Gold catalysis: Highly functionalized cyclopentadienes prepared by intermolecular cyclization of ynamides and propargylic carboxylates

Article abstract of DOI:10.1002/anie.201301382

When an ynamide meets a gold carbenoid: Highly electrophilic gold carbenoids available from propargylic esters by means of 1,2-acyloxy migration open up new reaction pathways for ynamide gold chemistry. In this way highly functionalized cyclopentadiene derivatives become accessible (see scheme; EWG=electron-withdrawing group). Copyright

Full text of DOI:10.1002/anie.201301382

Angewandte
Chemie
DOI: 10.1002/anie.201301382
Gold Carbenoids
Gold Catalysis: Highly Functionalized Cyclopentadienes Prepared by
Intermolecular Cyclization of Ynamides and Propargylic
Carboxylates**
Eva Rettenmeier, Andreas M. Schuster, Matthias Rudolph, Frank Rominger, Christoph A. Gade,
and A. Stephen K. Hashmi*
Following the first preparation of an ynamide in 1972,[1] this
class of substrates remained nearly unexplored for almost
thirty years. This changed tremendously at the beginning of
the new millennium when a real “ynamide boom” was
triggered by new synthetic methods, which enabled the
synthesis of starting materials under mild conditions and
with a higher functional group tolerance. Follow-
For initial experiments, we prepared ynamide 1a accord-
ing to Hsungꢀs protocol[3d] and performed a short screening
with propargylic acetate 3a (Table 1). Even though 1,2-
acetate shifts are also reported for AuIII compounds,[11] only
a complex mixture was obtained in the case of simple AuCl3
(Table 1, entry 1), while no conversion of the starting material
ing initial work with hypervalent iodine reagents
[2]
,
more and more copper-catalyzed ynamide
syntheses have dominated the field.[3] Based on
the easy availability of ynamides as starting
materials, an array of useful organic transforma-
tions have been published in the recent years
leading to a variety of valuable synthetic building
blocks.[4] Due to the exceptional ability of gold to
activate multiple bonds,[5] ynamides have also
been used as substrates in gold-catalyzed reac-
tions for a number of useful transformations.[6]
All of these reactions are initiated by the
attack of a nucleophile at the p-activated triple
Scheme 1. Known and proposed reactions of ynamides with metal carbenoids.
Table 1: Initial catalyst screening.[a]
bond of the ynamide. An exception is a gold-catalyzed
dimerization of ynamides that was recently reported by
Gagosz et al.[7] This exciting transformation utilizes the
ambivalent nature of the ynamide. Owing to the adjacent
heteroatom, an ynamide can also react as a nucleophile at its
b-carbon atom. Inspired by a contribution by Li and Hsung,
who reported on a synthesis of aminofurans from the reaction
of ynamides with diazomalonate or phenyliodonium ylides,[8]
we envisioned that gold carbenoids derived from propargylic
esters would be electrophilic enough for a selective reaction
in which the nucleophilic nature of ynamides could be
exploited. In addition to their easy and mild access,[9] these
Entry
1
Catalyst
AuCl3
Yield [%]
complex mixture
2
–
À
gold carbenoids possess an additional C C double bond,
which should enable a different reaction pathway leading to
highly substituted cyclopentadiene derivatives (Scheme 1).[10]
3
4
Ph3PAuCl/AgNTf2
Ph3PAuCl/AgSbF6
4
2
[*] Dipl.-Chem. E. Rettenmeier, Dr. A. M. Schuster, Dr. M. Rudolph,
Dr. F. Rominger, C. A. Gade, Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
5
/AgNTf2
13
6
6/AgSbF6
8
Ruprecht-Karls-Universitꢀt Heidelberg
7
8
9
10
11
SPhosAuCl/AgNTf2
SPhosAuCl/AgSbF6
IPrAuCl/AgNTf2
IPrAuCl/AgSbF6
AgNTf2
23
33
58
32
–
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
[**] We thank Umicore AG & Co. KG for the generous donation of gold
salts.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301382.
[a] SPhos= 2-dicyclohexylphosphino-2’,6’-dimethoxybiphenyl, Tf=tri-
flate, Ts=tosyl.
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was observed for the pyridine carboxylate AuIII complex 5.[12]
Triphenylphosphanegold(I) chloride activated by either
AgNTf2 or AgSbF6 gave only traces of product (Table 1,
entries 3 and 4). First promising results were obtained using
NAC (NAC = nitrogen acyclic carbene) complex 6 activated
with AgNTf2 but still only a 13% yield of product was
detected (Table 1, entry 5). The yield significantly increased
when the SPhos ligand was used (Table 1, entries 7 and 8). But
by far the best results were obtained with the commercially
available [IPrAuCl] (IPr= 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene), which delivered good yields in combina-
tion with AgNTf2 as a halide scavenger (Table 1, entry 9). In
these reaction with the IPr ligand a strong influence of the
counterion was observed. The yield increased from 32% to
Table 2: Synthesis of cyclopentadienes 4 under the optimized condi-
tions.[a]
Entry
1
Acetate
Ynamide
Product
Yield [%]
53
1a
À
58% when SbF6 was used instead of NTf2À. The control
experiment with only silver triflimide resulted in no con-
version (Table 1, entry 11).
After this short optimization process we performed the
conversion on a preparative scale. A product was obtained in
53% yield which indeed combined the two starting materials
in a completely atom-economical[13] fashion. Based on NMR
analysis using two-dimensional NMR techniques (HMBC,
HSQC) we determined the product to be the desired highly
functionalized cyclopentadiene derivative 4a (Table 2,
entry 1). Final proof was provided by an X-ray crystal
structure analysis of compound 4a.[14]
2
3
1a
1a
47
56
To evaluate the scope and limitations of this transforma-
tion, we synthesized a series of starting ynamides and reacted
them with different propargylic carboxylates under the
optimized conditions. The first experiments were conducted
with ynamide 1a and a series of differently substituted
propargylic acetates. Substrates with para-substituted phenyl
groups in the propargylic position showed similar reactivity
(Table 2, entries 2–4, 30–56%), but unfortunately the yield
dropped significantly when the aromatic moiety had a meta-
methoxy group (Table 2, entry 5, 17%). In this case the
methoxy group at the arene has no mesomeric effect on the
p system of the alkyne. An even more deactivated system
with an electron-withdrawing bromo substituent in meta-
position did not lead to the desired product (Scheme 2).
Instead dimerization of the ynamide was observed. Interest-
ingly, this ynamide-dimerization product was only observed if
no reactions with the acetate precursors took place. This
indicates that whenever the carbenoid is formed, a fast
reaction suppresses the competing reaction pathway or the
major part of the catalyst is not available for ynamide
dimerization as it is incorporated in the gold carbenoid. This is
4
5
6
7
8
1a
1a
1b
1c
1c
30
17
70
71
82
9
1c
56
Scheme 2. Dimerization as competing pathway.
2
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Table 2: (Continued)
Table 2: (Continued)
Entry
Acetate
Ynamide
Product
Yield [%]
52
Entry
Acetate
Ynamide
Product
Yield [%]
67
21
1d
10
1c
11
12
1d
1d
49
82
underlined by the observation that in all other cases even if
yields of the intermolecular cyclization products were not
perfect, no traces of dimerization were visible in the crude
1H NMR spectra. Nevertheless, substrates substituted with
moieties that do not contain hydrogen atoms for a competing
hydride shift led to a significant increase in yields, from 56%
(Table 2, entry 3) to 70% (entry 6) for the tert-butyl group. It
seems likely that competing pathways initiated by an initial
hydride shift might also occur for the intermolecular cycliza-
tion reactions, but also simply the increased steric environ-
ment might favor the product-forming pathway.
13
14
1d
1d
72
55
Next we changed the alkyl substituent at the ynamide
nitrogen. On shifting from methyl to benzyl, no big electronic
effects but rather the increased bulkiness of the benzyl group
should have an effect (Table 2, entries 7–9). Indeed, the
intermolecular cyclization reaction turned out to be more
efficient than for the methyl-substituted ynamide. A signifi-
cant increase in yields could be obtained for 3a, 3, and 3d
(71%, 82%, and 56%). Even propargylic carboxylate 3 f
bearing an electron-deficient p-chlorobenzyl substituent
could be transferred in acceptable yields (Table 2, entry 10,
52%). Next we tested a series of triisopropylsilyl-subtituted
ynamides in combination with differently substituted prop-
argylic acetates. For 3c and 3d yields were much higher than
for the linear alkyl-substituted counterparts (Table 2,
entries 12 and 13, 82% and 72%). This is in accordance
with the previously mentioned increase in yield for substrates
with moieties that are unable to undergo competing hydride-
shift pathways. The only exception was 3a which provided 4k
in only 49% yield (Table 2, entry 11). To complete the series
of halogen-substituted arenes, we included bromo-substituted
3g and 3h. Not unexpectedly, yields for the para-substituted
substrate (Table 2, entry 14, 55%) were higher than for the
meta-substituted (entry 15, 34%) which can also be rational-
ized by increased steric interactions as well as an electronic
interactions. Substrates containing two bulky groups, the
triisopropylsilyl and the benzyl group, were also investigated.
The obtained yields were also good to moderate (Table 2,
entries 16 and 17, 60% and 74%), despite the steric demand
that is evident in the solid -tate molecular structure of 4p.[14]
The even more demanding mesitylsulfonyl group in ynamide
1 f led to a decrease of the yield (Table 2, entry 18, 63%).
Interestingly, a shift to the sterically more demanding pivaloyl
group as the migrating carboxyl group did not significantly
affect the yields, which were comparable to those with the
acetyl-substituted derivatives (Table 2, entries 19–21, 60%,
82%, and 67%).
15
16
1d
1e
34
60
17
1e
74
18
19
20
1 f
63
60
82
1d
1d
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In summary, highly substi-
tuted cyclopentadienes can be
obtained through the activation
of ynamides by highly electro-
philic gold carbenoids. The
products are formed under
mild reaction conditions and
with complete atom economy.
This methodology opens up new
possibilities to exploit the
potential of ynamides in combi-
nation with gold catalysis.
Received: February 16, 2013
Published online: && &&,
&&&&
Scheme 3. Mechanistic proposal.
Keywords: cyclopentadienes · dimerization · gold ·
propargylic carboxylates · ynamides
A short mechanistic rationale[15] for the cyclopentadiene
synthesis is depicted in Scheme 3. After the initial formation
of gold carbenoid III through 1,2-acetal shift, two mechanistic
scenarios seem reasonable. Pathway a describes a cycloprope-
nation reaction of the electron-rich ynamide. This is an
unknown reaction for gold carbenoids derived from prop-
argylic acetates, but recently Davies and Briones reported on
the cyclopropenation of internal alkynes with gold carbenoids
derived from diazo acetates.[16] The next step comprises a ring
opening which is triggered by the lone pairs at the nitrogen
atom. The resulting zwitterionic keteniminium cation/allylic
anion VI then undergoes ring closure to form the final
product 4. This cyclopropene to cyclopentadiene rearrange-
ment might also be a concerted[17] or a gold-assisted pro-
cess.[18] The alternative pathway b comprises a stepwise
reaction in which the nucleophilic carbon atom of the
ynamide attacks the electrophilic carbene carbon to deliver
the keteniminum intermediate VII. After that, ring closure by
attack of the electron-rich double bond onto the iminium
carbon takes place. Elimination of the [Au]+ fragment and
liberation of product 4 complete the catalytic cycle.
To demonstrate the synthetic potential of the obtained
cyclopentadienes, compounds of type 4 were converted under
acidic conditions (Scheme 4). To our surprise cyclopente-
nones 8 were obtained as single products. The position of the
double bond (which is not in conjugation with the benzene
system) was assigned by an X-ray crystal structure analysis of
compound 8a.[14] Cyclopentenones are useful targets as they
are potential intermediates for the synthesis of biologically
important compounds.[19]
.
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Communications
Gold Carbenoids
E. Rettenmeier, A. M. Schuster,
M. Rudolph, F. Rominger, C. A. Gade,
A. S. K. Hashmi*
&&&& — &&&&
Gold Catalysis: Highly Functionalized
Cyclopentadienes Prepared by
Intermolecular Cyclization of Ynamides
and Propargylic Carboxylates
When an ynamide meets a gold carbe-
noid: Highly electrophilic gold carbe-
noids available from propargylic esters by
means of 1,2-acyloxy migration open up
new reaction pathways for ynamide gold
chemistry. In this way highly functional-
ized cyclopentadiene derivatives become
accessible (see scheme; EWG=electron-
withdrawing group).
6
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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