Functionalizing Heterocycles by Electron-Deficient Bonding to a Triosmium Cluster

Abstract

A new synthetic methodology for adding carbon-based nucleophiles to the carbocyclic ring of quinolines has been developed, based on the electron-deficient bonding of the C(8) carbon and the protective coordination of the nitrogen atom to the metal core in the complexes Os3(CO)9(μ3-η2-C9H5(R)N)(μ-H), 1a−1h. These compounds react with a wide range of carbanions (e.g., R‘Li) to give the nucleophilic addition products Os3(CO)9(μ3-η3-C9H7(5-R‘)N)(μ-H), 2a−2l, and Os3(CO)9(μ3-η3-C9H6(3-, 4-, or 6-R)(5-R‘)N)(μ-H), 3b−3g, after quenching with trifluoroacetic acid, in isolated yields of 25−86%. In the 6-substituted derivatives, this addition is stereoselective, forming only the cis-diastereomer. In the case the 6-chloro derivative, a second product is obtained, Os3(CO)9(μ3-η2-C9H5(6-Cl)(5-C(CH3)2CN)N)(μ-H)2, 4, the result of protonation at the metal core and rearrangement of the carbocyclic ring. The trans-diastereomer of the 6-substituted derivatives can be obtained by quenching the intermediate anion of the unsubstituted complex with (CH3O)2SO2 or acetic anhydride. Nucleophilic addition to the 5-chloro complex occurs across the 3,4-bond to give Os3(CO)9(μ3-η2-C9H6(5-Cl)(4-C(CH3)2CN)N)(μ-H), 5. The addition products, types 2 and 3, can be rearomatized by reaction with diazobicyclononane (DBU)/dichlorodicyanoquinone (DDQ) or by reaction of the intermediate anion with trityl cation or DDQ. The resulting rearomatized complexes can be cleanly cleaved from the cluster by heating in acetonitrile under a CO atmosphere, yielding the functionalized quinoline and Os3(CO)12 as the only two products. Solid structures of cis-3e, trans-3e, 4, and 5 are reported.