REACTIONS OF POLYCYCLIC ARENE SYSTEMS STABILIZED AND ACTIVATED BY CYCLOPENTADIENYLRUTHENIUM CATIONS

One vinylidene group of [(CpRu)(eta6-dibenzo-p-quinodimethane)]+PF6- (1) (Cp = eta5-cyclopentadienyl), in which highly reactive dibenzo-p-quinodimethane is stabilized by coordination to ruthenium, underwent Diels-Alder cycloaddition from the exo face with 2,3-dimethyl-1,3-butadiene to give a novel spiro derivative of dibenzo-p-quinodimethane (2). X-ray analysis of 2 (C27H27F6PRu) confirmed its structure and showed it to crystallize in the monoclinic space group P2(1)/n with a = 16.079(5) angstrom, b = 11.682(3) angstrom, c = 13.508(4) angstrom, beta = 91.03(3)-degrees, and Z = 4. The Cp* (Cp* = eta5-pentamethylcyclopentadienyl) analogue of 1 did not react with 2,3-dimethyl-1,3-butadiene. The eta6-localization effect of cyclopentadienylruthenium cations can also be used to activate polycyclic aromatic hydrocarbons toward Diels-Alder and catalytic hydrogenation reactions. [(Cp*Ru)(eta6-anthracene)]+TfO- (3) underwent quantitative Diels-Alder reaction from the exo face with maleic anhydride under mild conditions (83-degrees-C) to give [trans-(endo-Cp*Ru)(eta6-9,10-dihydroanthracene-9,10-alpha,beta-succinic acid anhydride)]+TfO- (4) selectively. Reaction of anthracene under identical conditions gave only 19% conversion. Reaction of maleic anhydride with [CpRu(eta6-naphthalene)]+PF6- (6) was not successful. However, hydrogenation of 6 in methanol (1 atm, 24-degrees-C, 6 h) took place smoothly in the presence of a heterogeneous Pd/C catalyst to give [CpRu(eta6-1,2,3,4-tetrahydronaphthalene)]+PF6- (7) in high yield. The Cp* analogue of 6 gave similar results. Noncomplexed naphthalene remained largely unreacted under identical conditions (61 % remaining), and a substantial proportion remained (32%) after 24 h. The Cp*Ru and CpRu groups in 4 and 7 can be removed by photolysis in acetonitrile to cleanly give 9,10-dihydroanthracene-9,10-alpha,beta-succinic acid anhydride (5) and 1,2,3,4-tetrahydronaphthalene (8), respectively, as well as generating Cp- and Cp*-ruthenium tris(acetonitrile) salts, which can be used to make more complexed arene.