POTASSIUM ON ALUMINA 1
Potassium on Alumina
H K/Al2O3
hexane
(1)
20 °C
K/Al2O3
100%
H
[7440-09-7] K (MW 39.10)
InChI = 1/K
InChIKey = ZLMJMSJWJFRBEC-UHFFFAOYAX
K/Al2O3
heptane
rt, 30 min
(catalyst for hydrogenations,1,7 double bond isomerizations,1 4
(2)
dehydrations, and skeletal rearrangements;2 4 metalating agent;5 100% by GC
effects reductive decyanation6)
Physical Data: blue powder if the metal content is in the range of
K/Al2O3 (5 10% w/w)
2 15%; at higher loading the reagent has a gray to black appear-
(3)
ance. The X-ray spectrum of a 14% K on Al2O3 reagent shows
no observable reflections due to potassium.6 A metal content
in the range of 2 15% usually leads to the highest catalytic
10% 90%
activity.1
Preparative Method: by adding potassium to thoroughly dried
basic or neutral alumina under argon with vigorous stirring
K/Al2O3
ć%
72 °C, 3 h
at temperatures >100 C until a homogeneous appearance is
(4)
reached. 96 98%
Handling, Storage, and Precautions: nonpyrophoric solid which
can be stored under argon for extended periods of time; must be
handled under inert atmosphere; catalytic activity may decrease
K/Al2O3
if impure or moist solvents are used; can be safely destroyed
(5)
rt, 5 min
by slowly adding isopropanol to a suspension of the reagent in
hexane with good stirring.
Macrocyclic 1,3-cycloalkadienes produced by isomerization of
1,n-cycloalkadienes are slowly reduced to cycloalkenes even in
the absence of external hydrogen.1 Residual water or  OH groups
on the alumina in combination with the adsorbed potassium may
Catalytic Activity K/Al2O3 is the most efficient among the
serve as the hydrogen source in this process. Under a hydrogen
series of alkali metals finely dispersed on alumina, although
atmosphere (1 atm), however, this selective hydrogenation of con-
Sodium Alumina (sometimes termed  high surface sodium )
jugated dienes by M/Al2O3 (M = Na, K) is considerably accel-
essentially effects the same types of transformations. In a model
erated, with no overreduction to the respective cycloalkane being
system, the following order of activity for the different alkali
observed.1
metals supported on alumina has been established: K e" Rb
K/Al2O3 exhibits a distinct propensity to catalyze transannular
Cs = Na.1a
reactions of unsaturated macrocyclic systems as well as skeletal
K/Al2O3 readily effects both configurational as well as posi- rearrangements leading to ring contraction, as shown with (+)-
tional alkene isomerizations with the following three trends being
longifolene as substrate.2
observed. Firstly, alkene groups are usually shifted (with few
exceptions) towards higher degrees of substitution (eq 1).2
Catalytic Cascades Six-membered rings bearing two alkene
Secondly, an alkene of accentuated conformational preference will
and/or cyclopropyl groups in the vicinity are smoothly aromatized
accumulate; this is evidenced, for example, by the formation of the
when exposed to M/Al2O3 (M = Na, K) as catalyst by a sequence
thermodynamically more stable (Z,Z)-cyclodeca-1,6-diene from
of double-bond isomerizations followed by dehydrogenation
(E,Z)-cyclodeca-1,5-diene (eq 2),1 as well as by the preponder- (eq 6).2,3
ance of (-)-aristolene in the equilibrium mixture obtained upon
treatment of (+)-calaren with K/Al2O3 (eq 3).2 Thirdly, the double
Na/Al2O3
bonds in 1,n-(cyclo)alkadienes are shifted towards conjugation in-
100 °C
dependent of their initial position in the starting material (eqs 4
 H2
and 5).1,2 Similarly, 1,2,4-trivinylcyclohexane quantitatively af-
H
fords 1,2,4-triethylbenzene in a highly exothermic process.3 Al-
i-Pr
though the mechanism responsible for such positional changes
is not yet elucidated, allyl anion intermediates are likely.1 This
picture is supported by the observation that successive treatment
++(6)
of Å‚-Alumina with Sodium Hydroxide and Na leads to a solid
superbase (pKb e" 37), which effects the same types of alkene
i-Pr i-Pr i-Pr
isomerizations via allyl anion species.8
20% 30% 50%
Avoid Skin Contact with All Reagents
2 POTASSIUM ON ALUMINA
In a one-pot procedure (Z,E,E)-cyclododeca-1,5,9-triene as CN K/Al2O3 (5 equiv)
hexane
substrate runs through a cascade of catalytic processes induced O O
rt, 15 min
by Na/Al2O3. This sequence comprises a transannular reaction,
80%
double bond isomerizations, and selective hydrogenation of the
conjugated diene produced in the presence of hydrogen. Final
ozonolysis of the crude reaction mixture afforded cyclododeca-
O O
(8)
1,7-dione in good yield (eq 7).4
Na/Al2O3 cat Na/Al2O3 cat
K/Al2O3 (5 equiv)
heptane H2 (1 atm)
hexane
Me(CH2)7 (CH2)7Me
", 4 h 5 10 h rt, 5 min
CN
70%
"
+ isomers
(9)
ozonolysis
Me(CH2)7 (CH2)7Me
(7)
+ isomers OO
1. (a) Hubert, A. J., J. Chem. Soc. (C) 1967, 2149. (b) Hubert, A. J.; Dale,
J., J. Chem. Soc. (C) 1968, 188. (c) Haag, W. O.; Pines, H., J. Am. Chem.
Soc. 1960, 82, 387. (d) Shabtai, J.; Gil-Av, E., J. Org. Chem. 1963, 28,
Organometallic Synthesis K/Al2O3 has been used as base to
2893.
metalate ketones, ethyl phenylacetate, alkyl nitriles, aldehyde-
2. Rienäcker, R.; Graefe, J., Angew. Chem., Int. Ed. Engl. 1985, 24, 320.
N,N-dimethylhydrazones, or N-cyclohexylketimines.5 However,
3. Ruckelshauss, G.; Kosswig, K., Chem.-Ztg. 1977, 101, 103.
an excess of the reagent was necessary and the yields reported
4. Alvik, T.; Dale, J., Acta Chem. Scand. 1971, 25, 1153.
for alkylation of the intermediate potassium carbanions were
moderate. In the case of alkyl nitriles as starting materials, the 5. Savoia, D.; Tagliavini, E.; Trombini, C.; Umani-Ronchi, A., J.
Organomet. Chem. 1981, 204, 281.
choice of solvent turned out to be decisive for the reaction path:
6. Savoia, D.; Tagliavini, E.; Trombini, C.; Umani-Ronchi, A., J. Org.
while deprotonation of these substrates predominates in THF, they
Chem. 1980, 45, 3227.
are readily decyanated when treated with K/Al2O3 in hexane as
7. Tazuma, J. J.; Zadra, M. D., J. Catal. 1978, 51, 435.
the reaction medium.6 Residual  OH groups on the alumina may
8. Suzukamo, G.; Fukao, M.; Minobe, M., Chem. Lett. 1987, 585.
be the proton sources in this reductive C C bond cleavage. While
9. Singh, S.; Dev, S., Tetrahedron 1993, 49, 10959.
the reaction leaves acetal groups and disubstituted alkene moi-
10. Scott, F.; van Heerden, F. R.; Raubenheimer, H. G., J. Chem. Res. (S)
eties in the substrates unaffected (eq 8), terminal double bonds
1994, 144.
are rearranged to internal ones during the decyanation process
11. (a) Stadtmüller, H.; Greve, B.; Lennick, K.; Chair, A.; Knochel, P.,
(eq 9).6 Recently, Na/Al2O3 has been used as reducing agent for
Synthesis 1995, 69. (b) Fürstner, A.; Seidel, G., Synthesis 1995, 63.
ketones, esters, and oximes.9 It also serves as a catalyst for the
Tischenko coupling of benzaldehyde to benzyl benzoate,10 and
Alois Fürstner
may be employed for preparing activated zinc and titanium
Max-Planck-Institut für Kohlenforschung, Mülheim,
samples.11
Germany
A list of General Abbreviations appears on the front Endpapers