Mark Gandelman

Electron-rich, bulky PNN-type ruthenium complexes: synthesis, characterization and catalysis of alcohol dehydrogenation.

Reaction of the electron-rich, bulky tridentate PNN ligand (PNN=2-di-tert-butylphosphinomethyl-6-diethylaminomethylpyridine) with Ru(PPh3)3Cl2 at 65 degrees C resulted in formation of the mononuclear dinitrogen complex (PNN)Ru(Cl)2N2 (minor) and the N2 bridged Ru(II) dinuclear complex [(PNN)Ru(Cl)2]2(micro-N2) (major). These complexes can be interconverted; passing argon through a solution of the mixture resulted in formation of pure . The cationic square-pyramidal [(PNN)Ru(PPh3)Cl]OTf was obtained by the reaction of complex with silver triflate followed by PPh3. Reaction of complex with CO yielded (PNN)Ru(CO)Cl2, which upon reaction with one equiv. of AgBF4 gave the cationic [(PNN)Ru(CO)Cl]BF4. The dicationic [(PNN)Ru(CO)(H2O)(acetone)](BF4)2 was obtained from with 2 equiv. of AgBF4 in acetone solution. Complexes , and were structurally characterized by X-ray crystallography. Complexes and upon addition of an equivalent of base, catalyzed the dehydrogenation of secondary alcohols to the corresponding ketones and primary alcohols to esters in good yields and high selectivity accompanied with the evolution of hydrogen gas.

Pincer Click Ligands

Tridentate pincer-type ligands of the general form DCD (where D and D are groups containing coordinating atoms) have been used to spectacular effect in coordination, mechanistic, synthetic, and supramolecular chemistry, as well as in nanoscience and in the development of sensors and molecular switches. Most significantly, the realization that pincer ligands offer both a unique, highly protective environment for the coordinated metal center and opportunities to finetune the steric and electronic properties of the metal atom has generated extensive research into the use of these complexes as catalysts. As a result, many important and challenging catalytic processes based on such systems have been developed. It is generally accepted that the reactivity, selectivity, and catalytic performance of pincer-based systems rely to a great extent on the characteristics of the donor groups D in the carefully selected ligand. The optimization of tailor-made catalysts involves extensive experimental investigation, in which the laborious synthesis of the ligands is often a serious bottleneck. In particular, the synthesis of nonsymmetrically substituted DCD ligands (D and D are different groups) represents a considerable challenge, as their preparation usually includes a series of steps and separations that commonly result in low yields. Consequently, the development of efficient and powerful methods for the rapid synthesis of a wide variety of tailormade ligands is of high importance. Although several methods for the preparation of bidentate ligand libraries have been reported, a strategy for the building of a tridentate ligand library is, to the best of our knowledge, still unknown. Here, we report a conceptually new general approach for the efficient and facile preparation of a novel family of tridentate pincer ligands of the DCD type. The tridentate mode of coordination was shown by the preparation and structural characterization of transition-metal complexes of these new ligands. Palladium complexes of this readily prepared set of representative ligands proved to be highly efficient catalysts in the Heck reaction. Traditionally, pincer ligands are prepared by attaching donor atoms to a ligand backbone. We developed an entirely different synthetic route that allowed access to a broad range of tailor-made pincer ligands. In designing our approach, we considered a methodology which would allow the selective and facile incorporation of two complementary monomeric donor groups D and D by covalent assembly to afford a pincer-type system DCD. The resulting molecule must also have a potential carbanion between the donor groups so as to bind the metal center in a pincer-type mode (Scheme 1a).

Nitrenium ions as ligands for transition metals

Unlike N-heterocyclic carbenes (NHCs), which are now used ubiquitously in metal-based chemistry, the nitrogen-derived analogue (in which a carbon is replaced with the isoelectronic nitrogen cation, a nitrenium ion) has remained elusive as a ligand for metals. This is especially intriguing, because several other main-group analogues of NHCs have been prepared, and have been shown to coordinate with transition-metal complexes. Here, we describe the preparation of several N-heterocyclic nitrenium ions that are isoelectronic and isostructural to NHCs, and study their ligand properties. The formation of relatively strong nitrenium-metal bonds is unambiguously confirmed, in solution by selective (15)N-labelling experiments, and in the solid state by X-ray crystallography. Experimental and computational studies of the electronic properties of this novel type of ligand suggest that they are poor σ-donors and good π-acceptors.

Efficient Synthesis of Secondary Alkyl Fluorides via Suzuki Cross-Coupling Reaction of 1-Halo-1-fluoroalkanes

Organofluorine compounds have found extensive applications in various areas of science. Consequently, the development of new efficient and selective methods for their synthesis is an important goal in organic chemistry. Here, we present the first Suzuki cross-coupling reaction which utilizes dihalo compounds for the preparation of secondary alkyl fluorides. Namely, an unprecedented use of simple 1-halo-1-fluoroalkanes as electrophiles in C(sp(3))-C(sp(3)) and C(sp(3))-C(sp(2)) cross-couplings allows for the formal site-selective incorporation of F-group in the alkyl chain with no adjacent activating functional groups. Highly effective approach to the electrophilic substrates, 1-halo-1-fluoroalkanes, via iododecarboxylation of the corresponding α-fluorocarboxylic acids is also presented. The conceptually new route to organofluorides was used for the facile preparation of biomedically valuable compounds. In addition, we demonstrated that an asymmetric version of the developed reaction for the stereoconvergent synthesis of chiral secondary alkyl fluorides is feasible.

Enantioselective Suzuki Cross-Couplings of Unactivated 1-Fluoro-1-haloalkanes: Synthesis of Chiral β-, γ-, δ-, and ε-Fluoroalkanes

The incorporation of fluorine atom into a stereogenic center is a highly challenging transformation with current methodologies offering access mainly to chiral α- and β-fluoroalkanes. In this article, the development of a novel general approach to construct β-, γ-, δ-, and ε- fluoroalkanes with good enantioselectivity is described. Different directing groups, such as benzyl, ketone, and sulfonyl, were shown to give good enantioselectivity under Suzuki cross-coupling conditions in the presence of a Ni catalyst and chiral diamine ligand. It includes the first examples of enantioselective synthesis of chiral fluorine-containing centers at as distant as δ or ε positions from the functional groups.

Cation–cation bonding in nitrenium metal complexes

Nitrenium ligands provide an excellent platform for the straightforward and efficient synthesis of extremely rare complexes that possess positively charged ligands coordinated to positively charged metals. Examples of stable cation–cation and cation–dication coordination bonds are demonstrated. Computational studies show that such bonding is greatly stabilized by its incorporation into a tridentate frame, as well as the use of polar solvents.

Coordination chemistry of N-heterocyclic nitrenium-based ligands.

Comprehensive studies on the coordination properties of tridentate nitrenium-based ligands are presented. N-heterocyclic nitrenium ions demonstrate general and versatile binding abilities to various transition metals, as exemplified by the synthesis and characterization of Rh(I) , Rh(III) , Mo(0) , Ru(0) , Ru(II) , Pd(II) , Pt(II) , Pt(IV) , and Ag(I) complexes based on these unusual ligands. Formation of nitrenium-metal bonds is unambiguously confirmed both in solution by selective (15) N-labeling experiments and in the solid state by X-ray crystallography. The generality of N-heterocyclic nitrenium as a ligand is also validated by a systematic DFT study of its affinity towards all second-row transition and post-transition metals (Y-Cd) in terms of the corresponding bond-dissociation energies.

Nitrogen Lewis Acids

Being a major conception of chemistry, Lewis acids have found countless applications throughout chemical enterprise. Although many chemical elements can serve as the central atom of Lewis acids, nitrogen is usually associated with Lewis bases. Here, we report on the first example of robust and modifiable Lewis acids centered on the nitrogen atom, which provide stable and well-characterized adducts with various Lewis bases. On the basis of the reactivity of nitrogen Lewis acids, we prepared, for the first time, cyclic triazanes, a class of cyclic organic compounds sequentially bearing three all-saturated nitrogen atoms (N-N-N motif). Reactivity abilities of these N-Lewis acids were explained by theoretical calculations. Properties and future applications of nitrogen Lewis acids are intriguing.

Aliphatic C–H Bond Iodination by a N-Iodoamide and Isolation of an Elusive N-Amidyl Radical

Contrary to C-H chlorination and bromination, the direct iodination of alkanes represents a great challenge. We reveal a new N-iodoamide that is capable of a direct and efficient C-H bond iodination of various cyclic and acyclic alkanes providing iodoalkanes in good yields. This is the first use of N-iodoamide for C-H bond iodination. The method also works well for benzylic C-H bonds, thereby constituting the missing version of the Wohl-Ziegler iodination reaction. Mechanistic details were elucidated by DFT computations, and the N-centered radical derived from the used N-iodoamide, which is the key intermediate in this process, was matrix-isolated in a solid argon matrix and characterized by UV-vis as well as IR spectroscopy.

Homogeneously catalyzed, chelate assisted hydrogenolysis of an amine C–N bond

Reaction of [RhCl(COE)2]2 with an excess of the aromatic aminophosphine 1-(diethylaminomethyl)-3-(di-tert-butylphosphinomethyl)-2,4,6-tri methylbenzene (2) in dioxane under mild H2 pressure results in selective catalytic activation of an unstrained C–N single bond.