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.

Trapped in a noble cube: A novel family of noble metalates has been discovered in which a 3d metal ion M (M = Mn(II), Fe(III), Co(II), Cu(II), Zn(II)) is encapsulated by a 12 palladium-oxo cage {Pd(12)O(32)}, which is capped by eight phosphate groups. Such discrete nanocubes were further investigated by EPR spectroscopy, electrochemistry, and in homogeneous hydrogenation catalysis.

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Multivariate curve resolution techniques allow to uncover from a series of spectra (of a chemical reaction system) the underlying spectra of the pure components and the associated concentration profiles along the time axis. Usually, a range of feasible solutions exists because of the so‐called rotational ambiguity. Any additional information on the system should be utilized to reduce this ambiguity. Sometimes the pure component spectra of certain reactants or products are known, or the concentration profiles of the same or other species are available. This valuable information should be used in the computational procedure of a multivariate curve resolution technique. The aim of this paper is to show how such supplemental information on the components can be exploited. The knowledge of spectra leads to linear restrictions on the concentration profiles of the complementary species and vice versa. Further, affine–linear restrictions can be applied to pairs of a concentration profile and the associated spectrum of a species. These (affine) linear constraints can also be combined with the usual non‐negativity restrictions. These arguments can reduce the rotational ambiguity considerably. In special cases, it is possible to determine the unknown concentration profile or the spectrum of a species only from these constraints. Copyright © 2012 John Wiley & Sons, Ltd.

The reaction of rac-(ebthi)Zr(η2-Me3SiC2SiMe3) (1) [ebthi = 1,2-ethylene-1,1‘-bis(η5-tetrahydroindenyl)] with an excess of ethylene at room temperature leads to the corresponding zirconacyclopentane 3, which was isolated as stable yellow crystals and characterized by an X-ray crystal structure analysis. At 203 K complex 1 reacts with only 1 equiv of ethylene to form the zirconacyclopentene 2, possibly an intermediate in the reaction from 1 to 3. Upon activation with tris(pentafluorophenyl)borane under an ethylene atmosphere, complex 3 was shown to be an effective catalyst for ethylene polymerization. The reaction of 2 equiv of styrene with the alkyne complex 1 leads to the unsymmetrically substituted zirconacyclopentane 6.

This contribution gives an overview of the synthesis of chiral -amino acids via asymmetric hydrogenation of the corresponding dehydroamino derivatives. Literature results are discussed regarding substrate synthesis and catalyst performance and how it is affected by substrate and catalyst structure as well as experimental parameters. A tentative mechanistic concept for the hydrogenation step is also presented. IntroductionEnantiomerically pure -amino acids and their derivatives not only exhibit broad biological activity but are also the building blocks for the synthesis of -peptides. The latter are characterized by a high enzymatic stability and show interesting three-dimensional structures. 1 The cyclization of -amino acids leads to the important family of the -lactams. Scheme 1 shows examples of pharmaceutically interesting structures containing a -aryl-substituted -amino acid as a common structural component. 2 Methods for the preparation of optically enriched -amino acids are predominantly based on stoichiometric reactions with chiral auxiliary agents and to a clearly smaller extent on stereoselective catalytic reactions. 2d,e,3 One of the most promising methodologies, also regarding industrial application, is the asymmetric hydrogenation of the appropriate -dehydroamino acid precursors catalyzed by homogeneous Rh or Ru complexes containing chiral phosphane ligands. In contrast to the synthesis of R-amino acid precursors where it is a standard method with many industrial applications, 4

The present work provides an overview of possible activation and deactivation phenomena in homogeneous catalytic processes promoted by different types of rhodium complexes containing diphosphine ligands.

As the workhorses for many applications, neutral dimeric μ2‐X‐bridged diphosphine rhodium complexes of the type [{Rh(diphosphine)(μ2‐X)}2] (X=Cl, OH) are usually prepared in situ by the addition of diphosphine ligands to the rhodium complex [{Rh(diolefin)(μ2‐X)}2] (diolefin=cyclooctadiene (cod) or norbornadiene (nbd)) or [{Rh(monoolefin)2(μ2‐Cl)}2] (monoolefin=cyclooctene (coe) or ethylene (C2H4)). The in situ procedure has been investigated for the diphosphines 2,2′‐bis(diphenylphosphino)‐1,1′‐binaphthyl (BINAP), 5,5′‐bis(diphenylphosphino)‐4,4′‐bi‐1,3‐benzodioxole (SEGPHOS), 5,5′‐bis[di(3,5‐xylyl)phosphino]‐4,4′‐bi‐1,3‐benzodioxole (DM‐SEGPHOS), 5,5′‐bis[di(3,5‐di‐tert‐butyl‐4‐methoxyphenyl)phosphino]‐4,4′‐bi‐1,3‐benzodioxole (DTBM‐SEGPHOS), 2,2′‐bis(diphenylphosphino)‐1,1′‐dicyclopentane (BICP), 1‐[2‐(diphenylphosphino)ferrocenyl]ethyldi‐tert‐butylphosphine (PPF‐PtBu2), 1,1′‐bis(diisopropylphosphino)ferrocene (DiPPF), 1,2‐bis(diphenylphosphino)ethane (DPPE), 1,2‐bis(o‐methoxyphenylphosphino)ethane (DIPAMP), 4,5‐bis(diphenylphosphinomethyl)‐2,2‐dimethyl‐1,3‐dioxalane (DIOP), 1,2‐bis(2,5‐dimethylphospholano)benzene (Me‐DuPHOS), 1,4‐bis(diphenylphosphino)butane (DPPB), and 1,3‐bis(diphenylphosphino)propane (DPPP); the resulting complexes have been characterized by 31P NMR spectroscopy and, in most cases, also by X‐ray analysis. Depending on the diphosphine ligand, the solvent, the temperature, and the rhodium precursor, species other than the desired one [{Rh(diphosphine)(μ2‐X)}2] are formed, for example, [(diolefin)Rh(μ2‐Cl)2Rh(diphosphine)], [Rh(diphosphine)(diolefin)]+, [Rh(diphosphine)2]+, and [Rh(diphosphine)(diolefin)(Cl)]. The results clearly show that the in situ method commonly applied for precatalyst preparation cannot be regarded as an optimal strategy for the formation of such neutral [{Rh(diphosphine)(μ2‐X)}2] complexes.