Organometallics 27, 5948-5959 (2008).

© American Chemical Society

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

William J. Leigh,* Gregory D. Potter, Lawrence A. Huck, and Adroha Bhattacharya

Contribution from the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

Direct irradiation of 1,1-diphenyl-, 1,1-bis[4-(trifluoromethyl)phenyl]- and 1,1-bis[3,5-bis(trifluoromethyl)phenyl]germacyclobutane (2, 4 and 5, respectively) in methanolic C₆D₁₂ solution affords products consistent with the competing formation of the corresponding 1,1-diarylgermene and diarylgermylene, along with ethylene and cyclopropane. The relative yields of the two Ge-containing primary products (germene:germylene) vary with the extent of CF₃-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 (λ_max ~ 325 nm) and quenching studies with MeOH, tert-butanol (t-BuOH), acetic acid (AcOH), n-butyl amine (n-BuNH₂), 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 and 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 ~ n-BuNH₂ < 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.

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