MAGNESIUM AMALGAM 1
Magnesium Amalgam 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-
Mg(Hg)
ization of imines include TiCl4 Mg,13. SmI2,14,15 Niobium(IV)
Chloride,16 Indium,17 Ytterbium(0),18 and Zinc.19
[37237-15-3]
NHR2 NHR2
R2
InChI = 1/Mg
Mg(Hg) TiCl4
N
R1 R1
+ (4)
InChIKey = FYYHWMGAXLPEAU-UHFFFAOYAI
R1 R1
THF, 0 �C, 12 h
R1 H
NHR2 NHR2
(reduction of metal halides, particularly for the synthesis of
syn (ą) anti (meso)
(a) R = Ph, R2 = Me (63%)
organometallic compounds; preparation of a reduced titanium
(b) R1 = Me, R2 = CH2Ph (67%)
species from TiCl4 that is useful for the reductive dimerization of
ketones, aldehydes, their derived imines, and the chemoselective
RiviŁre and Satge20 reported the Mg(Hg)-mediated reductive
reduction of nitro groups; preparation of bis(bromomagnesio)-
dimerizations of organogermanes. For example (eq 5), treatment
methanes; reductive dimerization of germane derivatives; deoxy-
ć%
of phenylchlorogermane with Mg(Hg) (THF, 20 C) affords the
genation of epoxides)
diphenyl-1,2-digermane in 75% yield. Similarly, diphenylchloro-
Preparative Methods: prepared fresh, or generated in situ, from
germane affords tetraphenyl-1,1,2,2-digermane in 73% yield.
Magnesium metal and typically 1 3 mol % of a mercury(II) salt
Ph Ph Ph
Mg(Hg)
such as Mercury(II) Chloride in THF or aromatic hydrocarbon
(5)
R Ge Cl R Ge Ge R
solvent.
THF, 20 60 �C
H H H
Handling, Storage, and Precautions: mercury salts are toxic.
R = H (75%), Ph (73%)
Proper disposal is required.
The Mg(Hg) TiCl4 mixture is an effective reagent for the
chemoselective reduction of the nitro group.21 For example
Reductions and Reductive Dimerizations. Mg(Hg) is the
(eq 6), chloro-, cyano-, and carboxy-substituted nitro aromat-
classical reagent for the reductive dimerization of acetone to form
ics are each reduced in high yield by the Mg(Hg) TiCl4 mix-
pinacol,1 and still finds occasional use in that regard.2,3 More re-
ć%
ture (THF t-butanol, 0 C, 1 h). Aliphatic nitro compounds
cently, the pinacol coupling has been effected by the combination
are also efficiently reduced. Others have used this procedure
of Titanium(IV) Chloride and 2 equiv of Mg(Hg).4 For exam-
successfully.22 24 Other reagents for this transformation include
ple, using the latter mixture, cyclohexanone is dimerized in 93%
Titanium(III) Chloride,25 TiCl4 Te(i-Bu)2,26 TiCl4 Sodium
yield (eq 1). The intramolecular dimerization can be facile, as il-
Borohydride,27 Palladium on Carbon Ammonium Formate,28
lustrated by the reductive cyclization of 2,5-hexanedione to the
Raney Nickel Hydrazine,29 Chromium(II) Chloride,30 Tin(II)
cis-cyclobutanediol (eq 2), and the intramolecular ketoaldehyde
Chloride,31 and Nickel Boride (Ni2B).32
cross coupling illustrated in eq 3. Other reagents for pinacol-type
coupling of carbonyl derivatives include a variety of reduced tita-
O2N H2N
Mg(Hg) TiCl4
nium reagents,5 7 Samarium(II) Iodide,8 Niobium Trichloride,9
(6)
THF, 0 �C, 12 h
and Aluminum Chloride Zinc Acetic Acid.10
R R
R = Cl (92%), CN (94%), C(O)OCH2CH=CH2 (96%)
O HO OH
Mg(Hg) TiCl4
(1)
THF, 0 �C, 0.5 h
Mg(Hg) has been used occasionally for the reduction of
93%
1,2-dihalides33 and enones,34 and has been extensively used for
the preparation of reduced organometallic complexes.35 40
Me
Preparation of 1,1-Diorganometallic Reagents. Mg(Hg) has
Mg(Hg) TiCl4
O OH
(2)
been widely used for the preparation of 1,1-dimagnesium
O THF, 0 �C, 2.5 h OH
81% reagents41 by reaction with methylene dibromides and diiodides.
Me
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(bromo-
H H
magnesio)methane reacts with CO2 to afford the malonic acid
Mg(Hg) TiCl4
(3)
and with aldehydes and ketones to give alkenes. This reagent
THF, 0 �C, 1.5 h
O OH
90% has found use in organic synthesis43 47 and offers an alternative
CHO
OH
to Methylenetriphenylphosphorane, Trimethylsilylmethyllith-
ium, Trimethylsilylmethylmagnesium Chloride, the Tebbe
The Mg(Hg) TiCl4 mixture is also an effective reagent reagent(�-Chlorobis(cyclopentadienyl)(dimethylaluminum)-�-
for the stereoselective reductive dimerization of aldimines to methylenetitanium), dimethyltitanocene (Bis(cyclopentadienyl)
1,2-diamines.11,12 For example (eq 4), treatment of the aldimine dimethyltitanium),48 CH2I2 CrCl2,49 CH2I2 Zn Me3Al,50
derived from benzaldehyde leads to a 80:20 mixture of syn (ą) CH2I2 Zn TiCl4,51,52 and CH2(AlR2)2,53 reagents. Similarly,
and anti (meso) isomers in 63% yield. Simple reduction of the 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)trimethylsilylmethane54 and bis 23. Sanchez, I. H.; Larraza, M. I.; Rojas, I.; Brena, F. K.; Flores, H. J.;
Jankowski, K., Heterocycles 1985, 23, 3033.
(bromomagnesio)bis(trimethylsilyl)methane.55
24. Pakusch, J.; Ruechardt, C., Chem. Ber. 1990, 123, 2147.
R1
X MgX 25. Ho, T.-L.; Wong, C. M., Synthesis 1974, 45.
Mg(Hg) R1C(O)R2
(7)
26. Suzuki, H.; Manabe, H.; Enokiya, R.; Hanazaki, Y., Chem. Lett. 1986,
X = Br, I
X MgX
R2
1339.
27. Kano, S.; Tanaka, Y.; Sugino, E.; Hibino, S., Synthesis 1980, 695.
R1 = C11H23, R2 =H (65%)
28. Ram, S.; Ehrenkaufer, R. E., Tetrahedron Lett. 1984, 25, 3415.
R1 = Ph, R2 = H (70%)
R1, R2 = -(CH2)5- (68%) 29. Yuste, F.; Saldana, M.; Walls, F., Tetrahedron Lett. 1982, 23, 147.
30. Varma, R. S.; Varma, M.; Kabalka, G. W., Tetrahedron Lett. 1985, 26,
3777.
Bickelhaupt and co-workers56 reported the use of bis(bro-
31. Bellamy, F. D.; Ou, K., Tetrahedron Lett. 1984, 25, 839.
momagnesio)methane as a reagent for the preparation of 1,3-
dimetallacyclobutanes. For example (eq 8), treatment of bis- 32. Ganem, B.; Osby, J. O., Chem. Rev. 1986, 86, 763.
33. Wiberg, K. B.; Ubersax, R. W., J. Org. Chem. 1972, 37, 3827.
(bromomagnesio)methane (prepared using Mg(Hg) as described
34. Celina, M.; Lazana, R. L. R.; Luisa, M.; Franco, T. M. B.; Herold, B. J.,
above) with Cp2TiCl2 and Dichlorodimethylsilane affords the
J. Chem. Soc., Faraday Trans. 1993, 1327.
novel mixed 1,3-dimetallacyclobutane in quantitative yield.
35. McVicker, G. B., Inorg. Synth 1976, 16, 56.
36. McVicker, G. B., Inorg. Chem. 1975, 14, 2087.
CH2MgBr
MgBr
Cp2TiCl2 Me2SiCl2
Cp2Ti
Cp2Ti SiMe2 (8) 37. Green, M. L. H.; Wong, L. L., J. Chem. Soc., Chem. Commun. 1989,
quantitative
571.
CH2MgBr
MgBr
38. Hill, A. F.; Honig, H. D.; Stone, F. G. A., J. Chem. Soc., Dalton Trans.
1988, 3031.
Miscellaneous. Mg(Hg) has been used in combination with
39. Morancais, J. L.; Hubert-Pfalzgraf, L. G., Transition Met. Chem. 1984,
9, 130.
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Trans. 1981, 2550.
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A list of General Abbreviations appears on the front Endpapers