The cyclization of parent and cyclic hexa-1,3-dien-5-ynes - A combined theoretical and experimental study

Article abstract of DOI:10.1002/1521-3765(20011015)7:20<4386::AID-CHEM4386>3.0.CO;2-S

The thermal cycloisomerization of both parent and benzannelated hexa-1,3-dien-5-yne, as well as of carbocyclic 1,3-dien-5-ynes (ring size 7-14), was investigated by using pure density functional theory (DFT) of Becke, Lee, Yang, and Parr (BLYP) in connection with the 6-31G^* basis set and the Brueckner doubles coupled-cluster approach [BCCD(T)] with the cc-pVDZ basis set for the parent system. The initial cyclization product is the allenic cyclohexa-1,2,4-triene (isobenzene), while the respective biradical is the transition structure for the enantiomerization of the two allenes. Two consecutive [1,2]-H shifts further transform isobenzene to benzene. For the benzannelated system, the energetics are quite similar and the reaction path is the same with one exception: the intermediate biradical is not a transition state but a minimum which is energetically below isonaphthalene. The cyclization of the carbocyclic 1,3-dien-5-ynes, which follows the same reaction path as the parent system, clearly depends on the ring size. Like the cyclic enediynes, the dienynes were found to cyclize to products with reduced ring strain. This is not possible for the 7- and 8-membered dienynes, as their cyclization products are also highly strained. For 9- to 11-membered carbocycles, all intermediates, transition states, and products lie energetically below the parent system; this indicates a reduced cyclization temperature. All other rings (12- to 14-membered) have higher barriers. Exploratory kinetic experiments on the recently prepared 10- to 14-membered 1,3-dien-5-ynes rings show this tendency, and 10- and 11-membered rings indeed cyclize at lower temperatures.

Full text of DOI:10.1002/1521-3765(20011015)7:20<4386::AID-CHEM4386>3.0.CO;2-S

FULL PAPER
The Cyclization of Parent and Cyclic Hexa-1,3-dien-5-ynesÐ
A Combined Theoretical and Experimental Study
Matthias Prall,[a] Anke Krüger,[b] Peter R. Schreiner,*[a, c] and Henning Hopf*[b]
Abstract: The thermal cycloisomeriza-
tion of both parent and benzannelated
hexa-1,3-dien-5-yne, as well as of carbo-
cyclic 1,3-dien-5-ynes (ring size 7 ± 14),
was investigated by using pure density
functional theory (DFT) of Becke, Lee,
Yang, and Parr (BLYP) in connection
with the 6 ± 31G* basis set and the
Brueckner doubles coupled-cluster ap-
proach [BCCD(T)] with the cc-pVDZ
basis set for the parent system. The
initial cyclization product is the allenic
isobenzene to benzene. For the benzan-
nelated system, the energetics are quite
similar and the reaction path is the same
with one exception: the intermediate
biradical is not a transition state but a
minimum which is energetically below
isonaphthalene. The cyclization of the
carbocyclic 1,3-dien-5-ynes, which fol-
lows the same reaction path as the
parent system, clearly depends on the
ring size. Like the cyclic enediynes, the
dienynes were found to cyclize to prod-
ucts with reduced ring strain. This is not
possible for the 7- and 8-membered
dienynes, as their cyclization products
are also highly strained. For 9- to 11-
membered carbocycles, all intermedi-
ates, transition states, and products lie
energetically below the parent system;
this indicates a reduced cyclization tem-
perature. All other rings (12- to 14-
membered) have higher barriers. Ex-
ploratory kinetic experiments on the
recently prepared 10- to 14-membered
1,3-dien-5-ynes rings show this tendency,
and 10- and 11-membered rings indeed
cyclize at lower temperatures.
cyclohexa-1,2,4-triene
(isobenzene),
while the respective biradical is the
transition structure for the enantiomeri-
zation of the two allenes. Two consec-
utive [1,2]-H shifts further transform
Keywords: ab initio calculations ´
cycloaromatization ´ enynes ´ iso-
benzene ´ macrocycles
Introduction
FVP
The thermochemical cyclization of hexa-1,3-dien-5-yne de-
rivatives is of major importance for the synthesis of bowl-
shaped molecules like corannulene 2 (Scheme 1)[1±3] and
semibuckminsterfullerenes.[4] Examining and understanding
the parent reaction in detail will aid in optimizing and
extending these synthetic strategies.
1
2
[a] Prof. Dr. P. R. Schreiner, Dipl.-Chem. M. Prall
Institut für Organische Chemie, Georg-August-Universität Göttingen
Tammannstrasse 2, 37077 Göttingen (Germany)
E-mail: prs@chem.uga.edu
4
E-3
Z-3
[b] Prof. Dr. H. Hopf, Dr. A. Krüger
Institut für Organische Chemie
Technischen Universität Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
Fax : (‡49) 531-391-5388
Scheme 1. Synthesis of bowl-shaped corannulene 2 from hexa-1,3-dien-5-
yne derivative 1, and the thermal cycloisomerization of 3 to 4.
In 1969, Hopf and Musso were able to show that hexa-1,3-
dien-5-yne 3 undergoes a thermal cycloisomerization to give
rise to benzene 4.[5] The reaction starts above 2748C, at which
point isomerization between E-3 and Z-3 takes place. From a
geometrical point of view, Z-3 was believed to be the species
which readily cyclizes to 4 (Scheme 1).
E-mail: h.hopf@tu-bs.de
[c] Prof. Dr. P. R. Schreiner
Current address:
Computational Center for Molecular Structure and Design
Department of Chemistry, University of Georgia
Athens, Georgia 30602-2556 (USA)
Fax.: (‡1)706-542-9454
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/chemistry/ or from the author.
As 3 posesses the same number of atoms as 4, a hydrogen
shift or rearrangement has to occur prior to or after
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4386±4394
cyclization. Scheme 2 represents the four most likely path-
ways connecting the structures 3 and 4:
[1,2]-H-shift leading to the vinylidene carbene 5 (1A') and
*
then to 4.
*
thermal cyclization with rearrangement of the p-system
giving rise to isobenzene (cyclohexa-1,2,4-triene) 6 which
undergoes hydrogen shift to give 4.
thermal cyclization to the biradical 7 (1A'') and hydrogen
*
shift to 4.
*
addition of a hydrogen radical (8), cyclization, and
*
subsequent loss of H to give 4.
Scheme 3. Schematic representation of the enediyne and dienyne carbo-
cycles (9a ± f, 11a ± h) and their cyclization products (10a ± f, 12a ± h,
respectively).
3
detected in traces after thermolysis of 13 at 1808C, but its
formation could indirectly be deduced by its cycloisomeriza-
tion product benzocyclobutene 12b (Scheme 4). Besides 11b,
the only representative of carbocyclic hexa-1,3-dien-5-ynes
ever reported is cyclodeca-1,3-dien-5-yne 11d; its NMR
structural assignment, however, is doubtful.[25, 26] Experimen-
tal work on the medium-sized carbocycles 11 still has to be
done to characterize this substance class more accurately.
+H
H
H
H
H
H
H
H
5 (1A')
7 (1A'')
6 (C1)
8 (C1)
N
-H
Se
N
13
11b
12b
4
Scheme 4. Synthesis of nonisolable eight membered dienyne 11b from
selenadiazol 13 and cyclization product benzocyclobutene 12b.
Scheme 2. Possible pathways for the cycloisomerization of 3 to 4.
Experimental and theoretical studies report that intermedi-
ates 6 or 7 are mainly formed during the cyclization at lower
temperatures (200 ± 4008C).[6±8] The role of allene 6 and
biradical 7 is, however, unclear.[9, 10] At higher temperatures,
processes including the intermediates 5 and 8 are more
favorable; this is indicated by the increased formation of
pentafulvene as a by-product.[11±15]
With respect to the formation of fullerene-like moieties or
building blocks, the effect of benzannelation on the barriers
and enthalpies of formation is also important. Other groups,
as well as two of the authors, have shown that the barrier of
the corresponding benzannelated enediyne is merely raised,
while the reaction enthalpy is ꢀ10 kcalmol 1 more endother-
mic.[16±20] As the cycloisomerization of 3 is apparently not a
one-step process, like the Bergman reaction, benzannelation
may have a different effect on this reaction.
Another important aspect is the behavior of the dienyne
moiety embedded in a carbocyclic system. The structurally
similar cyclic enediynes 9a ± f show a correspondence be-
tween ring strain and activation enthalpies.[21, 22] In analogy to
the Bergman reaction of 9a ± f, at least some of the smaller
strained dienyne carbocycles 11a ± h are expected to have
lower cyclization barriers (Scheme 3).
The synthesis of cycloocta-1,3-dien-5-yne (11b) from selen-
adiazol 13 is a first example for cyclic dienynes.[23, 24] Because
of its short lifetime, the highly strained 11b could only be
In the present study, the cycloisomerization of the Z-hexa-
1,3-dien-5-yne parent system 3 to benzene 4 is discussed in
detail, including all possible intermediates and transition
structures. The computational results are compared with
experiment, and conclusions are drawn considering the
involvement of isobenzene 6 or biradical 7. The effect of
benzannelation on the cycloisomerization also is investigated,
and predictions regarding the energetics for this reaction are
made.
In the third part, ring strain effects on the cycloisomeriza-
tion reaction are discussed theoretically on the basis of the
E,Z-hexa-1,3-dien-5-yne carbocycles 11a ± h and the results
are again compared with experimental data.
Computational methods: All calculations were performed
with Gaussian98[27] by using Beckeꢁs pure gradient-corrected
exchange functional[28, 29] and the Lee ± Yang ± Parr nonlocal
correlation functional[30, 31] (BLYP) with a 6 ± 31G*[32] basis
set. A restricted approach was used for geometry optimiza-
tions, energy evaluations, and frequency analyses of the
closed-shell reactants and transition structures, while the
biradical structures were computed by an unrestricted bro-
ken-spin approach (BS-UBLYP). For the parent and the
benzannelated system, additional single-point energies were
evaluated by using the Brueckner-doubles (BD) coupled-
cluster approach BCCD(T)[33±37] [RBCCD(T) for closed shell,
Chem. Eur. J. 2001, 7, No. 20
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P. R. Schreiner, H. Hopf et al.
FULL PAPER
UBCCD(T) for open shell spe-
cies] and employing Dunningꢁs
correlation-consistent valence
double-z (cc-pVDZ) basis
set.[38] All thermal corrections
to obtain enthalpy values are
taken from BLYP/6 ± 31G* fre-
quency calculations. For com-
parison of experimental with
calculated enthalpies for the
parent system, the corrections
for these transition states (TSs)
were taken from frequency
analyses at 623 K (the actual
experimental reaction temper-
ature). The use of a larger basis
set (6 ± 311 ‡ G**) for the key
structures of the parent system
H
H
H
H
H
H
(S)-6
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
3
14
7 (1A'')
H
H
H
H
H
H
(R)-6
Scheme 5. Cyclization of dienyne 3 giving rise to isobenzenes (S)-6 and (R)-6, connected by biradical TS 7.
raised the barriers by only 1 ±
1
2 kcalmol . Although DFT is a single-reference method in
the formal sense, it has proven to be quite capable of
describing structures with multireference character, such as
carbenes[39±44] and biradicals.[21, 22, 45±50] The reasons for this are
not entirely clear, but is likely to be due to the fact that the
density may be well described by a single configuration in
many cases. Hence, this makes the use of DFT for biradicals a
case-to-case decision.[20, 47, 51, 52] Based on our and othersꢁ
recent experience, the choice of functional/basis set combi-
nation used in the present work seems, therefore, well
justified. We would like to point out that feasible multi-
the respective chemistry of 3. The result is that dienyne 3
cyclizes to isobenzene 6, which racemizes readily via 7
(Scheme 5).
1
The barrier height between 3 and 6 is 29.2 kcalmol at
1
BLYP/6 ± 31G* and 31.7 kcalmol
at the BCCD(T)/cc-
pVDZ level of theory, in good agreement with the exper-
1
imental result of 30.3 kcalmol (Table 1). Experimentally,
Table 1. Results of the computed enthalpy values for the cycloisomeriza-
tion of 3 compared with experimental data (all enthalpies in kcalmol 1).
Structure
DRH298/DH=623
(BLYP/6 ± 31G*)
DRH298/DH=623
(BCCD(T)/cc-pVDZ)[a]
Experiment[6]
reference
treatments
(CASSCF(8,8)/6 ± 31G*
and
CASMP2(8,8)/6 ± 31G*) are not significantly better in de-
3
14
6
0.0
29.2
12.6
18.5
38.8
44.1
57.4
73.8
19.8
20.6
24.2
21.5
65.3
65.1
0.0
31.7
7.9
0.0
30.3
scribing these types of reactions. For instance, the error in
1
describing the barrier (7 ± 11 kcalmol ) and reaction enthal-
20.4
1
7
17.1
38.4
45.8
54.6
75.0
22.0
21.3
21.7
22.6
58.8
67.9
py (2 ± 9 kcalmol ) for the well-known Bergman cyclization
17
18
19
20
21
22
23
24
25
4
is rather large.[53] Higher-level treatments, such as multi-
reference CI, are currently not feasible.[44]
40.5
±
±
±
±
±
Results and Discussion
Parent system: The parent Z-hexa-1,3-dien-5-yne, 3, can
cycloisomerize along different paths (Scheme 2); at lower
temperatures (200 ± 4008C), electrocyclic ring closure is
favored (via 6 or 7). Previous work shows that there is a
disagreement regarding the intermediate formed along this
path. The first experimental reports propose isobenzene
structure 6;[54] an early AM1 study indicated that biradical 7
is not a ground state but rather a transition structure that
connects the two enantiomeric isobenzene structures (S)-6
and (R)-6 (C1 point group).[55] On the basis of the enthalpies of
formation, the intermediate was suggested to have biradical
structure 7.[6]
A very detailed B3LYP/6 ± 31G* study has now thrown
some light on this dilemma.[56] These computations, which
agree nicely with our own BLYP/6 ± 31G* results, show that
biradical 7 is indeed a transition structure for the enantiomer-
ization of 6. Whereas the B3LYP study concentrates on the
isomerization of 6 to 4, our own study combines this issue with
64.9
[a] Thermal corrections taken from the BLYP/6 ± 31G* computations.
the formation of the intermediate is 20.4 kcalmol 1 endother-
mic; this correlates best with the value for biradical 7 (BLYP:
1
1
18.5 kcalmol , BCCD(T): 17.1 kcalmol ). However, extend-
ed computations with a large number of theoretical levels
(BLYP,[28, 30, 31] G96LYP,[30, 31, 57] BPW91,[28, 58] HF,[59±61] MP2,[62]
and CASSCF(8,8)[63, 64] with a 6 ± 31G* basis set) show that
planar 7 is a TS (NImag ˆ 1), in which the imaginary vector
and IRC calculations indicate movement of the allenic
hydrogen atoms out-of-plane toward a twisted allene. Follow-
ing the path of the dihedral angle of these hydrogens from 08
(7) to 988 (6) by optimizing the rest of the structure with
BLYP/6 ± 31G* and subsequently BCCD(T)/cc-pVDZ sin-
gle-point calculations show the transition-state nature of 7
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Cyclization of Cyclic 1,3-dien-5-ynes
4386±4394
subsequent [1,4]-H shift via
25 to 4.
3) [1,2] hydrogen shift from the
sp3-hybridized carbon via TS
17 to the allenic carbon
leading to intermediate 21/
22 and through a subsequent
[1,2]-H shift via 23 to 4.
4) [1,2] hydrogen shift of the
allenic hydrogen via TS 20
giving rise to the same inter-
mediate 21/22 and [1,2]-H
shift via 23 to 4.
As noted, path 3 is energetically
most favorable. With a barrier
1
of 38.8 kcalmol for BLYP or
1
38.4 kcalmol
for BCCD(T)
above 3, this path agrees nicely
with the experimental result of
1
Figure 1. Singlet (bottom) and triplet (top) energy surface of 6 depending on dihedral angle. All points are
BCCD(T)/cc-pVDZ//BLYP/6 ± 31G* single-point energies.
40.5 kcalmol . The barrier of
the perhaps most obvious path,
1, is ꢀ16 kcalmol and that of
1
beyond any doubt (Figure 1). The triplet state lies
1
1.9 kcalmol above the open shell singlet and is a true
H
H
H
H
minimum. Additionally, an IRC calculation performed on TS
14 shows the connection between 3 and 6.
This situation is reminiscent of the enantiomerization of
cycloheptatetraene 15 via singlet cycloheptatrienylidene 16
(1A2, Scheme 6).[65±67] The latter carbene may be considered a
one-center biradical, which was long believed to be an
intermediate. It is clear now,
(R)-15
16 (1A2)
(S)-15
Scheme 6. Racemization of cycloheptatetraene 15 via planar cyclohepta-
trienylidene 16.
however, that only the triplet is
low-lying and that all low-lying
singlets are transition structures.
In contrast to previous calcu-
lations, we find the biradical TS
H
H
H
H
H
H
H
H
H
H
H
1)
2)
3)
4)
H
19
7
higher
in
energy
4
1
(5.9 kcalmol
9.2 kcalmol
at BLYP or
at BCCD(T)
1
H
H
H
H
H
above
6
compared with
H
H
H
H
H
H
H
H
H
H
H
H
H
1
ꢀ2 kcalmol ); the racemiza-
tion, however, is still fast.[55, 56]
The difference between theory
18
24 (1A'')
25
and experiment of about
6
1
10 kcalmol
remains, and
more experiments on the cycli-
zation of 3 are needed. For the
isomerization of 6 to 4, differ-
ent pathways are possible
(Scheme 7):
1) direct [1,5] hydrogen shift
from the sp3-hybridized car-
bon to the allenic carbon via
transition structure 19.
2) [1,2] hydrogen shift from the
sp3-hybridized carbon via TS
18 leading to biradical inter-
mediate 24 and through a
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
17
21 (C1)
22 (C1)
23
H
H
H
H
H
H
20
Scheme 7. Possible isomerization paths of isobenzene 6 to benzene 4.
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P. R. Schreiner, H. Hopf et al.
FULL PAPER
path 4 more than 34 kcalmol 1 higher than for path 3. The first
of the other two pathways are too high in energy to play a role
in the hydrogen transfer. The highest barrier of the two-step
1
barrier of path 2 is only ꢀ5 kcalmol higher than in path 3,
1
1
1
but a second barrier of more than 26 kcalmol makes this
shift (BLYP: 44.1 kcalmol , BCCD(T): 46.0 kcalmol ) is
6 kcalmol 1 above the respective barrier of the parent system
and would require a higher reaction temperature. The
intermediate B21/B22 is again a problem (it is a biradical
B21 at BLYP but a carbene at BCCD(T)). The barrier
between B21/B22 and B4 is very low and cannot be
determined by a cyclization experiment. The overall exother-
micity is a little lower than that of the parent reaction
reaction mode likewise unfavorable.
The structure of intermediate 21/22 is difficult to character-
ize. The BLYP energies favor the biradical 21 over the
carbenoid structure 22, while the BCCD(T) data show that 22
is the ground state. Since the difference in energy is within the
computational error bars, a clear conclusion cannot be drawn.
1
As the singlet ± triplet separation is only 1.9 kcalmol , a
1
1
biradical description of the intermediate seems to be indi-
cated. The intermediate undergoes a second [1,2] hydrogen
shift leading to 4. As the barrier is quite small (BLYP:
4.4 kcalmol 1 above 21, BCCD(T): 0.4 kcalmol 1 above 22) it
has not been detected experimentally. The overall experi-
(ꢀ60 kcalmol vs. ꢀ67 kcalmol ) because of the aromatic
ring already present in B3. While 3 gains the full aromatiza-
tion energy on giving rise to 4, B3 only experiences partial
aromatization energy on building B4 from the already
aromatic B3 (Table 2).
mental exothermicity of the cycloisomerization of 3 to 4
1
Table 2. Results of the computed enthalpy values of the cycloisomeriza-
(
(
64.9 kcalmol ) again nicely agrees with the BLYP
tion of B3 (all enthalpies in kcalmol 1).
1
1
65.1 kcalmol ) and BCCD(T) ( 67.9 kcalmol ) results.
Structure
DRH298/DH=298
DRH298/DH=298
(BCCD(T)/cc-pVDZ)[a]
In summary, we could show that 3 undergoes a thermocyclic
(BLYP/6 ± 31G*)
reaction to one of the enantiomeric isobenzene forms 6, which
are interconnected via biradical TS 7. Structure 6 isomerizes
through two subsequent [1,2]-H shifts to benzene along path 3
via the intermediate 21/22. The comparison between exper-
imental and computational data shows that BLYP/6 ± 31G*
and BCCD(T)/cc-pVDZ describe the cycloisomerization of
the parent system under consideration quite well; except for
the 6/7 problem where more computations and experiments
are perhaps needed. The time-saving BLYP DFT method
does an especially good job, therefore it is sensible to use this
method on the large carbocyclic dienynes 11a ± h and their
cycloisomerization products, for which the more sophisticated
methods are too time-consuming.
B3
B14
B6
0.0
32.1
25.3
22.8
44.1
20.8
21.8
24.8
59.3
0.0
36.4
25.5
[b]
B7
±
B17
B21
B22
B23
B4
46.0
[b]
±
20.7
22.0
60.1
[a] Thermal corrections taken from the BLYP/6 ± 31G* computations.
[b] Owing to restricted computer power, the open-shell single points could
not be calculated.
Cyclic dienynes: By analogy to enediynes 9a ± f, the cyclic
dienynes 11a ± h should be greatly affected by their inherent
ring strain. As known experimentally, nine- and ten-mem-
bered carbocyclic enediynes 9c and 9d readily cyclize at room
temperature, while the parent enediyne needs elevated
temperatures (t1/2 ꢁ 1 h at 1558C). The reason is the reduction
in ring strain by forming the less-strained cyclization prod-
uct.[18, 21, 22] For similar reasons, the barriers for small cyclic
dienynes 11 should decrease and make the molecules cyclize
at lower temperatures than parent dienyne 3.
Benzannelated system: For the benzannelated system, exper-
imental values are not yet available and, therefore, accurate
predictions for the cyclization of B3 would be useful
1
(Scheme 8). The cyclization barrier (BLYP: 32.1 kcalmol ,
1
BCCD(T): 36.4 kcalmol ) is only slightly affected by the
The following examination deals with the E,Z-carbocyclic
dienynes 11a ± h and their cyclization paths that lead to
benzocycloalkenes 12a ± h because the E,Z species were
synthesized and experimentally investigated. Additionally,
the structures of Z,Z-11a ± h were computed for comparison.
While 11a and 11b are much more stable in the Z,Z- than in
Scheme 8. Benzannelated dienyne B3 and cyclization products B6 and
B7.
benzene ring and lies a little higher in energy than in the
parent system. On the other hand, the effect on the cyclization
product B6 or B7 (1A'') is large. In contrast to the parent
system, biradical B7 is not a TS but a stable structure and lies
1
the E,Z-conformation (38.2 and 16.1 kcalmol , respectively),
11c and 11d are only slightly more stable in the Z,Z-form (7.7
1
and 2.9 kcalmol , respectively). In 11e both isomers have
1
almost the same energy, while the E,Z-isomer is preferred by
the larger rings 11 f ± h.
below the the allenic structure B6 (2.5 kcalmol for BLYP).
One explanation may be that the benzylic p-electron of B7
can delocalize over a larger system, while B6 loses its
aromaticity during cyclization by forming a o-quinoide
system. Both structures B6 and B7 are higher in energy than
the parent system.
The E,Z-isomers 11a ± h can occur in the cis- or in the trans
form (Scheme 9), with a preference for the former by small
rings and the latter by large. The cyclization to 26a ± h can, of
course, only start from the cis-conformer. As calculations on
the 14-membered ring 11 h show, the trans conformation is
only 1.4 kcalmol 1 more stable than the cis-conformation, and
Further isomerization to naphthalene B4 also occurs
through two consecutive [1,2] hydrogen shifts. The barriers
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Chem. Eur. J. 2001, 7, No. 20

Cyclization of Cyclic 1,3-dien-5-ynes
4386±4394
regarded further. The eight-membered 11b can, in contrast to
11a, form a trans allene 26b, it is, however, highly strained as
1
well; this makes the barrier of 28.0 kcalmol
and an
1
endothermicity of 27.8 kcalmol quite reasonable. The bar-
rier for the first [1,2]-H shift is 37.0 kcalmol 1; this makes 11b
a little more reactive than the parent dienyne 3.
The nine-membered dienyne 11c has a cyclization barrier
Scheme 9. Structures 11a ± h in cis- and trans conformation.
1
of 20.2 kcalmol , and the highest H-shift barrier is
26.7 kcalmol 1; this renders 11c much more reactive than 3.
1
the barrier for interconversion is only 5.6 kcalmol . Since
11 h has the most stable trans configuration, all other rings
either prefer the cis-conformation or have lower barriers than
11 h. That is, the cis/trans isomerization before the actual
cyclization can be neglected kinetically.
1
Only the energy of the allene trans-26c (14.4 kcalmol ) is
comparable to that of 6 because of the additional ring strain of
a trans configured five-membered ring in 26c. The cyclization
1
barrier of ten-membered 11d (22.9 kcalmol ) is slightly
In general, the E,Z-isomers of 11a ± h initially give rise to
the allenes trans-26a ± h via transition structures 25a ± h, while
the Z,Z-isomers of 11a ± h always lead to cis-26a ± h. The two
diastereomeric forms of allenes 26a ± h are connected with
each other by biradical TS 27a ± h. The trans allenes 26a ± h
are then transformed through a [1,2]-H shift to intermediates
29a ± h and through a second [1,2]-H shift further to benzo-
cycloalkenes 12a ± h (Scheme 10). As the second [1,2]-H shift
has a much lower barrier than the first one, it is not important
kinetically and will therefore only be mentioned in Table 3.
higher than that of 11c, while the H-shift barrier is increased
to 34.3 kcalmol 1: still well below the barriers of 3. Allene
trans-26d is not much affected by additional ring strain
through the trans configuration, and is therefore lower in
1
energy than trans-26c. With an energy of 12.1 kcalmol ,
trans-26d is the lowest-lying of allenes 26. The eleven-
membered 11e has nearly the same energetics as the parent
1
system. With 28.1 and 39.5 kcalmol for the cyclization and
H-shift barriers, respectively, 11e is only slightly more
reactive than 3. All remaining cyclic dienynes 11 f ± h have
higher barriers and energies
than the parent system, and
are therefore less reactive.
A point worth mentioning is
that apart from 11a and 11b,
which suffer from ring strain in
the cyclization products, all bar-
riers and energies increase with
the enlargement of the ring, so
that the potential energy surfa-
ces do not cross each otherÐ
except, as already mentioned,
trans-26c (Figure 2).
Scheme 10. Cyclization of cyclic dienynes 11a ± h.
Experimental Results
Table 3. Results of the computed enthalpy values of the cycloisomerization of
11a ± h [Gibbs free enthalpies (DG298) in kcalmol 1].
As reported elsewhere, two of the authors successfully
synthesized a series of cyclo-1,3-dien-5-ynes 11d ± h.[68] These
compounds can be obtained in overall yields between 2 and
Ring size
Structure
8
b
9
c
10
d
11
e
12
f
13
g
14
h
Parent
System
11
25
trans-26
27
28
29
30
12
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
32.4
14.9
21.1
42.7
21.9
28.0
62.1
22% by following
a synthetic route with five steps
28.0
27.8
±
37.0
15.9
23.1
65.8
20.2
14.4
±
26.7
9.5
22.9
12.1
13.8
34.3
14.5
19.4
70.1
28.1
15.1
24.3
39.5
21.4
24.7
64.8
37.3
20.2
33.5
51.4
29.2
37.7
49.8
38.6
22.4
37.5
56.2
34.8
41.3
47.1
44.5
30.7
42.4
62.1
42.6
45.8
43.7
(Scheme 11); they are colorless, volatile liquids, which are
stable when stored under argon and cooled.
First experiments on the thermocyclization of 11d ± h have
been carried out. The starting materials were heated in
o-C6D4Cl2, and the thermocyclizations were monitored by
1H NMR spectroscopy. These first studies clearly showed a
correlation between the ring size of the cyclodienyne (and
therefore ring strain) and the cyclization temperature. Cyclo-
deca-1,3-dien-5-yne (11d) begins to cyclize at room temper-
ature; as expected the higher homologues require higher
cyclization temperatures (Table 4). For cyclotetradeca-1,3-
dien-5-yne, 11 h, no cycloaromatization occurred up to 2108C
(b.p. of dichlorobenzene).
13.0
76.7
The reactions of seven- and eight-membered dienynes 11a
and 11b are special cases and have to be considered
separately from the other carbocycles. Trans allene 26a
cannot be built from 11a because of the large strain associated
with the trans configured cyclopropane ring. For the same
reason, TS 27a cannot be reached. As the reaction to 12a
would only be possible from trans-26a this cyclization is not
Exploratory kinetic experiments of these cyclizations gave
preliminary rate constants and activation barriers. The
Chem. Eur. J. 2001, 7, No. 20
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4391

P. R. Schreiner, H. Hopf et al.
FULL PAPER
cyclic system 11a ± h, other hy-
drogen shifts can occur that are
not possible in the parent sys-
tem 3 because of the absence of
the aliphatic chain. In order to
give a more satisfactory explan-
ation, additional computational
and experimental work on this
problem must be carried out.
Conclusion
In the present work we exam-
ined the thermal cycloisomeri-
zation of parent and benzannel-
ated carbocyclic hexa-1,3-dien-
5-ynes by employing a density
functional approach and addi-
Figure 2. Potential energy surface of the cycloisomerization of 11c ± h to 12c ± h via TSs 26c ± h and 29c ± h.
Table 4. Minimal cyclization temperatures for 11d ± h.
tional coupled-cluster energy single points for the parent
system. The applicability of the BLYP DFT functional and of
the Brueckner doubles [BCCD(T)] approach is also shown to
work well for these systems.
Substance
Tmin [8C]
Cyclodeca-1,3-dien-5-yne
Cycloundeca-1,3-dien-5-yne
Cyclododeca-1,3-dien-5-yne
Cyclotrideca-1,3-dien-5-yne
Cyclotetradeca-1,3-dien-5-yne
11d
11e
11 f
11g
11 h
RT
100
150
traces at 2108C
no reaction up to 2108C
measurements were carried out in o-C6D4Cl2 and the product/
starting material ratio was calculated from the integrated
1H NMR spectra recorded at different times. Table 5 shows
the results obtained; as expected the data are consistent with a
first-order reaction (see Figure 3). It is important to note
again that the given values represent only approximate
results, and that further work has to be done in order to
allow a detailed discussion of the experimental results.
Table 5. Results of kinetic experiments for the thermocyclization of
11d ± f.
Figure 3. Kinetic diagram of the thermocyclization of 11 f.
1
Substance
Ttherm. [8C (K)]
106 k [s
]
t1/2 [h]
11d (CDCl3)
11e (o-C6D4Cl2)
11f (o-C6D4Cl2)
40 (313.15)
165 (438.15)
165 (438.15)
7.1 Æ 0.3
70 Æ 5
27.0 Æ 1.0
2.8 Æ 0.2
55.0 Æ 1.5
For the cycloisomerization of the parent system 3, both
BLYP and BCCD(T) reproduce the experimental values very
well. The initial step is a cyclization that forms a six-
membered intermediate. It was shown that the formerly
proposed biradical intermediate 7 is a transition structure that
connects the two enantiomeric isobenzenes 6, and that 6 is
indeed a true intermediate. Two consecutive [1,2]-H shifts
transform 6 into benzene 4; all other shifts, including the
direct [1,5]-H shift, have much higher barriers and can be
excluded.
3.5 Æ 0.1
The experimental results agree well with the computed
cyclization barriers. On the other hand, all computed [1,2]-
shift barriers are apparently too high to allow the formation of
benzocycloalkenes at temperatures lower than 2008C, in
contrast to our experimental findings; this problem has not
yet been fully solved. One possible explanation is that, in the
Scheme 11. Synthesis of cyclo-1,3-dien-5-ynes 11d ± h: a) (COCl)2, DMSO, CH2Cl2, NEt3, 608C; b) [1,3]Dioxolan-2-ylmethyltriphenylphosphonium
bromide, tBuKO, THF, RT; c) nBuLi, diethyl ether, 508C, DMF, 708C, d) TiCl3(DME)1.5 , Zn/Cu, DME, reflux.
4392
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Cyclization of Cyclic 1,3-dien-5-ynes
4386±4394
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reactions, we computed the cyclization energies of carbocyclic
1,3-dien-5-ynes with ring sizes from 7 to 14 carbons (11a ± h).
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Acknowledgments
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Received: March 12, 2001 [F3122]
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