Mechanistic switch in dual gold catalysis of diynes: C(sp3)-H activation through bifurcation - Vinylidene versus carbene pathways

Article abstract of DOI:10.1002/anie.201208777

The other side of the mountain: Changing the framework of diyne systems opens up new cyclization modes for dual gold catalysis. Instead of a 5-endo cyclization and gold vinylidenes a 6-endo cyclization gives rise to gold-stabilized carbenes as key intermediates for selective C-H insertions. Copyright

Full text of DOI:10.1002/anie.201208777

Angewandte
Chemie
DOI: 10.1002/anie.201208777
Dual Gold Catalysis
Mechanistic Switch in Dual Gold Catalysis of Diynes: C(sp3)–H
Activation through Bifurcation—Vinylidene versus Carbene
Pathways**
Max M. Hansmann,* Matthias Rudolph, Frank Rominger, and A. Stephen K. Hashmi*
À
Transition-metal-catalyzed C H activa-
tion and functionalization is an area of
broad interest as it enables the rapid
creation of complex molecules in an
atom-economic fashion.[1] The selective
À
formation of C C bonds by means of
3
À
À
C H activation of unactivated C(sp ) H
bonds without any directing groups in
close proximity is still a challenging task
in transition-metal catalysis and a highly
active field of current research.[2]
In the last decade gold catalysis has
developed into a powerful synthetic tool
in organic synthesis.[3] While there are
some reports on the heterolytic cleavage
3
À
of C(sp ) H bonds by employing a gold-
3
À
Scheme 1. Au-catalyzed diyne cyclizations involving C(sp ) H activation.
catalyzed hydride transfer, usually spe-
cial electronically or structurally facili-
tated hydride-transfer steps are required.[4] Very recently,
Zhangꢀs group and our group could show that gold vinyl-
precursors, which have a terminal C C bond, could lead to
ꢀ
the unprecedented 6-endo-dig cyclization mode; the inter-
mediate of the 5-exo-dig cyclization with the anellation of two
five-membered rings should be less stable than the inter-
mediate of the 6-endo-dig cyclization with the anellation of
one five- and one six-membered ring. This would further
expand the scope of the gold s,p dual-activation chemistry.[8]
Instead of the expected thiofulvene substrates, benzothio-
phenes should be observed (Scheme 1, bottom). The scope of
this reaction and mechanistic investigations, indicating an
alternative pathway besides the vinylidene chemistry, are
discussed herein.
idenes[5] as highly reactive intermediates can efficiently
activate C(sp ) H bonds,[6] even of alkanes in an intermolec-
3
À
ular manner.[6c]
In the intramolecular reaction of alkyl chains attached to
the aromatic diyne substrates, selective activation of the b-
hydrogen atom was observed, providing products with
a fulvene substructure (Scheme 1, top). Based on computa-
tional results of Zhang et al., who reported on a bifurcation[7]
pathway in the cyclization event of diynes,[6a] we envisioned
that the use of 2,3-diethynylthiophenes as heteroaromatic
Heteroaromatic backbones were of particular interest for
us as, apart from the electronic effect, the widening of the
alkyne–alkyne angle from 608 to 708 should have a strong
effect on the chemoselectivity.[9] Our initial experiment
addressed the transformation of 2,3-diethynylthiophene 1a
with IPrAuNTf2 (IPr= 1,3-bis(2,6-diisopropylphenyl)imida-
zol-2-ylidene, Tf = trifluoromethylsulfonyl) as the catalyst
(Scheme 2, top). To our delight, instead of a thiofulvene that
would arise from the 5-endo-pathway (the analogue of the
product formed from the corresponding benzene derivative),
indanothiophene 2a was obtained as the exclusive product in
[*] M. Sc. M. M. Hansmann, Dr. M. Rudolph, Dr. F. Rominger,[+]
Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
[+] Crystallographic investigation.
[**] We thank Umicore AG & Co. KG for the generous donation of gold
salts. M.M.H. is grateful to the Fonds der chemischen Industrie for
a Chemiefonds scholarship and the Studienstiftung des deutschen
Volkes. The computational studies were supported by bwGRiD,
member of the German d-Grid initiative, funded by the Ministry for
Education and Research (Bundesministerium fꢁr Bildung und
Forschung) and the Ministry for Science, Research and Arts Baden-
Wꢁrttemberg.
ꢀ
moderate yield. Shifting the alkyl substituent from the C C
ꢀ
bond at the 3-position of the thiophene unit to the C C bond
at the 2-position (1b) resulted in the clean formation of
regioisomer 2b in significantly higher yield. As in the reaction
with 1a, not even traces of the five-membered pentalene
product were observed (Scheme 2). So far reports on the
transition-metal-catalyzed cyclization of enediynes with five-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208777.
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Table 1: Au-catalyzed synthesis of indanothiophenes, dibenzothio-
phenes, and benzofurans.[a]
Entry
1
Substrate 1
Product 2
t
[h]
Yield
[%]
Scheme 2. Au-catalyzed synthesis of indanothiophenes: role of the
position of the terminal alkyne.
1.5
2
81
90
87
membered heteroaromatic units are not very common.[10]
There are reports on the Au-catalyzed addition of external
or internal nucleophiles to enediynes, which undergo cycliza-
tion after the first addition event, but these systems are not
2
3
À
undergoing C H bond activation and the reaction follows
a very different mechanism.[11] The connectivity in the cyclo-
isomerization product is unusual as well. With six-membered-
ring substrates PtII and sometimes Ru generate products with
2,3 instead of 3,4 ring fusion; other Ru catalysts give only poor
yields of products with similar connectivity.[12]
2
4
5
6
7
2.5
2
50
After synthesizing a small library of 2,3-diethynylthio-
phenes by a novel sequential one-pot tandem Sonogashira
strategy,[9] we performed catalyst-screening experiments on
two substrates.[9] To our delight, the dual-activation catalysts
previously developed in our group[13a] turned out to be ideal
catalysts. Bulky phosphane ligands such as BrettPhosAuNTf2
and XPhosAuNTf2 (BrettPhos = 2-(dicyclohexylphosphino)-
3,6-dimethoxy-2’,4’,6’-triisopropyl-1,1’-biphenyl, XPhos = 2-
74
1.5
2.5
73
dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl)
per-
formed equally well in terms of selectivity, but in general
longer reaction times were required. This is in accordance
with a prolonged initiation phase for the generation of the
gold acetylide.[5a] Simple precatalysts such as AuCl and PtCl2
gave no desired product.
61[b]
Next we investigated the scope of this transformation
(Table 1). By choosing the appropriate alkyl chain length, it is
8
9
6
2
56[c]
3
À
possible to perform a C(sp ) H insertion into a methyl group,
generating the unsubstituted indano[5,6-b]thiophene scaffold
(Table 1, entry 1). Extending the alkyl chain length results in
the selective activation of a methylene group, regardless of
whether unactivated (Table 1, entries 2 and 3) or benzylic
hydrogens (entry 5) are available. If enantiotopic methyl
groups are present, it is possible to create a stereocenter in the
6-position of the indanothiophene substructure (Table 1,
entry 4). It is remarkable that a methine group can also be
selectively activated giving access to higher substituted
81
10
6.5
56
À
products with quaternary centers (Table 1, entry 6). C H
[a] Reactions were performed on a 0.2 mmol scale in toluene (0.2m) at
708C. All yields refer to isolated product. [b] Single diastereomer.
[c] d.r.=1:3.
groups in a cycloalkyl substituent can be activated as well.
While the cyclopentane fragment smoothly delivered the
polycyclic indanothiophene exclusively as the cis diastereo-
mer (Table 1, entry 7), a mixture of diastereomers was
obtained for the cyclohexane analogue (entry 8). Instead of
starting with the 2,3-diethynylthiophenes, it is equally possible
to employ the corresponding benzothiophenes. After the gold
catalysis benzo[b]indano[5,6-b]thiophene 2j was obtained in
good yield (Table 1, entry 9). The transformation could also
be extended to furans, but the yield dropped significantly
(Table 1, entry 10). The assignment of the indanothiophene
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Table 2: Au-catalyzed synthesis of fluorenothiophenes.[a]
structure was unambiguously proven by an X-ray crystal
structure analysis[9,14] of compound 2j.
3
À
As we could selectively address the g-C(sp ) H bond in
3
À
the presence of other C(sp ) H bonds, we were interested in
À
achieving a benzylic C H activation even in the presence of
2
[15]
À
a C(sp ) H bond in b-position to the alkyne. While the
corresponding (2-ethynylphenyl)alkynes insert into the b-
Entry
Substrate 1
Product 2
Yield
[%]
2
[6c]
À
C(sp ) H bond giving access to dibenzopentalenes, fluo-
renothiophenes were obtained as single products for the five-
membered thiophene unit (Table 2). As these compounds
have a high potential for applications in material science[16]
and are difficult to access otherwise, this new approach is
highly interesting. The scope of this reaction is broad as well,
and different substitution patterns on the aromatic ring are
tolerated. Electron-rich diynes (Table 2, entries 1–4) as well
as electron-poor (entries 5 and 6) substrates can be employed.
It is noteworthy that with the very electron-rich substrate 1o
only a few turnovers could be achieved with the dual-
activation catalyst. A possible explanation could be the
formation of a highly stable gem-diaurated intermediate
inhibiting the catalytic cycle; a similar stabilization by donor
atoms was also observed by Gagnꢁ and co-workers.[17] This
assumption was underlined by using catalytic amounts of
tBuXPhosAuNTf2, a catalyst with a sterically demanding
ligand which should destabilize gem-diaurated Au compounds
and therefore restore catalytic activity. Indeed with 1o
complete conversion and good yields were observed. Elec-
tron-deficient fluorinated compounds 1p and 1q could also be
transformed into the corresponding fluorinated fluorenothio-
phenes in good yields (Table 2, entries 5 and 6). The X-ray
crystal structure analysis of compound 2l clearly proves the
connectivity of the product.[9,14]
1
77
74
2
3
4
5
82
84[b]
92
With 2,6-dimethylphenyl-substituted diyne 1r no forma-
tion of the desired fluorenothiophene was observed; instead
a solvent molecule was incorporated into the product,
forming benzothiophene 2ra (Scheme 3). This gold-catalyzed
hydroarylating aromatization takes a different reaction path-
way, which is in accordance with the recent discoveries in the
field of Au–vinylidene chemistry.[5a] A possible reason for this
reaction pathway is the steric hindrance of the two methyl
groups in ortho position which lead to a twist of the aromatic
ring out of the plane of the molecule. Instead of the
intramoleculary available hydrogen atoms, which are out of
reach, a solvent molecule is attacked by the electrophilic
center, leading to a product of formal intermolecular
6
79
[a] Reactions were performed on a 0.2 mmol scale in toluene (0.05m) at
708C. All yields refer to isolated product. [b] 5 mol% tBuXPhosAuNTf2
was used instead.
2
À
C(sp ) H activation. (Incorporation of solvent molecules in
the reactions of the substrates shown in Table 2 was never
evident by GC–MS or NMR methods.) Interestingly, when
deuterated benzene was employed the b-product 2rb formed
exclusively and also the deuterium was selectively transferred
to only one of the two possible positions. When we used a 1:1
mixture of benzene/[D6]benzene, we observed a 1:1 mixture
of 2ra and 2rb without formation of any crossover products.
To gain further mechanistic insight, stoichiometric experi-
ments were conducted. Treating the gold acetylide of
thiophene diyne 5 with a stoichiometric amount of IPrAuNTf2
resulted in the formation of the gem-diaurated species 6
(Scheme 4). Slow vapor diffusion of pentane into a solution of
6 in dichloromethane at À308C gave UV/Vis-fluorescent
Scheme 3. Experiments with the sterically hindered substrate 1r.
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(even in the solid state)[9,18] orange crystals suitable for X-ray
diffraction.[9,14] Several IR spectroscopic measurements of the
X-ray sample and the bulk material proved that only this gem-
diaurated compound is formed.[9] Interestingly, when the
same procedure was employed for the regioisomer 7, the gem-
diauration occurred at the opposite position. In this case
crystals suitable for X-ray diffraction could be obtained as
well.[9,14] These structures confirm the connectivity of the
products and unambiguously prove the relevance of diaurated
Au compounds in these transformations. In accordance to
earlier results,[5,6] these species can be seen as resting states of
the dual-catalysis cycle.
Scheme 4. Stoichiometric experiments capturing arylgold intermedi-
ates as diaurated species.
Furthermore, we performed deuterium-labeling studies to
gain insight into the nature of the proton transfer leading to
the two benzothiophene H atoms (Scheme 5). Deuterium
ꢀ
labeling of the terminal C C unit resulted in high deuterium
enrichment on the arene position that was diaurated in the
stoichiometric reaction. The second arene proton was unaf-
fected. The gold-catalyzed reaction of the deuterated com-
pound 11 resulted in exclusive deuterium enrichment at the
other arene position. This clearly indicates a completely
selective reaction mechanism.
Our experiments clearly indicate a gold-catalyzed dual-
activation mode (Scheme 6). In order to gain further inside
and to support our outlined mechanism, we performed DFT
calculations at the B3LYP/cc-pVDZ level of theory on the
main intermediates and transition states (Figure 1; for
a complete energy profile see the Supporting Information).[9]
In line with the related transformations,[5,6] the s,p dual-
activation cycle is initiated by the precatalyst, which generates
the gold-bonded thiophene acetylide II. According to the
work of our group and others, the Au fragment coordinates
Scheme 5. Deuterium-labeling experiments.
Scheme 6. Proposed catalytic cycle:6-endo-dig versus 5-endo-dig cyclization.
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hydride shift lead to the final
diaurated compound 8 (this is
also in accord with the labeling
experiments).
Alternatively, we cannot
rule out a 5-endo-dig cycliza-
tion forming the gold(I)–vinyl-
idene IX, which after selective
3
À
insertion into the d-C(sp ) H
bond (X) and subsequent ring
expansion would lead to the
same intermediate VI. Based
on our computational results
this pathway is energetically
disfavored.[9]
In summary, we report
a gold-catalyzed cycloisomeri-
zation of 2,3-diethynylthio-
ꢀ
phenes with a terminal C C
bond affording indanothio-
phenes and fluorenothio-
phenes. Variation of the back-
bone of the diyne system opens
up a new pathway for the
3
À
intramolecular C(sp ) H acti-
vation event. Based on theo-
retical work we postulate
a switch in selectivity along
a bifurcation pathway, for the
first time leading to products
through s,p dual activation
and a 6-endo diyne cyclization.
Figure 1. Calculated energy profile for the Au-catalyzed diyne cycloisomerization. TS=transition state,
E+ZPE=electronic energy+zero point energy.
preferentially to the Au acetylide as shown in I.[13,19] An
additional coordination to the sulfur atom of the thiophene
potentially shifts the equilibrium to II (which also would
explain the higher reactivity of substrates bearing the alkyl-
substituted alkyne in the 2-position of the thiophene). For the
generation of product 2 two potential pathways must be
considered: a 6-endo-dig cyclization leading to intermediate
III and a 5-endo-dig cyclization generating the gold–vinyli-
dene IX. Until now, six-membered aromatic products were
Deuterium-labeling studies together with the capture and
characterization of gem-diaurated gold intermediates, which
serve as a potential resting state in the catalytic cycle, are in
agreement with the new reactivity of the dual gold catalysis.
Received: November 1, 2012
Published online: January 30, 2013
Keywords: alkynes ·
.
À
C H activation · computational chemistry · dual catalysis · gold
À
never observed to form when C H bonds were available for
an intramolecular reaction. The experiments described here
indicate a switch in the bifurcation pathway by a slight change
of the aromatic backbone.[20] Indeed, we could locate
a transition state connecting the 6-endo cyclization product
III and the 5-endo cyclization product IX typical for a ridge–
valley situation which would be in accordance with the work
of Zhang et al.[6a] We assume that the two cyclization products
equilibrate via this transition state and that the selectivity is
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Huang, A. Goeke, C. J. Nevado, J. Am. Chem. Soc. 2010, 132,
4720 – 4730; b) Z. J. Wang, D. Benitez, E. Tkatchouk, W. A.
Goddard III, F. D. Toste, J. Am. Chem. Soc. 2010, 132, 13064 –
13071; c) E. L. Noey, X. Wang, K. N. Houk, J. Org. Chem. 2011,
76, 3477 – 3483.
[9] See the Supporting Information.
[21] We could locate two energetically (less than 1 kcalmolÀ1 apart)
and structurally very closely related transition states leading to
the 5-endo and 6-endo products, which can be seen as one
transition state leading to the posttransition state.[9]
[22] The optimized structure of carbene IV shows a high degree of
distortion towards a bent Au–allene, which is stabilized through
the para-digold substitution, and this distorsion acts as driving
force.
[10] The only two examples are tellurium-mediated cyclizations:
a) H. Ebata, E. Miyazaki, T. Yamamoto, K. Takimiya, Org. Lett.
2007, 9, 4499 – 4502; and a platinum-catalyzed reaction: b) C.-Y.
Lo, M. P. Kumar, H.-K. Chang, S.-F. Lush, R.-S. Liu, J. Org.
Chem. 2005, 70, 10482 – 10487.
[11] a) S. Naoe, Y. Suzuki, Y. Inaba, S. Oishi, N. Fujii, H. Ohno, J.
Org. Chem. 2012, 77, 4907 – 4916; b) G. Ferrara, T. Jin, K. Oniwa,
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Angew. Chem. Int. Ed. 2013, 52, 2593 –2598