Prolyl 4-Hydroxylase: Substrate Isosteres in Which an (E)- or (Z)-Alkene Replaces the Prolyl Peptide Bond.

Collagen prolyl 4-hydroxylases (CP4Hs) catalyze a prevalent posttranslational modification, the hydroxylation of (2S)-proline residues in protocollagen strands. The ensuing (2S,4R)-4-hydroxyproline residues are necessary for the conformational stability of the collagen triple helix. Prolyl peptide bonds isomerize between cis and trans isomers, and the preference of the enzyme is unknown. We synthesized alkene isosteres of the cis and trans isomers to probe the conformational preferences of human CP4H1. We discovered that the presence of a prolyl peptide bond is necessary for catalysis. The cis isostere is, however, an inhibitor with a potency greater than that of the trans isostere, suggesting that the cis conformation of a prolyl peptide bond is recognized preferentially. Comparative studies with a Chlamydomonas reinhardtii P4H, which has a similar catalytic domain but lacks an N-terminal substrate-binding domain, showed a similar preference for the cis isostere. These findings support the hypothesis that the catalytic domain of CP4Hs recognizes the cis conformation of the prolyl peptide bond and inform the use of alkenes as isosteres for peptide bonds.

Detection of Ethylene with Defined Metal Complexes: Strategies and Recent Advances.

Despite its relative simplicity, ethylene is an interesting molecule with wide-ranging impact in modern chemistry and biology. Stemming from ethylene's role as a critical plant hormone, there has been significant effort to develop selective and sensitive molecular sensors for ethylene. Late transition metal complexes have played an important role in detection strategies due to ethylene's lack of structural complexity and limited reactivity. Two main approaches to ethylene detection are identified: (1) coordination-based sensors, wherein ethylene binds reversibly to a metal center, and (2) activity-based sensors, wherein ethylene undergoes a reaction at a metal center, resulting in the formation and destruction of covalent bonds. Herein, we describe the advantages and disadvantages of various approaches, and the challenges remaining for sensor development.

Steric Modulation of CAACs Controls Orientation and Ethenolysis Performance.

In this issue of Chem Catalysis, Sytniczuk, Kajetanowicz, and Grela report sterically tuned Cyclic(Alkyl)(Amino)Carbene (CAAC) ligands to protect the requisite Ru-methylidene ([Ru]=CH2) intermediate present during ethenolysis of renewable fatty acid methyl esters (FAME). Surprising structural characteristics of the Ru-CAAC complexes resulted in TON up to 740,000 and sub-ppm catalyst loadings.

Advancing the mechanistic understanding of an enantioselective palladium-catalyzed alkene difunctionalization reaction.

The mechanism of an enantioselective palladium-catalyzed alkene difunctionalization reaction has been investigated. Kinetic analysis provides evidence of turnover-limiting attack of a proposed quinone methide intermediate with MeOH and suggests that copper is involved in productive product formation, not just catalyst turnover. Through examination of substrate electronic effects, a Jaffé relationship was observed correlating rate to electronic perturbation at two positions of the substrate. Ligand effects were evaluated to provide evidence of rapid ligand exchange between palladium and copper as well as a correlation between ligand electronic nature and enantioselectivity.

Evaluation of catalyst acidity and substrate electronic effects in a hydrogen bond-catalyzed enantioselective reaction.

A modular catalyst structure was applied to evaluate the effects of catalyst acidity in a hydrogen bond-catalyzed hetero Diels-Alder reaction. Linear free energy relationships between catalyst acidity and both rate and enantioselectivity were observed, where greater catalyst acidity leads to increased activity and enantioselectivity. A relationship between reactant electronic nature and rate was also observed, although there is no such correlation to enantioselectivity, indicating the system is under catalyst control.

Palladium-catalyzed enantioselective addition of two distinct nucleophiles across alkenes capable of quinone methide formation.

A sequential intramolecular-intermolecular enantioselective alkene difunctionalization reaction has been developed which is thought to proceed through Pd-catalyzed quinone methide formation. The synthesis of new chiral heterocyclic compounds with adjacent chiral centers is achieved in enantiomeric ratios up to 99:1 and diastereomeric ratios up to 10:1.

Mechanistic approaches to palladium-catalyzed alkene difunctionalization reactions.

Alkene difunctionalization, the addition of two functional groups across a double bond, exemplifies a class of reactions with significant synthetic potential. This emerging area examines recent developments of palladium-catalyzed difunctionalization reactions, with a focus on mechanistic strategies that allow for functionalization of a common palladium alkyl intermediate.

Palladium(II)-catalyzed enantio- and diastereoselective synthesis of pyrrolidine derivatives.

A palladium-catalyzed enantio- and diastereoselective synthesis of pyrrolidine derivatives is described. Initial intramolecular nucleopalladation of the tethered protected amine forms the pyrrolidine moiety and a quinone methide intermediate. A second nucleophile adds intermolecularly to afford diverse products in high enantio- and diastereoselectivity.