Selective activation of organocatalysts by specific signals

Chemical Information Processing by a Responsive Chemical System

Nature has an extraordinary capacity to precisely regulate the chemical reactivity in a highly complex mixture of molecules that is present in the cell. External stimuli lead to transient up- and downregulation of chemical reactions and provide a means for a cell to process information arriving from the environment. The development of synthetic chemical systems with life-like properties requires strategies that allow likewise control over chemical reactivity in a complex environment. Here, we show a synthetic system that mimics the initial steps that take place when a natural signal transduction pathway is activated. Monophosphate nucleosides act as chemical triggers for the self-assembly of nanoreactors that upregulate chemical reactions between reagents present at low micromolar concentrations. Different nucleotides template different assemblies and hence activate different pathways, thus establishing a distinct connection between input and output molecules. Trigger-induced upregulation of chemical reactivity occurs for only a limited amount of time because the chemical triggers are gradually removed from the system by enzymes. It is shown that the same system transiently produces different output molecules depending on the chemical input that is provided.

On-Demand Release of Secondary Amine Bases for the Activation of Catalysts and Crosslinkers

Dynamic covalent (DCv) ureas have been used abundantly to design self-healing materials. We demonstrate that apart from self-healing materials, the species present in the equilibrium of DCv ureas can be employed as responsive organocatalysts. Easily controllable stimuli like heat or addition of water shift the equilibrium towards isocyanate and free base which can function as an in situ released reagent. We demonstrate this application of DCv ureas with two examples. Firstly, we use the liberated base to catalytically activate a latent organocatalyst for acylhydrazone formation. Secondly, this base can be employed in an equimolar manner to trigger the release of nitrile-N-oxides from chlorooximes, which react with acrylate-terminated polymers to form an isoxazoline polymer gel.

Switchable aqueous catalytic systems for organic transformations

In living organisms, enzyme catalysis takes place in aqueous media with extraordinary spatiotemporal control and precision. The mechanistic knowledge of enzyme catalysis and related approaches of creating a suitable microenvironment for efficient chemical transformations have been an important source of inspiration for the design of biomimetic artificial catalysts. However, in "nature-like" environments, it has proven difficult for artificial catalysts to promote effective chemical transformations. Besides, control over reaction rate and selectivity are important for smart application purposes. These can be achieved via incorporation of stimuli-responsive features into the structure of smart catalytic systems. Here, we summarize such catalytic systems whose activity can be switched 'on' or 'off' by the application of stimuli in aqueous environments. We describe the switchable catalytic systems capable of performing organic transformations with classification in accordance to the stimulating agent. Switchable catalytic activity in aqueous environments provides new possibilities for the development of smart materials for biomedicine and chemical biology. Moreover, engineering of aqueous catalytic systems can be expected to grow in the coming years with a further broadening of its application to diverse fields.

Mechanoresponsive Carbamoyloximes for the Activation of Secondary Amines in Polymers

Mechanophores are molecular moieties that are incorporated into polymers and respond to force with constitutional, configurational, or conformational bond rearrangements to enable functionality. Up to today, several chemically latent motifs have been activated by polymer mechanochemical methods, but the generation of secondary amines remains elusive. Here we report carbamoyloximes as mechanochemical protecting groups for secondary amines. We show that carbamoyloximes undergo force-induced homolytic bond scission at the N-O oxime bond in polymers thus producing the free amine, as the reaction proceeds via the carbamoyloxyl and aminyl radicals, analogously to its photochemical counterpart. Eventually, we apply the carbamoyloxime motif in a force-activated organocatalytic Knoevenagel reaction. We believe that this protecting strategy can be universally applied for many other secondary and primary amines in polymer materials.

Visible-Light-Initiated Palladium-Catalyzed Cross-coupling by PPh3 Uncaging from an Azobenzene Ruthenium-Arene Complex

Photo-release of triphenylphosphine from a sulfonamide azobenzene ruthenium-arene complex was exploited to activate PdII Cl2 into Pd0 catalyst, for the photo-initiation of Sonogashira cross-coupling. The transformation was initiated on demand - by using simple white LED strip lights - with a high temporal response and the ability to control reaction rate by changing the irradiation time. Various substrates were successfully applied to this photo-initiated cross-coupling, thus illustrating the wide functional-group tolerance of our photo-caged catalyst activator, without any need for sophisticated photochemistry apparatus.

Transient Host-Guest Complexation To Control Catalytic Activity

Signal transduction mechanisms are key to living systems. Cells respond to signals by changing catalytic activity of enzymes. This signal responsive catalysis is crucial in the regulation of (bio)chemical reaction networks (CRNs). Inspired by these networks, we report an artificial signal responsive system that shows signal-induced temporary catalyst activation. We use an unstable signal to temporarily activate an out of equilibrium CRN, generating transient host–guest complexes to control catalytic activity. Esters with favorable binding toward the cucurbit[7]uril (CB[7]) supramolecular host are used as temporary signals to form a transient complex with CB[7], replacing a CB[7]-bound guest. The esters are hydrolytically unstable, generating acids and alcohols, which do not bind to CB[7], leading to guest reuptake. We demonstrate the feasibility of the concept using signal-controlled temporary dye release and reuptake. The same signal controlled system was then used to tune the reaction rate of aniline catalyzed hydrazone formation. Varying the ester structure and concentration gave access to different catalyst liberation times and free catalyst concentration, regulating the overall reaction rate. With temporary signal controlled transient complex formation we can tune the kinetics of a second chemical reaction, in which the signal does not participate. This system shows promise for building more complex nonbiological networks, to ultimately arrive at signal transduction in organic materials.

Metal-Mediated Organocatalysis in Water: Serendipitous Discovery of Aldol Reaction Catalyzed by the [Ru(bpy)2(nornicotine)2]2+ Complex

The [Ru(bpy)2(Nor)2]2+ complex (Nor = nornicotine) is an efficient catalyst for the aldol reaction of acetone with activated benzaldehydes in a buffered aqueous solution. The metal plays the role of an activator for the nornicotine organocatalyst ligands. The resulting catalytic activity is potentiated by a factor of about 4.5 as compared to free nornicotine. Similar rate enhancements can be achieved by using Zn(II) cations as the activator. The observations are rationalized with the reduced basicity of the pyrrolidine N in nornicotine due to the enhanced electron withdrawal of the metal-complexed pyridyl moiety.

On the use of catalysis to bias reaction pathways in out-of-equilibrium systems

Catalysis is an essential function in living systems and provides a way to control complex reaction networks. In natural out-of-equilibrium chemical reaction networks (CRNs) driven by the consumption of chemical fuels, enzymes provide catalytic control over pathway kinetics, giving rise to complex functions. Catalytic regulation of man-made fuel-driven systems is far less common and mostly deals with enzyme catalysis instead of synthetic catalysts. Here, we show via simulations, illustrated by literature examples, how any catalyst can be incorporated in a non-equilibrium CRN and what their effect is on the behavior of the system. Alteration of the catalysts' concentrations in batch and flow gives rise to responses in maximum conversion, lifetime (i.e. product half-lives and t90 - time to recover 90% of the reactant) and steady states. In situ up or downregulation of catalysts' levels temporarily changes the product steady state, whereas feedback elements can give unusual concentration profiles as a function of time and self-regulation in a CRN. We show that simulations can be highly effective in predicting CRN behavior. In the future, shifting the focus from enzyme catalysis towards small molecule and metal catalysis in out-of-equilibrium systems can provide us with new reaction networks and enhance their application potential in synthetic materials, overall advancing the design of man-made responsive and interactive systems.

Visible Light-Gated Organocatalysis Using a RuII -Photocage

The light-gated organocatalysis via the release of 4-N,N-dimethylaminopyridine (DMAP) by irradiation of the [Ru(bpy)₂ (DMAP)₂ ]²⁺ complex with visible light was investigated. As model reaction the acetylation of benzyl alcohols with acetic anhydride was chosen. The pre-catalyst releases one DMAP molecule on irradiation at wavelengths longer than 455 nm. The photochemical process was characterized by steady-state irradiation and ultrafast transient absorption spectroscopy. The latter enabled the observation of the ³ MLCT state and the spectral features of the penta-coordinated intermediate [Ru(bpy)₂ (DMAP)]²⁺ . The released DMAP catalyzes the acetylation of a wide range of benzyl alcohols with chemical yields of up to 99 %. Control experiments revealed unequivocally that it is the released DMAP which takes the role of the catalyst.

Conditional Copper-Catalyzed Azide-Alkyne Cycloaddition by Catalyst Encapsulation

Supramolecular encapsulation is known to alter chemical properties of guest molecules. We have applied this strategy of molecular encapsulation to temporally control the catalytic activity of a stable copper(I)-carbene catalyst. Encapsulation of the copper(I)-carbene catalyst by the supramolecular host cucurbit[7]uril (CB[7]) resulted in the complete inactivation of a copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction. The addition of a chemical signal achieved the near instantaneous activation of the catalyst, by releasing the catalyst from the inhibited CB[7] catalyst complex. To broaden the scope of our on-demand CuAAC reaction, we demonstrated the protein labeling of vinculin with the copper(I)-carbene catalyst, to inhibit its activity by encapsulation with CB[7] and to initiate labeling at any moment by adding a specific signal molecule. Ultimately, this strategy allows for temporal control over copper-catalyzed click chemistry, on small molecules as well as protein targets.

Caged Proline in Photoinitiated Organocatalysis

Organocatalysis is an emerging field, in which small metal-free organic structures catalyze a diversity of reactions with a remarkable stereoselectivity. The ability to selectively switch on such pathways upon demand has proven to be a valuable tool in biological systems. Light as a trigger provides the ultimate spatial and temporal control of activation. However, there have been limited examples of phototriggered catalytic systems. Herein, we describe the synthesis and application of a caged proline system that can initiate organocatalysis upon irradiation. The caged proline was generated using the highly efficient 4-carboxy-5,7-dinitroindolinyl (CDNI) photocleavable protecting group in a four-step synthesis. Advantages of this system include water solubility, biocompatibility, high quantum yield for catalyst release, and responsiveness to two-photon excitation. We showed the light-triggered catalysis of a crossed aldol reaction, a Mannich reaction, and a self-aldol condensation reaction. We also demonstrated light-initiated catalysis, leading to the formation of a biocide in situ, which resulted in the growth inhibition of E. coli, with as little as 3 min of irradiation. This technique can be broadly applied to other systems, by which the formation of active forms of drugs can be catalytically assembled remotely via two-photon irradiation.

Orthogonal Stimuli Trigger Self-Assembly and Phase Transfer of FeII4L4 Cages and Cargoes

Two differently protected aldehydes, A and B, were demonstrated to deprotect selectively through the application of light and heat, respectively. In the presence of iron(II) and a triamine, two distinct Fe^II₄L₄ cages, 1 and 2, were thus observed to form from the deprotected A and B, respectively. The alkyl tails of B and 2 render them preferentially soluble in cyclopentane, whereas A and 1 remain in acetonitrile. The stimulus applied (either light or heat) thus determines the outcome of self-assembly and dictates whether the cage and its ferrocene cargo remain in acetonitrile, or transport into cyclopentane. Cage self-assembly and cargo transport between phases can in this fashion be programmed using orthogonal stimuli.