Ben S. Pilgrim

Post-assembly Modification of Tetrazine-Edged FeII4L6 Tetrahedra

Post-assembly modification (PAM) is a powerful tool for the modular functionalization of self-assembled structures. We report a new family of tetrazine-edged Fe(II)4L6 tetrahedral cages, prepared using different aniline subcomponents, which undergo rapid and efficient PAM by inverse electron-demand Diels-Alder (IEDDA) reactions. Remarkably, the electron-donating or -withdrawing ability of the para-substituent on the aniline moiety influences the IEDDA reactivity of the tetrazine ring 11 bonds away. This effect manifests as a linear free energy relationship, quantified using the Hammett equation, between σ(para) and the rate of the IEDDA reaction. The rate of PAM can thus be adjusted by varying the aniline subcomponent.

Synthesis of substituted isoquinolines utilizing palladium-catalyzed α-arylation of ketones

The utilization of sequential palladium-catalyzed α-arylation and cyclization reactions provides a general approach to an array of isoquinolines and their corresponding N-oxides. This methodology allows the convergent combination of readily available precursors in a regioselective manner and in excellent overall yields. This powerful route to polysubstituted isoquinolines, which is not limited to electron rich moieties, also allows rapid access to analogues of biologically active compounds.

Modular isoquinoline synthesis using catalytic enolate arylation and in situ functionalization.

A methyl ketone, an aryl bromide, an electrophile, and ammonium chloride were combined in a four-component, three-step, and one-pot coupling procedure to furnish substituted isoquinolines in overall yields of up to 80%. This protocol utilizes the palladium catalyzed α-arylation reaction of an enolate, followed by in situ trapping with an electrophile, and aromatization with ammonium chloride. tert-Butyl cyanoacetate participated in a similar protocol; after functionalization and decarboxylation, 3-amino-4-alkyl isoquinolines were prepared in high yield.

Osmium-catalyzed oxidative cyclization of dienes and their derivatives.

The development and application of novel methods for accomplishing the synthesis of heterocycles via osmium-catalyzed oxidative cyclization onto an alkene is described in this Perspective. Beginning with a fortuitous discovery, an extensive examination of the possible mechanism of cyclization has been carried out, and the method was continuously developed until it had been transformed into an extremely efficient and powerful new catalytic reaction for the formation of tetrahydrofurans and pyrrolidines with complete control over all aspects of relative and absolute stereochemistry. By working with Os(VI) rather than the more familiar Os(VIII), a highly potent yet mild set of reaction conditions were developed. In addition to the method development studies, this work also sets out some synthetic challenges against which the methodology was tested. Pleasingly, the catalytic oxidative cyclization has proved itself to be an efficient and functional group tolerant process that was pivotal to the completion of several natural product syntheses.

Pathway-Dependent Post-assembly Modification of an Anthracene-Edged MII4L6 Tetrahedron

Fe(II)4L6 tetrahedral cage 1 undergoes post-assembly modification (PAM) via a Diels-Alder cycloaddition of the anthracene panels of the cage with tetracyanoethylene (TCNE). The modified cage 2 possesses an enclosed cavity suitable for encapsulation of the fullerene C60, whereas original cage 1 forms a unique covalent adduct through a Diels-Alder cycloaddition of three of its anthracene ligands with C60. This adduct undergoes further PAM via reaction of the remaining three ligands with TCNE, enabling the isolation of two distinct products depending on the order of addition of C60 and TCNE. Modified cage 2 was also able to bind an anionic guest, [Co(C2B9H11)2](-), which was not encapsulated by the original cage, demonstrating the potential of PAM for tuning the binding properties of supramolecular hosts.

Short and Efficient Syntheses of Protoberberine Alkaloids using Palladium‐Catalyzed Enolate Arylation

A concise synthesis of the biologically active alkaloid berberine is reported, and a versatile palladium-catalyzed enolate arylation is used to form the isoquinoline core. The overall yield of 50 % is a large improvement over the single, previous synthesis. By design, this modular route allows the rapid synthesis of other members of the protoberberine family (e.g., pseudocoptisine and palmatine) by substitution of the readily available aryl bromide and ketone coupling partners. Moreover, by combining enolate arylation with in situ functionalization, substituents can be rapidly and regioselectively introduced at the alkaloid C13 position, as demonstrated by the total synthesis of dehydrocorydaline. The avoidance of electrophilic aromatic substitution reactions to make the isoquinoline allows direct access to analogues possessing more varied electronic properties, such as the fluorine-containing derivative synthesized here.

Palladium nanoparticle-modified carbon nanotubes for electrochemical hydrogenolysis in ionic liquids

The electrochemical hydrogenolysis of benzyl and carboxybenzyl protecting groups is described. Optimisation of a synthetic protocol afforded well-defined palladium (Pd) nanoparticles on the surface of multiwall carbon nanotubes (CNTs). The high surface area composite was investigated, and differences observed in the voltammetry for the same composite in aqueous and ionic liquid systems are discussed. When used to reduce acidic protons in the ionic liquid (IL) 1-ethy-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) reactive palladium hydride was formed efficiently, which rapidly performed hydrogenolysis. The compounds 1-(3-(benzyloxy)propyl)-4-methoxybenzene, benzyl octyl carbonate, N,N′-bis(benzyloxycarbonyl)-L-lysine and N-benzyl-L-prolinol were investigated. Extended electrolysis of bis(trifluoromethylsulfonyl)imide (H[NTf2]) in [C2mim][NTf2] containing alcohol groups protected with benzyl and carboxybenzyl groups afforded the alcohol product in high yield. The probable hydrogenolysis of the similarly protected amine groups is also reported.

Significant changes in pKa between bulk aqueous solution and surface immobilized species: ortho-hydroquinones

The pKa values of two ortho-hydroquinone H2Q, systems immobilized at a carbon electrode are investigated using voltammetric methods and compared with those of the corresponding molecules in bulk aqueous solution. A large and significant increase of both pKa values for the successive equilibria H2Q ⇆ HQ− + H+ ⇆ Q2− + 2H+ are observed, which highlight the fact that very significant differences of thermodynamic and chemical behaviour exist between molecules in bulk solution and immobilized at interfaces, both in general and in the case of quinones which are regularly used as electron transfer mediators in electroanalysis and electrocatalysis.

A green approach to Fenton chemistry: mono-hydroxylation of salicylic acid in aqueous medium by the electrogeneration of Fenton's reagent

First, the electrochemical characteristics of Fentons reagent (Fe(III), Fe(II) and hydrogen peroxide), including its catalytic (EC′) behaviour were investigated. Second, the electrogeneration of hydrogen peroxide by a two-electron reduction of dissolved oxygen was conducted at a carbon electrode in a divided electrolysis cell and the concentration of hydrogen peroxide obtained was determined by the titration. The two approaches were then combined to give a one-pot, relatively green approach to aromatic hydroxylation reactions, with the electro-Fenton method allowing the iron to be used catalytically and the oxygen/water employed as the hydroxylation reagent by the in situ production of hydrogen peroxide. In particular, the preparative-scale hydroxylation of salicylic acid has been studied in an oxygen-saturated 0.1 M Na2SO4 pH 3.0 solution containing 5 mM Fe(II) and 5 mM salicylic acid at the controlled potential of −1.0 V vs. SCE. The study showed that after the attack of the hydroxyl radical produced in situ by the electro-Fenton process on salicylic acid, 2,3- and 2,5-dihydroxybenzoic acids were detected as primary monohydroxylated products with the highest yield of ca. 21.6 ± 2.5%.

Covalent Post-assembly Modification Triggers Multiple Structural Transformations of a Tetrazine-Edged Fe4L6 Tetrahedron

Covalent post-assembly modification (PAM) reactions are useful synthetic tools for functionalizing and stabilizing self-assembled metal-organic complexes. Recently, PAM reactions have also been explored as stimuli for triggering supramolecular structural transformations. Herein we demonstrate the use of inverse electron-demand Diels-Alder (IEDDA) PAM reactions to induce supramolecular structural transformations starting from a tetrazine-edged FeII4L6 tetrahedral precursor. Following PAM, this tetrahedron rearranged to form three different architectures depending on the addition of other stimuli: an electron-rich aniline or a templating anion. By tracing the stimulus-response relationships within the system, we deciphered a network of transformations that mapped different combinations of stimuli onto specific transformation products. Given the many functions being developed for self-assembled three-dimensional architectures, this newly established ability to control the interconversion between structures using combinations of different stimulus types may serve as the basis for switching the functions expressed within a system.