Dimeric phenolic compounds - lignans and dilignols - form in the so-called oxidative coupling reaction of phenols. Enzymes such as peroxidases and laccases catalyze the reaction using hydrogen peroxide or oxygen, respectively, as oxidant generating phenoxy radicals which couple together according to certain rules. In this thesis, the effects of the structures of starting materials - monolignols - and the effects of reaction conditions such as pH and solvent system on this coupling mechanism and on its regio- and stereoselectivity have been studied. After the primary coupling of two phenoxy radicals a very reactive quinone methide intermediate is formed. This intermediate reacts quickly with a suitable nucleophile which can be, for example, an intramolecular hydroxyl group or another nucleophile such as water, methanol, or a phenolic compound in the reaction system. This reaction is catalyzed by acids. After the nucleophilic addition to the quinone methide, other hydrolytic reactions, rearrangements, and elimination reactions occur, leading finally to stable dimeric structures called lignans or dilignols. Similar reactions occur also in the so-called lignification process when monolignol (or dilignol) reacts with the growing lignin polymer. New kinds of structures have been observed in this thesis. The dimeric compounds with a so-called spirodienone structure have been observed to form both in the dehydrodimerization of methyl sinapate and in the -1-type cross-coupling reaction of two different monolignols. This -1-type dilignol with a spirodienone structure was the first synthesized and published dilignol model compound, and at present, it has been observed to exist as a fundamental construction unit in lignins. The enantioselectivity of the oxidative coupling reaction was also studied for obtaining enantiopure lignans and dilignols. A rather good enantioselectivity was obtained in the oxidative coupling reaction of two monolignols with chiral auxiliary substituents using peroxidase/H2O2 as an oxidation system. This observation was published as one of the first enantioselective oxidative coupling reaction of phenols. Pure enantiomers of lignans were also obtained by using chiral cryogenic chromatography as a chiral resolution technique. This technique was shown to be an alternative route to obtain enantiopure lignans or lignin model compounds in a preparative scale.
|Award date||27 Sep 2008|
|Place of Publication||Espoo|
|Publication status||Published - 2008|
|MoE publication type||G5 Doctoral dissertation (article)|
- oxidative coupling reactions
Setälä, H. (2008). Regio- and stereoselectivity of oxidative coupling reactions of phenols. Spirodienones as construction units in lignin: Dissertation. VTT Technical Research Centre of Finland. http://www.vtt.fi/inf/pdf/publications/2008/P689.pdf