Huck and Leigh, Can. J. Chem., 89, 241-255 (2011).

Can. J. Chem., 89, 241-255 (2011).

Kinetic and Mechanistic Studies of the Reactions of Diarylgermylenes and Tetraaryldigermenes with Carbon Tetrachloride

Lawrence A. Huck and William J. Leigh*

Contribution from the Department of Chemistry & Chemical Biology, McMasterUniversity, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

The mechanisms of the reactions of diphenylgermylene (GePh₂) with CCl₄ in hydrocarbon solvents and in THF solution have been studied, employing 3,4-dimethyl-1,1-diphenylgermacyclopent-3-ene (6a) and 1,1-diphenylgermacyclobutane (17) as photochemical precursors to GePh₂. In hydrocarbon solvents, the reaction produces Ph₂GeCl₂ (10) and Ph₂Ge(Cl)CCl₃ (12) in a ratio of 10/12 ≈ 7, along with a variety of other radical-derived products and small amounts of Ph₂GeH(D)Cl (11), which is formed partly by reaction of GePh₂ with adventitious HCl. The reaction is much cleaner in THF, where 12 is formed as the major product (10/12 ≈ 0.8); a similar product distribution is obtained in hexanes containing 0.05 M THF, while 12 is the exclusive product in hexanes containing 3 mM NEt₃. Rate constants for the reactions of CCl₄ with GePh₂ and five ring-substituted derivatives were determined by laser flash photolysis, as well as Arrhenius parameters for reaction of the parent (GePh₂), in the two solvents. The reactions of GePh₂ with CCl₄ and CHCl₃ have also been studied in 3-methylpentane at 78-90K. Different reaction mechanisms are clearly operative in hydrocarbon and complexing solvents, but both involve modest charge donation from germanium to the substrate in the transition state for the rate determining step. The Arrhenius parameters for reaction of the free germylene in hexanes indicate a weakly bound but highly structured transition state; the data are consistent with inner-sphere electron transfer following or in concert with weak Lewis acid-base complexation. A similar mechanism is proposed for the reaction in THF solution, in competition with a second involving nucleophilic attack of the germylene-THF complex at a chlorine atom of the substrate. Rate constants were also determined for reaction of CCl₄ with the corresponding tetraaryldigermenes at low halocarbon concentrations in hexanes, along with Arrhenius parameters for the parent (Ge₂Ph₄). These reactions also proceed via initial Cl-atom abstraction, based on the identity of the products formed in the reaction of CCl₄ with tetramesityldigermene.

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