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-Pr-i (1), C(6)H(3)Me(3) (2), C(6)Me(6) (3)) have been prepared by reaction of [RuCl2(arene)](2) precursors with the enetetraamines (RN)C6H4(RN)C=C(NR)C6H4(NR) I (R=Me) and II (R=Et). Ru=C(NCH(2)Ph)CH2CH2(NCH(2)Ph)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)C(6)H(4)NMe)(C(6)Me(6)) (5). The cyclic voltammograms of derivatives 1-3 show that they are oxidized in the range E(1/2)=1.03-1.31 V vs SCE and are more electrophilic than the isoelectronic RuCl2(PR(3))(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.