Cobalt-Catalyzed Hydroalkynylation of Vinylaziridines

Article abstract of DOI:10.1002/adsc.202001575

Transition metal-catalyzed hydroalkynylation reactions are efficient transformations allowing the straightforward formation of functionalized alkynes. Therein, we disclose the cobalt-catalyzed hydroalkynylation of vinylaziridines giving rise to both linea

Full text of DOI:10.1002/adsc.202001575

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DOI: 10.1002/adsc.202001575
Cobalt-Catalyzed Hydroalkynylation of Vinylaziridines
a
a
a
a,
Bohdan Biletskyi, Lingyu Kong, Alphonse Tenaglia, and Hervé Clavier *
a
Aix Marseille University, CNRS, Centrale Marseille, iSm2, Marseille, France
Phone: +33 (0)413 945 630,
E-mail: herve.clavier@univ-amu.fr
Manuscript received: December 18, 2020; Revised manuscript received: February 16, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202001575
Abstract: Transition metal-catalyzed hydroalkynylation reactions are efficient transformations allowing the
straightforward formation of functionalized alkynes. Therein, we disclose the cobalt-catalyzed hydro-
alkynylation of vinylaziridines giving rise to both linear and branched enynes. The optimization of the reaction
conditions allowed to determine the key parameters of the cobalt-based catalytic system. The scope
investigation showed that the cobalt-catalyzed hydroalkynylation is mainly limited to phenylacetylene
derivatives. In most cases branched and linear enynes were isolated in equimolar proportions (20 examples).
Additional experiments allowed us to propose a plausible mechanism. Finally, the hydroalkynylation products
were further subjected to an iodocyclization affording a 2,3-dihydropyrrole derivative and a platinum-catalyzed
cycloisomerization.
Keywords: 1,4-Enynes; Alkynes; Cobalt; Small ring; Vinylaziridine
Introduction
nylation of cyclopropenes; albeit limited to alkyl-
[11]
substituted alkynes (Scheme 1a).
Later, Tenaglia
Alkynes are fundamental and versatile building blocks demonstrated that the Herrmann-Beller phosphapalla-
in organic synthesis as they can be straightforwardly dacyle catalyst enabled the hydroalkynylation of cyclo-
derivatized into various functional groups through a propenes with alkyl-, aryl- and silyl-substituted
[12]
wide array of transformations such as reduction, alkynes. Recently, efficient enantioselective versions
hydration, halogenation, oxidation, cycloaddition or were disclosed using gadolinium- and palladium-based
[1]
[13]
cycloisomerization among many others. Alkynylation catalysts. In 2009, two independent studies on the
represents an efficient methodology to introduce hydroalkynylation of methylenecyclopropane deriva-
nucleophilic alkynyl groups to unsaturated substrates. tives were published. Sugimone found that a nickel(0)-
However, the low reactivity of alkynylides limits their based catalytic system could perform addition of
application to addition reactions onto aldehydes, (triisopropylsilyl)acetylene without ring-opening of the
[14]
ketones and imines. Metal alkynylide nucleophiles cyclopropane moiety (Scheme 1b). With benzylide-
circumvented this drawback and allowed the develop- necyclopropane substrates and palladium(0)/
ment of enantioselective alkynylations of carbonyl and phosphite catalytic system, López and Mascareñas
a
[2]
imine compounds. Further improvements were ob- showed that the hydroalkynylation occured with a
tained with transition metal (TM)-based complexes ring-opening of the cyclopropane. The ratio of the
[3]
allowing catalytic hydroalkynylation through nucleo- resulting three isomers depended on the nature of the
1
[15]
philic addition onto less reactive substrates. Dimeriza- R substituent (Scheme 1c). During the investigation
tion of alkynes could thus be promoted by many TM of a platinum-catalyzed [3+2] cycloaddition between
[
4]
[16]
complexes. TM-catalyzed hydroalkynylations of car- alkylidenecyclopropanes and alkynes, we observed,
bon-carbon double bonds were further extended to as a side reaction, an hydroalkynylation process
[5]
[6]
[7]
allenes, activated or unactivated alkenes, styrene leading to the distal bond cleavage of the alkylidene
[
8]
[8a,9]
derivatives,[
1,3-dienes
and
norbornene moiety.
As a part of our interest for metal-catalyzed
8a,c,10]
derivatives.
Other ring-strained olefins such as
[17,18]
cyclopropenes were also investigated. For example, activation of small rings,
Chisholm disclosed a palladium-catalyzed hydroalky- unprecedented hydroalkynylation of vinylaziridines
we disclose herein an
[19]
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showed that no reaction occurred without Co(OAc)
2
[21]
(entry 9). Vinylaziridine 1a was almost completely
recovered and only traces of enone 5a, resulting from
[22]
the oxidation of 1a, were detected.
Among the
various metal acetate salts that were tested, only
Ni(OAc) displayed a similar reactivity (entry 10) and
2
only DMSO was found suitable to carry out the
[23]
hydroalkynylation.
dppe was mandatory and the
most competent ligand for this reaction (entry 11).
Indeed, the catalytic activity decreased when the bite
angle of the diphosphine ligand increased (entries 12
and 13). For large bite angle diphosphines, such as
1
,1’-bis(diphenylphosphino)ferrocene (dppf), no reac-
[23]
tion occured (entry 14).
Unfortunately, the ligand
screening did not enable to improve the selectivity
toward the formation of either the branched enyne or
the linear one, respectively 4aa and 3aa. Chiral
ligands were also scrutinized, but only low chiral
[23]
inductions were observed (up to 32% ee).
Next, the scope of the cobalt-catalyzed hydro-
alkynylation was examined with respect to the alkyne
partner (Table 2). Phenylacetylene derivatives substi-
tuted in para position with electron donating groups
Scheme 1. TM-catalyzed hydroalkynylation of 3-membered
cycle derivatives (TIPS=triisopropylsilyl).
such as OMe, NMe , or NMeOAc were tolerated
2
providing moderate to good overall yields (entries 2, 4
and 5). Branched and linear enynes were isolated in
equimolar ratios with good E selectivities for linear
catalyzed by a cobalt salt associated to a phosphorus- ones. Of note, it was not possible to separate enynes
based ligand. This strategy led to the ring-opening of 3ae and 4ae by silica gel chromatography and thus to
vinylaziridines and gave rise to branched and linear determine the E/Z ratio of 3ae (entry 5). A good
enynes (Scheme 1d).
overall yield was obtained with phenylacetylene
derivative 2f, substituted by a chlorine atom in para
position, i.e. a weak electron withdrawing group (64%,
entry 6). However, alkynes bearing strong electron
Results and Discussion
We first investigated the hydroalkynylation of vinyl- withdrawing groups such as NO and CF were found
2
3
aziridine 1a with phenylacetylene 2a as benchmark completely inefficient in this transformation (entries 7
substrates using the cobalt-based catalytic system and 8). With tolylacetylenes, similar results were
[5h,10d,20]
developed by Nishimura and Hayashi.
Using observed with either para- or ortho substituents
5
mol% of cobalt(II) acetate in combination with 1,2- providing equimolar mixtures of enynes in 24–25%
bis(diphenylphosphino)ethane (dppe) and zinc metal as overall yields (entries 9 and 10). Unexpectedly, with a
reducing agent in DMSO at 80°C, both linear enyne methyl group in meta-position, the catalytic system
3
aa and branched enyne 4aa were isolated in similar displayed a higher reactivity (60% overall yield,
quantities (1:1 ratio) and a good overall yield (Table 1, entry 11). This observation led us to reconsider the
entry 1). Evaluation of the temperature influence influence of the substituents position on the reactivity.
determined that the temperature could be decreased to Whereas a CF group in para position of the phenyl-
3
2
5°C, albeit an increase of the reaction time to 4 h was acetylene derivative did not allow the formation of
required to achieve full consumption of 1a (entries 2– hydroalkynylation products, enynes 3al and 4al, with
4
). The reaction temperature did not have a noticeable a CF in meta position were isolated, albeit in low
3
influence on the branched/linear ratio or on the E/Z yields, 13% and 10% respectively (entries 8 and 12).
selectivity of 3aa. Unexpectedly, the cobalt-based The cobalt-catalyzed hydroalkynylation was also found
catalytic system also proceeded without zinc as tolerant toward ester group in meta position (entry 13),
reducing agent, albeit a heating at 60°C was required 1-naphthylacetylene (entry 14), and alkynes containing
in this case (entries 5–7). Therefore, zinc was removed a heterocycle such as 2-thienyl or 1-tosylindolyl
for the continuation of the study. Freshly distilled (entries 15–16). In all the above mentioned examples,
phenylacetylene 2a was also used to improve signifi- no noticeable selectivity between linear and branched
cantly the overall yield while maintaining the same enynes was observed, and the E/Z selectivity in linear
linear/branched ratio (entry 8). Control experiments enynes 3a was found moderate to good (4:1 to 10:1).
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[a]
Table 1. Optimization of the reaction conditions.
Entry
Change from the “standard conditions”
3aa
4aa
Overall yield (%)
Yield (%)
Yield (%)
[b]
(E/Z ratio)
1
2
3
4
5
6
7
None
34 (4:1)
42 (5:1)
44 (4:1)
42 (6:1)
33 (6:1)
36 (7:1)
–
44 (6:1)
–
41 (6:1)
–
36
40
38
31
34
28
–
48
–
43
–
70
82
82
73
67
64
NR
92
NR
84
at 60°C, 4 h
at 40°C, 4 h
at 25°C, 4 h
No Zn
No Zn, at 60°C, 4 h
No Zn, at 40°C, 20 h
No Zn
[
c]
c]
[
8
9
1
[
No Co(OAc) , No dppe, no Zn
2
c]
c]
0
Ni(OAc) instead of Co(OAc) , no Zn
No dppe, no Zn
dppp instead of dppe, no Zn
dppb instead of dppe, no Zn
dppf instead of dppe, no Zn
2
2
[
1
1
NR
69
26
[
c]
c]
c]
1
1
1
2
3
4
38 (7:1)
11 (8:1)
–
31
15
–
[
[
NR
[
a]
Reaction conditions: vinylaziridine 1a (56 mg, 0.25 mmol, 1 equiv.), phenylacetylene 2a (102 mg, 1 mmol, 4 equiv.), Co(OAc)2
2 mg, 5 mol%), dppe (5 mg, 5 mol%), Zn (8 mg, 50 mol%), DMSO (0.5 mL).
Determined by H NMR.
(
[
b]
c]
1
[
freshly distilled phenylacetylene. NR=no reaction.
Despite a longer reaction time (20 h), no reaction vinylaziridine bearing a Ph P(O) was interesting since
2
occurred with 1-cyclohexenylacetylene, 1-butyne or only the branched enyne 4ga was obtained (entry 6).
propargyl acetate (entries 17–19). With silyl-substi- The influence of the nature of the electron withdrawing
tuted acetylene, no hydroalkynylation product was group on the regioselectivity of the hydroalkynylation
formed under the optimized reaction conditions. How- was further confirmed with vinylaziridine 2h (entry 7).
ever, upon addition of Zn as reducing agent The benzyl carbamate group favored the formation of
(50 mol%), enynes 3at and 4at were isolated even the branched enyne 4ha over the linear product 3ha
though in moderate overall yield (entry 20). The scope (55% vs. 26%) for an overall yield of 81%. Interest-
investigation highlighted the strong influence of the ingly, the minor product 3ha was also formed with an
alkyne nature on the reactivity as previously reported excellent stereoselectivity (E/Z ratio up to 20:1).
[5h,8,10d,f,11,13b,24]
in hydroalkynylation reactions.
With the methyl-substituted vinylaziridine 6a, no
The scope of the cobalt-catalyzed hydroalkynyla- product of hydroalkynylation was detected under the
tion was then evaluated with various vinylaziridines as optimized reaction conditions. Only enone 7a was
depicted in Table 3. Sulfonyl-substituted vinylaziri- isolated in 33% yield after 3 h at 80°C (Scheme 2).
dines reacted in moderate to good overall yields
The same reactivity was observed with the cyclic
(entries 1–4), even with sterically hindered sulfonyl vinylaziridine 6b; enone 7b was the only product
groups (entries 3 and 4). Vinylaziridine 2f bearing an formed in 26% yield (Scheme 3a). Formation of the
electron withdrawing phthalimidyl group reacted enone was suspected to arise from the action of DMSO
slowly, and even after 20 h of reaction at 80°C, tiny as oxidant. Hence, a control experiment was performed
amounts of linear and branched enynes were formed in the absence of the cobalt catalyst. As depicted in
(entry 5). Despite a low yield, the hydroalkynylation of Scheme 3b, treatment of 6b with 10 equivalents of
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[a]
Table 2. Investigation of the scope of the cobalt-catalyzed hydroalkynylation with various alkynes.
Entry
R
3a
4a
Yield
Overall yield
[b]
Yield (E/Z ratio)
1
2
3
4
5
6
7
8
9
Ph
44% (6:1)-3aa
30% (7:1)-3ab
40% (10:1)-3ac
14% (7:1)-3ad
27% (ND)-3ae
33% (10:1)-3af
–
48%-4aa
36%-4ab
41%-4ac
16%-4ad
27%-4ae
31%-4af
–
92%
66%
81%
30%
54%
64%
–
X=OMe
X=OAc
X=NMe2
X=NMeAc
X=Cl
X=NO2
X=CF3
X=Me
[c]
–
–
–
25%
13% (6:1)-3ai
12%-4ai
10
12% (5:1)-3aj
12%-4aj
24%
1
1
X=Me
X=CF3
29% (6:1)-3ak
13% (4:1)-3al
25% (6:1)-3am
31%-4ak
10%-4al
27%-4am
60%
23%
52%
12
3
1
X=CO Et
2
14
15
16
17
34% (7:1)-3an
32% (7:1)-3ao
25% (5:1)-3ap
–
34%-4an
29%-4ao
28%-4ap
–
68%
61%
53%
–
[d]
[
[
[
d]
18
19
20
nBu
–
–
–
d]
–
–
–
d,e]
Si(iPr) (TIPS)
16% (4:1)-3at
17%-4at
33%
3
[a]
Reaction conditions: vinylaziridine 1a (56 mg, 0.25 mmol, 1 equiv.), alkyne 2 (1 mmol, 4 equiv.), Co(OAc) (2 mg, 5 mol%),
2
dppe (5 mg, 5 mol%), DMSO (0.5 mL), 80°C, 2 h.
[b]
[c]
[d]
[e]
1
Determined by H NMR.
branched and linear enynes were unseparable by silica gel chromatography.
reaction time=20 h.
with Zn (8 mg, 50 mol%). ND=not determined.
oxidants (pyridine N-oxide, N-bromosuccinimide, ceric
[22]
ammonium nitrate, 2-iodoxylbenzoic acid).
More-
over, the absence of reactivity using a phenylaziridine
substrate highlighted the requirement of the vinyl
moiety to achieve the hydroalkynylation reaction.
[23]
Scheme 2. Reactivity of substituted vinylaziridine 6a.
To shed light on the mechanism of this cobalt-
catalyzed hydroalkynylation, additional experiments
DMSO in DCE at 60°C for 20 h gave 63% of 7b. Of were carried out. In order to probe the formation of a
note, several reports in the literature detail the cobalt(II)-acetylide intermediate, as suggested in sev-
[5h,10d,20b,25]
oxidation of vinylaziridines into enones with various eral literature reports,
deuterium incorpora-
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[a]
Table 3. Scope of the cobalt-catalyzed hydroalkynylation with various vinylaziridines.
Entry
EWG
3a
4a
Yield
Overall yield
[b]
Yield (E/Z ratio)
1
2
SO Me
29% (15:1)-3ba
29% (6:1)-3ca
32%-4ba
24%-4ca
61%
53%
2
SO Ph
2
3
4
28% (7:1)-3da
27% (8:1)-3ea
36%-4da
23%-4ea
64%
50%
[c]
5
3% (ND)-3fa
3%-4fa
6%
6
7
–
27%-4ga
55%-4ha
27%
81%
26% (>20:1)-3ha
[a]
Reaction conditions: vinylaziridine 1 (0.25 mmol, 1 equiv.), phenylacetylene 2a (102 mg, 1 mmol, 4 equiv.), Co(OAc) (2 mg,
2
5
mol%), dppe (5 mg, 5 mol%), DMSO (0.5 mL), 80°C, 2 h.
[b]
[c]
[d]
1
Determined by H NMR.
reaction time=3 h.
reaction time=20 h. ND=not determined.
acetylide formation. The deuterium incorporation also
took place with propargylacetate 2s (54% after 2 h at
8
0°C, c).
Scheme 5 depicts the catalytic cycle proposed for
the cobalt-catalyzed hydroalkynylation of vinylaziri-
dines. Through a ligand exchange, the cobalt acetylide
B would be formed from cobalt acetate A and alkyne
2. B would then coordinate vinylaziridine 1 either
through the carbon-carbon double bond or the nitrogen
atom, leading to intermediates C1 and C2, respec-
tively. Subsequent acetylide addition would give
intermediates D1 and D2. The alkynylation selectivity
would be related to the nature of the electron with-
drawing group on the nitrogen atom as observed
Scheme 3. Reactivity of cyclic vinylaziridine 6b.
tion experiments were performed using 5 equivalents experimentally. Finally, a protodemetalation step
of acetic acid-d . After 2 hours of heating at 80°C, would release products 3 and 4 and regenerate active
4
phenylacetylene was recovered with 75% of deuterium species A.
labelling (Scheme 4,a). Of note, the deuterium incor-
To illustrate the synthetic usefulness of the hydro-
poration also occurred at 25°C but slowly (20% after alkynylation adducts, the branched enyne 4aa was first
6
h). Similar partial deuterium incorporation occurred treated with iodine in acetonitrile in presence of silver
in benzene (Scheme 4,b) highlighting that the nature of acetate and potassium carbonate (Scheme 6). This
the solvent might not play a role during the Co(II) iodocyclization afforded the iododihydropyrrole 8 in
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Scheme 6. Iodocyclization of branched enyne 4aa.
Scheme 4. Hydrogen-deuterium exchange experiments.
Scheme 7. Platinum-catalyzed cycloisomerization of enynes 9a
and 9b.
moderate yield through a 5-endo-dig cyclization
process.
Linear 1,4-enyne 3aa was further functionalized
[26]
with alkyne moieties to give 1,6-enynes 9a and 9b as isolated in good overall yields. Scope investigation
potential substrates for metal-catalyzed cycloisomeri- showed that this reaction was strongly influenced by
zations. Several cationic gold complexes were then the electronic effect of the arylalkyne substituents. The
tested but only degradation was observed. Upon utility of both linear and branched enynes was high-
heating in toluene in the presence of 10 mol% of PtCl2, lighted in the synthesis of cyclic product 8 by
enyne 9a rapidly decomposed whereas enyne 9b gave iodocyclization or bicyclic compound 10b through
the expected cyclic product 10b in moderate yield platinum-catalyzed cycloisomerization. Further devel-
(Scheme 7).
opments are currently underway in our laboratory and
will be reported in due course.
Conclusion
Experimental Section
In summary, we have described an unprecedented
cobalt-catalyzed hydroalkynylation of vinylaziridines. General procedure for cobalt-catalyzed hydroalkynylation
This transformation represents one of the rare exam- reaction: In a glovebox, an oven-dried Schlenk tube was
charged with anhydrous cobalt (II) acetate (2 mg, 0.012 mmol,
ples of cobalt-catalyzed transformations allowing the
ring opening of small rings. Despite a low selectivity
between linear and branched adducts, the latters were
[27]
5 mol%)
and
1,2-bis(diphenylphosphino)ethane
(5 mg,
0
.012 mmol, 5 mol%). Outside of the glovebox, anhydrous
Scheme 5. Proposed catalytic cycle for the cobalt-catalyzed hydroalkynylation of vinylaziridines.
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DMSO (0.3 mL), vinylaziridine 1 (0.25 mmol, 1 equiv.) and
alkyne 2 (1 mmol, 4 equiv.) were added in turn followed by an
additional amount of anhydrous DMSO (0.2 mL). The reaction
mixture was heated to 80°C and stirred for 1–3 h. The reaction
was monitored by TLC and after complete consumption of
vinylaziridine 1, the reaction mixture was cooled to room
temperature and diluted with AcOEt. The organic layer was
X. Guo, T. Sawano, T. Nishimura, T. Hayashi, Tetrahe-
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[
6] For comprehensive reviews, see: a) Z.-X. Wang, X.-Y.
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washed with water (2 times), brine and dried over MgSO . After
4
concentration under reduced pressure, the residue containing a
mixture of linear and branched adducts, 3 and 4 respectively,
was purified by silica gel column chromatography with a
[
[
9
312–9320.
8] a) M. Shirakura, M. Suginome, J. Am. Chem. Soc. 2008,
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Acknowledgements
This work was supported by the Ministère de l’Enseignement
Supérieur et de la Recherche (BB Ph.D. grant), the CNRS,
AMU, and Centrale Marseille. L. K. thanks the China Scholar-
ship Council for a Ph.D. grant. We thank Dr. Christophe
Chendo and Dr. Valérie Monnier for mass spectrometry
analyses (Spectropole, Fédération des Sciences Chimiques de
Marseille) and Marion Jean and Dr. Nicolas Vanthuyne (Aix
Marseille Université) for chiral HPLC analyses. We acknowl-
edge Dr. Julie Broggi (Aix Marseille Université) for valuable
comments on the manuscript.
[
9] M. Shirakura, M. Suginome, Angew. Chem. 2010, 122,
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© 2021 Wiley-VCH GmbH
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These are not the final page numbers!

FULL PAPER
Cobalt-Catalyzed Hydroalkynylation of Vinylaziridines
Adv. Synth. Catal. 2021, 363, 1–9
B. Biletskyi, L. Kong, A. Tenaglia, H. Clavier*