No abstract

No abstract

No abstract

No abstract

The relative stabilization of 10 4-monoand 24 4,4'-disubstituted triphenylmethyl radicals 1 has been measured by recording the degree of dissociation of the corresponding quinonoid dimers 2 by means of ESR. The following substituents or combinations of two of them have been used: H, CF3, r-Bu, OMe, OPh, CN, COPh, COMe, Ph, SMe, and N02. Both donors and acceptors enhance the stability in the ground state of the radicals, which is evaluated in terms of ' values and a Hammett-like equation. Two donors act additively, as do two acceptors. No specific synergism of a donor with an acceptor (capto-dative stabilization) has been found. Most efficient for the relative stabilization are the electroneutral substituents Ph and SMe. Most of the ESR spectra of these trityls are new. An exact assignment became possible via the corresponding ENDOR spectra, which are listed in detail. Many of the substituted trityls including dimers and precursors have been prepared

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

Die Ti telradikale (4-R -CJ&),C' (I), R = F, C1, Ph, OMe, NO2, tBu, OEt, SMe, CN, CFs, wurden dargestellt, davon die vier letzten erstmalig, und mittels ESR-Spektroskopie vermessen. a!, 4 und u; werden angegeben. Die ESR-Signale bleiben im gesamten zugangljchen Temperaturbereich -30 bis + 100°C in ihrer Intensitat unverandert. Daraus folgt, dal3 alte diese Radikale im genannten Temperaturbereich kinetisch stabil sind, also nicht wie andere Trityle zu Dimerisierungen, etwa o p , up-, a,a-, neigen.

No abstract