are metalated ortho to the directing group with loss of methane and carbon monoxide (eq 5).5,41,46-48 The same types of cyclometalated ketone are available from (CO)5MnR and Diphenylmercury (R = alkyl).48 The products are stable in air and may be purified by column chromatography.

Methyl Transfer.

Reaction with the highly nucleophilic Fe(CO)42- dianion results in clean transfer of Me+ from manganese to iron, giving MeFe(CO)4- and Mn(CO)5-.49

Protonolysis.

Alkanes may be liberated from pentacarbonylmanganese alkyls by protonation with strong acids.50


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M. G. Finn

University of Virginia, Charlottesville, VA, USA



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