The synthesis and photochemistry of bicyclo[4.2.0]octa-2,4,7-trienes

Two new crystalline bicyclo[4.2.0]octa-2,4,7-trienes have been prepared by photodecarbonylation of the appropriate tricyclo[4.2.1.0^2,5]nona-3,7-dien-9-ones and their structures confirmed by ¹³C-n.m.r. The title compounds all undergo thermal ring opening to cyclooctatetraenes, but photolysis is substituent dependent.     

Calculations of the effects of methyl groups on the energy differences between cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene and between their iron tricarbonyl complexes

In accord with experiment, DFT calculations find that cyclooctatetraene (COT, 1a) is lower in energy than its valence isomer, bicyclo[4.2.0]octa-2,4,7-triene (BCOT, 3a) and that the iron tricarbonyl complex of COT [COT-Fe(CO)(3), 2a] is lower in energy than the iron tricarbonyl complex of BCOT [BCOT-Fe(CO)(3), 4a]. Also in agreement with experiment are the DFT findings that 1,3,5,7-tetramethylCOT (TMCOT, 1b) is lower in energy than 1,3,5,7-tetramethylBCOT (TMBCOT, 3b), but that the iron tricarbonyl complex of TMCOT [TMCOT-Fe(CO)(3), 2b] is higher in energy than the iron tricarbonyl complex of TMBCOT [TMBCOT-Fe(CO)(3), 4b]. Calculations of the energies of isodesmic reactions allow the effect of each of the four methyl groups in 1b-4b to be analyzed in terms of its additive contribution to the relative energies of TMCOT (1b) and TMBCOT (3b) and to the Fe(CO)(3) binding energies in TMCOT-Fe(CO)(3) (2b) and TMBCOT-Fe(CO)(3) (4b). Our calculations also predict that the eight methyl groups in octamethylCOT-Fe(CO)(3) [OMCOT-Fe(CO)(3), 2c] should have much more than twice the effect of the four methyl groups in TMCOT-Fe(CO)(3) (2b) on raising the energy of OMCOT-Fe(CO)(3) (2c), relative to that of OMBCOT-Fe(CO)(3) (4c). The effects of the interactions between the methyl groups in OMCOT-Fe(CO)(3) (2c) and OMBCOT-Fe(CO)(3) (4c) are dissected and discussed.     

The facile isomerization of benzo[8,9]bicyclo[5.2.0]nona-2,4,8-triene to benzo[7,8]bicyclo[4.2.1]nona-2,4,7-triene promoted by L3M(CO)3 complexes (M = Cr,Mo, W). The crystal structure of the tricarbonyl complex of benzo[7,8]bicyclo[4.2.1]nona-2,4,7-triene

Benzo[8,9]bicyclo[5.2.0]nona-2,4,8-triene (I) isomerizes to benzo[7,8]bicyclo[4.2.1]nona-2,4,7-triene (II) in the presence of L₃M(CO)₃ complexes (L = CO, NH₃, CH₃CN; M = Cr, Mo, W). In THF at 40°C with (CH₃CN)₃CrCO₃ the uncomplexed II is formed together with smaller amounts of the corresponding η⁶-tricarbonylchromium complex, TCC-II. In n-butyl ether at 120°C with (NH₃)₃Cr(CO)₃ the main product is the latter complex. The structure of TCC-II has been determined by NMR and X-ray analysis.     

Heterohomoconjugation versus heterobicycloconjugation in 9-thiabicyclo[4.2.1]nona-2,4,7-triene

The bridged heterocycles, 9-thiabicyclo[4.2.1]nonane (3), 9-thiabicyclo[4.2.1]non-7-ene (4), 9-thiabicyclo[4.2.1]nona-2,4-diene (5) and 9-thiabicyclo[4.2.1]nona-2,4,7-triene (2), were examined by photoelectron spectroscopy and the various MO levels theoretically located in each case by CNDO/S calculation. The combined information is best interpretable in terms of localized π interaction between sulfur and ethylene but not butadiene. The conclusion therefore is that thiabicyclotriene 2 is not a heterobicycloconjugated molecule but rather one best described as heterohomoconjugated.     

A Facile Synthesis of Bicyclo[4.2.0]octa-1,3,5-trien-3-ol

Bicyclo[4.2.0]octa-1,3,5-trien-3-ol (2) has been synthesized in good yield via a Baeyer-Villiger reaction of the corresponding aldehyde 5 with permonophosphoric acid.     

Direct and sensitized photolysis of bicyclo[4.2.0]octa-2,4-diene in the gas phase

The direct photolysis of bicyclo[4.2.0]octa-2,4-diene in the gas phase at 280–300 nm produces mainly 1,3,5-cyclooctatriene and benzene plus ethylene. The yield of the former product is enhanced by added gases, and it is proposed that it is formed in a vibrationally excited state which can revert to bicyclooctadiene unless the excess energy is removed in collisions. Computer modelling of the direct photolysis yielded quantitative agreement with the experimental results, but only when large, arbitrary adjustments were made to the calculated rate constants for the interconversion of cyclooctratriene and bicyclooctadiene. The Hg(6³P₁) sensitized reaction of bicyclooctadiene produces mainly benzene plus ethylene, a process which is also enhanced by added gases.