New (carbene)ruthenium-arene complexes: Preparation and uses in catalytic synthesis of furans

A variety of neutral ruthenium-carbene complexes RuCl2(carbene)(arene) 1 (carbene = C(NR)C6H4(NR), R = Me (a), Et (b); arene = p-Me-C6H4-iPr (1), C6H3Me3 (2), C6Me6 (3)) have been prepared by reaction of [RuCl2(carene)]2 precursors with the enetetraamines (RN)C6H4(RN)C=C(NR)C6H4(NR) I (R = Me) and II (R = Et). Ru=C(NCH2Ph)CH2-CH2(NCH2Ph)Cl 2(p-cymene) (4) was prepared similarly, whereas the reaction of [RuCl2-(cycloocta-1,5-diene)]n with I led to the formation of the 16-electron neutral complex RuCl2[=C(NMe)C6H4(NMe)]3 (6). One of them (3a) was transformed into the dihydride derivative RuH2(C(NMe)C6H4NMe)(C6Me 6) (5). The cyclic voltammograms of derivatives 1-3 show that they are oxidized in the range E1/2 = 1.03-1.31 V vs SCE and are more electrophilic than the isoelectronic RuCl2(PR3)(arene) complexes. Derivatives 1a, 3a, and 3b act as efficient catalyst precursors for the electrophilic activation of the C≡C bond of (Z)-3-methylpent-2-en-4-yn-1-ol to afford 2,3-dimethylfuran in good yield via intramolecular cyclization.