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Distinguished Professor, Physical Chemistry
Ph.D. 1969, Texas A&M University
Email: cdsteve@ilstu.edu
Phone: (309)438-7300
Office: 219 Science Laboratory Building
Picture of the Group
Left to Right: Stevenson, Gard, Kiesewetter, Reiter
[8]annulyne
Benzyne, or [6]annulyne, is an intermediate replete throughout
benzene chemistry, but this belies the fact that benzyne was the only known
annulyne. When left to itself it undergoes a [2+2] dimerization to form biphenylene,
or bi-[6]annulenylene. There is ostensibly no reason why the
chemistry of benzene cannot be paralleled in the larger annulenes, including [8]annulene.
Conceivably, the dehydrohalogenation of mono-bromocyclooctatetraene should lead to the
formation of [8]annulyne, which could be trapped as its anion radical or dimerized
to form bi-[8]annulenylene. It is under these hypotheses that we sought the second annulyne.
Indeed, it is possible to trap [8]annulyne as its anion
radical (in THF) via reduction with alkali metal. Interestingly, when the metal of reduction is
of greater ionic diameter than is the length of the triple bond in the
annulyne, the metal (those with an ionic diameter larger than Na) becomes bound
into the pz orbitals of the triple bond resulting in an unusually
large EPR metal splitting. The presence
of 18-crown-6 does not prevent ion association, and two anionic species are
observed in equilibrium. If, instead of
immediately reducing the annulyne to trap it as its anion radical, it is
allowed to react with itself, it undergoes the reaction analogous to the
dimerization of benzyne and forms bi-[8]annulenylene. By virtue of the fact that more than one
annulyne could now be formed, it was possible to trap, as its anion radical,
the asymmetric annulenylene, [6]-[8]annulenylene,
Scheme 1. Because each
annulenylene features a central cyclobutadiene moiety, the neutral molecules
could not be isolated. The
above reactions serve to demonstrate that benzyne is not unique in its
chemistry, and that, perhaps, these reactions apply to the larger annulynes as
well.
Scheme 1
When benzyne is allowed to react with itself at even
higher concentrations, it undergoes a pseudo-trimerization to form
triphenylene. This particular molecule
has proven to be quite useful, and of considerable theoretical interest, but it
was the only known member of the triannulenylenes since its discovery in 1867. Again, the eight-membered ring analog proved
possible to generate via the dehydrohalogenation of mono-bromocyclooctatetraene
in higher concentrations than those in the dimerization reaction. The anion radical of tri-[8]annulenylene
features the odd electron localized on one COT moiety with the other two COT
rings folded out of the plane of the benzene ring in opposite directions. The stability of the central benzene ring
allows for the isolation of neutral tri-[8]annulenylene;
NMR and DFT studies of which indicate that two COT moieties are displaced above
and one below the plane of the benzene ring, reaction 1 and Figure 1. Tri-annulenylene
represents only the second of a host of possible members in the triannulenylene
family, 165 in all (considering asymmetrical members using annulenes up to [22]annulene).
Figure 1. 1H NMR spectrum (400 MHz) of the reoxidized solution that exhibited
the EPR spectrum shown in Figure 1. The simulation (lower) was
generated using the chemical shifts and the J couplings shown.
Coming off of our success with tri-[8]annulenylene
(which received an Editor’s Choice
in Science), we were interested in continuing the analogy of [8]annulyne
and benzyne with the possibility of making the eight-membered analog of parent
cycloproparene. We were ultimately able
to isolate the neutral molecule, Scheme 2, which proved to possess many
interesting properties. The cyclopropyl
group caused the COT moiety to assume a nearly-planar conformation, resulting
in an observed paratropic ring current on the NMR spectrum of the
compound. Whimsically, cycloprop-[8]annulene has a sweet olefinic
odor unlike the oft-cited extreme malodor of cycloprop-[6]annulene. Much like tri-[8]annulenylene,
cycloprop-[8]annulene has opened the door to the other cyclopropannulenes, some
of which may prove to possess interesting NMR chemical shifts and, perhaps, a
pleasant bouquet as well. We are
currently attempting to continue to story of [8]annulene
by generating [8]annuldiyne from di-bromocyclooctatetraene.
Scheme 2
Currently,
we have working on a collaborative project with Raj Rathore (Marquette
University). Our lab carried out anion
radical studies on some polyfluorenes that he synthesized and with which he has
been involved for some time. Firstly,
the anion radical spectrum of the co-facially p-stacked
tetra-fluorene, F-4, demonstrated rapid electron exchange between its two
central fluorene moieties immediately following its one reduction. This kinetically controlled product
eventually gives way to the thermodynamically controlled product, where the odd
electron presides exclusively on an external fluorine moiety, reaction 2. Secondly, di-fluorene
(F-2), when reduced to its diamagnetic dianion,
demonstrated s-bond activation resulting in proton exchange between
its four ortho positions. A similar effect was observed in F-3 and F-4 and
was confirmed via deuteriation studies.
[12]annulene
No
class of compounds has contributed more to our understanding of conjugation and
the concept of aromaticity than have the annulenes. Yet, of all the small annulenes
(n < 20) there is only one, [12]annulene, that
remains controversial and understudied.
Having been deemed nearly unobtainable, our poor understanding of [12]annulene was not born from a lack of interest.
The origin of the controversy and elusiveness
lies in the electron count (4n p-electrons), which acts
synergistically with the bond angle strain and steric
interactions involving the internal protons.
These factors render [12]annulene very
thermodynamically unstable. However, the
addition of an extra electron, creating a 4n+1
p-electron system, can adequately stabilize [12]annulene
to render it spectroscopically observable.
Some Abstracts from some current and recent projects:
Low temperature (-100°C)
dehydrohalogenation of
1,2,5,6,9,10-hexabromocyclododecane (a common fire retardant) with potassium tert-butoxide in THF followed by one-electron
reduction yields the anion radical of the di-trans
form of [12]annulene. This system yields a well-resolved EPR signal that
reveals that most of the spin density resides on one side (the planar side) of
the anion radical. Five of the carbons in this [12]annulene
system are twisted from the plane of the remaining seven carbons, and the rate
of rearrangement between the degenerate conformations is on the EPR time scale
(k = 106-107 s-1). Warming of the
solution results in the formation of a s-bond
between the two internal carbons, loss of molecular hydrogen, and consequent
generation of the anion radical of heptalene.
The room temperature
potassium reduction of 1,2,3-triscyclooctatetraeneoxypropane, in
hexamethylphosphoramide (HMPA), yields an anion radical,
which disproportionates so strongly to the dianion diradical that the anion
radical cannot be observed via EPR. The dianion
diradical has one unpaired electron in a primary and one in
a secondary ring system, and it can be readily reduced to the corresponding trianion triradical. An analogous
reduction of 1,2,3,4-tetrakiscyclooctatetraeneoxybutane
does produce an observable anion radical, but it also is readily reduced to the
system corresponding to one electron per eight-membered
ring (the tetraanion tetraradical).
These results and those obtained from systems containing two cyclooctatetraene (COT) moieties are explained in terms of
the geometry changes COT undergoes upon one-electron reduction, the
interactions between reduced and adjacent unreduced ring systems, and the
electron- electron repulsion present in the polyanion
polyradicals.
Stevenson, C. D.; Reiter, R. C., Szczepura, L. F.;
Peters, S. J.
"Polycyclooctatetraeneoxy Alkane Polyanionic Polyradicals" J. Am. Chem. Soc. ASAP
The one-electron reduction of tertiary N,N'-dimethyl-N,N'-diarylureas,
in HMPA, results in anion radicals that undergo novel intramolecular reductive
elimination reactions leading to the formation of the anion radicals of the
corresponding biaryls. These results are due to face
to face p-
p
stacking interactions involving the two aromatic rings in the urea systems. The
overlapping pp
orbitals on the ipso carbons of opposing aryl groups evolve into a s-bond leading to the formation of the biaryl
anion radical. In the case of the N,N'-dimethyl-N,N'-di-2-pyrenylurea
system, there is a node in the LUMO of the number 2 carbon, and the parent
anion radical remains intact.
Kurth, T. L.; Lewis,
F. D.; Hattan, C. M.; Reiter, R. C.; Stevenson, C. D. "N,N'-Dimethyl-N,N'-diarylurea
Anion Radicals: An Intramolecular Reductive Elimination" J. Am. Chem.
Soc. 2003, 125, 1460.
A series of N-methylated polyarylurea
oligomers have been reduced with potassium metal in
HMPA. These reductions result in the transient formation of
arylurea anion radicals, which undergo reductive
elimination of the urea linkages. The aryl moieties appear in the products as
the anion radicals of oligoaryl systems. The reaction
is intramolecular, and the sequencing in the polyaryl
anion radical remains the same as in the polyarylureas
due to the urea-enforced pi-pi stacking
interactions.
Lewis, F. D.;
Kurth, T. L.; Hattan, C. M.; Reiter, R. C.; Stevenson, C. D. "Polyaryl Anion Radicals via Alkali Metal
Reduction of Arylurea Oligomers" J. Am. Chem. Soc. 2003, 125, 1460.
Matt Gard doing some science
Having
just made an apparatus, Matt Kiesewetter thought it looked like a gun.
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SELECTED PUBLICATIONS
Stevenson, C. D.; Reiter, R. C., Szczepura, L. F.; Peters, S. J. "Polycyclooctatetraeneoxy Alkane Polyanionic Polyradicals,"
Journal of the American Chemical Society, 2005, 127, 421.
Kiesewetter, M.K.; Reiter, R.C.; Stevenson, C.D. "The Second Triannulenylene: Tri-[8]annulenylene," Journal of the American Chemical Society 2004, 126, 8884.
Featured as an Editor’s Choice in Science: "Beyond Triphenylene," Science, 2004, 305, 311.
Gard, M. N.; Reiter, R. C.; Stevenson, C. D. "Anion Radicals of Di-trans-[12]annulene and Heptalene in a One-Pot Synthesis from a Common Fire Retardant," Organic Letters, 2004, 6(3), 393.
Stevenson, C.D.; Kiesewetter, M.K.; Peters, S.J. "Transient [8]Annulenyl Carbanion from the Anion Radical of Bromo-[8]annulene," Journal of Physical Chemistry A, 2004, 108, 2278.
Lewis, F. D.; Kurth, T. L.; Hattan, C. M.; Reiter, R. C.; Stevenson, C. D. "Polyaryl Anion Radicals via Alkali Metal Reduction of Arylurea Oligomers," Organic Letters 2004, 6, 1605-1608.
Peters, S.J.; Turk, M.R.; Kiesewetter, M.K.; Reiter, R.C.; Stevenson, C.D. "The Cyclooctatriene-η2-ynyl Potassium Zwitterionic Radical: Evidence for a Potassium Organometallic," Journal of the American Chemical Society, 2003, 125, 11212.
Peters, S.J.; Turk, M.R.; Kiesewetter, M.K.; Stevenson, C.D. "Single-Electron Entrapment of [8]Annulyne, Biannulenylenes, and an Annulenoannulene," Journal of the American Chemical Society, 2003, 125, 11264.
Kurth, T. L.; Lewis, F. D.; Hattan, C. M.; Reiter, R. C.; Stevenson, C. D. "N,N'-Dimethyl-N,N'-diarylurea Anion Radicals: An Intramolecular Reductive Elimination," Journal of the American Chemical Society 2003, 125, 1460.
Stevenson, C. D.; Gard, M. N.; Reiter, R. C. "Spin Densities in Dialkoxy-[16]annulene Anion Radicals: Dimerization of Alkoxy-[8]annulenes," Journal of Organic Chemistry 2003, 68(4), 1464
Peters, S. J.; Reiter, R. C.; Stevenson, C. D. "Interannular Communication in the Radical Anions of Bis-cyclooctatetraene Systems," Organic Letters, 2003, 5(6), 937.
C.D. Stevenson, L. Heinle, and R.C. Reiter, "Tunneling in a Substituted
[8]Annulene," Journal of the American Chemical Society, 2002, 124, 2704.
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