Gold for the generation and control of fluxional barbaralyl cations

Article abstract of DOI:10.1002/anie.201207682

Fluxional molecules which rapidly pass back and forth between a large number of constitutional isomers through low-energy rearrangements have fascinated chemists owing to their role in the study of fundamental theoretical concepts[ 2] and their potential to adapt their chemical structures in response to their environment or to act as prototypical molecular transport systems. They represent a facet of systems chemistry that is relatively unexplored, in which a dynamic structural library can be contained within a single molecule. The 9-barbaralyl cation (1) is a hugely fluxional C₉H₉ ⁺ hydrocarbon that exists as a mixture of 181 400 degenerate forms which interconvert rapidly at temperatures as low as -135 °C-each carbon atom may exchange with every other carbon atom in the structure through a series of pericyclic reactions. Unlike the neutral homologues semibullvalene (2; two degenerate tautomers) and bullvalene (3; 1209600 degenerate tautomers), which are stable compounds under ambient conditions, 1 is highly reactive and undergoes irreversible rearrangement to 1,4-bishomotropylium cation (4) above -125 °C. Functionalized barbaralanes may be suitable candidates for switchable, fluxional molecules. However, the difficulty in handling these compounds coupled with the low-yielding, multistep syntheses and harsh reaction conditions (typically featuring strongly or super acidic media) employed in the generation of 1 and its derivatives have so far limited the extent to which the chemistry of this fascinating dynamic carbon skeleton has been explored.

Full text of DOI:10.1002/anie.201207682

Angewandte
Chemie
DOI: 10.1002/anie.201207682
Fluxional Intermediates
Gold for the Generation and Control of Fluxional Barbaralyl Cations**
Paul R. McGonigal, Claudia de Leꢀn, Yahui Wang, Anna Homs, Cꢁsar R. Solorio-Alvarado, and
Antonio M. Echavarren*
Fluxional molecules which rapidly pass back and forth
between a large number of constitutional isomers through
low-energy rearrangements have fascinated chemists[1] owing
to their role in the study of fundamental theoretical con-
cepts[2] and their potential to adapt their chemical structures
in response to their environment[3] or to act as prototypical
molecular transport systems.[4] They represent a facet of
systems chemistry[5] that is relatively unexplored, in which
a dynamic structural library can be contained within a single
molecule. The 9-barbaralyl[6] cation (1) is a hugely fluxional
Gold complexes are potent activators of p bonds towards
nucleophilic attack by nucleophiles[13] and in the cyclization of
1,n-enynes,[13f,14] in which gold exerts exquisite control over
the competitive reaction pathways by the stabilization of
intermediates with carbenic character.[15–17] As part of a study
of the gold-catalyzed activation of cycloheptatrienes,[18] we
examined the reactivity of their alkynyl derivatives. Herein
we report the generation of fluxional barbaralyl intermediates
from 7-alkynyl cycloheptatrienes 5 under very mild condi-
tions. The gold-stabilized barbaralyl intermediates evolve to
furnish 1- or 2-substituted indenes, depending on the catalyst.
The formation of 1-substituted indenes involves a remarkable
transformation in which the alkyne carbon atoms end up at
the bridge of the indene.
We first investigated the reaction of 5a in the presence of
gold salts and other known p Lewis acids as catalysts
(Table 1). The treatment of 5a with a variety of gold
complexes led to rapid isomerization to indene products 6a
and 7a (Table 1, entries 1–9). Other common p acids were not
+
C9H9 hydrocarbon that exists as a mixture of 181400
degenerate forms[7] which interconvert rapidly at temper-
atures as low as À1358C[8]—each carbon atom may exchange
with every other carbon atom in the structure through a series
of pericyclic reactions. Unlike the neutral homologues semi-
bullvalene (2; two degenerate tautomers)[2a,9a] and bullvalene
(3; 1209600 degenerate tautomers),[1–3] which are stable
compounds under ambient conditions, 1 is highly reactive
and undergoes irreversible rearrangement to 1,4-bishomo-
tropylium cation (4)[9] above À1258C.[8] Functionalized bar-
baralanes may be suitable candidates for switchable, fluxional
molecules. However, the difficulty in handling these com-
pounds coupled with the low-yielding, multistep syntheses[10]
and harsh reaction conditions (typically featuring strongly or
super acidic media) employed in the generation of 1 and its
derivatives[10–12] have so far limited the extent to which the
chemistry of this fascinating dynamic carbon skeleton has
been explored.
Table 1: Screening of gold and other Lewis acid catalysts.[a]
Entry
Catalyst
Conversion [%][b]
6a/7a[b]
[*] Dr. P. R. McGonigal, C. de Leꢀn, Y. Wang, A. Homs,
C. R. Solorio-Alvarado, Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paꢁsos Catalans 16, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
1
2
3
4
5
6
7
8
9
[Ph3PAuNCMe]SbF6
8
9
10
>99
>99
>99
>99
>99
>99
50
>99
>99
0
48:52
84:16
80:20
62:38
84:16
87:13
96:4
>99:1
0:100
–
11
Prof. A. M. Echavarren
Departament de Quꢂmica Analꢂtica i Quꢂmica Orgꢃnica
Universitat Rovira i Virgili
AuCl3
AuCl
12
C/Marcel·li Domingo s/n, 43007 Tarragona (Spain)
13
[**] We thank the MICINN (CTQ2010-16088/BQU and FPI fellowship to
A.H.), the AGAUR (2009 SGR 47), AtMol (Contract No. ICT-
270028), and the ICIQ Foundation for financial support. We also
thank the ICIQ X-ray diffraction unit for the solid-state structures of
16/16’, 17, 18, and 7v.
10
11
12
PtCl2
GaCl3
AgOTf
0
0
–
–
[a] For full details, see the Supporting Information. [b] Conversion and
the product ratio were determined by 1H NMR spectroscopy.
Cy =cyclohexyl, Tf=trifluoromethanesulfonyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207682.
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13093

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Angewandte
Communications
catalytically active (Table 1, entries 10–12). The ratio of
regioisomers 6a/7a was sensitive to the gold source
employed. Gold trichloride and cationic phosphine–gold(I)
and N-heterocyclic carbene–gold(I) complexes generated
mixtures (Table 1, entries 1–6). However, AuCl and complex
12 exhibited excellent bias towards regioisomer 6a (Table 1,
entries 7 and 8). Remarkably, use of the highly electrophilic
phosphite–gold(I) complex 13[16b] led exclusively to 2-(p-
tolyl)-1H-indene (7a; Table 1, entry 9).[19]
The catalyst-controlled regioselectivity observed in the
cycloisomerization of 5a was retained for the majority of
substrates, although in two instances mixtures were obtained
(Table 2, entries 4 (under conditions B) and 16 (under
withdrawing substituents were tolerated (Table 2, entries 5
and 6). Fused aromatic and heteroaromatic alkynyl deriva-
tives were also suitable substrates (Table 2, entries 13 and 14),
although slightly more forcing conditions were necessary with
the thienyl enyne 5n when catalyst 12 was used. The
ferrocenyl alkyne 5o underwent cycloisomerization under
conditions B, albeit in low yield (Table 2, entry 15), and the
benzyl substrate 5p underwent successful isomerization to
indene products (Table 2, entry 16). The homobenzyl deriv-
ative 5q and terminal alkyne 5r were both unreactive under
conditions A, but could be converted into indenes in moder-
ate yield with catalyst 13 (Table 2, entries 17 and 18).
We also discovered a method that allowed this reaction to
be conducted in a single step (Table 2, entries 19–21). Catalyst
8 (1 mol%) mediated the nucleophilic substitution reaction of
7-methoxycyclohepta-1,3,5-triene with terminal alkynes to
form 5 in situ (presumably via a gold acetylide), which
subsequently underwent cycloisomerization to indenes 6 and
7. The overall reaction was slower than the cycloisomerization
of premade 5, and the yields were modest; however, it is
straightforward method by which to access the cycloisomeri-
zation pathway.
Table 2: Substrate scope.[a]
Entry
Substrate
Yield [%][b] (product)
conditions A[c] conditions B[d]
We hypothesized that the intermediacy of barbaralyl
species,[20] generated by an initial 1,6-enyne cyclization,[13f,14]
could be responsible for the observed reactivity and hoped to
gain insight into the process by trapping intermediates with
tethered olefins (Scheme 1).[21] Substrates 14 and 15, which
feature pendant olefins, underwent cycloisomerization with
the cationic gold catalyst 11 to form barbaralanes 16/16’ and
17, respectively. The tautomeric barbaralanes 16 and 16’
interconvert rapidly on the NMR timescale at room temper-
ature in CDCl3 through a strain-assisted Cope rearrange-
1
2
3
4
5
5a, R=p-Tol
5b, R=Ph
5c, R=m-Tol
5d, R=o-Tol
86 (6a)
64 (6b)
55 (6c)
76 (6d)
69 (6e)
63 (6 f)
80 (6g)
82 (6h)
77 (6i)
88 (6j)
85 (6k)
60 (6l)
75 (6m)
66 (6n)
–
60 (7a)
51 (7b)
50 (7c)
48 (1:1.8 6d/7d[e])
30 (7e)
5e, R=p-MeOC6H4
5 f, R=p-FC6H4
5g, R=p-ClC6H4
5h, R=p-BrC6H4
5i, R=o-PhC6H4
5j, R=o-ClC6H4
5k, R=o-BrC6H4
5l, R=2,4-F2C6H4
5m, R=2-naphthyl
5n, R=3-thienyl
5o, R=ferrocenyl
5p, R=Bn
6[f]
7
8
9
10
11
12
13
14[f]
15
16
17
18
60 (7 f)
59 (7g)
57 (7h)
76 (7i)[g]
35 (7j)
38 (7k)
40 (7l)
51 (7m)
41 (7n)
23 (7o)
57 (7p)
31 (7q)
54 (7r)[g]
76 (1:2 6p/7p[e])
–
5q, R=CH2Bn
5r, R=H
–
19[h]
20[h]
21[h]
5j
5k
5l
41 (6j), 5 (7j)
47 (6k), 6 (7k)
29 (6l), 2 (7l)
[a] For full details, see the Supporting Information. [b] Yield of the
isolated product. [c] Conditions A: 12 (1 mol%), CH2Cl2 (0.3m), 08C,
1 h. [d] Conditions B: 13 (5 mol%), PhMe (0.3m), 08C, 1 h. [e] Isolated
as a mixture. [f] 12 (5 mol%), 238C, 2 h. [g] The yield was determined by
1H NMR spectroscopy. [h] The substrate was generated in situ from the
terminal-alkyne precursor (3 equiv) and 7-methoxycyclohepta-1,3,5-
triene (1 equiv); reaction conditions: 8 (1 mol%), CH2Cl2 (0.3m), 238C,
24 h. Bn=benzyl.
conditions A)). In general, reactions were complete within
1 h at 08C, and the picolinate catalyst 12 afforded 1-
substituted indenyl products in good yields (55–88%),
whereas catalyst 13 produced the 2-substituted indenyl
isomers in lower yields (23–76%), probably as a result of
partial decomposition in the presence of the more reactive
phosphite complex. Substrates bearing aryl groups with
substituents in the 2-, 3-, or 4-positions performed similarly
(Table 2, entries 1–12), and electron-donating or electron-
Scheme 1. Trapping and labeling experiments for the elucidation of the
intermediate species in the gold-catalyzed cycloisomerization of 7-
alkynyl cyclohepta-1,3,5-trienes and stick representation of the solid-
state structure of 18. Ts=p-toluenesulfonyl.
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ment[22] and were detected as a 1:1 mixture in the crystal
state,[23,24] whereas 17 was observed as a single tautomer.[24]
Additional evidence for the involvement of barbaralyl
gold intermediates was obtained from the cyclization of 5m in
MeOH/CH2Cl2 (1:1) to form the barbaralane methyl ether 18
in 40% yield, along with minor amounts of indenes 6m and
7m (Scheme 1). The structure of 18, a product of alkoxycyc-
lization,[14] was confirmed by crystallographic analysis.[24]
To determine whether the short-lived, cationic barbaralyl
species evolve through a short series of steps or whether they
undergo numerous rearrangements, we prepared two alkynyl
cycloheptatrienes containing a 13C label as part of the alkyne
either at the position adjacent to the cycloheptatriene moiety
(red dot, 5m’) or attached to the naphthyl ring system (blue
dot, 5m’’). If the barbaralyl intermediate could undergo
manifold pericyclic rearrangements, the 13C label would be
expected to be dispersed throughout the barbaralane skel-
eton.
Cycloisomerization of 5m’ and 5m’’ under conditions A
afforded the single isotopomers 6m’ and 6m’’, respectively,
with the 13C label at the bridging positions of the indene core
and thus separated from the naphthalene unit by an extra
bond relative to its position in the starting alkynyl cyclo-
heptatriene (Scheme 1). The transformation of 5m’ under the
catalysis of phosphite complex 13 (conditions B) led to
product 7m’ as a mixture of two isotopomers with the 13C
label located in either the 1- or the 3-position of the indene.
The methoxycyclization of 5m’ or 5m’’ generated 18’ or 18’’, in
which the 13C label had been transferred with complete
fidelity to a single position.
These results are indicative of a mechanism that proceeds
through a small number of steps. Experimental and theoret-
ical studies have established that the C9H9+ barbaralyl cation
(1) fluctuates between degenerate isomers through a unique
network of reaction paths involving 30240 degenerate
divinylcyclopropylcarbinyl cation–divinylcyclopropylcarbinyl
cation rearrangements (dvcpc–dvcpc; one cyclopropyl bond is
broken as another forms) and 90720 rearrangements which
pass through a bicyclo[3.2.2]nona-3,6,8-trien-2-yl transition
state.[7] Although the number of energetically accessible
nondegenerate isomers, and hence the number of possible
rearrangements, is reduced in substituted systems,[12c] such as
ours, the key mechanistic features remain the same. The most
concise mechanistic rationale that accounts for our exper-
imental observations was formulated on the basis of these
known steps (Scheme 2). We propose that coordination of the
catalyst to the alkyne initiates the cyclization to afford the
barbaralyl–gold complex II,[25] pictured by invoking the gold–
carbene resonance form.[15,16a] Intermediate II can be trans-
formed irreversibly into the homoaromatic bicyclo-
[4.3.0]nonatrienyl cation III[9b] (red arrow), which can
undergo deauration with a concomitant 1,2-shift via TS-III
to establish aromaticity through the formation of the 1-
substituted indene 6. As a consequence of the 1,2-shift, the R
group is separated from the carbon atoms that originated
from the alkyne of 5 (red and blue dots), as detected in
products 6m’ and 6m’’ (Scheme 1). Alternatively, a reversible
dvcpc–dvcpc rearrangement of II (black arrows) via transition
state TS-I generates the isomeric barbaralyl–gold complex IV,
Scheme 2. Mechanistic rationale for the formation and evolution of
barbaralyl gold cations.
a precursor to the methoxycyclization product 18. By analogy
+
with the reversible isomerization process in C9H9 (1),[7] the
barbaralyl–gold complex VII can be accessed by the evolution
of TS-I via V and TS-II. The homoaromatic bicyclo-
[4.3.0]nonatrienyl cation VIII is then formed irreversibly
(blue arrow) and can undergo aromatization through two
pathways to generate either IX or X. Protodeauration of
either intermediate liberates the same 2-substituted indene
product 7. The preceding steps are only distinguishable in the
case of the 13C-labeled indene 7m’, which was isolated in
a near 1:1 mixture of isotopomers.
Gold-stabilized barbaralyl cations generated in this reac-
tion can fluctuate between a minimum of three structures: II,
IV, and VII. In the case of reactions performed with catalyst 8,
experimental evidence shows that all three of these inter-
mediates are accessed, as indenes 6 and 7 and barbaralane 18
have all been isolated. It is therefore remarkable that the
picolinate complex 12 and phosphite complex 13 are able to
exercise such a high level of control over the pathway that is
followed. It is possible that other low-energy, transient
barbaralyl intermediates are present during the course of
these reactions but have escaped detection, as they are not
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direct precursors to any of the compounds isolated in this
study.
The sheer number of conceivable permutations in the
isomerization of barbaralyl cations makes it impractical to
perform calculations to determine the relative energies of all
possible isomers;[7,8,11a,12b] however, DFT modeling of our
proposed intermediates with the M06 functional showed in all
cases[25] that the minimum structures correspond to 1-aryl 9-
barbaralyl gold species, such as IIa: intermediate structures
between gold-stabilized barbaralyl cations and barbaralyl
À
gold carbenes. The C1 C9 bond lengths of 1.38–1.41 ꢀ in
these structures are between those of single and double
À
carbon–carbon bonds (Scheme 2). The shorter C Au bond
distance in IIa, with a donating chloride ligand, relative to
that in the analogous cationic species with P(OMe)3 as the
ligand (2.00 versus 2.07 ꢀ)[26] corresponds to a more metal-
carbene-like structure for the neutral intermediate.[16a]
In summary, we have found a new method for the direct
generation of fluxional barbaralyl cations either from readily
prepared 7-alkynyl cyclohepta-1,3,5-trienes 5 or in one pot
from aryl alkynes and 7-methoxycyclohepta-1,3,5-triene.
These reactions are catalyzed by gold complexes under very
mild conditions. In the absence of nucleophiles, substituted
indenes are produced in good to high yields with catalyst-
controlled regioselectivity. The nature of the gold catalyst
affects the fluxionality and evolution the cationic intermedi-
ates. Such influence over the speciation of barbaralyl cations
is unprecedented. Our methodology for the preparation of
barbaralanes and barbaralyl cations may be of use in the field
of shape-shifting molecules. We are therefore currently
investigating the expansion of this methodology to the
preparation of stabilized barbaralyl cations.
[19] For ligand-controlled selectivity in gold-catalyzed transforma-
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Received: September 23, 2012
Published online: November 19, 2012
Keywords: barbaralyl cations · cyclization · enynes · fluxionality ·
.
gold catalysis
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[24] Detailed procedures and characterization data are available in
the
Supporting
Information.
X-ray
crystallography:
CCDC 897922 (16/16’), 897923 (17), 897920 (18), and 897921
(7v; see the Supporting Information) contain the supplementary
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.
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corresponding norcaradiene tautomer. See the Supporting
Information for the reaction coordinate for the transformation
of Id into IId (L = PMe3) and for additional computational
details.
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with AuCl, AuCl3, AuPMe3, and AuP(OMe3). In these structures,
gold, rather than the phenyl substituent, stabilizes the carbocat-
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