Urs Gellrich

Tandem Rhodium‐Catalyzed Hydroformylation–Hydrogenation of Alkenes by Employing a Cooperative Ligand System

The hydroformylation of olefins is one of the largest industrial applications of homogeneous catalysis and results in the production of millions of tons of aldehydes per year. These aldehydes constitute useful intermediates, but are rarely the final objective of the industrial chemist, because they are usually reduced to the corresponding alcohols. In particular linear alcohols have tremendous industrial applications as solvents but also as raw materials for plasticizers and detergents. In most of the cases, these valuable materials are produced in two separate steps from the terminal alkenes, namely by a hydroformylation that employs syngas as an inexpensive one-carbon source, followed by a reduction step that uses molecular hydrogen and a second catalyst. The cost of the alcohol is further increased by the requirement to purify the aldehyde. Many approaches have been investigated in an attempt to shorten this sequence, ideally by designing a one-pot tandem hydroformylation/hydrogenation protocol, in which the alcohol would be directly isolated from the reaction mixture. In 1966, chemists from the Shell Oil Company pioneered such a process and reported the use of cobalt catalysts that are capable of converting alkenes into alcohols under CO/H2 atmosphere. The main limitations are the moderate yields and the somewhat harsh reaction conditions that are required. Many other catalytic systems that are composed of a ligand associated with a metal, such as Co, Pd, Rh, or Ru, have been reported as potential solution to this highly relevant industrial issue. However, none of these systems operates with satisfying chemoselectivity (alcohol vs. alkane; the latter resulting from competing direct reduction of the alkene) and regioselectivity (linear/branched (l/b) regioisomers resulting from unselective hydroformylation). Recently, Nozaki and co-workers described an elegant approach that relies on the cooperative use of rhodiumand rutheniumbased catalysts and results in the formation of the desired linear alcohols with excellent linear/branched selectivities in good yields. Our group also reported an example that fulfills these requirements by using a supramolecular rhodium catalyst based on the bifunctional ligand L. While the system showed high activity, its regioselectivity toward the linear alcohol product was not optimal. In recent years, the use of cooperative catalysis has enabled the development of many tandem processes or cascade reactions that combine the use of two or more separate catalytic systems, which work either in a cooperative or successive manner. Such tandem protocols can be the result of combining Lewis acid catalysis with Brønsted acid catalysis or Lewis base catalysis with Brønsted base catalysis, but also any other possible combination involving organometallic catalysis and organocatalysis. For example, Cole-Hamilton and co-workers have reported an elegant approach for the synthesis of alcohols from alkenes by combining use of rhodium, Xantphos, and triethylphosphine. We herein report a unique multifunctional rhodium catalyst system that enables the simultaneous catalysis of two distinct transformations in a highly selective manner, controlled by the cooperative action of two different ligands 1a and 2a (Scheme 1). These ligands stem from two

Enantioselective Titanium(III)-Catalyzed Reductive Cyclization of Ketonitriles†

Reduction, please! The title reaction affords α-hydroxyketones, a common structural motif in biologically active natural products, in good yields and high enantioselectivities at room temperature. The commercially available ansa-titanocene 1 was found to be an efficient catalyst for this process, which presumably proceeds by addition of a ketyl radical to a nitrile.

Mechanistic Investigations of the Rhodium Catalyzed Propargylic CH Activation

Previously we reported the redox-neutral atom economic rhodium catalyzed coupling of terminal alkynes with carboxylic acids using the DPEphos ligand. We herein present a thorough mechanistic investigation applying various spectroscopic and spectrometric methods (NMR, in situ-IR, ESI-MS) in combination with DFT calculations. Our findings show that in contrast to the originally proposed mechanism, the catalytic cycle involves an intramolecular protonation and not an oxidative insertion of rhodium in the OH bond of the carboxylic acid. A σ-allyl complex was identified as the resting state of the catalytic transformation and characterized by X-ray crystallographic analysis. By means of ESI-MS investigations we were able to detect a reactive intermediate of the catalytic cycle.

Mechanistic Insights into a Supramolecular Self-Assembling Catalyst System: Evidence for Hydrogen Bonding during Rhodium-Catalyzed Hydroformylation†

The structural integrity and flexibility provided by intermolecular hydrogen bonds leads to the outstanding properties of the 6-diphenylphosphinopyridin-(2H)-1-one ligand (see scheme) in the rhodium-catalyzed hydroformylation of terminal alkenes, as demonstrated by the combination of spectroscopic methods and DFT computations. Hydrogen bonds were also detected in a competent intermediate of the catalytic cycle.

Manganese-Catalyzed N-Formylation of Amines by Methanol Liberating H2: A Catalytic and Mechanistic Study

The first example of a base metal (manganese) catalyzed acceptorless dehydrogenative coupling of methanol and amines to form formamides is reported herein. The novel pincer complex (iPr-PNH P)Mn(H)(CO)2 catalyzes the reaction under mild conditions in the absence of any additives, bases, or hydrogen acceptors. Mechanistic insight based on the observation of an intermediate and DFT calculations is also provided.

The Reaction of White Phosphorus with NO+/NO2+[Al(ORF)4]−: The [P4NO]+ Cluster Formed by an Unexpected Nitrosonium Insertion†

Despite decades of intense research into polyphosphorus chemistry, our knowledge of homoleptic polyphosphorus cations is still limited to the results of mass spectrometry and quantum chemical calculations. In general, the diamagnetic cage cations with an odd number of phosphorus atoms are more stable, with P9 , composed of two C2v symmetric P5 cages joined by a common phosphonium atom having special stability. This cage was found in one of the few types of simple inorganic phosphorus cluster cations that are known, that is, [P5R2] + (R = Cl, Br, I, Ph, DippN(Cl)NDipp (Dipp = 2,6-diisopropylphenyl)). Those P5 cages are formed by the formal insertion of carbene-analogous PR2 + fragments into the P P bond of P4 (see Ref. [9, 10] for Reviews on P4 activation). Stable carbenes also interact with P4, leading to compounds including P1 up to P12 moieties, depending on the electronic nature of the carbene. Larger cationic P7 cages were recently prepared, but all preparative approaches to true Pn + ions remained futile. However, we expected that an appropriate one-electron oxidant should be able to oxidize P4 (ionization energy (IE) 9.34 eV) and lead to phosphorus cluster cations Pn . Herein we give an account of the reaction of P4 with the salts [NO] [Al(OC(CF3)3)4] [13] (1; IE NO = 9.26 eV) and [NO2] [Al(OC(CF3)3)4] (2 ; IE NO2 = 9.59 eV. At least 2 was expected to be a strong enough oxidant to yield Pn + cations. The novel salt 2 was synthesized in 94 % yield from NO2[BF4] and Li[Al(OC(CF3)3)4] in SO2 solution with precipitation of insoluble Li[BF4]; it was fully characterized by X-ray diffraction and vibrational and NMR spectroscopy (for details, see the Supporting Information). Unexpectedly, the reactions of 1 and 2 with P4 in CH2Cl2 show an analogous process, regardless of the ratios of phosphorus to oxidant employed (between 3P:1 NOx + and 9P:1 NOx ). They form a red intermediate and yield the same yellow final product ([P4NO] [Al(OC(CF3)3)4] (3 ; Scheme 1). Compound 3 may be viewed as the insertion

Ligand Self-Assembling through Complementary Hydrogen-Bonding in the Coordination Sphere of a Transition Metal Center : the 6-Diphenylphosphanylpyridin-2(1H)-one System

Motivated by previous findings which had shown that transition metal catalysts based on the 6-diphenylphosphanylpyridone ligand (6-DPPon, 2) display properties as a self-assembling bidentate ligand-metal complex, we have performed a thorough study on the bonding situation of this ligand, alone and in the coordination sphere of a late transition metal. Thus, combining a number of spectroscopic methods (UV-vis, IR, NMR, X-ray), we gained insights into the unique structural characteristics of 2. These experimental studies were corroborated by DFT calculations, which were in all cases in good agreement with the experimental results. The free ligand 2 prefers to exist as the pyridone tautomer 2A and dimerizes to the pyridone-pyridone dimer 4A in solution as well as in the crystal state. The corresponding hydroxypyridine tautomer 2B is energetically slightly disfavored (ca. 0.9 kcal/mol within the up-conformer relevant for metal coordination); hence, hydrogen bond formation within the complex may easily compensate this small energy penalty. Coordination properties of 2 were studied in the coordination sphere of a platinum(II) center. As a model complex, [Cl(2)Pt(6-DPPon)(2)] (11) was prepared and investigated. All experimental and theoretical methods used prove the existence of a hydrogen-bonding interligand network in solution as well as in the crystal state of 11 between one 6-DPPon ligand existing as the pyridone tautomer 2A and the other ligand occupying the complementary hydroxypyridine form 2B. Dynamic proton NMR allowed to determine the barrier for interligand hydrogen bond breaking and, in combination with theory, enabled us to determine the enthalpic stabilization through hydrogen-bonding to contribute 14-15 kcal/mol.

Reductive Cleavage of CO2 by Metal-Ligand-Cooperation Mediated by an Iridium Pincer Complex.

A unique mode of stoichiometric CO2 activation and reductive splitting based on metal-ligand-cooperation is described. The novel Ir hydride complexes [((t)Bu-PNP*)Ir(H)2] (2) ((t)Bu-PNP*, deprotonated (t)Bu-PNP ligand) and [((t)Bu-PNP)Ir(H)] (3) react with CO2 to give the dearomatized complex [((t)Bu-PNP*)Ir(CO)] (4) and water. Mechanistic studies have identified an adduct in which CO2 is bound to the ligand and metal, [((t)Bu-PNP-COO)Ir(H)2] (5), and a di-CO2 iridacycle [((t)Bu-PNP)Ir(H)(C2O4-κC,O)] (6). DFT calculations confirm the formation of 5 and 6 as reversibly formed side products, and suggest an η(1)-CO2 intermediate leading to the thermodynamic product 4. The calculations support a metal-ligand-cooperation pathway in which an internal deprotonation of the benzylic position by the η(1)-CO2 ligand leads to a carboxylate intermediate, which further reacts with the hydride ligand to give complex 4 and water.

Mechanistic Investigations of the Catalytic Formation of Lactams from Amines and Water with Liberation of H2

The mechanism of the unique lactam formation from amines and water with concomitant H2 liberation with no added oxidant, catalyzed by a well-defined acridine-based ruthenium pincer complex was investigated in detail by both experiment and DFT calculations. The results show that a dearomatized form of the initial complex is the active catalyst. Furthermore, reversible imine formation was shown to be part of the catalytic cycle. Water is not only the oxygen atom source but also acts as a cocatalyst for the H2 liberation, enabled by conformational flexibility of the acridine-based pincer ligand.

A Systematic Investigation of Coinage Metal Carbonyl Complexes Stabilized by Fluorinated Alkoxy Aluminates

The reaction of Cu(I), Ag(I), and Au(I) salts with carbon monoxide in the presence of weakly coordinating anions led to known and structurally unknown non-classical coinage metal carbonyl complexes [M(CO)n][A] (A = fluorinated alkoxy aluminates). The coinage metal carbonyl complexes [Cu(CO)n(CH2Cl2)m](+)[A](-) (n = 1, 3; m = 4-n), [Au2(CO)2Cl](+)[A](-), [(OC)nM(A)] (M = Cu: n = 2; Ag: n = 1, 2) as well as [(OC)3Cu⋅⋅⋅ClAl(OR(F))3] and [(OC)Au⋅⋅⋅ClAl(OR(F))3] were analyzed with X-ray diffraction and partially IR and Raman spectroscopy. In addition to these structures, crystallographic and spectroscopic evidence for the existence of the tetracarbonyl complex [Cu(CO)4](+)[Al(OR(F))4](-) (R(F) = C(CF3)3) is presented; its formation was analyzed with the help of theoretical investigations and Born-Fajans-Haber cycles. We discuss the limits of structure determinations by routine X-ray diffraction methods with respect to the C-O bond lengths and apply the experimental CO stretching frequencies for the prediction of bond lengths within the carbonyl ligand based on a correlation with calculated data. Moreover, we provide a simple explanation for the reported, partly confusing and scattered CO stretching frequencies of [Cu(I)(CO)n] units.