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Fluxionality Induced by High Pressure [9-D]9-Homocubyl Triflate a Pressure-Sensing Molecule.

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Fluxionality Induced by High Pressure:
[PD]9-Homocubyl Triflate, a Pressure-Sensing
Molecule""
Urs P. Spitz and Philip E. Eaton*
d @
The solvolysis of 9-homocubyl triflate (1) and similar
. .......,.
derivatives occurs by ionization
to a tight ion pair containing
the
o-bridged
homocubyl
1
2
cation (2) . [ I %21 Recently, and in
accord with this interpretation,
we reported that [9-D]9-homocubyl triflate ([9-D]-1) on exposure to heat or to polar solids like silica gel undergoes multiple
automerizations that result in scrambling of the deuterium label
over five positions of the hoinocubyl skeleton [Equation (a)].[']
Starting material recovered from incomplete solvolyses of [9-D]1 has the label scrambled in the same way. It follows from these
OTf
OTf
PI-1
[9-D]-1
observations that only C - C bonds anti to the ionizing group
undergo Wagner -Meerwein shifts. Continual automerization
results in a formal rotation of a cyclopentyl cation against a
cyclobutane ring. Herein we report that these automerizations
occur simply upon pressurizing solutions of [9-D]9-homocubyl
triflate.
Samples of labeled triflate [9-D]-1 ['I in dichloromethane were
subjected to hydrostatic pressures of about 10000 atm.I31 After
various periods of time the triflate was recovered, and the label
distribution was determined by proton and deuterium NMR
spectroscopy. Under the conditions of high pressure, as is apparent from the data in Table 1 , a nonsolvolytic, highly stereoseTable 1 . Pressure-induced scrambling in labeled triflate [9-D]-1 as a function of
time.
OTf
OTf
Entry
1
2
3
4
5
D at position [a]
Pressure
[atm]
Time
[hl
C(9)
C(1.8)
(32.3)
10000
10 700
0.25
1.75
6
12
72
70
52
46
23
20
30
41
41
40
40
-0
9 400
10000
10000
7
13
37
40
lective scrambling process took place.[41Exposure for several
days (entry 5 ) led to essentially statistical distribution of the
deuterium label 120% C(9), 40% C(1,8) 40% C(2,3)] over the
syn face. At normal pressure under otherwise the same conditions no significant scrambling of the label occurred. Although
the solution of the triflate darkened during the pressure-induced
scrambling process, no significant decomposition could be detected by N M R spectroscopy. This is remarkable because at
least ten automerizations, each involving the formation of the
ion pair and internal return of the triflate, are necessary to
spread the label evenly (k3 %) over the five positions. In contrast, thermally induced scrambling of the label of [9-D]-1, for
example in hot toluene, was accompanied by complete decomposition of the triflate well before the statistical distribution of
the label could be reached.[']
Pressure has two very particular effects on chemical react i o n ~ . ~The
' ~ first, intuitively obvious, follows the ancient principle of Le Chatelier: Increasing pressure is countered by a
decrease in volume. Thus, reaction rates are strongly enhanced
by pressure if in the transition state molecules associate (negative volume of activation). The second important effect, electrostriction, is less obvious: The polarization induced by a
charge (an ion) in a dielectric (a solvent) increases intermolecular forces. This causes the dielectric medium to pack more
densely and decrease its volume. Consequently, processes involving charge separation can be promoted by pressure. We
suppose that the electrostriction effect is the operating principle
in the present examples. In this case, quite extraordinarily, pressure controls the relative rate with which a cyclobutane rotates
relative to a cyclopentane ring. This pressure-regulated "molecular gear" is certainly unique.
Received: May 8. 1995 [Z79671E]
German version: Anyew. Cheni. 1995. 107. 2206-2207
Keywords: automerization * carbocations
fluxionality high-pressure chemistry
*
electrostriction
-
[I] U. P. Spitz, J. Am. Cheni. Soc. 1993, 115, 10174.
[2] a) P. von R. Schleyer. J. J. Harper. G. L. Dunn. V. J. DiPdsquo, J. R. E. Hoover,
J. Am. Chc,m. So<. 1967, 89, 698; b) R. E. Leone, P. vonR. Schleyer, Angew.
Chem. 1970.82,889: Anyew. Chem. Int. Ed. Engl. 1970, 9,860; c) R. E. Leone.
J. C. Barborak, P. vonR. Schleyer, Curhonium Ions. Wiley Interscience, New
York. 1973. pp. 1863-1869; d) A. P. Marchand, Chem. Rev. 1989.89, 1011; e)
J. C . Barbordk. R. Pettit. J. Am. ChPm. Soc. 1967. 89, 3080; f) P. Ahlberg, G.
JonsHII, C. Engdahl, A h . Phys. Ory. Chem. 1983, 223.
[3] The apparatus (LECCO model PG-200 HPC) consisted of a steel chamber
containing a pressure transmission fluid (castor oil) in which a sample holder
was immersed. The system was pressurized by a hydrdUllCdlly driven differential
area piston The sample was contained in Teflon tubing sealed with glass stoppers.
14) Significant adiabatic heat is produced during pressurization and might affect the
experimental results. To reduce the problem, the experiments were run with
samples pre-cooled in ice water, and the pressure was raised relatively slowly.
[5] For an excellent and detailed overview of the effects of pressure on organic
reactions, see W. J. IeNoble in High Pressure Organic Chemistry. Vol 37, (Ed.:
W. J. IeNoble). Elsevier, New York, 1988, 1
[a] In % ( T 3%); determined by integration of ' H NMR signals. The comparison
with data from the *H N M R spectra was satisfactory.
[*I
[**I
Prof. P. E. Eaton, Dr. U. P. Spitz
Department of Chemistry
The University of Chicago
5735 S. Ellis Avenue, Chicago, IL 60637 (USA)
Telefax: Int. code + (312)702-0805
e-mail: pde2(11midway.uchicago.edu
This work was supported by the National Science Foundation
2030
(('3
VCH Verlugsgexlls~haftmhH, 0-69451 Weinherin, tYYS
0S70-0~33/9S/34t8-2030
$ 10.00+ .2S/0
Angew. Chem. Int. Ed. Engl. 1995. 34. N o . 18
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