CYCLOBUTENE PHOTOCHEMISTRY - ADIABATIC PHOTOCHEMICAL RING-OPENING OF ALKYLCYCLOBUTENES

The photochemistry of the cis and trans isomers of a series of dimethy lbicyclo[n.2.0]alk-(n+2)-enes (n = 2-5) (bicyclic cyclobutene derivati ves in which the C=C bond is shared by the two rings) in pentane solut ion is described. Irradiation of these compounds using monochromatic 1 93- or 214-nm light sources results in ring opening to yield the corre sponding 1,2-bis(1-ethylidene)cycloalkanes (C-4-C-7) in high chemical and quantum yields. In all cases, the reaction proceeds with a high (7 0-90%) degree of disrotatory stereoselectivity. Quantum yields for dir ect cis,trans photoisomerization of the isomeric E,E- and E,Z-1,2-bis( 1-ethylidene)cycloalkanes have also been determined. The product distr ibutions from irradiation of the cyclobutenes are wavelength dependent , but for 214-nm excitation the isomeric diene distributions obtained from cyclobutene ring opening agree fairly closely with those calculat ed from the quantum yields for cis, trans photoisomerization of the is omeric dienes on the assumption that the process involves purely disro tatory ring opening to yield a single diene isomer in the lowest excit ed singlet state. The results are consistent with an orbital-symmetry- controlled, adiabatic mechanism for ring opening.