MAGNESIUM AMALGAM

1

Magnesium Amalgam

Mg(Hg)

[37237-15-3]
InChI = 1/Mg
InChIKey = FYYHWMGAXLPEAU-UHFFFAOYAI

(reduction of metal halides, particularly for the synthesis of
organometallic compounds; preparation of a reduced titanium
species from TiCl

4

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; deoxy-

genation 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 re-

cently, the pinacol coupling has been effected by the combination
of Titanium(IV) Chloride and 2 equiv of Mg(Hg).

4

For exam-

ple, using the latter mixture, cyclohexanone is dimerized in 93%
yield (eq 1). The intramolecular dimerization can be facile, as il-
lustrated 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 tita-
nium reagents,

5

–

7

Samarium(II) Iodide,

8

Niobium Trichloride,

9

and Aluminum Chloride–Zinc–Acetic Acid.

10

O

HO OH

(1)

THF, 0 °C, 0.5 h

93%

Mg(Hg)–TiCl

4

O

O

Me

Me

OH

OH

(2)

THF, 0 °C, 2.5 h

81%

Mg(Hg)–TiCl

4

H

O

CHO

H

OH

OH

(3)

THF, 0 °C, 1.5 h

90%

Mg(Hg)–TiCl

4

The Mg(Hg)–TiCl

4

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-ben-
zylimine derived from acetaldehyde affords a 90:10 syn:anti
mixture in 67% yield. Other reagents for the reductive dimer-
ization of imines include TiCl

4

–Mg,

13

. SmI

2

,

14,15

Niobium(IV)

Chloride,

16

Indium,

17

Ytterbium(0),

18

and Zinc.

19

(4)

N

H

R

1

R

1

R

2

R

1

NHR

2

NHR

2

NHR

2

NHR

2

R

1

R

1

+

syn

(±)

anti

(meso)

(a) R = Ph, R

2

= Me (63%)

(b) R

1

= Me, R

2

= CH

2

Ph (67%)

THF, 0 °C, 12 h

Mg(Hg)–TiCl

4

Rivière and Satge

20

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, diphenylchloro-
germane affords tetraphenyl-1,1,2,2-digermane in 73% yield.

R Ge

H

Ph

Cl

R

H

Ph

R

Ge

H

Ph

Ge

(5)

R = H (75%), Ph (73%)

THF, 20–60 °C

Mg(Hg)

The Mg(Hg)–TiCl

4

mixture is an effective reagent for the

chemoselective reduction of the nitro group.

21

For example

(eq 6), chloro-, cyano-, and carboxy-substituted nitro aromat-
ics are each reduced in high yield by the Mg(Hg)–TiCl

4

mix-

ture (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

TiCl

4

–Te(i-Bu)

2

,

26

TiCl

4

–Sodium

Borohydride,

27

Palladium on Carbon–Ammonium Formate,

28

Raney Nickel–Hydrazine,

29

Chromium(II) Chloride,

30

Tin(II)

Chloride,

31

and Nickel Boride (Ni

2

B).

32

O

2

N

R

H

2

N

R

(6)

R = Cl (92%), CN (94%), C(O)OCH

2

CH=CH

2

(96%)

THF, 0 °C, 12 h

Mg(Hg)–TiCl

4

Mg(Hg) has been used occasionally for the reduction of

1,2-dihalides

33

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
reagents

41

by reaction with methylene dibromides and diiodides.

For example (eq 7), Cainelli and co-workers

42

found that

treatment of Dibromomethane or Diiodomethane with Mg(Hg)
affords a solution of organomagnesium reagent. Bis(bromo-
magnesio)methane reacts with CO

2

to afford the malonic acid

and with aldehydes and ketones to give alkenes. This reagent
has found use in organic synthesis

43

–

47

and offers an alternative

to Methylenetriphenylphosphorane, Trimethylsilylmethyllith-
ium, Trimethylsilylmethylmagnesium Chloride, the Tebbe
reagent(µ-Chlorobis(cyclopentadienyl)(dimethylaluminum)-µ-
methylenetitanium), dimethyltitanocene (Bis(cyclopentadienyl)
dimethyltitanium),

48

CH

2

I

2

–CrCl

2

,

49

CH

2

I

2

–Zn–Me

3

Al,

50

CH

2

I

2

–Zn–TiCl

4

,

51,52

and CH

2

(AlR

2

)

2

,

53

reagents. Similarly,

Mg(Hg) has been used for the preparation of the 1,1-dimagnesium

Avoid Skin Contact with All Reagents

2

MAGNESIUM AMALGAM

species bis(bromomagnesio)trimethylsilylmethane

54

and bis

(bromomagnesio)bis(trimethylsilyl)methane.

55

(7)

R

1

C(O)R

2

R

1

= C

11

H

23

, R

2

=H (65%)

R

1

= Ph, R

2

= H (70%)

R

1

, R

2

= -(CH

2

)

5

- (68%)

X

MgX

MgX

R

1

X

R

2

Mg(Hg)

X = Br, I

Bickelhaupt and co-workers

56

reported the use of bis(bro-

momagnesio)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 Cp

2

TiCl

2

and Dichlorodimethylsilane affords the

novel mixed 1,3-dimetallacyclobutane in quantitative yield.

(8)

Cp

2

TiCl

2

Cp

2

Ti

CH

2

MgBr

CH

2

MgBr

Cp

2

Ti

SiMe

2

MgBr

MgBr

Me

2

SiCl

2

quantitative

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, Lincoln, NE, USA

A list of General Abbreviations appears on the front Endpapers