Enantioselective Hydrogenation of Enones with a Hydroformylation Catalyst

Abstract

Use of a typical rhodium precatalyst for hydroformylation results in the enantioselective hydrogenation of cyclic enones with up to 90% ee. Extensive screening of chiral ligands reveals the simple ligand Chiraphos as the best ligand, so far. The hydrogenation shows high chemoselectivity. Exclusive formation of saturated, chiral b-branched ketones is observed. It is proposed that the catalyst follows a frustrated hydroformylation pathway ("monohydride-based mechanism") and differs by that from the classical cationic Schrock-Osborn type rhodium precatalysts ("dihydride-based mechanism") for enantioselective hydrogenation. The catalyst operates under neat conditions and is easily recyclable by simply distilling off the reaction mixture and treatment with syn gas prior to hydrogenation.

Interplay between Cationic and Neutral Species in the Rhodium‐Catalyzed Hydroaminomethylation Reaction

“…In the absence of BF 4 − , acetylacetonate acts as a counter anion. The formation of cationic rhodium species with diphosphine ligands and acac − as the counter anion have recently been proposed 30. Attempts to isolate this complex were unsuccessful.…”

Chiral Hetero‐ and Carbocyclic Compounds from the Asymmetric Hydrogenation of Cyclic Alkenes

“…Although high enantioselectivities have been obtained in the transfer hydrogenation of this type of alkenes,29 their direct catalytic hydrogenation is less efficient and the few examples reported to date have shown modest selectivity 30. Scheuermann née Taylor and Jaekel have developed a rhodium‐based system that yields 3‐substituted cyclic ketones in moderate to good selectivity,31 but iridium catalysts have not proven efficient in this reaction. We obtained the targeted 3‐substituted ketones and lactones in high selectivity by using catalysts bearing the electron‐rich ligand 1 D (Table 3, entries 4 and 5).…”

Manufacture of Citronellal by the Rhodium‐Catalyzed Homogeneous Hydrogenation of Neral

“…We speculate that after coordination to rhodium, the large bite angle of diphosphane Xantphos more strongly inhibits the formation of bis‐ligand rhodium complexes as found for the corresponding Chiraphos complexes. The latter displays a smaller bite angle, and therefore, CO has the function of loosely coordinating to the metal and preventing the formation of catalytically inactive bis‐ligated complexes ,. The mild reaction conditions found may offer a possibility for a more facile enantioselective version of the reaction, provided that suitable chiral Xantphos‐type ligands with the correct electronic and steric structure become available.…”

“…Up to now, the role of CO has not been fully clarified. NMR spectroscopic investigations by Scheuermann and Jaekel with a catalyst prepared from Rh(acac)(CO) 2 and Chiraphos using cyclic enones as the substrate indicated that only by the addition of a carefully adjusted amount of CO is the formation of a catalytically inactive bis(diphosphane)Rh complex prevented . Moreover, it was claimed that CO‐free metal sources such as Rh(acac)(cod) (cod=1,5‐cyclooctadiene) or [Rh(cod) 2 ]BF 4 did not catalyze the hydrogenation even at high H 2 pressure.…”