hrvatski

Sinteza i fotokemija stiril-derivata tiofena

U okviru proučavanja fotokemijskih reakcija heterostilbenskih spojeva, u ovom radu studirani su stiril-tiofeni. U tu svrhu pripravljeni su različito supstituirani -tiofenski derivati o-divinilbenzena 1-3, kao i '-ditiofenski derivati o-divinilbenzena, s atomom sumpora na položaju 2 (20-22) i položaju 3 (23-24). Za usporedbu je priređen i '-distirilni derivat 25. Novi spojevi sintetizirani su Wittigovom reakcijom, nakon čega su podvrgnuti fotokemijskim reakcijama u neutralnom i kiselom mediju u intramolekularnim uvjetima. Osvjetljavanjem -tiofenskih derivata o-divinilbenzena 1-3, kao glavni produkti izolirani su biciklo[3.2.1]oktadienski derivati 6, 14, odnosno 18, formirani intramolekularnom reakcijom [2+2]-cikloadicije i 1, 6-zatvaranjem nastalog biradikalnog intermedijera. Ukoliko se atom sumpora nalazi na položaju 3 tiofenskog prstena, poput spoja 2, osim biciklo[3.2.1]oktadienskog derivata 14, dobiven je i biciklo[2.1.1]heksenski derivat 15, i to 1, 4-zatvaranjem istog rezonancijom stabiliziranog biradikalnog intermedijera. Studij 2-supstituiranih '-ditiofenskih derivata o-divinilbenzena 20-22 pokazao je da je, uz izomerizaciju dvostrukih veza, 6π-elektrociklizacija i nastajanje dihidronaftalenskih derivata 26-28 glavni fotokemijski proces. Osvjetljavanjem 3-supstituiranih ditiofenskih derivata 23 i 24 izolirani su fuzionirani triciklički spojevi 41, odnosno 42, čije je nastajanje objašnjeno sekundarnom [4+2]-cikloadicijom istog primarno nastalog dihidronaftalenskog intermedijera. Osvjetljavanjem distirilnog derivata 25 nisu nađeni intramolekularni fotoprodukti. Izolirani su samo dimerni derivati, nastali kao posljedica intermolekularne reakcije glava-glava i glava-rep cikloadicije. Studij ditiofenskih derivata 20-24 u kiselom mediju ukazuje na niz intramolekularnih procesa kojima nastaje niz fotoprodukata u različitim udjelima uz dihidronaftalenski derivat. Pretpostavka je da su produkti nastali kao posljedica 1, 5- ili 1, 6-zatvaranja karbokationskog intermedijera, nastalog protoniranjem etenske veze početnog spoja, ili protoniranjem dihidro-intermedijera. Svi novi spojevi okarakterizirani su spektroskopskim metodama. Ključne riječi: tiofen, benzotiofen, biciklo[3.2.1]oktadien, dihidronaftalen, dimeri, ciklofan, fotokemija, intramolekularna i intermolekularna fotocikloadicija, fotokemijska elektrociklizacija, sinteza, Wittigova reakcija, kiselinski katalizirane reakcije, stilbeni, stiriltiofeni, spektroskopske metode. SUMMARY Styryl-thiophenes were studied to investigate the photochemical reactions of heterostilbene compounds. Different substituted -thiophene derivatives of o-divinylbenzenes 1-3 and '-dithiophene derivatives of o-divinylbenzenes 20-24 with sulphur at position 2 (20-22) and position 3 (23-24) were prepared. For comparison, '-distyryl derivative 25 was also prepared. New compounds were synthesized using the Wittig reaction and then their intramolecular photochemical reactions were investigated under neutral and acidic conditions. -Thiophene derivatives of o-divinylbenzenes 1-3 gave bicyclo[3.2.1.]octadiene derivatives 6, 14 and 18, respectively, as the main products upon irradiation. These are formed by initial [2+2]-cycloaddition and subsequent 1, 6-ring closure of the biradical intermediate. Irradiation of 3-substituted thiophene derivative 2 gave bicyclo[3.2.1.]octadiene derivative 14, accompanied by the bicyclo[2.1.1.]hexene derivative 15, formed via the same type of resonance stabilized biradical intermediate and 1, 4-ring closure. The photochemistry of 2-substituted '-dithiophene derivatives of o-divinylbenzenes 20-22 showed that along with the isomerization of double bonds the main photochemical process was 6π-electrocyclization and formation of dihydronaphtalene derivatives 26-28, respectively. Irradiation of 3-substituted dithiophene derivatives 23 and 24 provided the fused tricyclic compounds 41 and 42, respectively, via the same dihydronaphtalene intermediate by [4+2]-cycloaddition with thiophene. No intramolecular photoproduct was formed when the distyryl derivative 25 was irradiated. Only dimeric compounds were formed via intermolecular head-to-head or head-to-tail cycloaddition reactions. The photochemistry of dithiophene derivatives 20-24 in acidic medium showed, in addition to dihydronaphthalene derivatives, a series of photoproducts, depending on the starting compounds. The formation of the products may be explained by 1, 5- or 1, 6-ring closure of carbocation intermediate formed either by protonation of the ethenic bond of the starting compound or protonation of dihydro-intermediate. All compounds were characterized by spectroscopic methods.

tiofen ; benzotiofen ; biciklo[3.2.1]oktadien ; dihidronaftalen ; dimeri ; ciklofan ; fotokemija ; intramolekularna i intermolekularna fotocikloadicija ; fotokemijska elektrociklizacija ; sinteza ; Wittigova reakcija ; kiselinski katalizirane reakcije ; stilbeni ; stiriltiofeni ; spektroskopske metode

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