Automerization of Homocubylidene: Proof of a Reversible Olefin/CarbeneRearrangement

摘要

Eaton and Hoffmann found that decomposition of cubyl-phenyldiazomethane gives 9-phenylhomocub-1(9)-ene (1) and discovered that this extremely twisted, anti-Bredt olefin rearranges to 1-phenylhomocubylidene (2), a singlet carbene. Jones and Chen reported thereafter similar behavior for the desphenyl compound. Herein we demonstrate that such extraodinary olefin-to-carbene rearrangements are reversible. We now know that the thermal decomposition of cubyl phenyl ketone tosylhydrazone (3) and of 1-phenylhomocubanone tosylhydrazone (4) in refluxing ethanolic sodium ethoxide give the identical distribution of ethyl ethers. As entry into the reaction from the olefin side (i.e., from 3) gives the same product distribution as entry from the carbene side (i.e., from 4), the reaction manifold would seem to allow for homocubene 1 and homocubylidene 2 to come into equilibrium. However, 2 might not be formed directly from the salt of 4. Carbon migration could proceed concomitantly with loss of dinitrogen; this would give 1 without the intervention of 2, which would arise only subsequently from 1. Under this less-than-simple circumstance, the observation of an identical product ratio from the decompositions of 3 and 4 does not require that equilibrium be reached between the olefin and the carbene, nor even that these be interconvertible. We demonstrate now that at least the latter does in fact pertain. For unambiguous proof of the interconversion, an unequivocal source of homocubylidene is needed.