Magnesium Amalgam

Mg(Hg)

[37237-15-3]  · Mg  · Magnesium Amalgam

(reduction of metal halides, particularly for the synthesis of organometallic compounds; preparation of a reduced titanium species from TiCl4 that is useful for the reductive dimerization of ketones, aldehydes, their derived imines, and the chemoselective reduction of nitro groups; preparation of bis(bromomagnesio)methanes; reductive dimerization of germane derivatives; deoxygenation of epoxides)

Preparative Methods: prepared fresh, or generated in situ, from Magnesium metal and typically 1-3 mol % of a mercury(II) salt such as Mercury(II) Chloride in THF or aromatic hydrocarbon solvent.

Handling, Storage, and Precautions: mercury salts are toxic. Proper disposal is required.

Reductions and Reductive Dimerizations.

Mg(Hg) is the classical reagent for the reductive dimerization of acetone to form pinacol,1 and still finds occasional use in that regard.2,3 More recently, the pinacol coupling has been effected by the combination of Titanium(IV) Chloride and 2 equiv of Mg(Hg).4 For example, using the latter mixture, cyclohexanone is dimerized in 93% yield (eq 1). The intramolecular dimerization can be facile, as illustrated by the reductive cyclization of 2,5-hexanedione to the cis-cyclobutanediol (eq 2), and the intramolecular ketoaldehyde cross coupling illustrated in eq 3. Other reagents for pinacol-type coupling of carbonyl derivatives include a variety of reduced titanium reagents,5-7 Samarium(II) Iodide,8 Niobium Trichloride,9 and Aluminum Chloride-Zinc-Acetic Acid.10

The Mg(Hg)-TiCl4 mixture is also an effective reagent for the stereoselective reductive dimerization of aldimines to 1,2-diamines.11,12 For example (eq 4), treatment of the aldimine derived from benzaldehyde leads to a 80:20 mixture of syn (±) and anti (meso) isomers in 63% yield. Simple reduction of the aldimine competes with the dimerization. Similarly, the N-benzylimine derived from acetaldehyde affords a 90:10 syn:anti mixture in 67% yield. Other reagents for the reductive dimerization of imines include TiCl4-Mg,13 SmI2,14,15 Niobium(IV) Chloride,16 Indium,17 Ytterbium(0),18 and Zinc.19

Rivière and Satge20 reported the Mg(Hg)-mediated reductive dimerizations of organogermanes. For example (eq 5), treatment of phenylchlorogermane with Mg(Hg) (THF, 20 °C) affords the diphenyl-1,2-digermane in 75% yield. Similarly, diphenylchlorogermane affords tetraphenyl-1,1,2,2-digermane in 73% yield.

The Mg(Hg)-TiCl4 mixture is an effective reagent for the chemoselective reduction of the nitro group.21 For example (eq 6), chloro-, cyano-, and carboxy-substituted nitro aromatics are each reduced in high yield by the Mg(Hg)-TiCl4 mixture (THF-t-butanol, 0 °C, 1 h). Aliphatic nitro compounds are also efficiently reduced. Others have used this procedure successfully.22 -24 Other reagents for this transformation include Titanium(III) Chloride,25 TiCl4-Te(i-Bu)2,26 TiCl4-Sodium Borohydride,27 Palladium on Carbon-Ammonium Formate,28 Raney Nickel-Hydrazine,29 Chromium(II) Chloride,30 Tin(II) Chloride,31 and Nickel Boride (Ni2B).32

Mg(Hg) has been used occasionally for the reduction of 1,2-dihalides33 and enones,34 and has been extensively used for the preparation of reduced organometallic complexes.35-40

Preparation of 1,1-Diorganometallic Reagents.

Mg(Hg) has been widely used for the preparation of 1,1-dimagnesium reagents41 by reaction with methylene dibromides and diiodides. For example (eq 7), Cainelli and co-workers42 found that treatment of Dibromomethane or Diiodomethane with Mg(Hg) affords a solution of organomagnesium reagent. Bis(bromomagnesio)methane reacts with CO2 to afford the malonic acid and with aldehydes and ketones to give alkenes. This reagent has found use in organic synthesis43-47 and offers an alternative to Methylenetriphenylphosphorane, Trimethylsilylmethyllithium, Trimethylsilylmethylmagnesium Chloride, the Tebbe reagent (m-Chlorobis(cyclopentadienyl)(dimethylaluminum)-m-methylenetitanium), dimethyltitanocene (Bis(cyclopentadienyl)dimethyltitanium),48 CH2I2-CrCl2,49 CH2I2-Zn-Me3Al,50 CH2I2-Zn-TiCl4,51,52 and CH2(AlR2)2,53 reagents. Similarly, Mg(Hg) has been used for the preparation of the 1,1-dimagnesium species bis(bromomagnesio)trimethylsilylmethane54 and bis(bromomagnesio)bis(trimethylsilyl)methane.55

Bickelhaupt and co-workers56 reported the use of bis(bromomagnesio)methane as a reagent for the preparation of 1,3-dimetallacyclobutanes. For example (eq 8), treatment of bis(bromomagnesio)methane (prepared using Mg(Hg) as described above) with Cp2TiCl2 and Dichlorodimethylsilane affords the novel mixed 1,3-dimetallacyclobutane in quantitative yield.

Miscellaneous.

Mg(Hg) has been used in combination with Magnesium Bromide as a reagent for the deoxygenation of epoxides.57 Yields are typically modest, for example 50% for the formation of cyclohexene from cyclohexene oxide, and several good alternative reagents are available for this transformation.58


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James M. Takacs

University of Nebraska, Lincoln, NE, USA



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