PALLADIUM TRIETHYLAMINE FORMIC ACID 1
Palladium Triethylamine Formic Acid directly from o-nitrophenylacetic acid and o-nitrocinnamic acid,
respectively, in 72 75% yields.3 Some selectivity was observed in
the reduction of one nitro group in dinitro aromatic compounds.
Pd Et3N HCO2H
The best results were observed with 2,4-dinitrotoluene, in which
(Pd)
2-nitro-4-aminotoluene was obtained in 92% yield. The yields of
[7440-05-3] Pd (MW 106.42)
other dinitro compounds ranged from 24 to 77%.4
InChI = 1/Pd
Hydrogenolyses of aryl benzyl ethers and allylic acetates or
InChIKey = KDLHZDBZIXYQEI-UHFFFAOYAH
amines have been reported to give phenols and alkenes, res-
(Et3N)
pectively.5 Hydrogenolysis of aryl halides with triethylammo-
[64-18-6] C6H15N (MW 101.22)
nium formate in the presence of heterogeneous catalysts is an-
InChI = 1/C6H15N/c1-4-7(5-2)6-3/h4-6H2,1-3H3
other useful synthetic method. Some interesting selectivity was
InChIKey = ZMANZCXQSJIPKH-UHFFFAOYAU
observed when the heterogeneous catalysts were replaced with
(HCO2H)
homogeneous catalysts. For example, reduction of 1-nitro-3-
[121-44-8] CH2O2 (MW 46.03)
bromobenzene in the presence of 5% Pd/C gave a mixture of ni-
InChI = 1/CH2O2/c2-1-3/h1H,(H,2,3)/f/h2H
trobenzene (46%) and aniline (15%). Replacement of the 5% Pd/C
InChIKey = BDAGIHXWWSANSR-QEZKKOIZCL
with Palladium(II) Acetate and 2 equiv of tri-o-tolylphosphine
gave nitrobenzene (81%) and only 2% of aniline. Similarly, the
(transfer hydrogenation system used with a variety of palladium
yield of benzonitrile from 4-bromobenzonitrile was improved
catalysts for hydrogenation and hydrogenolysis reactions; hydro-
from 53 to 83% with the homogeneous catalyst.3
genation of alkynes, alkenes, nitro compounds; hydrogenolysis
The homogeneous catalytic system, on the other hand, is also
of N-benzyl and O-benzyl groups; hydrogenolysis of allyic esters
able to deoxygenate phenols via their aryl sulfonates (eq 1).6 By
and allylic carbonates)
varying the phosphine ligands, the phenol starting materials could
be regenerated and thus the sulfonyl group only acts as a protecting
Alternate Name: triethylammonium formate (TEAF).
group (eq 2). Similarly, aryl triflates are also hydrogenolyzed in
Physical Data: see Palladium on Carbon, Triethylamine, and
excellent yields (eq 3).7 Selective hydrogenolysis of the triflates
Formic Acid.
was realized in the presence of alkenes.
Solubility: heterogeneous catalyst system.
Allyl esters (eq 4) and carbonates (eq 5) are converted to alkenes
under the homogeneous catalysis reaction conditions.8 The least
substituted alkene is formed preferentially.
Hydrogenation and Hydrogenolysis. Triethylammonium
formate, prepared most frequently in situ from triethylamine and
O OSO2R Pd(OAc)2 O
formic acid, has been used in conjunction with both heterogeneous
dppp
TEAF
and homogeneous palladium catalysts for a variety of chemical
(1)
transformations. With heterogeneous palladium catalysts, such as 90%
palladium on charcoal, both hydrogenation and hydrogenolysis
O O
are observed. When used in combination with homogeneous cat-
alysts, hydrogenolysis is mainly observed. Many of these reactions
can also be carried out by replacing triethylammonium formate O OSO2R Pd(OAc)2 O OH
dppm
with Ammonium Formate or hydrogen. The use of a hydrogen
TEAF
transfer agent, such as triethylammonium formate, enables the (2)
95%
use of ordinary laboratory glassware and no special hydrogena-
tion equipment is required. O O
The following reactions have been carried out with heteroge-
neous palladium catalysts. The reduction of alkynes and conju-
TfO O O O O
gated enynes with Pd/C or Pd/CaCO3 catalyst was not stereose-
TEAF
(3)
lective and gave mixtures of (E)- and (Z)-dienyl esters along with
82%
substantial amounts of over-reduced byproducts. The yields of the
dienyl esters ranged from 59 to 84%.1 A combination of tri-
Pd(OAc)2
ethylammonium formate and Pd/C was used to reduce alkynes
OCHO
Bu3P
to alkenes or alkanes. Reduction of 3-hexyne was not selective,
TEAF
(4)
giving a mixture of 3-hexenes (70%) and hexanes (18%). Dipheny-
86%
OTHP OTHP
lacetylene, on the other hand, gave cis-stilbene (93%) with only
2% of bibenzyl. Selective reduction of the double bond in Ä…,²-
unsaturated carbonyl compounds, such as citral (91%), cro-
Pd(OAc)2
tonaldehyde (81%), 2-cyclopentenone (83%), methyl crotonate OCO2Me
Bu3P
(83%), and methyl cinnamate (86%), provided good yields of the
TEAF
(5)
carbonyl compounds.2
82%
The facile reduction of aromatic nitro compounds to anilines
t-Bu t-Bu
has been reported and oxindole and benzolactam can be formed
Avoid Skin Contact with All Reagents
2 PALLADIUM TRIETHYLAMINE FORMIC ACID
The Pd-catalyzed Carroll rearrangement of allyl ²-ketocar- 1. Weir, J. R.; Patel, B. A.; Heck, R. F., J. Org. Chem. 1980, 45, 4926.
boxylates to give Ä…-allyl ketones was realized in THF (eq 6).9 2. Cortese, N. A.; Heck, R. F., J. Org. Chem. 1978, 43, 3985.
3. Cortese, N. A.; Heck, R. F., J. Org. Chem. 1977, 42, 3491.
Pd(OAc)2
4. Terpko, M. O.; Heck, R. F., J. Org. Chem. 1980, 45, 4992.
Ph3P, THF
O TEAF
5. Krishnamurty, H. G.; Ghosh, S.; Sathyanarayana, S., Indian J. Chem.,
44% Sect. B 1986, 25B, 1253.
O O
6. Cabri, W.; Bernardinis, S. D.; Francalanci, F.; Penco, S., J. Org. Chem.
1990, 55, 350.
(6)
7. Cacchi, S.; Ciattini, P. G.; Morera, E.; Ortar, G., Tetrahedron Lett. 1986,
O
27, 5541.
Switching the solvent to MeCN provided Ä…,²-unsaturated 8. (a) Tsuji, J.; Yamakawa, T., Tetrahedron Lett. 1979, 613. (b) Mandai, T.;
Suzuki, S.; Murakami, T.; Fujita, M.; Kawada, M.; Tsuji, J., Tetrahedron
ketones instead (eq 7).10
Lett. 1992, 33, 2987.
Pd(OAc)2
O O
9. (a) Shimizu, I.; Yamada, T.; Tsuji, J., Tetrahedron Lett. 1980, 21, 3199.
O Ph3P, MeCN
(b) Tsuda, T.; Chujo, Y.; Nishi, S.; Tawara, K.; Saegusa, T., J. Am. Chem.
TEAF
(7) Soc. 1980, 102, 6381.
O
85%
10. Shimizu, I.; Tsuji, J., J. Am. Chem. Soc. 1982, 104, 5844.
11. Tsuji, J.; Nisar, M.; Shimizu, I., J. Org. Chem. 1985, 50, 3416.
Allyl ²-keto esters can also be hydrogenolyzed to form ketones
without allylation (eq 8).11
Anthony O. King & Ichiro Shinkai
Merck & Co., Inc., Rahway, NJ, USA
O
C5H11 Pd(OAc)2 O
Ph3P, THF
O
TEAF
C5H11 (8)
O 100%
A list of General Abbreviations appears on the front Endpapers