Basicity and bulkiness effects of 1,8-diaminonaphthalene, 8-aminoquinoline and their alkylated derivatives on the different efficiencies of η5-C5H5 and η5-C5Me5 ruthenium precatalysts in allylic etherification reactions

Abstract

The different behaviours of Ru(η⁵-C₅H₅) and Ru(η⁵-C₅Me₅) precatalysts, [Ru(η⁵-C₅R₅)(NCMe)(N,N)]PF₆ (R = H, Me), in the allylic etherification reaction of cinnamyl chloride using the phenoxide anion as a nucleophile was considered. The N,N ligands are the commercial products 1,8-diaminonaphthalene and 8-aminoquinoline, and their derivatives obtained by alkylation of the amino nitrogen atoms: alkyl substituents that are also bulky chiral C₂-symmetric frameworks allow modulation of the basicity and steric demand of the ligands. Some of the precatalysts, [Ru(η⁵-C₅R₅)(NCMe)(N,N)]PF₆ (R = H, Me), were also synthesized and characterized. The cinnamyl phenyl ether isomers were obtained with very high B/L regioselectivity values, either with Ru(η⁵-C₅H₅) or Ru(η⁵-C₅Me₅) precatalysts. The highest B/L regioselectivity values achieved with Ru(η⁵-C₅Me₅) precatalysts were found with the N,N ligand 1,8-diaminonaphthalene and its derivatives; with Ru(η⁵-C₅H₅) precatalysts best B/L values were obtained with ligands derived from 8-aminoquinoline. A correlation between the B/L regioselectivity, and the σ-donor power and bulkiness of the substituents at the nitrogen atoms of the N,N coordinated ligand was established, but the Ru(η⁵-C₅H₅) or Ru(η⁵-C₅Me₅) precatalysts followed an opposite trend. It was also found that the low ee values did not depend on the diastereomeric composition of the chiral-at-metal precatalyst [Ru(η⁵-C₅R₅)(NCMe)(N,N)]PF₆.