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"Rhodium - chemistry"
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Biotinylated Rh(III) Complexes in Engineered Streptavidin for Accelerated Asymmetric C-H Activation
Enzymes provide an exquisitely tailored chiral environment to foster high catalytic activities and selectivities, but their native structures are optimized for very specific biochemical transformations. Designing a protein to accommodate a non-native transition metal complex can broaden the scope of enzymatic transformations while raising the activity and selectivity of small-molecule catalysis. Here, we report the creation of a bifunctional artificial metalloenzyme in which a glutamic acid or aspartic acid residue engineered into streptavidin acts in concert with a docked biotinylated rhodium(III) complex to enable catalytic asymmetric carbon-hydrogen (C-H) activation. The coupling of benzamides and alkenes to access dihydroisoquinolones proceeds with up to nearly a 100-fold rate acceleration compared with the activity of the isolated rhodium complex and enantiomeric ratios as high as 93:7.
Journal Article
Rhodium-catalysed syn-carboamination of alkenes via a transient directing group
Alkenes are the most ubiquitous prochiral functional groups--those that can be converted from achiral to chiral in a single step--that are accessible to synthetic chemists. For this reason, difunctionalization reactions of alkenes (whereby two functional groups are added to the same double bond) are particularly important, as they can be used to produce highly complex molecular architectures. Stereoselective oxidation reactions, including dihydroxylation, aminohydroxylation and halogenation, are well established methods for functionalizing alkenes. However, the intermolecular incorporation of both carbon- and nitrogen-based functionalities stereoselectively across an alkene has not been reported. Here we describe the rhodium-catalysed carboamination of alkenes at the same (syn) face of a double bond, initiated by a carbon-hydrogen activation event that uses enoxyphthalimides as the source of both the carbon and the nitrogen functionalities. The reaction methodology allows for the intermolecular, stereospecific formation of one carbon-carbon and one carbon-nitrogen bond across an alkene, which is, to our knowledge, unprecedented. The reaction design involves the in situ generation of a bidentate directing group and the use of a new cyclopentadienyl ligand to control the reactivity of rhodium. The results provide a new way of synthesizing functionalized alkenes, and should lead to the convergent and stereoselective assembly of amine-containing acyclic molecules.
Journal Article
Catalytic activation of carbon-carbon bonds in cyclopentanones
In the chemical industry, molecules of interest are based primarily on carbon skeletons. When synthesizing such molecules, the activation of carbon-carbon single bonds (C-C bonds) in simple substrates is strategically important: it offers a way of disconnecting such inert bonds, forming more active linkages (for example, between carbon and a transition metal) and eventually producing more versatile scaffolds. The challenge in achieving such activation is the kinetic inertness of C-C bonds and the relative weakness of newly formed carbon-metal bonds. The most common tactic starts with a three- or four-membered carbon-ring system, in which strain release provides a crucial thermodynamic driving force. However, broadly useful methods that are based on catalytic activation of unstrained C-C bonds have proven elusive, because the cleavage process is much less energetically favourable. Here we report a general approach to the catalytic activation of C-C bonds in simple cyclopentanones and some cyclohexanones. The key to our success is the combination of a rhodium pre-catalyst, an N-heterocyclic carbene ligand and an amino-pyridine co-catalyst. When an aryl group is present in the C3 position of cyclopentanone, the less strained C-C bond can be activated; this is followed by activation of a carbon-hydrogen bond in the aryl group, leading to efficient synthesis of functionalized α-tetralones-a common structural motif and versatile building block in organic synthesis. Furthermore, this method can substantially enhance the efficiency of the enantioselective synthesis of some natural products of terpenoids. Density functional theory calculations reveal a mechanism involving an intriguing rhodium-bridged bicyclic intermediate.
Journal Article
Engineering a dirhodium artificial metalloenzyme for selective olefin cyclopropanation
Artificial metalloenzymes (ArMs) formed by incorporating synthetic metal catalysts into protein scaffolds have the potential to impart to chemical reactions selectivity that would be difficult to achieve using metal catalysts alone. In this work, we covalently link an alkyne-substituted dirhodium catalyst to a prolyl oligopeptidase containing a genetically encoded L-4-azidophenylalanine residue to create an ArM that catalyses olefin cyclopropanation. Scaffold mutagenesis is then used to improve the enantioselectivity of this reaction, and cyclopropanation of a range of styrenes and donor-acceptor carbene precursors were accepted. The ArM reduces the formation of byproducts, including those resulting from the reaction of dirhodium-carbene intermediates with water. This shows that an ArM can improve the substrate specificity of a catalyst and, for the first time, the water tolerance of a metal-catalysed reaction. Given the diversity of reactions catalysed by dirhodium complexes, we anticipate that dirhodium ArMs will provide many unique opportunities for selective catalysis.
Journal Article
Pt-, Rh-, Ru-, and Cu-Single-Wall Carbon Nanotubes Are Exceptional Candidates for Design of Anti-Viral Surfaces: A Theoretical Study
As SARS-CoV-2 is spreading rapidly around the globe, adopting proper actions for confronting and protecting against this virus is an essential and unmet task. Reactive oxygen species (ROS) promoting molecules such as peroxides are detrimental to many viruses, including coronaviruses. In this paper, metal decorated single-wall carbon nanotubes (SWCNTs) were evaluated for hydrogen peroxide (H
O
) adsorption for potential use for designing viral inactivation surfaces. We employed first-principles methods based on the density functional theory (DFT) to investigate the capture of an individual H
O
molecule on pristine and metal (Pt, Pd, Ni, Cu, Rh, or Ru) decorated SWCNTs. Although the single H
O
molecule is weakly physisorbed on pristine SWCNT, a significant improvement on its adsorption energy was found by utilizing metal functionalized SWCNT as the adsorbent. It was revealed that Rh-SWCNT and Ru-SWCNT systems demonstrate outstanding performance for H
O
adsorption. Furthermore, we discovered through calculations that Pt- and Cu-decorated SWNCT-H
O
systems show high potential for filters for virus removal and inactivation with a very long shelf-life (2.2 × 10
and 1.9 × 10
years, respectively). The strong adsorption of metal decorated SWCNTs and the long shelf-life of these nanomaterials suggest they are exceptional candidates for designing personal protection equipment against viruses.
Journal Article
Preparation of chiral-at-metal catalysts and their use in asymmetric photoredox chemistry
Asymmetric catalysis is a powerful approach for the synthesis of optically active compounds, and visible light constitutes an abundant source of energy to enable chemical transformations, which are often triggered by photoinduced electron transfer (photoredox chemistry). Recently, bis-cyclometalated iridium(III) and rhodium(III) complexes were introduced as a novel class of catalysts for combining asymmetric catalysis with visible-light-induced photoredox chemistry. These catalysts are attractive because of their unusual feature of chirality originating exclusively from a stereogenic metal center, which offers the prospect of an especially effective asymmetric induction upon direct coordination of the substrate to the metal center. As these chiral catalysts contain only achiral ligands, special strategies are required for their synthesis. In this protocol, we describe strategies for preparing two types of chiral-at-metal catalysts, namely the Λ- and Δ-enantiomers (left- and right-handed propellers, respectively) of the iridium complex IrS and the rhodium complex RhS. Both contain two cyclometalating 5-tert-butyl-2-phenylbenzothiazoles in addition to two acetonitrile ligands and a hexafluorophosphate counterion. The two cyclometalated ligands set the propeller-shaped chiral geometry, but the acetonitriles are labile and can be replaced by substrate molecules. The synthesis protocol consists of three stages: first, preparation of the ligand 5-tert-butyl-2-phenylbenzothiazole; second, preparation of salicylthiazoline (used for iridium) and salicyloxazoline (used for rhodium) chiral auxiliaries; and third, the auxiliary-mediated synthesis of the individual enantiopure Λ- and Δ-configured catalysts. This class of stereogenic-only-at-metal complexes is of substantial value in the field of asymmetric catalysis, offering stereocontrolled radical reactions based on visible-light-activated photoredox chemistry. Representative examples of visible-light-induced asymmetric catalysis are provided.
Journal Article
Cooperative activation of cyclobutanones and olefins leads to bridged ring systems by a catalytic 4 + 2 coupling
Bridged ring systems are widely found in natural products, and successful syntheses of them frequently feature intramolecular Diels-Alder reactions. These reactions are subclassified as either type I or type II depending on how the diene motif is tethered to the rest of the substrate (type I are tethered at the 1-position of the diene and type II at the 2-position). Although the type I reaction has been used with great success, the molecular scaffolds accessible by the type II reactions are limited by the strain inherent in the formation of an sp(2) carbon at a bridgehead position. Here, we describe a complementary approach that provides access to these structures through the C-C activation of cyclobutanones and their coupling with olefins. Various alkenes have been coupled with cyclobutanones to provide a range of bridged skeletons. The ketone group of the products serves as a convenient handle for downstream functionalization.
Journal Article
A site-selective amination catalyst discriminates between nearly identical C-H bonds of unsymmetrical disubstituted alkenes
C-H activation reactions enable chemists to unveil new retrosynthetic disconnections and streamline conventional synthetic approaches. A long-standing challenge in C-H activation is the inability to distinguish electronically and sterically similar C-H bonds. Although numerous synergistic combinations of transition-metal complexes and chelating directing groups have been utilized to distinguish C-H bonds, undirected regioselective C-H functionalization strategies remain elusive. Here we report a regioselective C-H activation/amination reaction of various unsymmetrical dialkyl-substituted alkenes. The regioselectivity of C-H activation is correlated to the electronic properties of allylic C-H bonds indicated by the corresponding
J
coupling constants. A linear relationship between the difference in the
J
coupling constants of the two competing allylic C-H bonds (Δ
J
) and the C-H activation barriers (ΔΔG
) has also been determined.
Journal Article
Metal-catalysed 1,2-diamination reactions
The 1,2-diamine motif is present in a number of natural products with interesting biological activity and in many important pharmaceutical agents. Chiral 1,2-diamines are also widely used as the control elements in asymmetric synthesis and catalysis. Such compounds are thus an attractive target for the synthetic chemist. Although the diamination of an alkene seems an obvious route to these structures, far less research has been devoted to it than to the analogous dihydroxylation or aminohydroxylation reactions that are well-established processes in asymmetric synthesis. Here, we examine recent advances in metal-catalysed diamination reactions and their asymmetric variants. Given the prevalence of these structures, it seems likely that they will find extensive application in the construction of natural products and drug molecules in the near future.
Journal Article