Solvent-free Ru-catalyzed isomerization of allyloxyalcohols: Methods for highly selective synthesis of 1-propenyloxyalcohols

An excellent, selective and very convenient method for the synthesis of hydroxyalkyl vinyl ethers of HO–A–O–CHCHCH3 type via isomerization of appropriate readily available allylic substrate of HO–A–O–CH2CHCH2 type mediated by various soluble ruthenium complexes under solvent-free conditions has been presented. Our research has demonstrated that (CH2)n or (CH2CH2O)n or ortho-phenylene, but neither –CH2CCCH2– nor –CH2CHCHCH2– can play a role of an A linker. The use of detailed screening experiments allowed discovering new, efficient catalysts for a highly selective isomerization of allyloxyalcohols such as [RuH2(CO)(PPh3)3], [RuH2(PPh3)4] or [Ru(CO)3(PPh3)2]. For these ruthenium complexes, the only reaction products were enol ethers, and an intra- and/or intermolecular addition of an OH group to the double bond has not been observed. The use of the complexes containing Cl ligand as catalysts precursors ([RuClH(CO)(PPh3)3], [RuCl2(PPh3)3], RuCl3·3H2O and other) and surprisingly also [Ru3(CO)12] led to a consequent cyclization and/or polymerization of enol ethers. The addition to the catalytic system of trialkyl amines, K2CO3, CaH2 and others as scavengers of HCl (generated in situ from Ru–Cl complexes) has also allowed to eliminate fully consequent reactions. The scale-up of the selected method of isomerization has been successfully performed and the best results were obtained for [RuH2(CO)(PPh3)3] and [RuCl2(PPh3)3] + 5 K2CO3. Finally, several pathways of the reaction between allyloxyalcohols and the ruthenium complexes of various structures have been proposed. The double bond migration and addition of the OH group to the allyl or the vinyl ether moiety could proceed according to the transition metal catalysis mechanisms and tandems the transition metal catalysis-Lewis acid catalysis or the transition metal catalysis-Brønsted acid catalysis.

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► The soluble ruthenium complexes were tested as pre-catalysts.
► Solvent-free highly selective synthesis of terminal 1-propenyloxyalcohols was studied.
► An easy scale-up with the practically quantitative yields of product can be applied.
► Distillation of product from above catalyst was a highly efficient purification method.
► Pathways of Ru-catalyzed transformations of hydroxyalkyl allyl ethers were presented.