Photochemical synthesis 29. Photoannelation and cyclohexenone

Abstract

Values of 1.7 x 10' s-I and 2.5 x lo7 M-' s-' for k,, the unimolecular rate constant for decay, and k,, the rate constant for addition to cyclohexene, have been obtained for cyclohexenone. These indicate that this cyclenone decays somewhat faster and reacts significantly slower with the same substrate (in the same solvent) than does cyclopentenone. Evidence is adduced, from sensitization experiments and the phosphorescence quenching of a range of carbonyl compounds, that the reactive triplet and lowest triplet of cyclohexenone lie close together, if they are indeed not identical, and the energy of this triplet is in the region of 66-68 kcal/mole. This is higher than that indicated by photostationary state measurements. This latter method, applied to cyclenones, may be suspect, and triplet energies so derived subject to doubt (that of cyclopentenone included) or have a different significance from those obtained by other methods. Further attempts to demonstrate the existence of a lower, cycloadditively unreactive, triplet in cyclopentenone have been negative. It now seems far more likely that the original interpretation was in error, that the lowest energy triplet is near 73 kcal/mole, and that a special mechanism for quenching benzophenone triplets by ground state cyclopentenone exists. The evidence for other multiple reactive triplets is re-examined as is the evidence for the nature of the states involved.

Photochemical synthesis. 67. Mechanism of enone photoannelation: activation energies and the role of exciplexes

“…Cyclopentenone (Aldrich), cyclohexene (Phillips), m-dichloroethylene (Eastman), cyclohexenone, trans-3-hexene, and 2,5-dimethyl-2,4-hexadiene (Aldrich) were purified as previously described. 6 The method of Liu10was used in preparing bicyclo[4.2.0]oct-7-ene (bp 132 °C). c/s-Piperylene (K and K Laboratories) and trans-piperylene (Aldrich) were purified by distillation over sodium wire and then passed over neutral alumina.…”

Proton Induced Structural Reorganization of a Few Carbonyl Molecules in the Ground and Excited States

“…Acrolein. The effect of protonation on the CO bond length follows the expected trend; [14][15][16] the proton polarizes the π electron density within the CO bond and toward the oxygen atom of the CdO moiety (Figure 1 and Figure 2). The response of electron density in the C-O bond to the added proton is however seen to exhibit some dependence on the orientation of the -CHO unit about the C-C single bond indicating the relative stabilities of cis and trans geometries.…”

Primary Bond Formation in the Addition of Cyclopentenone to Chloroethylenes

“…Previous experience (26) suggests that the difference would not be great and so the values obtained (@,. ;,.D~E = 0.35 ; @ ,ra,,s.~c~ = 0.18)' incline us to believe that the cis addition is reversing less than the /runs (provided that no other energy-wasting reversible intermediate lies on the reaction pathway before the tetramethylene).…”