Structure Identification of Precatalytic Copper Phosphoramidite Complexes in Solution

Copper-Catalyzed Asymmetric Arylation of N-Heteroaryl Aldimines: Elementary Step of a 1,4-Insertion

Copper complexes of monodentate phosphoramidites efficiently promote asymmetric arylation of N-azaaryl aldimines with arylboroxines. DFT calculations and experiments support an elementary step of 1,4-insertion in the reaction pathway, a step in which an aryl-copper species adds directly across four atoms of C=N-C=N in the N-azaaryl aldimines.

Elusive transmetalation intermediate in copper-catalyzed conjugate additions: direct NMR detection of an ethyl group attached to a binuclear phosphoramidite copper complex

Copper-catalyzed asymmetric conjugate addition reactions are a very powerful and widely applied method for enantioselective carbon-carbon bond formation. However, structural and mechanistic insight into these famous reactions has been very limited so far. In this article, the first direct experimental detection of transmetalation intermediates in copper-catalyzed reactions is presented. Special combinations of (1)H,(31)P HMBC spectra allow for the identification of complexes with chemical bonds between the alkyl groups and the copper complexes. For the structural characterization of these transmetalation intermediates, a special approach is applied, in which samples using enantiopure ligands are compared with samples using enantiomeric mixtures of ligands. It is experimentally proven, for the first time, that the dimeric copper complex structure is retained upon transmetalation, providing an intermediate with mixed trigonal/tetrahedral coordination on the copper atoms. In addition, monomeric intermediates with one ligand, but no intermediates with two ligands, are detected. These experimental results, in combination with the well-known optimal ligand-to-copper ratio of 2:1 in synthetic applications, allow us to propose that a binuclear transmetalation intermediate is the reactive species in copper-catalyzed asymmetric conjugate addition reactions. This first direct experimental insight into the structure of the transmetalation intermediate is expected to support the mechanistic and theoretical understanding of this important class of reactions and to enable their further synthetic development. In addition, the special NMR approach presented here for the identification and characterization of intermediates below the detection limit of (1)H NMR spectra can be applied also to other classes of catalyses.

Structures and interligand interaction pattern of phosphoramidite Pd complexes by NMR spectroscopy: modulations in extended interaction surfaces as stereoselection mode of a privileged class of ligands

During the last decade, phosphoramidites have been established as a so-called privileged class of ligands in various transition metal catalyses. However, the interactions responsible for their favorable properties have hitherto remained elusive. To address this issue, the formation trends, structural features, and interligand interaction patterns of several trans- and cis-[PdLL'Cl2] complexes have been investigated by NMR spectroscopy. The energetic contribution of their interligand interactions has been measured experimentally using the supramolecular balance for transition-metal complexes. The resulting energetics combined with an analysis of the electrostatic potential surfaces reveal that in phosphoramidites not only the aryl groups but the complete (CH)CH3 Ph moieties of the amine side chains form extended quasi-planar CH-π and π-π interaction surfaces. Application of the supramolecular balance has shown that modulations in these extended interaction surfaces cause energetic differences that are relevant to enantioselective catalysis. In addition, the energetics of these interligand interactions are quite independent of the actual structures of the complexes. This is shown by similar formation and aggregation trends of complexes with the same ligand but different structures. The extended quasi-planar electrostatic interaction surface of the (CH)CH3 Ph moiety explains the known pattern of successful ligand modulation and the substrate specificity of phosphoramidites. Thus, we propose modulations in these extended CH-π and π-π interaction areas as a refined stereoselection mode for these ligands. Based on the example of phosphoramidites, this study reveals three general features potentially applicable to various ligands in asymmetric catalysis. First, specific combinations of alkyl and aryl moieties can be used to create extended anisotropic interaction areas. Second, modulations in these interaction surfaces cause energetic differences that are relevant to catalytic applications. Third, bulky substituents with matching complementary interaction surfaces should not only be considered in terms of steric hindrance but also in terms of attractive and repulsive interactions, a feature that may often be underestimated in asymmetric catalysis.

NMR-spectroscopic and solid-state investigations of cometal-free asymmetric conjugate addition: a dinuclear paracyclophaneimine zinc methyl complex

We present herein the first indications for dimeric structures in cometal-free asymmetric conjugate addition reactions of dialkylzinc reagents with aldehydes. These are revealed by nonlinear effect (NLE) studies. A monomer-dimer equilibrium can be assumed which explains the increase of the ee value in the product over time. Also, DOSY NMR spectroscopic measurements indicate the existence of the catalyst as [LZnEt](n) complexes in solution. Additionally, the first X-ray structure of a zinc complex with a [2.2]paracyclophane ligand was determined. The structures of the zinc complexes are supported by DFT calculations of monomeric and dimeric species.

Phosphoramidites: privileged ligands in asymmetric catalysis

Asymmetric catalysis with transition-metal complexes is the basis for a vast array of stereoselective transformations and has changed the face of modern synthetic chemistry. Key to this success has been the design of chiral ligands to control the regio-, diastereo-, and enantioselectivity. Phosphoramidites have emerged as a highly versatile and readily accessible class of chiral ligands. Their modular structure enables the formation of ligand libraries and easy fine-tuning for a specific catalytic reaction. Phosphoramidites frequently show exceptional levels of stereocontrol, and their monodentate nature is essential in combinatorial catalysis, where a ligand-mixture approach is used. In this Review, recent developments in asymmetric catalysis with phosphoramidites used as ligands are discussed, with a focus on the formation of carbon-carbon and carbon-heteroatom bonds.

Improved applicability of DOSY experiments by high resolution probes combined with gradient amplifiers of diffusion units

Short relaxation times and small diffusion coefficients often impede reliable diffusion experiments due to insufficient signal to noise ratios, especially in low temperature studies, where in the case of small diffusion coefficients in combination with very short T(2) times and substantial convection, double-stimulated-echo experiments are required as last resort of convection-compensating DOSY pulse sequences. Therefore, the combination of a strong gradient amplifier of a diffusion unit with Z-gradient high resolution probes is tested for low temperature applications to combine the advantages of high (1)H resolution, flexible temperature, and multinuclear applications with short gradient durations and diffusion delay. The experimental spectra on phosphoramidite ligands and phosphoramidite-copper complexes show that signal to noise improvements up to 176% were achieved despite longer eddy current delays and increased noise levels. Calculational estimations of the enhancement factors predict special benefits for systems with short transversal relaxation times and small diffusion coefficients and promise even higher enhancement factors for noise level optimized combinations. In addition, an easy way is presented to find fast and effectively relaxation optimized DOSY parameters for the convection-compensated double-stimulated-echo pulse sequence of Jerschow and Müller.

Recent advances in enantioselective copper-catalyzed 1,4-addition

A comprehensive overview of recent literature from 2003 concerning advances in enantioselective copper catalysed 1,4-addition of organometallic reagents to alpha,beta-unsaturated compounds is given in this critical review. About 200 ligands and catalysts are presented, with a focus on stereoselectivities, catalyst loading, ligand structure and substrate scope. A major part is devoted to trapping and tandem reactions and a variety of recent synthetic applications are used to illustrate the practicality and current state of the art of 1,4-addition of organometallic reagents. Finally several mechanistic studies are discussed (162 references).

Influence of Copper Salts, Solvents, and Ligands on the Structures of Precatalytic Phosphoramidite Copper Complexes for Conjugate Addition Reactions

For copper-catalyzed enantioselective conjugate addition reactions of organozinc reagents, the available knowledge about the mechanism and the structures involved is still insufficient to understand in detail the strong influences of solvent, salt, and ligand size, or to enable a rational control of this reaction. Screening with three phosphoramidite ligands and four copper(I) salts using NMR spectroscopy has revealed a binuclear copper complex with mixed trigonal/tetrahedral stereochemistry as the basic structural motif of the ground state of precatalysts with highly stereoselective ligands. Ligands with smaller amine moieties allow higher coordination numbers and higher aggregation levels, leading to reduced ee values. Since the ESI mass spectra of several precatalytic copper halide complexes show a striking correlation with the structures observed in solution, ESI-MS may be used as a fast tool to determine the maximum number of phosphoramidite ligands attached to copper.