Fluoro-ene reaction versus [2+2] cycloaddition in the thermal C 2-C6 cyclization of enyne-allenes: An experimental and theoretical investigation

Article abstract of DOI:10.1002/ejoc.201402438

A series of fluoroalkyl-substituted enyne-allenes have been synthesized by a new route with the aim of elucidating the possibility of a fluoro-ene reaction via an intermediate fulvenyl diradical generated in the thermal C ²-C⁶ (Schmittel) cyclization reaction. As a result of the strong C-F bond, fluorine atom transfer was not observed. Instead, 1H-cyclobuta[a]indenes were formed in good yields despite the high strain energy. DFT calculations at the B3LYP level of theory indicated that although the fluoro-ene reaction is the most exothermic reaction available, cyclobutene formation is kinetically favored over the [1,5]-fluorine shift by 5-15 kcal mol^-1 in various model compounds. Copyright

Full text of DOI:10.1002/ejoc.201402438

FULL PAPER
DOI: 10.1002/ejoc.201402438
Fluoro-Ene Reaction Versus [2+2] Cycloaddition in the Thermal C2–C6
Cyclization of Enyne-Allenes: An Experimental and Theoretical Investigation
M. Emin Cinar,[a] Chandrasekhar Vavilala,[a][†] Ralph Jaquet,*[a] Jan W. Bats,[b] and
Michael Schmittel*[a]
Keywords: Fluorine migration / Enynes / Allenes / Cyclization / Radicals / Density functional calculations
A series of fluoroalkyl-substituted enyne-allenes have been
synthesized by a new route with the aim of elucidating the
possibility of a fluoro-ene reaction via an intermediate ful-
venyl diradical generated in the thermal C2–C6 (Schmittel)
cyclization reaction. As a result of the strong C–F bond, fluor-
ine atom transfer was not observed. Instead, 1H-cyclobuta-
[a]indenes were formed in good yields despite the high strain
energy. DFT calculations at the B3LYP level of theory indi-
cated that although the fluoro-ene reaction is the most exo-
thermic reaction available, cyclobutene formation is kinet-
ically favored over the [1,5]-fluorine shift by 5–15 kcalmol–1
in various model compounds.
zation)[2] to the C2–C6 cyclization (later often denoted as
the Schmittel cyclization) in enyne-allenes upon replacing
Introduction
In 1995, Schmittel and co-workers[1] described a switch the hydrogen atom at the alkyne terminus by aryl or steri-
from the well-studied Myers–Saito cyclization (C2–C7 cycli- cally bulky groups (tBu, SiR3). The C2–C6 cyclization has
Scheme 1. Thermal C2–C7 (Myers–Saito) and C2–C6 (Schmittel) cyclization of enyne-allenes.
[a] Department Chemie-Biologie, Universität Siegen,
Adolf-Reichwein-Str., 57068 Siegen, Germany
been extensively studied, both mechanistically[3] and theore-
E-mail: schmittel@chemie.uni-siegen.de
tically,[4] in particular due to its fascinating use in the syn-
ralph.jaquet@uni-siegen.de
http://www.chemie-biologie.uni-siegen.de/oc/oc1/
thesis of complex carbocycles,[5] in photochemical applica-
[b] Institut für Organische Chemie und Chemische Biologie,
tions,[6] in nonstatistical dynamics,[7] and for DNA cleav-
Johann Wolfgang Goethe-Universität,
age.[8] Depending on the nature of the substituent at the
Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany
[†] Deceased 2010
terminal allene carbon, the C2–C6 cyclization develops
through different product channels. Whereas an alkyl group
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402438.
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Thermal C2–C6 Cyclization of Enyne-Allenes
leads to a rapid [1,5]-hydrogen shift to furnish benzo-
fulvenes in a formal ene reaction,[1a] an aryl group leads
to benzofluorenes through a formal Diels–Alder reac-
tion.[3b,8a] However, in the absence of either group, 1H-cy-
clobuta[a]indenes form in a stepwise [2+2] cycloaddition re-
action (Scheme 1).[9]
The first computational studies of the C2–C6 cyclization/
ene reaction at the (U)B3LYP/6-31G(d) level of theory led
to the proposal of a stepwise mechanism when a radical-
stabilizing group is present at the alkyne terminus and a
concerted process when H and tBu are present.[10] Subse-
quent kinetic isotope effect studies[11] and radical-trapping
experiments[12] supported the stepwise mechanism for en-
yne-allenes with at least one radical-stabilizing unit at either
the alkyne or allene terminus. Like the Myers–Saito cycliza-
tion, the Schmittel cyclization can supersede the usual di-
radical paradigm.[3k,13]
Figure 1. (a) σ,π-Diradical intermediate formed by the thermal C2–
C6 cyclization of enyne-allenes suggests a possible [1,5]-fluorine
shift. (b) Enyne-allenes EA1–EA5 selected for this study.
Results and Discussion
Synthesis and Thermolysis of Enyne-allenes
Sigmatropic rearrangements, particularly [1,5]-shifts, are
one of the most important reaction classes in organic chem-
istry, and in the past four decades, numerous experimental
and theoretical studies have been devoted to their under-
standing. Although the intramolecular [1,5]-migration of
alkyl, aryl, acyl, vinyl, alkoxy, ester, cyano, nitroso, and
other groups has been examined by a number of groups in
recent decades,[14] very few examples of [1,5]-halogen shifts
have been reported in the literature. A chlorine shift in the
dibenzotropylidine system has been described by Looker[15]
and another example, under photolytic conditions, was in-
vestigated later by using perchlorinated cycloheptatriene.[16]
Barlow et al. observed [1,3]- and [1,5]-fluorine shifts in the
pyrolysis of 1,2,3,4,5,6-hexafluorocyclohepta-1,3,5-triene at
450 °C,[17] and [1,5]-fluorine shifts in the pyrolysis of the
N,1-dichloroperfluoro(1,2-dihydro-2-naphthylideneamine)
at 570 °C and hexafluorobicyclo[2.2.0]hexa-2,5-diene at
450 °C.[18] The pyrolysis of polyfluorocyclohexadienes at
480 °C[19] and tetradecafluorotricyclo[6,2,2,02,7]dodeca-
2,6,9-triene at a temperature of 580 to 680 °C also led to
[1,5]-fluorine migration.[20] More recently, Lectka and
co-workers reported a [1,5]-fluoride ion shift in the o,oЈ-
diphenyl system at a moderate temperature (40 °C) through
the use of an aryl carbocationic system,[21] and [1,5]-fluor-
ine migration via excited-state molecules has been seen in
the photolysis of dodecafluorotricyclo[5.2.2.0]undeca-2,3,8-
triene to dodecafluorotricyclo[5.2.2.0]undeca-2,5,8-tri-
ene.[22]
As a first candidate for a [1,5]-fluorine shift, we chose to
prepare a perfluoro analogue of our earlier systems,[12a] for
example, EA1. The protio analogue of EA1 has been shown
to undergo a clean conversion to the corresponding ene
product at moderate temperatures.[12a] To synthesize EA1,
we adapted a convenient four-step protocol, the reaction
sequence of which is outlined in Scheme 2. Accordingly,
Sonogashira cross-coupling of o-iodobenzaldehyde with
TIPS-CϵCH afforded aldehyde 2.[23] The propargyl alcohol
3 was obtained in moderate yield by nucleophilic addition
of 1-perfluorohexyne to aldehyde 2 by using the Grignard
methodology.[24] The reaction of 3 with acetic anhydride at
room temperature in the presence of 4-(N,N-dimeth-
ylamino)pyridine (DMAP) provided propargyl acetate 4.[25]
Enyne-allene EA1 was finally obtained by the Pd-promoted
reaction of p-AnZnCl with 4 at –60 °C.[26] The1H, 13C, and
19F NMR, IR, and HRMS data convincingly supported the
structural assignment of EA1.
In this work we investigated the possibility and chances
of a [1,5]-fluorine shift in the thermal ene cyclization of
perfluoroalkyl-substituted enyne-allenes. We envisioned
that the putative diradical of the C2–C6 (Schmittel) cycliza-
tion should constitute a favorable intermediate for a [1,5]-
fluorine shift due to 1) weakening of the scissile C–F bond
by the adjacent radical center, 2) a vinyl radical center ready
to form a strong Csp2–F bond, and 3) a six-membered tran-
Scheme 2. Synthesis of EA1.
Thermolysis of enyne-allene EA1 in toluene at 110 °C
sition state. Therefore the specially designed enyne-allenes furnished the formal Diels–Alder product DA1 (Scheme 3).
EA1–EA5 (Figure 1) were prepared and subjected to ther- The 1H NMR spectrum shows a characteristic signal at δ =
mal conditions. Furthermore, we calculated the energy pro- 4.30 ppm corresponding to the fluorenyl CH2 group. The
files for both the concerted and stepwise mechanisms of the structure of DA1 was unambiguously determined by X-ray
fluoro-ene and [2+2] cycloaddition pathways.
crystallography (Figure 2). Thermolysis of EA1 at a higher
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temperature (250 °C) also provided only formal Diels– with atom C6 being 0.18 Å out of the plane through C7–
Alder products (as a mixture of DA1 and DA1Ј, Scheme 3) C8–C13–C14. Also, the central six-membered ring is non-
and not the desired fluoro-ene product.
planar with atoms C6 and C14 deviating in the same direc-
tion by 0.21 and 0.31 Å, respectively, from the plane
through C15–C16–C4–C5. The Si atom deviates by 0.37 Å
from the C14–C15–C16 plane due to steric effects. Two of
the methyl groups of the triisopropylsilyl group show rather
short repulsive intramolecular contacts with the benzo[b]-
fluorene core: d(C25–H···C16) = 2.54 Å, d(C25–H···C17) =
2.67 Å, d(C31–H···C13) = 2.65 Å, and d(C31–H···C14) =
2.68 Å. The crystal packing shows a number of weak inter-
molecular C–H···F interactions.
Scheme 3. Thermal transformation of EA1.
According to the X-ray data, the benzo[b]fluorene group
in DA1 is nonplanar (Figure 2), with only the two outer
six-membered rings being approximately planar. The angle
between the planes of these two rings is 23.4°. The five-
membered ring has an approximate envelope conformation
To explore the possibility of a [1,5]-fluorine shift further,
we selected enyne-allenes EA2–EA5, which are charac-
terized by perfluoroalkyl and tBu groups at the allene ter-
minus and a bulky subunit (Ar, tBu) at the alkyne terminus
(Scheme 4). Bulky substituents at this position are known
to steer the regioselectivity of enyne-allene cyclizations
towards the C2–C6 mode, and a formal Diels–Alder reac-
tion is precluded due to the absence of an aryl group at the
allene terminus.
Although thermolysis of EA2–EA5 at 150 °C led to the
anticipated C2–C6 cyclizations, no product from a fluorine
atom transfer (i.e., fluoro-ene product) was observed.
Rather, the intramolecular [2+2] cycloaddition dominated
to furnish 1H-cyclobuta[a]indenes CB2–CB5 (Scheme 5).
1
Their structures were confirmed by H, 13C, and 19F NMR
analysis, and the 19F-19F COSY spectra of CB2, CB4, and
CB5 clearly indicate the presence of an intact n-perfluoro-
butyl chain. Moreover, the structure of CB2 was unequivo-
cally established by X-ray analysis (Figure 3).
According to the X-ray analysis of CB2, the 1H-cyclo-
buta[a]indene unit is almost planar. Only atoms C11 and
C1 deviate from the plane by 0.04 and 0.10 Å, respectively.
The C1–C11 bond length of 1.607(8) Å is very long for a
Figure 2. X-ray structure of benzofluorene DA1.
Scheme 4. Synthesis of EA2–EA5.
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Thermal C2–C6 Cyclization of Enyne-Allenes
Scheme 5. Thermolysis of EA2–EA5.
Scheme 6. Synthesis of the protio analogue of EA2 (EA2Ј).
Scheme 7. Thermolysis of EA2Ј as the protio analogue of EA2.
reaction would have been possible considering the substitu-
ents, thus suggesting that out of all the possible pathways,
the fluoro-ene reaction is the least likely.
Computations
Owing to the successful radical trapping in earlier C2–
C6 cyclizations of enyne-allenes,[12] we proposed a stepwise
mechanism for when a radical-stabilizing group is attached
Figure 3. X-ray structure of 1H-cyclobuta[a]indene CB2.
single bond between sp2 and sp3 carbon atoms, presumably to the allene or alkyne terminus.[12] As established earlier,[3d]
due to the bulky substituents at C1 and C11. A long C– however, a stepwise mechanism would involve a diradical
C bond has also been observed in other substituted 1H- with the radical centers mainly located at C1 and C7, which
cyclobuta[a]indenes.[3f] The large displacement parameters should greatly facilitate a [1,5]-fluorine or hydrogen shift.
at the terminal end of the nonafluorobutyl side-chain shows Although hydrogen transfer has been demonstrated in nu-
this group to be disordered. 1H-Cyclobuta[a]indenes as merous experimental (see also above for EA2Ј) and compu-
products of thermal enyne-allene cyclizations have been de- tational studies, the reluctance of the systems EA1–EA5 to
scribed earlier by Gillmann[9a] and Krause,[27] and later by undergo any [1,5]-fluorine shift required further insight.
Wang and our group.[3f,3g,3i,3j,9b]
Hence, we decided to calculate the energy profiles of the
At a higher temperature (250 °C), thermolysis of EA2 fluoro-ene reaction of enyne-allenes (EAn) to benzofulvenes
and EA5 also only provided 1H-cyclobuta[a]indenes. How- (BFn; Scheme 7) and of the competing [2+2] cycloaddition
ever, after heating CB2 at 300 °C, most of this compound to 1H-cyclobuta[a]indenes (CBn; Scheme 5). As in the ex-
was recovered, and we observed no fluoro-ene product. To perimental systems, we equipped our computed model sys-
investigate whether the protio analogue of EA2 would also tems with a phenyl group at the alkyne terminus as a radi-
follow the [2+2] cycloaddition route, EA2Ј was prepared cal-stabilizing group.
similarly to its perfluoro analogue (Scheme 6).
The structures of the reactants, transition states, radical
The thermolysis of enyne-allene EA2Ј in toluene fur- intermediates, and products were fully optimized by using
nished only the benzofulvene BF2 by ene cyclization the B3LYP functional with the 6-31(d) basis set, partly in
(Scheme 7). The 1H NMR spectrum of BF2 exhibits charac- the restricted (R) and unrestricted (U) versions. The calcu-
teristic signals at δ = 5.61 (t, 1 H), 6.39 (s, 1 H), and lations were performed by using the GAUSSIAN (G98/03)
[36]
6.98 ppm (s, 1 H), which correspond to Hb, Ha, and Hc,
respectively.
and TURBOMOLE (TM)[37] program packages and a
part of the discussion will focus on the results of the DFT
These experimental results strongly indicate that the bar- calculations. Owing to the problem of the spin contami-
riers for the stepwise hydrogen-ene reaction and the Diels– nation of several structures (mostly transition states), se-
Alder process are lower than those of the fluoro-ene and lected transition-state structures were subjected to 2-elec-
[2+2] cycloaddition reactions. [2+2] Cycloadducts were tron/2-orbital CASSCF investigations to quantify the mag-
formed from EA2–EA5 exclusively, although a fluoro-ene nitude of the energy barrier.
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For our computational work, we chose the experimen-
tally studied enyne-allene EA2 (C4F9 side-chain) and a sim-
plified variant thereof, EA2a (CF3 side-chain; Figure 4).
Unfortunately, we were not able to locate the exact post-
diradical transition-state structures 9 and 10 at the (U)
B3LYP level for EA2 due to strong spin contamination and
a large degree of conformational freedom in the molecule.
To understand these problems, we calculated the relevant
part of the potential energy surfaces of EA2a with regard
to the fluoro-ene reaction that yields BF2a (see Figure S1
in the Supporting Information) and the formation of CB2a
(see Figure S2). Moreover, we looked at the simplified
model compounds EA6 and EA6a lacking benzannulation
Figure 4. Enyne-allenes subjected to computational studies.
with residues H and CF3 or H and CFH2 at the allene ter- level, the TS(D)s for EA2 and EA2a are Ͼ29 (not con-
minus, while maintaining a Ph group at the alkyne terminus. verged) and 29.2 kcalmol–1, respectively. Starting from D2
Furthermore, for comparison, the parent enyne-allene EA7 and D2a we were unable to locate the transition states for
(no benzannulation; CH3, H at allene and CH3 at alkyne BF2, BF2a and CB2 and CB2a. The height of the TS struc-
terminus) was subjected to computations to evaluate the ture 9a was estimated from CASSCF calculations fully opti-
potential surface of the concerted and stepwise [2+2] cyclo- mized along the reaction path (F shift). In the case of D2a,
addition pathways (see Figure S5).
the barrier for the formation of the fluoro-ene product is
The computed diradical pathway for the thermal reaction substantially higher (by 14.5 kcalmol–1) than that for the
of EA2 and EA2a is outlined in Scheme 8. Starting from [2+2] cycloaddition.
EA2 and EA2a we were able to locate the transition states
We were able to locate all the transition-state structures
8 and 8a for diradical formation, but our attempts to locate for EA6 and EA6a. Similarly to EA2a, the barrier for the
the TS for the concerted fluoro-ene reaction failed, which fluoro-ene reaction is higher than that of the [2+2] cycload-
is indicative of a high-order saddle point. At the (U)B3LYP dition. For comparison, we examined the energy profile of
Scheme 8. Computed pathways for the cyclization of EA2 and EA2a at the (R)B3LYP/6-31G(d) [results obtained at the (U)B3LYP/6-
31G* level are given in parentheses] level of theory. Compound 9a was estimated at the CASSCF level (energies in kcalmol–1).
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Thermal C2–C6 Cyclization of Enyne-Allenes
the hydrogen-ene process and the [2+2] cycloaddition for (29.2 kcalmol–1).[30] In contrast, the barrier for the
enyne-allene EA7. The formation of the Schmittel diradical hydrogen shift was calculated to be 56.1 kcalmol–1 at both
intermediate from EA7 requires 2–3 kcalmol–1 more energy levels of theory.
than from EA2–EA6, which possess a σ-C–F acceptor prox-
To the best of our knowledge, there is no theoretical
imate to the allene terminus, possibly providing some hyper- study of [1,5]-fluorine shifts reported in the literature. As
conjugative assistance.[28] Concerning the subsequent EA2 did not yield the fluoro-ene but the [2+2] cycload-
reaction of diradical D7 with
a relative energy of dition product upon thermolysis, we were particularly inter-
13.5 kcalmol–1, the barrier for the [1,5]-hydrogen shift, for ested in the activation barrier for the [1,5]-fluorine shift
which the TS has a relative energy of 15.3 kcalmol–1, is ex- from the diradical. Unfortunately, we were unable to locate
tremely low (1.8 kcalmol–1) compared with that of cyclobu- the exact transition structures 9 at the single determinant
tene formation with a TS relative energy of 29.9 kcalmol–1 B3LYP level for both EA2 and EA2a because of varying
(16.4 kcalmol–1; Table 1).
competition between closed- and open-shell electronic con-
figurations before and after the TS. As an alternative, we
determined the full energy hypersurface (neglecting the spin
contamination problems around the TS 9) of the fluoro-ene
reaction of EA2a at the (U)BP86/SV(P) level of theory.
From this it becomes evident that the stepwise process is
favored by far over a concerted fluoro-ene mechanism (see
Figure S1 in the Supporting Information). Formation of the
cyclized product BF2 is approximately 20 kcalmol–1 more
exothermic than the formation of CB2. As thermolysis of
enyne-allene EA2 at high temperatures (250 or 300 °C), and
also for longer periods of time, did only provide CB2 as an
isolable product, we have to assume that the CB2Ǟ D2Ǟ
BF2 route is also precluded.
Table 1. Calculated energies for the enyne-allene cyclization deter-
mined at the B3LYP/6-31G* (UB3LYP/6-31G*) level of theory and
energies are given with respect to the energy of enyne-allene.
Comp.
Erel [kcalmol–1
]
TS(D)
Diradical TS(ene)
TS(2+2)
BF
CB
EA2
–
(Ͼ29[a]
–
(29.2)
28.5
29.5
31.2
–
–
–
–31.8
(–31.4)
–24.0
(–24.1)
–27.8
–10.0
(–10.0)
–17.1
(–17.5)
–19.2
)
(31.4)
28.5
(28.4)
17.3
13.6
13.5
EA2a
47.5
(–)
39.4
33.7
15.3
33.0
EA6
EA6a
EA7
33.5
33.3
29.9
–42.9
–41.1
–15.2
–16.6
[a] Not converged.
The computed activation energies (E‡) for the formation
The above results demonstrate both experimentally and of diradicals D6 and D6a are 28.5 and 29.5 kcalmol–1,
computationally that even in an optimized setting, as real- respectively (Scheme 9). Interestingly, the replacement of
ized in diradicals D1–D6, a [1,5]-fluorine shift requires such two fluorine atoms by hydrogens has an opposite effect on
a high barrier that otherwise slow reactions occur. With ref- the kinetics and thermodynamics of diradical formation; al-
erence to the diradical, the barrier for a [1,5]-fluorine shift though the barrier is higher by around 1 kcalmol–1, the di-
is quite large (18–22 kcalmol–1) compared with that of a radical D6a is approximately 3.7 kcalmol–1 more stable
hydrogen shift (1.8 kcalmol–1 in EA7), whereas cyclobutene than D6. For the formation of the two products, BF6/BF6a
formation shows a barrier of 16.4 kcalmol–1 (29.9 kcalmol–1 versus CB6/CB6a, the barriers of the reactions of diradicals
with respect to EA7). This explains why two structurally D6/D6a are important. The barriers relative to the energy
rather similar enyne-allenes, that is, EA2 and EA2Ј, furnish of the diradicals are 22.1 and 20.1 kcalmol–1 for the forma-
upon thermolysis two constitutionally isomeric products, tion of BF6 and BF6a, and 16.2 and 19.7 kcalmol–1 for
that is, 1H-cyclobuta[a]indene CB2 in a stepwise [2+2] cy- CB6 and CB6a, respectively.
cloaddition[9,27] and benzofulvene BF2Ј[3a,8a] by a stepwise
Again, a closer look at all the stationary points along the
ene reaction, respectively.
[2+2] cycloaddition energy hypersurface (see Figure S2 in
The much higher barrier for the [1,5]-fluorine than for the Supporting Information), except for the TS of the con-
the [1,5]-hydrogen shift is in good agreement with results certed process, was achieved through a computational study
from related processes. The high barrier for a fluorine shift of EA6 and EA6a at the B3LYP level of theory so that a
is mainly attributed to its high bond strengths. North and comparison with the fluoro-ene reaction channel becomes
co-workers calculated C–H and C–X (X = F, Cl, Br) bond possible. The barrier for the formation of the [2+2] cycload-
dissociation energies (BDEs) in haloalkanes, who deter- duct from D6 is less than that for the fluoro-ene reaction
mined a C–F bond dissociation energy of 119.3 kcalmol–1 by 5.9 kcalmol–1, despite the fact that the fluoro-ene prod-
in CH2F–F at the MP2/cc-pVtz//MP2/6-311+G* level of uct BF6 is more stable than the cyclobutene CB6 (system-
theory, whereas the BDE(C–H) in CH3–H is atic name: bicyclo[3.2.0]hepta-1,3,5-trienes). In contrast,
102.8 kcalmol–1.[29] Although reports on fluorine shifts in- starting from diradical D6a, the transition states for both
volving intermediate carbon radicals are rare, a few [1,2]- reaction channels exhibit almost identical energies. This is
fluorine shifts are known.[30,31] Interestingly, in the case of mainly due to a lowering of the energy of TS 12a compared
[1,2]-shifts, the fluorine shift is much more facile than that with 12 by almost 6 kcalmol–1, whereas the TS energies of
of hydrogen, as demonstrated experimentally.[31,32] This is 13 and 13a are essentially the same (Scheme 9). It seems
also reflected in computational studies on the [1,2]-fluorine likely that the lowering of the relative energy of 12a is a
shift. The semi-empirical calculations of Kotaka et al.[32] result of thermodynamic contributions in the TS as the pro-
suggested a barrier ΔE‡ of 25.9 kcalmol–1, similar to that cess to BF6a is much more exothermic than that leading to
obtained from DFT calculations by Dolbier and co-workers BF6.
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[1,5]-hydrogen shift, followed by the formal Diels–Alder
ring closure, then the [2+2] cycloaddition, and finally the
fluoro-ene reaction pathway. In addition, the product aris-
ing from the ene reaction (e.g., BF7) is also thermodynami-
cally more stable than the [2+2] cycloaddition product (e.g.,
CB7). Another factor controlling the subsequent reaction
of D has not been addressed much, namely the conforma-
tion of D. Of all the four pathways, only ring closure to the
[2+2] cycloadduct is accessible in any possible conformation
of the diradical. This should provide some entropic advan-
tage to the formation of cyclobutene over the fluoro-ene
process, and would have to be considered if the two pro-
cesses are enthalpically similar, as may be the case for
EA6a.
Conclusions
This study has sought to utilize the C2–C6 diradical inter-
mediate as a suitable starting point to realizing the hitherto
unknown [1,5]-fluorine shift. We had hypothesized that the
diradical intermediate D should clearly enhance the likeli-
hood of a [1,5]-fluorine shift as compared with those arising
from monoradical or concerted processes. Experimental
and computational results have led to the conclusion that
the fluoro-ene is an inaccessible, high-energy process. In-
stead, an alternative pathway, the stepwise [2+2] cycload-
dition, becomes the dominating reaction channel giving a
high yield of product. To realize the conceived fluorine
shift, one would need to design a system with a very high
barrier for the [2+2] cycloaddition due to, for example, large
steric impediments that would still allow the fluoro-ene
pathway to occur.
Scheme 9. Computed pathways for the cyclization of EA6 and
EA6a at the B3LYP level of theory (energies in kcalmol–1).
For EA7, the geometrical changes and activation barriers
associated with the concerted and stepwise hydrogen-ene re-
actions are quite similar.[12] In addition, we have roughly
calculated the relevant part of the PES for the [2+2] cyclo-
addition (see Figure S5 in the Supporting Information).
Analogously to EA6a, the stepwise [2+2] mechanism is
much more favorable than the concerted process although
a radical-stabilizing substituent is now missing. Hence, un-
like the hydrogen-ene reaction that exhibits both stepwise
and concerted reaction scenarios, the [2+2] cycloaddition
reaction always seems to proceed by a stepwise mechanism
involving a diradical intermediate independent of the ter-
minal substituents.
Moreover, the computations unambiguously suggest that
the [2+2] cycloadducts and [1,5]-fluoro-ene products arise,
in all examples, from the diradical intermediate and not
from concerted processes. Further investigations of other
migrations in this special setting are underway in our
laboratories.
Experimental Section
NMR spectra were recorded with a Bruker AC 400 spectrometer.
Chemical shifts are reported in δ (ppm) relative to TMS. Owing to
the complex splitting patterns and low intensity, all the carbon
atoms of the C4F9 side-chains have been excluded from the 13C
NMR data reported herein, but are given in the Supporting Infor-
mation. IR spectra were recorded with a Perkin–Elmer 1750 FT-
For all the enyne-allenes, the fluoro-ene reaction starting
from D exhibits a higher barrier than that for the [2+2]
cycloaddition. For an enyne-allene with a TIPS substituent
at the alkyne terminus and n-butyl and triarylamine groups IR spectrometer. Anhydrous THF and diethyl ether were distilled
from sodium/benzophenone. Dichloromethane and triethylamine
were freshly distilled from calcium hydride under N2. Anhydrous
toluene was distilled from potassium. Commercial reagents were
used as received. Analytical TLC was carried out on Merck silica
gel 60 F254 TLC plates. Column chromatography was performed
on Merck silica gel 63–200 μm. All solvents were distilled prior to
use for column chromatography. 3,3,4,4,5,5,6,6,6-Nonafluorohex-
1-yne,[34] 2-(2-phenylethynyl)benzaldehyde,[24] 2-[2-(p-tolyl)ethynyl]-
benzaldehyde,[1b] 2-(3,3-dimethylbut-1-ynyl)benzaldehyde,[35] and 1-
[2-(2-phenylethynyl)phenyl]pentan-1-ol[8b] were synthesized accord-
at the allene terminus, the barrier for the Diels–Alder clo-
sure from diradical D has been calculated to be around
5 kcalmol–1.[33] This is substantially lower than the barrier
for the fluorine shifts (20.1–22.1 kcalmol–1) or [2+2] cyclo-
additions (16.2–19.7 kcalmol–1) computed for D6 and D6a.
Thus, an aryl group at the allene terminus precludes any
high-barrier processes, such as the fluoro-ene reaction or
[2+2] cycloaddition due to the low barrier of the Diels–
Alder process. Among the competing reaction channels of
diradicals D, there is hence a clear kinetic preference for the ing to literature procedures.
5172
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 5166–5177

Thermal C2–C6 Cyclization of Enyne-Allenes
4,4,5,5,6,6,7,7,7-Nonafluoro-1-{2-[2-(triisopropylsilyl)ethynyl]phen-
yl}hept-2-yn-1-ol (3): A solution of ethylmagnesium bromide [pre-
pared from ethyl bromide (1.06 g, 8.74 mmol) and magnesium turn-
ings (212 mg, 8.74 mmol) in dry THF (20 mL)] was added to a
solution of perfluorohex-1-yne (2.22 g, 8.74 mmol) in dry THF
(10 mL) under nitrogen at 0 °C. After stirring for 4 h at room tem-
perature, a solution of 2 (1.00 g, 3.49 mmol) in dry THF (10 mL)
was added dropwise. After stirring for 3 h, the reaction mixture was
quenched with a saturated ammonium chloride solution (25 mL).
The aqueous layer was extracted with diethyl ether (2ϫ 25 mL)
and the combined organic layers were dried with Na2SO4, and con-
centrated. After purification by column chromatography (silica gel,
n-pentane/diethyl ether = 9:1, Rf = 0.59), compound 3 was isolated
(400 MHz, C6D6): δ = 1.15 (d, J = 7.1 Hz, 21 H), 3.22 (s, 3 H),
6.62 (d, J = 8.7 Hz, 2 H), 6.78 (t, J = 7.5 Hz, 1 H), 6.89 (t, J =
7.5 Hz, 1 H), 7.34–7.45 (m, 3 H), 7.55 (d, J = 7.8 Hz, 1 H), 7.83
(t, J = 4.0 Hz, 1 H) ppm. 13C NMR (100 MHz, C6D6): δ = 11.6,
18.8, 54.7, 97.3, 99.7, 104.6, 105.2, 114.6, 122.2, 122.9, 127.1, 128.5,
129.3, 130.0, 133.4, 133.6, 160.5, 210.1 ppm. 19F NMR (376 MHz,
CDCl3): δ = –126.2 (2 F), –121.7 (2 F), –105.7 (2 F), –81.6 (3
F) ppm. HRMS (EI): calcd. for C27H33OSi 401.230 [M – C4F9]+;
found 401.230.
Triisopropyl[7-methoxy-10-(perfluorobutyl)-11H-benzo[b]fluoren-5-
yl]silane (DA1): Enyne-allene EA1 (30.0 mg, 48.5 μmol) was dis-
solved in dry toluene (30 mL), degassed by three freeze–thaw cycles,
and heated at 110 °C in a sealed tube for 12 h. After removal of
toluene under reduced pressure and purification by chromatog-
raphy (preparative TLC, silica gel 60 F254, n-pentane, Rf = 0.72),
as a colorless oil (1.24 g, 67%). IR (neat): ν = 3440, 3064, 2943,
˜
2891, 2251, 2153, 1701, 1650, 1593, 1463, 1352, 1235, 1135, 1075,
1027, 995, 883 cm–1. 1H NMR (400 MHz, CDCl3): δ = 1.17 (m, 21
H), 3.14 (d, J = 6.3 Hz, 1 H), 6.01 (dt, J = 6.3, 4.3 Hz, 1 H), 7.34
(td, J = 7.5, 1.4 Hz, 1 H), 7.40 (td, J = 7.6, 1.5 Hz, 1 H), 7.56 (dd,
J = 7.6, 1.3 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 11.2,
18.6, 62.9, 72.0, 91.8, 98.3, 103.3, 121.6, 126.4, 128.9, 129.2, 133.4,
139.8 ppm. 19F NMR (376 MHz, CDCl3): δ = –126.4 (2 F), –124.5
(2 F), –109.3 (1 F), –99.4 (1 F), –81.6 (t, 3 F) ppm. MS (EI, 70 eV):
m/z = 530.1 [M]+.
DA1 was isolated (27 mg, 90%). IR (neat): ν = 2954, 2868, 1619,
˜
1512, 1464, 1384, 1340, 1233, 1131, 1038, 883, 823, 733 cm–1. 1H
NMR (400 MHz, CDCl3): δ = 1.08 (d, J = 7.5 Hz, 18 H), 1.98
(sept, J = 7.5 Hz, 3 H), 3.98 (s, 3 H), 4.30 (t, J = 3.8 Hz, 2 H), 7.19
(dd, J = 9.5, 2.7 Hz, 1 H), 7.32–7.38 (m, 2 H), 7.51–7.56 (m, 1 H),
7.58 (d, J = 2.7 Hz, 1 H), 7.95 (m, 1 H), 8.12 (d, J = 9.5 Hz, 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 14.6, 20.1, 37.7, 55.5,
109.5, 110.1, 117.2, 123.5, 124.5, 125.3, 125.5, 126.3, 127.9, 135.4,
141.5, 141.8, 142.0, 144.0, 151.0, 155.6 ppm. 19F NMR (376 MHz,
CDCl3): δ = –126.4 (2 F), –121.0 (2 F), –101.8 (2 F), –81.4
(3 F) ppm. HRMS (EI): calcd. for C28H26F9OSi 577.161 [M –
C3H7]+; found 577.160.
4,4,5,5,6,6,7,7,7-Nonafluoro-1-{2-[2-(triisopropylsilyl)ethynyl]phen-
yl}hept-2-ynyl Acetate (4): DMAP (23 mg, 0.18 mmol), dry triethyl-
amine (380 mg, 3.76 mmol), and acetic anhydride (212 mg,
2.07 mmol) were added to a solution of 3 (1.00 g, 1.88 mmol) in
dry dichloromethane (20 mL). After stirring at room temperature
for 12 h, the reaction mixture was washed with water. The aqueous
layer was extracted with dichloromethane (2 ϫ 25 mL) and the
combined organic layers were washed with water and brine, dried
with Na2SO4, and concentrated under atmospheric pressure.
Attempts to purify 4 by chromatography on silica gel led to decom-
position. Thus, compound 4 was further used without any purifica-
Triisopropyl[7-methoxy-10-(perfluorobutyl)-11H-benzo[b]fluoren-11-
yl]silane (DA1Ј): Enyne-allene EA1 (30.0 mg, 48.5 μmol) was dis-
solved in dry toluene (30 mL), degassed by three freeze–thaw cycles,
and heated at 250 °C in a sealed tube (volume 100 mL) for 7 h.
After removal of toluene under reduced pressure and purification
by chromatography (preparative TLC, silica gel 60 F254, n-pentane,
Rf = 0.67), compounds DA1 and DA1Ј were isolated (DA1: 14 mg,
tion. IR (neat): ν = 3072, 2946, 2894, 2262, 2156, 1759, 1464, 1371,
˜
1235, 1137, 1019, 997, 881 cm–1. 1H NMR (400 MHz, CDCl3): δ =
1.16 (br. s, 21 H), 2.14 (s, 3 H), 6.98 (t, J = 4.3 Hz, 1 H), 7.37 (td,
J = 7.5, 1.5 Hz, 1 H), 7.42 (td, J = 7.6, 1.6 Hz, 1 H), 7.56 (dd, J =
7.5, 1.3 Hz, 1 H), 7.63 (dd, J = 7.7, 1.4 Hz, 1 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 11.2, 18.5, 20.5, 62.7, 72.2, 89.4, 98.0,
102.6, 122.9, 127.2, 129.0, 129.4, 133.2, 135.9, 168.7 ppm. 19F
NMR (376 MHz, CDCl3): δ = –126.2 (2 F), –124.0 (2 F), –99.5 (2
F), –81.8 (3 F) ppm. HRMS (EI): calcd. for C23H22F9O2Si 529.125
[M– C3H7]+; found 529.124.
47%; DA1Ј: 9 mg, 30%). DA1Ј: IR (neat): ν = 2943, 2865, 1631,
˜
1444, 1382, 1297, 1122, 1076, 845 cm–1 1H NMR (400 MHz,
.
CDCl3): δ = 0.72 (d, J = 7.5 Hz, 9 H), 0.78 (dd, J = 7.4, 1.8 Hz, 9
H), 1.12–1.18 (m, 3 H), 3.96 (s, 3 H), 4.63 (d, J = 3.8 Hz, 0.5 H),
4.66 (d, J = 3.8 Hz, 0.5 H), 7.20 (dd, J = 9.5, 2.8 Hz, 1 H), 7.28
(d, J = 2.7 Hz, 1 H), 7.31–7.37 (m, 2 H), 7.56 (d, J = 6.6 Hz, 1 H),
7.90 (d, J = 6.7 Hz, 1 H), 8.18 (d, J = 9.6 Hz, 1 H), 8.34 (s, 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 12.4, 18.5, 18.6, 18.9,
41.8, 41.9, 55.2, 106.8, 119.0, 119.1, 120.2, 121.7, 124.5, 125.2,
125.3, 125.4, 126.5, 126.6, 127.2, 134.4, 138.2, 140.4, 147.1,
156.9 ppm. 19F NMR (376 MHz, CDCl3): δ = –126.4 (2 F), –121.2
(2 F), –100.0 (1 F), –97.3 (1 F), –81.4 (3 F) ppm. HRMS (EI):
calcd. for C31H33AgF9OSi 727.121 [M + Ag]+; found 727.121.
(2-{2-[4,4,5,5,6,6,7,7,7-Nonafluoro-3-(4-methoxyphenyl)hepta-1,2-
dienyl]phenyl}ethynyl)triisopropylsilane (EA1): A p-anisylmagnes-
ium bromide solution [prepared from Mg (85.0 mg, 3.52 mmol)
and p-bromoanisole (658 mg, 3.52 mmol)] in dry THF (10 mL) was
added dropwise to a 1 m solution of ZnCl2 (915 mg in 3.5 mL of
dry diethyl ether) at room temperature under nitrogen. The reac-
tion mixture was stirred for 30 min at room temperature. Then it
2-[2-(Naphthalen-1-yl)ethynyl]benzaldehyde (5b): [PdCl2(Ph3P)2]
(60.7 mg, 86.5 μmol) and CuI (82.3 mg, 432 μmol) were added to
a mixture of o-iodobenzaldehyde (1; 2.00 g, 8.54 mmol) and 1-eth-
was cooled to –60 °C and [Pd(PPh3)4] (50 mg, 440 μmol) in dry ynylnaphthalene (960 mg, 9.40 mmol) in triethylamine (50 mL).
THF (5 mL) was added dropwise. After stirring for 15 min at this
temperature, propargyl acetate 4 (500 mg, 880 μmol) in dry THF
(10 mL) was added dropwise. After stirring for 16 h at room tem-
perature, the reaction mixture was quenched with a saturated am-
monium chloride solution. The aqueous layer was extracted with
diethyl ether (2ϫ 25 mL) and the combined organic layers were
dried with Na2SO4 and concentrated under reduced pressure. After
purification by column chromatography (silica gel, n-pentane, Rf =
0.55), compound EA1 was isolated as a colorless oil (310 mg, 57%
After stirring for 14 h at room temperature, the reaction mixture
was quenched with an aqueous saturated ammonium chloride solu-
tion. The aqueous layer was washed with pentane (2ϫ 50 mL) and
the combined organic layers were dried with sodium sulfate and
evaporated under reduced pressure. The product was purified by
column chromatography (silica gel, n-hexane/diethyl ether = 99:1,
R = 0.48) to yield 5b (1.91 g, 86%). IR (neat): ν = 3058, 2952,
˜
f
2840, 2745, 2258, 1696, 1586, 1485, 1397, 1235, 1136, 869,
774 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.45–7.49 (m, 2 H),
7.54–7.65 (m, 3 H), 7.75 (d, J = 7.7 Hz, 1 H), 7.81 (dd, J = 7.1,
over two steps). IR (neat): ν = 2944, 2866, 2153, 1948, 1608, 1513,
˜
1
1486, 1463, 1385, 1297, 1135, 1035, 883 (m), 832 cm–1. H NMR 0.9 Hz, 1 H), 7.88 (d, J = 8.2 Hz, 2 H), 7.99 (dd, J = 7.8, 1.4 Hz,
Eur. J. Org. Chem. 2014, 5166–5177
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5173

M. E. Cinar, C. Vavilala, R. Jaquet, J. W. Bats, M. Schmittel
1 H), 8.41 (d, J = 8.3 Hz, 1 H), 10.8 (s, 1 H) ppm. 13C NMR After 1 h, the resulting solution was diluted with water (50 mL)
FULL PAPER
(100 MHz, CDCl3): δ = 89.7, 94.5, 119.9, 125.2, 125.8, 126.6, 127.1,
127.4, 128.4, 128.6, 129.5, 130.9, 133.0, 133.1, 133.4, 133.7, 135.7,
153.9, 191.5 ppm. HRMS (EI): calcd. for C19H12O 256.089 [M]+;
found 256.088.
and extracted with diethyl ether (2ϫ 50 mL). The combined or-
ganic layers were dried and concentrated under reduced pressure.
After purification by column chromatography (silica gel, n-hexane/
diethyl ether = 99:1, Rf = 0.84), propargyl benzoate 7a was isolated
as colorless oil (950 mg, 77%). IR (neat): ν = 3056, 2930, 2872,
˜
4,4,5,5,6,6,7,7,7-Nonafluoro-1-[2-(2-phenylethynyl)phenyl]hept-2-yn-
1-ol (6a): Compound 6a was prepared following the procedure used
2262, 2218, 1729, 1602, 1495, 1452, 1351, 1316, 1239, 1179, 1090,
1
1026, 1008, 942, 874 cm–1. H NMR (400 MHz, CDCl3): δ = 7.29
for the synthesis of 3, yield 55%. IR (neat): ν = 3064, 2946, 2258,
˜
(t, J = 4.3 Hz, 1 H), 7.32–7.35 (m, 3 H), 7.40–7.47 (m, 4 H), 7.48–
7.52 (m, 2 H), 7.57 (t, J = 7.56 Hz, 1 H), 7.62–7.65 (m, 1 H), 7.75
(dd, J = 6.6, 1.8 Hz, 1 H), 8.06 (d, J = 6.2 Hz, 2 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 63.4, 72.8, 85.4, 89.0, 95.9, 123.0, 127.7,
128.2, 128.3, 128.5, 128.7, 128.9, 129.6, 129.7, 130.0, 131.6, 132.7,
133.6, 135.6, 164.8 ppm. 19F NMR (376 MHz, CDCl3): δ = –125.7
(2 F), –120.7 (2 F), –99.2 (2 F), –81.7 (3 F) ppm. HRMS (EI):
calcd. for C28H15F9O2 554.093 [M]+; found 554.093.
1
2218, 1602, 1494, 1354, 1235, 1138, 1079, 1027, 915, 873 cm–1. H
NMR (400 MHz, CDCl3): δ = 3.56 (d, J = 5.9 Hz, 1 H), 6.07 (d,
J = 5.1 Hz, 1 H), 7.36–7.44 (m, 5 H), 7.57–7.67 (m, 4 H) ppm. 13
C
NMR (100 MHz, CDCl3): δ = 62.7, 71.8, 85.7, 92.1, 95.7, 121.6,
122.4, 126.5, 128.4, 128.9, 129.0, 129.1, 131.5, 132.7, 139.3 ppm.
19F NMR (376 MHz, CDCl3): δ = –126.1 (2 F), –123.7 (2 F), –98.6
(2 F), –81.7 (3 F) ppm. HRMS (EI): calcd. for C21H11F9O 450.067
[M]+; found 450.066.
4,4,5,5,6,6,7,7,7-Nonafluoro-1-{2-[2-(naphthalen-1-yl)ethynyl]-
phenyl}hept-2-ynyl Benzoate (7b): Compound 7b was prepared fol-
lowing the procedure used for the synthesis of 7a, yield 61%. IR
4,4,5,5,6,6,7,7,7-Nonafluoro-1-{2-[2-(naphthalen-1-yl)ethynyl]-
phenyl}hept-2-yn-1-ol (6b): Compound 6b was prepared following
the procedure used for the synthesis of 3, yield 43%. IR (neat): ν
˜
(neat): ν = 3061, 2960, 2873, 2261, 2210, 1732, 1602, 1586, 1508,
˜
= 3065, 2944, 2257, 2218, 1589, 1494, 1354, 1234, 1137, 1036, 974,
1
1458, 1452, 1353, 1336, 1316, 1234, 1179, 1104, 1089, 1027, 1009,
943, 874, 817 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.35–7.54
(m, 7 H), 7.56 (m, 2 H), 7.78 (d, J = 8.4 Hz, 2 H), 7.82 (m, 1 H),
7.86 (d, J = 8.8 Hz, 2 H), 8.09 (d, J = 7.2 Hz, 2 H), 8.39 (m, 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 63.6, 72.8, 89.1, 90.2,
94.0, 120.1, 123.1, 125.2, 126.0, 126.5, 126.9, 127.8, 128.3, 128.5 (2
C), 128.9, 129.0, 129.3, 129.7, 129.9, 130.0 (2 C), 130.9, 132.9,
133.1, 133.6, 135.5, 164.8 ppm. 19F NMR (376 MHz, CDCl3): δ =
–125.9 (2 F), –123.6 (2 F), –99.2 (2 F), –81.5 (3 F) ppm. HRMS
(EI): calcd. for C32H17AgF9O2 711.014 [M + Ag]+; found 711.013.
873 cm–1. H NMR (400 MHz, CDCl3): δ = 3.22 (d, J = 5.7 Hz, 1
H), 6.21 (d, J = 4.7 Hz, 1 H), 7.45 (td, J = 6.6, 1.4 Hz, 2 H), 7.50
(d, J = 7.7 Hz, 1 H), 7.59 (t, J = 7.5 Hz, 1 H), 7.67 (t, J = 7.5 Hz,
1 H), 7.72 (t, J = 8.9 Hz, 2 H), 7.83 (d, J = 7.1 Hz, 1 H), 7.91 (dd,
J = 7.6, 1.7 Hz, 2 H), 8.47 (d, J = 8.3 Hz, 1 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 62.8, 72.1, 90.4, 92.1, 93.8, 119.9, 121.8,
125.2, 125.9, 126.5, 126.6, 127.1, 128.4, 129.1, 129.2, 129.5, 130.8,
132.9, 133.1, 133.2, 139.2 ppm. 19F NMR (376 MHz, CDCl3): δ =
–126.1 (2 F), –123.6 (2 F), –98.6 (2 F), –81.5 (3 F) ppm. HRMS
(EI): calcd. for C25H13F9O 500.082 [M]+; found 500.082.
4,4,5,5,6,6,7,7,7-Nonafluoro-1-{2-[2-(p-tolyl)ethynyl]phenyl}hept-2-
ynyl Benzoate (7c): Compound 7c was prepared following the pro-
4,4,5,5,6,6,7,7,7-Nonafluoro-1-{2-[2-(p-tolyl)ethynyl]phenyl}hept-2-
yn-1-ol (6c): Compound 6c was prepared following the procedure
cedure used for the synthesis of 7a, yield 75%. IR (neat): ν = 3065,
˜
used for the synthesis of 3, yield 45%. IR (neat): ν = 3451, 3055,
˜
2929, 2873, 2263, 2216, 1732, 1601, 1514, 1453, 1336, 1316, 1234,
1180, 1089, 1026, 1007, 943, 874, 817 cm–1. 1H NMR (400 MHz,
CDCl3): δ = 2.37 (s, 3 H), 7.14 (d, J = 7.9 Hz, 2 H), 7.29 (t, J =
4.3 Hz, 1 H), 7.39–7.47 (m, 6 H), 7.57 (t, J = 7.4 Hz, 1 H), 7.61–
7.63 (m, 1 H), 7.74–7.77 (m, 1 H), 8.07 (dd, J = 8.3, 1.3 Hz, 2
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 21.5, 63.4, 72.7, 84.8,
89.1, 96.2, 119.3, 123.2, 127.6, 128.5, 128.6, 128.9, 129.1, 129.6,
129.9, 131.5, 132.6, 133.6, 135.4, 139.0, 164.8 ppm. 19F NMR
(376 MHz, CDCl3): δ = –125.9 (2 F), –123.7 (2 F), –99.2 (2 F),
–81.5 (3 F) ppm. HRMS (EI): calcd. for C29H17F9O2 568.108
[M]+; found 568.108.
3032, 2924, 2258, 2216, 1571, 1353, 1298, 1234, 1137, 1040, 973,
872, 707 cm–1. 1H NMR (400 MHz, CDCl3): δ = 2.42 (s, 3 H), 3.52
(m, 1 H), 6.06 (d, J = 4.6 Hz, 1 H), 7.22 (d, J = 8.1 Hz, 2 H), 7.38
(td, J = 7.5, 0.8 Hz, 1 H), 7.41 (td, J = 7.5, 0.8 Hz, 1 H), 7.50 (d,
J = 8.0 Hz, 2 H), 7.61 (dd, J = 7.2, 1.7 Hz, 1 H), 7.64 (dd, J = 7.2,
1.4 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 21.3, 62.8,
71.8, 85.1, 92.1, 96.0, 104.5, 107.0, 115.9, 118.7, 119.3, 121.8, 126.5,
128.8, 128.9, 129.0, 129.2, 131.4, 132.6, 139.2 ppm. 19F NMR
(376 MHz, CDCl3): δ = –126.1 (2 F), –123.7 (2 F), –98.6 (2 F),
–81.7 (3 F) ppm. HRMS (EI): calcd. for C22H13F9O 464.082 [M]+;
found 464.082.
4,4,5,5,6,6,7,7,7-Nonafluoro-1-[2-(3,3-dimethylbut-1-ynyl)phenyl]-
hept-2-ynyl Benzoate (7d): Compound 7d was prepared following
4,4,5,5,6,6,7,7,7-Nonafluoro-1-[2-(3,3-dimethylbut-1-ynyl)phenyl]-
hept-2-yn-1-ol (6d): Compound 6d was prepared following the pro-
the procedure used for the synthesis of 7a, yield 82%. IR (neat): ν
˜
cedure used for the synthesis of 3, yield 42%. IR (neat): ν = 3440,
˜
= 3069, 2971, 2870, 2261, 2216, 1733, 1690, 1602, 1514, 1453, 1336,
3071, 2973, 2871, 2258, 1599, 1475, 1450, 1235, 1137, 1079, 1040,
919, 869 cm–1. 1H NMR (400 MHz, CDCl3): δ = 1.36 (s, 9 H), 3.21
(d, J = 5.0 Hz, 1 H), 5.85 (q, J = 4.6 Hz, 1 H), 7.29–7.36 (m, 2 H),
7.45 (d, J = 8.2 Hz, 1 H), 7.49 (d, J = 7.9 Hz, 1 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 28.3, 30.5, 63.2, 71.6, 75.9, 92.1, 105.6,
122.0, 126.4, 128.3, 128.8, 132.7, 139.2 ppm. 19F NMR (376 MHz,
CDCl3): δ = –126.1 (2 F), –123.8 (2 F), –98.8 (2 F), –81.7 (3
F) ppm. HRMS (EI): calcd. for C19H15F9O 430.098 [M]+; found
430.098.
1
1316, 1234, 1180, 1089, 1026, 1007, 943, 874, 817 cm–1. H NMR
(400 MHz, CDCl3): δ = 1.28 (s, 9 H), 7.16 (t, J = 4.3 Hz, 1 H),
7.37–7.40 (m, 2 H), 7.44–7.50 (m, 3 H), 7.59 (t, J = 7.4 Hz, 1 H),
7.71–7.73 (m, 1 H), 8.08 (dd, J = 8.3, 1.3 Hz, 2 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 28.1, 30.6, 63.4, 72.5, 89.3, 105.5, 123.9,
127.2, 128.0, 128.4, 129.0, 129.4, 129.7, 129.9, 132.5, 133.6, 135.4,
164.8 ppm. 19F NMR (376 MHz, CDCl3): δ = –126.1 (2 F), –123.8
(2 F), –99.3 (2 F), –81.7 (3 F) ppm. HRMS (EI): calcd. for
C26H19F9O2 534.124 [M]+; found 534.123.
4,4,5,5,6,6,7,7,7-Nonafluoro-1-[2-(2-phenylethynyl)phenyl]hept-2-
ynyl Benzoate (7a): nBuLi (2.5 m in hexane, 970 µL, 2.44 mmol) was
added to alcohol 6a (1.00 g, 2.22 mmol) in THF (30 mL) at –78 °C.
After stirring for 30 min, the solution was transferred over 5 min to
{2-[2-(3-tert-Butyl-4,4,5,5,6,6,7,7,7-nonafluorohepta-1,2-dienyl)-
phenyl]ethynyl}benzene (EA2): A solution of tBuLi (1.7 m in pent-
ane, 3.2 mL, 5.41 mmol) was slowly added to a stirred solution of
a solution of BzCl (342 mg, 2.44 mmol) in THF (30 mL) at 0 °C. CuCN (240 mg, 2.70 mmol) in diethyl ether (20 mL) at –78 °C. Af-
After stirring for 3 h, a 10% Na2CO3 solution (50 mL) was added. ter 5 min, the reaction mixture was warmed to 0 °C, and after
5174
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 5166–5177

Thermal C2–C6 Cyclization of Enyne-Allenes
15 min it was cooled again to –78 °C and a solution of benzoate
7a (500 mg, 900 μmol) in diethyl ether (10 mL) was then slowly
added. The reaction mixture was stirred for 1 h and then warmed
to 0 °C. After 1 h, water (50 mL) was added and the mixture was
extracted with diethyl ether (3ϫ 30 mL). The combined organic
layers were dried and concentrated under reduced pressure. After
purification by column chromatography (silica gel, n-hexane, Rf =
0.75), enyne-allene EA2 was isolated as a colorless oil (225 mg,
under reduced pressure and purification by chromatography (pre-
parative TLC, silica gel 60 F254, n-pentane, Rf = 0.69), compound
CB2 was isolated (47.0 mg, 94%). IR (neat): ν = 3055, 2982, 1578,
˜
1547, 1509, 1481, 1438, 1352, 1306, 1266, 1235, 1132, 1052, 909
1
(m) cm–1. H NMR (400 MHz, CDCl3): δ = 1.25 (s, 9 H), 6.25 (s,
1 H), 7.19 (td, J = 7.4, 1.3 Hz, 1 H), 7.33 (td, J = 7.4, 1.0 Hz, 1
H), 7.37–7.45 (m, 2 H), 7.51 (t, J = 7.5 Hz, 2 H), 7.78 (d, J =
7.4 Hz, 1 H), 7.87 (d, J = 7.5 Hz, 2 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 29.5, 38.0, 69.3, 113.8, 122.3, 124.2, 124.5, 128.7,
51%). IR (neat): ν = 3056, 2969, 2873, 2215, 1947, 1595, 1509,
˜
1484, 1446, 1422, 1368, 1351, 1266, 1234, 1134, 1055, 956, 129.0, 129.6, 129.9, 133.7, 141.0, 147.6, 147.7, 149.5, 151.9 ppm.
1
826 cm–1. H NMR (400 MHz, CDCl3): δ = 1.31 (s, 9 H), 7.25 (t, 19F NMR (376 MHz, CDCl3): δ = –125.7 (2 F), –119.8 (2 F),
J = 7.2 Hz, 2 H), 7.32 (dt, J = 7.8, 1.2 Hz, 1 H), 7.37–7-39 (m, 3
H), 7.44 (d, J = 7.8 Hz, 1 H), 7.54–7.59 (m, 3 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 30.3, 35.7, 86.9, 94.6, 98.8, 121.8, 122.9,
126.4, 127.8, 128.4 (2 C), 128.6, 128.8, 131.6, 132.6, 133.5,
206.7 ppm. 19F NMR (376 MHz, CDCl3): δ = –125.8 (2 F), –120.7
(2 F), –101.5 (2 F), –81.5 (3 F) ppm. HRMS (EI): calcd. for
C25H19F9 490.134 [M]+; found 490.134.
–106.6 (1 F), –102.9 (1 F), –81.3 (3 F) ppm. HRMS (EI): calcd. for
C25H19F9 490.134 [M]+; found 490.135.
1-tert-Butyl-2-(naphthalen-1-yl)-1-(perfluorobutyl)-1H-cyclobuta-
[a]indene (CB3): Compound CB3 was prepared following the pro-
cedure used for the synthesis of CB2, yield 78%. IR (neat): ν =
˜
3060, 2966, 1578, 1548, 1509, 1481, 1438, 1351, 1306, 1234, 1132,
1
1052, 969, 829 cm–1. H NMR (400 MHz, CDCl3): δ = 1.24 (s, 9
1-{2-[2-(3-tert-Butyl-4,4,5,5,6,6,7,7,7-Nonafluorohepta-1,2-dienyl)- H), 6.36 (s, 1 H), 7.10–7.21 (m, 2 H), 7.34 (d, J = 7.3 Hz, 1 H),
phenyl]ethynyl}naphthalene (EA3): Compound EA3 was prepared
following the procedure used for the synthesis of EA2, yield 34%.
7.41 (d, J = 7.5 Hz, 1 H), 7.58–7.65 (m, 3 H), 7.99 (d, J = 7.3 Hz,
3 H), 8.45 (dd, J = 6.1, 3.3 Hz, 1 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 29.5, 38.5, 70.3 (t), 121.2, 122.1, 124.1, 125.0, 125.3,
IR (neat): ν = 3061, 2969, 2910, 2874, 2216, 1948, 1596, 1508, 1488,
˜
1463, 1446, 1422, 1368, 1351, 1234, 1207, 1134, 1055, 973, 126.4, 127.5, 129.1, 129.3, 129.5, 130.3, 130.5, 130.9, 130.9, 133.9,
856 cm–1. 1H NMR (400 MHz, CDCl3): δ = 1.37 (s, 9 H), 7.34 (td, 141.2, 145.5, 145.4, 151.8, 152.1 ppm. 19F NMR (376 MHz,
J = 7.5, 1.3 Hz, 1 H), 7.40 (td, J = 7.7, 1.5 Hz, 1 H), 7.46 (t, J =
CDCl3): δ = –125.7 (2 F), –119.0 (2 F), –103.6 (1 F), –102.5 (1 F),
5.2 Hz, 1 H), 7.50–7.67 (m, 4 H), 7.72 (dd, J = 7.6, 1.4 Hz, 1 H), –81.2 (3 F) ppm. HRMS (EI): calcd. for C29H21F9 540.150 [M]+;
7.86 (dd, J = 7.1, 1.0 Hz, 1 H), 7.91 (t, J = 7.1 Hz, 2 H), 8.49 (d,
J = 7.8 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 30.3,
35.7, 91.8, 92.8, 98.9, 120.6, 122.0, 125.3, 126.1, 126.5 (2 C), 126.9,
127.9 128.4, 128.9, 129.1, 130.5, 132.7 (2 C), 133.2, 133.3, 133.6,
206.8 (t) ppm. 19F NMR (376 MHz, CDCl3): δ = –125.7 (2 F),
–120.7 (2 F), –101.4 (2 F), –81.4 (t, 3 F) ppm. HRMS (EI): calcd.
for C28H18F9 525.126 [M – CH3]+; found 525.126.
found 540.149.
1-tert-Butyl-1-(perfluorobutyl)-2-(p-tolyl)-1H-cyclobuta[a]indene
(CB4): Compound CB4 was prepared following the procedure used
for the synthesis of CB2, yield 84%. IR (neat): ν = 3059, 2964,
˜
1608, 1547, 1509, 1436, 1352, 1306, 1234, 1132, 1055, 969, 820
1
(m) cm–1. H NMR (400 MHz, CDCl3): δ = 1.24 (s, 9 H), 2.44 (s,
3 H), 6.23 (s, 1 H), 7.18 (td, J = 7.4, 1.2 Hz, 1 H), 7.30–7.35 (m, 3
1-(3-tert-Butyl-4,4,5,5,6,6,7,7,7-nonafluorohepta-1,2-dienyl)-2-(p- H), 7.37 (d, J = 7.5 Hz, 1 H), 7.75 (d, J = 8.3 Hz, 3 H) ppm. 13C
tolylethynyl)benzene (EA4): Compound EA4 was prepared follow-
ing the procedure used for the synthesis of EA2, yield 55%. IR
NMR (100 MHz, CDCl3): δ = 21.6, 29.5, 38.0, 69.3, 113.3, 122.2,
124.1, 124.4, 127.9, 128.7, 129.0, 129.4, 129.6, 131.0, 140.5, 147.9,
(neat): ν = 3054, 2969, 2873, 2215, 1949, 1595, 1511, 1480, 1446, 148.5, 151.8 ppm. 19F NMR (376 MHz, CDCl3): δ = –125.7 (2 F),
˜
1422, 1368, 1266, 1234, 1134, 1055, 933, 876 cm–1 1H NMR –119.9 (2 F), –106.6 (1 F), –103.1 (1 F), –81.3 (3 F) ppm. HRMS
.
(400 MHz, CDCl3): δ = 1.35 (s, 9 H), 2.43 (s, 3 H), 7.23 (d, J =
7.8 Hz, 2 H), 7.26–7.37 (m, 3 H), 7.48 (dd, J = 7.8, 0.9 Hz, 1 H),
7.52 (d, J = 8.1 Hz, 2 H), 7.59 (dd, J = 7.6, 1.0 Hz, 1 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 21.5, 30.3, 35.7, 86.3, 94.8, 98.8,
119.9, 122.0, 126.3, 127.8, 128.6, 129.2, 130.5, 131.5, 132.5, 133.4,
138.8, 206.7 (t) ppm. 19F NMR (376 MHz, CDCl3): δ = –125.7 (2
F), –120.7 (2 F), –101.3 (2 F), –81.4 (3 F) ppm. HRMS (EI): calcd.
for C26H21F9 504.150 [M]+; found 504.150.
(EI): calcd. for C26H21F9 504.150 [M]+; found 504.150.
1,2-Di-tert-butyl-1-(perfluorobutyl)-1H-cyclobuta[a]indene (CB5):
Compound CB5 was prepared following the procedure used for the
synthesis of CB2, yield 86%. IR (neat): ν = 3060, 2966, 1578, 1548,
˜
1509, 1481, 1438, 1351, 1306, 1234, 1132, 1052, 969, 829 (m) cm–1.
1H NMR (400 MHz, CDCl3): δ = 1.22 (br. s, 9 H), 1.43 (s, 9 H),
6.10 (s, 1 H), 7.12 (td, J = 7.4, 1.4 Hz, 1 H), 7.25–7.32 (m, 2 H),
7.57 (dd, J = 7.5, 0.7 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3):
1-(3-tert-Butyl-4,4,5,5,6,6,7,7,7-nonafluorohepta-1,2-dienyl)-2-(3,3- δ = 29.5, 30.4, 31.0, 36.0, 36.4, 112.8 (t), 121.7, 123.7, 124.5, 128.6,
dimethylbut-1-ynyl)benzene (EA5): Compound EA5 was prepared
following the procedure used for the synthesis of EA2, yield 47%.
128.9, 132.4, 140.2, 151.6, 151.9 ppm. 19F NMR (376 MHz,
CDCl3): δ = –125.7 (2 F), –119.0 (2 F), –103.6 (1 F), –102.5 (1 F),
–81.2 (3 F) ppm. MS (EI, 70 eV): m/z = 470.2 [M]+.
IR (neat): ν = 3054, 2971, 2872, 2235, 1949, 1597, 1511, 1476, 1462,
˜
1351, 1265, 1207, 1134, 1055, 956, 933, 856 cm–1 1H NMR
.
1-[2-(2-Phenylethynyl)phenyl]hept-2-ynyl Benzoate (7aЈ): Com-
pound 7aЈ was prepared following the procedure used for the syn-
(400 MHz, C6D6): δ = 1.15 (br. s, 9 H), 1.18 (br. s, 9 H), 6.82 (td,
J = 7.6, 1.3 Hz, 1 H), 6.94 (td, J = 7.6, 1.3 Hz, 1 H), 7.37 (dd, J =
7.8, 1.1 Hz, 1 H), 7.51–7.55 (m, 2 H) ppm. 13C NMR (100 MHz,
C6D6): δ = 30.3, 30.9, 31.2, 35.7, 77.6, 88.1, 99.6, 104.5, 123.4,
126.5, 127.2, 128.2, 128.4, 132.9, 207.1 ppm. 19F NMR (376 MHz,
CDCl3): δ = –125.7 (2 F), –120.7 (2 F), –101.3 (2 F), –81.4 (3
F) ppm. MS (EI, 70 eV): m/z = 470.2 [M]+.
thesis of 7a, yield 87%. IR (neat): ν = 3062, 2958, 2932, 2871, 2239,
˜
2151, 1723, 1601, 1571, 1494, 1451, 1379, 1315, 1259, 1176, 1094,
1
1069, 1026, 927, 905, 757 cm–1. H NMR (400 MHz, CDCl3): δ =
0.91 (t, J = 7.3 Hz, 3 H), 1.45 (sext, J = 7.1 Hz, 2 H), 1.56 (quint.,
J = 7.5 Hz, 2 H), 2.32 (td, J = 7.0, 1.8 Hz, 2 H), 7.22 (s, 1 H), 7.32
(t, J = 3.1 Hz, 3 H), 7.39 (t, J = 7.7 Hz, 3 H), 7.43 (td, J = 7.6,
1.0 Hz, 1 H), 7.50–7.54 (m, 3 H), 7.61 (d, J = 7.3 Hz, 1 H), 7.94
(d, J = 7.5 Hz, 1 H), 8.09 (d, J = 7.4 Hz, 2 H) ppm. 13C NMR
1-tert-Butyl-1-(perfluorobutyl)-2-phenyl-1H-cyclobuta[a]indene
(CB2): Enyne-allene 2a (50.0 mg, 102 μmol) was dissolved in dry
toluene (30 mL) and after degassing the resulting solution was (100 MHz, CDCl3): δ = 13.5, 18.6, 21.9, 30.4, 65.0, 76.2, 86.3, 88.7,
heated at 150 °C in a sealed tube for 7 h. After removal of toluene 95.0, 122.8 (2 C), 127.9, 128.1, 128.2, 128.3, 128.4, 128.6, 129.8,
Eur. J. Org. Chem. 2014, 5166–5177
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5175

M. E. Cinar, C. Vavilala, R. Jaquet, J. W. Bats, M. Schmittel
FULL PAPER
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2003, 68, 5832–5837; i) Y. H. Yang, J. L. Petersen, K. K. Wang,
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130.0, 131.6, 132.3, 132.9, 138.8, 165.3 ppm. HRMS (EI): calcd.
for C28H24AgO2 499.083 [M + Ag]+; found 499.083.
1-(3-tert-Butylhepta-1,2-dienyl)-2-(2-phenylethynyl)benzene (EA2Ј):
Compound EA2Ј was prepared following the procedure used for
the synthesis of EA2, yield 71%. IR (neat): ν = 3090, 2960, 2860,
˜
2399, 1938, 1598, 1493, 1443, 1361, 825, 754 cm–1. 1H NMR
(400 MHz, CDCl3): δ = 0.96 (t, J = 7.2 Hz, 3 H), 1.23 (br. s, 9 H),
1.38–1.58 (m, 4 H), 2.14–2.21 (m, 2 H), 6.94 (t, J = 3.2 Hz, 1 H),
7.19 (td, J = 7.5, 1.1 Hz, 1 H), 7.32 (td, J = 7.5, 1.3 Hz, 1 H), 7.38–
7.43 (m, 3 H), 7.54 (dd, J = 7.8, 1.6 Hz, 1 H), 7.58 (dd, J = 7.5,
1.1 Hz, 1 H), 7.64 (dd, J = 7.5, 1.3 Hz, 2 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 14.0, 22.8, 27.2, 29.4, 30.4, 34.5, 87.9, 93.8,
94.8, 118.3, 120.5, 123.4, 125.5, 125.9, 128.2, 128.3, 128.4, 131.5,
132.4, 138.0, 202.2 ppm. HRMS (EI): calcd. for C25H28 328.219
[M]+; found 328.219.
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1-Benzylidene-2-(1-tert-butylpent-1-enyl)-1H-indene (BF2): Com-
pound BF2 was prepared following the procedure used for the syn-
[5]
[6]
thesis of CB2, yield 92%. IR (neat): ν = 3062, 2959, 2867, 1599,
˜
1492, 1451, 1357, 908, 875, 734, 698 cm–1. 1H NMR (400 MHz,
CDCl3): δ = 0.90 (t, J = 7.4 Hz, 3 H), 1.17 (br. s, 9 H), 1.38 (sext,
J = 7.3 Hz, 2 H), 1.88 (qd, J = 7.2, 1.6 Hz, 2 H), 5.76 (t, J = 7.2 Hz,
1 H), 6.53 (s, 1 H), 6.94 (td, J = 7.5, 1.0 Hz, 1 H), 7.09 (s, 1 H),
7.20 (t, J = 7.3 Hz, 1 H), 7.25 (d, J = 7.2 Hz, 1 H), 7.38–7.47 (m,
4 H), 7.53 (d, J = 7.3 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 13.8, 23.0, 30.0, 32.0, 36.0, 120.2, 123.0, 124.1, 126.9, 127.8,
128.0, 128.3, 129.0, 129.1, 133.6, 134.1, 137.1, 141.6, 143.2, 143.5,
144.1 ppm. HRMS (EI): calcd. for C25H28 328.219 [M]+; found
328.219.
[7]
[8]
[9]
CCDC-997237 (for DA1) and -997238 (for CB2) contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
[10]
[11]
1
cle): H, 13C, and 19F NMR spectra, crystallographic data as well
as structure refinement details, cartesian coordinates, transition-
state structures, and potential energy surfaces.
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P. W. Peterson, R. K. Mohamed, I. V. Alabugin, Eur. J. Org.
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Acknowledgments
The authors are very much indebted to the Deutsche Forschungs-
gemeinschaft (DFG) (Schm 647/18-1) and the University of Siegen,
Germany for continued support of our research and to the com-
puter centers of NIC-Jülich and HRZ Siegen, Germany for com-
puter time.
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Received: April 17, 2014
Published Online: July 17, 2014
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