Synthesis, computational modeling and photochemistry of substituted butadiene chromophores

Butadienski kromofori sintetizirani su Wittigovom reakcijom kao smjese (E) i (Z) izomera te podvrgnuti intramolekulskoj fotokemijskoj reakciji u cilju dobivanja fotoprodukata s reaktivnom slobodnom dvostrukom vezom. Na fotoproduktima je provedena epoksidacija te otvaranje epoksidnog prstena s ciljem da se priprave odgovarajući alkoholi. Računalnim modeliranjem posebno je istražena Wittigova reakcija za dobivanje butadienskih kromofora. Reakcija o-ksililen-a,a'-difosfonijevog bromida s cimetnim aldehidom i njegovim derivatima ima općenito nisku diastereoselektivnost pa nastaju (E/Z) smjese supstituiranih butadiena. Stereokemija bifunkcionalnog reagensa je slabo istražena. U slučaju dva neovisna propelerska stereocentra, očekuju se homokiralni (P,P)/(M,M) par i heterokiralna (P,M) meso-forma. Konformacijska analiza ilid-fosfonijeve vrste kvantno-kemijskim računima na temelju teorije funkcionala gustoće (DFT) pokazuje da je heterokiralna (P,M) meso-forma za 5,2 kcal/mol stabilnija od homokiralne (M,M)-forme. Intramolekulsko transilidiranje modelirano je na meso-obliku. Prijelazno stanje za translokaciju karbaniona putem prijenosa protona lokalizirano je na plohi potencijalne energije te su njegova geometrija, kao i geometrije odgovarajućih minimuma, optimizirane. Karakteriziranje kratkih kontakata omogućuje identifikaciju nekonvencionalnih C–H···p i C–H···C− intramolekulskih vodikovih veza koje određuju konformacije ilida. Struktura prijelaznog stanja za cikloadiciju dobivena računalnim modeliranjem pokazuje da prisutnost zaostale trifenilfosfonijeve soli u ključnom stereogenom stupnju Wittigove reakcije utječe na osobito nisku selektivnost u ovom redu polustabiliziranih ilida. Piramidalizacija karbaniona i unutarnja rotacija oko ilid-arilne veze važne su karakteristike. Gauche interakcija između fenilnoga o-položaja i atoma fosfora određuje preferiranu konformaciju benzilida (torzijski ∠P+CH−CiCo 110 – 140°) u prijelaznom stanju [2+2]-cikloadicije. Ova strukturna značajka vrijedi i za različite slučajeve opisane u literaturi. Sterička zapriječenost gauche usmjerenim fenilnim prstenom u neposrednoj blizini C–C veza koje tek počinju nastajati dobro objašnjava stereokemijski tijek reakcija.Butadiene chromophores prepared by Wittig reaction as mixtures of (E) and (Z) isomers were subjected to an intramolecular photochemical reaction with the intention to prepare products containing a reactive free double bond. The photoproducts were epoxidized and converted to the corresponding alcohols by ring opening. The synthesis of butadiene chromophores was thoroughly examined by computational modeling. Wittig reaction of o-xylylene-a,a'- diphosphonium bromide with cinnamaldehyde and related aldehydes generally proceeds with low diastereoselectivity to give an (E/Z) mixture of substituted butadienes. Stereochemistry of the bifunctional reagent has received little attention. For two independent propeller-shaped stereocenters, a homochiral (P,P)/(M,M) pair and heterochiral (P,M) meso-form, are to be expected. Conformational analysis of the ylide-phosphonium species by quantum-chemical calculations based on density functional theory (DFT) reveals that the heterochiral (P,M) mesoform is 5.2 kcal/mol more stable than the homochiral (M,M)-form. Computational modeling of intramolecular transylidation was performed on the meso-form. The transition state involved in carbanion translocation via proton transfer is localized on the potential energy surface, and its geometry, as well as geometries of corresponding minima, are optimized. Characterization of short contacts aids identification of unconventional C–H···p and C–H···C− intramolecular hydrogen bonds that control the ylide conformations. Computationally obtained structures of the cycloaddition transition states show that the presence of the residual triphenylphosphonium salt in the key stereogenic stage of the Wittig reaction may be responsible for the particularly low selectivity in this series of semi-stabilized ylides. Carbanion pyramidalization accompanied by an internal rotation about the ylide-aryl bond are noteworthy features. The gauche interaction between the phenyl o-carbon and phosphorus atoms in benzylide exerts a control over the molecular conformation (torsion ∠P+CH−CiCo 110 – 140°) in [2+2] cycloaddition transition states. This structural trend holds true for various cases described in the literature. The steric hindrance in the region close to the incipient C–C bond formation by the gauche exposed phenyl moiety nicely accounts for the stereocourse of the reactions