N-heterocyclic superelectrophiles and evidence for single electron transfer chemistry

Article abstract of DOI:10.1021/ja8062953

In reactions with benzene and related substrates, an oxazole-based superelectrophile is found to be significantly more reactive than a related monocationic species. Theoretical calculations estimate that the lowest unoccupied molecular orbital (LUMO) for the superelectrophile is about 4 eV lower in energy than the LUMOs of comparable monocations. When the oxazole-based superelectrophile is reacted with ferrocene, a dimeric product is formed in high yield. The dimerization occurs by a single electron transfer reaction between the dicationic superelectrophile and ferrocene, followed by coupling of the radical cations. Copyright

Full text of DOI:10.1021/ja8062953

Published on Web 10/08/2008
N-Heterocyclic Superelectrophiles and Evidence for Single Electron Transfer
Chemistry
Kiran Kumar Solingapuram Sai, Matthew J. Tokarz, Andrew P. Malunchuk, Chong Zheng,
Thomas M. Gilbert, and Douglas A. Klumpp*
Department of Chemistry and Biochemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115
Received August 18, 2008; E-mail: dklumpp@niu.edu
Since the pioneering work of Olah in the 1970s, superelec-
trophilic chemistry has been an active area of research.1
Superelectrophiles have been the basis for many unusual
synthetic transformations, for example leading to functionalized
hydrocarbons,2 heterocycles,3 and even polymers.4 Among the
many superelectrophiles described in the literature, the proto-
+
nitronium ion (HNO22+) is typical.5 When nitronium ion (NO2
)
salts are dissolved in excess Lewis acids or Brønsted superacids,
the resulting species exhibit greatly enhanced electrophilic
reactivities. In the case of Brønsted acids, this is considered to
be the result of protosolvation of the nitronium ion (partial or
complete proton transfer to the oxygen). As a result of
protonation, the ion develops an increasing positive charge, and
in the limiting case it may form the dicationic protonitronium
ion. The superelectrophilic nitronium ion is capable of reacting
with exceptionally weak nucleophiles, such as alkanes and
strongly deactivated arenes.5 Besides synthetic studies, the
protonitronium ion has been studied computationally,6a and in
accord with calculations, it has been observed as a kinetically
stable species in gas-phase experiments.6
When the oxazole derivative 1 is reacted with ferrocene in
CF3SO3H, the product from electrophilic aromatic substitution
is not obtained, but rather a dimerization product (11) is
generated in good yield (eq 2). Gas chromatography-mass
spectral analysis indicates that two isomeric dimers are produced
(ca. 20:1 ratio) in the reaction. The minor component of the
product mixture could not be isolated, but the major component
was isolated and a single crystal X-ray diffraction structure was
determined (see Supporting Information). Compound 11 crystal-
lizes as a mixture of the two enantiomers. Decamethyl ferrocene
also reacts with 1 in superacid to give a high yield of product
11.
Computational studies have shown superelectrophiles to
possess low-lying LUMOs, high electronegativities, and delo-
calized positive charges.7 These properties contribute to their
high electrophilic reactivities. Interestingly, these same structural
and electronic properties could also lead to efficient single
electron transfer (SET) reaction pathways with suitable combina-
tions of electron donors.8 Although the possibility of SET
chemistry with superelectrophiles has been suggested previ-
ously,9 to the best of our knowledge no evidence has been
reported to date confirming these types of reactions. In the
following communication, we describe our studies of the
chemistry of N-heterocyclic superelectrophiles and report ex-
amples of SET reactions involving these superelectrophiles.
In work related to the synthesis of triarylmethane products,
our studies began with reactions of 9,10-dihydro-2-methyl-4H-
benzo[5,6]cyclohept[1,2-d]oxazol-4-ol (1) and arenes in the
Brønsted superacid CF3SO3H (triflic acid, TfOH, Ho ) -14.1).10
For example, compound 1 reacts with C6H6 in CF3SO3H to give
product 3 in high yield (eq 1). Ionization of 1 leads to formation
of the superelectrophile 2, which is capable of reacting with
benzene. Other arenes also reacted and gave products 4-7,
including those from moderately deactivated arenes such as 1,3-
dichlorobenzene. Intramolecular cyclizations were also ac-
complished, for example involving a phenylethyl derivative to
produce 8. In contrast to the high reactivity of the superelec-
trophile (2), the analogous monocationic species (9; generated
from dibenzosuberol in CF3SO3H) does not react with benzene
or 1,3-dichlorobenzene.
We propose that dimer 11 is the result of a SET reaction
between the superelectrophilic dication (2) and ferrocene, a good
single electron donor (eq 2).11 The SET reaction produces the
radical cation 10, which then dimerizes to give 11 as a
racemate.12 The strong preference for the racemate over the meso
product suggests that the dimerization step occurs by stacking
the cationic ring over the neutral ring (12). Similar dimerization
products (13,14) were formed in reactions of a thiazole-based
alcohol and a pyridine-based alcohol. In products 13 and 14,
the stereochemistry of the bridging carbons has not yet been
determined. The dimer 11 is not formed in reactions of 1 with
other arenes in CF3SO3H (i.e., benzene or 1,2-dimethoxyben-
zene), nor is it formed in the absence of ferrocene. This suggests
that 2-electron reactions are occurring with these arenes, by a
conventional Friedel-Crafts mechanism. If the SET reaction
9
14388 J. AM. CHEM. SOC. 2008, 130, 14388–14389
10.1021/ja8062953 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
made to the Department of Education (Grant P116Z020095) and
the State of Illinois for support of our computational facilities. We
also thank Mr. Patrick Kindelin for preliminary studies.
Supporting Information Available: Complete ref 13; NMR spectra
of new compounds, representative experimental procedures, crystal-
lographic data, computational data. This material is available free of
charge via the Internet at http://pubs.acs.org.
does take place, then it requires the rapid collapse of the two
radical cations (i.e., 10 and the radical cation of benzene).
The chemistry of the oxazole-based superelectrophile (2) was
also examined by calculations.13 Energies of LUMOs and HOMOs
of dication 2 and two analogous monocations 9 and 15 were
estimated at the HF/6-31G(d) level (Figure 1). As expected, oxazole-
based superelectrophile 2 is characterized by a low-lying LUMO,
one significantly lower than the LUMOs of the monocations. From
calculations at the B3LYP/6-311G(d,p) level, gas-phase single
electron reduction energies (∆ERED) of the ions were estimated by
comparing their ZPE-corrected DFT model energies with those of
the respective radical or radical cation reduction products (Figure
1). All three exhibit strongly exothermic ∆ERED values; however,
reduction of superelectrophile 2 releases 90 kcal/mol more energy
than for either monocation. This large energy of one-electron
reduction suggests a certain measure of stability for the product
radical cation (10), as back electron transfer (from 10 to the
ferrocenium cation) should be highly unfavorable.
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Figure 1. Calculated EHOMO and ELUMO levels (eV; HF/6-31G(d) level)
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Acknowledgment. Grateful acknowledgement is made to the
donors of the American Chemical Society Petroleum Research Fund
(PRF No. 44697-AC1) and the National Science Foundation (Grant
CHE-0749907) for support of this work; acknowledgement is also
JA8062953
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