Highly Selective Syn Addition of 1,3-Diones to Internal Ynamides Catalyzed by Zinc Iodide

Article abstract of DOI:10.1002/ejoc.201701690

1,3-Diones are already established as nucleophiles to perform additions to ynamides; the highly selective hydroalkoxylation of internal ynamides is now described. Several catalytic systems have been compared to perform this transformation, including transition-metal-based catalysts and Lewis acids. ZnI₂ was found to be both very active and highly selective providing only E adducts through a syn addition. Investigation of the scope and limitations showed that this catalyst was compatible with various functional groups. In addition to seventeen examples of ynamide hydroalkoxylation, one example of ynamide hydroarylation is reported.

Full text of DOI:10.1002/ejoc.201701690

A Journal of
Accepted Article
Title: Highly selective syn addition of 1,3-diones to internal ynamides
catalyzed by zinc iodide
Authors: Hervé Clavier, Rémi Plamont, and Lionel V. Graux
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201701690
Link to VoR: http://dx.doi.org/10.1002/ejoc.201701690
Supported by

10.1002/ejoc.201701690
European Journal of Organic Chemistry
COMMUNICATION
Highly selective syn addition of 1,3-diones to internal ynamides
catalyzed by zinc iodide
Rémi Plamont, [a] Lionel V. Graux,[a] and Hervé Clavier*[a]
Dedication ((optional))
Abstract: Having previously established that 1,3-diones could be
used as nucleophiles to perform additions to ynamides, highly
selective hydroalkoxylation of internal ynamides is now described
herein. Several catalytic systems were compared to carry out this
transformation including transition metal-based catalysts or Lewis
acids. ZnI2 was found to be both very active and highly selective giving
only E adducts through a syn addition. Scope and limits investigation
showed that this catalyst was compatible with various functional
groups. In addition to 17 examples of ynamide hydroalkoxylation, one
example of ynamide hydroarylation is reported.
carbon double bond was not clearly discussed (Scheme 1.a).
During the course of our research directed toward the use of
ynamides in [2+1] cycloadditions,[15] we discovered that
Herrmann-Beller catalyst (H-B cat) was able to promote the
hydroalkoxylation of ynamides with 1,3-diones.[16] Although good
yields were obtained with terminal ynamides (Scheme 1.b), the
resulting products were easily hydrolysable under acid conditions.
With internal ynamides, alkoxy-substituted enamide adducts were
significantly more stable to acid conditions but their preparation
led to mixtures of E- and Z-isomers in low to moderate ratios
(Scheme 1.c).
Introduction
Among the myriad of reactions using ynamides as substrate,[1]
those consisting of the heteronucleophile introduction to the
ynamide-carbon are particularly interesting from a synthetic
point of view. Thus, according to the nature of the nucleophile, as
well as the pending functional groups of the ynamides, various
structurally different products can be formed. With oxygen
nucleophiles, various reactions can be performed[2-4] such as the
hydration of ynamide that can be achieved under acid conditions
to straightforwardly give rise to functionalized amides.[5]
Skrydstrup even reported a gold-catalyzed hydration of dimerized
ynamides leading to the formation of 2,5-diamidofurans.[6]
Subsequently to Lam’s report on palladium-catalyzed
hydroacyloxylation of ynamide,[7] it was demonstrated that the
addition of carboxylic acids could be performed without
catalyst.[5b,8]
In comparison to the previously cited reactions, the
hydroalkoxylation of ynamides has been less studied whereas it
represents a straightforward access to unprecedented alkoxy-
substituted enamide patterns. Unfortunately with simple alcohols,
such as benzyl alcohol, hydroalkoxylation was unsuccessful.[9,10]
Hsung disclosed that with allylic[11] and propargylic alcohols[12] the
hydroalkoxylation was possible using a substoichiomietric amount
of p-nitrobenzenesulfonic acid under thermal activation. However,
since [3,3]-sigmatropic rearrangements took place, only the
corresponding amides were isolated.[13] Very recently, Swamy
reported a single example of hydroxyalkylation of an ynamide with
phenol using a palladium-based catalyst and a base upon thermal
activation.[14] Unfortunately, the stereochemistry of the carbon-
Scheme 1. Hydroalkoxylation reactions of ynamides affording alkoxy-
substituted enamides.
To investigate the synthetic potential of alkoxy-substituted
enamides,[8b,17] the hydroalkoxylation adducts from internal
ynamides represent more interesting substrates due to their
higher stability. Nevertheless, it would be better to obtain them as
single isomer. Therefore, we decided to further investigate the
addition of 1,3-diones to internal ynamides with the primary
objective of developing a highly selective catalytic system
promoting this reaction.
[a]
http://ism2.univ-amu.fr/fr/annuaire/chirosciences/clavierherve
Supporting information for this article is given via a link at the end of
the document.
Results and Discussion
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We started to survey various catalytic systems using as
benchmark substrates methyl-substituted ynamide 1a and
dimedone 2a (Table 1). In the absence of catalyst, the reaction
required a heating at 80 °C in dichoroethane (DCE) to observe
the adduct formation (entries 1 and 2). Despite a good E/Z ratio
(19:1), the reaction required a prolonged heating time to reach a
moderate yield. The use of the Herrmann-Beller catalyst (H-B cat)
allowed the decrease of the reaction temperature, but only poor
E/Z selectivities were observed (entries 3 and 4). With the gold
complex [Au(PPh3)NTf2], the addition proceeded at lower
temperature (40 °C, entry 5) but despite a long reaction time (16
h), the yield of 3aa was disappointing (45%). At higher
temperature, the gold catalyst showed a better efficiency but in
detriment of the E/Z selectivity (entry 6).[18] Alternatively, we
investigated several Lewis acids in order to efficiently activate the
ynamide or/and the 1,3-dione.[19] In(OTf)3 and Sn(OTf)2 were
found very active with a complete conversion in 1 h at 25 °C but
low selectivities were obtained (entries 7 and 8). Copper(I) or (II)
triflate slightly increased the E/Z selectivity (around 10:1, entries
9 and 10). Whereas Zn(OTf)2 gave only a moderate selectivity
(5.8:1, entry 11), ZnCl2 or ZnBr2 both allowed a substantial
increase of the E/Z selectivity (around 25:1, entries 12 and 13)
with a comparable efficiency. Finally, in presence of zinc(II) iodide,
we were able to successfully achieve the addition at 25 °C in 1 h
to give rise to exclusively E-adduct 3aa (E/Z > 50:1) in 77% yield
(entry 14). Concerning the difference of selectivies observed as a
function of the zinc salt used, we assume that the halide or
pseudohalide dissociation is favored in the case of ZnI2 and might
lead to the formation of chelate intermediates with reaction
partners that trigger the exclusive syn addition of 1,3-dione 2a.
The influence of the solvent was then investigated (Table 2).
The reaction performed well in DCE, dichloromethane (DCM) and
toluene with a complete selectivity in favor of the E adduct (entries
1-3). The use of moderately coordinating solvents such as THF or
dioxane led to a slight decrease of both reaction yields and
selectivities (entries 4 and 5). With the strongly acetonitrile, the
drop of the selectivity was even more drastic (entry 6). No reaction
occurred in DMF in spite of a prolonged reaction time (5 h, entry
7). These results, both in terms of efficiency and also selectivity,
support the assumption of an ynamide activation by ZnI2 and the
formation of chelates with the reactants. Despite lower yield and
E/Z ratio, the catalytic system was found compatible with MeOH
and no product resulting from the competitive addition of MeOH
was detected (entry 8).
Table 2. Solvent screening in ZnI2-catalyzed addition of dimedone 2a to
ynamide 1a[a]
Entry
Solvent
Yield (%)
E/Z[b]
1
2
DCE
DCM
77
82
72
58
57
54
NR
49
>50:1
>50:1
>50:1
33:1
31:1
9.3:1
-
3
Toluene
THF
4
5
1,4-Dioxane
MeCN
Table 1. Optimization of the reaction conditions[a] [20]
6
7[c]
DMF
8
MeOH
24:1
[a] Reaction conditions: ynamide 1a (150 mg, 0.5 mmol), dimedone 2a (70
mg, 0.5 mmol, 1 equiv.), ZnI2 (8 mg, 0.025 mmol, 5 mol%), solvent (3 mL,
0.17 M), 25 °C, 1 h. [b] Determined by 1H NMR. [c] 5 h of reaction. NR = No
reaction.
Having established the optimal reaction conditions, we further
investigated the scope of the hydroalkoxylation with a range of
1,3-diones (Scheme 2). Similarly to dimedone 2a, the furyl
derivate 2b gave rise rapidly to the E isomer of adduct 3ab in a
good yield. Owing to the poor solubility of the phenalene-1,3-
dione 2c at room temperature, the reaction mixture was heated at
40 °C for 5 h to reach a satisfactory yield of 3ac. 2-Substitued 1,3-
diones such 2h did not react despite an increase of reaction
temperature to 40 °C. Cyclopentane-1,3-dione based substrates
2d and 2e were good candidates affording the expected adducts
in good yields and as E isomers only. Unfortunately, indanedione
2i afforded a complex mixture of products. The ZnI2-catalyzed
addition was also efficiently achieved with Meldrum acid 2f or 4-
hydroxycoumarine 2g. With 2g, we obtained a separable mixture
of compounds 3ag and 3ag’ in a 1.7:1 ratio resulting from the
competitive addition of the two nucleophilic oxygen atoms. The
main limitation of this transformation remained that only cyclic 1,3-
diones can be used.
Time
(h)
Yield
(%)
Entry
Catalyst
T (°C)
E/Z[b]
1
2
None
None
60
80
60
80
40
60
25
25
25
25
25
25
25
25
5
16
5
NR
61
45
66
45
58
90
65
92
91
80
73
68
77
-
19:1
2:1
3
H-B cat
4
H-B cat
5
2:1
5
[Au(PPh3)NTf2]
[Au(PPh3)NTf2]
In(OTf)3
16
5
19:1
0.8:1
2.8:1
5.7:1
10.4:1
11.6:1
5.8:1
25:1
27:1
>50:1
6
7
1
8
Sn(OTf)2
Cu(OTf)2
Cu(OTf)·C6H6
Zn(OTf)2
ZnCl2
1
9
1
10
11
12
13
14
1
1
1
ZnBr2
1
ZnI2
1
[a] Reaction conditions: ynamide 1a (150 mg, 0.5 mmol), dimedone 2a (70
mg, 0.5 mmol, 1 equiv.), catalyst (5 mol%; 2.5 mol% for dimeric H-B cat),
DCE (3 mL, 0.17 M). [b] Determined by 1H NMR. NR = No reaction.
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10.1002/ejoc.201701690
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Scheme 3. Scope investigation with various internal ynamides (Ns = SO2(p-
NO2-C6H4)).
Scheme 2. Scope investigation of the ZnI2-catalyzed addition with various 1,3-
diones.
nucleophiles have already been well studied,[21] we focused on
carbon nucleophiles. To the best of our knowledge
intermolecular[22] hydroarylation of ynamides had only been
reported with indole derivatives.[23] Therefore, we decided to test
the hydroarylation with electron-rich trimethoxybenzene 4. To our
delight, the expected adduct 5a was obtained with 41% yield as a
single Z-isomer after 20 h of reaction at 40 °C (Scheme 4,
condition A). Alternatively, it was also found that [RuCl2(CO)3]2 in
association with a silver salt (condition B)[24] performed better,
since no thermal activation was required. In this case, 5a was
equally isolated as a single isomer. This transformation is
interesting, not only because it represents a straightforward
access to hindered and functionalized styrene derivatives but also
Results depicted in Scheme 3 showed that broad structural
variations of the ynamide partner can be accommodated. Except
in the case of the triisopropylsilyl-subsituted ynamide 1d, the
nature of the ynamide substituent did not influenced the efficiency
or selectivity outcomes of the additions. Corresponding adducts
were isolated as E-isomers only. These included functionalized
ynamides such as 1e, 1f and 1g containing ester, nitrile and
phtalimide groups respectively. For ynamides 1h and 1i bearing
nosyl and Boc (tert-butyloxycarbonyl) as electron withdrawing
group respectively, 5 h of reaction were necessary to obtain
moderate isolated yields but the addition remained exclusively
syn. Oxazolidinone-substituted ynamides 1j and 1k were also
found compatible with the reaction conditions and the
corresponding E adducts were isolated in moderate to good yields
(66% and 78%, respectively). Hydroalkoxylation of ynamide 1k
bearing a bulky group (tBu) indicated that the absence of reactivity
of 1d was probably due to electronic effects rather than a steric
congestion.
Finally, we explored the possibility to expand this reaction to
other nucleophiles. With the several catalytic systems
investigated, phenols were found unreactive. As nitrogen,
Scheme 4. Hydroarylation of ynamides.
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because it indicates clearly that the role of the ZnI2 catalyst is the
activation of the ynamide partner.
Conclusions
In summary, among different catalytic systems tested, including
transition metal-based catalysts and Lewis acids, we found that
ZnI2 was able to efficiently catalyze the syn hydroalkoxylation of
internal ynamides with 1,3-diones. Advantageously, this
transformation could be performed in various solvents. In a
general manner, reactions proceeded smoothly at room
temperature giving the adducts in good yields and as E-isomers
exclusively. As shown by the scope investigation, various
functional groups either on the ynamides or the 1,3-diones were
well tolerated. This transformation was successfully transposed to
the hydroarylation of ynamide with an electron-rich arene. This
result seems to indicate that ZnI2 activates the ynamides to trigger
the hydroalkoxylation. Further studies on the reaction mechanism
and the reactivity of adducts are currently undergoing in our
laboratory.
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[5]
Experimental Section
General procedure for the addition of 1,3-diones to terminal
ynamides: In a 5 mL Schlenk flask under nitrogen were added in turn ZnI2
(8.0 mg, 0.0125 mmol, 5 mol%), the ynamide (0.5 mmol, 1 equiv.), 1,3-
diketone derivatives (0.5 mmol, 1 equiv.) and DCM (3 mL). The reaction
mixture was allowed to stir at 25 or 40 °C for the indicated reaction time.
Volatiles were removed under reduced pressure and the crude residue
was purified by silica gel flash chromatography using petroleum ether and
ethyl acetate as eluent to give the pure desired product.
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Acknowledgements ((optional))
This work was supported by the Ministère de l'enseignement
supérieur et de la recherche (L.G. Ph.D. grant), ECM (R.P. ATER
grant), AMU and the CNRS. We thank Dr. Christophe Chendo and
Dr. Valérie Monnier for mass spectrometry analyses (Spectropole,
Fédération des Sciences Chimiques de Marseille). We
acknowledge Dr. Julie Broggi for useful comments on this
manuscript.
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Keywords: Hydroalkoxylation • Ynamide • 1,3-Dione • Lewis
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Ynamides*
Rémi Plamont, Lionel. V. Graux, Hervé
Clavier*
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Highly selective syn addition of 1,3-
diones to internal ynamides catalyzed
by zinc iodide
The hydroalkoxylation of internal ynamides with 1,3-diones was achieved in a highly
syn manner using zinc iodide as Lewis acid catalyst. The scope of this
transformation was found broad and compatible with various functional groups.
*one or two words that highlight the emphasis of the paper or the field of the study
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