Edwin Gerard Ijpeij

Methylaluminoxane as an Alternative for BArF in the Iridium-Catalyzed Asymmetric Hydrogenation of Imines

One of the methods for the preparation of enantiopure secondary amines is the asymmetric hydrogenation of imines. Over the last two decades, chiral catalysts based on iridium, rhodium, ruthenium, palladium, and titanium have been reported to be effective in the hydrogenation of C=N functionalities. Recently, we have published our results on the asymmetric hydrogenation of N-aryl imines using an iridium catalyst based on the use of the phosphoramidite ligand PipPhos L1 and the BArF counterion. In the hydrogenation of N-(3,5-dimethylphenyl)-(1-phenyl-ethylidene)-amine 1, full conversion over 4 h and >99% ee was achieved. Using a catalyst precursor that contained chloride as counter ion, the reaction was much slower and needed higher temperatures and higher pressures to reach full conversion. In addition, the product was isolated with lower enantioselectivity. This underscores the importance of BArF as counter ion in this type of asymmetric hydrogenation. In many hydrogenations the BArF counterion outperformed the PF6 counterion, which is attributed to its steric bulk. Phosphoramidites are cheap ligands that are easily made in only two synthetic steps. They have found many applications in asymmetric hydrogenation. However, the BArF counter ion is very expensive, which could hinder the industrial application of imine hydrogenation with Ir/PipPhos. In olefin polymerization, the common metallocene halide catalysts are made cationic by reaction with a large excess of methylaluminoxane (MAO). MAO is a poorly-defined oligomeric material roughly characterized by the formula (Al(CH3)xOy)n. MAO alkylates and then activates the metal-chloride pre-catalyst species by abstracting the chloride, thus forming an ion pair. It is prepared by a controlled hydrolysis of trimethylaluminium (TMA) and it always contains small amounts of TMA. TMA is a methylating agent and for that reason it is often removed by reaction with a bulky phenol, with which it forms AlMe(OAr)2. Although a large excess of MAO is necessary in these polymerizations, the low cost and the innocuous nature of this compound makes this procedure very attractive as a potential replacement of BArF in asymmetric hydrogenation reactions. In addition, in view of its oligomeric nature MAO can be viewed as a “bulky” anion. For these reasons the use of MAO was explored in combination with an iridium chloride catalyst precursor in the asymmetric hydrogenation of imines using PipPhos (L1) and the phosphine-oxazoline L2 developed by Pfaltz as ligands (Scheme 1). Two imine substrates were chosen, N-aryl imine 1 and cyclic imine 2. The hydrogenations were performed using

The oxidation of alcohols with trichloroisocyanuric acid: pyridine from base to organocatalyst

An alcohol oxidation method based on the use of pyridine or 3-cyanopyridine as an organocatalyst in combination with trichloroisocyanuric acid as the terminal oxidant is reported. It is demonstrated that secondary aliphatic alcohols can be selectively oxidized to the corresponding ketones in the presence of primary aliphatic alcohols. The method is also suitable for the oxidation of primary and secondary benzylic alcohols. N-Chloropyridinium cyanurates are proposed as the active alcohol-oxidizing species.