Palladium-Triethylamine-Formic Acid


[7440-05-3]  · Pd  · Palladium-Triethylamine-Formic Acid  · (MW 106.42) (Et3N)

[64-18-6]  · C6H15N  · Palladium-Triethylamine-Formic Acid  · (MW 101.22) (HCO2H)

[121-44-8]  · CH2O2  · Palladium-Triethylamine-Formic Acid  · (MW 46.03)

(transfer hydrogenation system used with a variety of palladium catalysts for hydrogenation and hydrogenolysis reactions; hydrogenation of alkynes, alkenes, nitro compounds; hydrogenolysis of N-benzyl and O-benzyl groups; hydrogenolysis of allyic esters and allylic carbonates)

Alternate Name: triethylammonium formate (TEAF).

Physical Data: see Palladium on Carbon, Triethylamine, and Formic Acid.

Solubility: heterogeneous catalyst system.

Hydrogenation and Hydrogenolysis.

Triethylammonium formate, prepared most frequently in situ from triethylamine and formic acid, has been used in conjunction with both heterogeneous and homogeneous palladium catalysts for a variety of chemical transformations. With heterogeneous palladium catalysts, such as palladium on charcoal, both hydrogenation and hydrogenolysis are observed. When used in combination with homogeneous catalysts, hydrogenolysis is mainly observed. Many of these reactions can also be carried out by replacing triethylammonium formate with Ammonium Formate or hydrogen. The use of a hydrogen transfer agent, such as triethylammonium formate, enables the use of ordinary laboratory glassware and no special hydrogenation equipment is required.

The following reactions have been carried out with heterogeneous palladium catalysts. The reduction of alkynes and conjugated enynes with Pd/C or Pd/CaCO3 catalyst was not stereoselective and gave mixtures of (E)- and (Z)-dienyl esters along with substantial amounts of over-reduced byproducts. The yields of the dienyl esters ranged from 59 to 84%.1 A combination of triethylammonium formate and Pd/C was used to reduce alkynes to alkenes or alkanes. Reduction of 3-hexyne was not selective, giving a mixture of 3-hexenes (70%) and hexanes (18%). Diphenylacetylene, on the other hand, gave cis-stilbene (93%) with only 2% of bibenzyl. Selective reduction of the double bond in a,b-unsaturated carbonyl compounds, such as citral (91%), crotonaldehyde (81%), 2-cyclopentenone (83%), methyl crotonate (83%), and methyl cinnamate (86%), provided good yields of the carbonyl compounds.2

The facile reduction of aromatic nitro compounds to anilines has been reported and oxindole and benzolactam can be formed 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. The best results were observed with 2,4-dinitrotoluene, in which 2-nitro-4-aminotoluene was obtained in 92% yield. The yields of other dinitro compounds ranged from 24 to 77%.4

Hydrogenolyses of aryl benzyl ethers and allylic acetates or amines have been reported to give phenols and alkenes, respectively.5 Hydrogenolysis of aryl halides with triethylammonium formate in the presence of heterogeneous catalysts is another useful synthetic method. Some interesting selectivity was observed when the heterogeneous catalysts were replaced with homogeneous catalysts. For example, reduction of 1-nitro-3-bromobenzene in the presence of 5% Pd/C gave a mixture of nitrobenzene (46%) and aniline (15%). Replacement of the 5% Pd/C with Palladium(II) Acetate and 2 equiv of tri-o-tolylphosphine gave nitrobenzene (81%) and only 2% of aniline. Similarly, the yield of benzonitrile from 4-bromobenzonitrile was improved from 53 to 83% with the homogeneous catalyst.3

The homogeneous catalytic system, on the other hand, is also able to deoxygenate phenols via their aryl sulfonates (eq 1).6 By varying the phosphine ligands, the phenol starting materials could be regenerated and thus the sulfonyl group only acts as a protecting group (eq 2). Similarly, aryl triflates are also hydrogenolyzed in excellent yields (eq 3).7 Selective hydrogenolysis of the triflates was realized in the presence of alkenes.

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.

The Pd-catalyzed Carroll rearrangement of allyl b-ketocarboxylates to give a-allyl ketones was realized in THF (eq 6).9

Switching the solvent to MeCN provided a,b-unsaturated ketones instead (eq 7).10

Allyl b-keto esters can also be hydrogenolyzed to form ketones without allylation (eq 8).11

1. Weir, J. R.; Patel, B. A.; Heck, R. F. JOC 1980, 45, 4926.
2. Cortese, N. A.; Heck, R. F. JOC 1978, 43, 3985.
3. Cortese, N. A.; Heck, R. F. JOC 1977, 42, 3491.
4. Terpko, M. O.; Heck, R. F. JOC 1980, 45, 4992.
5. Krishnamurty, H. G.; Ghosh, S.; Sathyanarayana, S. IJC(B) 1986, 25B, 1253.
6. Cabri, W.; Bernardinis, S. D.; Francalanci, F.; Penco, S. JOC 1990, 55, 350.
7. Cacchi, S.; Ciattini, P. G.; Morera, E.; Ortar, G. TL 1986, 27, 5541.
8. (a) Tsuji, J.; Yamakawa, T. TL 1979, 613. (b) Mandai, T.; Suzuki, S.; Murakami, T.; Fujita, M.; Kawada, M.; Tsuji, J. TL 1992, 33, 2987.
9. (a) Shimizu, I.; Yamada, T.; Tsuji, J. TL 1980, 21, 3199. (b) Tsuda, T.; Chujo, Y.; Nishi, S.; Tawara, K.; Saegusa, T. JACS 1980, 102, 6381.
10. Shimizu, I.; Tsuji, J. JACS 1982, 104, 5844.
11. Tsuji, J.; Nisar, M.; Shimizu, I. JOC 1985, 50, 3416.

Anthony O. King & Ichiro Shinkai

Merck Research Laboratories, Rahway, NJ, USA

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