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The Protonation of Cubane Revisited.

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Communications
Cubane Rearrangements
The Protonation of Cubane Revisited**
Andrey A. Fokin,* Boryslav A. Tkachenko,
Pavel A. Gunchenko, and Peter R. Schreiner*
Dedicated to Professor A. G. Yurchenko
on the occasion of his 70th birthday
Protonation of saturated hydrocarbons is the simplest and
practically most important electrophilic reaction.[1] Small
alkanes like methane, ethane, and isobutane[2] form welldefined protonated species, and their experimental proton
affinities (PAs) are excellently reproduced computationally.[3?5] However, the current data on the protonations of
strained hydrocarbons are rather controversial, and facile
rearrangements are thought to account for the difficulties
encountered in measuring the PAs.[6] Cyclopropane (strain
energy(Estr) = 27.2 kcal mol 1) is an exception because the
number of energetic downhill paths is very limited and the
barrier for the ring opening of corner-protonated C3H7+ to the
2-propyl cation is high. The experimental PA of cyclopropane
(179.4 kcal mol 1 [7]) is well reproduced computationally
(179.3 kcal mol 1 at CCSD(T)/cc-pVTZ//CCSD(T)/cc-pVDZ
for corner-protonated C3H7+).[8] In contrast to cyclopropane
which displays secondary C H bonds only, a surprisingly
small range of PAs is characteristic for a number of hydrocarbons with tertiary C H bonds. These hydrocarbons differ
considerably in their strain energies, for example, dodecahedrane (PA = 201.7 kcal mol 1,[9] Estr = 65.4 kcal mol 1 [10]), adamantane (PA = 175.7 kcal mol 1,[11] Estr = 6.3 kcal mol 1 [12]),
and cubane[13] (PA = ca 200 kcal mol 1,[7, 9] Estr = 161 kcal
mol 1 [14]). For these systems the question remains: How
much does the strain energy affect the proton affinity of a
saturated hydrocarbon and to what extent should isomer[*] Prof. A. A. Fokin, Dr. P. A. Gunchenko
Department of Organic Chemistry
Kiev Polytechnic Institute
pr. Pobedy 37, 03056 Kiev (Ukraine)
Fax: (+ 038) 44-236-97-74
E-mail: aaf@xtf.ntu-kpi.kiev.ua
Dr. B. A. Tkachenko, Prof. Dr. P. R. Schreiner
Institute of Organic Chemistry
Justus-Liebig University
Heinrich-Buff-Ring 58, 35392 Giessen (Germany)
Fax: (+ 49) 641-99-34309
E-mail: prs@org.chemie.uni-giessen.de
and
Department of Chemistry
The University of Georgia
Athens, GA 30602-2556 (USA)
Fax: (+ 1) 706-542-9454
E-mail: prs@chem.uga.edu
[**] This work was supported by the Volkswagenstiftung, the Ukrainian
Science Foundation, and the NATO Science Program. We thank
A. G. Marshall for helpful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
146
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
izations be considered? The experimental PA of cubane (1)
measured by ion-cyclotron resonance spectroscopy[9] is surprisingly low and was recently questioned by Koppel et al.[15]
on the basis that 1 rearranges rapidly to cuneane so that the
PA is attributed to the energy change of the entire protonation/rearrangement/deprotonation process. However,
cuneane is also highly strained, so where one does stop?
Are there any other rearrangement paths for protonated
cubane? Herein we address both computationally and
experimentally the question to what extent the cubane cage
is able to survive electrophilic attack[16] and what the most
favorable paths for the reaction of 1 with a proton are.
Despite enormous strain, cubane[15, 17, 18] is kinetically quite
stable because breaking just one C C bond causes only minor
structural changes and hence only little relief of strain.
However, cleavage of a second C C bond is highly exothermic and followed by rapid rearrangements. For instance,
thermolysis[16, 19] and single-electron oxidation[20] of 1 leads to
cyclooctatetraene. In contrast, mild radical reagents[21] lead to
substitution products with conservation of the cubane cage.[22]
Electrophilic cubane conversions are limited to reactions of
soft electrophiles like metal ions,[18] and a number of
fascinating rearrangements (e.g., to syn-tricyclooctadiene
(2) with Pd2+ and to cuneane (3) with Ag+ and Li+) are
preparatively useful.[18, 23]
The attack of a proton on cubane is highly exergonic and
all of our and previous[9, 15, 16, 24] attempts to locate C8H9+
protonated minima retaining the cubane cage at various
levels of density functional theory (DFT) failed. When DFT
methods are used, edge-protonated cubane (4, Scheme 1) is
not a minimum and converges to the tetracyclo[4.2.02,4.03,8]oct-5-yl cation (5)[25] upon optimization without symmetry constrains. This structure is energetically much
too low to account for the ?experimental? gas-phase basicity
of 1 (ca. 200 kcal mol 1).[7, 9] The explanation for this discrepancy was proposed[15] to lie in the rapid conversion of 1 to 3 via
5 followed by deprotonation. However, our computations[26]
at this level indicate that there is a low-energy path from 5 via
transition structure (TS) 6 to the lower-lying cation 7, which
can subsequently rearrange to the more stable bicyclo[3.3.0]octadienyl 8 via TS 9. The discrepancies between
experiment and theory are simply unacceptable and require
careful scrutiny of both the high-energy part of the C8H9+
potencial enery surface (PES) and the theoretical treatment.
First of all, the gas-phase protonation of hydrocarbons
with tertiary C H bonds results in weakly bound complexes
(R+иииH2) as shown both experimentally[2] and computationally[5] for isobutane as well as computationally for adamantane;[11] the situation for cubane is expected to be similar.
Small, strained hydrocarbons like cyclopropane[8] and cyclobutane[4] form structures protonated at a C C bond. To test
the DFT findings we used MЭller-Plesset perturbation theory
(MP2) utilizing a correlation-consistent Dunning basis set (ccpVDZ) and considered (Scheme 1) the attack of a proton on 1
at a C H bond (top), corner (middle), and edge (bottom);
MP2 is known to be very suitable for the description of highly
delocalized carbocations, even when DFT approaches fail.[27]
As the formation of the cubyl cation in the condensed phase is
well documented,[28] we also searched for the products of the
DOI: 10.1002/anie.200461042
Angew. Chem. Int. Ed. 2005, 44, 146 ?149
Angewandte
Chemie
Scheme 1. The computed paths for the protonation of cubane (1) (DH0 in kcal mol 1 at CCSD(T)/cc-pVDZ//MP2/cc-pVDZ + DZPVE) and the
structure of edge-protonated cubane 4 at MP2/cc-pVDZ and CCD/cc-pVDZ (in italics).
formal ?hydride? abstraction with H+. Similar to isobutane
and adamantane, 1 forms a weakly bound complex with H2
(10) upon protonation.[29] The absolute value of the reaction
enthalpy[30] (DH0 = 158.0 kcal mol 1) is approximately
40 kcal mol 1 higher than the ?experimental? gas-phase
basicity and proton affinity of 1; in other words, 10 cannot
be the high-energy C8H9+ species resulting from protonation.
Complex 10 is connected via TS 11 with the complex of the 2cuneyl cation with H2 (12), which is 195.0 kcal mol 1 lower in
energy than 1. Although this value is only slightly lower than
the ?experimental? PA[7, 9] of cubane, the barrier for the
rearrangement of 10 to 12 is too high (25.3 kcal mol 1) for it to
occur in a weakly bound complex such as 10.
Corner-protonated cubane 13 in not a minimum and
converges upon optimization without symmetry constrains to
14, which is connected with protonated syn-tricyclooctadiene
15 via TS 16. Cation 14 is also too stable (DDH0 = 219 kcal mol 1) to stand for the ?experimental? PA of cubane. In
contrast, edge-protonated cubane 4 is a true minimum that is
190.4 kcal mol 1 below 1 + H+ (Scheme 1) at MP2/cc-pVDZ.
Reoptimization at the coupled cluster CCD/cc-pVDZ as well
as at the decisive CCSD(T)/cc-pVDZ levels of theory also
identifies 4 as a minimum. The cubane cage opening to the
?boat? structure 17 through TS 18 is barrierless. As the ZPVE
correction is larger than the difference in electronic energies,
the TSs appear to have a lower energy than the starting
minima. This simply means that the reaction is barrierless and
Angew. Chem. Int. Ed. 2005, 44, 146 ?149
highly exothermic; 17 is connected with cation 5 via another
low-lying TS 19.
The PA of 1 previously was determined with a bracketing
experiment in which the equilibrium reaction between cubane
and protonated 2-propyl ether was examined using the FT/
ICR technique.[9] Computations show that edge-protonated
cubane 4 is not able to survive under these experimental
conditions (155 8C, time from 0.1 to 25 s),[9] as its rearrangement is barrierless. To define the most favorable path for the
proton transfer, we modeled the reaction of cubane with
protonated dimethyl ether. Because of the size of the system
we employed both DFT (B3LYP/6-311 + G**) and MP2/ccpVDZ (for selected highly symmetrical structures), which
both gave very similar results for this part of the PES
(Scheme 2). Encounter complex 20 forms mildly exergonically ( 2.1 kcal mol 1, B3LYP/6-311 + G**) and proceeds to a
complex of edge-protonated cubane with methyl ether 21 via
TS 22.[31] The barrier for the alternative corner attack (TS 23)
is 7.8 kcal mol 1 larger. The geometry of the hydrocarbon
moiety of 21 is virtually identical to that of 4.
Thus, in the FT-ICR experiment the protonation of
cubane should give cation 8 (Scheme 1 and Scheme 3).[32]
This is followed by deprotonation to give 1,8-dihydropentalene (24) after back proton transfer to the base,[33] and this
hydrocarbon probably forms in the ICR chamber. The proton
affinity of 24 computed at different levels is about 200 kcal
mol 1, which is in perfect agreement with the experimental
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
147
Communications
Scheme 4. The experimentally found rearrangement of cubane in its
reaction with CrO2Cl2.
Scheme 2. The proton transfer from protonated dimethyl ether to
cubane (DG298 in kcal mol 1, B3LYP/6-311 + G**, MP2/cc-pVDZ, in italics) and the structure of edge-protonated cubane in the complex with
dimethyl ether (21) at B3LYP/6-311 + G** and MP2/cc-pVDZ (in italics).
ized over the entire s cage. However, under the experimental
gas-phase conditions protonated cubane 4 can only survive
the early protonation stages very briefly because the barriers
for rearrangements are simply too low. Careful scrutiny of the
C8H9+ PES starting from cubane gives 1,8-dihydropentalene
(24) as the final product along the most favorable protonation/isomerization/deprotonation downhill path. We suggest
that hydrocarbon 24 is responsible for the ?experimental?
proton affinity of cubane measured previously by the ICR
technique.
Received: June 22, 2004
.
Keywords: computational chemistry и cubane и proton affinity и
rearrangements
Scheme 3. The rearrangement of cubane in the protonation/deprotonation process.
value obtained previously for the reaction of cubane with
protonated bases.[7, 9]
The key conclusion derived from Scheme 1 is that the
most favorable follow-up transformation of 4 is the rearrangement to 5 and further to the low-lying cation 7. Thus, the
reaction of cubane with electrophiles should first give a
derivative of the tetracyclic cation 7, which is stable towards
deprotonation (to the anti-Bredt olefin) and may be stabilized
in solution due to more effective solvation relative to that of
cation 8. These conclusions are fully supported by our
experimental observation that the reaction of cubane with
the electrophile CrO2Cl2 (Scheme 4) gives a single diastereomer of dichloride 25.[34] This product forms through
trapping of the exo-4-substituted tetracyclo[3.3.0.02,8.03,6]oct7-yl cationic structure with the reagent.
In summary, we find that both H+ and (CH3)2OH+ favor
an edge attack on 1 leading to C C bond protonated minima
(4 and 21, respectively) where the positive charge is delocal-
148
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[29] Formation of tight C8H7+иииH2 complexes is in agreement with
the experimental observation of an ion at m/z 103 derived from
protonated cubane [1+H+ H2] in the low-temperature chemical
ionization mass spectra.[16]
[30] We use DH0 values at CCSD(T)/cc-pVDZ//MP2/cc-pVDZ +
DZPVE because these are closely related to the experimental
proton affinities (PAs). The relative energies DG298 (Supplementary Information) differ only by 1.5 kcal mol 1.
[31] A sizable imaginary mode (150i), which still present at B3LYP/6311 + G** for 21 in C1 (even with tight convergence criteria)
show that this method still does not handle this flat part of the
PES properly; however, at MP2/6-31G* this structure is a true
minimum.
Angew. Chem. Int. Ed. 2005, 44, 146 ?149
[32] Further rearrangement of cation 8 to the more stable homotropylium cation (protonated cyclooctatetraene) is precluded by
a high barrier (ca. 30 kcal mol 1).
[33] Deprotonation of 8 may lead to semibullvalene, which, however,
is ca. 10 kcal mol 1 less stable than 24.
[34] NMR 1H (CDCl3): d = 1.75 (m, 1 H), 1.89 (m, 1 H), 2.12 (m, 1 H),
2.62 (m, 1 H), 2.87 (br s, 1 H), 3.05 (m, 1 H), 4.06 (s, 1 H), 4.13 ppm
(m, 1 H); NMR 13C (CDCl3): d = 17.4, 18.8, 33.5, 47.8, 48.0, 49.1,
63.5, 64.7 ppm; MS (m/z): 174, 139, 125, 103 (100 %), 77, 63, 51.
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