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

Abstract

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 C6D12 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 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 (λmax ∼325 nm) and quenching studies with MeOH, tert-butyl alcohol (t-BuOH), acetic acid (AcOH), n-butylamine (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, 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.

Kinetic and mechanistic studies of the reactions of diarylgermylenes and tetraaryldigermenes with carbon tetrachloride

“…[13]). 64 We anticipated that GePh 2 should be detectable from 17 under these conditions since its co-product (cyclopropane) is inert; we have previously shown that the photolysis of 6a in low-temperature glasses does not afford Table 2. Second-order rate constants for the reactions of CCl4 with germylenes 8a-8f and digermenes 9a-9g in deoxygenated hexanes solution and with the corresponding germylene-THF complexes in THF at 25 8C.…”

“…4a). The absorption at 326 nm is assigned to germene 18, 64,67 whereas the 460 nm absorption band is assigned to GePh 2 . The latter spectrum agrees well with those obtained in low-temperature experiments using other precursors, such as the known disilylgermane 19 26,68 and disilyldigermane 20.…”

1,1-Bis[4-(trifluoromethyl)phenyl]germetane

“…For the synthesis of the title compound, see: Leigh et al (2008). For related compounds see: Tokitoh et al (1995); Eichler et al (1999); Meiners et al (2002); Tajima et al (2005).…”

“…Photolysis of 1,1-diarylgermetanes results in two competing modes of cycloreversion: (2 + 2) to yield the corresponding 1,1-diarylgermene (R 2 Ge=CH 2 ) and ethylene, and (3 + 1) to yield the corresponding diarylgermylene (R 2 Ge) and cyclopropane (Leigh et al, 2008). While the quantum yield for decomposition of the germetane remains roughly constant regardless of aromatic substitutent (Φ ca 0.10), decreasing the electron density at germanium by aromatic ring substitution favors formation of the germylene and cyclopropane (Leigh et al, 2008). The molecular structure of (I) is shown in Figure 1.…”

“…Compound (I) was synthesized as described elsewhere (Leigh et al, 2008). Single crystals for x-ray diffraction were obtained via repeated recrystallizations from methanol.…”