The photochemistry of 3,3′,4,4′-tetramethoxy-and 4-hydroxy-3,3′,4′-trimethoxystilbene — models for stilbene chromophores in peroxide-bleached, high-yield wood pulps

Abstract

The photochemistry of the title compounds has been investigated in ethanol and tetrahydrofuran solution under aerobic and anaerobic conditions. Direct irradiation of trans-3,3',4,4'-tetramethoxystilbene (tra~zs-1) in deoxygenated ethanol leads to the rapid establishment of a photostationary state with the cis isomer, and the slower formation of the ethyl ether corresponding to addition of ethanol across the olefinic C=C bond and cyclobutane dimers. The same products are formed upon photolysis in the presence of oxygen under the same conditions but, in addition, two isomeric tetramethoxyphenanthrenes and 3,4-dimethoxybenzaldehyde are formed. Photolysis of trans-1 in oxygenated tetrahydrofuran leads to the same products in different relative yields. Quantum yields for cis,rra~ls photoisomerization, phenanthrene formation, and addition of ethanol have been determined by ferrioxalate actinometry. Direct irradiation of trans-4-hydroxy-3,3',4'-trimethoxystilbene (tratls-2) in ethanol solution also results in rapid cis-tra~ls isomerization and the formation of (three) isomeric phenanthrene derivatives in photolyses carried out in the presence of oxygen, although the material balance is low. The various products of photolysis of trans-2 have been independently synthesized by desilylation of the products isolated from photolysis of trclns-4-tertbutyldimethylsiloxy-3,3',4'-trimethoxystbene (trans-3) under similar conditions. Fluorescence-quenching experiments have been carried out to determine the relative rates of quenching of the excited singlet states of trans-1 and tmtzs-2 by alcohols and oxygen. The formation of aldehydes is proposed to arise via reaction of superoxide ion with stilbene radical cations, which are formed by electron-transfer quenching of the stilbene excited singlet state by oxygen.Key words: stilbenes, lignin, photochemistry, photooxidation.RCsumC : On a CtudiC la photochimie des composCs mentionnks dans le titre en solutions dans 1'Cthanol et le titrahydrofurane, dans des conditions tant akrobiques qu'anakrobiques. L'irradiation direct du tmtzs-3,3',4,4'-tCtramkthoxystilbkne (tratzs-1) dans de 1'Cthanol dCsoxygCnC conduit h la crCation rapide d'un Ctat photostationnaire avec l'isomere cis et h une formation plus lente de l'tther Cthylique correspondant ii l'addition de 1'Cthanol sur la double liaison C=C olCfinique ainsi que des dimkres cyclobutanes. Les m&mes produits se forment lors de la photolyse dans les m&mes conditions, en prCsence d'oxygkne; toutefois, il se forme de plus deux tCtramtthoxyphCnanthrbnes isomkres ainsi que du 3,4-dimCthoxybenzaldthyde. La photolyse du tratzs-1 dans du titrahydrofurane oxygCnC conduit h la formation des m&me produits avec des rendements relatifs diffkrents. On a dCterminC les rendements quantiques de la photoisomCrisation cis, trans, de la formation du phknanthrkne et de l'addition de l'tthanol par actinomCtrie feriioxalate. L'irradiation directe du tmrzs-4-hydroxy-3,3',4'-trimCthoxystilbbne (trans-2) en solution dans 1'Cthanol effectuke en prtsence ...

Precipiton Reagents: Precipiton Phosphines for Solution-Phase Reductions

“…(The saturated solution was prepared by irradiation of a THF solution containing 0.024 M TBS-ether 1 Z with 350 nm light.) As expected, the ( E )-isomer underwent an irreversible (at 350 nm) [2 + 2] cycloaddition reaction leading to a 8.7:1 ratio of very soluble syn- and anti -cyclobutanes 2 − 5 , respectively (up to 1.0 M in hexanes, THF, ether, CH 2 Cl 2 , CHCl 3 , EtOAc). The syn- and anti - cyclobutanes were obtained as a 1:1 ratio of regioisomers (Figure ).…”

Photodegradation of Substituted Stilbene Compounds: What Colors Aging Paper Yellow?

“…One of the most lucid proposals regarding the type of chromophores involved suggests that the yellowing results from the formation of o -quinones from diguaiacylstilbenes. These stilbenes are generated during the mechanical grinding, disc refining, and alkaline peroxide treatment of spruce pulps. − The precursors of the stilbenes, in turn, are diarylpropane units of lignin, i.e., fragments of the amorphous lignin polymer that together with cellulose and hemicellulose makes up the main fraction of the wood fiber cell wall. In the context of this proposal, polyalkylated stilbenes have been shown to be important chromophores, , and it has been suggested that these compounds undergo photoinduced redox and degradation reactions producing phenoxy-type radicals that in turn react further to form the corresponding o -quinones with absorption characteristics yielding a yellow coloration.…”

Size-Selective Catalysis of Ester and Anilide Cleavage by the Dinuclear Barium(II) Complexes of cis- and trans-Stilbenobis(18-crown-6)

“…The experiment reported in Figure was started in the spectrophotometric cell compartment and the reaction progress monitored in the early stages. After 40 s the cuvette was removed from the spectrophotometer and rapidly introduced into a photoreactor, where it was irradiated for 30 s . The cuvette was then rapidly reintroduced into the spectrophotometer and the reaction progress monitored until reaction completion.…”