Hydrogenation Using cationic complexes

R. R. Schrock, and J. A. Osborn, Catalytic Hydrogenation Using Cationic Rhodium Complexes. I. Evolution of the Catalytic System and the Hydrogenation of Olefins, J. Am. Chem. Soc. 98, 2134-2143 (1976). 

Markovnikov addition [162]. The catalyst precursor is a cationic complex, [Rh(cod)2] BF4, used in the presence of 2 equiv. of PPhj. An equivalent amount of the hydrogenated vinylarene is formed (Eq. 4.38) [163-165]. 

Enantioselectivities of up to 47% ee were reported by Ruiz et al. in 1997 for the asymmetric hydrogenation of various prochiral dehydroamino acid derivatives and itaconic acid by using iridium cationic complexes of the novel chiral... 

Prochiral imines can be hydrogenated to the corresponding amines with extremely high enan-tioselectivities in H20/ethyl ethanoate biphasic systems, using Rh1 complexes of sulfonated phosphines 342 The cationic rhodium complex [Rh(NBD)(131)]+ was an active catalyst for hydrogenation of 2-ethanamido-propenoic acid in aqueous solution.343... 

The bis-DIOP complex HRh[(+)-DIOP]2 has been used under mild conditions for catalytic asymmetric hydrogenation of several prochiral olefinic carboxylic acids (273-275). Optical yields for reduction of N-acetamidoacrylic acid (56% ee) and atropic acid (37% ee) are much lower than those obtained using the mono-DIOP catalysts (10, II, 225). The rates in the bis-DIOP systems, however, are much slower, and the hydrogenations are complicated by slow formation of the cationic complex Rh(DIOP)2+ (271, 273, 274) through reaction of the starting hydride with protons from the substrate under H2 the cationic dihydride is maintained [cf. Eq. (25)] ... 

By using PHIP-NMR studies, various intermediates such as the previously elusive dihydrides of neutral and cationic hydrogenation catalysts, as well as hydrogenation product/catalyst complexes, have already been detected during the hydrogenation of styrene derivatives using cationic Rh catalysts. Information about the substituent effect on chemical shifts and kinetic constants has been obtained via time-resolved PASADENA NMR spectroscopy (DYPAS). 

Stability constants, from which AG° values are calculated, provide a direct measure of the extent of complexing in solution, and these values have been used to determine cation selectivity by macrocyclic compounds. Several of the methods commonly used to determine log K values cannot be used with many of these systems. Thus, procedures based on change in hydrogen ion concentration (pH titration, hydrogen electrode, etc.) cannot be used in those cases where the ligand is uncharged and its concentration is not pH dependent. Spectral methods generally have not been used because of the usual lack of favorable absorption characteristics by the compounds, cations or cation-complexes in the cases studied. 

In a more detailed study of the hydrogenation using [Ru(OAc)2(PPh3)2] (9) in benzene, methanol and methanol containing p-toluenesulfonic acid, the rate passed through a maximum at low acid concentration. This was attributed to the stepwise protonation of the acetate ligands, leading to cationic, methanol-solvated complexes (equation 17). 

The N-phenylimine of acetophenone was hydrogenated using an Ir cationic complex with a phosphinodihydrooxazole (S)-la in CH2CI2 under 100 atm of H2 to give the R amine in 87% ee (Table of Scheme 1) [6]. The reaction was completed with a substrate to catalyst molar ratio (S/C) of 667. The reactivity was further increased when the reaction was conducted in supercritical CO2 in-... 

I.4.I.2. Amino, Hydroxy, and Phenylthio Ketones Asymmetric hydrogenation of amino ketones, in either a neutral or hydrochloride form, has extensively been studied. Both Rh(I) and Ru(II) complexes with an appropriate chiral diphosphine give a high enantioselectivity. As described in Scheme 1.42, a-aminoacetophenone hydrochloride is hydrogenated using a cationic Rh complex with (R)-MOC-BIMOP, an unsymmetricaJ biaryl diphosphine, to give the... 

Unlike the heterogeneous variant, homogeneous hydrogenation of C=0 bonds has not developed at the rate of the corresponding C=C reduction process. This is due mainly to the lack of early success in the use of the Wilkinson catalyst in this respect. In work that has been described elsewhere192, the selection of the correct phosphine within a cationic complex was critical. Since these breakthroughs, progress in this area has been made at a tremendous pace, particularly in the area of asymmetric reductions, as will be described below. 

Nolan and coworkers have described the use of their cationic Ir complex 27 as an effective catalyst for transfer hydrogenations [73]. While complex 27 was found to be effective, the analog bearing the NHC ICy was found to be su-... 

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