34% yield. An increase in the diene concentration resulted in a decrease in the yield of the silacyclopentene, and none of this product was obtained when the hexamethylsilirane-2,3-dimethyl-l,3-butadiene reaction was carried out (1 5 hr at 70°) in the absence of a diluent, although the tetramethylethylene yield was 40%. With some other dienes (e.g., trans,trans-2,4-hexadiene) no dimethylsilylene adducts were obtained under the conditions found to be successful with 2,3-dimethyl-1,3-butadiene. One may speculate that the unconverted 1,3 diene present in these reactions intercepts either a diradical intermediate'2a,b of the rearrangement of the vinylsilacyclopropane or reacts (by cycloaddition?) with the product of the rearrangement. Further experiments are required on this point.This new, mild route to dimethylsilylene may allow the development of new silylene chemistry, and experiments directed toward this goal are in progress. However, it should be recognized that the scope of the application of hexamethylsilirane will be limited by the extremely high reactivity of the silacyclopropane ring system toward many classes of corn pound^,^^^^-'^ among which are some which might react with silylenes. In those cases reactions with the silirane starting material would preclude observation of the desired silylene reactions.This thermal dimethylsilylene extrusion from hexamethylsilirane finds parallels in some known cases of sulfur extrusion from thiiranes" and in thermal difluorocarbene extrusion from gem-difluorocyclopropanes.'* The other known silacycl~propanes'~ are much more thermally stable than hexamethylsilirane and do not serve as sources of dimethylsilylene at these low temperatures. Thus when 2 2 was heated in triethylsilane solution at reflux for 17 hr, no Et3SiSiMe2H was obtained, Instead, a 20% yield of a dimer of 2, presumably 3, was formed, along with nonvolatile polymeric material. 3No information concerning the mechanism of dimethylsilylene extrusion from hexamethylsilirane is available at present. A concerted process, the reverse of singlet state silylene addition to the C-C bond,I2 seems a good possibility, but a stepwise process proceeding via the diradical -Si-MezCMezCMe2-also must be considered.

No abstract

Samples of trans-polyacetylene, (CH)x, were doped with the magnetic ion, FeCl−4, by immersion in nitromethane solutions of FeCl3. The resulting dopant levels ranged over two orders of magnitude. ESR spectra and spin–lattice relaxation times, T1, were measured for the undoped polymer and lightly doped polymer over the temperature range 1.5–120 K. The ESR spectra of trans-(CH)x lightly doped (<1 mol % Fe−4 ) with FeCl−4 exhibit a narrow (ΔH≊5–10 G at 4.2 K) g=2.003 signal which decreases in relative amplitude with increasing dopant level, and broad signals at g=2 (ΔH≊500 G), which increases with increasing dopant level, and at g=4.3 (ΔH≊200 G). The narrow signal is the intrinsic trans-(CH)x signal; the broad signals are attributable to iron. T1 for the narrow signal is approximately one-half of the value of undoped trans-(CH)x at 4.2 K. dc susceptibilities for all of the doped samples were measured over the temperature range of 1.5–300 K. The magnetically doped samples all exhibit Curie law behavior, and an analysis of the magnetization data yields an effective moment, Jeff, equal to 2.77, close to the value 5/2 expected for Fe(III).

No abstract

No abstract