Chemistry of heavy carbene analogs R2M (M = Si, Ge, Sn). 12. Concerted and nonconcerted insertion reactions of dimethylgermylene into the carbon-halogen bond

Abstract

During the reaction of MezGe with CC13X (X = C1, Br), PhCHzX (X = Br, I), and PhzCHCl, ' H CIDNP is observed in the products of net insertion of MezGe into the carbon-halogen bond and in MezGeXz (X = C1, Br). It is concluded that a two-step radical reaction takes place by an abstraction-recombination mechanism. No reaction takes place with alkyl halides that have a C-X bond dissociation energy of more than about 70 kcal/mol. MezGe is generated thermally at 70-95 "C or photochemically from the 7-germabenzonorbornadiene 1 and reacts in both cases in the singlet state. The activation energy for forming MezGe from 1 is 19 kcal/mol for the reaction with CC14 Insertion products are also formed with the alkenyl halides CH2=CHCHzX, PhCH=CHX (X = C1, Br), and 2-bromobut-2-ene, but without showing CIDNP effects. Since MezGeXz was not found either, MezGe reacts in these cases in a nonradical manner. It does not react with 1-chlorocyclohexene, but it does react with MezGeXz under formation of digermanes and/or oligogermanes without CIDNP. Me Me , , 4 7Oo-15O0C ph& PIT f M~z G~:~ Ph Ph 1 PhCarbenes are known to undergo insertion into the carbon-hydrogen bond. From the beginning of carbene chemistry, two different reaction mechanisms were discussed. The first is a concerted one, meaning that the insertion occurs in a single step without further reaction intermediates via a three-center-type transition state (eq

Fast Processes and Intermediates in Photochemistry of 7-Dimethyl-germanorbornadiene

“…The TPN molecule could be in either the ground or the excited singlet state A long-wave GNB absorption band is localized in the range of 275 nm, which corresponds to an energy of absorbed quantum of about 4.5 eV. The effective thermal decomposition of GNB at about 100°C 16,47 indicates that the energy of the first Ge-C bond break is about 1.0-1.2 eV 48 (the break of the second Ge-C bond will require a much smaller amount of energy). About 3.3-3.5 eV can be spent to form the excited S 1TFN molecule, which is insufficient because a maximum of the longwave TPN absorption band is at 300 nm (4.1 eV).…”

“…[3][4][5][7][8][9][10][11][12][13][14] 7,7′-Dimethylgerma-1,4,5,6-tetraphenyl-2,3-benzo-norbornadiene (GNB) is a convenient source of Me 2 Ge; by exposing GNB to high temperatures or UV-irradiation, it decomposes into Me 2 Ge and 1,2,3,4-tetraphenylnaphthalene (TPN). 2,[15][16][17][18][19] However, data on the elementary stages of the photochemical transformations of GNB are rather contradictory. Therefore, an application of the 1 H CIDNP method suggests two different pathways of GNB phototransformation.…”

Reactions of an Isolable Dialkylsilylene with Aroyl Chlorides. A New Route to Aroylsilanes

“…The ionic mechanism is also applicable for the reaction of 1 with cyclopropylmethyl chloride, in which the intermediary cyclopropylmethyl cation or its equivalent 3-butenyl cation reacts with an extra silylene 1 forming 3-butenylsilyl cation and then finally 6; the 3-butenylsilyl cation would be stabilized by the coordination of the terminal π bond. Chloroalkanes with less electron donating substituents like CHCl 3 and CCl 4 destabilize the carbocation intermediates and instead yield 5 after the homolysis of the C-Cl bond [54]. The insertion reactions of silylene 1 into the Si-Cl bonds of chlorosilanes have been found to occur cleanly [49,50]; hence, the concerted mechanism via three-membered cyclic transition states has been proposed.…”

Re-examination of 7,7′-dimethyl-7-germanorbornadiene photocleavage in solution and in organic glasses

“…7,7 0 -dimethyl-1,4,5,6-tetraphenyl-2,3-benzo-7-germanorbornadiene (GeNB) is the most frequently used source of dimethylgermylene for chemical synthesis. This compound is thermally or photochemically decomposed with the formation of dimethylgermylene and 1,2,3,4-tetraphenylnaphthalene (TPN) [1,[30][31][32][33].…”