Primena anodne oksidacije hlorida, bromida i cirkonijuma u organskoj sintezi

Abstract

U ovom radu ispitani su reakcioni uslovi pod kojima anodna oksidacija može da se primeni kao sredstvo za postizanje određenih sintetičkih ciljeva, pri čemu se sama sintetička reakcija ne odvija na elektrodama. Tokom istraživanja obuhvaćenih izradom ove teze detaljno su razrađeni postupci upotrebe anode kao oruđa za izvršenje tri konkretna sintetička zadatka: - sinteza hlorovanih sekundarnih metabolita nekih lišajeva hlorovanjem fisciona (48), prirodnog, polifunkcionalnog antrahinona; - sinteza prirodnog proizvoda kurkumenskog etra ((±)-52); - sinteza 2-nezasićenih peracetilovanih glikozida iz odgovarajućih glikala (Ferijeovo premeštanje), kao i 3-tia- i 3-azaketona iz odgovarajućih konjugovanih karbonilnih jedinjenja primenom cirkonijumovih jedinjenja kao katalizatora (hetero-Majklova adicija). Uloga anode u ostvarivanju ova tri cilja je različita: kod hlorovanja fisciona (48) anoda služi da bi se na njoj generisalo sredstvo za hlorovanje (slobodni hlor) iz nekog hlorida, u sintezi kurkumenskog etra ((±)-52) na njoj se generiše medijator za sprovođenje ključne reakcije (fenilselenoeterifikacija), a kod Ferijeovog premeštanja i hetero-Majklove reakcije na anodi se generiše katalizator reakcije. Hlorovanje fisciona (1,8-dihidroksi-6-metil-3-metoksi-antracen-9,10-diona; 48) anodno generisanim hlorom zamišljeno je kao elektroliza rastvora nekog hlorida u podesnom rastvaraču, u prisustvu fisciona. Da bi se tome pristupilo, sprovedena su obimna ciklovoltametrijska merenja u dva rastvarača – sirćetnoj kiselini i dihlormetanu (osnovni elektrolit - tetrabutilamonijum-perhlorat). Njihov cilj bio je da se utvrdi da li se pri oksidaciji hlorida odigrava neki sporedni proces, npr., da li i fiscion na potencijalima bliskim redoks potencijalu hlorida podleže oksidaciji. Na osnovu ciklovoltamograma fisciona (48), tetraetilamonijum-hlorida i ove dve supstance zajedno, utvrđeno je da u širokoj oblasti potencijala sa radnom elektrodom elektrone razmenjuju samo hloridi (dajući hlor) i (tako dobijeni) hlor (dajući nazad hloride). Ti rezultati omogućili su pristupanje anodnom generisanju hlora u sredini koja sadrži fiscion (48) (preparativnoj elektrolizi) bez bojazni od sporednih elektrodnih reakcija ovog antrahinona.

In this work, the reaction conditions of the application of anodic oxidation as a means to achieving certain synthetic tasks were studied, whereby the primary synthetic reaction does not take place at the electrodes. Throughout these investigations, the protocols of the use of the anode as a tool in the realization of three specific goals were optimized in detail: - The synthesis of chlorinated secondary metabolites of some lichens by chlorination of physcion (48), a natural, polyfunctionalized anthraquinone; - The synthesis of a natural product curcumene ether ((±)-52); - The synthesis of 2-unsaturated peracetylated glycosides from the corresponding glycals (Ferrier rearrangement), as well as 3-thia- and 3-azaketones from the corresponding conjugated carbonyl compounds by using zirconium compounds as the catalyst (hetero Michael reaction). The role of the anode in these three tasks differed: in the chlorination of physcion (48) it served to generate a chlorinating agent (free chlorine) from chloride, in the synthesis of curcumene ether ((±)-52) a mediator was generated at the anode in order to promote the key reaction (phenylselenoetherification), and a catalyst was generated at the anode that could promote Ferrier rearrangement and hetero Michael addition. The chlorination of physcion (1,8-dihydroxy-3-methoxy-6-methylanthracene-9,10- dione; 48) by chlorine generated at the anode was conceptualized as the electrolysis of a solution of a chloride in an appropriate solvent, in the presence of physcion. In order to do this, comprehensive cyclovoltammetric measurements in two solvents – acetic acid and dichloromethane (tetrabutylammonium perchlorate as the supporting electrolyte) - were carried out, aimed to establish whether any side processes take place during the oxidation of chlorides, e.g., whether also physcion undergoes oxidation at the potentials near to the chloride redox potential. On the basis of the obtained cyclovoltammograms of physcion (48), tetraethyammonium chloride and these substances together, it was found that, in a broad potential window, chlorides were the only species exchanging electrons with the working electrode (giving chlorine), as well as the thus obtained chlorine (giving back chloride). These results have enabled access to the anodic generation of chlorine in the media containing physcion (48) (a preparative electrolysis) without the risk of electrode side reactions occurring with this anthraquinone.