Hans J. Schäfer

New Syntheses with Oils and Fats as Renewable Raw Materials for the Chemical Industry.

Oils and fats are the most important renewable raw materials for the chemical industry. Hitherto, industrial oleochemistry has concentrated predominantly on the carboxy functionality of fatty acids but, more recently, modern synthetic methods have been applied extensively to fatty compounds for the selective functionalization of the alkyl chain. Radical, electrophilic, nucleophilic, and pericyclic as well as transition metal catalyzed additions to the C-C double bond of, for example, oleic acid as the prototype of a readily accessible, unsaturated fatty acid have led to a large number of novel fatty compounds from which interesting properties are expected. Functionalization of C-H bonds in the alkyl chain is also feasible with remarkable selectivity. Effective and highly versatile catalysts for the metathesis of esters of unsaturated fatty acids have been developed, which lead to new and interesting omega-unsaturated fatty acids. The epoxidation of unsaturated fatty acids has been developed extensively. Enzymatic reactions allow syntheses with high selectivity and yield of mono- and diglycerides and esters of carbohydrates with a variety of surfactant properties. Regio- and enantioselective microbial hydrations and hydroxylations widen the spectrum of selective reactions. Of considerable significance is that, with the use of gene technology, natural oils and fats have been improved significantly and will be improved still further, insofar as they show a more uniform and often unusual fatty acid spectrum. Numerous fatty acids are now available in a purity which makes them attractive for synthesis and as raw materials for the chemical industry.

Oils and Fats as Renewable Raw Materials in Chemistry

Oils and fats of vegetable and animal origin have been the most important renewable feedstock of the chemical industry in the past and in the present. A tremendous geographical and feedstock shift of oleochemical production has taken place from North America and Europe to southeast Asia and from tallow to palm oil. It will be important to introduce and to cultivate more and new oil plants containing fatty acids with interesting and desired properties for chemical utilization while simultaneously increasing the agricultural biodiversity. The problem of the industrial utilization of food plant oils has become more urgent with the development of the global biodiesel production. The remarkable advances made during the last decade in organic synthesis, catalysis, and biotechnology using plant oils and the basic oleochemicals derived from them will be reported, including, for example, ω-functionalization of fatty acids containing internal double bonds, application of the olefin metathesis reaction, and de novo synthesis of fatty acids from abundantly available renewable carbon sources.

Neue Synthesen mit Ölen und Fetten als nachwachsende Rohstoffe für die chemische Industrie

Ole und Fette sind die wichtigsten nachwachsenden Rohstoffe der chemischen Industrie. Die industrielle Oleochemie konzentriert sich bisher uberwiegend auf die Carboxyfunktion der Fettsauren, doch wurden in letzter Zeit auch moderne Synthesemethoden zur selektiven Funktionalisierung in der Alkylkette auf Fettstoffe angewandt. Additionen an die C-C-Doppelbindung z. B. von Olsaure als Prototyp einer gut zuganglichen ungesattigten Fettsaure fuhrten zu einer Vielzahl neuartiger Fettstoffe mit interessanten Eigenschaften. Auch Funktionalisierungen von C-H-Bindungen in der Alkylkette sind bemerkenswert selektiv moglich. Es wurden effektive Katalysatoren fur die Metathese ungesattigter Fettsaureester entwickelt, die zu vielfaltig einsetzbaren ω-ungesattigten Fettsauren fuhrt. Die Epoxidierung ungesattigter Fettsauren wurde wesentlich weiterentwickelt. Enzymatische Reaktionen erlauben mit hoher Selektivitat und Ausbeute die Synthese von Mono- und Diglyceriden und von Kohlenhydratestern mit interessanten oberflachenaktiven Eigenschaften. Regio- und enantioselektive mikrobielle Hydratisierungen und Hydroxylierungen erweitern das Spektrum der selektiven Reaktionen. Die nativen Ole und Fette wurden durch Pflanzenzuchtung (auch gentechnisch) betrachtlich verbessert, und zahlreiche Fettsauren stehen heute in genugender Reinheit fur die Synthese und als Rohstoff fur die chemische Industrie zur Verfugung.

Electrolysis for the benign conversion of renewable feedstocks

A large variety of C,C-bond forming reactions and functional group interconversions can be achieved by electron transfer. For the conversion of renewable feedstocks, electrolysis has been applied to coupling of radicals generated by anodic decarboxylation of fatty acids and carboxylic acids of carbohydrates. Furthermore, a derivative of L-gulonic acid is converted nearly quantitatively into L-xylonolacton. Trimethyl aconitate from trimethyl citronate is dimerized stereoselectively at the cathode in 72 % yield to a cyclic hexamethyl ester by an inter- and intramolecular Michael addition. Two acetoxy groups are added anodically to methyl conjuenate (obtained from methyl linoleate) to form methyl (E)-9,12-diacetoxy-10-octenoate and methyl (E)-10,13-diacetoxy-11-octenoate in 85 % yield. The primary hydroxy groups in mono- and disaccharides can be oxidized to carboxylic acid groups in good yield and high selectivity by anodic oxidation with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as mediator. The results demonstrate that electrolysis is in good accordance with many of the 12 principles of green chemistry.

Synthesis of the salutaridine and aporphine skeleton via palladium(0) catalyzed cyclization and SRN1 reaction of 2′-bromoreticulines

Abstract Two intramolecular aryl-aryl-coupling reactions of 2′-bromoreticulines are described. Their regioselectivity depends on the cyclization method. The palladium(0) catalyzed reaction of 22 leads preferentially to the salutaridine derivative 27, whilst via the photochemically induced SRN1 reaction of 22 the aporphine skeleton 24 is obtained.

New methods for reductive free-radical cyclizations of α-bromoacetals to 2-alkoxytetrahydrofurans with activated chromium(II)-acetate

Abstract A method for the synthesis of γ-butyrolactones 6 is described in which the key step is a free-radical cyclization of α-bromoacetals 4 to 2-alkoxytetrahydrofurans 5 in 54–93% yield induced by activated chromium(II)-acetate. Four new methods have been developed in order to activate the transition metal. Two of them require only catalytic amounts of chromium(II)-acetate, because it can be regenerated in situ chemically or electrochemically. The diastereoselectivity of the cyclization depends on the substitution pattern of 4 and ranges between 30.4:1 and 1:54.8.

Cleavage of olefinic double bonds by mediated anodic oxidation

Abstract Seven alkenes, e.g. 1-decene, methyl oleate, cyclododecene, norbornene, are cleaved by indirect anodic oxidation with IO 4 − /RuCl 3 as mediator to carboxylic acids. The best performance was achieved with two alternative ex cell-methods. Periodate is regenerated from iodate in a divided cell at a PbO 2 /Ti-anode. In the chemical reactor alkene and the produced carboxylic acid are immobilized in a chromatography column on Chromosorb W and oxidized with IO 4 − /RuO 4 in CH 3 CN/water. In the alternative version the alkene is oxidized in an emulsion generated by sonication and the organic phase is retained in the reactor by a separator. Acids and diacids are obtained in 61–91% chemical yield and good current yields. The amount of consumed periodate can be reduced to less than 5% of the amount needed for the chemical oxidation. The mediated anodic cleavage of alkenes is altogether an interesting alternative to ozonolysis.

Oxidation of partially protected carbohydrates at the nickel hydroxide electrode

Abstract Primary hydroxy groups in pyranoses are oxidized in excellent yields to the corresponding carboxylic acids. In furanose 3 the yield of acid is only moderate. Secondary hydroxy groups are inert, aside from lactols. The different reactivity of secondary and primary hydroxy groups allows the chemoselective oxidation of 8 and 10.

Recent synthetic applications of the Kolbe electrolysis

Abstract The Koble electrolysis is an effective and simple method for the coupling of carboxylates. Experimental conditions and substituents in the carboxylate that favour dimerisation are specified and representative preparative examples are given. Cross-coupling of carboxylates with halfesters of dicarboxylic acids affords an easy access to fatty acids; additive dimerisation with butadiene yields unsaturated diesters. Some recent applications of the Kolbe reaction are the preparation of 1,5-dienes by dimerisation of β, γ-unsaturated acids and the synthesis of pheromones, e.g. muscalure, looplure, brevicomin or disparlure by mixed coupling of unsaturated carboxylic acids with halfesters of dicarboxylic acids.

Anodic intramolecular arylation of enaminones

Abstract N-Benzyl- and β-phenethyl-enaminones are cyclized at the anode to isoquinolines and benzazepines.