Efficient Trapping of 1,2-Cyclohexadienes with 1,3-Dipoles

Article abstract of DOI:10.1002/chem.201602201

1,2-Cyclohexadienes are transient intermediates that undergo rapid dimerization and intermolecular trapping with activated olefins and heteroatomic nucleophiles. Fluoride-mediated desilylative elimination of readily accessible 6-silylcyclohexene-1-triflates allows the mild, chemoselective, and functional-group tolerant generation of cyclic allene intermediates, which undergo efficient trapping reactions with stable 1,3-dipoles. The reactions proceed with high levels of both regio- and diastereoselectivity. The reaction of cyclic allenes with azides is accompanied by the facile loss of dinitrogen, resulting in the formation of tetrahydroindoles or polycylic aziridines depending on the azide employed.

Full text of DOI:10.1002/chem.201602201

A Journal of
Accepted Article
Title: Efficient Trapping of 1,2-Cyclohexadienes With 1,3-Dipoles
Authors: Verner A. Lofstrand; F. G. West
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Chemistry - A European Journal
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COMMUNICATION
Efficient Trapping of 1,2-Cyclohexadienes with 1,3-Dipoles
Verner A. Lofstrand[a] and Frederick G. West*[a]
Dedication ((optional))
Abstract: 1,2-Cyclohexadienes are transient intermediates that
undergo rapid dimerization and intermolecular trapping with
Previous work:
activated olefins and heteroatomic nucleophiles. Fluoride-mediated
X
Br
Br
desilylative elimination of readily accessible 6-silylcyclohexene-1-
triflates allows the mild, chemoselective, and functional group
tolerant generation of cyclic allene intermediates, which undergo
efficient trapping reactions with stable 1,3-dipoles. The reactions
proceed with high levels of both regio- and diastereoselectivity. The
reaction of cyclic allenes with azides is accompanied by the facile
loss of dinitrogen, resulting in the formation of tetrahydroindoles or
polycylic aziridines depending on the azide employed.
R
KOt-Bu (R = H)
or F– (R = SiR3)
CH3Li
R
O
Ph
R
R
O
H
Apart from the commonly encountered charged and radical
species, transient molecules whose high reactivity derives from
ring strain offer an attractive means for the rapid assembly of
molecular complexity.[1][2] Cyclic alkynes have received
considerable attention recently as strategic intermediates for bio-
orthogonal tagging and synthetic methodology;[3] however, there
have been comparatively few studies dedicated to the synthetic
utility of the isomeric cyclic allenes. The generation and
functionalization of 1,2-cyclohexadiene 1 eluded chemists until
Wittig and Fritze reported the [4+2] cycloaddition of 1,3-
diphenylisobenzofuran with 1,2-cyclohexadiene via an E2
elimination of HBr from 1-bromo-1-cyclohexene (Scheme 1).[4]
Subsequent research in this area has principally focused on
methods to generate these highly strained intermediates–
studies which have historically relied upon large excesses of
styrenes, furans, 1,3-dienes, alkoxides, and enolates as
intermolecular traps of putatitve cyclic allene intermediates in
order to avoid rapid dimerization to 2.[5] These challenges have
imposed limitations on the range of substrates and conditions
available for broader study of cyclic allene reactivity and its
exploitation in synthesis. Herein, we report the mild generation
of 1,2-cyclohexadienes via the fluoride-mediated desilylative
elimination of vinylic leaving groups (3), and their synthetically
useful cycloaddition reactions with various 1,3-dipoles.
R
1
(
)
n
RXM
OM
XR
(
)
n
OH
H
H
H
2
This report:
R
N
Z
R
Bu4NF or CsF
CH3CN, rt
OTf
Y
SiMe2Ph
Y
R
H
N
Z
3
high regio- and
R
(Z = O, NR) diasteroselectivity
Scheme 1. Known generation and trapping processes of 1,2-cyclohexadiene.
leaving group as reported by Quintana et al. was seen as a
logical improvement of the allene generation method and served
as the starting point for our investigations.[8,9] The readily
accessible enol triflate derivative (4)[10] of cyclohexane-1,2-dione
was found to be a convenient starting material for the synthesis
of cyclic allene precursors (Scheme 2). 1,2-Reduction of the
enone,[11] derivatization to allylic tosylate 5 under modified
conditions,[12] and copper-mediated allylic substitution with
To circumvent the complicated elimination of vinylic
leaving groups under strongly basic conditions previously
investigated and the limited functional group tolerance of the
Doering-Moore-Skattebøl reaction,[6] we chose to investigate the
fluoride-mediated elimination conditions first reported by
Johnson and Shakespeare.[7] Further, the use of a vinyl triflate
OAc
OTs
SiMe2Ph
OTf
O
[a]
V. A. Lofstrand and Prof. F. G. West
Department of Chemistry
OTf
OTf
d
a
b, c
OTf
University of Alberta
SiMe2Ph
E3-43 Gunning-Lemieux Chemistry Centre
Edmonton, AB. T6G 2G2 Canada
Fax: (+1) 780-492-8231
3b
4
5
3a
3 steps from
cyclohexanone
E-mail: frederick.west@ualberta.ca
We thank NSERC for support of this work, and Dr. Robert McDonald
(U. of Alberta X-ray Crystallography Lab) for obtaining the X-ray
crystal structure of x.
[b]
Scheme 2. Reaction conditions: a) CuBr•SMe2, PhMe2Si-Cl, Li(0), THF, –
78 °C; Ac2O (86 %), b) CeCl3•7H2O, NaBH4, MeOH, c) TsCl, NMe3•HCl, NEt3,
PhCH3, 0 °C, (75 % over two steps), d) CuCN, PPh3, PhMe2Si-Cl, Li(0), 0 °C
(85 %).
Supporting information for this article is given via a link at the end of
the document.

Chemistry - A European Journal
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COMMUNICATION
dimethylphenylsilyllithium provided an efficient and scalable
synthesis of allylic silane 3a,[13] equivalent to the previously
described precursors.[8,9] Direct copper-mediated conjugate
addition using dimethylphenylsilyllithium and capping of the
enolate with acetic anhydride[14] afforded allylic silane 3b, a
potential precursor to an acetoxy-substituted 1,2-cyclohexadiene.
The suitability of allylic silanes 3a and 3b as precursors of
the desired cyclic allenes was evaluated via treatment with a
THF solution of tetrabutylammonium fluoride (TBAF) in the
presence of furan (Scheme 3). Both precursors furnished the
corresponding [4+2]-cycloadducts 6a and 6b as mixtures of
diastereomers favoring the endo diastereomer (~11:1), in
agreement with previously reported cycloadditions of 1,2-
cyclohexadiene with furan.[15] The reaction of 3b is notable:
carbocyclic allenes bearing oxygen substituents have not
previously been reported, and the complete selectivity for
reaction via the non-oxygenated  bond leaves a synthetically
useful enol acetate moiety intact in the cycloadduct 6b.
Table 1. Reaction of nitrile oxide, nitrone and azomethine imine 1,3-dipoles
with cyclic allenes.[a]
R1 R2
N
R
Ar
R3
OTf
N
N
CsF (5 equiv.), 7
N
PhMe2Si
X
O
O
X
X
X
O
H
H
CH3CN, RT, 15–24 h
3a/3b
9a-f
10a-d
8a,b
R3
R2
7b (R1 = Bn, R2 = Ph, R3 = H)
H3C
R1
N
O
N
O
N
C
CH3
N
R
O
H3C
7a
7e (R = Ph)
7f (R = i-Pr)
7c (R1 = i-Bu, R2 = i-Pr, R3 = H)
7d (R1 = i-Pr, R2 = R3 = Me)
Entry
X
1,3-dipolar trap
Product
Yield[e]
d.r.[a]
1
2
3
H
OAc
H
7a[b]
7a[b]
7b[c]
8a
8b
43 %
69 %
62 %
n.a.
n.a.
9a (R1=Bn, R2=Ph,
>20:1
R3=H)
4
5
6
7
8
OAc
H
7b[c]
7c[c]
7c[c]
7d[c]
7d[c]
9b (R1=Bn, R2=Ph,
34 %
54 %
40 %
58 %
>20:1
>20:1
>20:1
n.a.
R3=H)
9c (R1=i-Bu, R2=i-Pr,
R3=H)
Scheme 3. [4+2] Cycloaddition of furan with unsubstituted and acetoxy-
substituted cyclic allenes generated in situ.
OAc
H
9d (R1=i-Bu, R2=i-Pr,
R3=H)
9e (R1=i-Pr,
R2=R3=Me)
With a convenient approach to the generation of cyclic
allene intermediates in hand, their reactivity with 1,3-dipoles was
examined. Upon addition of cesium fluoride to a solution of 3a or
3b and 2,4,6-trimethylbenzonitrile oxide 7a,[16] the 4-alkylidene
isooxazoline 1,3-dipolar cycloadducts 8a and 8b were isolated in
good yields (Table 1, entry 1–2). Notably, each was obtained as
a single regioisomer as determined by 1H NMR analysis. As
previously noted by Engels and Christl,[17] and Werstiuk[18] cyclic
allenes can be viewed as weak nucleophiles at the central
carbon of the allene; thus, preferential bonding of this carbon
with the electrophilic carbon terminus of the 1,3-dipole is not
surprising. Complete selectivity for reaction via the non-
oxygenated alkene of the acetoxy-substituted allene derived
from 3b was a welcome observation, and the higher yield seen
with this example indicates that polarizing substituents on the
allene moiety may enhance reaction with 1,3-dipoles, in accord
with the orbital considerations noted by Houk and coworkers.[19]
Remarkably low loadings of the nitrile oxide trap (5
equivalents) could be used, indicating the comparatively more
effective reactivity match versus the conventionally employed
dienes and alkenes. However, this approach was incompatible
with in situ generated nitrile oxides only yielding material derived
from the dimerization of the cyclic allenes.
9f (R1=i-Pr,
–
n.a.
[f]
OAc
R2=R3=Me)
9
H
7e[d]
7e[d]
7f[d]
7f[d]
10a (R=Ph)
10b (R=Ph)
10c (R=i-Pr)
10d (R=i-Pr)
46 %
51 %
63 %
44 %
11:1
>20:1
>20:1
>20:1
10
11
12
OAc
H
OAc
[a] In all cases d.r. was determined by comparison of the 1H NMR signals of
the isolated mixtures, [b] 5 equiv. of nitrile oxide were used, [c] 5 equiv. of
nitrone were used, [d] 3 equiv. of azomethine imine were used, [e] All yields
are for isolated products after purification, [f] only dimerization and
decomposition were observed (see text).
In contrast to earlier trapping examples, reaction of cyclic
allenes with nitrones and nitrile oxides affords new carbon-
oxygen bonds along with the C–C bond generated at the central
allene carbon, broadening the synthetic potential of these
intermediates. In order to further expand the versatility of this
approach, we also examined trapping of cyclic allenes with
isolable azomethine imines as an entrée to strategic carbon-
nitrogen bonds.[22] The reaction of 3-oxopyrazolidin-1-ium-2-ides
with cyclic allenes generated in situ showed complete
regioselectivity, olefin selectivity (in the case of the acetoxy-
substituted cyclic allene) and only slight decreases in the
diastereoselectivity relative to the nitrone examples to give
tricyclic products 10a-d (Table 1, entry 9–12). The relative
configuration and regiochemistry were established by similar
means to the nitrone adducts (vide supra) and further confirmed
by single crystal X-ray diffraction analysis of pyrazolidino[1,2-
a]indazol-3-one 10a.[23]
The analogous nitrones also underwent 1,3-dipolar
cycloaddition with cyclic allenes to produce alkylidene
isoxazolidines 9a-f in good yields, and with complete
regiocontrol and excellent diastereoselectivity (Table 1, entry 3–
8).[20] Given the greater bench stability of nitrones more
variability in the 1,3-dipole structure was available for study.
1
Only a single diastereomer was observed by analysis of the H
NMR spectral data regardless of the substituents on the nitrone,
which was determined to be anti- via TROESY correlations.[21]

Chemistry - A European Journal
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COMMUNICATION
Table 2. Reaction of organoazide 1,3-dipolar compounds with cyclic allenes.[a]
The high levels of stereoselectivity seen in these
cycloadditions is consistent with a concerted pericyclic process,
but the intervention of short-lived biradical or zwitterionic
intermediates cannot be ruled out at this point, indicating fruitful
future studies. Concerted reaction is also consistent with the
high degree of stereochemical fidelity reported by Christl and
Engels in the furan trapping of substituted 1,2-cyclohexadienes
derived from enantioenriched precursors.[24] In analogy, we
posit a concerted dipolar cycloaddition in these cases, with the
observed diastereoselectivity arising from an endo-like
transitions state (Figure 1, TS 2).
Entry
Azide
11a
11b
11a
14a
14b
14c
14d
Conditions[a]
Product
12a
Yield[b]
52 %
1
2
3
4
5
6
7
A
A
H
H
12b
13
51 %
H
N
A
34 %[c]
47 %
H
N
N
N
H
H
B
15a
O
O
1D TROESY
Irradiation
Relative configuration
B
15b
15c
66 %
R
A/B
B
34 %/43 %
15 %
X
N
H
H
X
H
N
Ph
H
R
Ph
15d
H
H
H
H
H
H
H
H
[a] Conditions: A: Azide (5 equiv.) and 3a (1 equiv.) were dissolved in dry
MeCN and CsF (5 equiv.) was added as a granular solid. The reaction was
stirred for 18–24 h; B: Azide (5 equiv.) and 3a (1 equiv.) were dissolved in dry
THF (0.2 M) and TBAF solution (0.1 in THF) was added via syringe pump
(2mL/h). Reaction was stirred for 2 h, [b] all yields are for isolated product after
purification, [c] product was only obtained after an acidic aqueous workup.
H
H
H
H
H
H
10a
TS 1 (exo)
TS 2 (endo)
Figure 1. TROESY correlations corroborated by X-ray crystal structure of
azomethine imine adduct 10a. The relative configuration of the cycloadducts is
consistent with endo transition state, (TS 2).
given the proposed accessibility of open-shell electronic states
in the case of strained cyclic allenes.[27] Regardless of their
provenance, the formation of 12a,b is notable given the
presumed combination of two short-lived reactants (cyclic allene
+ alkylidene aziridine) and the introduction of amine functionality
at a synthetically challenging angular position.
Reaction of 1,2-cyclohexadienes with stable and readily
accessible azide 1,3-dipolar partners was also of interest.[25]
Initial studies indicated that only the electron-deficient aromatic
azides 11a-b furnished characterizable products (Table 2,
entries 1–2). Treatment of 2a with cesium fluoride in the
presence of these traps afforded unexpected 2:1/cyclic
allene:azide serial [2+1]/[2+2] cycloadducts 12a and 12b. The
relative configuration of these structurally complex polycyclic
products was determined by TROESY correlations of both the
cycloadducts as well as the ring-opened amino alcohol 13,
which was obtained upon acidic workup. Unfortunately, reaction
of acetoxy-substituted cyclic allene precursor 3b and organic
azides only gave complicated mixtures in the presence of
fluoride.
Ar
Ar
N
N
Ar
N
N
N
N
N
Ar
.
..
.
11a,b
N
14a-d
N
N
–N2
..
.
.
.
.
1
B
A
C
1
Ar
N
Ar
Ar
..
Ar
H
N
NH
N
N2
–N2
H
12a,b
E
D
15a-d
Intramolecular cycloaddition of acyclic allenes with azides was
studied extensively studied by Feldman and coworkers.[26] The
initially
formed
5-methylene-4,5-dihydro-1,2,3-triazole
Scheme 4. Proposed mechanism for the formation of 2:1 adducts 12 and 2-
aryltetrahydroindoles 15 via reaction with azide traps.
intermediates were found to be thermally unstable, undergoing a
retro-[3+2] cycloaddition, extruding dinitrogen and furnishing an
azatrimethylene methane intermediate, which could be
intercepted with tethered olefins. The 2:1 adducts 12a,b appear
to result from a [2+2]-cycloaddition of alkylidene aziridine B with
a second cyclic allene (Scheme 4). This aziridine may arise
from an azatrimethylene methane (A) analogous to those
postulated by Feldman and coworkers; however, it is not clear
whether A would be formed via cycloreversion from an initial
alkylidenetriazoline, since this process typically requires high
reaction temperatures. A possible stepwise process involving
biradical intermediates could intervene under these conditions,
The intermediacy of aza-trimethylenene methane
intermediate A was investigated via reaction of 3a with 11a and
CsF in the presence of excess styrene (Scheme 5). However,
only known [2+2]-cycloadduct 16 was observed.[28] This result
indicated that reaction of styrene with the cyclic allene is
kinetically favored over that of the aryl azide. With this in mind,
we then examined allene trapping with styryl azides 14a-d
(Table 2, entry 3–6). These traps, combining styrene and azide
functionalities, afforded 2-aryl-4,5,6,7-tetrahydroindoles 15a-
d.[28] The reaction tolerated a range of electronic environments;

Chemistry - A European Journal
10.1002/chem.201602201
COMMUNICATION
4764–4806; d) N. J. Agard, J. A. Prescher, C. R. Bertozzi, J. Am. Chem.
Soc. 2004, 126, 15046–15047.
however, incorporation of methyl substituent at the -styryl
position completely extinguished any reaction between the trap
and the cyclic allene, indicating a high sensitivity to steric
demand.
[4]
[5]
a) G. Wittig, P. Fritze, Angew. Chem. Int. Ed. 1966, 5, 846; b) G. Wittig,
P. Fritze, Liebigs Ann Chem. 1968, 711, 82–87.
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Taskesenligil, In Advances in Strained and Interesting Organic
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Scheme 5. Attempted interception of the allene/azide [2+1+2] cycloaddition
with styrene.
[7]
[8]
[9]
W. C. Shakespeare, R. P. Johnson, J. Am. Chem. Soc. 1990, 112,
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Previously studied reactions of substituted styrenes with
cyclic allenes strongly suggest the reactions involve diradical
intermediates.[28,30,31] In analogy, the present reaction likely
occurs via the radical addition of the central cyclic allene carbon
atom into the styrene to generate a new biradical intermediate C
(Scheme 4). Upon ring closure, spontaneous extrusion of
dinitrogen would occur furnishing tetrahydroindole E which could
rearrange to the aromatic product 15 via a series of proton
transfers.
We have described a succinct, convenient and scalable
route to highly reactive 1,2-cyclohexadienes, exemplified here
by the parent compound and the 1-acetoxy derivative. In
addition to well-established [4+2] reaction with furan, we have
shown that nitrile oxide, nitrone and azomethine imine 1,3-
dipoles can serve as efficient traps to furnish polycyclic
heterocyclic products with high regio- and stereoselectivity. In
contrast, azide traps afford unique reactivity, involving formation
of unusual 2:1 adducts in the case of aryl azides, and
tetrahydroindoles in the case of styryl azides. Further efforts to
intercept these fascinating intermediates in synthetically useful
transformations and attempts to elucidate reactions mechanisms
and the origins of the high selectivities are ongoing and will be
described elsewhere.
During preparation of this manuscript a similar study concerning nitrone
trapping of the parent 1,2-cyclohexadine was reported: J. S. Barber, E.
D. Styduhar, H. V. Pham, T. C. McMahon, K. N. Houk, N. K. Garg, J.
Am. Chem. Soc. 2016, 138, 2512–2515.
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[21] See supporting information for full characterization.
[22] Only catalyzed reactions have been reported for azomethine imine/
allene cycloadditions. C. Nájera, J. M. Sansano, M. Yus, Org. Biomol.
Chem. 2015, 13, 8596–8636.
Acknowledgements
V.A.L. acknowledges the financial support of the Natural
Science and Engineering Research Council of Canada (NSERC)
for a Post Graduate Fellowship as well as the University of
Alberta for a Queen Elizabeth II Graduate Scholarship. The
authors also thank Dr. Robert McDonald (University of Alberta
X-ray Crystallography Facility) for the X-Ray crystal structure of
10a and thank NSERC for partial research support.
Acknowledgment is also made to the Donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research.
[23] CCDC 1457161 (10a) contains the supplementary crystallographic data
for this paper. These data may be obtained free of charge from the
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
[24] M. Christl, H. Fischer, M. Arnone, B. Engels, Chem. Eur. J. 2009, 15,
11266–11272.
[25] For earlier examples of 1,3-dipolar cycloadditions of azides with acyclic
and cyclic allenes, see: a) W. Ried, H. Mengler, Liebig Ann. Chem.
1964, 678, 95–113; b) D. K. Wedegaertner, R. K. Kattak, I. Harrison, S.
K. Cristie, J. Org. Chem. 1991, 56, 4463–4467.
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4591; b) C. S. López, O. N. Faza, K. S. Feldman, M. R. Iyer, D. K.
Hester, J. Am. Chem. Soc. 2007,129, 7638–7646.
Keywords: Cyclic Allene • Dipolar Cycloaddition • Heterocycles
• Reactive Intermediate
[27] R. O. Angus, Jr, M. W. Schmidt, R. P. Johnson, J. Am. Chem. Soc.
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[28] W. R. Moore, W. R. Moser, J. Org. Chem. 1970, 35, 908–912.
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Chemistry - A European Journal
10.1002/chem.201602201
COMMUNICATION
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COMMUNICATION
1,2-Cyclohexadienes–strained, highly
reactive, short-lived intermediates–are
readily generated from silyl triflate
precursors. These species undergo
efficient reaction with several classes
of 1,3-dipole to afford polycyclic
products with high regio- and
Verner A. Lofstrand and F. G. West*
Page No. – Page No.
Efficient Trapping of 1,2-
Cyclohexadienes with 1,3-Dipoles
stereoselectivity. In the case of azide
traps, unusual new products are
formed via loss of dinitrogen.