Competing Germene and Germylene Extrusion from Photolysis of 1,1-Diarylgermacyclobutanes. Substituent Effects on Germene Reactivity

Direct irradiation of 1,1-diphenyl-, 1,1-bis[4-(trifluoromethyl)phenyl]-, and 1,1-bis[3,5-bis(trifluoromethyl)phenyl]germacyclobutanes (2, 4, and 5, respectively) in methanolic C(6)D(12) solution affords products consistent with the competing formation of the corresponding 1,1-diarylgermenes and diarylgermylenes, along with ethylene and cyclopropane. The relative yields of the two Ge-containing primary products (germene:germylene) vary with the extent of CF(3) substitution on the aryl rings, decreasing in the order 2 > 4 > 5. As was reported previously for 2, laser flash photolysis of 4 and 5 in hexane, acetonitrile, or tetrahydrofuran solution allows the detection of the corresponding transient 1, 1-diarylgermenes (6 and 7, respectively), which have been identified on the basis of their UV/vis spectra (lambda(max), similar to 325 nm) and quenching studies with MeOH, tert-butyl alcohol (t-BuOH), acetic acid (AcOH), n-butylamine (n-BuNH(2)), and acetone. In carefully dried hexane solution, weak transient absorptions assignable to the corresponding germylenes and their respective (digermene) dimers are also observed; in the case of 5, these assignments have been confirmed by the results of steady-state and laser photolysis experiments with 1,1-bis[3,5-bis(trifluoromethyl)phenyl]-2,3-dimethyl-1-germacyclopent-3-ene (14c), which affords the germylene exclusively, in substantially higher quantum yield. The reactivities of the germenes toward each of the various substrates studied vary modestly with aryl substituent, increasing in the order acetone < t-BuOH < MeOH approximate to n-BuNH(2) < AcOH. The rate constants increase with increasing trifluoromethyl substitution in the cases of alcohol and acetone addition but decrease correspondingly in the case of AcOH addition. Substrate acidity thus plays a much more dominant role in the reactions of the Ge=C bond with nucleophilic reagents than is the case with the homologous silene derivatives, whose reaction kinetics are controlled primarily by substrate nucleophilicity.