Pd(II)-Catalyzed Regio‑, Enantio‑, and Diastereoselective 1,4-Addition of Azlactones Formed in Situ From Racemic Unprotected Amino Acids and Acetic Anhydride

A multicomponent reaction is reported generating highly enantioenriched and diastereomerically pure quaternary amino acid derivatives via 1,4-addition of azlactones to enones. The azlactone intermediates are generated in situ from unprotected α-amino acids and acetic anhydride. Previous attempts using bis-palladacycle catalysts required the use of a large excess of benzoic anhydride (which is very difficult to remove from the products), since acetic anhydride provided regioisomeric product mixtures. Key for the high regioselectivity is a pentaphenylferrocene monopalladacycle catalyst

Selective C–C and C–H Bond Activation/Cleavage of Pinene Derivatives: Synthesis of Enantiopure Cyclohexenone Scaffolds and Mechanistic Insights

The continued development of transition-metal-mediated C–C bond activation/cleavage methods would provide even more opportunities to implement novel synthetic strategies. We have explored the Rh­(I)-catalyzed C–C activation of cyclobutanols resident in hydroxylated derivatives of pinene, which proceed in a complementary manner to the C–C bond cleavage that we have observed with many traditional electrophilic reagents. Mechanistic and computational studies have provided insight into the role of C–H bond activation in the stereochemical outcome of the Rh-catalyzed C–C bond activation process. Using this new approach, functionalized cyclohexenones that form the cores of natural products, including the spiroindicumides and phomactin A, have been accessed

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20 février 19371937/02/20 (A66).Appartient à l’ensemble documentaire : PoitouCh

Nontargeted Volatile Metabolite Screening and Microbial Contamination Detection in Fermentation Processes by Headspace GC-IMS

Gas chromatography combined with ion mobility spectrometry (GC-IMS) is a powerful separation and detection technique for volatile organic compounds (VOC). This combination is characterized by exceptionally low detection limits in the low ppbv range, high 2-dimensional selectivity, and robust operation. These qualities make it an ideal tool for nontarget screening approaches. Fermentation broths contain a substantial number of VOC, either from the medium or produced by microbial metabolism, that are currently not regularly measured for process monitoring. In this study, Escherichia coli, Saccharomyces cerevisiae, Levilactobacillus brevis, and Pseudomonas fluorescens were exemplarily used as model organisms and cultivated, and the headspace was analyzed by GC-IMS. Additionally, mixed cultures for every combination of two of the microorganisms were also characterized. Multivariate data analysis of the GC-IMS data revealed that it is possible to differentiate between the microorganisms using PLS-DA with a prediction accuracy of 0.92. The mixed cultures could be separated from the pure cultures with accuracies between 0.87 and 1.00 depending on the organism. GC-IMS data correlate with the optical density and can be used to follow and model growth curves. The root mean squared errors ranged between 10 and 20% of the maximum value, depending on the organism. Peak identification with reference compounds did not reveal specific marker compounds, rather the pattern was found to be responsible for the model performance

Monomeric Ferrocene Bis-Imidazoline Bis-Palladacycles: Variation of Pd–Pd Distances by an Interplay of Metallophilic, Dispersive, and Coulombic Interactions

Monomeric ferrocene bis-imidazoline bis-palladacycles (FBIP) have recently been reported to be efficient bimetallic catalysts in different sorts of asymmetric reactions by the cooperation of two Pd­(II) centers. A crucial parameter regarding the efficiency of reactions catalyzed in a bimetallic mode isin generalthe intermetallic distance of both catalytically relevant metal centers. In this article we describe the structural elucidation of the monomeric FBIP catalyst type (usually generated in situ from a catalytically inactive dimeric chloride bridged precatalyst) by X-ray crystal structure analysis. Two dicationic monomeric complexes are compared to a neutral complex. The solid-state structures reveal varying Pd–Pd distances ranging from 3.15 to 5.27 Å for the doubly charged complexes, whereas for the neutral complex a distance of 3.28 Å has been found. This variability is supposed to be one of the key advantages of a ferrocene backbone in a bimetallic catalyst system, since the Fe–Cp bonds allow the bimetallic complex to readily open and close like a pair of scissors, employing just a few degrees of rotational freedom. In addition, on the basis of the nature of the reported catalyst species, we suggest that a permanent switch among neutral and mono- and dicationic catalyst species by a Brønsted acid such as acetic acid might facilitate different elementary steps in a catalytic cycle. By DFT calculations we have found that weak d⁸–d⁸ interactions contribute to short Pd–Pd distances but are less important than dispersive interactions, which can even overcome the Coulombic repulsion of two cationic palladium centers

Monomeric Ferrocene Bis-Imidazoline Bis-Palladacycles: Variation of Pd–Pd Distances by an Interplay of Metallophilic, Dispersive, and Coulombic Interactions

Monomeric ferrocene bis-imidazoline bis-palladacycles (FBIP) have recently been reported to be efficient bimetallic catalysts in different sorts of asymmetric reactions by the cooperation of two Pd­(II) centers. A crucial parameter regarding the efficiency of reactions catalyzed in a bimetallic mode isin generalthe intermetallic distance of both catalytically relevant metal centers. In this article we describe the structural elucidation of the monomeric FBIP catalyst type (usually generated in situ from a catalytically inactive dimeric chloride bridged precatalyst) by X-ray crystal structure analysis. Two dicationic monomeric complexes are compared to a neutral complex. The solid-state structures reveal varying Pd–Pd distances ranging from 3.15 to 5.27 Å for the doubly charged complexes, whereas for the neutral complex a distance of 3.28 Å has been found. This variability is supposed to be one of the key advantages of a ferrocene backbone in a bimetallic catalyst system, since the Fe–Cp bonds allow the bimetallic complex to readily open and close like a pair of scissors, employing just a few degrees of rotational freedom. In addition, on the basis of the nature of the reported catalyst species, we suggest that a permanent switch among neutral and mono- and dicationic catalyst species by a Brønsted acid such as acetic acid might facilitate different elementary steps in a catalytic cycle. By DFT calculations we have found that weak d⁸–d⁸ interactions contribute to short Pd–Pd distances but are less important than dispersive interactions, which can even overcome the Coulombic repulsion of two cationic palladium centers