POTASSIUM CARBONATE 18-CROWN-6 1
O K2CO3
Potassium Carbonate 18-Crown-6
18-crown-6
Cl
+ (2)
Ph
MeCN
40 °C
EtO2C
O CO2Et
89%
Ph
O O
O
K2CO3 +
O O
O K2CO3
18-crown-6
Ph CO2Et
ClCH2CO2Et + PhCHO (3)
MeCN
(K2CO3) O
130 °C
[584-08-7] CK2O3 (MW 138.21)
72%
InChI = 1/CH2O3.2K/c2-1(3)4;;/h(H2,2,3,4);;/q;2*+1/p-2/fCO3
.2K/q-2;2m
In addition to the Darzens reaction, it is also possible to prepare
InChIKey = BWHMMNNQKKPAPP-VPFIEVHZCO
dibromocarbene, ethers by the Williamson synthesis, phospho-
(18-crown-6)
nates by Michaelis Becker alkylation, and alkenes by the Wit-
[17455-13-9] C12H24O6 (MW 264.36)
tig reaction. In this last process, the conditions are sufficiently
InChI = 1/C12H24O6/c1-2-14-5-6-16-9-10-18-12-11-17-8-7-15-
mild that solvent effects can be utilized to control product stereo-
4-3-13-1/h1-12H2
chemistry.3 When nonstabilized ylides are involved, the use of
InChIKey = XEZNGIUYQVAUSS-UHFFFAOYAP
THF leads predominantly to the cis-alkenic isomer in a manner
(basic reagent combination that operates efficiently under cat- akin to  salt-free conditions (eq 4). In CH2Cl2 solution the prod-
alytic two-phase conditions, yet is sufficiently gentle not to hy- uct distribution is reversed. If the ylide is stabilized, either solvent
drolyze or otherwise destroy sensitive products;1,2 capable of leads to the trans-alkene.
promoting trans-alkene formation during Wittig condensations;3
heightens selectivity and reactivity of carbanions in nonpolar
+
PhCHO + Ph3PCH2Me Br + (4)
solvents4)
Ph
Ph
Physical Data: prepared in situ.
Solubility: while 18-crown-6 is soluble in aprotic solvents, potas- K2CO3, 18-crown-6, THF, ", 96% 85:15
22:78
K2CO3, 18-crown-6, CH2Cl2, ", 93%
sium carbonate exhibits very limited solubility. As a conse-
quence, it is believed that proton abstraction occurs on the
surface of the solid carbonate.1 For this reason, it is partic-
Dendritic polymers having an average molecular weight in ex-
ularly important to recognize that K2CO3 is a finely pow-
cess of 100 000 have been produced by initiating polycondensation
dered material that exhibits no tendency to aggregate and form
with the title reagent system.5
lumps.
Handling, Storage, and Precautions: this reagent combination
Methylation and Silylation Reactions. Neat dimethyl car-
eliminates the need to prepare anhydrous solvents and is con-
bonate can supplant Dimethyl Sulfate as methylating agent when
venient because the prior preparation of carbanions is unnec-
reaction is promoted by potassium carbonate and 18-crown-6
essary. Furthermore, hazardous and expensive ingredients are
(eq 5).6 The same catalyst system appears ideally suited to the
not involved.
methylation of carboxylic acids and phenols with methyl trichloro-
acetate (eqs 6 and 7).7,8 Since the byproducts are CHCl3 and
CO2, product isolation is conveniently realized. Primary and al-
lylic trichloroacetates can be substituted with equal success. An
Efficiency as a Base. Phase-transfer catalysis of reactions by
added major advantage of this technology is its lack of dependence
aqueous hydroxide in an inert organic medium is often disad-
on stoichiometric base.
vantaged by competing saponification if the starting materials are
prone to hydrolysis. To regain efficiency, it is advantageous to turn
instead to a liquid solid two-phase system constituted of Potas- Ph Ph
N O N
sium Carbonate and an aprotic solvent, such as CH2Cl2 or ben-
K2CO3
+ (5)
zene, to which 18-Crown-6 is added to facilitate transport of the
Ph Ph
MeO OMe 18-crown-6
N N
conjugate base into the organic phase. Under these conditions,
100 °C
H Me
91%
Diethyl Malonate readily undergoes highly selective monoalky-
lation (eq 1).1 Other notably sensitive esters respond well to this
rather mild base (eqs 2 and 3),1 despite the fact that somewhat
O
O
K2CO3
higher temperatures are sometimes required.
+
OH
MeO CCl3 18-crown-6
90 150 °C
K2CO3
CO2Et CO2Et
100%
18-crown-6
Br
+ (1)
MeCN O
CO2Et CO2Et
90 °C
(6)
94% OMe
Avoid Skin Contact with All Reagents
2 POTASSIUM CARBONATE 18-CROWN-6
O O
K2CO3
+ O
Cl OH
MeO CCl3 18-crown-6
K2CO3
150 °C
P(OMe)2 H
90%
18-crown-6
O O
THF, 40 °C
92% (12)
Cl OMe (7)
O O
P(OMe)2
O
O
O
O O K2CO3
The companion reagent trimethylsilyl trichloroacetate has been
TBDMSO
developed into a convenient tool for the  salt-free silylation of TBDMSO
O 18-crown-6
OMe
toluene, 70 °C
OMe
compounds carrying an acidic hydrogen (carboxylic acids, phe-
80%
nols, amides, thiols, terminal alkynes, etc). As before, a catalytic
O
amount of potassium carbonate/18-crown-6 gives rise to CO2 and
CHCl3 in addition to product (eq 8).9 When aldehydes or ketones
are involved, trimethylsilyl trichloromethyl carbinols are formed
efficiently (eq 9). (13)
O
O O
O O
O OTMS
TBDMSO
OMe
O
O OTBDMS
K2CO3
OMe
+ (8)
O NH O N
Cl3C OTMS 18-crown-6
100 °C
90%
Formation of Carbonate Esters. Historically, it has not been
O practical to prepare organic carbonates from inorganic carbonate.
OTMS
K2CO3
H3O+
O + However, the advent of solid liquid phase techniques has opened
Cl3C OTMS 18-crown-6
CCl3
the door to striking developments in this area. A catalyst other
100 °C
90% than potassium carbonate is necessary for acceleration and the
realization of high-yield conversions. Suitable candidates are Tri-
OH
butylchlorostannane,13 hexabutyldistannoxane,13 hexabutyldis-
(9)
tannathiane (eq 14),13 potassium hydrogen carbonate,14 and qua-
CCl3
ternary ammonium salts.15 Polycarbonates have similarly been
produced from Ä…,Ä… -dibromo-p-xylene.13
O
Intramolecular Wadsworth Emmons Cyclizations.
(Bu3Sn)2S
Attempts to prepare bicyclo[3.3.0]oct-1-en-3-one and derivatives Br
+ K2CO3 (14)
O O
Br
18-crown-6
thereof by intramolecular cyclization have been fraught with
MeCN, 80 °C
complications because of the sensitivity of both the starting
93%
material and product to basic conditions. This problem has
been resolved by making recourse to potassium carbonate Cyclic carbonates are likewise readily available via the reaction
ć%
and 18-crown-6 in benzene or toluene at 60 C (eq 10).2 The of oxiranes with Carbon Dioxide under phase-transfer catalysis of
gentle nature of this reagent has proven highly serviceable in a potassium carbonate/18-crown-6 (eq 15).16 Homopolymerization
number of related ring closures (eqs 11 and 12),10,11 including is not a serious complication as long as the crown ether is present.
macrocyclizations (eq 13).12
Cl
O
K2CO3
+ CO2
Cl O O (15)
O
K2CO3
18-crown-6
O O (10)
120 °C
O
18-crown-6
98%
P(OMe)2 C6H6, 60 °C
O
66%
O
P(OMe)2
K2CO3
1. Fedorynski, M.; Wojciechowski, K.; Matacz, Z.; Makosza, M., J. Org.
O
O
18-crown-6
Chem. 1978, 43, 4682.
toluene, 75 °C
2. Aristoff, P. A., Synth. Commun. 1983, 13, 145.
THPO
OTHP 65%
3. Boden, R. M., Synthesis 1975, 784.
O
4. Renga, J. M.; Wang, P.-C., Synth. Commun. 1984, 14, 69.
5. (a) Uhrich, K. E.; Hawker, C. J.; Fréchet, J. M. J.; Turner, S. R.,
Macromolecules 1992, 25, 4583. (b) Spindler, R.; Fréchet, J. M. J., J.
(11)
H
Chem. Soc., Perkin Trans. 1 1993, 913.
6. Lissel, M., Liebigs Ann. Chem. 1987, 77.
THPO
OTHP
7. Renga, J. M.; Wang, P.-C., Synth. Commun. 1984, 14, 77.
A list of General Abbreviations appears on the front Endpapers
POTASSIUM CARBONATE 18-CROWN-6 3
8. Renga, J. M.; Wang, P.-C., Synth. Commun. 1984, 14, 69. 13. Fujinami, T.; Sato, S.; Sakai, S., Chem. Lett. 1981, 749.
9. Renga, J. M.; Wang, P.-C., Tetrahedron Lett. 1985, 26, 1175. 14. Lissel, M.; Dehmlow, E. V., Chem. Ber. 1981, 114, 1210.
10. Aristoff, P. A., J. Org. Chem. 1981, 46, 1954. 15. Cella, J. A.; Bacon, S. W., J. Org. Chem. 1984, 49, 1122.
11. Dauben, W. G.; Walker, D. M., Tetrahedron Lett. 1982, 23, 16. Rokicki, G.; Kuran, W.; Pogorzelska-Marciniak, B., Monatsh. Chem.
711. 1984, 115, 205.
12. Nicolaou, K. C.; Seitz, S. P.; Pavia, M. R., J. Am. Chem. Soc. 1982, 104,
2030.
Leo A. Paquette
The Ohio State University, Columbus, OH, USA
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