17O-EPR determination of the structure and dynamics of copper single-metal sites in zeolites

The bonding of copper ions to lattice oxygens dictates the activity and selectivity of copper exchanged zeolites. By 17O isotopic labelling of the zeolite framework, in conjunction with advanced EPR methodologies and DFT modelling, we determine the local structure of single site CuII species, we quantify the covalency of the metal-framework bond and we assess how this scenario is modified by the presence of solvating H216O or H217O molecules. This enables to follow the migration of CuII species as a function of hydration conditions, providing evidence for a reversible transfer pathway within the zeolite cage as a function of the water pressure. The results presented in this paper establish 17O EPR as a versatile tool for characterizing metal-oxide interactions in open-shell systems.

Fructosyl transfer from sucrose and oligosaccharides during fructan synthesis in excised leaves of Lolium temulentum L.

Fructan biosynthesis begins with the transfer of a fructosyl moiety from one sucrose molecule to another to yield a trisaccharide. Trisaccharides may also arise by the reversible transfer of a fructosyl moiety from higher oligomers to sucrose but in this case there is no net fructan synthesis. Short‐term and long‐term exposure of detached illuminated leaf blades of Lolium temulentum (L.I to 14CO2 was used to examine the mechanism of transfer of fructosyl residues to sucrose. Two trisaccharides, 1‐kestose and neokestose, were found to be radioactive when leaves excised and illuminated for 15 h ‐were exposed to NCO2 for 30 min. The label increased in neokestose during the chase period, while that in 1‐kestose increased for the first 2 h of the chase period then declined for the remaining 4h. With a longer exposure to 14CO2 during the first 6 h of the induction period, three trisaccharides, neokestose, 1‐kestose and 6‐kestose were radiolabelled. The label turned over in neokestose and 1‐kestose, but continued to accumulate in 6‐kestose during a subsequent 18 h chase period. The specific activities of glucose and fructose of the sucrosyl portion and the terminal fructosyl moiety of the various trisaccharides were compared. In the rapid pulse‐chase experiment the specific activity of the1 terminal fructosyl moiety was consistently less than that of the sucrosyl moiety. During the chase period, the specific activity of the terminal and internal fructose moieties became similar. These results indicate that in addition to trisaccharide formed by transfer of fructosyl units from sucrose, substantial amounts of both neokestose and 1‐kestose are made by transfer of fructosyl units from higher oligomers onto sucrose in reactions probably localized in the vacuole. Copyright © 1991, Wiley Blackwell. All rights reserved

Fructosyl transfer from sucrose and oligosaccharides during fructan synthesis in excised leaves of Lolium temulentum L.

Fructan biosynthesis begins with the transfer of a fructosyl moiety from one sucrose molecule to another to yield a trisaccharide. Trisaccharides may also arise by the reversible transfer of a fructosyl moiety from higher oligomers to sucrose but in this case there is no net fructan synthesis. Short‐term and long‐term exposure of detached illuminated leaf blades of Lolium temulentum (L.I to 14CO2 was used to examine the mechanism of transfer of fructosyl residues to sucrose. Two trisaccharides, 1‐kestose and neokestose, were found to be radioactive when leaves excised and illuminated for 15 h ‐were exposed to NCO2 for 30 min. The label increased in neokestose during the chase period, while that in 1‐kestose increased for the first 2 h of the chase period then declined for the remaining 4h. With a longer exposure to 14CO2 during the first 6 h of the induction period, three trisaccharides, neokestose, 1‐kestose and 6‐kestose were radiolabelled. The label turned over in neokestose and 1‐kestose, but continued to accumulate in 6‐kestose during a subsequent 18 h chase period. The specific activities of glucose and fructose of the sucrosyl portion and the terminal fructosyl moiety of the various trisaccharides were compared. In the rapid pulse‐chase experiment the specific activity of the1 terminal fructosyl moiety was consistently less than that of the sucrosyl moiety. During the chase period, the specific activity of the terminal and internal fructose moieties became similar. These results indicate that in addition to trisaccharide formed by transfer of fructosyl units from sucrose, substantial amounts of both neokestose and 1‐kestose are made by transfer of fructosyl units from higher oligomers onto sucrose in reactions probably localized in the vacuole. Copyright © 1991, Wiley Blackwell. All rights reserved

Overcoming Selectivity Issues in Reversible Catalysis – A Transfer Hydrocyanation Exhibiting High Kinetic Control

<p><i>Typically, reversible catalytic reactions operate under thermodynamic control and thus establishing a selective catalytic system poses a considerable challenge. In this manuscript, we report a reversible yet kinetically selective transfer hydrocyanation protocol. Selectivity is achieved by exploiting the lower barrier for C–CN oxidative addition and reductive elimination at benzylic positions in the absence of co-catalytic Lewis acid. The design of a novel type of HCN donor was crucial to realizing this practical, branched-selective, HCN-free transfer hydrocyanation. The synthetically useful resolution of a mixture of branched and linear nitrile isomers was also demonstrated to underline the value of reversible and selective transfer reactions. In a broader context, this work demonstrates that high kinetic selectivity can be achieved in reversible transfer reactions, thus opening new horizons for their synthetic applications.</i></p>

Overcoming Selectivity Issues in Reversible Catalysis – A Transfer Hydrocyanation Exhibiting High Kinetic Control

<p><i>Typically, reversible catalytic reactions operate under thermodynamic control and thus establishing a selective catalytic system poses a considerable challenge. In this manuscript, we report a reversible yet kinetically selective transfer hydrocyanation protocol. Selectivity is achieved by exploiting the lower barrier for C–CN oxidative addition and reductive elimination at benzylic positions in the absence of co-catalytic Lewis acid. The design of a novel type of HCN donor was crucial to realizing this practical, branched-selective, HCN-free transfer hydrocyanation. The synthetically useful resolution of a mixture of branched and linear nitrile isomers was also demonstrated to underline the value of reversible and selective transfer reactions. In a broader context, this work demonstrates that high kinetic selectivity can be achieved in reversible transfer reactions, thus opening new horizons for their synthetic applications.</i></p>

Para-(aza,thio)xanthenylated anilines in the pereaminination reaction

It is known that the biochemical enzymatic reaction of the reversible transfer of an amino group from an amino-acid to a keto-acid is called a transamination reaction. However, the transamination reaction is applicable not only for biochemical enzymatic reactions, but is also often used in organic synthesis to produce aromatic azomethines. As objects of study in the transamination reaction, we selected substituted N-benzylidenanilines (imines, Schiff bases) and anilines, containing a biologically active heterocyclic fragment in the para-position of the aniline ring. We have shown the feasibility of transamination of substituted N-benzylidenanilines (N-benzylidenaniline, N-benzyliden-4-(5H-benzopyrano[2,3-b]pyridin-5-yl)aniline, N-benzyliden-4-methoxyaniline), heterocyclic anilines (4-(9H-xanthen-9-yl)aniline, 4-(9H-thioxanthen-9-yl)aniline or 4-(5H-benzopyrano[2,3-b]pyridin-5-yl)aniline). It was found that the interaction of 4-(9H-xanthen-9-yl)aniline, 4-(9H-thioxanthen-9-yl)aniline or 4- (5H-benzopyrano[2,3-b]pyridin-5-yl)aniline with N-benzylidenanilines, the imine aniline cycle is replaced by the corresponding fragment of heterylated aniline, with the formation of new N-benzylidenanilines, the structure of which is proved by a breakdown of mixed melting and H1 NMR spectroscopy. However, the transamination reaction does not proceed with the use of N-benzyliden-4-methoxyaniline. This, apparently, is associated with the presence of an electron-donating substituent at the para-position of the aniline imines fragment. Thus, a series of activity of the studied compounds in the transamination reaction of substituted anilines was experimentally established. The most active of these is 4-methoxyaniline, followed by 4-(9H-xanthen-9-yl)aniline, 4-(9H-thioxanthen-9-yl) aniline, 4-(5H-benzopyrano[2,3-b]pyridin-5-yl)aniline, and closes the series of the least active, unsubstituted aniline. The synthesis method proposed in this work allows one to obtain new substituted N-benzylidenanilines, and the studied series of activity allows one to predict the behavior of anilines containing various electron-donating and electron-withdrawing substituents in the transamination reaction with N-benzylidenanilines.

Simultaneous optimization of donor and acceptor substrate specificity for transketolase by a small but smart library

A narrow substrate range is a major limitation in exploiting enzymes more widely as catalysts in synthetic organic chemistry. For enzymes using two substrates, the simultaneous optimization of both substrate specificities, is also required for the rapid expansion of accepted substrates. Transketolase catalyses the reversible transfer of a C2-ketol unit from a donor substrate to an aldehyde acceptor and suffers the limitation of narrow substrate scope for widely industrial applications. Herein, transketolase from E. coli was engineered to simultaneously accept both pyruvate as a novel donor substrate, and unnatural acceptor aldehydes, including propanal, pentanal, hexanal and 3-formylbenzoic acid. Twenty single-mutant variants were firstly designed and characterized experimentally. Beneficial mutations were then recombined to construct a small but smart library. Screening of this library identified the best variant with a 9.2-fold improvement in the yield towards pyruvate and propionaldehyde, relative to WT. Pentanal and hexanal were used as acceptors to determine stereoselectivities of the reactions, which were found to be higher than 98% ee for the S configuration. Three variants were identified to be active for the reaction between pyruvate and 3-formylbenzoic acid. The best variant was able to convert 47% of substrate into product within 24 h, whereas no conversion was observed for WT. Docking experiments suggested a cooperation between the mutations responsible for donor and acceptor acceptances, that would promote the activity towards both the acceptor and donor. The variants obtained have the potential to be used for developing catalytic pathways to a diverse range of high-value products.