Electron deficient bonding of benzoheterocycles to triosmium clusters: synthesis and applications to organic chemistry

Abstract

A new synthetic methodology for the addition of carbon based nucleophiles to the carbocyclic ring of quinolines has been developed which is 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 [Os 3(CO) 9(μ 3-η 2-C 9H 5(R)N)(μ-H)] (R can be a wide range of substituents in the 3-, 4-, 5- and 6-positions of the quinoline ring). These complexes react with a wide range of carbanions (R′Li, R′=Me, n Bu, t Bu, Bz, Ph, CHCH 2, C 2(CH 2) 3CH 3, CH 2CN, (CH 3) 2CCN, –CHS(CH 2) 2S–; CH 2CO 2 t Bu, R′MgBr, R′=CH 3, CH 2CHCH 2) to give the nucleophilic addition products [Os 3(CO) 9(μ 3-η 3-C 9H 7(5-R′)N)(μ-H)] ( 2a– 2l), after quenching with trifluoracetic acid, in isolated yields of 43–86%. Substitution at the 3- or 4-position is well tolerated giving the expected nucleophilic addition products [Os 3(CO) 9(μ 3-η 3-C 9H 6(3 or 4-R)(5-R′)N)(μ-H). The 6-substituted derivatives give >95% of the cis-diastereomer, [(Os 3(CO) 9(μ 3-η 3-C 9H 6(5-R′)(6-R)N)(μ-H)]. The stereochemistry is preserved even in the case of less bulky carbanions (R′=CH 3). In the case of the 6-Cl derivative, a second product is obtained on reaction with the isobutyryl carbanion, [Os 3(CO) 9(μ 3-η 2-C 9H 5(6-Cl)(5-C(CH 3) 2CN)N)(μ-H) 2] ( 4) which is the result of protonation at the metal core and rearrangement of the carbocyclic ring. The trans-diastereomer of the addition products obtained from the 6-substituted derivatives can be synthesized by reaction of the unsubstituted complex with R′Li followed by quenching with (CH 3O) 2SO 2. Acetic anhydride can also be used as the quenching electrophile for the intermediate anions generated from R′Li [( trans-Os 3(CO) 9(μ 3-η 3-C 9H 6(6-CH 3CO)(5-CH 3)N)(μ-H)] ( 5). Nucleophilic addition occurs across the 3,4-bond in the cases where the 5-position is occupied by a substituent. The nucleophilic addition products can be rearomatized by reaction with DBU/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 reflux in acetonitrile under a CO atmosphere yielding the functionalized quinoline and Os 3(CO) 12 as the only two products. An overview of this previously reported work along with additional examples of this novel chemistry is given here as well as an extention of the synthesis of the electron deficient triosmium clusters to a wide range of heterocycles structurally related to quinoline. These complexes include those containing the heterocycles: phenanthridine ( 7), 5,6-benzoquinoline ( 8), 2-CH 3-benzimidazole ( 9), 2-methyl benzotriazole ( 10), 2-methyl-benzoxazole ( 11), 2-R-benzothiazole (R=H, 12; CH 3, 13) and quinoxaline ( 14). The solid state structures of 7– 10, 12, and 14 are reported.