R. Daniel Little

Organic electrosynthesis: a promising green methodology in organic chemistry

Over the last decade, organic electrosynthesis has become recognized as one of the methodologies that can fulfill several important criteria that are needed if society is to develop environmentally compatible processes. It can be used to replace toxic or dangerous oxidizing or reducing reagents, reduce energy consumption, and can be used for the in situ production of unstable and hazardous reagents. These are just a few of the most important attributes that render electrochemistry environmentally useful. In this review the main characteristics of electrochemistry as a promising green methodology for organic synthesis are described and exemplified. Herein we provide basic information concerning the nature of electrosynthetic processes, paired electrochemical reactions, electrocatalytic reactions, reactions carried out in ionic liquids, electrogeneration of reactants, electrochemical reactions that use renewable starting materials (biomass), green organic electrosynthesis in micro- and nano-emulsions, the synthesis of complex molecules using an electrosynthetic key step, and conclude with some insights concerning the future. Throughout the review the “green aspects” of these topics are highlighted and their relationship with the twelve green chemistry principles is described.

MIRC (Michael Initiated Ring Closure) Reactions Formation of Three, Five, Six and Seven Membered Rings

Abstract The term MIRC reaction is defined. Examples of the construction of three, five, six and seven membered rings using alkylthiolates and LDA as nucleophiles are presented.

Novel Triarylimidazole Redox Catalysts: Synthesis, Electrochemical Properties, and Applicability to Electrooxidative C–H Activation

A new class of metal-free, easy to synthesize redox catalysts based on a triarylimidazole framework is described. With those synthesized thus far, one can access a potential range of ca. 410 mV. They proved to be useful mediators for the activation of benzylic C-H bonds under mild conditions.

Optimizing Electron Transfer Mediators Based on Arylimidazoles by Ring Fusion: Synthesis, Electrochemistry, and Computational Analysis of 2-Aryl-1-methylphenanthro[9,10-d]imidazoles

A significant improvement of the properties of redox catalysts based on the triarylimidazole framework can be achieved with a simple structural modification. By linking the ortho-carbons of the aromatics positioned at C-4 and C-5, a fused framework is generated, removing the distortion from planarity and enhancing the influence of the substituents on the redox properties. This modification leads not only to a much broader range of available redox potentials for the resulting phenanthro[9,10-d]imidazoles but also to improved stability of the corresponding radical cation. These concepts were verified with eight new phenanthro[9,10-d]imidazole derivatives, using cyclic voltammetry and DFT calculations. For this purpose, an optimized and general synthetic route to the desired compounds was developed. An excellent linear correlation of the calculated effective ionization potentials with the experimental oxidation potentials was obtained, allowing for an accurate prediction of oxidation potentials of derivatives yet to be synthesized. Moreover, high catalytic activity was found for electro-oxidative C-H activation reactions.

Oxidative desulfonylation. Phenyl vinyl sulfone as a ketene synthetic equivalent

Abstract α-Sulfonyl carbanions undergo oxidative desulfonylation to form ketones upon treatment with molybdenum peroxide MoO 5 ·Py·HMPA (MoOPH) in THF at −78°C.

Electrochemically Initiated Oxidative Amination of Benzoxazoles Using Tetraalkylammonium Halides As Redox Catalysts

An electrochemically promoted coupling of benzoxazoles and amines has been developed, leading directly to the formation of 2-aminobenzoxazoles. The chemistry utilizes catalytic quantities of a tetraalkylammonium halide redox catalyst and is carried out under constant current conditions in a simple undivided cell. The use of excess chemical oxidant or large amounts of supporting electrolyte is avoided. This greatly simplifies the workup and isolation process and leads to a reduction in waste.

Electrocatalytic Aziridination of Alkenes Mediated by n-Bu4NI: A Radical Pathway

Efficient electrocatalytic aziridination of alkenes has been achieved for the first time. A structurally broad range of aziridines was easily accessed using an undivided cell operated at constant current and mediated by a catalytic quantity of n-Bu4NI. The electrocatalytic reaction also proceeded in the absence of additional conducting salt. The aziridination is proposed to follow a radical mechanism.

1,3-Diyl trapping reactions. Fundamental investigations with application to the synthesis of linearly fused tricyclopentanoids

Abstract A comparison of the inter- and intramolecular diyl trapping routes to linearly fused tricyclopentanoids is presented. In addition, several of the factors which are responsible for the stereoselectivity which is associated with the intramolecular process are examined and it is concluded that conformational rather than electronic (secondary orbital) factors play the dominant role. It is shown that gem methyl groups located on the acyclic chain which joins the diyl and diylophile (in reference to 32 and 35, but not to 47) have no practical effect upon the outcome of the trapping reaction. The intramolecular process is stereospecific with respect to diylophile geometry, and highly stereoselective with respect to the ring junction stereochemistry. Finally, an abortive attempt to synthesize the marine natural product Δ9(12)-capnellene (19) as well as a successful synthesis of the mold metabolite d,l- hirsutene (18) is presented.

Electroreductive cyclization reactions. Stereoselection, creation of quaternary centers in bicyclic frameworks, and a formal total synthesis of quadrone.

The electroreductive cyclization reaction has been successfully applied to an efficient formal total synthesis of quadrone (1). Two of the four rings were created electrochemically, a third via a novel oxidativedecarboxylation and cyclization onto a remote nitrile functional group (22a24).

Polymeric Ionic Liquid and Carbon Black Composite as a Reusable Supporting Electrolyte: Modification of the Electrode Surface

One of the major impediments to using electroorganic synthesis is the need for large amounts of a supporting electrolyte to ensure the passage of charge. Frequently this causes separation and waste problems. To address these issues, a polymeric ionic liquid-Super P carbon black composite has been formulated. The system enables electrolyses to be performed without adding an additional supporting electrolyte, and its efficient recovery and reuse. In addition, the ability of the composite to modify the electrode surface in situ leads to improved kinetics. A practical consequence is that one can decrease catalyst loading without sacrificing efficiency.