Catalytic Dehydrative Allylation of Alcohols

Abstract

An environmentally benign synthesis of allyl ethers has been developed which can be applied to a highly chemoselective protection of hydroxyl groups. A highly efficient [CpRu]–2‐quinolinecarboxylic acid (L) catalytic system was successfully employed for the dehydrative allylation of various alcohols without additional activators and solvent (see scheme; R=alkyl, aryl, multifunctional alkyl, Cp=C5H5).

[CpRu(IV)(π‐C3H5)(2‐quinolinecarboxylato)]PF6 Complex: A Robust Catalyst for the Cleavage and Formation of Allyl Ethers

“…The complex can be quantitatively prepared via an easy and simple one-pot synthesis and can be kept for a long time without protection from air and moisture with no loss of catalytic activity. In comparison to the original Ru(II)-based method, [4,6] the advantages afforded by the new complex 6 should further enhance the practical value of allyl ethers for hydroxy protection in organic synthesis.…”

“…[4,6] The CpRu(II)-4-catalyzed reactions are thought to proceed via an intermediate catalyst -substrate complex, which is generated via olefin-mediated coordination of 2-propan-1-ol onto a CpRu(II)/2-quinolinecarboxylic acid species 5. Here, a hydrogen bond is formed between the COOH moiety of the catalyst part and the OH group of 2-propen-1-ol, and the s-donative sp 2 -N atom of the quinoline moiety and the monoanionic h 5 -Cp ligand strongly coordinate to the Ru ion.…”

“…[6] We have established the reliable, practical, and quantitative preparation of 6 by taking advantage of the higher stability of Ru(IV). The robustness as well as the catalytic reactivity in the title reactions have also been investigated.…”

“…Furthermore, by using non-alcoholic solvents or no solvent, the highly efficient catalytic dehydrative allylation of alcohols with 2-propen-1-ol can be achieved. [6,7] A novel CpRu(II) catalyst would certainly increase the utility of allyl ethers in protecting group chemistry but, from a practical viewpoint, the instability of Ru(II) catalysts is a drawback.…”

“…Here, a hydrogen bond is formed between the COOH moiety of the catalyst part and the OH group of 2-propen-1-ol, and the s-donative sp 2 -N atom of the quinoline moiety and the monoanionic h 5 -Cp ligand strongly coordinate to the Ru ion. [4,6] These simultaneous events synergistically enhance the electrophilicity of C(3) and the nucleophilicity of Ru, [9] thereby accelerating the generation of the cationic CpRu(IV)(p-C 3 H 5 ) carboxylate species 6.…”