Organometallics, 26, 1339-1348 (2007).
© American Chemical Society
Lawrence A. Huck and William J. Leigh*
Contribution from the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.
A series of three 1,1-diaryl-3,4-dimethylgermacyclopent-3-ene derivatives bearing polar substituents in the para-positions of the 1,1-diphenyl rings have been synthesized and their photochemistry has been studied by steady state and laser flash photolysis methods. Photolysis in hydrocarbon solvents affords the corresponding diarylgermylenes in chemical and quantum yields similar to those of the parent (1,1-diphenyl) compound, as shown by the results of trapping experiments with methanol (MeOH) and acetic acid (HOAc). The germylenes exhibit UV/vis absorption spectra (lmax 480-500 nm) and kinetic behavior (t ~ 2 ms) similar to that of the parent, diphenylgermylene (GePh2), in hexane solution. Their decay results in the formation of new transient products assigned to the corresponding tetraaryldigermenes (Ge2Ar4), whose absorption spectra (lmax = 440 nm) and lifetimes are also similar to those exhibited by the parent, Ge2Ph4. Both the germylenes and the corresponding digermenes are quenched rapidly by added HOAc and Et2NH, with rate constants that increase with increasing electron-withdrawing power of the aryl substituents, consistent with reaction mechanisms that involve initial nucleophilic attack at germanium in every case. The Lewis acid-base complexation of the three germylenes with THF and ethyl acetate (EtOAc) have also been studied, and support a two-step mechanism for the reaction with HOAc involving initial, rate determining nucleophilic attack at germanium followed by rapid proton transfer. A similar mechanism is proposed for reaction of this substrate with Ge2Ph4 on the basis of the measured Hammett r-value. Tetramesityldigermene reacts with HOAc in THF solution to afford the 1,2-addition product, 1-acetoxy-1,1,2,2-tetramesityldigermane, while no reaction could be detected with Et2NH.
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