Anke Spannenberg

Well-Defined Iron Catalyst for Improved Hydrogenation of Carbon Dioxide and Bicarbonate

The most efficient, stable, and easy-to-synthesize non-noble metal catalyst system for the reduction of CO(2) and bicarbonates is presented. In the presence of the iron(II)-fluoro-tris(2-(diphenylphosphino)phenyl)phosphino]tetrafluoroborate complex 3, the hydrogenation of bicarbonates proceeds in good yields with high catalyst productivity and activity (TON > 7500, TOF > 750). High-pressure NMR studies of the hydrogenation of carbon dioxide demonstrate that the corresponding iron-hydridodihydrogen complex 4 is crucial in the catalytic cycle.

Development of a General Palladium-Catalyzed Carbonylative Heck Reaction of Aryl Halides

The first general palladium-catalyzed carbonylative vinylation of aryl halides with olefins in the presence of CO has been developed. Applying a catalyst system consisting of [(cinnamyl)PdCl](2) and bulky imidazolyl-phosphine ligand L1 allows for the efficient and selective synthesis of α,β-unsaturated ketones under mild reaction conditions. Starting from easily available aryl halides and olefins, versatile building blocks can be prepared in a straightforward manner. The generality and functional group tolerance of this novel protocol is demonstrated.

Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by Well-Defined Manganese Pincer Complexes

Hydrogenations constitute fundamental processes in organic chemistry and allow for atom-efficient and clean functional group transformations. In fact, the selective reduction of nitriles, ketones, and aldehydes with molecular hydrogen permits access to a green synthesis of valuable amines and alcohols. Despite more than a century of developments in homogeneous and heterogeneous catalysis, efforts toward the creation of new useful and broadly applicable catalyst systems are ongoing. Recently, Earth-abundant metals have attracted significant interest in this area. In the present study, we describe for the first time specific molecular-defined manganese complexes that allow for the hydrogenation of various polar functional groups. Under optimal conditions, we achieve good functional group tolerance, and industrially important substrates, e.g., for the flavor and fragrance industry, are selectively reduced.

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).

A convenient and efficient procedure for the palladium-catalyzed cyanation of aryl halides using trimethylsilylcyanide

Abstract Benzonitriles are easily accessible from the corresponding aryl bromides catalyzed by a palladium-complex using trimethylsilylcyanide (TMSCN) as cyanating agent under mild conditions. The key of success for the cyanation protocol is the slow dosage of the TMSCN to the reaction mixture. This new method is applicable on both activated and deactivated aryl and heteroaryl bromides giving the corresponding benzonitriles in good to excellent yield.

A General and Efficient Catalyst for Palladium-Catalyzed C−O Coupling Reactions of Aryl Halides with Primary Alcohols

An efficient procedure for palladium-catalyzed coupling reactions of (hetero)aryl bromides and chlorides with primary aliphatic alcohols has been developed. Key to the success is the synthesis and exploitation of the novel bulky di-1-adamantyl-substituted bipyrazolylphosphine ligand L6. Reaction of aryl halides including activated, nonactivated, and (hetero)aryl bromides as well as aryl chlorides with primary alcohols gave the corresponding alkyl aryl ethers in high yield. Noteworthy, functionalizations of primary alcohols in the presence of secondary and tertiary alcohols proceed with excellent regioselectivity.

Palladium-Catalyzed Formylation of Aryl Bromides: Elucidation of the Catalytic Cycle of an Industrially Applied Coupling Reaction

The first comprehensive study of the catalytic cycle of the palladium-catalyzed formylation of aryl bromides with synthesis gas (CO/H2, 1:1) is presented. The formylation in the presence of efficient (Pd/PR2(n)Bu, R = 1-Ad, (t)Bu) and nonefficient (Pd/P(t)Bu3) catalysts was investigated. The main organometallic complexes involved in the catalytic cycle were synthesized and characterized, and their solution chemistry was studied in detail. Comparison of stoichiometric and catalytic reactions using P(1-Ad)2(n)Bu, the most efficient ligand known for the formylation of aryl halides, led to two pivotal results: (1) The corresponding carbonylpalladium(0) complex [Pd(n)(CO)(m)L(n)] and the respective hydrobromide complex [Pd(Br)(H)L2] are resting states of the active catalyst, and they are not directly involved in the catalytic cycle. These complexes maintain the concentration of most active [PdL] species at a low level throughout the reaction, making oxidative addition the rate-determining step, and provide high catalyst longevity. (2) The product-forming step proceeds via base-mediated hydrogenolysis of the corresponding acyl complex, e.g., [Pd(Br)(p-CF3C6H4CO){P(1-Ad)2(n)Bu}]2 (8), under mild conditions (25-50 degrees C, 5 bar). Stoichiometric studies using the less efficient Pd/P(t)Bu3 catalyst resulted in the isolation and characterization of the first stable three-coordinated neutral acylpalladium complex, [Pd(Br)(p-CF3C6H4CO)(P(t)Bu3)] (10). Hydrogenolysis of 10 needed significantly more drastic conditions compared to that of dimeric 8. In the presence of amine base, complex 10 gave a catalytically inactive diamino acyl complex, which explains the low activity of the Pd/P(t)Bu3 catalyst formylation of aryl bromides.

Synthesis of Monocarbenepalladium(0) Complexes and Their Catalytic Behavior in Cross‐Coupling Reactions of Aryldiazonium Salts

The first monocarbenepalladium(0) complexes with benzoquinone and naphthoquinone as additional ligands have been prepared. As demonstrated by NMR spectroscopy and X-ray analysis, the complexes show a unique coordination mode giving quinone-bridged dimers. The monocarbenepalladium(0) complexes allow efficient cross-coupling reactions of aryldiazonium salts with olefins (Heck reaction) and arylboronic acids (Suzuki reaction).

Hydrogenation of Esters to Alcohols Catalyzed by Defined Manganese Pincer Complexes

The first manganese-catalyzed hydrogenation of esters to alcohols has been developed. The combination of Mn(CO)5 Br with [HN(CH2 CH2 P(Et)2 )2 ] leads to a mixture of cationic and neutral Mn PNP pincer complexes, which enable the reduction of various ester substrates, including aromatic and aliphatic esters as well as diesters and lactones. Notably, related pincer complexes with isopropyl or cyclohexyl substituents showed very low activity.

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