MERCURY 1
Mercury ened and shifted when the mercury is dissolved. Perfect matching
of the emitter and absorber in the vapor phase, where both are
vapor-phase mercury atoms, leads to efficient energy transfer in
Hg
the vapor; energy transfer is probably poor for dissolved Hg. The
use of a high-pressure mercury lamp (or even allowing the tem-
[7439-97-6] Hg (MW 200.59)
perature of the lamp to become abnormally high) causes the selec-
InChI = 1/Hg tivity of the reaction to change sharply for the worse, presumably
InChIKey = QSHDDOUJBYECFT-UHFFFAOYAU because absorption can now take place in the liquid phase and
so condensation no longer protects the condensate from further
(photosensitizer for C H bond cleavage reactions;1-6 solvent for
reaction.
metals to give amalgams which can act as selective reducing
The absorption coefficient for mercury vapor is so large that all
agents;8-19 heterogeneous catalyst poison, especially for Group
the light is absorbed within a few micrometers of the inside surface
10 metal catalysts;22 electrode material in electrosynthesis24)
of the reactor and so the bulk of the vapor is protected from irradia-
tion. This means that  normal organic photochemistry is severely
ć% ć%
Physical Data: mp -38.87 C; bp 356.6 C; d 13.6 g cm-3.
suppressed; essentially all the energy is absorbed by the mercury
Solubility: sparingly soluble in organic solvents and water.
atoms, leaving none to excite the much more weakly absorbing
Form Supplied in: silvery liquid; widely available; electronic
organic species. Absorption of the 254 nm line is responsible for
grade (foreign metals d"1 ppm) and ACS grade (99.9995%) are
the chemistry described here, and so the excited state responsible
available, normally making purification unnecessary.
3
is the P1 state of mercury. The quartz glassware is transparent to
Handling, Storage, and Precautions: as a volatile heavy metal,
the 254 nm line, allowing for efficient transmission to the reaction
care must be taken to prevent long term inhalation of the va-
zone.
por. Carrying out reactions in a fume hood, cleaning up spills,
A useful modification for certain substrates is the use of a reac-
and assuring that the air handling system of the laboratory is
tive atmosphere. In this case, the temperature is typically adjusted
operating normally is generally sufficient.
so that the vapor pressure of the substrate is 100 Torr. This is nec-
essary so that the reactive gas has a substantial partial pressure
(P) in the reaction zone; in the case where the substrate has a
P(substrate) of 100 Torr, the P(reactive gas) would be 660 Torr.
Original Commentary
The two most useful reactive gases are H2 (Hg*/H2 conditions)4
Robert H. Crabtree
and NH3 (Hg*/NH3 conditions).5 The reason for moving to these
Yale University, New Haven, CT, USA
reactive gases is that the standard conditions, with reflux under N2
(Hg* conditions), completely fail when the Hg* attacks a func-
Mercury Photosensitization. This allows the dehydrodimer- tional group in the substrate. This typically happens for substrates
ization of a variety of volatile organic molecules on a 1 to 50 g
having multiple bonds or N lone pairs. Under Hg*/H2 or Hg*/NH3
scale without a solvent in a simple apparatus which can be put to- conditions, H atoms take over from Hg* as the active species, and
gether from standard laboratory photochemical equipment.1-5 A
a much wider range of substrates are reactive. For example, NEt3
254 nm low-pressure mercury lamp is used, together with quartz
fails to react at all under Hg* conditions, presumably because Hg*
glassware. The reactive triplet excited state of mercury is usually
attacks the N lone pair, and energy transfer leads to thermal ex-
designated Hg*. The reaction happens in the vapor phase at rt and
citation of the substrate but not to productive chemistry. Under
pressure, but no special precautions have to be taken; the normal
Hg*/H2 conditions the compound undergoes dehydrodimeriza-
vapor pressure of the substrate is usually enough to replenish the
tion at the C H bond Ä… to N. Arenes seem to work best under Hg*
vapor above the substrate when reaction has taken place. Mercury
conditions, probably because H atom addition leads to undesired
is supplied in the form of a small mercury drop in the substrate.
and unselective partial saturation of the aromatic ring. In practice
Normal laboratory glassware can contain sufficient adsorbed mer- there is little point in attempting to predict what will happen with
cury so that Hg photosensitized reactions can occur. To obtain a
a given substrate because it is easy to try Hg* conditions first and
valid control experiment in the absence of mercury, it is some- then move to Hg*/H2 conditions if the results are unsatisfactory.
times necessary to anneal the glassware in a glassblower s oven.
In each case, the crude product is collected by removing the
This implies that care needs to be taken in interpreting the results
volatile starting material by rotary evaporation after the reaction
of what are ostensibly normal or nonmercury photosensitized re- is over. The main limitation is that the substrate be volatile, but
actions where special care to remove trace mercury may not have
compounds with up to 16 nonhydrogen atoms have been success-
been taken.
fully dimerized. The reaction proceeds under reduced pressure
The liquid phase is unreactive and this has important effects on
if this is useful to vaporize the substrate. Another useful modifi-
the selectivity of the reaction. Dehydrodimerization or function- cation is to use steam distillation to bring the substrate into the
alization always leads to the condensation of the product, which
vapor phase; in this variant, water is added to the substrate and
protects it from further reaction and so the substrate only un- the mixture is refluxed. The weakest C H (or X H) bond in the
dergoes one homolysis even if there are several C H bonds of
molecule is homolyzed and the resulting C-centered (or, in gen-
similar strength. To take a simple example, cyclohexane dimer- eral, X-centered) radicals recombine. That part of the radical pool
izes to bicyclohexyl with no further oligomerization. The origin
which disproportionates instead of recombining does not in gen-
of the nonreactivity of the liquid phase is probably that the very
eral lead to lower chemical yield because the H atoms present add
narrow atomic absorption line for vapor-phase mercury is broad- to the alkene disproportionation product and re-form the initial
Avoid Skin Contact with All Reagents
2 MERCURY
radical. Quantum yields of 0.04 0.8 are usual and the majority of amalgam) and to dilute the active metal (and so moderate its ther-
substrates have values in the range 0.2 0.6. Chemical yields are modynamic reducing potential), which can improve the selectivity
good to excellent (40 98%). Conversions depend on photolysis of the reduction.
time.
The great advantage of the method is that it allows a num- Sodium Amalgam. Sodium Amalgam is a liquid up to 1%,
ber of difficult synthetic transformations to be carried out in one semisolid at 1.2%, and a pulverizable solid at higher concentra-
step. The synthesis of a few simple compounds that are otherwise tions, except in a narrow range around 40% Na, where the material
ć%
very difficult to make is shown below. In the diamine synthesis, is a low melting (< 30 C) solid. These materials can be made from
Hg*/NH3 conditions gave the best results (eq 1). elemental Na and Hg (caution: much heat is evolved) and analyzed
by titration with acid.8 Na/Hg is useful for the reduction of Ä…,²-
unsaturated carboxylic acids to the saturated forms, and for the
NH2
Hg*/NH3
Emde degradation of a quaternary amine (eq 5).9 The reduction
Me2CHNH2 (1)
of aldonolactones to aldoses with Na/Hg is a key transformation
NH2
in sugar chemistry.10 Oximes are readily reduced to amines.11
Na/Hg
A variant, hydrodimerization of alkenes, takes place under
ArNMe3Cl ArNMe2 (5)
Hg*/H2 conditions (eq 2). The H atoms add to the terminal carbon
of the alkene to give the intermediate radical shown.
Aluminum Amalgam. Aluminum Amalgam, readily
Hg*/H2
prepared12 by treating base-etched elemental aluminum
RfCF=CF2 RfCF(CHF2) RfCF(CHF2)CF(CHF2)Rf (2)
with aqueous mercury(II) salts, is a useful replacement for
Na/Hg when the compound to be reduced is base sensitive.
Diethyl oxaloacetate can be reduced to diethyl malate (70 80%
Another useful feature is the facility with which two different
yield) (eq 6) and aryl ketones can be reduced to the corresponding
substrates cross dimerize (eq 3).
pinacols (30 60% yield) (eq 7) in this way.12 Desulfurization of
disulfides is also possible.13
R1H + R2H R1R1 + R1R2 + R2R2 (3)
Al/Hg
CO2Et CO2Et
(6)
EtO2C EtO2C
In suitable cases, the volatility or polarity differences among
O OH
the three allow easy separation by distillation or chromatography
(eq 4).
OH
Al/Hg R
Ar
(7)
ArCOR
Ar
O CHO
Hg* H+ R
O
OH
+ (4)
O O
O O
Dienes undergo what is effectively a 1,4-addition of H2 to give
the monoenes, and cumulenes undergo a 1,2-reduction.14 The C S
The intermediate radicals can undergo rearrangement in special
bond in Ä…,²-unsaturated phenyl sulfones can be hydrogenolyzed
cases, as in the case of hexenyl radicals which cyclize. Trapping the
stereospecifically to give the alkene in excellent yield.15 Net hy-
intermediate radicals with CO, SO2, and O2 has proved possible,
drogenolysis of a P=C bond is involved in the sequence shown in
giving aldehydes, ketones, sulfonic acids, and hydroperoxides.6 8, in which an acyl halide is converted to a keto ester.16a
eq
Since methane has strong C H bonds, it only reacts well under
Hg*/NH3 conditions to give CH2=NH as product.5a Arenes do
COR COR
R3P=CH(CO2Et) Al/Hg, H+
not undergo cleavage of the strong aryl C H bonds but benzylic
(8)
RCOCl R3P
C H bonds of side chain alkyl groups can be cleaved under Hg*
CO2Et CO2Et
conditions; neither Hg*/H2 nor Hg*/NH3 conditions seem to be
useful for arenes, however, probably because H atoms readily add
In a recent synthesis of mannostatin A, King and Ganem have
to arene rings to give a complex mixture of products.
shown how the N O bond of a cyclic acyl-nitroso compound can
be hydrogenolyzed by Al/Hg (eq 9).16b
Reduction. Ultrasonically dispersed mercury reduces Ä…,Ä… -
dibromo ketones to an intermediate that is believed to be a mercu-
NHCOR
rated 2-oxyallyl species which gives a 4-methylene-1,3-dioxolane
MeS
1. Al/Hg
with acetone.7
(9)
SMe
COR
N
2. Ac2O
O
Amalgams. This is a traditional and well-established appli-
OAc
cation of Hg0. Metallic mercury readily forms amalgams with
most metals but Na/Hg, Al/Hg, and Zn/Hg are the most useful in
organic chemistry. The mercury serves both to keep the surface Zinc Amalgam. The classic use of Zinc Amalgam is the
of the metal clean (because inorganic salts adhere poorly to the Clemmensen reduction of ArCOR to ArCH2R.17 Variants of this
A list of General Abbreviations appears on the front Endpapers
MERCURY 3
method have proved successful for specific substrates.18 The ni- Hg(0) helps distinguish homogeneous from heterogeneous
troalkene closure shown in eq 10 is a more recent application of catalysis.25 Mercury electrodes are common inert electrodes in
Zn/Hg.19 electrochemical reductions.26
In thioglycosides protected as the O-sulfonate ester, the
sulfonyl group is easily removed with sodium amalgam in
OH
2-propanol.27
H
O2N
N
Zn/Hg, HCl
(10)
1. Brown, S. H.; Crabtree, R. H., Tetrahedron Lett. 1987, 28, 5599.
2. Ferguson, R. R.; Boojamra, C. G.; Brown, S. H.; Crabtree, R. H.,
Heterocycles 1989, 28, 121.
3. (a) Brown, S. H.; Crabtree, R. H., J. Am. Chem. Soc. 1989, 111, 2935.
Ultrasound20 and Rieke21 methods are increasingly being used
(b) Brown, S. H.; Crabtree, R. H., J. Am. Chem. Soc. 1989, 111, 2946.
as an alternative to Hg amalgamation for activating metals, a
4. Muedas, C. A.; Ferguson, R. R.; Brown, S. H.; Crabtree, R. H., J. Am.
trend encouraged by disposal problems of mercury-contaminated
Chem. Soc. 1991, 113, 2233.
wastes.
5. (a) Michos, D.; Sassano, C. A.; Krajnik, P.; Crabtree, R. H., Angew.
Chem., Int. Ed. Engl. 1993, 32, 1491. (b) Krajnik, P.; Ferguson, R. R.;
Crabtree, R. H., Nouv. J. Chim. 1993, 17, 559. (c) Krajnik, P.; Michos,
Catalyst Poison. Mercury selectively poisons heterogeneous
D.; Crabtree, R. H., Nouv. J. Chim. 1993, 17, 805.
catalysts, particularly of the platinum group metals (PGM). This
6. Ferguson, R. R.; Crabtree, R. H., J. Org. Chem. 1991, 56, 5503.
can be useful when a homogeneous PGM catalyst decomposes
7. Fry, A. J.; Ginsburg, G. S.; Parente, R. A., J. Chem. Soc., Chem. Commun.
with time to give the free metal; in such a case, Hg0 can suppress
1978, 1040.
the heterogeneous component of the reaction.22 This can improve
8. Fieser & Fieser 1967, 1, 1033.
selectivity or give mechanistic information about which products
9. Emde, H., Justus Liebigs Ann. Chem. 1912, 391, 88.
are attributable to which pathway.
10. Fischer, E., Chem. Ber. 1890, 23, 930.
11. Hochstein, F. A.; Wright, G. F., J. Am. Chem. Soc. 1949, 71, 2257.
Potential Route to Organomercury Compounds from Hg0.
12. (a) Wislicenus, H.; Kaufmann, L., Chem. Ber. 1895, 28, 1323.
Organomercury compounds are synthetically accessible23 from
(b) Newman, M. S., J. Org. Chem. 1961, 26, 582.
metallic mercury by a number of routes, including reaction of
13. Johnson, J. R.; Buchanan, J. B., J. Am. Chem. Soc. 1953, 75, 2103.
elemental mercury with alkenes in acid medium and with acyl
14. Kuhn, R.; Fischer, H., Chem. Ber. 1961, 94, 3060.
and alkyl halides (under thermal or photochemical conditions).
15. (a) Pascali, V.; Umani-Ronchi, A., J. Chem. Soc., Chem. Commun. 1973,
Organic synthetic applications of this chemistry seem to be very
351. (b) Mukaiyama, T.; Narasaka, K.; Maekawa, K.; Furusato, M., Bull.
rare, however.
Chem. Soc. Jpn. 1971, 44, 2285.
16. (a) Cooke, M. P., Jr., J. Org. Chem. 1982, 47, 4963. (b) King, S. B.;
Electrolysis at Mercury Cathodes. The high overvoltage of a
Ganem, B., J. Am. Chem. Soc. 1991, 113, 5089.
mercury surface in several electrochemical processes is often used
17. Staschewski, D., Angew. Chem. 1959, 71, 726.
to advantage; for example, proton reduction to H2 is kinetically
18. (a) Schwarz, R.; Hering, H., Org. Synth., Coll. Vol. 1963, 4, 203.
disfavored relative to electron transfer to an organic substrate. An
(b) Caesar, D., Org. Synth., Coll. Vol. 1963, 4, 695.
example of an organic electrochemical application is provided by
19. Yamada, F.; Makita, Y.; Suzuki, T.; Somei, M., Chem. Pharm. Bull. 1985,
reduction of a number of alkyl halides, RX, to the radical, R" ,
33, 2162.
which dimerizes to R2, disproportionates to RH and the corre-
20. (a) Erdik, E., Tetrahedron 1987, 43, 2203. (b) Kitazume, T.; Ishikawa,
sponding alkene, and also leads to the formation of R2Hg.24
N., Chem. Lett. 1981, 1679. (c) Han, B.-H.; Boudjouk, P., J. Org. Chem.
1982, 47, 5030.
21. Rieke, R. D.; Li, P. T-J.; Burns, T. P.; Uhm, S. T., J. Org. Chem. 1981,
46, 4323.
First Update
22. Anton, D. R.; Crabtree, R. H., Organometallics 1983, 2, 855.
23. Wardell, J. L. In Comprehensive Organometallic Chemistry; Wilkinson,
Robert H. Crabtree
G., Ed.; Pergamon:Oxford, 1982; Vol. 2, Chapter 17.
Yale University, New Haven, CT, USA
24. Mbarak, M. S.; Peters, D. G., J. Org. Chem. 1982, 47, 3397 and references
cited therein.
The rise of Green Chemistry has seen a decline in the use
25. Jansat, S.; Gomez, M.; Philippot, K.; Muller, G.; Guiu, E.; Claver, C.;
of heavy metals, especially mercury. Alternative reductants to
Castillon, S.; Chaudret, B., J. Am. Chem. Soc. 2004, 126, 1592.
amalgams are now readily available for most purposes (e.g.,
26. Vanalabhpatana, P.; Peters, D. G., Tetrahedron Lett. 2003, 44, 3245.
ultrasound20 and Rieke21 metals), although mercury photosen-
27. Crich, D.; Picione, J., Org. Lett. 2003, 5, 781.
sitization has few alternatives.
Avoid Skin Contact with All Reagents