The isomers of [12]annulyne and their reactive relationships to heptalene and biphenyl

Article abstract of DOI:10.1002/anie.200803863

(Chemical Equation Presented) [12]Annulyne not like benzyne: The base-initiated condensation of hexadiyne in nonpolar solvents leads directly to the symmetrical isomers of [12]annulyne, i.e. the all cis isomer, which exists as its cumulene, and the 6,9-tr

Full text of DOI:10.1002/anie.200803863

Communications
DOI: 10.1002/anie.200803863
Antiaromatic Rings
The Isomers of [12]Annulyne and their Reactive Relationships to
Heptalene and Biphenyl**
Brad D. Rose, Richard C. Reiter, and Cheryl D. Stevenson*
The colorful and interesting chemistry of the annulenes
represents a still living vestige (“Renaissance of Annulene
Chemistry”)[1] of the days of the intense investigation of small
hydrocarbon ring systems.[2] The smallest possible “antiaro-
matic” dehydrogenated annulene, [8]annulyne,[3] can be
trapped as its radical anion, thus avoiding destabilization
due to the p-electron count rule, and can be studied by EPR
spectroscopy.[3] The situation turns out to be much more
complex and intriguing in the case of the [12]annulyne system.
[12]Annulyne, which can be thought of as the double ring
expanded o-benzyne,[4] is the smallest “antiaromatic” annu-
lyne (4n p-electrons) capable of isomerization and having
internal protons.The [12]annulynes are also subject to
destabilization from the p-electron count rule, but upon
one-electron reduction they undergo, much to our amaze-
ment, rearrangement to form either the radical anion of
biphenyl or of heptalene, depending upon the particular
isomer being reduced.
1,5-Hexadiyne was exposed to a mixture of potassium tert-
butoxide and [18]crown-6 in an evacuated sealed apparatus
using the same procedure that produced 1 and 2,[5] except that
the [D8]THF solvent system was replaced with perdeuterated
benzene.Cooling an attached NMR sample tube causes the
C6D6 and any volatile products to cleanly distill into the tube,
which can subsequently be sealed from the apparatus.The
now clear, colorless, freshly distilled C6D6 solution exhibits a
1H NMR signal revealing two new C12H10 isomers: 3 (6,9-di-
int-di-trans-[12]annulyne) and 4 (a cumulene structure with
four additional cis double bonds) (Figure 1).The “impurity”
signal at d = 5.6 ppm is greatly diminished as is the ratio [3]/[4]
when the solution is distilled through a column warmed to
1508C.These products are clearly kinetically controlled and
ion association, in the intermediates, plays a role.
It was recently reported that the facile base condensation
of 1,5-hexadiyne with potassium tert-butoxide in tetrahydro-
furan (THF) leads directly to the formation of two isomers of
[12]annulyne:[5] 5,10-di-int-di-trans-[12]annulyne (1) and 4,11-
di-int-di-trans-[12]annulyne[6] (2) in a 1:1 ratio (Scheme 1).
Figure 1. Lower: 1H NMR spectrum (258C, 400 MHz) obtained from
KOC(CH3)3 initiated condensation of hexadiyne in C6D6. Upper: Com-
puter simulated spectra of the major species (3) and minor species
(4) generated using the J splittings (arrows; in Hz) and chemical shifts
(in ppm). The ratio [3]/[4] is 5. As the temperature is elevated up to
858C the only observable change in the spectrum is a slight increase
in the ratio [3]/[4] and some decomposition. The arrows in the
spectrum point out two small impurity resonances. Replacing C6D6
with [D12]cyclohexane gives the same spectrum.
Scheme 1. Base condensation of 1,5-hexadiyne in THF.
Since the products from the base condensation of
hexadiyne vary with ion association conditions,[7] we thought
that we might be able to establish the conditions whereby the
remaining possible symmetrical isomers of [12]annulyne (6,9-
di-int-di-trans-[12]annulyne and the all cis-[12]annulyne)
would be revealed.
The internal protons of 3 appear at d = 6.99 ppm. This is
close to the conventional aromatic region for benzenoid
systems, which suggests some paratropicity.Downfield shifts
were also observed for the internal protons of 1 and 2 at d =
6.87 and 6.63 ppm, respectively. The paratropic shifts are not
[*] B. D. Rose, Prof. Dr. R. C. Reiter, Prof. Dr. C. D. Stevenson
Department of Chemistry, Illinois State University
Normal, IL 61790-4160 (USA)
Fax: (+1)309-438-5538
E-mail: cdsteve@ilstu.edu
À
as prominent in 4, because the C C bond angle strain
prevents the approach to planarity.
[**] We thank the National Science Foundation and the Petroleum
Research Fund administered by the American Chemical Society for
support.
The requirement that the larger three bond J couplings
apply across a true double bond demands that compound 4 be
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in an all cis cumulene KekulØ structure (1,2,3,5,7,9,11-cyclo-
dodecaheptaene).Such a structure, that fits well to the NMR
data, was indeed found using the B3LYP/6-31G* protocol.
To see if the compound with the highest solution electron
affinity (presumably 3) could be trapped as its corresponding
radical anion and observed by EPR spectroscopy (as was
immediately (within 1 min) placed into the cavity of the X-
band spectrometer.Indeed, a radical anion intermediate was
observed in the presence of biphenylCÀ, but it is (surprisingly)
not that of 3.Further, it could be clearly seen that the
concentration of this new radical anion (5CÀ) is decreasing
during the course of the scan, while that of the biphenyl
radical anion is growing at its expense.
[8]annulyne),
dehydrated
hexamethylphosphoramide
(HMPA)[8a,b] was distilled into the C6D6 solution.Subse-
quently, the 50:50 HMPA/C6D6 solution was briefly exposed
to a potassium metal mirror yielding a paramagnetic solution.
In summary, treatment of 1,5-hexadiyne with KOC(CH3)3 in
benzene followed by the addition of HMPA and reduction
with potassium metal leads to an radical anion solution
exhibiting a very strong, well resolved EPR signal due to the
radical anion of biphenyl (Figure 2, upper panel).When the
same experiment is carried out using [18]crown-6 in place of
HMPA the biphenyl radical anion is not formed, but that of
heptalene is (Figure 2, lower panel).
Complete analysis of this new spectrum reveals that it is
due to a system exhibiting ten individual electron–proton
hyperfine interactions, indicating a C12H10 with C1 symmetry.
The validity of our measured aH values (Figure 3) is illustrated
in an expanded view of this spectrum (see inset of Figure3).
Figure 3. Upper: X-band EPR spectrum of the radical anions measured
immediately after the one-electron reduction of the isomers of
[12]annulyne that yielded the NMR spectrum shown in Figure 1. The
simulation (lower panel) was generated using a 1:7 mixture of 5CÀ to
biphenylCÀ. Note that the concentration of the [12]annulyne anion
radical is decreasing relative to that of the biphenyl during the EPR
scan. Anion radical 5CÀ is clearly asymmetric, as ten different electron–
proton couplings (each from a single proton) were used in the
simulation: 0.98, 2.245, 4.085, 4.145, 5.37, 5.49, 5.95, 7.02, 7.04, and
7.06 G with a line width of 0.17 G. The first 16 G of the EPR spectrum
and the computer generated simulation are shown in the upper and
lower inset spectra, respectively.
Figure 2. Upper: X-band EPR spectrum of the radical anion measured
about 1 h after the one-electron reduction of the isomers of [12]annu-
lyne that were produced by the KOC(CH3)3 initiated condensation of
hexadiyne in benzene. This spectrum is due to the radical anion of
biphenyl. Lower: X-band EPR spectrum of the radical anion measured
after one-electron reduction of the isomers of [12]annulyne that were
produced by KOC(CH3)3 initiated condensation of hexadiyne in THF.
This spectrum is due to the radical anion of heptalene.
We found a conformer of C12H10 that conforms to this set of
EPR coupling constants (spin densities), which can be easily
formed by the bond rotations illustrated in Scheme 2.The
B3LYP/6-31+G* predicted carbon pz spin densities for 5CÀ
(shown in italics in Scheme 2) are in good agreement[7c] with
the experimental spin densities (upright numbers in
Scheme 2) obtained from the aH values (Figure 3) and the
McConnell relationship (aH = Q1 with Q = 55 G).[3,7,10] The
unsymmetric form of 5CÀ is predicted (B3LYP/6-31+G*) to be
nearly planar because of the attenuated conflict between the
internal protons.This fully conjugated system is the kineti-
cally controlled reduction product, and it can be formed by
the facile bond rotations indicated by arrows in Scheme 2.
It was strongly anticipated that the one-electron reduction
of the 3+4 mixture would initially lead to the formation of 3CÀ,
as 3 is much more planar than is 4.However, once 3CÀ is
formed electron transfer to 4 would lead to its radical anion,
which could immediately rearrange to the radical anion of
biphenyl.B3LYP/6-31 + G* analysis of 4CÀ reveals non-
classical p-py–p-pz interactions which can readily evolve into
s bonds.Analogously, biaryl radical anions have been
produced by one-electron reduction of tetraphenylmethane,
triphenylamine, triphenylboron, triphenyl phosphine oxide,
phenyl ethers, diphenylsilanes, trinapthyl borane, biarylureas,
etc.[9]
À
À
À
*
Electron transfer from 5C (5C +4)À5+4C ) produces a finite
concentration of 4CÀ.
In an attempt to actually observe the radical anion of 3
prior to biphenylCÀ formation, the reduction was carried out in
the proximity of the EPR spectrometer with the sample
The Q value and spectral width (Figure 3) is unusually
large suggesting an unusually efficient transfer of spin from
the p to the s system.[10] The proposed structure for 5CÀ is
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and yields a strong well-resolved EPR signal due to, not
biphenylCÀ, but the radical anion of heptalene (Figure 2, lower
panel).Further, no intermediate system could be observed,
even when EPR analysis was carried out immediately.Also,
when a THF containing [18]crown-6 solution of 3 and 4 is
briefly exposed to a potassium metal mirror heptaleneCÀ is
again formed.Keep in mind that HMPA effectively prevents
ion association, and [18]crown-6 simply forms a host–guest
complex with the cation leaving the crown–cation complex
ion associated with the radical anion.[8c] In the presence of ion
association 4CÀ, which can rearrange to biphenylCÀ, is not
formed.Ion association inhibits electron transfer to 4 and
promotes the formation of heptaleneCÀ.
Scheme 2. Reduction of 6,9-di-int-di-trans-[12]annulyne leads to the
formation of the 5,9-di-int-di-trans-[12]annulyne anion radical, which
can accommodate a much more planar morphology.
In solution, intramolecular reactions of [6]annulyne are
practically non-existent,[4c] while those of its twelve carbon
counterpart are vast.Based upon solvent choice, two different
pairs of isomers come from the base condensation of
hexadiyne, and, upon reduction (after HMPA addition), the
distinct isomer pairs rearrange very differently: one to
biphenylCÀ and one to heptaleneCÀ (Scheme 3).On the other
supportive of such an interaction, because C(1) is only 2.42 Š
from the nearest internal proton.The p y–1s overlap increases
the efficiency of spin transfer from the p to the s system.
Although DFT-predicted s orbital spin densities are not
adequate for accurate coupling constant predictions, a
comparison of the sum of all of the H atom 1s spin densities
over a homologous series should lead to a general under-
standing of the efficiency of the p to s spin transfer.A survey
of the possible isomers of [12]annulyne reveals one with an
extraordinarily large set of negative s orbital spin densities.
The predicted total hydrogen spin density in 5CÀ is À0.0451
which is, indeed, an unusually large negative number.The
same calculation yields predicted total hydrogen spin den-
sities for 1, 2, and 3 that are all close to À0.022. In total there
are, in principle, five possible unsymmetrical (needed to
obtain ten non-equivalent electron–proton couplings) struc-
tures with one internal proton and 40 with two internal
protons.Not all are stable, and only one ( 5CÀ) is predicted to
have the spin density profile supporting the proposed
structure for the observed radical anion.
Thus, it appears that one-electron reduction of the
mixture in HMPA produces an anion system that has access
(through electron transfer) to the all-cis cumulene conforma-
tion, 4CÀ (Figure 4).Once the radical anion in this conforma-
tion is formed, the system undergoes immediate transforma-
tion to the radical anion of biphenyl.
In stark contrast, when a THF/HMPA or THF containing
[18]crown-6 solution of 1 and 2 is briefly exposed to a
potassium metal mirror, the solution becomes dark purple
Scheme 3. Base condensation of hexadiyne and reduction of the
products leads to different radical anion systems, depending on the
solvent system.
hand, both pairs are reduced to form heptaleneCÀ when ion
association is present.Perhaps the intermolecular chemistry
of [12]annulyne will prove to be as practicable and interesting
as that of [6]annulyne.Now that these four [12]annulynes are
readily available, this can be investigated.We are still in the
process of unraveling the mechanistic details of the strong
solvent effect, but to borrow a recent quote from Whitesides:
“It is a poor decision to regard chemistry as the conversion of
reactants to products rather than the conversion of reactants
and solvent to products.”[11] This is certainly true in the case of
the [12]annulyne system and probably in many other, yet to
be observed, annulyne systems.The small resonances
between d = 5.6 and 5.7 ppm in Figure 1 are apparently due
to heptalene; this is also a topic of current investigation.
Figure 4. B3LYP/6-31+G* computed structures of 4CÀ. In the radical
anion of the all-cis-[12]cumannulene the p orbitals in the 2/9 and 3/8
positions could evolve into s bonds while the central double bond in
the cumulene moiety reduces to a single s bond. The predicted
distances C(2)–C(9) and C(3)–C(8) are both 3.08 Š. Actually the C(2)–
C(8) and C(3)–C(9) distances are only 3.05 Š, but they are not
arranged in mutual configurations for proper orbital overlap. Keep in
mind that the transformation need not be concerted.
Experimental Section
Preparation of NMR samples: A break tube with fragile ends
containing 120 mL of 1,5-hexadiyne (50% in pentanes) was added to
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tube A in Figure 5.Next 112 mg of potassium tert-butoxide was also
added to tube A.Then, a vacuum was applied to the system, and
approximately 1 mL of perdeuterated benzene or perdeuterated
Figure 6. Apparatus used for the generation of the [12]annulynes and
their subsequent one-electron reduction. When THFserves as the
primary solvent, the radical anion observed upon EPR analysis is that
of heptalene. However, when benzene serves as the primary solvent,
EPR analysis reveals only the radical anion of biphenyl (see Figure 2).
with the reduced mixture and submitted to X-band EPR analysis.
When the spectrum is recorded immediately, that shown in Figure 3 is
obtained.However, if there is a delay, only the EPR spectrum of the
biphenyl radical anion can be observed (see Figure 2, upper panel).If
THF instead of benzene is distilled into tube A, the only EPR
spectrum that can be observed is that due to the radical anion of
heptalene (lower spectrum in Figure 2).
Figure 5. Apparatus used for the generation of the [12]annulynes.
cyclohexane, which was stored in an evacuated bulb containing
“NaK2”, was distilled onto the potassium tert-butoxide.The apparatus
was then sealed from the vacuum system at B, while the benzene
solution was kept frozen in liquid nitrogen.The apparatus was
brought to ambient temperature and vigorously shaken so as to break
the tube containing the 1,5-hexadiyne.After a period of about 1 h, the
U tube was immersed in a hot oil bath (1508C), and the attached
NMR tube was immersed in ice water.This caused the solvent and
reaction mixture to cleanly distill into the NMR tube, which could
then be sealed from the apparatus and submitted to NMR analysis
(see Figure 1).Some heptalene appears to be formed during the
reaction.Neutral heptalene is unstable at elevated temperatures, and
the warm U tube diminishes the resonance from the (what we believe
to be) heptalene.
Preparation of EPR samples: A glass tube with fragile ends
containing 120 mL (0.39 mmol) of 1,5-hexadiyne (50% in pentanes)
was carefully placed into tube A (Figure 6) along with 50 mg
(0.45 mmol) of potassium tert-butoxide.Then approximately 2 g of
potassium metal was added to test tube D, and the apparatus was
sealed at point E.The apparatus was then evacuated, and the
potassium metal was distilled to create a mirror in tube C.Tube D was
sealed from the apparatus at point I.Approximately 1 mL of benzene
(C6D6) was then distilled from an evacuated bulb containing “NaK2”
into tube A, and the apparatus was sealed from the vacuum system at
point F.The apparatus was then vigorously shaken, at ambient
temperature, to rupture the tube containing the 1,5-hexadiyne.This
resulted in a thick tan colored solution, which was distilled directly
into cooled tube B.Tube A was then sealed from the apparatus at
point G.The apparatus was then reconnected to the vacuum system
and the break seal was ruptured.This allowed the distillation of
approximately 1 mL of dehydrated hexamethylphosphoramide
(HMPA) into tube B.The apparatus was then sealed from the
vacuum system at point H.The apparatus was then tilted to allow the
solution to come into contact with the potassium mirror resulting in
the reduction of the [12]annulynes.The EPR tube could then be filled
Received: August 5, 2008
Published online: October 2, 2008
Keywords: annulene · EPR spectroscopy · heptalene ·
.
NMR spectroscopy · radical anions
[1] E.L.Spitler, C.A.Johnson II, M.M.Haley,
Chem. Rev. 2006,
106, 5344 – 5386.
[2] For a then popular example see: M.P.Cava, M.J.Mitchell,
Cyclobutadiene and Related Compounds, Academic Press, New
York, 1967.
[3] S.J.Peters, M.R.Turk, M.K.Kiesewetter, R.C.Reiter, C.D.
Stevenson, J. Am. Chem. Soc. 2003, 125, 11264 – 11268.
[4] a) H.Heaney, Chem. Rev. 1962, 62, 81 – 97; b) S.L.Buchwald,
R.B. Nielsen, Chem. Rev. 1988, 88, 1047 – 1058; c) in the gas
phase there is good evidence for the formation of cyclopenta-
dienylidene carbene from benzyne, see: R.J. Brown, R.F.C.
Brown, F.W.Eastwood, M.E.Romyn, Aust. J. Chem. 1979, 32,
1767 – 1774.
[5] M.N.Gard, M.K.Kiesewetter, R.C.Reiter, C.D.Stevenson,
J.
Am. Chem. Soc. 2005, 127, 16143 – 16150.
[6] The numerals followed by “int” describe the location of the
internal protons without ambiguity.
[7] a) M.K.Kiesewetter, M.N.Gard, R.C.Reiter, C.D.Stevenson,
J. Am. Chem. Soc. 2006, 128, 15618 – 15624; b) C.D.Stevenson,
T.L.Kurth, J. Am. Chem. Soc. 2000, 122, 722 – 723; c) note that
the largest disagreement between the DFT predicted and
empirical spin density is only 0.03 (at C(10)).
[8] HMPA is an excellent solvent for radical anion systems, and ion
association involving hydrocarbon radical anions is absent in
HMPA, see: a) G.Levin, J.Jagur-Grodzinski, M.Szwarc, J. Am.
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Chem. Soc. 1970, 92, 2268 – 2275; b) C.D. Stevenson, L.
d) F.D. Lewis, T.L. Kurth, C.M. Hattan, R.C. Reiter, C.D.
Stevenson, J. Am. Chem. Soc. 2003, 125, 1460 – 1461; e) F.
Gerson, Top. Curr. Chem. 1983, 115, 57 – 60.
Echegoyen, L.R.Lizardi, J. Phys. Chem. 1972, 76, 1439 – 1442;
c) the addition of [18]crown-6 to ethereal radical anion systems
does not eliminate ion association, see: M.T.Watts, M.L.Lu,
M.P.Eastman, J. Phys. Chem. 1973, 77, 625 – 628.
[10] J.A. Weil, J.R. Bolton, J.E. Wertz,
Electron Paramagnetic
Resonance: Elementary Theory and Practical Applications,
Wiley, New York, 1994.
[9] a) A.D.Britt, M.M.Urberg, E.T.Kaiser,
J. Org. Chem. 1966,
31, 1661 – 1662; b) Y.-P.Wan, D.H.OꢀBrien, F.J.Smentowski,
Am. Chem. Soc. 1972, 94, 7680 – 7686; c) H.J. Shine, L.D.
J.
[11] G.Whitesides, Seminar delivered at Stanford University,
Department of Chemistry, February 27, 2008.
Hughes, P.Gesting, J. Organomet. Chem. 1970, 24, 53 – 61;
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