Selectivity for Alkynyl or Allenyl Imidamides and Imidates in Copper-Catalyzed Reactions of Terminal 1,3-Diynes and Azides

Article abstract of DOI:10.1021/acs.orglett.0c03861

Copper-catalyzed reactions of terminal 1,3-diynes with electron-deficient azides to generate either 3-alkynyl or 2,3-dienyl imidamides and imidates are described. The selectivity depends on the diyne substituents and the nucleophile that reacts with the ketenimide intermediate generated from the corresponding triazole precursor. Reactions of 1,3-diynes containing a propargylic acetate afford [3]cumulenyl imidamides, while reactions using methanol as the trapping agent selectively generate 2,3-dienyl imidates. Five-membered heterocycles were obtained from 1,3-diynes containing a homopropargylic hydroxyl or amine substituent.

Full text of DOI:10.1021/acs.orglett.0c03861

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Selectivity for Alkynyl or Allenyl Imidamides and Imidates in
Copper-Catalyzed Reactions of Terminal 1,3-Diynes and Azides
Sourav Ghorai and Daesung Lee*
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ABSTRACT: Copper-catalyzed reactions of terminal 1,3-diynes
with electron-deficient azides to generate either 3-alkynyl or 2,3-
dienyl imidamides and imidates are described. The selectivity
depends on the diyne substituents and the nucleophile that reacts
with the ketenimide intermediate generated from the correspond-
ing triazole precursor. Reactions of 1,3-diynes containing a
propargylic acetate afford [3]cumulenyl imidamides, while reactions using methanol as the trapping agent selectively generate
2,3-dienyl imidates. Five-membered heterocycles were obtained from 1,3-diynes containing a homopropargylic hydroxyl or amine
substituent.
Scheme 1. Two Different Approaches to the Formation of
Closely Related Functional Groups
llenes constitute a distinct class of organic compounds
with two orthogonal π-bonds. There are numerous
A
natural products and bioactive molecules that contain allene
substructures.1−3 It has been demonstrated that allene-
substituted bioactive compounds, like steroids,4,5 prostaglan-
dins,6,7 carbacyclins,7 nucleosides,8 and unnatural amino
acids,9 display higher potency, increased metabolic stability,
and bioavailability. Because of the strained nature of the
cumulene structure, allenes have been engaged in numerous
synthetic transformations as a versatile building block to form a
variety of carbo- and heterocyclic frameworks.10−19 Axial-to-
central chirality transfer is an efficient method for generating
chiral compounds containing one or more stereogenic centers
from chiral allenes.19−22
Due to the important utility of allenes, it is highly desirable
to develop new synthetic methods to generate functionalized
allenes from readily available starting materials.23−28 One of
the traditional approaches to the synthesis of allenes involves
1,2-elimination of vinyl derivatives under strong basic or metal-
catalyzed conditions.29 SN2′-type reactions with propargylic
alcohol derivatives (FG−CH2−CC)30−33 or isomerization
of enyne moieties34 also constitutes an efficient method for
generating allenes. Transition metal-catalyzed coupling be-
tween terminal alkynes and carbonyl moieties in the presence
of a secondary amine is also a well-established method,35 and a
Cu(I)-catalyzed cross-coupling of terminal alkynes with diazo
compounds is another efficient protocol for generating
allenes.36 Trisubstituted allenes can be accessed via metal-
catalyzed cationic Heck coupling of alkynes with aryl halide/
triflate.37−39 Enones and ynones are also explored as a
precursor of allenes.40,41
and others found that the alkynes containing a heteroatom
substituent at the propargylic or homopropargylic carbon
center preferentially generate the allenoate, and in particular in
Received: November 22, 2020
Published: January 14, 2021
In 2004, Fu reported a Cu(I)-catalyzed coupling of alkynes
and diazoacetate under mild conditions to generate 3-alkynyl
carboxylate and only small amounts of the corresponding
allenoates were observed (Scheme 1A).42 On the contrary, Lee
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© 2021 American Chemical Society
Org. Lett. 2021, 23, 697−701
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the presence of a base such as triethyl amine, allenoate became
the exclusive product.43 Fox also reported a similar reaction
that selectively generated allenoate as the product.44 Although
the exact mechanisms of these reactions are not well
understood, on the basis of other related reactions, we propose
a copper acetylide-mediated mechanism for the formation of
the 3-alkynoate and allenoate. Relying on this copper acetylide-
mediated mechanism, we envision that the copper-catalyzed
coupling of terminal 1,3-diyne 1 and azide would generate 3-
alkynyl and allenyl imidamide/imidate 2/3 (Scheme 1B). We
surmise that the terminal 1,3-diyne would form the
corresponding copper acetylide, which will participate in a
[3+2] cycloaddition45 with tosylazide to form triazole IN-1.
The subsequent loss of molecular nitrogen will lead to two
equilibrating organocopper aza-cummulenes IN-2 and IN-3,46
which then react with a nucleophile such as an amine or
alcohol to generate imidamide47 or imidate48 containing either
an alkyne (2) or an allene moiety (3). A unique feature of this
transformation is the selective transformation of the terminal
alkyne moiety for the construction of imidamide and imidate
functionality. Although starting from a different set of starting
materials, the formation of closely related products from
transformations in A and B is noteworthy. Herein, we describe
our exploration of copper-catalyzed reactions of terminal 1,3-
diyne 1 with tosylazide, which generated functionalized 1,3-
disubstituted 2,3-dienyl imidamides and imidates 3 with good
selectivity over the corresponding alkyne derivative 2. Also, it
was found that the types of nucleophiles, the base additives,
and the substituent patterns of the 1,3-diynes not only affect
the ratio of 2 to 3 but also promote the formation of alternative
products such as [3]cumulenes and triple-bond-migrated
products.
2:1 ratio (entry 1). Reactions of 1a with other azides such as
diphenyl phosphoryl azide49 and mesyl azide are also efficient,
affording 3-alkynyl imidamide 2ab and 2ac as the major
products (entries 2 and 3, respectively). On the contrary,
employing piperidine as a nucleophile under otherwise
identical conditions, 1a provided 2ad exclusively (entry 4).
1,3-Diynes with a secondary (1b) or tertiary alkyl group (1c)
provided good yields of 2/3ba and 2/3ca but low selectivity
with a slight preference for alkynyl products 2ba and 2ca
(entries 5 and 6, respectively). 1-Cyclohexenyl-substituted 1,3-
diyne provided 2/3da in 48% yield with a preference for
compound 3da. 1,3-Diyne 1e containing a propargylic
hydroxyl group afforded allene derivative 3ea predominantly
(entry 8), whereas 1,3-diyne 1f containing a trimethylsilyl
group provided alkyne derivative 2fa selectively but in marginal
yield (entry 9).
Once a general trend for the efficiency and selectivity
between 2 and 3 had been revealed from the entries in Table 1,
we next explored the selective formation of 3-alkynyl
imidamide 2 (Scheme 2). As piperidine exclusively generated
Scheme 2. Selective Formation of 3-Alkynyl Imidamides
with Assorted Nucleophiles, Azides, and 1,3-Diynes
Our exploration commenced with the assessment of the
efficiency and selectivity for the formation of 3-alkynyl and
allenyl imidamide 2 and 3 (Table 1). Under the conditions
that include a copper catalyst (CuI, 10 mol %), an azide (1.2
equiv), and an amine (1.2 equiv), 1,3-diynes containing
different substituents were examined. A hexyl-substituted
terminal 1,3-diyne 1a and TsN3 provided a mixture of 3-
alkynyl and allenyl imidamides 2aa and 3aa in 75% yield with a
a
Yields in parentheses represent those of the corresponding 2,3-dienyl
isomer 3.
3-alkynyl imidamide, reactions of n-decyl- and homobenzyl-
substituted 1,3-diynes with piperidine were performed,
resulting in the corresponding 3-alkynyl imidamides 2ac′ and
2ac″ in 57% and 40% yields, respectively. Reaction with
phosphoryl azide and alkyl-substituted diynes predominantly
generated 3-alkynyl imidamides (2ab′, 2ab″, and 2bb) in
moderate 45−60% yields. Electron-rich 1,3-diynes with silyl
substituents selectively delivered 3-alkynyl imidamides. TES-,
TIPS-, and TBS-substituted 1,3-diynes provided the corre-
sponding alkynyl derivatives 2fa′−2fa‴ in 85−94% yields.
Reactions of TIPS-substituted 1,3-diyne with phosphoryl and
mesyl azide also generated 3-alkynyl imides 2fb″ and 2fc″
exclusively in 59% and 82% yields, respectively. It is evident
from Scheme 2 that reactions with cyclic amine, phosphoryl
azide, and electron-rich silyl-substituted 1,3-diynes tend to
generate 3-alkynyl imidamide predominantly or exclusively.
Next, we examined reactions of different 1,3-diynes to
selectively form 2,3-dienyl imidamide 3 (Scheme 3). On the
basis of the initial observation with propargylic alcohol-
containing diyne 1e that selectively generated 2,3-dienyl
imidamide, we further tested the reactivity of structurally
diversified propargyl alcohols, (thio)ethers, amines, and
Table 1. Efficiencies and Product Distributions with
Assorted Nucleophiles, Azides, and 1,3-Diynes of Different
Substituents
yield (%)
a
entry
R
Nu−H
azide
TsN3
(PhO)2PON3
MsN3
TsN3
TsN3
(alkyne:allene)
1
2
3
4
5
6
7
a, n-Hex
a, n-Hex
a, n-Hex
a, n-Hex
b, c-Hex
c, t-Bu
i-Pr2NH
i-Pr2NH
i-Pr2NH
piperidine
i-Pr2NH
i-Pr2NH
i-Pr2NH
2/3aa, 75% (2:1)
2/3ab, 66% (5:1)
2/3ac, 54% (2:1)
b
2ad, 56% (1:0)
2/3ba, 79% (2:3:1)
2/3ca, 71% (1:3:1)
2/3da, 48% (1:2:5)
TsN3
TsN3
d,
1-cyclohex
8
9
e, CH2OH
f, SiMe3
i-Pr2NH
i-Pr2NH
TsN3
TsN3
2/3ea, 73% (1:15)
2fa, 45% (1:0)
a
b
Isolated yield. H2O (1 equiv) and NH2OH·HCl (1−2 mol %) were
used as additives.
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Although the selectivity of forming allenyl imidamides is
good, the formation of alkyne isomer could not be suppressed
in many cases. At this juncture, we surmised that trapping the
ketenimine intermediate with alcohols may have different
product distributions.50,51 Indeed, under identical conditions
except for the replacement of i-Pr2NH (1.2 equiv) with MeOH
(10 equiv) and Et3N (2 equiv), the reaction of 1,3-diynes
selectively provided 2,3-dienyl imidates without a vestige of the
alkyne isomer (Table 2). The lower stoichiometry of methanol
Scheme 3. Formation of Allenyl Imidamides from Diverse
1,3-Terminal Diynes
Table 2. Synthesis of Allenyl Imidates by Trapping with
Methanol
a
led to the lower efficiency of the reaction. Alkyl and alkenyl
diynes, which provided a mixture of allenyl and alkynyl
imidamides previously, selectively generated allenyl imidates
(3ae, 3ce, and 3de) in moderate to good yield (entries 1−3,
respectively). Even though 1,3-diyne 1g containing a free
secondary alcohol led to decomposition (entry 4), the
corresponding acyl-, benzyl-, and tert-butyldimethylsilyl-
protected 1,3-diynes provided 2,3-dienyl imidates (3he, 3he′,
and 3he″, respectively) in good yield (entries 5−7,
respectively). Similarly, 1,3-diyne with benzyl-protected
secondary alcohol 1i delivered 2,3-dienyl imidate 3ie in 72%
yield.
Subsequently, we observed that under standard conditions,
1,3-diynes 4 containing an an acetoxy or benzoyloxy
substituent at the propargylic position provided mono-, di-,
and trisubstituted [3]cumulenes (Table 3).52−56 For example,
1,3-diynes 4a−4c afforded trisubstituted cumulenes 5a−5c,
respectively, in 76−80% yields (entries 1−3, respectively). 1,3-
Diynes substituted with a cycloalkyl moiety afforded the
corresponding cumulenes 5d−5f in good yields (entries 4−6,
respectively). Unexpectedly, while the acetate derivative of
tertiary alcohol was afforded, the corresponding primary and
secondary acetates provide a mixture of the expected
[3]cumulenes and the corresponding acetoxy allene deriva-
tives. However, upon replacement of the acetate with p-
nitrobenzoate (4g−4i), only cumulenes 5g−5i were obtained
(entries 7−9, respectively). We believe this is the consequence
of the better leaving group capacity of a benzoate compared to
that of an acetate. [3]Cumulenes make up a special class of
polyene organic compounds whose synthetic utilities have
been little explored.57−60 Thus, the current mild protocol to
allow the preparation of [3]cumulenes containing various
Yields in parentheses represent those of the corresponding 3-alkynyl
compound 2.
amides. 1,3-Diynes containing secondary or tertiary alcohols
afforded 2,3-dienyl imidamides (3ga, 3ga′, 3ga″, 3ga‴, 3ja,
3ja′, 3ja″, and 3ka) as a predominant or exclusive product in
65−76% yields. The selectivity between the allenyl and alkynyl
isomer depends on the substituent, but no clear trend has been
found. 1,3-Diynes with a tertiary alcohol and a cycloalkyl
substituent delivered 2,3-dienyl imidamides (3la, 3la′, and
3la″) in 70−80% yields with a roughly 10:1 selectivity.
Carvone-containing 1,3-diyne afforded allene 3ma (7:1
allene:alkyne ratio) in 69% yield with a 1.4:1 diastereomeric
ratio. While a vinyl-conjugated 1,3-diyne containing a free
hydroxyl group provided allene 3na in 46% yield with a low
selectivity (4:1), 1,3-diynes with benzyl-, 3-MeO-Ph-, and
THP-protected primary alcohols exclusively generated allene
derivatives (3oa, 3oa′, and 3oa″, respectively) in 54−70%
yields. Similarly, a cholesteryl ether-substituted 1,3-diyne
generated allene 3pa in 56% yield with an 8:1 allene:alkyne
ratio. On the contrary, the corresponding thioether afforded
3qa contaminated with the alkyne isomer (7.5:1 allene:alkyne).
1,3-Diynes containing aniline and tosylamido substituents at
the propargylic position selectively generated allenes 3ra and
3sa in 58% and 88% yields, respectively. A gem-dimethyl,
however, decreased the yield and selectivity for 3sa′ (68%,
8:5:1). An indole-substituted 1,3-diyne provided only allene
3ta in 68% yield, and N-allyl tosyl-substituted 1,3-diyne
provided single isomer 3ua in 67% yield. On the contrary,
phenyl- and 4-MeO-phenyl-substituted 1,3-diynes provided
moderate yields and selectivity provided allenes 3va (64%,
4.6:1) and 3va′ (64%, 3.6:1).
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isomer 8 was not observed under the conditions regardless of
the reaction time. However, because of the thermodynamic
preference for 2-alkynyl isomers, we surmised that the
isomerization of 2 or 3 to 8 would happen if a base stronger
than secondary amines is used (Scheme 4).66 Indeed, treating a
Table 3. Synthesis of [3]Cumulenyl Imidamides via
Eliminating the Acetoxy or Benzoyloxy Group of a Putative
Allene Intermediate
Scheme 4. Isomerization of 3-Alkynyl and 2,3-Dienyl
Imidamides to the Corresponding 2-Alkynyl Isomers
a
b
Decomposition of starting materials. 3-Alkynyl imidamide 2 was
recovered.
substituent patterns from readily available building blocks is of
highly synthetically useful.
We envision that with a suitably tethered nucleophile, the
conversion of 1,3-diynes to the corresponding conjugated
allenyl imidamide and imidates would promote an intra-
molecular Michael-type addition (Table 4).61−65 Under
mixture of 2 and 3 with DBU (0.1 equiv) rapidly induced
isomerization to provide 2-alkynyl imidamides 8a−8d. On the
contrary, phenyl- and silyl-substituted 3-alkynyl imidiamides 2
and 3 did not isomerize to the corresponding 2-alkynyl isomers
8e and 8f, respectively.
In conclusion, we have developed efficient protocols to
generate discrete isomers of 2-alkynyl, 3-alkynyl, 2,3-dienyl,
and 2,3,4-trienyl imidamides and imidates from copper-
catalyzed reactions of 1,3-diynes and tosylazide. The selectivity
between 3-alkynyl and 2,3-dienyl imidamides could be
controlled by a heteroatom substituent at the propargylic
position of the 1,3-diynes and employing different trapping
reagents such as amines and alcohols. [3]Cumulene derivatives
were also generated by employing 1,3-diynes that contain an
acetoxy or bezoyloxy substituent at the propargylic position.
While trapping of the putative azacumulene intermediates with
amines provided either 3-alkynyl or 2,3-dienyl imidamides
depending on the structure of the trapping amines, trapping
with methanol selectively generated 2,3-dienyl imidates. It was
found that both 3-alkynyl and 2,3-dienyl imidamides could be
isomerized to selectively generate the corresponding 2-alkynyl
isomers under equilibrating conditions with a stronger base
such as DBU. Intramolecular trapping of the putative
azacumulene intermediates provided five-membered hetero-
cyclic products if the 1,3-diyne substrates contained a
homopropargylic hydroxyl or amino substituent. A unique
feature of these unprecedented reactions is that under mild
reaction conditions, terminal 1,3-diynes could be selectively
converted to different unsaturated carboxylic acid derivatives
with good selectivity.
Table 4. Synthesis of Heterocycles via Intramolecular
Trapping of the Putative Allene Intermediate
a
Conditions: (A) TsN3 (1.2 equiv), Et3N (2 equiv), MeOH (10
equiv), 12 h; (B) TsN3 (1.2 equiv), i-Pr2NH (1.2 equiv), 4 h.
standard conditions, homopropargyl alcohol-containing 1,3-
diynes 6a and 6b were smoothly converted to tetrahydrofur-
anylidene imidates 7a and 7b in 48% and 87% yields,
respectively (entries 1 and 2, respectively). The corresponding
homopropargyl sulfonamide 6c led to 1-tosylpyrrolidinylidene
imidamide 7c in 72% yield (entry 3); however, the formation
of a six-membered ring 7d from 6d failed (entry 4). 1,3-Diynes
6e and 6f substituted with a phenyl group containing an o-OH
or NH2 participated in the cascade reaction to generate
benzofuranyl imidamide 7e in 70% yield and indolyl
imidamide 7f in 64% yield (entries 5 and 6, respectively).
Although expected, migration of the triple bond from 3-
alkynyl or 2,3-dienyl isomers to the corresponding 2-alkynyl
ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03861.
Experimental procedures and spectroscopic data for all
new compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
Daesung Lee − Department of Chemistry, University of Illinois
at Chicago, Chicago, Illinois 60607, United States;
orcid.org/0000-0003-3958-5475; Email: dsunglee@
uic.edu
Author
Sourav Ghorai − Department of Chemistry, University of
Illinois at Chicago, Chicago, Illinois 60607, United States
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Wang, A.-X. Org. Lett. 2008, 10, 5585.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03861
Notes
(41) Maekawa, H.; Takano, A.; Watanabe, M. Tetrahedron Lett.
2014, 55, 6208.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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The authors thank the National Science Foundation (CHE-
1764141) for financial support. The Mass Spectrometry
Laboratory at the University of Ilinois at Chicago is
acknowledged.
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