| Popis: |
The formation of a 1,2,4-cyclohexatriene derivative (cf. 7) from the singlet-excited cis-butenyne is thought to be the primary reaction step in the photocyclization of 1,4-diarylbutenynes. This very reactive intermediate can yield the end-product along various pathways: (i) directly via a [1,5] hydrogen shift; (ii) via radical intermediates, formed either by acceptance of a hydrogen atom from the solvent (e.g. benzene, hexane) or by release of a hydrogen atom in the presence of oxygen; (iii) via ionic intermediates, formed either by acceptance of a proton (e.g. from methanol) or by release of a proton in the presence of amines. The rates of the cyclization are approximately equal for various butenynes. Differences in product quantum yields are mainly due to differences in the rates of the ring-opening reaction of the cyclic allene. Using laser flash photolysis two transients of the cyclization could be observed. The effect of amines on the transients is described. |
