Lutz H. Gade

Chiral N-Heterocyclic Carbenes as Stereodirecting Ligands in Asymmetric Catalysis

In recent years, N-heterocyclic carbenes (NHC) have proved to be a versatile class of spectator ligands in homogeneous catalysis. Being robust anchoring functions for late transition metals, their ligand donor capacity and their molecular shape is readily modified by variation of the substituents at the N-atoms and the structure of the cyclic backbone. After the first attempts to use chiral NHC ligands in asymmetric catalysis in the late 1990s, which initially met with limited success, several novel structural concepts have emerged during the past two years which have led literally to an explosion of the field. With a significant number of highly selective chiral catalysts based on chiral NHCs having been reported very recently, several general trends in the design of new NHC-containing molecular catalysts for stereoselective transformations in organic synthesis emerge.

Highly Polar Metal-Metal Bonds in "Early-Late" Heterodimetallic Complexes.

Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations.

Highly Enantioselective Copper-Catalyzed Electrophilic Trifluoromethylation of β-Ketoesters

Enantioselective Cu-catalyzed trifluoromethylation of β-ketoesters using commercially available trifluoromethylating reagents is reported. A number of α-CF(3) β-ketoesters are obtained with up to 99% ee. The trifluoromethylated products were then transformed diastereoselectively to α-CF(3) β-hydroxyesters with two adjacent quaternary stereocenters via a Grignard reaction.

Enantioselective Iron-Catalyzed Azidation of β-Keto Esters and Oxindoles

The first example of Fe-catalyzed enantioselective azidations of β-keto esters and oxindoles using a readily available N3-transfer reagent is reported. A number of α-azido-β-keto esters were obtained with up to 93% ee, and this methodology also generates 3-substitued 3-azidooxindoles with high enantioselectivities (up to 94%).

The Nature of the Catalytically Active Species in Olefin Dioxygenation with PhI(OAc)2: Metal or Proton?

Evidence for the protiocatalytic nature of the diacetoxylation of alkenes using PhI(OAc)(2) as oxidant is presented. Systematic studies into the catalytic activity in the presence of proton-trapping and metal-complexing agents indicate that protons act as catalysts in the reaction. Using triflic acid as catalyst, the selectivity and reaction rate of the conversion is similar or superior to most efficient metal-based catalysts. Metal cations, such as Pd(II) and Cu(II), may interact with the oxidant in the initiation phase of the catalytic transformation; however, 1 equiv of strong acid is produced in the first cycle which then functions as the active catalyst. Based on a kinetic study as well as in situ mass spectrometry, a mechanistic cycle for the proton-catalyzed reaction, which is consistent with all experimental data presented in this work, is proposed.

Copper-boxmi complexes as highly enantioselective catalysts for electrophilic trifluoromethylthiolations.

The enantioselective trifluoromethylthiolation of β-ketoesters using chiral copper-boxmi complexes as catalysts is reported. A number of α-SCF3-substituted β-ketoesters have been obtained with up to >99% enantiomeric excess (ee), and the trifluoromethylthiolated products were then transformed diastereoselectively to α-SCF3-β-hydroxyesters with two adjacent quaternary stereocenters.

New transition metal imido chemistry with diamido-donor ligands

Abstract This short review presents an account of the synthesis and chemistry to date of new Groups 4, 5 and 6 imido complexes derived from the diamido–pyridine or diamido–amine ligand precursors MeC(2-C5H4N)(CH2NHSiMe2R)2 (abbreviated as H2N2Npy for R=Me or H2N2Npy* for R=But) and Me3SiN(CH2CH2NHSiMe3)2 (abbreviated as H2N2Nam). Reaction of the dilithium salts Li2N2Npy, Li2N2Npy* or Li2N2Namwith metal imido synthons of the general type [M(NR)xCly(LB)z] (M=Groups 4–6 transition metals; R=alkyl or aryl substituents; LB=Lewis base) gives a diverse range of products among which are the following complexes: [Ti(NR)(N2Npy)], [M(NR)(LN2N)(LB)] (M=Ti or Zr; LN2N=a general diamido-donor ligand), [M(NR)(N2Npy)Cl(py)] (M=Nb or Ta), [Nb(μ-NBut)(N2Nam)(μ-Cl)]2, [W(NR)(LN2N)Cl2] (LN2N=N2Npy or N2Nam), [Mo(NR)2(LN2N)] (LN2N=N2Npy or N2Nam). The diamido–pyridine supported titanium imido complexes [Ti(NR)(N2Npy)] and [Ti(NR)(N2Npy)(py)] in particular have reactive Ti=NR linkages and these undergo a wide range of coupling reactions with the following unsaturated organic substrates: RNC, MeCN, ButCP, ArNCO, RC2Me, and RCHCCH2. Many of these transformations are the first, or among the first, of their type in transition metal imido chemistry. The combined diamido–pyridine–imido donor set also forms a useful supporting ligand environment for new chemistry at Groups 5 and 6 metal centres, especially in the Group 5 complexes [M(NR)(N2Npy)Cl(py)] (M=Nb or Ta) in which the chloride can be substituted by a range of N-, O- and C-donor ligands.

The Synthesis of a New Class of Chiral Pincer Ligands and Their Applications in Enantioselective Catalytic Fluorinations and the Nozaki–Hiyama–Kishi Reaction

A new class of chiral tridentate N-donor pincer ligands, bis(oxazolinylmethylidene)isoindolines (boxmi), was synthesized in three steps starting from readily available phthalimides. Their reaction with ethyl (triphenylphosphoranylidene)acetate by means of a key-step Wittig reaction gave the ligand backbones, which were condensed with amino alcohols and then cyclized to obtain the corresponding ligands. These ligands were subsequently applied in the nickel(II)-catalyzed enantioselective fluorination of oxindoles and β-ketoesters to obtain the corresponding products with enantioselectivities of up to >99% ee and high yields. Application of the chiral pincer ligands in the chromium-catalyzed enantioselective Nozaki-Hiyama-Kishi reaction of aldehydes gave the corresponding alcohols with an optimal enantioselectivity of 93%.

Aggregation and Contingent Metal/Surface Reactivity of 1,3,8,10-Tetraazaperopyrene (TAPP) on Cu(111)

The structural chemistry and reactivity of 1,3,8,10-tetraazaperopyrene (TAPP) on Cu(111) under ultra-high-vacuum (UHV) conditions has been studied by a combination of experimental techniques (scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy, XPS) and DFT calculations. Depending on the deposition conditions, TAPP forms three main assemblies, which result from initial submonolayer coverages based on different intermolecular interactions: a close-packed assembly similar to a projection of the bulk structure of TAPP, in which the molecules interact mainly through van der Waals (vDW) forces and weak hydrogen bonds; a porous copper surface coordination network; and covalently linked molecular chains. The Cu substrate is of crucial importance in determining the structures of the aggregates and available reaction channels on the surface, both in the formation of the porous network for which it provides the Cu atoms for surface metal coordination and in the covalent coupling of the TAPP molecules at elevated temperature. Apart from their role in the kinetics of surface transformations, the available metal adatoms may also profoundly influence the thermodynamics of transformations by coordination to the reaction product, as shown in this work for the case of the Cu-decorated covalent poly(TAPP-Cu) chains.

Metal Silylenes Generated by Double Silicon–Hydrogen Activation: Key Intermediates in the Rhodium-Catalyzed Hydrosilylation of Ketones†

Rhodium silylenes, which are generated by double Si-H activation at the metal, are involved in a low-activation-barrier mechanism of the hydrosilylation of ketones with R(2)SiH(2). A DFT-based study of reaction mechanisms accounts for the experimental observations, notably the rate enhancement for R(2)SiH(2) over R(3)SiH and an inverse kinetic isotope effect.