Scheme 9. Kolbe-Schmitt carboxylation of phenol to salicylic acid. [40] Scheme 10. Kolbe-Schmitt-type carboxylation of phenols with CO 2 . [42c] Scheme11. Carboxylation of ortho-alkylphenyl ketones with CO 2 under UV light irridiation. [44] Scheme 22. Ring-opening of exo-vinylene carboantes with amines and alcohols [107] Scheme 23. Polymers formed from CO 2 -based bis-exo-vinylene carbonates with diols and diamines. [107]
A one-pot two-step degradation of lignin β-O-4 model compounds initiated by preferred oxidation of the primary over the secondary hydroxyl groups with a TEMPO/DAIB system has been developed [TEMPO=2,2,6,6-tetramethylpiperidine-N-oxyl, DAIB=(diacetoxy)iodobenzene]. The oxidised products are then cleaved by proline-catalysed retro-aldol reactions. This degradation methodology produces simple aromatics in good yields from lignin model compounds at room temperature with an extension to organosolv beech-wood lignin (L1) resulting in known cleavage products.
A mechanochemical oxidation and cleavage reaction in lignin β-O-4 model compounds and lignin catalyzed by HO−TEMPO/ KBr/Oxone (TEMPO is 2,2,6,6-tetramethyl-1-piperidinyloxy) has been developed under milling conditions. The studies on nonphenolic lignin β-O-4 model compounds led to selective oxidations of the benzylic hydroxyl groups. Subjecting phenolic lignin model compounds to the oxidative conditions in a ball mill initiated aryl−C α bond cleavage reactions leading to the formation of the corresponding quinones and phenol derivatives. Transferring the mechanochemical protocol to lignin resulted in the simultaneous oxidation and cleavage of bonds with varied selectivity for monomeric products. Finally, a scale-up approach of the oxidative procedure by using vibrating disc mill technology enabled the mechanochemical protocol to be applied in gram-scale batch reactions under reduced milling time, while affording a similar extent of oxidation.
By using silver complexes with bulky
ligands such as DavePhos or N-heterocyclic carbenes,
propargylic alcohols are smoothly
converted with CO2 into a unique class of unexplored cyclic
alkylidene carbonates. These systems, for the first time, also achieve
the direct carboxylative cyclization of primary propargylic alcohols.
The silver-DavePhos catalyst is further applied for the bis-carboxylative
cyclization of primary propargyl derivatives, thereby providing an
effective route to a series of previously inaccessible and industrially
relevant α-alkylidene cyclic carbonates.
Using the bulk chemical 1,4-butynediol, readily available epoxides and carbon dioxide, a new series of unsubstituted exovinylene carbonates were synthesised.
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