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In conclusion, our results suggest that the chemistry of 7 and 12 in solution is the chemistry of "carbene complexes"2s affording nearly exclusively 6, while in the "gas phase" the typical products of 5 become dominant.26 Carbenoid chemistry the alkyllithium way can be shifted toward chemistry expected from free carbenes with our new method. Thus, the geminal dibromides studied in the gas phase give higher yields of hydrocarbons-thought to stem from free carbenes-than in solution. In cases in which a tosylhydrazone cannot be synthesized, the decomposition of geminal dibromides may be an alternative to the Bamford-Stevens reaction. From the product ratios of vinylallene (3) and cyclopentadiene (15), A H and AM' of the competing cyclopropylidene-allene and carbene-carbene rearrangement 1 -2 can be determined. Acknowledgment.We gratefully acknowledge support of this work by the Deutsche Forschungsgemeinschaft. We thank K. Gomann for skillful experimental and technical assistance. (25) Hoeg, D. F.; Lusk, D. I.; Crumbliss, A. L. J. Am. Chem. SOC. 1965, 87, 4147. (26) Seebachs' I3C NMR experiments demonstrate that there is a smooth transition of carbenoids to carbenes: Seebach, D.; Siegel, H.; Miillen, K.; Hiltbrunner, K. Angew. Chem., Inr. Ed. Engl. 1979, 18, 844. Siegel, H.; Hiltbrunner, K.; Seebach, D. Ibid. 1979, 18, 845.

The Rh is octahedrally coordinated. Four positions are taken up by the porphyrin pyrrole nitrogen atoms (Rh-N average distance 2.04 A); a chloride ion (Rh-Cl distance of 2.35 A) and a phenyl group that is -bonded to the Rh (Rh-C distance 2.05 A); complete the coordination. The phenyl group is approximately perpendicular to the mean porphyrin plane.The mechanism of formation and chemistry of this interesting species is presently under investigation.23 (23) This research was supported by grants from the National Science Foundation and National Institutes of Health.(24) Alfred P. Sloan Fellow.

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Alder reaction pathway which does not involve nitrogen loss. We now wish to record evidence that exo-6,7diazatricyclo[3.2.1.02'4]-6-octene (4) decomposes by yet another mechanism. In this connection, 4 shows an enormous reactivity ratio of 1011 compared to structurally related 2.We also considered the possibility that cyclooctatetraene might rearrange to the bicyclo[3.3.0]octadienyl dication (7) in the super acid medium HFS03-SbF6.6 This was not the case, for when cyclooctatetraene was added to 1:1 fluorosulfuric acid-antimony pentafluoride in sulfur dioxide solution, a poorly resolved spectrum of the homotropylium ion was obtained.7 It was possible to prepare a solution of this ion giving a well-resolved spectra by adding cyclooctatetraene to fluorosulfuric acid in sulfur dioxide and then adding antimony pentafluoride. Aside from slight changes in chemical shifts, the spectrum was identical with that reported for the homotropylium cation. We are continuing our studies on pentalenyl dications and bicyclo[3.3.0.]octadienyldications.