1,1-divinylallene

Article abstract of DOI:10.1002/anie.201105541

How to hit a triple: The title hydrocarbon, one of the most π-bond-rich small molecules, has been synthesized and characterized for the first time. This highly reactive hydrocarbon undergoes a new diene-transmissive, triple Diels-Alder cycloaddition seque

Full text of DOI:10.1002/anie.201105541

DOI: 10.1002/anie.201105541
p-Bond-Rich Hydrocarbons
1,1-Divinylallene**
Katie M. Cergol, Christopher G. Newton, Andrew L. Lawrence, Anthony C. Willis,
Michael N. Paddon-Row,* and Michael S. Sherburn*
Studies into fundamental hydrocarbon structures have led to
a deeper understanding of structure, reactions and mecha-
nism. Such investigations have also led to the invention of
new synthetic methods, strategies and tactics.[1] While many
families of designed molecules have succumbed to synthesis, a
significant number have not yet been made. Members of the
dendralene family (1 and 2, Figure 1) of branched acyclic
oligoalkenic hydrocarbons higher than the tetraene, for
example, were prepared for the first time in 2000.[2] More
recently, practical syntheses of the dendralenes allowed the
unexpected alternation in behavior in odd and even members
of the family to be uncovered.[3] It has recently been shown
that [4]- and [5]dendralenes have significant synthetic poten-
tial in the rapid generation of polycyclic systems.[4,5] Herein
we report the first synthesis of a considerably less stable
substituted derivative. Thus, Hopf and co-workers prepared
a methyl-substituted analogue of 3 but its instability pre-
cluded isolation and characterization.[10,11]
The Hopf group has also attempted to prepare the parent
1,1-divinylallene 3.[10,11] In one unsuccessful approach, flash
vacuum pyrolysis of diacetate 4 gave the ethynyl-1,3-buta-
diene 5, a result that was ascribed to a sigmatropic 1,5-
hydrogen shift of the presumed divinylallene intermediate 3
(Scheme 1). Ethynyl-1,3-butadiene 5 is the only reported
decomposition product of a 1,1-divinylallene. We suspected
that the sigmatropic rearrangement was a result of the high
temperature involved in the pyrolysis experiment. Never-
theless, much lower temperatures have been employed to
bring about related thermal hydrogen shifts in allenes,[12] so
we thought it prudent to calculate the activation parameters
for this transformation using the very accurate multilevel ab
initio G4 method[13] before devising a synthetic plan to the
hydrocarbon. The lowest-energy G4 transition state[14] for the
=
relative of the dendralenes and one of the most C C bond-
rich small molecules, 1,1-divinylallene 3. We also demonstrate
that it surpasses the dendralenes—on a “bonds formed per
atom” basis—in its ability to generate structural complexity.
Only a small number of 1,1-divinylallenes have been
reported in the literature.[6–11] Most are highly substituted[6–9]
and information on their physical and chemical properties is
scant. Only one study has been reported with a less
À À
1,5-H shift exhibits a C C C bond angle bent significantly
(1318) from the linear geometry of the starting allene and a
À
long (1.55 ꢀ) developing C H bond. The computed activa-
tion parameters, DH2°98K and DG2°98K are 177.0 and
184.4 kJmolÀ1, respectively, which are sufficiently large to
guarantee thermal stability of 1,1-divinylallene towards the
1,5-H sigmatropic shift at temperatures as high as 2508C.
Our computational results indicate that, under standard
laboratory conditions (i.e. < 2508C), 1,1-divinylallene 3
should be stable towards the only decomposition pathway
that has been previously documented. Nevertheless, alterna-
tive, highly facile, intermolecular pathways for degradation of
3, such as Diels–Alder dimerization—as seen previously with
[3]dendralene[15]—could necessitate special care with the
isolation of 3. G4 calculations provided valuable insight into
this problem. The G4 DG°298K value for the Diels–Alder
dimerization of 3 is 90.3 kJmolÀ1, which is substantially
=
Figure 1. Branched hydrocarbons that are rich in C C bonds.
[*] Dr. K. M. Cergol, C. G. Newton, Dr. A. L. Lawrence, Dr. A. C. Willis,[+]
Assoc. Prof. M. S. Sherburn
Research School of Chemistry, Australian National University
Canberra, ACT 0200 (Australia)
E-mail: sherburn@rsc.anu.edu.au
Homepage: http://rsc.anu.edu.au/research/sherburn.php
Prof. M. N. Paddon-Row
School of Chemistry, The University of New South Wales
Sydney, NSW 2052 (Australia)
E-mail: m.paddonrow@unsw.edu.au
[+] Crystallographer.
[**] Funding from the Australian Research Council (ARC) is gratefully
acknowledged. M.N.P.-R. acknowledges computing time from the
Australian Partnership for Advanced Computing (APAC) awarded
under the Merit Allocation Scheme. The authors thank Alan D.
Payne and Ryan W. Cox (ANU) for preliminary experiments.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105541.
Scheme 1. The G4 TS for Hopf’s sigmatropic rearrangement of 3.
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Communications
smaller than the G4 DG°298K value of 110.8 kJmolÀ1 for the
Diels–Alder dimerization of cyclopentadiene, neat solutions
of which have a lifetime in the order of days at room
temperature. Our calculations suggest that concentrated
solutions of 3 should have lifetimes three orders of magnitude
shorter than those for cyclopentadiene, on the timescale of
only minutes at room temperature. In light of this issue, we
elected to pursue an approach that would generate the
hydrocarbon in dilute solution.
Several of the more obvious approaches to 3, involving
cross-coupling reactions[16] or SN2’-type reactions[10,11,17] were
investigated. Frustratingly, none delivered the target. The
approach that was ultimately successful (Scheme 2) involved
Grieco–Sharpless elimination[18] of the selenide 10, derived
from alcohol 8, which was conveniently prepared on multi-
gram scale by nucleophilic addition of the Grignard reagent
derived from 2-chloro-[3]dendralene 6[19] to formaldehyde.
The addition proceeds with overall allylic transposition of the
Grignard reagent, presumably on account of the cyclic nature
of the mechanism of this process (see 7).[20] In fact, none of the
un-transposed isomer 9 could be detected in the reaction
mixture.[21]
Figure 2. 1H and 13C NMR spectra of 1,1-divinylallene 3 (800 MHz and
200 MHz, respectively, CDCl3 solvent, assignments from HSQC experi-
ments).
1
analysis[22] (see Figure 2 for H and 13C NMR spectra). 1,1-
Divinylallene 3 exhibits a single UV absorption maximum at
248 nm, a longer wavelength than that seen for vinylallenol 8
(215 nm), [3]dendralene 1 (two maxima, 206 and 231 nm)[3]
and closer in value to that of isotoluene 11 (242 nm).[23]
Hopf et al. reported an MP2/cc-pVTZ computational
study of the conformational analysis of 1,1-divinylallene and
concluded that the C2 conformation, with each vinyl group
approximately s-trans with respect to the allenic moiety, was
the most stable.[11] However, using the same MP2/cc-pVTZ
method, we find the s-cis,s-trans conformation—depicted in
the inset of Figure 2 (the MP2 dihedral angles made between
the vinyl groups with the allene moiety are À358 and 1738)—
to be the most stable conformation and not the C2 con-
formation, as stated by Hopf et al., the former being
enthalpically 3.9 kJmolÀ1 more stable than the C2 s-trans,s-
trans conformation. A variety of other methods—B3LYP,
M06-2X, CBS-QB3, G4—also predict the s-cis,s-trans con-
formation to be the most stable. We conclude, therefore, that,
of the four conformations of 3 that we have located,[24] the
global minimum energy conformation is the s-cis,s-trans
conformation.[25]
We have prepared samples of 1,1-divinylallene 3 in CDCl3
and [D6]DMSO through the Grieco–Sharpless elimination
reaction of selenide 10. As predicted through our calculations,
the hydrocarbon proved to be highly unstable. To minimize
loss of the hydrocarbon through decomposition and evapo-
ration, we find it most convenient to use these solutions
directly in further reactions. Nevertheless, for characteriza-
tion purposes, direct bulb-to-trap distillation (08C; 1 mmHg)
of the reaction mixture gave a dilute solution of 1,1-
divinylallene 3 in CDCl3 free of other impurities. A ca.
0.02m concentration solution of 1,1-divinylallene 3 has a half-
life of around 43 h at 258C. The decomposition pathway of 3 is
opaque at present, since a complex mixture of products is
generated. We suspect that Diels–Alder dimerizations are
1
involved. The hydrocarbon 3 was characterized by H NMR,
13C NMR, IR, UV spectroscopic and mass spectrometric
One molecule of 1,1-divinylallene 3 reacts with three
dienophile molecules to generate the tricyclic decahydro-
phenalene ring system 14 (Scheme 3). This unprecedented
triple diene-transmissive[26,27] sequence generates six new
À
C C bonds. It is striking that such a small cross-conjugated
=
hydrocarbon, with only four C C bonds, can generate such a
high degree of structural complexity so rapidly. Using N-
phenylmaleimide as dienophile, the overall triple cycloaddi-
tion sequence is high yielding and proceeds with the
generation of eight new stereocenters. The first and second
cycloadditions each give a single diastereomeric product,
within the limits of detection. The third gives a mixture but
with one diastereomeric product clearly dominating (major
diastereomer:other isomers = 65:35). The stereochemistry of
the major triple adduct 14 was determined through single
crystal X-ray analysis.[28]
The first cycloaddition occurs at either one of the two
equivalent vinylallene “diene” sites of 3 to give mono-adduct
12, a cyclic cross-conjugated triene.[29] The second addition
Scheme 2. First synthesis of the parent 1,1-divinylallene 3.
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endo-diastereomer, are depicted in Scheme 3. The calculated
TS energies are consistent with experimental observations,
with the one leading to the exo-isomer lower in energy by
5.3 kJmolÀ1.[30] The strain in the endo-TS is apparent from the
twisting of the succinimide ring introduced in the first
cycloaddition (curved arrow in endo-TS, Scheme 3).
In conclusion, 1,1-divinylallene 3 has been prepared for
the first time. The synthesis employs standard laboratory
equipment and techniques. In light of its volatility and
tendency towards decomposition, the hydrocarbon is best
handled in solution. 1,1-Divinylallene 3 is one of the most p-
bond rich hydrocarbons prepared, with only seven carbons
=
=
but four C C bonds. The arrangement of C C bonds in 3
permits a controlled sequence of three cycloaddition reac-
tions and the rapid construction of the phenalene framework.
The ability to isolate mono- and bis-adducts invites future
synthetic applications, which will undoubtedly mandate
different dienophiles for each of the three Diels–Alder
reactions. Studies into the application of this sequence in
total synthesis are underway.
Received: August 5, 2011
Published online: September 14, 2011
Keywords: 1,1-divinylallene · cycloaddition · Diels–
.
Alder reactions · hydrocarbons
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Scheme 3. A triple diene-transmissive Diels–Alder sequence involving
1,1-divinylallene 3. Lower right: the diene is boxed in the DFT model
TSs and developing bond lengths are shown.
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[21] The nucleophilic addition process is quite general and represents
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disclosure from our laboratories will detail these investigations.
[22] See the Supporting Information for full details.
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