POTASSIUM CARBONATE–18-CROWN-6

1

Potassium Carbonate–18-Crown-6

O

O

O

O

O

O

K

2

CO

3

+

(K

2

CO

3

)

[584-08-7]

CK

2

O

3

(MW 138.21)

InChI = 1/CH2O3.2K/c2-1(3)4;;/h(H2,2,3,4);;/q;2*+1/p-2/fCO3

.2K/q-2;2m

InChIKey = BWHMMNNQKKPAPP-VPFIEVHZCO
(18-crown-6)
[17455-13-9]

C

12

H

24

O

6

(MW 264.36)

InChI = 1/C12H24O6/c1-2-14-5-6-16-9-10-18-12-11-17-8-7-15-

4-3-13-1/h1-12H2

InChIKey = XEZNGIUYQVAUSS-UHFFFAOYAP

(basic reagent combination that operates efficiently under cat-
alytic two-phase conditions, yet is sufficiently gentle not to hy-
drolyze or otherwise destroy sensitive products;

1,2

capable of

promoting trans-alkene formation during Wittig condensations;

3

heightens selectivity and reactivity of carbanions in nonpolar

solvents

4

)

Physical Data:

prepared in situ.

Solubility:

while 18-crown-6 is soluble in aprotic solvents, potas-

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-

ularly important to recognize that K

2

CO

3

is a finely pow-

dered material that exhibits no tendency to aggregate and form
lumps.

Handling, Storage, and Precautions:

this reagent combination

eliminates the need to prepare anhydrous solvents and is con-
venient because the prior preparation of carbanions is unnec-
essary. Furthermore, hazardous and expensive ingredients are
not involved.

Efficiency as a Base. Phase-transfer catalysis of reactions by

aqueous hydroxide in an inert organic medium is often disad-
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-
sium Carbonate and an aprotic solvent, such as CH

2

Cl

2

or ben-

zene, to which 18-Crown-6 is added to facilitate transport of the
conjugate base into the organic phase. Under these conditions,
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

higher temperatures are sometimes required.

CO

2

Et

CO

2

Et

Br

MeCN

90 °C

94%

CO

2

Et

CO

2

Et

(1)

+

K

2

CO

3

18-crown-6

Ph

Cl

O

K

2

CO

3

18-crown-6

(2)

+

CO

2

Et

EtO

2

C

O

Ph

MeCN

40 °C

89%

ClCH

2

CO

2

Et

O

K

2

CO

3

18-crown-6

(3)

Ph

CO

2

Et

+

PhCHO

MeCN

130 °C

72%

In addition to the Darzens reaction, it is also possible to prepare

dibromocarbene, ethers by the Williamson synthesis, phospho-
nates by Michaelis–Becker alkylation, and alkenes by the Wit-
tig reaction. In this last process, the conditions are sufficiently
mild that solvent effects can be utilized to control product stereo-
chemistry.

3

When nonstabilized ylides are involved, the use of

THF leads predominantly to the cis-alkenic isomer in a manner
akin to ‘salt-free’ conditions (eq 4). In CH

2

Cl

2

solution the prod-

uct distribution is reversed. If the ylide is stabilized, either solvent
leads to the trans-alkene.

+

+

Ph

PhCHO

+

Ph

3

PCH

2

Me Br

–

Ph

(4)

K

2

CO

3

, 18-crown-6, THF,

∆, 96%

K

2

CO

3

, 18-crown-6, CH

2

Cl

2

,

∆, 93%

85:15
22:78

Dendritic polymers having an average molecular weight in ex-

cess of 100 000 have been produced by initiating polycondensation
with the title reagent system.

5

Methylation and Silylation Reactions. Neat dimethyl car-

bonate can supplant Dimethyl Sulfate as methylating agent when
reaction is promoted by potassium carbonate and 18-crown-6
(eq 5).

6

The same catalyst system appears ideally suited to the

methylation of carboxylic acids and phenols with methyl trichloro-
acetate (eqs 6 and 7).

7,8

Since the byproducts are CHCl

3

and

CO

2

, product isolation is conveniently realized. Primary and al-

lylic trichloroacetates can be substituted with equal success. An
added major advantage of this technology is its lack of dependence
on stoichiometric base.

N

N
H

Ph

Ph

MeO

OMe

O

N

N
Me

Ph

Ph

(5)

+

18-crown-6

100 °C

91%

K

2

CO

3

MeO

CCl

3

O

(6)

+

18-crown-6

90–150 °C

100%

OH

O

OMe

O

K

2

CO

3

Avoid Skin Contact with All Reagents

2

POTASSIUM CARBONATE–18-CROWN-6

MeO

CCl

3

O

(7)

+

18-crown-6

150 °C

90%

Cl

OH

OMe

Cl

K

2

CO

3

The companion reagent trimethylsilyl trichloroacetate has been

developed into a convenient tool for the ‘salt-free’ silylation of
compounds carrying an acidic hydrogen (carboxylic acids, phe-
nols, amides, thiols, terminal alkynes, etc). As before, a catalytic
amount of potassium carbonate/18-crown-6 gives rise to CO

2

and

CHCl

3

in addition to product (eq 8).

9

When aldehydes or ketones

are involved, trimethylsilyl trichloromethyl carbinols are formed
efficiently (eq 9).

Cl

3

C

OTMS

O

(8)

NH

O

O

N

O

OTMS

+

18-crown-6

100 °C

90%

K

2

CO

3

Cl

3

C

OTMS

O

+

O

CCl

3

OTMS

CCl

3

OH

18-crown-6

100 °C

90%

(9)

H

3

O

+

K

2

CO

3

Intramolecular

Wadsworth–Emmons

Cyclizations.

Attempts to prepare bicyclo[3.3.0]oct-1-en-3-one and derivatives
thereof by intramolecular cyclization have been fraught with
complications because of the sensitivity of both the starting
material and product to basic conditions. This problem has
been resolved by making recourse to potassium carbonate
and 18-crown-6 in benzene or toluene at 60

◦

C (eq 10).

2

The

gentle nature of this reagent has proven highly serviceable in a
number of related ring closures (eqs 11 and 12),

10,11

including

macrocyclizations (eq 13).

12

18-crown-6

C

6

H

6

, 60 °C

66%

P(OMe)

2

O

O

O

O

(10)

K

2

CO

3

O

O

THPO

OTHP

THPO

OTHP

(11)

O

P(OMe)

2

O

H

18-crown-6

toluene, 75 °C

65%

K

2

CO

3

(12)

P(OMe)

2

O

O

O

O

H

18-crown-6
THF, 40 °C

92%

K

2

CO

3

O

P(OMe)

2

O

O

TBDMSO

OMe

OMe

O

TBDMSO

O

O

O

O

O

O

O

O

O

OTBDMS

O

O

OMe

TBDMSO

OMe

(13)

18-crown-6

toluene, 70 °C

80%

K

2

CO

3

Formation of Carbonate Esters. Historically, it has not been

practical to prepare organic carbonates from inorganic carbonate.
However, the advent of solid–liquid phase techniques has opened
the door to striking developments in this area. A catalyst other
than potassium carbonate is necessary for acceleration and the
realization of high-yield conversions. Suitable candidates are Tri-
butylchlorostannane,

13

hexabutyldistannoxane,

13

hexabutyldis-

tannathiane (eq 14),

13

potassium hydrogen carbonate,

14

and qua-

ternary ammonium salts.

15

Polycarbonates have similarly been

produced from α,α

′

-dibromo-p-xylene.

13

Br

Br

O

O

O

K

2

CO

3

(14)

18-crown-6

MeCN, 80 °C

93%

(Bu

3

Sn)

2

S

+

Cyclic carbonates are likewise readily available via the reaction

of oxiranes with Carbon Dioxide under phase-transfer catalysis of
potassium carbonate/18-crown-6 (eq 15).

16

Homopolymerization

is not a serious complication as long as the crown ether is present.

O

O

O

CO

2

18-crown-6

120 °C

98%

O

Cl

Cl

(15)

+

K

2

CO

3

1.

Fedorynski, M.; Wojciechowski, K.; Matacz, Z.; Makosza, M., J. Org.
Chem. 1978

, 43, 4682.

2.

Aristoff, P. A., Synth. Commun. 1983, 13, 145.

3.

Boden, R. M., Synthesis 1975, 784.

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.

Chem. Soc., Perkin Trans. 1 1993

, 913.

6.

Lissel, M., Liebigs Ann. Chem. 1987, 77.

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.

9.

Renga, J. M.; Wang, P.-C., Tetrahedron Lett. 1985, 26, 1175.

10.

Aristoff, P. A., J. Org. Chem. 1981, 46, 1954.

11.

Dauben, W. G.; Walker, D. M., Tetrahedron Lett. 1982, 23,
711.

12.

Nicolaou, K. C.; Seitz, S. P.; Pavia, M. R., J. Am. Chem. Soc. 1982, 104,
2030.

13.

Fujinami, T.; Sato, S.; Sakai, S., Chem. Lett. 1981, 749.

14.

Lissel, M.; Dehmlow, E. V., Chem. Ber. 1981, 114, 1210.

15.

Cella, J. A.; Bacon, S. W., J. Org. Chem. 1984, 49, 1122.

16.

Rokicki, G.; Kuran, W.; Pogorzelska-Marciniak, B., Monatsh. Chem.
1984, 115, 205.

Leo A. Paquette

The Ohio State University, Columbus, OH, USA

Avoid Skin Contact with All Reagents