Di- and Trivalent Organogermanium Reactive Intermediates. Kinetics and Mechanisms of Some Reactions of Diphenylgermylene and Tetraphenyldigermene in Solution
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The reactivity of diphenylgermylene (Ph2Ge) with several classes of germylene scavengers has been studied in hexane solution at 23 degrees C by laser flash photolysis of 3,4-dimethyl-1,1-diphenyl-1-germacyclopent-3-ene (1a), a clean and highly efficient precursor to the germylene and its dimer, tetraphenyldigermene (2a). The reactions studied include M-H insertion reactions with Group 14 hydrides (M = Si, Ge, Sn), halogen atom abstractions from bromo- and chlorocarbons, Lewis acid-base complexation with 1 degrees, 2 degrees, and 3 degrees aliphatic amines, and reaction with an aliphatic alkene, alkyne and diene, and oxygen. Absolute rate constants for (irreversible) scavenging of the germylene could be obtained by direct measurement of the germylene decay kinetics for all but the least efficient scavengers (triethylsilane, oxygen, chloroform, and 1-bromopentane), for which estimates of the rate constants were obtained by Stern-Volmer analysis of the reduction in digermene yield as a function of scavenger concentration. Distinctly different kinetic behavior is observed for scavenging of Ph2Ge by isoprene, 4,4-dimethyl-1-pentene, and triethylamine; in these cases, the results suggest that reaction is rapid (k(Q) = 3-6 x 10(9) M(-1)s(-1)) but reversible (K(eq) = 2500 - ca. 20,000 M(-1)) over the range of scavenger concentrations studied. The reactions with the C-C unsaturated compounds proceed via the intermediacy of long-lived transient species absorbing at <290 nm, which are tentatively assigned to the corresponding three-membered germanocycles on the basis of their UV spectra and lifetimes. Upper limits for the absolute rate constants for reaction of tetraphenyldigermene (2a) toward many of these reagents are also reported.
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