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

A new synthetic methodology for the addition of carbon based nucleophiles t o the carbocyclic ring of quinolines has been developed which is based on t he electron deficient bonding of the C(8) carbon and the protective coordin ation of the nitrogen atom to the metal core in the complexes [Os-3(CO)(9)( mu(3)-eta(2)-C9H5(R)N)(mu-H)] (R can be a wide range of substituents in the 3-, 4-, 5- and 6-positions of the quinoline ring). These complexes react w ith a wide range of carbanions (R'Li, R'=Me, Bu-n, Bu-t, Bz, Ph, CN-CH2, C- 2(CH2)(3)CH3, CH2CN, (CH3)(2)CCN, -CHS(CH2)(2)S-; (CH2CO2Bu)-Bu-t, R'MgBr, R' = CH3, CH2CH=CH2) to give the nucleophilic addition products [Os-3(CO)(9 )(mu(3)-eta(3)-C9H7(5-R')N)(mu-H)] (2a-2l), after quenching with trifluorac etic acid, in isolated yields of 43-86%. Substitution at the 3- or 4-positi on is well tolerated giving the expected nucleophilic addition products [Os -3(CO)(9)(mu(3)-eta(3)-C9H6(3 or 4-R)(5-R')N)(mu-H)]. The 6-substituted der ivatives give >95% of the cis-diastereomer, [(Os-3(CO)(9)(mu(3)-eta(3)-C9H6 (5-R')(6-R)N)(mu-H)]. The stereochemistry is preserved even in the case of less bulky carbanions (R' = CH3). In the case of the 6-Cl derivative, a sec ond product is obtained on reaction with the isobutyryl carbanion, [Os-3(CO )(9)(mu(3)-eta(2)-C9H5(6-Cl)(5-C(CH3)(2)CN)N)(mu-H)(2)] (4) which is the re sult of protonation at the metal core and rearrangement of the carbocyclic ring. The trans-diastereomer of the addition products obtained from the B-s ubstituted derivatives can be synthesized by reaction of the unsubstituted complex with R'Li followed by quenching with (CH3O)(2)SO2. Acetic anhydride can also be used as the quenching electrophile for the intermediate anions generated from R'Li [(trans-Os-3(CO)(9)(mu(3)-eta(3)-C9H6(6-CH3CO)(5-CH3)N )(mu-H)] (5). Nucleophilic addition occurs across the 3,4-bond in the cases where the 5-position is occupied by a substituent. The nucleophilic additi on products can be rearomatized by reaction with DBU/DDQ or by reaction of the intermediate anion with trityl cation or DDQ. The resulting rearomatize d complexes can be cleanly cleaved from the cluster by reflux in acetonitri le 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 we ll as an extention of the synthesis of the electron deficient triosmium clu sters to a wide range of heterocycles structurally related to quinoline. Th ese complexes include those containing the heterocycles: phenanthridine (7) , 5,6-benzoquinoline (8), 2-CH3-benzimidazole (9), 2-methyl benzotriazole ( 10), 2-methyl-benzoxazole (11), 2-R-benzothiazole (R = H, 12; CH3, 13) and quinoxaline (14). The solid-state structures of 7-10, 12, and 14 are report ed. (C) 1999 Elsevier Science S.A. All rights reserved.