Bicyclobutonium on Zeolite Y
A R T I C L E S
temperature dependence of the 13C chemical shifts indicated the
presence of two carbocations, one of them in small amounts
but still in equilibrium with the major species. This conclusion
was supported by isotope perturbation experiments.18 The
classical cyclopropylcarbinyl cation and the nonclassical bicy-
clobutonium cation were considered as the most likely species
participating in this equilibrium.
Scheme 1. Possible Equilibrium between the Alkoxide and the
Carbocation on the Zeolite Surface
Scheme 2. Product Distribution from Solvolysis of
Cyclopropycarbinyl, Cyclobutyl, and Allylcarbinyl Chlorides
Alternatively, many theoretical methods have been employed
+
to elucidate the potential energy surface of the C4H7 in gas
phase19 and in solution.20 High-level ab initio calculations
suggest that, in the gas phase, there are three C4H7+ structures
as minima on the potential energy surface. These calculations
pointed to the bicyclobutonium and cyclopropylcarbinyl cations
as the most stable structures.
In this work, we report experimental and theoretical results
on the rearrangement and nucleophilic substitution of cyclo-
propylcarbinyl and cyclobutyl halides over NaY zeolite, aiming
inside the zeolite, which may also be related with the polarity
of the medium. We have been using cation-exchanged zeolites
as alkylating catalysts13 and to study the formation and reactivity
of the alkoxides.14 The main advantage over protonic zeolites
is that secondary side reactions, such as oligomerization, are
reduced. In these studies, the metal cation acts as a Lewis acid
site, coordinating with an alkylhalide to form a metal-halide
species and an alkoxide bonded to the zeolite structure.
However, it was neither possible to observe the carbocations
as discrete intermediates, nor to show the equilibrium between
the carbocation and the alkoxide (Scheme 1) in those studies.
The rearrangement of the cyclopropylcarbinyl chloride in
solution is well documented in the literature.15 In polar solvents,
three products, arisen from the nucleophilic substitution of the
solvent to the chloride, are usually detected. They are formed
via nucleophilic substitution of the chloride by the solvent. This
chemistry can be explained by the formation of a C4H7+ cation,
which acts as a tridentate ion, generating the three products
shown in Scheme 2.
+
at demonstrating the formation of C4H7 cations as discrete
intermediates inside the zeolite cavity. A new process of
nucleophilic substitution, which we have named halogen switch,
was encountered in these studies. This reaction supports the
hypothesis that carbocations are intermediates inside the zeolite
cavity and might be in equilibrium with alkoxides. This reaction
also gives additional information of the ability of the zeolite
cage to confine ionic species.
Experimental Section
The reactions were studied on a NaY zeolite (Si/Al ) 2.6 and surface
area of 704 m2 g-1) and on NaY impregnated with NaBr or NaCl. These
latter samples were prepared by soaking the NaY zeolite with an
aqueous solution of NaBr or NaCl, followed by water evaporation
in a rotary evaporator. The impregnation was performed to yield
about 10 wt % of chloride or bromide ions into the zeolite (in dry
base). In a typical procedure, 4.5 g (dry base) of the NaY zeolite was
treated with 75 mg of sodium chloride dissolved in 100 mL of deionized
water.
+
The 13C NMR spectrum of the C4H7 cation in superacid
solution shows a single peak for the three methylene carbon
atoms.16 This equivalence can be explained by a nonclassical
single symmetric (3-fold) structure. However, studies on the
solvolysis of labeled cyclopropylcarbinyl derivatives suggest a
degenerate equilibrium among carbocations with lower sym-
metry, instead of the 3-fold symmetrical species.17 A small
The reactions were carried out in a glass unit with a straight reactor
(fixed bed) at room temperature and atmospheric pressure. About 200
mg of the zeolite was initially pretreated for 30 min at 300 °C (2.5 °C
min-1), under N2 atmosphere (40 mL min-1). The reactor was cooled
to room temperature and 0.5 mL of the alkylhalide was injected in the
N2 flow, with the use of a syringe. The products were collected at the
reactor outlet, using a trap immersed in ice bath. The products were
separated by a gas chromatograph coupled to a mass quadrupole
spectrometer, using electron impact ionization (70 eV). For the halogen
switch, the experimental procedure was similar, except for the introduc-
tion of an equimolar mixture of an alkyl bromide and an alkyl chloride
of different structures.
(10) (a) Cozens, F. L.; Ortiz, W.; Schepp, N. P. J. Am. Chem. Soc. 1998, 120,
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A batch procedure was used for the kinetic experiments. About 500
mg of the zeolite was initially pretreated at 300 °C for 30 min in a
round-bottom flask. After cooling to room temperature a solution
containing 0.1 mmol of the alkylhalide in n-pentane, together with a
known amount of n-heptane used as internal standard, was introduced
into the flask. The kinetics, at 25 °C, were followed, withdrawing
samples of the liquid phase for analysis at specific time intervals.
To have some insight of the dispersion of the NaCl and NaBr into
the zeolite channels, we carried out a preliminary analysis using the
soft X-ray Spectroscopy (SXS) beam line at the National Laboratory
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