C O M M U N I C A T I O N S
Attempts to uncover thermal isomerizations of deuterium-labeled
1,1-dimethylcyclopropane taking place through tert-butylcarbene
or isobutylcarbene failed to provide any evidence implicating them
as mechanistically significant intermediates.12 The factors influenc-
ing possible cyclopropane-to-propylidene thermal transformations
in substituted systems remain to be clarified through theory and
experiments, and possibly related reactions of cyclopropene and
silacyclopropanes merit reconsideration.13
In his 1882 paper reporting the first synthesis of cyclopropane,
Freund explicitly considered 1-propylidene as a possible structure
for his newly prepared C3H6 compound.14 Cyclopropane and
1-propylidene have been closely associated from the beginning!
That 1 rearranges thermally to 4 may be viewed with surprise, given
the extended scrutiny the 1-to-2 isomerization has attracted over
more than a century. The present experimental work supports the
prediction based on coupled cluster methods in 19997 and re-
emphasizes the ineluctable fact that the level of theory used needs
to be appropriate to the issue being addressed if reliable predictions
are to be attained.
Acknowledgment. We thank the National Science Foundation
for support of this work through CHE-0211120.
Figure 1. Proton-decoupled 13C NMR spectrum of the methylene carbon
signals from labeled propenes formed through thermal rearrangements of
1-13C, d4. The singlet at 115.467 ppm is consistent with *CH2dCD-CD3.
The 1:1:1 triplet (1J13C-D ) 24 Hz) centered at 115.177 ppm is consistent
with 1-13C-1,2,3,3-d4-propene. All three kinetic runs gave nearly identical
spectra for product mixtures; unreacted 1-13C, d4 was unscrambled.
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run to run, in parallel with initial ratios of cyclopentane to labeled
cyclopropane.
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