Metal-mediated transformations of cyclooctatetraene to novel methylene-bridged, bicyclic compounds

The nucleophilic addition of Nu(1) to [(eta(5)-Cp)Fe(eta(6)-cot)]PF6 (1PF(6)) (cot = cyclooctatetraene) exclusively yields the neutral complex [(eta(5)-Cp)Fe(1,2,3,4,5-eta-C8H8-Nu(1))] (2) [Nu(1) = C(CO2Et)(2)CH2CHCH2 (2a), C(CO2Et)(2)(CH2)(2)CHCH2 (2b), C(CO2Et)(2)(CH2)(3)CHCH2 (2c)], with the nucleophiles linked to the cyclo-C-8 ligand solely and stereoselectively in exo position with respect to the metal center. The protonation of the neutral complexes 2a-c by addition of HBF4 reveals the new ionic product [(eta(5)-Cp)Fe(eta(6)-C(8)H(9)Nu(1))]BF4 (3BF(4)) with a 1,2,3,4,5,6-)7 coordination mode of the cyclo-C-8 ligand. The cationic complexes 3a-c are suitable for a second nucleophilic addition affording the exo-6,8-di substituted cyclooctadienyl complex [(eta(5)-Cp)Fe(1,2,3,4,5-eta-C8H9-6-Nu(1)-8-Nu(2))] (4) [Nu(1)/Nu(2) = C(CO2Et)(2)CH2CHCH2/C(CO2Et)(2)CH2CHCH(2) (4a), C(CO2Et)(2)CH2CHCH2/C(CO2Et)(2)(CH2)(2)CHCH2 (4b), C(CO2Et)(2)CH2CHCH2/C(CO2Et)(2)(CH2)(3)CHCH2 (4c), C(CO2Et)(2)CH2CHCH2/C(CO2Et)(2)(CH2)(2)CHCH2 (4d), C(CO2Et)(2)(CH2)(2)CHCH2/C(CO2Et)(2)(CH2)(3)CHCH2 (4e), C(CO2Et)(2)CH2CHCH2/C(CO2Et)(2)(CH2)(3)CHCH2 (4f). The cyclo-C-8 ligand can be cleaved as cis- 5,7-disubstituted cycloocta-1,3-diene (5) by protonation of the complexes 4a-f with CF3CO2H in acetonitrile: Nu(1)/Nu(2) = C(CO2Et)(2)CH2CHCH2/C(CO2Et)(2)CH2CHCH2 (5a), C(CO2Et)(2)CH2CHCH2/C(CO2Et)(2)(CH2)(2)CHCH2 (5b), C(CO2Et)(2)CH2CHCH2/C(CO2Et)(2)(CH2)(2)CHCH2 (5c), C(CO2Et)(2)(CH2)(2)CHCH2/C(CO2Et)(2)(CH2)(2)CHCH2 (5d), C(CO2Et)(2)(CH2)(2)CHCH2/C(CO2Et)(2)(CH2)(3)CHCH2 (5e), C(CO2Et)(2)(CH2)(3)CHCH2/C(CO2Et)(2)(CH2)(3)CHCH2 (5f). In an attempt to construct fused bicyclic ring systems with annelated 9- to 13-membered cycles employing the ring-closing metathesis (RCM), a novel ring opening-ring closing metathesis reaction was encountered to yield new methylene-bridged, bicyclic systems (7).