[5]Radialene

Article abstract of DOI:10.1021/jacs.5b07445

The [n]radialenes are a unique family of fundamental [n]-membered carbocyclic structures with radiating alkenes, which have attracted significant synthetic and theoretical attention. Whereas [3]-, [4]-, and [6]radialenes have been prepared and studied, all efforts to synthesize the five-membered ring compound have thus far met with failure. Here we describe the first synthesis of the fundamental hydrocarbon [5]radialene, C₁₀H₁₀. Our approach was a departure from previous radialene syntheses in that it utilized a low-temperature decomplexation of a stable organometallic compound, rather than high-temperature elimination or rearrangement. Our strategy was guided by analysis of previous radialene syntheses, which indicated rapid decomposition in oxygen, and ab initio calculations, which revealed an extraordinary susceptibility of [5]radialene to undergo Diels-Alder dimerization/polymerization. The origin of this susceptibility was traced to a small distortion energy associated with the formation of the transition structure geometry from the relaxed reactant monomers and to a narrow HOMO-LUMO gap.

Full text of DOI:10.1021/jacs.5b07445

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Article
[5]Radialene
Emily G. Mackay, Christopher G. Newton, Henry Toombs-Ruane, Erik Jan Lindeboom,
Thomas Fallon, Anthony C. Willis, Michael N. Paddon-Row, and Michael S. Sherburn
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b07445 • Publication Date (Web): 13 Sep 2015
Downloaded from http://pubs.acs.org on September 14, 2015
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[5]Radialene
Emily G. Mackay1, Christopher G. Newton1, Henry Toombs-Ruane1, Erik Jan Lindeboom1, Thomas Fallon1, Anthony C.
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Willis1, Michael N. Paddon-Row2* and Michael S. Sherburn1*
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1Research School of Chemistry, Australian National University, Canberra, ACT 2601, Australia,
2School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia.
ABSTRACT: The [n]radialenes are a unique family of fundamental [n]-membered carbocyclic structures with radiating
alkenes, which have attracted significant synthetic and theoretical attention. Whereas [3]-, [4]- and [6]radialenes have been
prepared and studied, all efforts to synthesize the five-membered ring compound have thus far met with failure. Here we
describe the first synthesis of the fundamental hydrocarbon [5]radialene, C10H10. Our approach was a departure from pre-
vious radialene syntheses in that it utilized a low temperature decomplexation of a stable organometallic compound, rather
than high temperature elimination or rearrangement. Our strategy was guided by analysis of previous radialene syntheses,
which indicated rapid decomposition in oxygen, and ab initio calculations, which revealed an extraordinary susceptibility of
[5]radialene to undergo Diels-Alder dimerization/polymerization. The origin of this susceptibility was traced to a small dis-
tortion energy associated with the formation of the transition structure geometry from the relaxed reactant monomers, and
to a narrow HOMO-LUMO gap.
the parent hydrocarbon, substituted [5]radialenes are well
known. The dehydrogenated C10 core of [5]radialene is the
INTRODUCTION
The radialenes (Fig. 1) are one of the four fundamental clas-
ses of hydrocarbons containing only sp2-hybridized carbons.1
Radialenes possess a single ring of carbons, each of which is
bonded to a methylene group outside of the ring, and have the
general molecular formula C2nH2n, where n>2, and where n
also specifies the number of C=C units. Structures of radi-
alenes are star-like when drawn flat, with C=C bonds radiating
outwards from the center of the carbocycle.
[3]Radialene,2 [4]radialene3 and [6]radialene4 have been
previously prepared. These syntheses involved classical elimi-
nations of Hofmann bases, hydrogen halides with bases, and
dihalides with zerovalent metals, and high-temperature rear-
rangements.
formal monomer of the sphero-hexameric buckminsterfuller-
ene C60, and other fully substituted [5]radialenes have been
prepared.6
RESULTS AND DISCUSSION
Our significant experience with the related acyclic hydro-
carbon family6e,7 (Fig. 1) led us to devise a preparation of
[5]radialene through cyclization of
a suitably-decorated
[5]dendralene. From the very outset, however, accurate high-
level G4(MP2) calculations8 warned us that [5]radialene
should display extraordinarily high reactivity towards Diels-
Alder dimerization or, perhaps, polymerization via a biradical
intermediate (Table 1). The G4(MP2) activation enthalpy and
free energy of activation for the concerted Diels-Alder dimeri-
zation of [5]radialene are 23.6 and 82.5 kJ/mol, respectively,
which are significantly lower than computed values for (a)
cyclopentadiene, which is known to dimerize over the course
of a couple of days at ambient temperature; (b) [3]dendralene,
which has an experimental half-life of 10 h neat at 25 °C;9 and
(c) 1,1-divinylallene, which has a half-life of 40 h as a 20 mM
concentration solution in CDCl3 at 25 °C.10 G4(MP2) calcula-
tions also predict six and three orders of magnitude increased
reactivity for [5]radialene relative to [4]radialene and
[6]radialene, respectively (Table 1).
annulenes
linear polyenes
[4]radialene
1962
[3]radialene
1965
cyclic, unbranched
acyclic, unbranched
dendralenes
radialenes
[5]radialene
THIS WORK
[6]radialene
1977
acyclic, branched
cyclic, branched
Figure 1. The [n]radialenes. On the left, the four fundamental
classes of conjugated oligo-alkenes, and, shaded on the right, the
first four members of the [n]radialene family. The year indicates
the date of the first reported synthesis.
Attempts to prepare [5]radialene along similar lines have
not been successful.1,5 Relatively little is known of the physi-
cal and chemical properties of the radialenes, aside from re-
ports of their susceptibility to polymerize when held neat
(even at 210 K)5 and a propensity to rapidly decompose2a–
e,3b,3c,4a,4c
and even spontaneously ignite in air.14 In contrast to
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ing effect of the Fe(CO)3 group upon otherwise unstable 1,3-
butadiene-containing organic species,11 and, more specifically,
b
ꢀH‡a
ꢀG‡
Diene
krel
1
2
3
4
5
6
7
8
our successful synthesis of a variety of iron tricarbonyl com-
plexes of [n]dendralenes,12 Fe(CO)3 complexes of [5]radialene
are attractive synthetic targets. Indeed, we predict the mono-
Fe(CO)3 complex of [5]radialene 2 to be more than five orders
of magnitude less reactive towards dimerization or polymeri-
zation than [5]radialene (Table 1) and the bis-Fe(CO)3 com-
plex of [5]radialene, lacking a free 1,3-butadiene moiety,
would be (presumably) indefinitely stable.
49.6
108.5
7.2
cyclopentadiene
c
54.0
29.5
113.4
91.8
1.0
[3]dendralene
9
•
6.1 × 103
c
The successful synthesis of a bis-Fe(CO)3 complex of
[5]radialene 11 is depicted in Scheme 1a. We were attracted to
the bis-Fe(CO)3 complex of 2,6-dichloro-3-oxa-[5]dendralene
(anti-4 and syn-8) as a starting point for this synthesis endeav-
or, due to both its close structural similarity to the requisite
precursor for cyclization, and its one step synthesis from 2,3-
dichloro-1,3-butadiene.13 The reported low yielding protocol
was reproducible in our hands and several one-gram batches
of the ketone were prepared. (We obtained a 1:1 mixture of
syn and anti diastereomeric forms 4 and 8, which was readily
separated by flash chromatography. See the Supporting Infor-
mation for details.) From here, in principle, only two steps
were needed to reach the bis-Fe(CO)3 complex of [5]radialene,
specifically a dechlorinative C–C bond-forming cyclization
and a ketone methylenation. In practice, difficulties stemming
from the steric burden of the two Fe(CO)3 groups thwarted all
efforts to secure a direct approach.
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1,1-divinylallene
[4]radialene
59.2
14.3
106.7
73.4
15
1.0 × 107
(1)d
(23.6)d (82.5)d
[5]radialene
(8.6 × 10–
(111.4)
(44.5)d
Fe(CO)3
d
6 d
)
2
Ultimately, both diastereomeric forms of the 2,6-dichloro-
oxa-[5]dendralene, anti-diastereomer 4 and syn-diastereomer
8, were taken through related but non-identical three step se-
quences, which were united at cyclized Petersen14 adduct
mono-Fe(CO)3 complex 7 (Scheme 1a). The anti-isomer 4
underwent Stille-Kelly coupling15 but the resulting oxa-
[5]radialene anti-bis-Fe(CO)3 complex 5, owing to the sub-
stantial steric bulk of Fe(CO)3 groups on both faces of the
molecule, could not be methylenated. Evidently, one of these
Fe(CO)3 groups had to be removed. Controlled exposure to
ceric ammonium nitrate16 delivered the sensitive mono-
Fe(CO)3 complex 6, which, on treatment with (trimethylsi-
lyl)methyllithium, provided 1,2-adduct 7 (only the product of
addition anti- to the Fe(CO)3 group was observed). In contrast,
the syn-diastereomer of dichloroketone 8 could not be cyclized
directly (to form the putative oxa-[5]radialene syn-bis-Fe(CO)3
complex). This compound underwent nucleophilic addition to
form tertiary alcohol 9 cleanly (only one diastereomer was
formed) but once again, all attempts to cyclize were met with
failure. Following selective decomplexation, mono-Fe(CO)3
complex 10 cyclized to form the common key intermediate 7,
thereby setting the scene for the final phase of the synthesis.
1.8 × 104
31.5
16.6
89.1
76.3
[6]radialene
3
3.2 × 106
1,2,3,4-tetramethylene
cyclopentane
kJ/mol, 298.15K. From ꢀG‡ and TS theory. Central double
bond is the dienophile. M06-3X/6-31G(d)//B3LYP/6-31G(d)
calculations.
Table 1. Gas phase G4(MP2) activation enthalpies (ꢀH‡),
free energies (ꢀG‡), and estimated relative rate constants
(krel) for Diels-Alder dimerization of some dienes.
a
b
c
d
The predicted rate constant for the Diels-Alder dimerization
of [5]radialene is seven and four orders of magnitude greater
than that for [3]dendralene and 1,1-divinylallene, respectively.
The experimentally determined half-life of 1,1-divinylallene,
together with our calculations and other considerations such as
statistical factors (which take into account the numbers of
equivalent dienophile double bonds, diene units and orienta-
tional regioselectivities) leads us to predict a half-life of less
than 5 seconds for a 20 mM solution of [5]radialene at 25 °C.
From these prognostications, it is no surprise that previous
synthetic efforts involving elimination processes have been in
Trimethylsilanol was eliminated from Petersen adduct 7 un-
der mildly acidic conditions at ambient temperature to gener-
ate a solution of the mono-Fe(CO)3 complex of [5]radialene 2.
As predicted computationally, this 1,2,3-trimethylene cyclo-
pentane derivative was prone to dimerization/polymerization
but nevertheless could be converted
vain. It would appear that
a successful synthesis of
[5]radialene should first generate the species as a stabilized
organometallic complex. Given the well-established stabiliz-
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Scheme 1. Chemical synthesis of [5]radialene, via bis-Fe(CO)3 complex 11. a, Approach to the bis-Fe(CO)3 complex of
[5]radialene 11 from diastereomeric Fe(CO)3 complexes 4 and 11, uniting at Petersen adduct 7. b, Low temperature decomplexation
of bis-Fe(CO)3 complex 11 provides [5]radialene. Good agreement is found between observed and predicted chemical shifts.
1
directly into the anti-bis-Fe(CO)3 complex of [5]radialene
11. A single crystal X-ray analysis of the bis-Fe(CO)3 complex
of [5]radialene 11 revealed a broadly in-plane conformation,
with the expected pyramidalization at the complexed meth-
ylene termini.
–20 °C, with no new signals discernable in the H NMR spec-
trum. We presume that the main mode of decomposition is
polymerization, perhaps promoted by the decomplexation
reagent, although similar observations have been reported for
[6]radialene in solutions apparently free of metal salts.4a
Decomplexation and generation of the free hydrocarbon was
achieved in dilute (ca. 0.1 mM concentration) solution at –78
°C with cerium ammonium nitrate in acetone (Scheme 1b).
Under these conditions, the hydrocarbon survives long enough
for NMR spectroscopic analysis to be performed. [5]Radialene
The molecular geometry of [5]radialene (Fig. 2) is interest-
ing. Several wavefunction theory and DFT calculations all
predict the D5h geometry to be a second-order saddle-point and
thus it is not a stable minimum energy structure. This is in
contrast to our calculations on [3]- and [4]radialenes, which
are predicted to have D3h and D4h global minimum energy
geometries, respectively. The distance between neighboring
methylene carbon atoms in [3]radialene (3.74 Å) and
[4]radialene (3.38 Å) lies well beyond the sum of their van der
Waals radii, whereas that in D5h [5]radialene (3.06 Å) is bor-
derline and appears to destabilize this structure, but barely so.
1
displays one singlet resonance in the H NMR spectrum at δ
5.68 ppm and two resonances in the 13C NMR spectrum at δ
104.5 and 143.6 ppm. HSQC and HMBC cross peaks confirm
this structural assignment.
As predicted computationally, [5]radialene exhibits a con-
spicuous tendency to decompose: aca. 30 µM concentration
solution of the hydrocarbon generated through this decom-
plexation method exhibited a half life of around 16 minutes at
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3.02Å
1
2
3.07Å
3.38Å
3
4
3.74Å
5
6
7
8
9
C2
D4h
D3h
[3]radialene
D3d
[6]radialene
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[4]radialene
[5]radialene
Figure 2. Top and side views of G4(MP2) minimum energy structures of the first four [n]radialenes. D3h [3]radialene, D4h
[4]radialene, C2 [5]radialene and D3d [6]radialene are shown, with distances between adjacent methylene carbons in Å.
The minimum energy structure of [5]radialene is calculated
to possess C2 symmetry but in which the distance between
adjacent methylene carbon atoms has increased by only 0.01
Å, compared to that in the D5h structure. The five-membered
ring of the C2 structure is fairly flat, with the five dihedral
angles between the vicinal ring bonds ranging between 3° and
10° (average value = 6°). The dihedral angles between pairs of
contiguous C=C bonds lie between 4° and 14°. The C=C bond
lengths in the C2 structure of [5]radialene are equal, to four
significant figures, at 1.337 Å and the C–C bond lengths lie
within the narrow range 1.486 – 1.487 Å; these values are
close to those calculated for the D5h structure, which are
1.338 and 1.488 Å, respectively. The similarity between the
C2 and D5h structures of [5]radialene is reflected in their nearly
equal energies, which differ by only ca. 0.1 kJ/mol
(G4(MP2)). (Although the electronic energy of the C2 struc-
ture is lower than that of D5h, inclusion of zero-point energy
reverses the ordering, by 0.3 kJ/mol. However, this is probably
due to an artefact arising from using harmonic frequencies to
calculate zero-point energies; see the SI for more discussion.)
From this analysis, we conclude that [5]radialene comprises a
degenerate set of five C2 structures that are rapidly intercon-
verting on the NMR timescale, even at very low temperatures,
and should give rise to NMR spectra consistent with average
The high degree of reactivity, predicted and observed, for
[5]radialene is under active investigation computationally, but
our initial results are indicative of this origin. At this stage, we
can say that the cause of the high reactivity of [5]radialene is
not due to the molecule possessing anti-aromatic character
arising from the cyclic fully cross-conjugated array of double
bonds in the radialene, for the following reasons. Firstly, the
radialene shows no sign of any ring current.18 The computed
1H NMR chemical shift in [5]radialene (δ 5.74 ppm) has al-
most the same magnitude as that (δ 5.56 ppm and δ 5.64 ppm)
calculated for the two pairs on protons associated with the two
internal methylene groups of the (unknown) cyclic interrupted
cross-conjugated tetramethylenecyclopentane molecule 3. In
addition, the calculated nucleus independent chemical shift,
NICS(1),19 for the radialene (–2.5 ppm) is small and compara-
ble to that for tetramethylenecyclopentane (–2.0 ppm), thereby
signifying negligible ring current in the former molecule. Sec-
ondly, the computed rate constant for the dimerization of tet-
ramethylenecyclopentane 3 is only three times smaller than
that for [5]radialene (Table 1). Thirdly, the B3LYP/6-31G(d)
HOMO-LUMO (H-L) energy gap uniformly decreases by a
constant ca 0.5 eV per additional methylene group along the
series n-methylenecyclopentanes: 5.2 eV (n = 2), 4.6 eV (n =
3), 4.2 eV (n = 4) and 3.8 eV for [5]radialene. Thus, there is
no anomalous sudden narrowing of the H-L gap in
[5]radialene, which would signify the presence of anti-
aromaticity in that molecule. This conclusion is also consistent
with our finding that the H-L gap in the ring-expanded, C2v-
constrained, pentamethylenecyclohexane (3.6 eV) is nearly the
same as that for [5]radialene.
1
D5h symmetry, which was observed experimentally. The H
and 13C chemical shifts for C2-symmetric [5]radialene were
calculated using the procedure of Tantillo et al.17 The calculat-
ed averaged shifts (experimental values in parentheses) are: 1H
NMR δ 5.74 (5.68) ppm, 13C NMR δ 104.3 (104.5) and 144.0
(143.6) ppm. The good agreement between the calculated and
experimental chemical shifts reinforces our conviction that the
observed NMR spectra are, in fact, those of [5]radialene.
Parenthetically, the lowest energy structure of [6]radialene
is predicted to be the D3d chair conformation with a 43° dihe-
dral angle between adjacent C=C bonds. This conformation
lies 3.2 kJ/mol below the D2 boat form and a substantial 71.5
kJ/mol below the D6h structure, which is a third-order saddle
point. The large energy difference between the chair and D6h
forms may be the result of steric compression between neigh-
boring pairs of methylene groups in the latter structure, where
the distance between the carbon atoms is only 2.85 Å, com-
pared to 3.02 Å in the chair conformation.
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the application of the distortion/interaction model.20 In this
model, the electronic energy of the transition state (TS), ꢀE‡,
is resolved into the sum of the distortion energy, ꢀE‡dist, and
the interaction energy, ꢀE‡int, where ꢀE‡dist is the energy that is
required to distort the reactants from their relaxed geometries
into the geometries adopted by them in the TS. From the data
in Table 2, it is clear that the enhanced dimerization reactivity
of [5]radialene, compared to the other cross-conjugated sys-
tems listed in Table 2, is due primarily to the comparatively
small magnitude of its distortion energy. Thus, even in the
case of [6]radialene, for which the interaction energy, ꢀE‡int, is
–11 kJ/mol more stabilizing than that for [5]radialene, this
energetic advantage is swamped by a much larger 32 kJ/mol
distortion energy for [6]radialene, relative to [5]radialene.
1
2
3
4
5
6
7
8
a
a
ꢀE‡
ꢀE‡
ꢀE‡a
79.7
22.3
Polyene
dist
int
102.2
63.3
-22.5
-41.0
[4]radialene
[5]radialene
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Inspection of the Diels-Alder dimerization transition struc-
tures (TSs) reveals the origin of the variation in ꢀE‡ within
dist
95.5
-52.1
43.3
[6]radialene
these cross-conjugated molecules. The lowest energy TSs for
these systems are highly asynchronous, each possessing an
advanced, short forming bond of ca. 2 Å and a very long form-
ing bond of 3.3 – 3.7 Å (represented schematically in Figure
3(a)). With the exception of [4]radialene, the TSs associated
with the remaining systems possess C2 symmetry and are bis-
pericyclic, displaying [4+2]/[2+4] dichotomy which is re-
solved at the bifurcation point into two enantiomeric product
channels.21 In fact, the aforementioned longer-forming bond in
these TSs is a member of a pair of equivalent very long pre-
bonds, which give rise to either [4+2] or [2+4] adducts. The
[4]radialene dimerization TS (Figure 3(d)) is asymmetric, with
unequal longer-forming bonds of 3.72 and 4.20 Å. The essen-
tial structural feature that is shared by all these TSs (including
that for [4]radialene dimerization) is that the reaction zone
may be regarded as comprising a pair of pentadienyl radical-
like moieties (represented schematically in Figure 3(b)), which
enter into a stabilizing through-bond interaction via the shorter
forming bond, as has been discussed in detail for the dimeriza-
tion of [3]dendralene.22 Taking the TS for the dimerization of
[3]dendralene as representative of the series of molecules stud-
ied here, the two pentadienyl moieties have essentially planar
W-shaped conformations (Figure 3(c)), with the two dihedral
angles between adjacent formal double bonds in each pentadi-
enyl unit being less than 3°. The methylene carbon atom of
each unit associated with the short forming bond is markedly
pyramidalized, as assessed by the 144° dihedral angle between
the two C=C–H planes of the central C=CH2 group of the pen-
tadienyl fragment (this angle is 180° for planar C=CH2).
3
1,2,3,4-
tetramethylene
64.7
92.4
-41.8
-37.5
22.9
46.3
cyclopentane
[3]dendralene
•
91.4
-44.0
47.4
1,1-divinylallene
a M06-2X/6-311+G(2df,p)//G4(MP2) energies in kJ/mol.
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Table 2. Distortion/Interaction Energy Analysis of the Di-
merization Reactions of Polyenesa
From the perspective of frontier MO theory, the markedly
narrower H-L gap for [5]radialene (3.8 eV), compared to those
for cyclopentadiene (5.5 eV), [3]dendralene (5.2 eV) and 1,1-
divinylallene (4.6 eV) is consistent with the finding that the
former is much more reactive than the latter three molecules.
However, the FMO argument is too simplistic because, alt-
hough the H-L gap for [4]radialene (3.9 eV) is the same as that
for [5]radialene, the calculated ꢀH‡ for the dimerization of
[5]radialene is 45 kJ/mol smaller than that for [4]radialene
(Table 1). Resolution of this problem was achieved through
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bis-pericyclic
DA TS schematic
(b)
[4]radialene DATS
bis-pentadienyl radical
DA TS schematic
(d)
[3]dendralene DA TS
(c)
(a)
1
2
3
4
in plane
W-shaped
pentadienyl
units
very long developing bonds
3.36 Å
3.36 Å
5
6
7
8
4.20 Å
2.08 Å
pyramidalization
at central
3.72 Å
1.87 Å
methylene
short developing bond
carbons
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(f) [6]radialene DATS
1,2,3,4-tetramethylene-
cyclopentane DA TS
(e) [5]radialene DA TS
(g)
(h)
1,1-divinylallene DATS
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3.33 Å
3.27 Å
3.38 Å
3.33 Å
3.38 Å
3.38 Å
3.27 Å
2.22 Å
2.04 Å
3.38 Å
2.05 Å
2.08 Å
(j)
anti-staggered conformation biradical
gauche-staggered conformation biradical
pentadienyl radical
(i)
new
C–C
bond
pentadienyl radical
Figure 3. Schematic representations (a,b) and G4(MP2) computed transition structures (TSs, c-h) of the Diels-Alder (DA) dimeri-
zation of polyenes, and UB3LYP/6-31G(d) optimized [5]radialene biradical dimers (i,j). Distances are in Å. W-shaped pentadienyl
radical-like moieties are highlighted in green in the schematic (b), the DA TSs (c-h), as are the pentadienyl radical sections in the biradical
species (i ,j).
The two major contributors to the distortion energy are (a)
the planarization of the carbon backbone of a cross-conjugated
triene unit in each reactant molecule which becomes a W-
shaped pentadienyl component of the TS and (b) the pyrami-
dalization of the central CH2 group in this unit. Considering
the dimerization of [5]radialene (Figure 3(e)), the planariza-
tion distortion energy is negligible, because of the near planar-
Thus, the cause of the predicted lower dimeriza-
tion/polymerization reactivity of [6]radialene, compared to
[5]radialene, is the planarization distortion energy in the for-
mer reaction. Similarly, planarization distortion energy was
found to be responsible for the lower reactivities of
[3]dendralene and 1,1-divinylallene (Figure 3(h)), compared to
[5]radialene, although a substantial proportion of this energy,
21 and 16 kJ/mol respectively, is required to convert the most
stable anti-gauche conformation of these molecules into their
reactive gauche-gauche forms.
ity of the reactant ring (Figure 3). Consequently, ꢀE‡ value
dist
of 63 kJ/mol for this reaction is attributed to the pyramidaliza-
tion process. As expected, ꢀE‡ and ꢀE‡ for the dimeriza-
dist
int
The greater magnitude of ꢀE‡ for the dimerization of
tion of the nearly planar tetramethylenecyclopentane (Figure
3(g)) have essentially the same values as the respective quanti-
ties for the [5]radialene dimerization.
dist
[4]radialene, compared to that for [5]radialene, is due, not to
planarization distortion energy owing to the planar nature of
the former, but to an increase in the pyramidalization distor-
tion energy in the [4]radialene reaction. Possibly on account of
the splaying of the double bonds in [4]radialene, compared to
[5]radialene (Figure 3), the longer forming bond in the
[4]radialene TS (Figure 3 (d)) blows out to 3.72 Å, about
0.33 Å longer than that in the [5]radialene TS (Figure 3(e)).
To compensate for, what must be, the absence of stabilizing
interactions in such a long forming "bond", the short forming
bond in the [4]radialene undergoes a significant 0.17 Å con-
In contrast to [5]radialene and tetramethylenecyclopentane,
[6]radialene (Figure 3(f)) has a non-planar chair conformation
with 43° dihedral angles between adjacent double bonds. Flat-
tening of a contiguous triene unit in [6]radialene costs 18.5
kJ/mol and is therefore 37 kJ/mol for two [6]radialene mole-
cules. Subtraction of this energy from the total ꢀE‡ for the
dist
dimerization of [6]radialene (Table 2) gives a pyramidaliza-
tion distortion energy of 58 kJ/mol, a value comparable to the
total distortion energy for the dimerization of [5]radialene.
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traction, from 2.04 Å, in the [5]radialene TS, to 1.87 Å. This and these findings prompted the implementation of a synthetic
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contraction is accompanied by increased pyramidalization of
both carbon atoms of the short forming bond, and thus, to an
increase in ꢀE‡dist, for [4]radialene dimerization, compared to
that for [5]radialene dimerization.
strategy distinct from all previous approaches towards radi-
alenes. The successful approach involved the preparation of a
stable Fe(CO)3 complex of [5]radialene, the decomplexation
of which led to the generation of the highly reactive molecule
in a controlled fashion. Our initial computational investiga-
tions lead us to conclude that the high degree of reactivity of
[5]radialene is primarily structural, in that the near planarity of
the molecule, together with the strain caused by the enforced
cis-configured contiguous double bonds, minimizes the distor-
tion energy for dimerization. In addition, the unusually narrow
HOMO-LUMO energy gap of 3.8 eV (cf. 5.5 eV for cyclopen-
tadiene) for [5]radialene is expected to further enhance the
reactivity of this molecule. This new strategy for [5]radialene
synthesis opens the door to this neglected family of hydrocar-
bons, and should permit the first detailed study into their be-
havior. The reasons for such studies are manifold, and include
the need for a deeper understanding of fundamental structure
and how it relates to properties. Specific applications will most
likely involve the applications of radialenes in the rapid gener-
ation of structural complexity. Studies with the related acyclic
cross-conjugated hydrocarbons recently culminated in the
shortest total synthesis of important natural products.23 It
would not be surprising were the radialenes to follow suit.
In the above discussion, we were equivocal concerning
whether the radialenes dimerized by a concerted mechanism or
via a biradical intermediate, which could lead to polymer for-
mation. Our G4(MP2) calculations favor the former mecha-
nism but this preference may be an artifact arising from our
using the restricted formalism. Our equivocation stems from
preliminary unrestricted B3LYP/6-31G(d) calculations on the
[5]radialene dimerization reaction. The unrestricted bis-
pericyclic TS is about 8 kJ/mol lower in free energy than the
restricted TS. Following the former TS, using IRC analysis,
led to a C2-symmetric biradical intermediate which has a
gauche arrangement of the radialene moieties about the newly
formed bond (Figure 3(i)). This biradical intermediate may
collapse, with an activation free energy of 27.5 kJ/mol, to give
either one of the enantiomeric Diels-Alder adducts, or it may
act as a seed for polymer formation. More salient to the poly-
mer production channel is the prediction that the activation
free energy for the formation of the anti conformation of the
biradical intermediate (Figure 3(j)) is favored over the bis-
pericyclic (gauche) TS by 1.7 kJ/mol. The anti biradical is
well disposed for initiating polymer production. This predic-
tion of preferred biradical production is consistent with our
inability to identify any dimeric adduct arising from the gener-
ation of free [5]radialene. A similar prediction was obtained
for [4]radialene, with an even stronger energetic preference, of
7.6 kJ/mol, for anti addition over gauche addition. As for
[4]radialene, the unrestricted gauche TS led to a gauche birad-
ical intermediate, its conversion into the [4+2] adduct requir-
ing an activation free energy (48.7 kJ/mol) nearly double that
calculated for the gauche biradical intermediate formed from
[5]radialene dimerization, which reflects the high degree of
strain in the [4+2] adduct from [4]radialene. In contrast to the
behavior of [4]radialene and [5]radialene, the restricted bis-
pericyclic TS for the dimerization of [6]radialene is the most
stable, lying 5.1 kJ/mol in free energy below the unrestricted
TS for formation of the anti biradical. Thus, of the three radi-
alenes studied, [6]radialene should offer the best prospect of
obtaining [4+2] adduct. A C2-symmetry-constrained path from
the bis-pericyclic TS, however, led to a C2-symmetric unre-
stricted gauche biradical, which may collapse to a [4+2] prod-
uct with a 21.3 kJ/mol activation free energy. Given the bifur-
cated nature of bis-pericyclic potential energy surfaces,15d it is
unlikely, from a molecular dynamics perspective, that the
gauche biradical intermediate would participate in radialene
dimerization reactions. Nevertheless, the gauche biradical is
predicted to lie on the Cope rearrangement path, which inter-
converts the two enantiomeric [4+2] adducts. Hence, Cope
rearrangements in [4+2] adducts from radialene dimerizations
are predicted to be non-concerted and we also find this to be
the case for the Cope rearrangement of the known15e
[3]dendralene [4+2] dimer. Of course, these provisional results
require validation using higher levels of theory.
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ASSOCIATED CONTENT
Supplementary information is available for synthetic and compu-
tational work. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*e-mail: michael.sherburn@anu.edu.au (synthetic),
m.paddonrow@unsw.edu.au (computational)
Author Contributions
This manuscript was written through contributions of all authors.
ACKNOWLEDGMENT
M.N.P.-R. acknowledges that this research was undertaken with
the assistance of resources provided at the NCI National Facility
through the National Computational Merit Allocation Scheme
supported by the Australian Government. This work was support-
ed by the Australian Research Council.
ABBREVIATIONS
DA, Diels–Alder, HOMO, highest occupied molecular orbital,
LUMO, lowest occupied molecular orbital, HMBC, heteronuclear
multiple-bond correlation spectroscopy, HSQC, heteronuclear
single quantum coherence spectroscopy, IRC, intrinsic reaction
coordinate, MO, molecular orbital, NMR, nuclear magnetic reso-
nance, TS, transition structure.
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In summary,
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SYNOPSIS TOC
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C10H10
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[5]radialene -
bis-Fe(CO)3 complex
[5]radialene
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