Synthesis of Allenamides and Structurally Related Compounds by a Gold-Catalyzed Hydride Shift Process

Article abstract of DOI:10.1002/adsc.201700615

A new procedure for the synthesis of allenamides and structurally related compounds is reported. Under gold catalysis, a series of ynamides possessing a benzyloxy group at the propargylic position were efficiently converted into the corresponding allenamides following a 1,5 hydride shift process. The scope of the reaction was shown to be extremely broad allowing the formation of γ-mono or γ-disubstituted allenes possessing various functional groups. Notably, and in contrast to previously reported methods, not only N-allenyl sulfonamido but also urea, phosphonamido and the rarely studied phthalimido, pyrrolo, indolo and carbazolo derivatives could be readily and efficiently accessed. (Figure presented.).

Full text of DOI:10.1002/adsc.201700615

Title: Synthesis of Allenamides and Structurally Related Compounds
by a Gold-Catalyzed Hydride Shift Process
Authors: Fabien Gagosz and Qing Zhao
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10.1002/adsc.201700615
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis of Allenamides and Structurally Related Compounds
by a Gold-Catalyzed Hydride Shift Process
Qing Zhao*a,b and Fabien Gagosz*a,b
a
Laboratoire de Synthèse Organique, UMR 7652 CNRS/Ecole Polytechnique, Route de Saclay, 91128 Palaiseau, France
Department of Chemistry and Biomolecular Sciences, University of Ottawa, K1N 6N5, Ottawa, Canada
b
Email: fgagosz@uottawa.ca, qzhao2@uottawa.ca. Fax: (+1) 613-562-5170
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
bromides as recently reported by Evano et al.[8]
Abstract.
A new procedure for the synthesis of
allenamides and structurally related compounds is reported. (Scheme 1B) represent so far, the most general
Under gold catalysis, a series of ynamides possessing a
benzyloxy group at the propargylic position were
efficiently converted into the corresponding allenamides
following a 1,5 hydride shift process. The scope of the
reaction was shown to be extremely broad allowing the
formation of -mono or -disubstituted allenes possessing
various functional groups. Notably, and in contrast to
previously reported methods, not only N-allenyl
sulfonamido but also urea, phosphonamido and the rarely
studied phthalimido, pyrrolo, indolo and carbazolo
derivatives could be readily and efficiently accessed.
methods to access allenamides.
Keywords: allenamide, gold, catalysis, hydride shift
Allenamides 1, which exhibit a higher stability
than their parent allenamines, have received
considerable attention from the synthetic organic
community during the last ten years.[1] Due to the
presence of the nitrogen atom which electronically
enriches and polarizes the allenic moiety, they
possess a singular reactivity as compared to simple
allenes and thus can be functionalized in a highly
regioselective manner. Their synthetic potential has
been highlighted in a great variety of regio- and
stereoselective transformations[1,2] as for examples
metal-mediated hydrofunctionalizations, cyclizations,
cycloadditions or aldol reactions.
In contrast with the ongoing increasing interest for
the chemistry of allenamides, relatively little effort
has been devoted to their synthesis. Currently
available methods to access them are not only
relatively limited, but they most often suffer from
lack of generality, what may eventually hamper
future synthetic developments. Classical routes such
as base-induced isomerizations,[3] sigmatropic
rearrangements[4] or amino-cyclizations[5] are either
poorly efficient, exhibit a poor substrate scope or do
not allow structural diversity (Scheme 1, top). The
copper-catalyzed coupling of nitrogen nucleophiles
with allenyl halides, developed by the Trost[6] and
Hsung[7] groups (Scheme 1A), or with propargyl
Scheme 1. Main access to allenamides and our approach to
their synthesis by gold catalysis.
While being generally efficient and complementary
in terms of polysubstitution of the allene moiety,[9]
these procedures are still suffering some limitations.
Access to the required halogenated coupling partners
is not always straightforward or even possible[10] and
limited structural functionalization has been reported.
1
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10.1002/adsc.201700615
Advanced Synthesis & Catalysis
Competitive formation of 1,3-diene byproducts is
also frequently encountered when allenyl halides are
employed and the use of propargyl bromides is
limited to the use of oxazolidinones or hydantoins
coupling partners. The alternative Cu-catalyzed direct
coupling of fluorosulfonimide with allenes described
by Zhang et al.[11] (Scheme 1C) is also limited in
terms of substitution both on the allene moiety and on
the nitrogen atom.[12]
electrophilic catalytic conditions and therefore
prevent the competitive formation of byproducts
frequently observed during the formation of
allenamides, d) advantageously, the phthaloyl group
could be easily removed in compounds resulting from
the
post-functionalization
of
the
targeted
phthalimidoallenes 6.
Our investigations started with phthalimidoalkyne
5a and we looked for suitable catalytic conditions
allowing its conversion into allene 6a. Main results
are compiled in Table 1. Substrate 5a was initially
treated with 5 mol% of [(XPhos)Au(NCCH3)]SbF6
(7) in dry CDCl3 and the reaction was monitored by
1H NMR spectroscopy.[20] While no reaction occurred
at 20°C (entry 1), a rapid (1.5 h) and complete
consumption of 5a took place when the temperature
was raised to 60°C (entry 2). We were pleased to
observe that, under these conditions, the desired
phthalimidoallene 6a was produced as a highly
predominant product (92% NMR yield) which could
be isolated in 86% yield. Several other gold(I)
complexes were screened but no improvement could
be made in terms of catalytic efficiency (entries 3-6).
While other biphenylphosphine gold(I) complexes
such as 8 and 9 were shown to be less selective
(entries 3 and 4), the NHC-gold complex 10 exhibited
In this context and following our interest in the
development of gold-catalyzed transformations,[13,14]
we conceived a novel approach to allenamides 2
based on a gold catalyzed hydride shift[15] from an
ynamide precursor 3 (Scheme 1, bottom). As
previously demonstrated,[16] the benzyloxy group in 3
could serve as a potent internal “hydride donor” for
the reduction of the -position to the nitrogen atom
upon activation of the alkyne moiety by an
electrophilic gold complex. The intermediate
oxocarbenium species 4 thus formed would then
evolve into allenamide 2 after loss of benzaldehyde
and concomitant regeneration of the catalyst. While
this approach might appear at first sight less
appealing than the direct copper-catalyzed procedures
previously reported, it offers several advantages that
motivated us to study its feasibility: a) ynamide
substrates 3 could be readily accessed by copper
catalysis with a wide variation of structure and
functionalization on both the nitrogen nucleophilic
partner and the alkyne, b) gold catalysis is known to
be tolerant of a large number of commonly used
functional groups, c) gold catalyzed experimental
conditions are generally mild enough to prevent the
formation of byproducts.
a
poor reactivity (entry 5) and the simple
[(Ph3P)Au(NCCH3)]SbF6 complex mainly led to
degradation products (entry 6).[21] Based on these
results, gold complex 7 was retained to study the
scope of the transformation.
Table 1. Optimization of the catalytic conditions [a,b]
Phthalimido derivatives of type 5 were initially
chosen as model substrates to validate our approach
(Scheme 2).
[a]
[b]
CDCl3 was dried over 4Å MS prior to use.
Yields
assessed by 1H NMR spectroscopy using 1,2-
[c]
dichloroethane as an internal standard.
Substrate concentration was 0.1 M.
Isolated yield.
[d]
Degradation was
Scheme 2. Phthalimidoalkynes as model substrates.
observed.
This choice was made on the following
considerations: a) 5 are easily accessible by Cu-
catalyzed coupling between phthalimide and terminal
alkynes under aerobic conditions,[17] b) the
corresponding phthalimidoallenes 6 have been rarely
studied and no general route to access them has been
reported to date,[18] c) importantly, the significant
contribution of resonance form 6’[19] in the allenyl
product was hypothesized to limit its activation under
As seen in Table 2, the transformation was shown to
be widely applicable. A series of phthalimidoalkynes
5a-b and 5d-v were efficiently (70-88%) and
generally rapidly (≈ 1-2 h) converted into the
corresponding phthalimidoallenes 6a-b and 6d-v.
Both -mono- and -disubstituted allenes could be
produced. It is worth noting that while substrates
monosubstituted at the propargylic position had to be
reacted at 60°C, the temperature could be lowered to
2
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10.1002/adsc.201700615
Advanced Synthesis & Catalysis
20°C in the case of disubstituted substrates (5h-v).
This can probably be related to the involvement of a
Thorpe-Ingold effect which would be more
pronounced in the case of disubstituted substrates: the
1,5-H shift step would be facilitated and the overall
efficiency of the reaction consequently increased
(yields > 90% for 6h-v).[22]
not reached with the previously reported procedures.
Indeed, the reaction was compatible with the
presence of an aromatic (6e, 6n, 6n-v), an ester (6k),
a silyl ether (6l), an imide (6m), a carbonate (6f), a
carbamate (6q), an ether (6k) or a cyano group (6k)
and with halogen atoms (6s-t and 6n). While most of
the phthalimidoallenes synthesized could be isolated
using regular silica gel chromatography, some of
them (6b,d,o) proved to be sensitive to both acidic
and basic conditions. The use of deactivated neutral
alumina was found appropriate for their efficient
isolation by chromatography.[24]
Table 2. Scope of the reaction with N-alkynyl phthalimide
substrates [a,b]
We then turned our attention on the possibility to
employ the same catalytic conditions to produce
allenamides
possessing
different
electron
withdrawing groups on the nitrogen atom. As shown
in Table 3, the process could be extended to the
synthesis of a series of other allenes 12a-i possessing
either a sulfonamide (12a-g), a urea (12h) or a
phosphonamide group (12i).[25] The reaction
proceeded with the same efficiency (82-95%) with
the exception of 11i which was converted into 12i in
a more moderate 46% yield. It should be noted that
the reactions were performed at 20°C in 1,2-
dichloroethane as significant amounts of 1,3-dienes
byproducts arising from a subsequent -isomerization
were formed when chloroform was used as the
solvent.[26]
Table 3. Scope of the reaction with sulfonamide,
phosphonamide and urea derivatives [a,b]
[a]
Reaction scale: 0.5 mmol. Chloroform was dried over
[b]
[c]
4Å MS prior to use. Isolated yields. Reaction run in
dry 1,2-dichloroethane.
[a]
Reaction scale: 0.5 mmol. 1,2-dichloroethane was dried
[b]
[c]
over 4Å MS prior to use.
Isolated yields.
Substrate
concentration was 0.1 M. [d] Reaction performed in acetone
Conversely, a limit in reactivity was found with
unsubstituted substrate 5c which delivered the
unsubstituted phthalimido allene 6c in low yield
(35%) after a prolonged reaction time (4 h).[23] It is
also important to note that the transformation exhibits
an impressive functional group tolerance, which was
using 10 mol% of 7.
With the idea to further extend the scope of the
transformation, the reactivity of N-alkynyl indoles,
3
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10.1002/adsc.201700615
Advanced Synthesis & Catalysis
carbazoles and pyrroles was investigated. We were
delighted to observe that a range of such substrates
13a-i could be rapidly and efficiently converted (82-
99%) into the corresponding allene derivatives 14a-i
under the same mild experimental conditions (5
mol% of 7 in 1,2-dichloroethane at 20°C) (Table
4).[25] It is worth mentioning that examples of such
polysubstituted allenes are extremely rare in the
literature probably as a lack of methods to access
them in an efficient manner.[27]
Table 4. Scope of the reaction with N-alkynyl pyrrole,
indole and carbazole substrates
Scheme 3. Gold-catalyzed cascade reactions involving N-
allenyl pyrrole or indole intermediates.
Finally, we demonstrated that -disubstituted allenes
12a-c, which were produced from 11a-c by the
present gold-catalyzed process, could be cleanly
converted in situ into 1,3-dienes 20a-c by a
subsequent treatment with Al2O3.
[a]
Reaction scale: 0.5 mmol. 1,2-dichloroethane was dried
[b]
[c]
over 4Å MS prior to use.
concentration was 0.1 M.
Isolated yields.
Substrate
We also took advantage of the capacity of
electrophilic gold species to activate allenes towards
their nucleophilic functionalization to perform
cascade reactions (Scheme 3). As examples, the
intermediate allenes formed from pyrrole or indole
derivatives 15, 16 and 13d could be converted in situ
into the corresponding polycyclic compounds 17, 18
and 19 by simply prolonging the reaction time. This
transformation, which involves the nucleophilic
addition of the nitrogen heteroaromatic motif on the
-position of the gold-activated allene moiety,
represents a rapid and efficient means to access the
pyrroloindole scaffold which is found in several
natural or biologically active compounds.
Scheme 4. One-pot, two-step synthesis of 1,3-dienes from
N-tosylynamides.
In conclusion, we have developed a new procedure
for the synthesis of allenamides and structurally
related compounds. While our method is not as direct
as the previously reported Cu-catalyzed processes, it
possesses several clear advantages. The reaction
conditions are mild enough to prevent the
4
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10.1002/adsc.201700615
Advanced Synthesis & Catalysis
overreaction or degradation of the allenamides which
can then be selectively produced and isolated. The
scope of the reaction is extremely broad allowing the
formation of -mono or -disubstituted allenes
possessing various functional groups. Notably, and in
contrast to previously reported methods, not only N-
allenyl sulfonamido but also urea, phosphonamido
and the rarely studied phthalimido, pyrrolo, indolo
and carabazolo derivatives can be readily and
efficiently accessed. This method, which offers new
opportunities for the design of polyfunctionalized
allenamides, should contribute to the ongoing
increasing interest in their use as building blocks in
organic chemistry.
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Experimental Section
Representative Procedure: To a stirred solution of ynamide
5a (0.50 mmol) in chloroform (5 mL) was added
XPhosAu(CH3CN)SbF6 (5 mol%) and the solution was
stirred at 60°C for 1h. The reaction mixture was cooled
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Acknowledgements
We are deeply appreciative of financial support from the Ecole
Polytechnique (grant for Q. Zhao) and CNRS. The authors thank
Dr. B. Bolte for preliminary studies and Dr. Chuang Wang for his
help in the preparation of substrates.
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Advanced Synthesis & Catalysis
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[21] Brønsted acids (HOTf) and silver salts (AgSbF6) were
not suitable catalysts for this reaction.
[22] At higher temperature (60°C) and under gold
catalysis, allenamides are prone to -isomerization and
polymerization what tends to lower their yields.
[23] This does not diminish the interest of the present
procedure since unsubstituted phthalimidoallene 6c can
be obtained by based-mediated isomerization of the
parent propargylic derivative but in low yield (24%).
See: V. N. G. Lindsay, D. Fiset, P. J. Gritsch, S. Azzi,
A. B. Charette, J. Am. Chem. Soc. 2013, 135, 1463.
[24] See supporting information for more details.
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6
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Advanced Synthesis & Catalysis
COMMUNICATION
Synthesis of Allenamides and Structurally Related
Compounds by a Gold-Catalyzed Hydride Shift
Process
Adv. Synth. Catal. Year, Volume, Page – Page
Qing Zhao*, Fabien Gagosz*
7
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