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Synthetic and Theoretical Studies of Cyclobuta[1,2:3,4]dicyclopentene. Organocobalt Intermediates in the Construction of the Unsaturated Carbon Skeleton and Their Transformation into Novel Cobaltacyclic Complexes by C−C Insertion
journal contribution
posted on 2004-04-02, 00:00 authored by Andrew G. Myers, Miki Sogi, Michael A. Lewis, Stephen P. ArvedsonTheoretical and synthetic studies of the tricyclic 10π-electron hydrocarbon cyclobuta[1,2:3,4]dicyclopentene (1), a nominally aromatic structure that has never been synthesized, are described.
Geometry optimization by density-functional-theory calculations (B3LYP/6-31G(d,p)) predict that
1 is a D2h symmetric structure with nonalternant C−C single and double bonds. The calculations
also predict that 1 is 4.7 kcal/mol higher in energy than the isomeric hydrocarbon 1,6-didehydro[10]annulene (2), a molecule known to isomerize to 1,5-didehydronaphthalene (4) above −50 °C.
Calculated enthalpic changes of homodesmotic reactions support the notion that 1 is an aromatic
molecule with a resonance stabilization energy (RSE) about half to two-thirds that of benzene on
a per-molecule basis. Investigations of potential synthetic pathways to 1 initially utilized as starting
material the tricyclic carbonate 11, the product of an intramolecular [2 + 2]-photocyclization
reaction. In these studies, 11 was transformed in several steps to the distannane 12, which upon
treatment with boron fluoride ethyl etherate is believed to have formed the unstable hydrocarbon
bicyclopentadienylidene (13). In an effort to avoid cleavage of the central, four-membered ring of
unsaturated tricyclo[5.3.0.02,6]decane intermediates (perhaps the result of 10-electron electrocyclic
ring opening of the tetraene 8), synthetic approaches to 1 employing cobalt−cyclobutadiene
complexes 18 and 19 were pursued. Treatment of 18 with excess methyllithium led to the novel
cobaltacyclic product 30, and dehydration of 19 in the presence of pyridine produced the ring-opening cobaltacyclic product 35. It is proposed that both processes may occur by a 10-electron
electrocyclic ring-opening reaction of η2-organocobalt intermediates. These processes may be related
to the hypothetical transformation of tetraene 8 to bicyclopentadienylidene (13).
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Novel Cobaltacyclic Complexeselectrocyclichomodesmotic reactions supportenthalpic changeselectronnovel cobaltacyclic product 30tricyclic carbonate 11distannane 12boron fluoride ethyl etheratedecane intermediatesGeometry optimizationresonance stabilization energyTheoretical StudiesD 2 hη 2tetraene 8calculationB 3LYPmoleculeUnsaturated Carbon Skeletonhydrocarbon bicyclopentadienylideneRSEorganocobalt intermediates
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