Armin Börner

A General Palladium‐Catalyzed Amination of Aryl Halides with Ammonia

A new robust palladium/phosphine catalyst system for the selective monoarylation of ammonia with different aryl bromides and chlorides has been developed. The active catalyst is formed in situ from [Pd(OAc)(2)] and air- and moisture-stable phosphines as easy-to-handle pre-catalysts. The productivity of the catalyst system is comparable to that of competitive Pd/phosphine systems; full conversion is achieved with most substrates with 1-2 mol % of Pd source and a fourfold excess of ligand (L).

The Synthesis of Chiral Phosphorus Ligands for use in Homogeneous Metal Catalysis

Recent developments in the synthesis of chiral phosphorus ligands are summarized. Main emphasis is given to the multitude of protocols for their synthesis starting from compounds of the chiral pool or by using other easily available chiral starting materials. Besides mono‐ and bidentate P‐ligands hybrid ligands bearing N, O, or olefinic groups as second ligating units are addressed. The syntheses of new P‐chirogenic ligands, as well as ligands with supramolecular structures are likewise reviewed. In appropriate cases structures of related privileged ligands are given for comparison. When available, the efficiency of the new ligands for metal catalyzed reactions are briefly assessed.

Improved palladium-catalyzed Sonogashira coupling reactions of aryl chlorides.

Christian Torborg, Jun Huang, Thomas Schulz, Benjamin Sch ffner, Alexander Zapf, Anke Spannenberg, Armin Bçrner, and Matthias Beller* The construction of Csp ACHTUNGTRENNUNG(aryl)Csp2 bonds is an important transformation in organic chemistry. The resulting aryl alkynes are building blocks often encountered within natural products, pharmaceutical products, and molecular materials. Due to the highly conjugated p system, this structural motif is found in organic semiconductors, and the respective products act as molecular sensors, light-emitting diodes, or polarizers for liquid-crystalline displays. In recent years polyaryleneethynylenes (PAEs) and oligoaryleneethynylenes (OAEs) such as 1 and 2 (Scheme 1) have become an established class of conjugated polymers in addition to poly(pphenylenevinylene)s (PPVs) and polyacetylenes. Moreover, arylene–ethynylene macrocycles (AEMs) (e.g. 3) and macromolecules such as 4 possess interesting electronic properties and lead to defined nanostructures. Apart from material science, the construction of aryl alkynes plays an important role in the synthesis of complex molecules of pharmaceutical and agrochemical interest (e.g. 5, 6, 7), even though the arylene–ethynylene structure itself does not often occur in natural products, which is in marked contrast to the corresponding vinylene–ethynylene motif. However, the alkynylation of aromatic halides and subsequent cyclization is a widely used method for the synthesis of carboand heterocycles as well as intermediates of natural products. It is undeniable that the most effective way to form aryl–alkyne bonds is still palladium-catalyzed coupling reactions of aromatic halides with alkynes in the presence of base and copper co-catalysts. Although this reaction was independently discovered by Cassar, Heck, and Sonogashira in 1975, today it is generally known as the Sonogashira reaction, and numerous catalytic systems have been reported for this

Iridium Phosphite−Oxazoline Catalysts for the Highly Enantioselective Hydrogenation of Terminal Alkenes

A modular library of readily available phosphite-oxazoline ligands (L1-L16a-f) has been successfully applied for the first time in the Ir-catalyzed asymmetric hydrogenation of a broad range of highly unfunctionalized 1,1,-disubstituted terminal alkenes. Enantioselectivities up to >99% and full conversions were obtained in several 1,1-disubstituted alkenes, including substrate classes that have never been asymmetrically hydrogenated before (i.e., 1,1-heteoraryl-alkyl, 1,1-diaryl, trifluoromethyl, etc.). The results indicated that these catalytic systems have high tolerance to the steric and electronic requirements of the substrate and also to the presence of a neighboring polar group. The asymmetric hydrogenations were also performed using propylene carbonate as solvent, which allowed the Ir catalyst to be reused and maintained the excellent enantioselectivities.

Amination of Aliphatic Alcohols and Diols with an Iridium Pincer Catalyst

Catalytic transformation of alcohols to amines is a fundamental transformation in synthetic organic chemistry. Since a variety of amines play an important role as precursors and final products employed in pharmaceuticals such as anticancer agents and DNA alkylators, agrochemicals, flavors and fragrances, cosmetics and toiletries, polymers, dyes, and other fine chemicals, the development of versatile and efficient amination methods is still an area of vital research. In recent years, a number of transition metal-catalyzed reactions for the synthesis of aliphatic and aromatic amines, such as hydroamination of alkenes or alkynes 4] and aryl halides, has been developed. Moreover, N-alkylation with alkyl halides is a well-known procedure, but the use of alkyl halides is undesirable from an environmental point of view, since it generates wasteful salts as byproducts. Another useful approach in the synthesis of amines is the reductive amination of aldehydes and ketones. However, this method requires the use of strong reducing reagents or dangerous hydrogen gas and is not always selective for monoalkylation of primary amines. Catalytic amination of alcohols is an alternative method for the preparation of amines (Scheme 1) and it is attractive from

Calix[4]arene‐based Bis‐phosphonites, Bis‐phosphites, and Bis‐O‐acyl‐phosphites as Ligands in the Rhodium(I)‐catalyzed Hydroformylation of 1‐Octene

New calix[4]arene-based bis-phosphonites, bis-phosphites and bis-O-acylphosphites were synthesized and characterized. Treatment of these P-ligands with selected rhodium and platinum precursors led to mononuclear complexes that were satisfactorily characterized. The solid state structure of the dirhodium(I) complex 14 has been determined by X-ray diffraction. The two rhodium centres are bridged by two chloro ligands; one rhodium atom is further coordinated by calix[4]arene phosphorus atoms and the other by cyclooctadiene. The new calix[4]arene P-ligands were tested in the Rh(I) catalyzed hydroformylation of 1-octene. All Rh(I) complexes catalyzed the reaction leading to high chemoselectivity with regard to the formation of aldehydes. Yields and n/iso-selectivities depended on the reaction conditions. Average yields of 80 % and n/iso-ratios of about 1.3 to 1.5 were observed. High yields of aldehydes can be achieved using the methoxy substituted P-ligands at low Rh:ligand ratios. Auf Calix[4]arenen basierende Bis-phosphonite, Bis-phosphite und Bis-O-acyl-phosphite als Liganden in der rhodium(I)-katalysierten Hydroformylierung von 1-Octen Neue auf Calix[4]arenen basierende Bis-phosphonite, Bis-phosphite und Bis-O-acyl-phosphite wurden synthetisiert und charakterisiert. Die Umsetzung dieser Phosphorliganden mit ausgewahlten Rhodium- und Platinvorstufen fuhrte zu mononuklearen Komplexen, die vollstandig charakterisiert wurden. Durch eine Rontgenstrukturanalyse konnte die Struktur des Dirhodium( I)-Komplexes 14 im festen Zustand bestimmt werden. Die beiden Rhodiumzentren sind durch zwei Chlorliganden verbruckt, wobei das eine Rhodiumatom an die Phosphoratome des Calix[4]-arens und das andere Rhodiumatom durch das Cyclooctadien koordiniert ist. Die neuartigen phosphorhaltigen Calix[4]aren-Liganden wurden in der Rhodium(I)-katalysierten Hydroformylierung von 1-Octen getestet. Alle Rh(I)-Komplexe katalysierten die Reaktion bei hoher Chemoselektivitat, bezogen auf die Bildung von Aldehyden. Ausbeuten und n/iso-Selektivitaten waren dabei von den Reaktionsbedingungen abhangig. Durchschnittliche Ausbeuten von 80 % und n/iso-Verhaltnisse von 1.3 bis 1.5 wurden beobachtet. Hohe Ausbeuten an Aldehyden bei geringen Rhodium:Ligand-Verhaltnissen konnten durch den Einsatz von methoxysubstituierten Phosphorliganden erzielt werden.

A versatile modular approach to new chiral C2-symmetrical ferrocenyl ligands: Highly enantioselective Rh-catalyzed hydrogenation of α-acetamidoacrylic acid derivatives☆

Abstract An easy, efficient, flexible modular synthesis of a new family of chiral C 2 -symmetrical ferrocenyl diphosphines (FERRIPHOS) is described. These new ligands gave high enantioselectivities (up to 99.4% ee ) in the rhodium-catalyzed hydrogenation of different enamide derivatives.

Rhodium(I) catalyzed asymmetric hydrogenation of enamines

The asymmetric hydrogenation of prochiral electron-rich enamines with rhodium(I) diphosphine and diphosphinite catalysts is described. The reaction is strongly sensitive to the ligand applied. Good results are observed with catalysts based on (R,R)-DIOP, Kβ+-OH, (R,R)-bdpch and other ligands forming seven-membered chelates with the metal. It is shown, that by conversion of a cyclic imine, which could not be hydrogenated, into the corresponding enamine a substrate is formed which is smoothly reduced at 1 bar initial hydrogen pressure by up to 72% ee.

The Effect of Internal Hydroxy Groups in Chiral Diphosphane Rhodium(I) Catalysts on the Asymmetric Hydrogenation of Functionalized Olefins

The unique effects of internal hydroxy groups in chiral rhodium(I) diphosphane catalysts on the asymmetric hydrogenation of functionalized olefins are summarized. In the first part, effects caused by the additional functional group on the rate and enantioselectivity of the catalytic reaction with RhI-diphosphane complexes are shown. For comparison, the results obtained with relevant parent catalysts or complexes bearing alkoxy groups are also considered. Subsequently, mechanistic studies which may explain the rate-reducing as well as the ee-enhancing effect of hydroxy groups are discussed in detail. Furthermore, the subtle competitive interplay between the hemilabile behaviour of hydroxy groups and their ability to establish hydrogen bonding within the framework of the ligand or between the ligand and the substrate is shown. Proof is given that conformationally flexible ligands bearing hydroxy groups can respond to varying electronic features of the catalytic centre and the substrate. Finally, the beneficial effect of hydroxy groups on the fine-tuning of diphosphane catalysts of low performance is demonstrated for some examples. This modification resulted in catalysts which afforded the chiral hydrogenation product in up to 99% ee.

Impressive Enhancement of the Enantioselectivity for a Hydroxy‐Containing Rhodium(I) Bisphosphine Catalyst in Aqueous Solution by Micelle‐Forming Amphiphiles

A selectivity increase for asymmetric hydrogenation in aqueous solution from 1.5%ee (blank experiment) up to 78%ee (amphiphiles present) may be accounted for by inclusion of the catalyst into micellar aggregates. The relative enantioselectivity Qa/b is distinctly higher for the rhodium chelate 1, carrying a hydroxy group, than for the related DIOP complex 2. The effect is minimized in the presence of increasing amounts of methanol, which is known to prevent micelle formation.