Intramolecular C-H activation through gold(I)-catalyzed reaction of iodoalkynes

Article abstract of DOI:10.1002/anie.201409970

The cycloisomerization reaction of 1-(iodoethynyl)-2-(1-methoxyalkyl)arenes and related 2-alkyl-substituted derivatives gives the corresponding 3-iodo-1-substituted-1H-indene under the catalytic influence of IPrAuNTf₂ [IPr = 1,3-bis(2,6-diisopropyl)phenylimidazol-2-ylidene; NTf₂ = bis(trifluoromethanesulfonyl)imidate]. The reaction takes place in 1,2-dichloroethane at 80°C, and the addition of ttbp (2,4,6-tritert-butylpyrimidine) is beneficial to accomplish this new transformation in high yield. The overall reaction implies initial assembly of an intermediate gold vinylidene upon alkyne activation by gold(I) and a 1,2-iodine-shift. Deuterium labeling and crossover experiments, the magnitude of the recorded kinetic primary isotopic effect, and the results obtained from the reaction of selected stereochemical probes strongly provide support for concerted insertion of the benzylic C-H bond into gold vinylidene as the step responsible for the formation of the new carbon-carbon bond.

Full text of DOI:10.1002/anie.201409970

Angewandte
Chemie
DOI: 10.1002/anie.201409970
Synthetic Methods
À
Intramolecular C H Activation through Gold(I)-Catalyzed Reaction
of Iodoalkynes**
Pablo Morꢀn-Poladura, Eduardo Rubio, and Josꢁ M. Gonzꢀlez*
Abstract: The cycloisomerization reaction of 1-(iodoethynyl)-
2-(1-methoxyalkyl)arenes and related 2-alkyl-substituted
derivatives gives the corresponding 3-iodo-1-substituted-1H-
indene under the catalytic influence of IPrAuNTf2 [IPr = 1,3-
bis(2,6-diisopropyl)phenylimidazol-2-ylidene; NTf2 = bis(tri-
fluoromethanesulfonyl)imidate]. The reaction takes place in
1,2-dichloroethane at 808C, and the addition of ttbp (2,4,6-tri-
tert-butylpyrimidine) is beneficial to accomplish this new
transformation in high yield. The overall reaction implies
initial assembly of an intermediate gold vinylidene upon
alkyne activation by gold(I) and a 1,2-iodine-shift. Deuterium
labeling and crossover experiments, the magnitude of the
recorded kinetic primary isotopic effect, and the results
obtained from the reaction of selected stereochemical probes
strongly provide support for concerted insertion of the benzylic
À
C H bond into gold vinylidene as the step responsible for the
formation of the new carbon–carbon bond.
T
he search for new ways to access key reactive intermediates
is key to advancing synthetic methodology. A number of
processes relying on catalytic generation of metal vinylidenes
are known.[1] C(sp ) C(sp ) catalytic coupling reactions
involving intermediate ruthenium vinylidenes have been
presented.[2] Gold(I) catalysis has also been shown to be
successful in a scenario in which dual gold catalysis plays
a prevalent role.[3] Iodoalkynes are an interesting class of
acetylenes endowed with remarkable structural features[4] and
reactivity,[5] and have been proposed as precursors for gold
3
2
À
3
À
Scheme 1. Basis for the proposed C(sp ) H bond activation triggered
by iodovinylidene generation from gold-catalyzed iodoalkyne isomer-
ization.
vinylidenes.[6] Herein, the merging of iodoalkynes with
3
À
gold(I)-catalysis to achieve C(sp ) H bond activation,
related non-iodine-substituted alkynes take part in gold(I)-
catalyzed carboalkoxylation reactions.[8] Data are collected in
Table 1 for gold-catalyzed reactions of 1a, thus illustrating an
initial screening in the search for efficient catalysts and
through prior generation of iodovinylidenes, is documented
(Scheme 1).
The reactivity of 2-(iodoethynyl)benzyloxy derivatives
towards gold(I) catalysts was explored to determine the
feasibility of product formation, arising from a gold iodo-
À
reaction conditions to accomplish the desired C H activation
process.
vinylidene[7] intermediate, through C H insertion chemistry
Previously, the N-heterocyclic gold complex IPrAuNTf2
was found to be an efficient and robust catalyst in the
hydroarylation reaction of iodoalkynes, reactions aimed at
the elaboration of heterocycles through 1,2-iodine shifts.[6b,c]
The reactivity of IPrAuNTf2 was tested in the target cyclo-
isomerization of 1a into 2a. At room temperature, 2a was not
observed. Heating 1a with IPrAuNTf2 (5 mol%) at 808C in
1,2-dichloroethane (DCE) gave rise to the full disappearance
of the iodoalkyne, but 2a could not be isolated from the crude
reaction mixture (entry 1). At this point two different actions
were undertaken. Both the reactivity of gold catalysts, based
on alternative ligands, and the effect of additives were
explored. While the first approach did not led to satisfactory
results (entries 2–4), the second was successful.[9] Gratifyingly,
addition of the hindered ttbp base (ttbp = 2,4,6-tri-tert-
À
rather than involving the oxygen atom in addition processes.
It is worth noting that these are demanding substrates, as the
[*] P. Morꢀn-Poladura, Dr. E. Rubio, Prof. Dr. J. M. Gonzꢀlez
Departamento de Quꢁmica Orgꢀnica e Inorgꢀnica and Instituto
Universitario de Quꢁmica Organometꢀlica “Enrique Moles”
Universidad de Oviedo
C/Juliꢀn Claverꢁa 8, Oviedo, 33006 (Spain)
E-mail: jmgd@uniovi.es
[**] Financial support by the Spanish MINECO (Grant CTQ2013-41511-
P) is acknowledged. P.M.-P. thanks the Spanish Government for
a FPU predoctoral fellowship. We thank Dr. Fernando Rodrꢁguez
(Univ. of La Rioja) for the X-Ray analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409970.
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Table 1: Cycloisomerization of 1a into 2a: Influence of the experimental
conditions.
À
Table 2: Indene derivatives through C H bond activation.
Entry
1
Method[a]
t
2
Yield
[%][b]
A
B
10 min
1 h
22
57
1
2
Entry[a]
Catalyst
Additive
(equiv)
t [h]
Conv.
[%][b]
Yield
[%][b]
A
B
15 min
1 h
26
91
[c]
1
2
3
4
5
6
7
8
9
10
IPrAuNTf2
–
–
–
–
1
24
24
24
9
100
10
8
JohnPhosAuNTf2
(RO)3PAuNTf2
Ph3PAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
0
0
0
62
56
60
61
48
6
A
B
1 h
1 h
79
91
14
3
4
5
ttbp (1)
ttbp (0.5)
ttbp (0.25)
ttbp (0.1)
ttbp (0.1)
ttbp (0.1)
100
100
100
100
100
52
3.5
2.5
1
A
B
10 min
1.5 h
45
87
9[d]
24[e]
A
B
15 min
1 h
90
97
[a] Reaction conditions: 0.1 mmol of 1a, 5 mol% of the catalyst, 0.1m in
DCE. [b] Yield and conversion estimated from 1H NMR analysis of the
crude reaction mixture upon addition of 1,3,5-trimethoxybencene, as
internal standard. [c] Final product decomposes under the reaction
conditions. [d] Reaction temperature 508C. [e] Reaction temperature
208C. DCE=1,2-dichloroethane, IPr=1,3-bis(2,6-diisopropyl)phenyl-
imidazol-2-ylidene; NTf2 =bis(trifluoromethanesulfonyl)imidate,
TBS=tert-butyldimethylsilyl.
A
B
1.5 h
2.25 h
89
93
6
7
A[c]
B
–
–
84
1.5 h
butylpyrimidine) allowed the identification of 2a in the crude
reaction mixtures (entries 5–10).[10] Further adjustment of the
amount of added ttbp and the reaction time furnished an
initial set of reaction conditions for the conversion of 1a into
2a (entry 8).[11] Decreasing the reaction temperature to 508C
required a significantly extended reaction time for the
complete consumption of 1a and thus resulted in a lower
yield (entry 9). The reaction conditions outlined in entry 8 of
Table 1 were then used to obtain and isolate several products
from different iodoalkynes 1.
Interestingly, the targeted reaction is associated with the
distinct reactivity of the iodine-substituted derivatives, and
also provides support for the hypothesis that the iodoalkyne–
iodovinylidene isomerization step takes place smoothly. The
synthesized indene derivatives 2 are shown in Table 2, which
also details the specific reaction conditions used for the
synthesis of each of the assembled carbocyclic frames.[12] The
cycloisomerization of 1 into 2 was conducted on a 0.2 mmol
scale of the parent compound. The outcome for the reaction
with and without addition of ttbp are reported (see methods B
and A, respectively), thus highlighting the beneficial effect of
this additive in accessing 2, for all the given examples. The
model compound 1a gives rise to the desired cyclization in
moderate yield, while the more sterically demanding 1b
affords the corresponding product in a significantly higher
yield.[13] The acetal 1c and the alkyl ether 1d also react under
these reaction conditions.[14]
A[d]
B[d]
4 h
18 h
28
84
8
A[c]
B
–
–
78
40 min
9
A
B
35 min
1 h
89
96
10
A
B
24 h
1.5 h
32
78
11
A
B
24 h
1.5 h
41[e]
91[e]
12
13
B[d]
4 h
72
À
Moreover, the activation of the benzylic C H bond in
[a] Method A: 0.2 mmol of 1, IPrAuNTf2 (5 mol%), 0.1m in DCE, 808C.
Method B: The same as for Method A, but adding ttbp (10 mol%).
[b] Yield of isolated 2. [c] The product decomposes under the reaction
conditions. [d] Additional 5 mol% of IPrAuNTf2 added. [e] 2k isomerizes
into 3k using Et3N treated silica gel for the isolation.
a simple linear alkyl chain gives the indene 2e in high yield
(Table 2, entry 5). Tertiary benzylic positions and substrates
showing additional remote functionality, including a carbazole
ring at the benzylic position, also undergo this reaction
2
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(entries 6–12). The reaction of 1l, the dideuterated-benzyl
analogue of 1j, gives 2l in reasonable yield, thus retaining the
deuterium, which becomes equally split between the benzylic
and vinylic positions. Notably, a more sluggish reaction for 1l,
in comparison to that of 1j, was observed and required
a longer reaction time and higher catalyst loading (entries 10
and 13).
The mechanism for this new reaction was analyzed by
using a crossover reaction, evaluating the magnitude of the
primary kinetic isotopic effect (PKIE) from an intramolecular
competition experiment[15] and testing the robustness of the
stereochemistry. Relevant information is depicted in
Scheme 2. As depicted in Scheme 2a, H/D scrambling was
À
Scheme 3. Proposed direct C H insertion reaction into gold iodo-
vinylidenes catalytically generated from iodoalkynes.
idenes generated upon activation of terminal alkynes using
platinum[2a,19] or ruthenium.[2a,b]
In short, a new catalytic transformation of iodoalkynes is
presented. The resulting iodine-substituted indenes are ver-
satile building blocks, which are not easy to assemble using
other approaches such as carbocycle iodination. From a mech-
anistic persepctive, this study reveals a distinct reaction path
À
for C H insertion reactions involving metal vinylidenes,
through use of an appropriate gold catalyst and switching
from hydrogen to iodine as the substituent in the parent
alkyne precursor. It also impacts ongoing efforts to access
gold vinylidene, a growing and promising research area, by
contributing to efforts in search of alternative precursors.
Scheme 2. Mechanistic insights: further experimental data. TIPS=tri-
isopropylsilyl.
Experimental Section
Method B (Table 2): The substrate 1 (1 equiv, 0.2 mmol) is dissolved
in 1,2-dichloroethane (2 mL) under argon atmosphere. Then, 2,4,6-
tri-tert-butyl pyrimidine (0.1 equiv, 0.02 mmol) and IPrAuNTf2
(0.05 equiv 0.01 mmol) are added consecutively. The mixture is
stirred and heated to 808C until total depletion (TLC) of 1 is
observed. The reaction cooled to room temperature, was concen-
trated under vacuum, and the residue was purified by flash
chromatography (specific conditions for each substrate are given in
the Supporting Information)
not observed when mixture of 1b and the doubly deuterium
labeled compound 1l was reacted with IPrAuNTf2. Inspection
of the crude reaction mixture by NMR spectroscopy was
conclusive in this regard.[12] This result suggests an intra-
molecular character of the hydrogen-transfer process. In the
same way, the catalytic activation of [D]-1b (Scheme 2b)
provides a value of 1.77 for kH/kD, which was easily estimated
upon recording NMR data for the mixture of compounds
obtained.[12] Furthermore, the reaction of the enantioenriched
chiral probe 1g provides key information to assess the timing
Received: October 10, 2014
Revised: November 28, 2014
Published online: && &&, &&&&
À
À
of the C H bond-cleavage and the C C bond-making events.
Interestingly, the stereochemistry remained unaltered.[16]
Overall, these data do not support alternative gold-catalyzed
À
Keywords: alkynes · C C coupling · cyclization · gold ·
reaction mechanisms
À
.
C C bond-forming events involving a stepwise hydride-
transfer process to give the intermediate gold vinylidene.[17]
At the same time, the absence of H/D scrambling processes,
the recorded PKIE value, which is in line with those reported
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À
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ruthenium(II) and platinum(II) vinylidenes[2a] generated from
terminal alkynes, as well as the retention of stereochemistry
for the enantioenriched probes are in keeping with the
mechanistic proposal outlined in Scheme 3. The proposed
generation of an iodovinylidene intermediate leading to
product formation in one step is in accordance with collected
data. The proposed mechanism differs from those docu-
mented for indene-formation processes involving metal vinyl-
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
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yields. Interestingly, in some cases it was possible to monitor by
the formation and fast decomposition of the product (by TLC) in
the absence of the base.
[11] Different metal catalysts were tested under this experimental
protocol, with AuCl and PtCl2 among them. For both the
consumption of 1a occurred after reacting for 1 h and 4 h,
respectively. The formation of 2a was evident by NMR analysis
of the crude reaction mixtures but, in both cases additional
products were present.
[12] For the synthesis and characterization of 1 and 2, including
copies of the spectra, and mechanistic details, see the Supporting
Information.
[13] 1a nicely isomerizes to 2a in high yield, however, competitive
formation of minor amounts of an alternative cyclization
product, likely from oxygen getting involved as nucleophile,
was noticed in the crude reaction mixture. These two products
turned out to be difficult to separate and 2a was isolated in
moderate yield. For the catalytic conversion of 1b into 2b, no
evidence of alternative by-product was found by NMR analysis
of the corresponding crude reaction mixture.
[14] 1-(bromoethynyl)-2-(methoxymethyl)benzene, an analogue of
1d, was tested under the same reaction conditions. No cycliza-
tion was observed and the starting material was recovered
essentially unaltered. The cyclization of 1-(iodoethynyl)-2-(1-
methoxyethyl)cyclohex-1-ene resulted in a sluggish reaction
where no cycloisomerization was observed.
[15] The magnitude of the internal intramolecular isotope effect
might be slightly slower than the intermolecular isotope effect.
See: K. B. Wiberg, L. H. Slaugh, J. Am. Chem. Soc. 1958, 80,
3033 – 3039.
[16] A stepwise transformation involving strong ion pairing could not
be safely disregarded on this basis. However, the counteranion
effect was tested without noticeable deviation of the behavior as
shown in Scheme 2c. Thus, when 1g (e.r. 86:14) was treated with
5 mol% of IPrAuCl/AgSbF6 and ttbp (10 mol%) in DCE at
808C, for 0.5 h, 2g was isolated in 53% yield (e.r. 81:19). This
result might provide additional evidence for the concerted
insertion step.
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[18] This value is in good agreement with those recorded and
3
À
[9] For details on reaction conditions see the Supporting Informa-
tion.
computed for insertion reactions of C(sp ) H bonds into metal
carbenes. See for instance: a) P. Wang, J. Adams, J. Am. Chem.
Soc. 1994, 116, 3296 – 3305; b) H. M. L. Davies, T. Hansen, M. R.
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[10] Other bases with different features were tested (see page 3 in the
Supporting Information). Only weak bases such as 2,6-dichloro-
pyridine or norbornene resulted in full conversion, although less
efficiently than ttbp to generate 2a. Less hindered or more basic
ones did not result in full conversion. It is likely that the role of
the base implies a subtle balance between the catalyst reactivity
and the decomposition of the product. Overall, the use of the
base results in a robust and reproducible reaction and higher
[19] Theoretical calculations predicted platinum vinylidene being
À
generated in the rate-limiting step for this tertiary C H benzylic
insertion reaction. See: G. B. Bajracharya, N. K. Pahadi, I. D.
Gridnev, Y. Yamamoto, J. Org. Chem. 2006, 71, 6204 – 6210.
4
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Synthetic Methods
P. Morꢁn-Poladura, E. Rubio,
J. M. Gonzꢁlez*
&&& — &&&
À
Intramolecular C H Activation through
Gold(I)-Catalyzed Reaction of
Iodoalkynes
Golden dance: The cycloisomerization
reaction of 1-(iodoethynyl)-2-(1-methoxy-
alkyl)arenes gives the corresponding 3-
iodo-1-substituted-1H-indene. Gold(I)
catalysis triggers a selective intramolec-
ular carbon–carbon bond-forming event,
which involves the insertion of benzylic
À
C H bonds in the catalytically assembled
gold(I) iodovinylidene intermediate.
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