Carboxylic acids can be decarboxylated by anodic oxidation to radicals (Kolbe-electrolysis) and/or carbocations (non-Kolbe electrolysis). The procedure and necessary equipment is simple, a scale-up easy, the choice of carboxylic acids wide, the selectivity towards radicals or carbocations can be controlled by reaction conditions and the structure of the carboxylic acid, the yields are in general good. The radical pathway can be used for the preparation of e.g. 1,n-diesters, pheromones, or rare fatty acids. Electrolysis in the presence of olefins affords additive dimers and monomers or by intramolecular addition five membered carbocycles and heterocycles. By non-Kolbe electrolysis carboxylic acids can be converted into ethers, acetals, olefins or acetamides. Rearrangements and fragmentations lead to stereospecifically substituted cyclopentanoids and one- or four-carbon ring extensions.

Recent Contributions of Kolbe Electrolysis to Organic Synthesis

Electrochemically generated anion radicals of aromatic nitriles and esters possess the remarkable property to reduce carbon dioxide to oxalate with negligible formation of carboxylated products. They may thus serve as selective homogeneous catalysts for the reduction of CO2 in an aprotic medium. The catalytic enhancement of the cyclic voltammetric peaks of these catalysts is used to determine the rate constant of the electron transfer from these aromatic anion radicals to CO2 as a function of the catalyst standard potential. Substituted benzoic esters allowed a particularly detailed investigation of the resulting activation-driving force relationship. Using 14 different catalysts in this series made it possible to finely scan a range of reaction standard free energies of 0.4 eV. Detailed analysis of the resulting data leads to the conclusion that the reaction is not a simple outer-sphere electron transfer. It rather consists in a nucleophilic addition of the anion radical on CO2, forming an oxygen (or nit...

Homogeneous Electron Transfer Catalysis of the Electrochemical Reduction of Carbon Dioxide. Do Aromatic Anion Radicals React in an Outer-Sphere Manner?

The near-unlimited availability of CO2 has stimulated a growing research effort in creating value-added products from this greenhouse gas. This paper presents the trends on the most important methods used in the electrochemical synthesis of carboxylic acids from carbon dioxide. An overview is given of different substrate groups which form carboxylic acids upon CO2 fixation, including mechanistic considerations. While most work focuses on the electrocarboxylation of substrates with sacrificial anodes, this review considers the possibilities and challenges of implementing other synthetic methodologies. In view of potential industrial application, the choice of reactor setup, electrode type and reaction pathway has a large influence on the sustainability and efficiency of the process.

https://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-10-260.pdf

Electrocarboxylation: towards sustainable and efficient synthesis of valuable carboxylic acids.

TosMIC is the acronym for 4-tolylsulfonylmethyl isocyanide or tosylmethyl isocyanide. It is the best-known member of a series of about 25 sulfonyl-substituted methyl isocyanides RSO2CH2NC collected in Chart 1 (p. 426). TosMIC is a multipurpose synthetic reagent. It is by far the most versatile synthon derived from methyl isocyanide. This chapter provides a complete account of the synthetic uses of TosMIC based on a literature search closed in January 1996, and supplemented with further data available to the authors. Also included are applications of some of the more important TosMIC homologs (TosCHRNC).



TosMIC is the only commercially available sulfonylmethyl isocyanide. It is a stable, colorless, practically odorless solid, which can be stored at room temperature without decomposition.



Four brief review papers on the chemistry of TosMIC have appeared, in 1980, 1987, 1993, and 1995. Several reviews on the chemistry of isocyanides in general are available.



An effort has been made to make this chapter as complete as possible, in coverage both by tables and by references, with respect to the immediate objective: a survey of the “Synthetic Uses of TosMIC.” Emphasis is on the primary products derived from reactions of TosMIC and related isocyanides, with limited reference to the further utilization of these products. Thus, the next section gives a complete account of reductive cyanation, the conversion of aldehydes or ketones into cyanides with the use of TosMIC, but for obvious reasons synthetic applications of the product cyanides are not treated in this chapter. One of the subsequent sections describes the application of TosMIC to the synthesis of pyrroles, including 2,3-divinylpyrroles, which are important precursors in a new synthesis of indoles. This latter transformation, made possible by the easy availability of the precursors through TosMIC chemistry, is afforded brief coverage.



As a rule, reference to preliminary papers has been omitted when the same information is available from the corresponding full papers. The patent literature is covered highly selectively and is cited only when providing new and relevant information. Negative results of reactions are reported in the text or tables only when at least some relevant information is available with respect to the conditions of such unsuccessful attempts.


Keywords:

tosylmethyl isocyanide;
4-tosylsulfonylmethyl isocyanide;
sulfonyl;
methyl isocyanide;
isocyanide;
synthetic application;
synthetic utility

Synthetic Uses of Tosylmethyl Isocyanide (TosMIC)

Durch Elektrolyse last sich die Reaktivitat eines Substrates andern oder sogar umpolen („Redox-Umpolung”). Damit werden das Kohlenstoffgerust und die funktionellen Gruppen eines Synthesebausteins okonomischer nutzbar, und zugleich lassen sich Reaktionsschritte in mehrstufigen Synthesen einsparen. Als Werkzeug fur die Elektrolyse werden die Zelle, die Stromquelle, die Elektroden und der Elektrolyt benotigt, wobei man Elektroden und Elektrolyt nach Kenntnis des Reduktions- oder Oxidationspotentials des Substrates auswahlt. Eine Reihe elektroanalytischer Methoden ermoglicht Aussagen uber den Mechanismus der Elektrodenreaktion. An der Anode dimerisieren Arene, Phenylether und elektronenreiche Olefine uber Radikalkationen als Zwischenstufe. Carbonsaure-Anionen decarboxylieren bei der Kolbe-Elektrolyse zu Radikalen, die kuppeln oder sich an Olefine addieren; die Kolbe-Elektrolyse eignet sich auch zur Synthese von Pheromonen. An der Kathode lassen sich aktivierte Olefine uber Radikalanionen hydrodimerisieren oder mit entsprechenden Reagentien acylieren, alkylieren und carboxylieren. Pinakole, gekreuzte Hydrodimere, cyclische und arylierte Verbindungen sind uber kathodisch erzeugte Radikale zuganglich, wahrend die Herstellung gespannter Kleinringe oder die reduktive Addition von Halogeniden an Carbonylverbindungen uber intermediare Carbanionen gelingt.

Ulrich Heß

Papers

Electrochemical synthesis of isocyanides

Reduktive Elektrocarboxylierung von Azomethinen

Zur reduktiven Elektrocarboxylierung von N‐Heterocyclen

Zur Kolbe-Reaktion von Aminosäuren

Zur anomischen Dimerisierung von N‐Isopronylacetanilid und N‐Isopropyl‐m‐methoxy‐acetanilid