Borylation and rearrangement of alkynyloxiranes: A stereospecific route to substituted α-enynes

Article abstract of DOI:10.3762/bjoc.15.141

1,3-Enynes are important building blocks in organic synthesis and also constitute the key motif in various bioactive natural products and functional materials. However, synthetic approaches to stereodefined substituted 1,3-enynes remain a challenge, as they are limited to Wittig and cross-coupling reactions. Herein, stereodefined 1,3-enynes, including tetrasubstituted ones, were straightforwardly synthesized from cis or trans-alkynylated oxiranes in good to excellent yields by a one-pot cascade process. The procedure relies on oxirane deprotonation, borylation and a stereospecific rearrangement of the so-formed alkynyloxiranyl borates. This stereospecific process overall transfers the cis or trans-stereochemistry of the starting alkynyloxiranes to the resulting 1,3-enynes.

Full text of DOI:10.3762/bjoc.15.141

Borylation and rearrangement of alkynyloxiranes:
a stereospecific route to substituted α-enynes
Ruben Pomar Fuentespina, José Angel Garcia de la Cruz, Gabriel Durin,
Victor Mamane*, Jean-Marc Weibel and Patrick Pale*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 1416–1424.
Laboratoire de Synthèse, Réactivité Organiques & Catalyse, associé
au CNRS, Institut de Chimie, Université de Strasbourg, 4 rue Blaise
Pascal, 67000 Strasbourg, France
doi:10.3762/bjoc.15.141
Received: 22 March 2019
Accepted: 07 June 2019
Published: 27 June 2019
Email:
Victor Mamane* - vmamane@unistra.fr; Patrick Pale* -
ppale@unistra.fr
Associate Editor: T. P. Yoon
*
Corresponding author
© 2019 Fuentespina et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
boron; enyne; lithium; oxirane; rearrangement
Abstract
1
,3-Enynes are important building blocks in organic synthesis and also constitute the key motif in various bioactive natural prod-
ucts and functional materials. However, synthetic approaches to stereodefined substituted 1,3-enynes remain a challenge, as they
are limited to Wittig and cross-coupling reactions. Herein, stereodefined 1,3-enynes, including tetrasubstituted ones, were straight-
forwardly synthesized from cis or trans-alkynylated oxiranes in good to excellent yields by a one-pot cascade process. The proce-
dure relies on oxirane deprotonation, borylation and a stereospecific rearrangement of the so-formed alkynyloxiranyl borates. This
stereospecific process overall transfers the cis or trans-stereochemistry of the starting alkynyloxiranes to the resulting 1,3-enynes.
Introduction
Lithiated oxiranes are useful intermediates in organic synthesis tetrasubstituted alkenes [6,7]. Capitalizing on these precedents,
[
1,2]. Such species can undergo different reactions, driven Aggarwal et al. showed that oxiranes could be homologated to
either by their carbanionic or carbenic behavior [3,4], thus diols through lithiation, borylation, rearrangement and oxida-
leading to various products. In contrast, the transmetalation of tion of the so-formed β-hydroxyboranes [8]. More recently,
lithiated oxiranes and the evolution of the resulting species have Blakemore et al. applied this sequence to sulfinyloxiranes [9].
been less explored. Utimoto’s group was probably the first who Alternatively, Unemaya et al. and more recently Buchwald et
reported on the treatment of oxiranyllithium with diethylzinc or al. described a Negishi cross coupling of aryl bromides or chlo-
trimethylaluminium [5]. Shimizu et al. demonstrated that in situ rides to α-CF3-oxiranyl zincates generated by lithiation of triflu-
formed trifluoromethyloxiranyllithium derivatives reacted with oromethyloxirane and transmetalation with zinc chloride
trimethylaluminium, dimethylzinc or organoboranes to yield [10,11].
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Beilstein J. Org. Chem. 2019, 15, 1416–1424.
In this context and based on the pioneering work of Shimizu These results showed that the steric bulk at the boron atom
[
6,7], and our experience on the metalation of alkynyloxiranes seems to facilitate the reaction. Indeed, the bulkier the boron
and reactivity studies of the so-formed alkynyloxirane anions substituent is, the more efficient is the process. Therefore, the
12,13], we explored the reaction of such anions with various substitution at the boron plays a key role, most probably at the
[
boron derivatives in order to produce stereodefined α-enynes boron rearrangement step (see below).
after Matteson-type rearrangement [14] and elimination [15]
(
Scheme 1). We also demonstrated the stereospecificity of these
Table 1: Borylation and rearrangement of ethynyloxirane 1a.a,b.
transformations [16].
Entry
Boronic ester
Timec
(min)
Yieldd
(%)
Scheme 1: Stereospecific formation of α-enynes from alkynyloxiranes.
t1
t2
α-Enynes (1,3-enynes) are an interesting class of compounds in
organic chemistry as these structures are found in various phar-
macologically active natural products [17], and in functional
materials [18]. α-Enynes are also important building blocks in
organic syntheses or platforms for further synthesis [19]. Ac-
cordingly, significant efforts have been made to develop effi-
cient syntheses of enynes [17-20]. The most common routes
relied on Wittig-type reactions [21,22] and transition-metal-cat-
alyzed coupling reactions of either terminal or organometallic
alkynes with vinyl halides or of alkynyl halides with vinylic
organometallics [20]. However, both routes may lead to mix-
tures of stereoisomers and synthetic approaches to stereo-
defined substituted α-enynes remain scarce and thus still repre-
sent a challenge in organic chemistry.
1
2
30
20
33
43
2
a
b
30
30
20
20
2
3
66
2
c
aReaction conditions: 1) 1.5 equiv n-BuLi, THF, −78 °C, t1;
) 1.5 equiv boronic ester 2, −78 °C, t2; 3) rt, 1 h; bTIPS: triisopropyl-
silyl); ct1 corresponds to the time of the lithiation step and t2 to the time
of the borylation step; dyields of pure isolated products, unless other-
wise stated.
2
Results and Discussion
Reaction set up
Stereoselectivity, scope and limitations
First we selected the simple 2-ethynyl-3-dimethyloxirane 1a to Next, the stereoselectivity of this rearrangement was evaluated
probe the feasibility of the reaction. In order to avoid unwanted using cis and trans-disubstituted alkynyloxiranes (Table 2). For
reactions upon metalation [23], the ethynyl group was silylated synthetic reasons [29,30], we selected cis-2,3-epoxy-5-triiso-
and a bulky silyl group was installed to avoid any silyl migra- propylsilylpent-4-ynol, protected with the monomethoxytrityl
tion [12]. As already shown [12,13], n-butyllithium proved to group (MMTr) 1b and prepared its trans-isomer to compare the
be very efficient, although stronger or complex bases are outcome of the reaction with both isomers.
usually required to produce oxiranyl anions [1,2,24-28]. Upon
deprotonation, the corresponding oxiranyllithium was suffi- The cis-configured compound cis-1b readily reacted with
ciently stable at −78 °C (see below) to be trapped by various pinacol phenylboronate 2c under the conditions set up above,
phenylboronic esters 2. The resulting borate intermediate providing enyne 4 as the sole product in high yield (Table 2,
afforded the tetrasubstituted enyne 3 in good to moderate entry 1). NMR analysis revealed the Z-stereochemistry of this
yields, depending on the boronic ester’s nature (Table 1). The enyne [31]. In contrast, the trans-isomer trans-1b afforded a
catechol ester 2a only provided a modest yield of 3 (Table 1, mixture of the 2 possible isomers, although in similar overall
entry 1), while the neopentylglycol ester 2b proved slightly yield (Table 2, entry 2 vs 1). Decreasing the temperature from
better (Table 1, entry 2). The pinacol ester 2c turned out to be −78 to −92 °C for the deprotonation–borylation steps reward-
the most effective reagent for this transformation, giving around ingly allowed the exclusive formation of the E-enyne 4 as a
7
0% of 3 after isolation and purification (Table 1, entry 3). single isomer (Table 2, entry 3).
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Table 2: Lithiation, borylation and rearrangement of cis or trans-disubstituted ethynyloxiranes 1b.a,b.
Entry
Oxirane
R
Temp.
°C)
Timec
(min)
Enyne
Yieldd
(%)
(
t1
t2
1
2
3
cis-1b
Ph
"
−78
−78
−92
30
30
60
20
20
90
78
80
70
(
Z)-4
trans-1b
trans-1b
(E)-4 + (Z)-4 4:1
"
(
E)-4
4
5
6
cis-1b
trans-1b
cis-1b
p-MeOPh
−78
−92
−78
30
60
30
20
90
20
85
88
81
(
Z)-5
"
(
E)-5
p-ClPh
(
Z)-6
7
8
9
trans-1b
cis-1b
"
Bn
"
−92
−78
−92
60
30
60
90
20
90
85
62
65
(
E)-6
(
Z)-7
trans-1b
(
E)-7
1
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Table 2: Lithiation, borylation and rearrangement of cis or trans-disubstituted ethynyloxiranes 1b.a,b. (continued)
1
1
0
1
cis-1b
allyl
−78
−92
30
60
20
90
57
78
(
Z)-8
trans-1b
"
(
E)-8
1
1
2
3
cis-1b
n-butyl
−78
−92
30
60
20
90
85
90
(
Z)-9
trans-1b
"
(
E)-9
aReactions were run under argon, c = 0.125 mol/L; bMMTr: monomethoxytrityl; ct1 corresponds to the time of the lithiation step and t2 to the time of
the borylation step; dyields of pure isolated products, unless otherwise stated.
After these encouraging results, the nature of the group trans- times needed at −78 °C. Surprisingly, the trans isomer of 1d
ferred during the rearrangement was explored. Electron-with- always provided a 4:1 mixture of the E/Z isomers of 11, while
drawing or electron-donating substituents on the phenyl group cis-1d gave the expected Z-isomer of 11 exclusively (Table 3,
did not alter the stereochemical course of the reaction, while entries 3 and 4). The two isomers could, however, be separated
slightly improving the yields (Table 2, entries 4–7 vs 1 and 2). by chromatography. Interestingly, the cis or trans-epoxydiynes
Benzyl or allyl groups could be transferred as well, although 1e stereospecifically and solely provided the corresponding and
slightly lower yields were observed (Table 2, entries 8–11). expected E or Z-isomers of enyne 12 in quite good yields
Even an alkyl group could be transferred and interestingly, the (Table 3, entries 5 and 6). However, in these reactions some
best yields were achieved with this kind of group (Table 2, degradation was observed, but the yields were satisfying
entries 12 and 13).
considering the sensitive polyunsaturated nature of the starting
compounds. The trisubstituted ethynyloxirane 1f proved also
It is worth noting that the same stereochemical relationship be- reactive, but longer reaction times were required, probably due
tween the starting oxirane and the enyne product was always to steric hindrance at some stage of the rearrangement (vide
obtained, whatever the group transferred from boron to the infra). Both isomers exclusively afforded the expected E or Z
oxirane carbon. These results revealed the stereospecificity of tetrasubstituted alkenes 13 in high yields (Table 3, entries 7 and
the reaction and suggested that no epimerization of the 8).
oxiranyllithium intermediate occurred.
Stereochemistry and mechanism
The present reaction was then further explored and extended to Based on the above results, we conclude that the alkynyl-
various ethynyloxiranes (Table 3). The non-functionalized cis oxirane-to-enyne reaction is stereospecific, maintaining the
or trans-3,4-epoxy-1-triisopropylsilyldec-1-ynes 1c stereospe- stereochemistry of the starting oxirane in the resulting enyne.
cifically gave the expected E or Z trisubstituted enynes 10 in This stereospecificity imposes the stereochemical integrity of
moderate to good yields (Table 3, entries 1 and 2). It is worth the oxiranyllithium species formed upon lithiation, but also the
mentioning that cis-1c contaminated with its trans-isomer (9:1 existence of at least one concerted step during the reaction
ratio) was converted to enyne 10 with the same E/Z ratio of 1:9. mechanism. This step most probably corresponds to the group
The cis or trans-phenyl analogues 1d were less reactive, as transfer in organoboron derivatives, as the stereoselectivity of
revealed by incomplete reactions at −92 °C and longer reaction such transfer has been demonstrated [32,33].
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Table 3: Lithiation, borylation and rearrangement of various cis or trans-ethynyloxiranes.a
Entry
Oxirane
Temp. (°C)
Enyne
Yield (%)b
55d
1
c
−78
(
Z)-10
cis-1c
trans-1c
cis-1d
2
−92
−78
−78
56
48
(
E)-10
3
4
(
Z)-11
63e,f
(
E)-11
trans-1d
5
−78
59g
cis-1e
(
Z)-12
6
7
−92
−78
57g
69h
(
E)-12
trans-1e
cis-1f
(
Z)-13
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Table 3: Lithiation, borylation and rearrangement of various cis or trans-ethynyloxiranes.a (continued)
8
−92
73h
(
E)-13
trans-1f
aReactions were run under argon, c = 0.125 mol/L; byields of pure isolated products, unless otherwise stated; cratio cis/trans 9:1; dratio Z/E 9:1; ea 4:1
mixture of stereoisomers was obtained; fthe yield refers to the isolated E-isomer; gsome degradation occurred; hafter addition of the boronic ester, the
mixture was stirred overnight at room temperature.
Regarding the oxiranyllithium stereochemical integrity and specificity (see entry 3 vs 2, Table 2). This observation sug-
stability, it has been demonstrated that solvent, oxirane substitu- gests that the corresponding oxiranyllithium species could be
ent’s nature and of course temperature play key roles [34], in labile. Trapping experiments with deuterated water were thus
addition to the initial cis or trans stereochemistry, if present conducted at −92 and −78 °C with cis-1b or trans-1b, respec-
[
35]. In our case, the oxiranyllithium derived from cis-alkynyl- tively. As expected, the cis-isomer gave the deuterated cis-de-
oxiranes are configurationally stable as it was established by us rivative 1b-d with an excellent 92% yield, whatever the temper-
12,13]. However, we found here that in case of trans-alkynyl- ature was (Scheme 2, entry 1). However, the trans-isomer at
[
oxiranes, a lower temperature was required to ensure stereo- −78 °C, produced a mixture of products (Scheme 2, entry 2).
Scheme 2: Trapping experiments of the oxiranyllithium derived from cis or trans-alkynyloxiranes 1b, and their interpretation.
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The expected deuterated trans-derivative was the major com- oxiranylborate complex B then undergoes an intramolecular
pound, the second product (2:1 ratio) was ascribed to ketone 14 SN2-like process (Matteson-type rearrangement) [14] transfer-
after isolation. In addition, traces of the deuterated cis-deriva- ring the non-oxygenated boron substituent and promoting
tive were detected by 1H NMR in the crude mixture, together oxirane ring opening [39]. The group’s migratory aptitude
with traces of other side-products. The formation of this set of cannot be directly compared to that known from cationic rear-
products clearly evidenced the carbenic behavior of the rangements, because the nature of the other boron substituents
involved oxiranyllithium (Scheme 2, entry 3) and revealed that plays a key role in migration, as revealed by calculations [40-
the trans-alkynyloxiranes are unstable at −78 °C, but perfectly 42]. From the results we so far gained, a migratory order could
stable at −92 °C.
be assessed for the present reaction: Bu ≥ EDG-substitued Ar >
EWG-substitued Ar ≥ Ar ≥ benzyl > allyl, and it appears that
As already proposed [36] and then demonstrated [12,13], depro- the larger the other substituents on the boron atom are, the
tonation of oxiranes occurred upon oxygen-directed metalation. easier the transfer is.
The resulting oxiranyllithium probably exists as a dimer in solu-
tion, as it has been nicely evidenced by structural characteriza- A possible transition state B could be envisaged with the large
tion and NMR studies of the lithiated o-trifluoromethylstyrene pinacol substituent facing the small oxirane ring and thus
oxide [37]. Such dimerization obviously stabilizes the oxiranyl- placing the migrating group antiparallel to the adjacent oxiranyl
lithium species. Applied to oxiranyllithium derived from cis or C–O bond. This may explain the role of the diol substituent on
trans-alkynyloxiranes, such dimer may easily form from the the reaction efficacy (see Table 1). Upon migration and oxirane
cis-oxirane (Scheme 2, entry 4), but not from the trans-isomer, ring opening, a β-boron alkoxide C is formed, which probably
or at least the large steric constrain present in it may hamper its isomerizes to a 1,2-oxaboretanide D, a known intermediate in
stability. Indeed, the long oxirane C–O bond adjacent to lithi- bora-Wittig reactions [7], whose fragmentation then stereospe-
um observed by X-ray diffraction in the lithiated o-trifluo- cifically forms the observed enyne.
romethylstyrene oxide [37] may be further elongated due to
hindrance introduced by the trans-substituent adjacent to the Conclusion
lithium atom of the second oxirane in the dimer (Scheme 2, To summarize, we reported here on a convenient access to
entry 5). Such effect may facilitate the formation of the corre- stereochemically defined α-enynes through lithiation, boryla-
sponding carbenoid, especially with increasing temperature.
tion and rearrangement of cis or trans-alkynyloxiranes. This
cascade reaction is stereospecific, probably due to the stereo-
Once the oxiranyllithium species is formed (A in Scheme 3), controlled group transfer from boron to the adjacent oxiranyl
oxiranylborate complexes B are obtained by the addition of carbon in an oxiranylborate complex intermediate, followed by
aryl-, benzyl-, allyl- or alkylboronic esters. This was clearly ev- an internal bora-Wittig reaction.
idenced by the fast disappearance of the coloration linked to the
oxiranyllithium species upon the addition of the boronic ester. This method is particularly appealing, providing with good to
Unfortunately, all attempts to detect or isolate such ate com- high yields a wide diversity of functionalized E or Z tri- or
plexes or related oxiranylboronic esters failed so far [38]. The tetrasubstituted α-enynes.
Scheme 3: Proposed mechanism for the rearrangement of alkynyloxiranes to α-enynes through metalation and borylation.
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ORCID® iDs
Ruben Pomar Fuentespina - https://orcid.org/0000-0002-4217-0601
Gabriel Durin - https://orcid.org/0000-0003-1836-6142
Victor Mamane - https://orcid.org/0000-0001-8996-7880
Patrick Pale - https://orcid.org/0000-0002-3924-530X
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3
8.For example, addition of pinacolboron derivatives carrying a leaving
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decomposition.
2
1
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39.It is worth noticing here that the lower stereoselectivity observed at −78
°
C with trans-1b could be explained by trapping of either the
corresponding trans and cis-oxyranyllithium and stereospecific
migration, or the carbenoid, which apparently formed under such
conditions, and further non-selective migration.
4
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