Formation and Elimination of Multiple Bond Functionalities

The multiple bond structures of importance in side chain modification reactions are carbonyl and ethylenic groups and the processes frequently involved in the formation and elimination of such groups are reduction, hydrogenation, oxidation, and dehydration, here arbitrarily restricted to reactions in which no accompanying carbon-carbon fragmentation occurs. 

In an alkali-promoted dehydration reaction, fi-l phenolic lignin units may give rise to ring-conjugated ethylenic groups, as indicated by the results of a 

Reactions that involve the formation and elimination of multiple-bond functional groups may significantly influence the color of residual lignin in bleached and unbleached pulps. Thus, ethylenic and carbonyl groups conjugated with phenolic or quinonoid structures may contribute to color as components of chromophore or leucochromophore systems. 

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Fig. 1.6. Reactions illustrating formation and elimination of multiple bond side chain functional groups...

The sulfone is a versatile functional group comparable to the carbonyl functionality in its ability to activate molecules for further bond construction, the main difference between these two groups being that the sulfone is usually removed once the synthetic objective is achieved. The removal most commonly involves a reductive desulfonylation process with either replacement of the sulfone by hydrogen (Eq. 1), or a process that results in the formation of a carbon-carbon multiple bond when a P-functionalized sulfone, for example a (3-hydroxy or (3-alkoxy sulfone, is employed (Eq. 2). These types of reactions are the Julia-Lythgoe or Julia-Paris-Kocienski olefination processes. Alkylative desulfonylation (substitution of the sulfone by an alkyl group, Eq. 3), oxidative desulfonylation (Eq. 4), and substitution of the sulfone by a nucleophile (nucleophilic displacement, Eq. 5) are also known. Finally, p-eliminations (Eq. 6) or sulfur dioxide extrusion processes (Eqs. 7, 8 and 9) have become very popular for the... 

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