Substituent Effects on the Reactions of Diarylgermylenes and Tetraaryldigermenes with Acetic Acid and Other Lewis Bases in Hydrocarbon Solution

Abstract

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 (λmax = 480−500 nm) and kinetic behavior (τ ≈ 2 μs) 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 (λmax = 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 all cases. The Lewis acid−base complexation of the three germylenes with THF and ethyl acetate (EtOAc) has also been studied and supports a two-step mechanism for the reaction with HOAc that involves 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 ρ-value. Tetramesityldigermene reacts with HOAc in toluene or THF solution to afford the 1,2-addition product, 1-acetoxy-1,1,2,2-tetramesityldigermane, while no reaction could be detected with Et2NH.

A combined kinetic and computational study of the reactions of transient germylenes with oxiranes and thiiranes in solution

“…The absorptions decay over several tens of microseconds with second-order decay kinetics with the concomitant growth of longer-wavelength absorption bands due to the corresponding digermenes (Ge 2 Me 4 and Ge 2 Ph 4 ). The spectra and kinetic behavior are quite similar to those reported previously for the Lewis acid-base complexes of the two germylenes with tetrahydrofuran (THF) and tetrahydrothiophene (THT) under similar experimental conditions 45,51 and are thus assigned to the corresponding Lewis acid-base complexes with CHO and PrS. In addition to the signals due to the GePh 2 -CHO complex and Ge 2 Ph 4 , another even longerlived product ( max ≤ 280 nm) was also detected, its growth kinetics indicating that it is most likely associated with a higher GePh 2 oligomer.…”

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

“…containing substrates, are thought to begin with the formation of a Lewis acid-base complex between the tetrylene and the substrate. 4 Indeed, transient silylene- 9,19,[22][23][24][25] and germylene- 20,21,[25][26][27] donor complexes have been detected directly in a large number of instances, both in lowtemperature matrix experiments and in solution at ambient temperatures by laser flash photolysis methods. Complexation with a nonreactive Lewis base such as THF reduces the reactivity of the tetrylene toward both dimerization and reactions with other substrates that require access to the vacant p orbital on the (ground-state singlet) tetrylene to proceed.…”

“…Arrhenius parameters have been determined, and the role of electronic factors in the transition states for the rate-determining steps of these reactions have been assessed through an examination of the effect of polar ring substituents on the absolute rate constants. As in earlier kinetic studies, 27,30,48 we employed the substituted 1,1-diaryl-3,4-dimethylgermacyclopent-3-enes 6a-6f as photochemical precursors to the corresponding diarylgermylenes (8a-8f) and tetraaryldigermenes (9a-9f; eq. [4]), each pair of which can be studied in the same experiment.…”

3,4-Dimethyl-1,1-bis[4-(trifluoromethyl)phenyl]-2,5-dihydro-1H-germole

“…Compound (I) was synthesized as described elsewhere (Huck & Leigh, 2007). Single crystals of (I) suitable for X-ray diffraction were obtained by slow evaporation of a hexane solution at room temperature.…”