ConspectusOxidation reactions are powerful tools for synthesis because they allow us to reverse the polarity of electron-rich functional groups, generate highly reactive intermediates, and increase the functionality of molecules. For this reason, oxidation reactions have been and continue to be the subject of intense study. Central to these efforts is the development of mechanism-based strategies that allow us to think about the reactive intermediates that are frequently central to the success of the reactions and the mechanistic pathways that those intermediates trigger. For example, consider oxidative cyclization reactions that are triggered by the removal of an electron from an electron-rich olefin and lead to cyclic products that are functionalized for further elaboration. For these reactions to be successful, the radical cation intermediate must first be generated using conditions that limit its polymerization and then channeled down a productive desired pathway. Following the cyclization, a second oxidation step is necessary for product formation, after which the resulting cation must be quenched in a controlled fashion to avoid undesired elimination reactions. Problems can arise at any one or all of these steps, a fact that frequently complicates reaction optimization and can discourage the development of new transformations. Fortunately, anodic electrochemistry offers an outstanding opportunity to systematically probe the mechanism of oxidative cyclization reactions. The use of electrochemical methods allows for the generation of radical cations under neutral conditions in an environment that helps prevent polymerization of the intermediate. Once the intermediates have been generated, a series of “telltale indicators” can be used to diagnose which step in an oxidative cyclization is problematic for less successful transformation. A set of potential solutions to address each type of problem encountered has been developed. For example, problems with the initial cyclization reaction leading to either polymerization of the radical cation, elimination of a proton from or solvent trapping of that intermediate, or solvent trapping of the radical cation can be identified in the proton NMR spectrum of the crude reaction material. Such an NMR spectrum shows retention of the trapping group. The problems can be addressed by tuning the radical cation, altering the trapping group, or channeling the reactive intermediate down a radical pathway. Specific examples each are shown in this Account. Problems with the second oxidation step can be identified by poor current efficiency or general decomposition in spite of cyclic voltammetry evidence for a rapid cyclization. Solutions involve improving the oxidation conditions for the radical after cyclization by either the addition of a properly placed electron-donating group in the substrate or an increase in the concentration of electrolyte in the reaction (a change that stabilizes the cation generated from the second oxidation step). Problems with the final cation typically lead to overoxidation. Solutions to this problem require an approach that either slows down elimination side reactions or changes the reaction conditions so that the cation can be quickly trapped in an irreversible fashion. Again, this Account highlights these strategies along with the specific experimental protocols utilized.

In electrochemical processes, an oxidation half-reaction is always paired with a reduction half-reaction. Although systems for reactions such as the reduction of CO2 can be coupled to water oxidation to produce O2 at the anode, large-scale O2 production is of limited value. One may replace a low-value half-reaction with a compatible half-reaction that can produce a valuable chemical compound and operate at a lower potential. In doing so, both the anodic and cathodic half-reactions yield desirable products with a decreased energy demand. Here we demonstrate a paired electrolysis in the case of the oxidative condensation of syringaldehyde and o-phenylenediamine to give 2-(3,5-dimethoxy-4-hydroxyphenyl)benzimidazole coupled with the reduction of CO2 to CO mediated by molecular electrocatalysts. We also present general principles for evaluating current–voltage characteristics and power demands in paired electrolyzers.

Cu(I)-catalyzed “click” reactions cannot be performed on a borate ester derived polymer coating on a microelectrode array because the Cu(II) precursor for the catalyst triggers background reactions between both acetylene and azide groups with the polymer surface. Fortunately, the Cu(II)-background reaction can itself be used to site-selectively add the acetylene and azide nucleophiles to the surface of the array. In this way, molecules previously functionalized for use in “click” reactions can be added directly to the array. In a similar fashion, activated esters can be added site-selectively to a borate ester coated array. The new chemistry can be used to explore new biological interactions on the arrays. Specifically, the binding of a v107 derived peptide with both human and murine VEGF was probed using a functionalized microelectrode array.

The placement of a peptide onto a microelectrode array is frequently complicated by the presence of multiple nucleophiles in the peptide. In the work reported here, we have found that the Chan–Lam coupling reactions used to site-selectively place thiol, alcohol, and amine nucleophiles onto diblock-copolymer-coated surfaces are chemoselective for the placement of thiol and alcohol nucleophiles on the arrays. This means that cysteine- and serine-containing peptides can be placed onto an array without any need to protect the N terminus of the peptide. Furthermore, it was found that placement of thiol groups onto an array with the Chan–Lam reaction using optimized reaction times leads to complete coverage of the electrodes. The extent of this coverage can be controlled by varying the reaction time in a manner that allows for the construction of arrays with a gradient of peptide concentrations.

A simplified analog (WU-07047) of the selective Gαq/11 inhibitor YM-254890 has been synthesized, and an initial probe of its activity conducted. In the analog, the two peptide-based linkers in the cyclic YM-254890 have been replaced with hydrocarbon chains. This enables a convergent approach to the synthesis of the analog. Biochemical assays showed that while the simplified analog is not as potent as YM-254890, it does still inhibit Gq.

Microelectrode arrays have great potential as analytical tools because currents can be independently measured at each electrode in the array. In principle, these currents can be monitored in order to follow in real time the binding events that occur between the members of a molecular library and a biological target. To capitalize on this potential, the surface of the array must be selectively functionalized so that each unique member of the molecular library is associated with a unique individually addressable electrode or set of electrodes in the array. To this end, this instructional review summarizes methods for coating the arrays with porous polymers that allow for the attachment of molecules to the surface of the array, selectively conducting reactions at individual electrodes in the array, characterizing molecules that are placed on the arrays, and running the analytical experiments needed to monitor in real time binding events between molecules on the array and a biological target.

The bipyrazine ligand is often employed in photoredox catalysts in order to increase the excited state oxidation potential of the catalyst. However, literature syntheses of the ligand are cumbersome and typically lead to low yields. This hampers use of the desired catalysts. We report here an efficient copper based synthesis of the bipyrazine ligand that affords the product in 65–76% yield on a multigram scale.

Either aldehyde or cinnamyl ether products can be selectively extracted from raw sawdust by controlling the temperature and pressure of a solvolysis reaction. These materials have been used as platform chemicals for the synthesis of 15 different synthetic substrates. The conversion of the initial sawdust-derived materials into electron-rich aryl substrates often requires the use of oxidation and reduction chemistry, and the role electrochemistry can play as a sustainable method for these transformations has been defined.

Inexpensive, readily available photovoltaic cells have been used to conduct indirect electrochemical oxidation reactions. The reactions retain the efficiency of the solar-electrochemical method while capitalizing on the unique opportunities for selectivity afforded by a chemical oxidant. The versatility of the electrochemical method allowed for the recycling of Os(VIII)-, TEMPO-, Ce(IV)-, Pd(II)-, Ru(VIII)-, and Mn(V)-oxidants all with the same very simple reaction apparatus.

Organometallic reagents and catalysts are powerful tools for site-selectively modifying the surface of a microelectrode array. In this context, the reagents or catalysts are generated at the electrodes in the array and then destroyed again in the solution above the array. In this way, they can be used to conduct reactions that are confined to the regions of the array immediately surrounding the electrodes used. In the review presented, this general strategy is applied to a series of organometallic reactions that serve to illustrate the scope of synthetic chemistry possible.

Oxidation reactions are powerful tools for synthesis because they allow for the functionalization of molecules. Here, we present a general method for conducting these reactions on a microelectrode array in a site-selective fashion. The reactions are run as a competition between generation of a chemical oxidant at the electrodes in the array and reduction of the oxidant by a “confining agent” in the solution above the array. The “confining agent” does not need to be more reactive than the substrate fixed to the surface of the array. In many cases, the same substrate placed on the surface of the array can also be used in solution as the confining agent.

Anodic oxidation reactions have been used to synthesize aryl- and biaryl-substituted C-glycosides. The reactions take advantage of the tendency for alcohol nucleophiles to trap nonpolar radical cations. The addition of the alcohol to the radical cation appears to be reversible, and the success of the cyclizations is dependent on the ease with which the resulting benzylic radical is oxidized.

Peptides have been site-selectively placed on microelectrode arrays with the use of both thiol-based conjugate additions and Cu(I)-coupling reactions between thiols and aryl halides. The conjugate addition reactions used both acrylate and maleimide Michael acceptors. Of the two methods, the Cu(I)-coupling reactions proved far superior because of their irreversibility. Surfaces constructed with the conjugate addition chemistry were not stable at neutral pHs, especially the surface using the maleimide acceptor. Once a peptide was placed onto the array, it could be monitored in “real-time” for its interactions with a biological receptor.

A new amino acid derived fluorescent linker for attaching molecules to the surface of a microelectrode array has been developed. Molecules to be monitored on an array are attached to the C-terminus of the linker, the N-terminus is then used to attach the linker to the array, and the side chain is used to synthesize a fluorescent tag. The fluorescent group is made with a one-step oxidative cycloaddition reaction starting from a hydroxyindole group. The linker is compatible with site-selective Cu(I)-chemistry on the array, it allows for quality control assessment of the array itself, and it is compatible with the electrochemical impedance experiments used to monitor binding events on the surface of the array.

Inexpensive, readily available photovoltaic cells have been used to conduct electrochemical oxidation reactions. In this way, the energy efficiency of sunlight-driven reactions can be combined with the versatility of electrochemistry to create new, sustainable methods for conducting oxidation reactions.

Intramolecular anodic olefin coupling reactions can be compatible with the presence of dithioketal protecting groups even though the dithioketal oxidizes at a lower potential than either of the groups participating in the cyclization. In such cases, product formation is controlled by the Curtin−Hammett Principle. In this study, the generality of such reactions is examined along with the use of alternative reaction conditions to suppress unwanted side reactions.

Site-selective Cu(I)-catalyzed reactions have been developed on microelectrode arrays. The reactions are confined to preselected electrodes on the arrays using oxygen as the confining agent. Conditions initially developed for the Cu(I)-catalyzed click reaction have proven general for the coupling of amine, alcohol, and sulfur nucleophiles to both vinyl and aryl iodides. Differences between reactions run on 1-K arrays and reactions run on 12-K arrays can be attributed to the 1-K array reactions being divided cell electrolyses and the 12-K array reactions being undivided cell electrolyses. Reactions on the 12-K arrays benefit from the use of a non-sugar-derived porous reaction layer for the attachment of substrates to the surface of the electrodes. The reactions are sensitive to the nature of the ligand used for the Cu catalyst.

A “safety-catch” linker strategy has been used to release a portion of the products of a Diels–Alder reaction conducted on a microelectrode array for characterization of stereochemistry. The attachment and cleavage of organic compounds from the surface of selected electrodes in the array can be accomplished by site-selective generation of base or acid at the electrode. It was found that the surface of the array had a minor influence on the stereochemistry of the Diels–Alder reaction, leading to slightly more of the exo-product relative to a similar solution-phase reaction.

A “safety-catch” linker strategy has been used to site-selectively cleave and characterize molecules from a microelectrode array. The linkers are attached to the array by means of an ester and contain either a protected amine or protected alcohol nucleophile that can be released using acid generated at the microelectrodes.

Anodic olefin coupling reactions using a tosylamine trapping group have been studied. The cyclizations are favored by the use of a less-polar radical cation and more basic reaction conditions. The most significant factor for obtaining good yields of cyclic product is the use of the more basic reaction conditions. However, a number of factors including the nature of both the solvent and the electrolyte used can influence the yield of the cyclizations. The cyclizations allow for the rapid synthesis of both substituted proline and pipecolic acid type derivatives.

Site-selective Pd(0)-catalyzed reactions have been developed to functionalize a microelectrode array. Heck, Suzuki, and allylation reactions have all been accomplished. The reactions are compatible with both 1K and 12K arrays and work best when a nonsugar porous reaction layer is used. Suzuki reactions are faster than the Heck reactions and thus require more careful control of the reactions in order to maintain confinement. The allylation reaction requires a different confining agent than the Heck and Suzuki reactions but can be accomplished nicely with quinone as an oxidant for Pd(0).

Catalytic intramolecular hydroamination of dithioketene acetals was developed for the synthesis of cyclic amino acid derivatives. Triggered by the addition of a catalytical amount of n-BuLi, the reaction proceeds to give proline and pipecolic acid derivatives in excellent yields and diastereoselectivity.

A convenient, two-step procedure has been developed for converting sugar derivatives into C-glycosides containing a masked aldehyde functional group. The chemistry takes advantage of an anodic coupling reaction between an electron-rich olefin and an alcohol. The sequence works for the formation of both furanose and pyranose derivatives if less polarized vinyl sulfide derived radical cation intermediates are used. With more polarized enol ether derived radical cations, the cyclizations work best for the formation of furanose derivatives where the rate of five-membered ring formation precludes elimination reactions triggered by the radical cation.

A new diblock copolymer-derived porous reaction layer for microelectrode arrays has been tested for its stability and its compatibility with both site-selective synthesis and electrochemical signaling experiments. The diblock copolymer consisted of a cinnamoyl-substituted polymethacrylate block for attachment to the surface of the array and a bromo-substituted polystyrene block for selective functionalization of the surface proximal to microelectrodes in the array. Site-selective Suzuki, Heck, and Cu(I)-coupling reactions were all performed on the new reaction layer along with electrochemical impedance studies.

A click-reaction was site-selectively carried out on 1000 and 12000 microelectrode arrays and characterized using TOF-SIMS.

A two-step, Michael reaction-based strategy for site-selectively placing molecules by unique electrodes in an addressable microelectrode array has been developed. The strategy is compatible with the use of polypeptide nucleophiles and works with microelectrode arrays having either 1024 electrodes/cm2 or 12 544 electrodes/cm2. The chemistry should allow for the transfer of existing molecular libraries to microelectrode array devices for analysis.

The synthesis, site-selective placement, and TOF-SIMS cleavage properties of a new, fluorescent linker for attaching molecules to a microelectrode array are reported. The linker was developed to provide a handle for quality control assessment of the microelectrode arrays being used to probe the binding of molecular libraries with biological receptors.

A competition experiment was designed so that the relative rates of anodic cyclization reactions under various electrolysis conditions can be determined. Reactions with ketene dithioacetal and enol ether-based substrates that use lithium methoxide as a base were shown to proceed through radical cation intermediates that were trapped by a sulfonamide anion. Results for the oxidative coupling of a vinyl sulfide with a sulfonamide anion using the same conditions were consistent with the reaction proceeding through a nitrogen-radical.

Oxidative condensation reactions are potentially valuable tools for the valorization of lignin derived materials. They are used in the two step conversion of raw sawdust into benzothiazole and benzimidazole ring skeletons. The reactions are also excellent candidates for use in paired electrochemical reactions that utilize the cathodic half-reaction to make the hydrogen gas needed for the conversion of lignin derived materials into indanones. However, the optimized oxidative condensation reactions used to date require the use of low currents that inhibit their utility in paired electrochemical reactions. In this manuscript, the optimization of current for the oxidative condensation is examined and this optimization is used to illustrate how the optimization of an electrochemical reaction relies on a traditional look at organic mechanisms.

A click-reaction was site-selectively carried out on 1000 and 12000 microelectrode arrays and characterized using TOF-SIMS.