Triethylammonium formate

In the reaction of Q,/3-unsaturated ketones and esters, sometimes simple Michael-type addition (insertion and hydrogenolysis, or hydroarylation, and hydroalkenylation) of alkenes is observed[53,54]. For example, a simple addition product 56 to methyl vinyl ketone was obtained by the reaction of the heteroaromatic iodide 55[S5]. The corresponding bromide affords the usual insertion-elimination product. Saturated ketones are obtained cleanly by hydroarylation of o,/3l-unsaturated ketones with aryl halides in the presence of sodium formate, which hydrogenolyses the R—Pd—I intermediate to R— Pd—H[56]. Intramolecular hydroarylation is a useful reaction. The diiodide 57 reacts smoothly with sodium formate to give a model compound for the afla-toxin 58. (see Section 1.1.6)[57]. Use of triethylammonium formate and BU4NCI gives better results. 

Formate is an excellent hydride source for the hydrogenolysis of aryl halides[682]. Ammonium or triethylammonium formate[683] and sodium formate are mostly used[684,685]. Dechlorination of the chloroarene 806 is carried out with ammonium formate using Pd charcoal as a catalyst[686]. By the treatment of 2,4,6-trichloroamline with formate, the chlorine atom at the /iiara-position is preferentially removed[687]. The dehalogenation of 2,4-diha-loestrogene is achieved with formic acid, KI, and ascorbic acid[688]. 

A method for protecting ketones and aldehydes is the formation of oximes, but sometimes further protection of the oximes is required. For this purpose, the oximes can be protected as allyl ethers. The oxime ethers ean be eleaved with triethylammonium formate in boiling dioxane[444]. The allyl ether of oximes is eleaved under mild conditions without attaeking the aeetal group in 677. 

Stereoselective and chemoselective semihydrogenation of the internal alkyne 208 to the ew-alkene 210 is achieved by the Pd-catalyzed reaction of some hydride sources. Tetramethyldihydrosiloxane (TMDHS) (209) i.s used in the presence of AcOH[116]. (EtO)3SiH in aqueous THF is also effective for the reduction of alkynes to di-alkenes[l 17], Semihydrogenation to the d.v-alkene 211 is possible also with triethylammonium formate with Pd on carbon[118]. Good yields and high cis selectivity are obtained by catalysis with Pd2fdba)3-Bu3P[119],... 

Triethylammonium formate is another reducing agent for q, /3-unsaturated carbonyl compounds. Pd on carbon is better catalyst than Pd-phosphine complex, and citral (49) is reduced to citronellal (50) smoothly[55]. However, the trisubstituted butenolide 60 is reduced to the saturated lactone with potassium formate using Pd(OAc)2. Triethylammonium formate is not effective. Enones are also reduced with potassium formate[56]. Sodium hypophosphite (61) is used for the reduction of double bonds catalyzed by Pd on charcoal[57]. 

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

Triethylammonium formate-formic acid complex, (HC02NHEtj)2(HC02H)3, is used in a one-pot reductive aldol condensation of Meldrum s acid with aldehydes to produce the alkylated derivative [51]. Although feasible, the process has not been adapted for use with a quaternary ammonium formate salt. 

Meldrum s acid (72 g, 0.5 mol) is added with stirring at room temperature to the aldehyde (0.5 mol) suspended in the triethylammonium formate complex (500 ml), or with the... 

The concurrent development of analytical techniques for the isolation, detection, and quantitation of indole alkaloids in small amounts of cellular material has been addressed by Hdfle and co-workers 134), who developed a reversed-phase system using either methanol-water-triethylamine or acetonitrile-triethylammonium formate buffer as the eluept. Two prepacked cartridge systems were found to be very effective, for the rapid preparation of the alkaloid mixture from the cell contents. ... 

Triethylammonium formate in the presence of palladium reduced the less hindered para nitro group in 2,4-dinitrotoluene but the more hindered ortho nitro group in 2,4-dinitrophenol, 2,4-dinitroanisole, 2,4-dinitroaniline and 2,4-dinitroacetanilide in yields of 24-92% [73]. 

For the synthesis of simple amines, including valuable resolving agents such as the (l-naphthyl)ethylamines, ATH can be useful but an N-hnked phosphorus-based electron withdrawing group was found to be a necessary addition to the substrate. Blacker and Martin demonstrated that the reduction may be run at very low levels of catalyst when triethylammonium formate was fed into the reactor and nitrogen gas passed into the system as shown in Figure 1.36. 

Moghaddam, F.M., Khakshoor, O. and Ghaffarzadeh, M., Microwave assisted reduction of Schiff bases by triethylammonium formate/formic acid system, /. Chem. Res. (S), 2001, 525-527. 

The phenolic OH group can be removed by Pd-catalysed hydrogenolysis of its triflate 522 with triethylammonium formate [261]. Naphthol can be converted to naphthalene by the hydrogenolysis of its triflate. The Ni-catalysed reduction of aryl mesylates 523 is possible using MeOH and Zn as the hydrogen donor [262]. Smooth removal of phenol groups as triflates and mesylates is not possible by any other means. 

The synthetic potential of reductions by formate has been extended considerably by the use of ammonium formate with transition metal catalysts like palladium and rhodium. This forms a safe alternative to use of hydrogen. In this fashion it is possible to reduce hydrazones to hydrazines, azides and nitro groups to amines, to dehalogenate chloro-substituted aromatics, and to carry out various reductive removals of functional groups. For example, phenol triflates are selectively deoxygenated to the aromatic derivatives using triethylammonium formate as reductant and a palladium catalyst. - These recent af li-cations have been reviewed. 

Hydrogenation of itaconic acid (14) with Rh(COD)Cl2 catalyst and commercially available triethylammonium formate as hydrogen source delivers (5)-(15) in good enantiomeric excess (equation 14) with hydrogen as reductant instead of ammonium formate a 94% ee is obtained. ... 

An interesting variation on conventional catalytic hydrogenation is catalytic transfer hydrogenation, in which formic acid or formate serves as the hydrogen source. Thus, indoles are smoothly reduced (and formylated) to the corresponding A -formylindoles (equations 42 and 43). Since it is difficult to avoid formylation of the indoline, it is best to employ triethylammonium formate to maximize yields of the reduction product (equation 43). 

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