Mechanism of Thermal Deazetization of DBH
J. Am. Chem. Soc., Vol. 122, No. 41, 2000 10175
hydrogens. These atoms move quite small distances, and the
reaction could very probably be accomplished with little
disturbance of surrounding solvent molecules.
The attractive interaction between CO2 and cyclopentane-
1,3-diyl that we have suggested as a possible explanation of
the unusual pressure dependence of ki/kr in that medium, results
in a detailed mechanism that is too complex to be susceptible
to quantitative fitting. However, it seems qualitatively plausible
that the formation of various CO2/biradical complexes could
explain the results. Presumably the rate of formation of such
complexes would increase with pressure. It also seems likely
that the efficiency of IVR between the components of a
relatively long-lived cluster would be greater than in a transient
encounter complex. If both of these postulates were correct,
the result would be a higher-order dependence of the IVR rate
on pressure than predicted by the simple collision model of
Scheme 5. That appears to be what we observe experimentally.
Experimental Section
Computational Details. CASSCF calculations were mostly carried
out with GAMESS4747 although some analytical frequency calculations
were carried out with Gaussian 98.48 CASPT2 calculations were carried
out with MOLCAS.49 All other calculations were carried out with
Gaussian 98. All calculations with the 6-31G(d) basis set used six
Cartesian d functions. Those using larger basis sets employed the
numbers and types of d and f functions that are the defaults for Gaussian
98.
Figure 8. Viscosity correlation of ki/kr for thermal deazetization of
1x (see ref 45). Open circles are data for supercritical propane; filled
diamonds are data for supercritical CO2.
It should be noted, however, that Adam et al.45 have very
recently demonstrated that the ratio of product stereoisomers
can be correlated with the solvent viscosity for photochemical
deazetization of a trialkyl derivative of DBH (endo-1-ethyl-4,7-
dimethylbicyclo[2.2.1]hept-2-ene). That raises the question of
whether the present results could be equally well-explained by
a simple viscosity argument. We think that they cannot, for two
reasons. First, Adam et al. showed that there is a linear
correlation between the logarithm of the product ratio and the
logarithm of the solvent viscosity. Since we know the viscosity
of our supercritical fluids at all of the pressures studied, we
can check for such a relationship. The results are shown for
propane and CO2 together in Figure 8. Not only is the
relationship highly nonlinear, the slope of the best-fit straight
line (R2 ) 0.68) is a factor of 2 larger than that found by Adam
et al.45
The second reason for doubting that changes in viscosity are
sufficient to explain the data is that we have previously shown6
that the ki/kr ratio is insensitive to temperature over a 50 °C
range, in three different liquids. Since the viscosities of organic
liquids typically change by a factor of 2-3 over this temperature
range,46 the lack of temperature dependence to the rate-constant
ratio does not seem compatible with a significant viscosity
dependence.
It is not obvious why the product ratio for deazetization of a
substituted DBH analogue should be sensitive to the medium
viscosity whereas that for the parent molecule is not. One
possibility is that the volume swept out during stereochemical
change is larger for the substituted molecule. The interconver-
sion of bicyclopentane stereoisomers may look like a reaction
that also involves significant atomic motion, even for the parent
molecule, if one focuses on the cyclopropane ring moving with
respect to a fixed cyclobutane moiety. However, the actual
motion of the atoms about the center of mass of the molecule
involves primarily the bridgehead carbons and their attached
Calculations were carried out on a 500 MHz dual-processor Compaq
Alpha DS-20E workstation, a 195 MHz SGI Impact-10000 workstation,
and a 466 MHz G3 Power Macintosh computer.
Synthesis of 2,3-Diazabicyclo[2.2.1]hept-2-ene-exo,exo-5,6-d2 (1x).
The synthesis of DBH-d2 (1x) was carried out as described previously.6
Synthesis of Menthol Methylxanthate. To a suspension of sodium
hydride (0.28 g, 11.52 mmol) in tetrahydrofuran (THF, 40 mL) at 0
°C under a N2 atmosphere was added (-)-menthol (1.5 g, 9.60 mmol)
in THF. The reaction was stirred for 30 min, warmed to room
temperature, and then stirred for another 30 min. Carbon disulfide (0.877
g, 11.52 mmol) was then added and the reaction mixture stirred for
another 1 h. After this period, iodomethane (1.635 g, 11.52 mmol) was
added and the reaction mixture stirred for an additional 1 h. The mixture
was partitioned between water (30 mL) and diethyl ether (50 mL). The
organic layer was separated and dried over sodium sulfate. The solvent
was removed on a rotary evaporator and the residual crude product
subjected to column chromatography on silica gel, with the products
being eluted with hexanes. Menthol methylxanthate (1.66 g, 70% yield)
was obtained as a foul-smelling yellow solid. 1H NMR (200 MHz,
CDCl3): δ 0.81 (d, J ) 7.0 Hz, 3H), 0.92 (d, J ) 7.1 Hz, 3H), 0.94 (d,
J ) 7.7 Hz, 3H), 1.01-1.87 (m, 8H), 2.19-2.28 (m, 1H), 5.53 (td, J
) 4.4, 10.8 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 16.97, 18.79,
20.56, 21.96, 23.77, 26.61, 31.33, 34.14, 39.61, 47.24, 84.49, 215.41.
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Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
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D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz J. V.; Baboul, A. G.; Stefanov, B. B.; Liu G.; Liashenko, A.; Piskorz,
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Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
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