Expedient synthesis of conjugated triynesviaalkyne metathesis

Article abstract of DOI:10.1039/d0sc01124j

The first synthesis of conjugated triynes by molybdenum-catalysed alkyne metathesis is reported. Strategic to the success of this approach is the utilization of sterically-hindered diynes that allowed for the site-selective alkyne metathesis to produce th

Full text of DOI:10.1039/d0sc01124j

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ARTICLE
Expedient Synthesis of Conjugated Triynes via Alkyne Metathesis
Idriss Curbet,a Sophie Colombel-Rouen,a Romane Manguin,a Anthony Clermont,a Alexandre
Quelhas,a Daniel S. Müller,a Thierry Roisnel,a Olivier Baslé,a† Yann Trolez*a and Marc Mauduit*a
Received 00th January 20xx,
Accepted 00th January 20xx
The first synthesis of conjugated triynes by molybdenum-catalysed alkyne metathesis is reported. Strategic to the success
of this approach is the utilization of sterically-hindered diynes that allowed for the site-selective alkyne metathesis to
produce the desired conjugated triyne products. The steric hindrance of alkyne moiety was found to be crucial in
preventing the formation of diyne byproducts. This novel synthetic strategy was amenable to self- and cross-metathesis
providing straightforward access to the corresponding symmetrical and dissymmetrical triynes with high selectivity.
DOI: 10.1039/x0xx00000x
Despite the mentioned advances in this field, the development
of versatile and efficient methodologies is still necessary.
Several well-defined W- and Mo-based catalysts (e.g. Cat-1
and Cat-2, Figure 2) can promote metathesis reactions of
alkynes, allowing for the formation of new carbon-carbon
triple bonds.8 Logically, extension of this methodology to
Introduction
Polyynes are an important class of natural products which
display a broad array of biological activities such as
antibacterial, antimicrobial, antitumor and anticancer
properties.1 Conjugated diynes and triynes (e.g. ivorenolide B
12 and ichthyothereol 23) represent the major part of this class
of compounds that can be isolated from plants, fungi or
bacteria (Figure 1, a). While the diyne core is easily
accessible,4 the triyne moiety appears more difficult to access
and represents a synthetic challenge.5 Several methods have
been developed to access symmetrical and unsymmetrical
triynes.6 For instance, Tykwinski and co-workers recently
reported an elegant synthetic strategy involving a Pd-catalysed
cross-coupling between 3 and various terminal alkynes that
lead to the desired triynes 4 (Figure 1, b).7
triyne substrates would represent
a
promising and
straightforward way to synthesize the highly desirable 1,3,5-
hexatriyne core.
Ph
W
pMeOPh
(tBuO)3SiO
(tBuO)3SiO
Mo
Ph3SiO
Ph3SiO
OSi(OtBu)3
OSiPh3
Cat-1
Cat-2
(a) Self-Metathesis (Tamm, 2012)
Cat-1 (2 mol %)
6 (97% yield)
Me
+
toluene, rt, 2 h
MS 5 Å
5
OH
HO
Me
Me
(a)
7 (DMDA)
(b) Cross-Metathesis (Tamm, 2013 & 2018)
R1 R1
O
O
O
Me
O
(1.0 equiv)
+
Cat-1 (2 mol %)
Ivorenolide B 1
(-)-Ichthyothereol 2
R1
R2
CH2Cl2, rt
R2
R2
(62 to 91% yield)
R1 = Ar or alkyl; R2 = nBu or SiMe3
(4.0 equiv)
(b)
Br
Br
Ar
H
Ar
(c) Ring-Closing Metathesis (Fürstner, 2015)
Et
Pd(OAc)2 (10 mol%)
OTBS
Et
OTBS
OTf
Cat-2 (40 mol%)
H
P(tBu)3/CuI (20 mol%)
iPr2NH
)
O
O
2
iPr3Si
iPr3Si
toluene, rt, 22 h
MS 4 Å / 5 Å
(
Me
3
O
O
4
9 (82% yield)
8
Figure 1 (a) Examples of natural products containing diynes and triynes. (b)
Synthesis of triynes as reported by Tykwinski and co-workers.
(d) This work: Alkyne metathesis with sterically-demanding diynes
R2
R1
R1
Me
Cat-2
+
+
a. Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS,
R1 = R2 or R1 = R2
ISCR
– UMR 6226, F-35000 Rennes, France. E-mail: marc.mauduit@ensc-
Me
Me
R2
Me
rennes.fr; yann.trolez@ensc-rennes.fr
† Present address: LCC-CNRS, Université de Toulouse, CNRS, UPS, Toulouse, France
† Electronic Supplementary Information (ESI) available: Experimental procedures,
NMR spectra. CCDC 1908076, 1908077, 1966970, 1908075, 1915266 and 1920652.
For ESI and crystallographic data see DOI: 10.1039/x0xx00000x
Figure 2 Previous works on diynes metathesis catalysed by Mo- or W-catalysts
and the proposed concept for the access of conjugated triynes (this work).
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This strategy was recently attempted by Tamm and co-workers Additionally, structures of diynes 13d and 13e weVrieewcAortnicfleirOmnleinde
using a tungsten-alkyne complex Cat-1, which unexpectedly by single crystal X-ray diffraction studDieOsI: 1(S0c.1h0e39m/De0S1C)0.11T2h4eJ
led to the exclusive formation of symmetrical diynes (e.g. 5) reactivity and behaviour of these more or less sterically-
and related dimethyldiacetylene 7 (Figure 2, a). The demanding diynes were then assessed in alkyne self- and
anticipated conjugated triynes could not be isolated.9 In 2013, cross-metathesis. First, we investigated the self-metathesis of
the authors extended the methodology to diyne cross- silylated diynes 13a-c,f (Table 1). Triisopropylsilylpentadiyne
metathesis (DYCM) (Figure 2, b)10,11 resulting in unsymmetrical 13a was exposed to alkyne metathesis catalyst Cat-2 (3 mol%)
diynes products along with traces of the triyne products upon at 40 °C over three hours. While a 50% conversion was
prolonged reaction time. In line with these pioneering works, observed, only traces of desired triyne 14a (1%) were detected
Fürstner and coworkers also observed a similar reactivity in by GC analysis (entry 1). Inspired by the work of Fürstner and
the catalytic ring-closing metathesis (RCM) of a bis-diyne 8 co-workers,12c the addition of 4 and 5 Å molecular sieves (MS)
with molybdenum-alkyne complex Cat-2 that produced the in the reaction media to trap 2-butyne could not only increase
corresponding cyclized diyne 9 (Figure 2, c), a key intermediate the conversion up to 96% but also allow for the formation of
in the total synthesis of Ivorenolide B 1 (Figure 1).12 In order to the desired symmetrical (TIPS)2triyne 14a in excellent 86% GC
enforce the appropriate CC triple bond to react, it was yield (Table 1, entry 2). Nevertheless, symmetrical (TIPS)2diyne
hypothesized that the introduction of bulky groups would 15a was also obtained in a respective triyne/diyne ratio of
influence the regioselectivity13 of diyne metathesis reactions 86:14 (see Supplementary Information, SI for details).
favouring the triyne products over the diyne products as Interestingly, at lower catalyst loading (2 and 1 mol%, entries 3
previously reported (Figure 2, d).9-12 We would like now to and 5) as well as at ambient temperature (entry 4), Cat-2
report on a methodology that allows for the unprecedented remained quite productive reaching respectively 80, 59 and
selective formation of symmetrical and dissymmetrical 74% GC yields.
conjugated triynes from alkyne metathesis.
Table 1 Self-metathesis of silylated diynes 13a-c,f catalysed by Mo-complex Cat-2aa
R
R
R
Results and discussion
R
Si
Si
R
Investigations began with the synthesis of diynes 13a-e
bearing various substituents following a robust methodology
from the literature (Scheme 1).14 Starting from terminal
alkynes 10, the Pd-catalysed Sonogashira coupling with (E)-1,2-
dichloroethylene 11 resulted in the corresponding chloro-
enynes 12. The latter was then reacted with n-Buli and
iodomethane to produce the expected diynes 13a-e in
moderate to good overall yields (Scheme 1).
R
Cat-2 (x mol%)
14a-c,f
R
R
MS 4Å/5Å
R
Si
Me
+
toluene
R
R
40 °C, 3 h
13a-c,f
R
Si
Si
R
R
R
a: R = iPr; b: R = Et; c: R = Ph; f: R = Me
15a-c,f
Cat-2
(mol%)
Conv.
(%)b
Ratio
Yield
Yield
entry diynes
14:15c
14b
15b
PdCl2(PPh3)2 (5 mol%)
1d
2
13a
13a
13a
13a
13a
13a
13f
3
3
2
2
1
3
3
3
3
50
96
91
89
75
96
91
>98
95j
nd
1
86
80
74
59
63g
-
-
Cl
CuI (10 mol%)
R1
H
Cl
+
R1
Cl
HN(iPr)2 (5 equiv)
86:14
90:10
90:10
95:5
-
-
THF, 50 °C, 14 h
12
10
11 (3 equiv)
3
nBuLi (2 equiv)
4e
-
THF, 0 °C
R2
Si
5
-
then MeI (4.5 equiv)
rt, 16 h
R2
Me
6f
7h
8
81:19
0:100
2:98
-
R2
R1
Me
48
56
19k,l
13a R2 = iPr (54%)a
13b R2 = Et (18%)a
13c R2 = Ph (47%)a
13b
13c
-
13
9i
40:60k
12k,l
a
Reaction conditions: diyne (0.04 mmol), catalyst (3 mol%), MS 4Å/5Å (40 mg),
Ph
Ph
Ph
b
toluene (0.35 mL), 40 °C, 3 h, under Ar. Determined by GC-analysis with
acetophenone as internal standard. Determined by GC-analysis. Performed
without MS 4Å/5Å. Performed at 20 °C. Performed at 0.55 mmol-scale.
Isolated yield after silica gel chromatography. Performed at 20 °C over 1 h.
Performed at 0.26 mmol-scale. Based on the recovered starting material.
Determined by quantitative 13C NMR spectroscopy. Estimated yield from an
Me
c
d
X-ray of 13d
e
f
g
13d (54%)a
h
i
j
k
l
Me
isolated mixture of 14c + 15c.
13e (42%)a
a Isolated yield after silica gel purification.
More importantly, the selectivity increased up to 95:5 with the
lowest catalyst loading, although the GC yield dropped to 59%
for triyne 14a (entry 5). By increasing the scale of the reaction,
it was possible to isolate 14a in good 63% isolated yield after
X-ray of 13e
Scheme 1 Synthetic access of sterically-demanding dienes 13a-e
2 | J. Name., 2012, 00, 1-3
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Ph
Ph
Ph
Ph
Ph
Cat-2 (3 mol%)
silica gel purification (entry 6). After demonstrating that the
presence of a bulky triisopropylsilyl (TIPS) group allowed for
the formation of the triyne as the major metathesis product, it
was investigated whether less sterically-hindered Si-
substituent such as trimethylsilyl (TMS), triethylsilyl (TES) or
triphenylsilyl (TPS) groups would give preferentially rise to the
symmetrical diyne or the triyne product. Hence, TMS-diyne 13f
was synthesized according to a protocol from the literature
and exposed to the aforementioned conditions (3 mol%, 40 °C,
3 h).15 Mo-based complex Cat-2 appeared quite productive,
although symmetrical (TMS)2diyne 15f was exclusively formed
(ratio 14f:15f = 0:100) in 48% GC yield (Table 1, entry 7), which
is consistent with previous observations from the Tamm group
(Scheme 1b).11 A similar behaviour was also observed with TES-
diyne 13b (ratio 14b:15b = 2:98) that led to symmetrical
(TES)2diyne 15b in a moderate 56% GC yield (entry 8).
Regarding diyne 13c featuring a bulkier Ph3Si group, the
expected symmetrical (TPS)2triyne 14c was formed more
significantly but (TPS)2diyne 15c remained predominant (ratio
14c:15c = 40:60, entry 9).16 Moreover, X-ray diffraction
analysis could be done allowing to confirm the solid-state
structure of desired triyne 14c that exhibited usual
geometrical features for this kind of compounds6b (Figure 3).
Encouraged by the promising good result reached with TIPS
substituent, we decided to investigate other substituents such
as triphenylmethyl (trityl) or mesityl (Mes) groups (Scheme 2).
To our delight, as seen with the TIPS-substrate 13a, Cat-2
demonstrated efficient catalytic activity in the self-metathesis
of the trityl-substituted diyne 13d resulting in the desired
symmetrical triyne product 16 in 54% isolated yield (Scheme 2,
a).
View Article Online
Ph
Me
Ph
(a)
(b)
DOI: 10.1039/D0SC01124J
MS 4Å/5Å
toluene, 40 °C, 3 h
Ph
Ph
16 (54%)a
13d
Mes
Mes
17 (40%)c
Cat-2 (3 mol%)
+
Me
MS 4Å/5Å
toluene, 40 °C, 3 h
Mes
Mes
Mes
Mes
18
19
13e
+
>95% conv.b
2
ratio 17:18:19 = 50:45:5b
Si
Me
Si
Mes
20 (41%)b
Cat-2 (3 mol%)
13a (1 equiv.)
MS 4Å/5Å
(c)
+
+
toluene, 40 °C, 3 h
>97% conv.b
Mes
Si
Me
21 (9%)b
13e (1 equiv.)
+ 14a (14%)b, 17 (12%)b
+ 15a (3%)b, 18 (9%)b, 19 (4%)b,d
Si
Me
Cat-2 (5 mol%)
13a (1 equiv.)
Ph
Ph
Ph
MS 4Å/5Å
(d)
Si
+
toluene, 40 °C, 3 h
Ph
Ph
Ph
22 (49%)a
Me
+ 14a (26%)a, 16 (3%)a
13d (1 equiv.)
Ph
Ph
Ph
Me
13d (1 equiv.)
Ph
Ph
Ph
Cat-2 (5 mol%)
Mes
(e)
+
MS 4Å/5Å
toluene, 40 °C, 3 h
23 (41%)a
Me
+ 17 (30%)e, 18 (16%)e, 16 (9%)a
13e (1 equiv.)
Scheme 2 Scope of self- and cross-metathesis catalysed by Mo-complex Cat-2. aIsolated
yield after silica gel chromatography. bDetermined by GC-MS analysis cYield determined
by GC-MS analysis with ndodecane as the internal standard. d(TIPS)2tetrayne and
TIPS/Mes-tetrayne were also detected (1-3%). eYield from an inseparable isolated
mixture of 17 + 18 (ratio = 55/45).
14c
16
Furthermore, X-ray diffraction analysis unambiguously
confirmed the structure of trityl-substituted triyne 16 (Figure
3). Additionally, the stability of trityl-triyne 16 toward the Mo-
benzylidyne catalyst was evaluated, as the competitive
conversion of triyne to higher polyynes was suspected. Triyne
16 was thus exposed to Mo-cat 2 (3 mol%) at 40 °C over 3
hours. Interestingly, 96% of the starting material were
recovered. It is worth to notice that a similar behaviour was
also observed with a mixture of triyne/diyne 14a/15a (81/19
ratio) without any alteration of the triyne/diyne ratio (see SI
for details). Those additional experiments highlighted the
remarkable stability of the triynes into the reactive media. Mo-
benzylidyne catalyst was also efficient toward mesityl-
substituted diyne 13e affording the expected symmetrical
triyne 17 in moderate 40% GC-MS yield (Scheme 2, b).
However, GC-MS analysis evidenced a significant formation of
symmetrical diyne 18 but also some traces of tetrayne 19
22
23
Figure
3 Solid-state structures of symmetrical triynes 14c and 16 and
dissymmetrical triynes 22 and 23 from single crystal X-ray diffraction.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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(ratio 17:18:19 =50:45:5).17 The steric hindrance of the mesityl with the quest for more efficient alkyne-metathesis complexes
View Article Online
20
DOI: 10.1039/D0SC01124J
group appeared thus less suitable to promote the selective demonstrating higher selectivity and productivity.
formation of the triyne metathesis product. The more
challenging cross-metathesis reaction was then studied. First,
by reacting TIPS- and Mes-diynes 13a and 13e, the formation
of the highly desirable dissymmetrical triyne 20 as the major
product was observed, reaching 41% GC yield (Scheme 2, c).
Unsurprisingly, some amounts of symmetrical triynes 14a and
17 were also observed (14 and 12%, resp.), as well as the
dissymmetrical diyne 21 (9%) and its symmetrical congeners
15 and 18 (3 and 9%, resp.). Of note, few traces (1-4%) of
symmetrical/dissymmetrical tetraynes were also detected (see
SI for details). Secondly, with trityl-diyne 13d as a partner,
TIPS-diyne 13a led to the expected dissymmetrical triyne 22 in
49% isolated yield while Mes-diyne 13e produced the
corresponding dissymmetrical triyne 23 in 41% isolated yield
(Scheme 2, d,e). Again, some amounts of symmetrical
triynes/diynes were also formed. Furthermore, solid-state
structures of these challenging dissymmetrical triynes 22 and
23 were also confirmed by X-ray diffraction analysis (Figure 3).
In order to show the synthetic potential of this methodology to
furnish valuable triyne building-blocks, the post-
transformation of dissymmetrical triyne 22 was then explored.
As depicted in Scheme 3, the TIPS fragment was successfully
removed using silver fluorine followed by direct iodination of
the resulting deprotected alkyne in the presence of N-
iodosuccinimide (NIS). The expected 7-iodohepta-2,4,6-triyne-
1-trityl 24 was isolated in good 69% isolated yield. Considering
the aforementioned experimental results, a plausible reaction
pathway for the self-metathesis of sterically-hindered diynes is
suggested in Scheme 4. Depending on the steric hindrance
brought by the RL-substituent of the diyne, and according to
Ph
Ph
Ph
Ph
Ph
Ph
NIS, AgF
Si
I
CH3CN/
acetone/CHCl3
24 (69%)a
22
r.t., 16 h
Scheme 3 Post-functionalization of dissymmetrical triyne 22.
Me
Ph(pOMe)


Mo(Ph3SiO)3
+


Cat-2
RL
Me
Ph(pOMe)


M
MCBD-
RL

Me
Ph(pOMe)
RL
Me
P2
Me
RL






Me

M

PS
RL
RL
Me
Me




A
MCBD-A
MCBD-A
M
M
(MCBD-A/MCBD-A


Me
RL

Me
the established mechanism of the alkyne metathesis,9-11,18
a

P1
catalytic cycle could be proposed. If RL is large enough, Mo-
based complex Cat-2 reacts preferably with the less hindered
CC triple bond to form the metallacyclobutadiene MCBD-
leading to propagating species PS. The latter is then engaged
through the catalytic cycle A by reacting again towards the less
hindered CC triple bond to produce the valuable symmetrical
(-triyne P1. In opposition, if PS reacts with the more
hindered internal alkyne, a catalytic cycle B already
described by Tamm and co-workers is involved and explains
the formation of the undesired diyne product P3.11


M
Me
RL
RL
RL



PS

MCBD-

Me
Ph(pOMe)
Me
RL

Me
RL

P4





Me

M
PS
Conclusions
Me
RL
RL
Me
In conclusion, it was demonstrated herein that the utilization
of sterically-hindered diynes can modify the selectivity of
alkyne metathesis to favour the formation of the desired
triynes. Therefore, for the first time, the synthesis of
symmetrical and dissymmetrical conjugated triynes by self-
and cross-metathesis was successfully achieved. By involving
an efficient molybdenum benzylidyne complex,19 remarkable
triyne:diyne ratios were reached (up to >95:5) affording
expected triynes in moderate to good yields (up to 86%).
These pioneer results pave the way to further developments




B
MCBD-B
MCBD-B
M
M
(MCBD-B/MCBD-B


RL
Me
Me


P3


M
Me
RL
RL
RL
PS




4 | J. Name., 2012, 00, 1-3
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Scheme 4 Proposed mechanism for the self-metathesis of sterically-hindered diynes
catalysed by Mo-complex Cat-2 leading to conjugated triynes (cycle A) or diynes (cycle
B). Reversibility is expected for each step. Only productive metathesis steps are
represented.
13 We previously reported that sterically-hindered alkynes
View Article Online
could promote high regioselectivity in DaOsyI:m10m.1e0t3r9ic/Dc0oSnCj0u1g1a24teJ
addition of enynones, see: M. Tissot, D. Poggiali, H. Hénon,
D. Müller, L. Guénée, M. Mauduit and A. Alexakis, Chem. Eur.
J., 2012, 18, 8731.
14 N. Kerisit, R. Ligny, E. S. Gauthier, J.-P. Guegan, L. Toupet, J.-
C. Guillemin and Y. Trolez, Helv. Chim. Acta., 2019, 102,
e1800232.
Conflicts of interest
15 V. Fiandanese, D. Bottalico, G. Marchese and A. Punzi,
Tetrahedron, 2006, 62, 5126.
There are no conflicts to declare.
16 Due to their higher molecular weight, all metathesis
reactions with 13d were monitored by quantitative 13C-NMR
spectroscopy.
Acknowledgements
17 Tetrayne 19 resulted from the self-metathesis of triyne 17.
18 T. J. Katz and J. McGinnis, J. Am. Chem. Soc., 1975, 97, 1592.
19 (a) W. Zhang, S. Kraft and J. S. Moore, J. Am. Chem. Soc.,
2004, 126, 329; (b) D. E. Gross and J. S. Moore,
Macromolecules 2011, 44, 3685; (c) P. Persich, J. Llaveria, R.
Lhermet, T. de Haro, R. Stade, A. Kondoh, A. Fürstner, Chem.
Eur. J., 2013, 19, 13047.
20 (a) M. Koy, I. Elser, J. Meisner, W. Frey, K. Wurst, J. Kästner,
M. R. Buchmeiser, Chem. Eur. J., 2017, 23, 15484; (b) J.
Hillenbrand, M. Leutzsch, A. Fürstner, Angew. Chem. Int. Ed.
2019, 58, 15690.
We are grateful to the CNRS, the Ecole Nationale Supérieure
de Chimie de Rennes (grant to RM and AC) and the Ministère
de l’Enseignement Supérieur, de la Recherche et de
l’Innovation (grant to RM and AQ). This work was also
supported by the region Bretagne (SAD 2016 N° 9639 –
RZSELECT; grant to DSM) and the FASO (grant to IC and DSM).
Prof. A. Fürstner is acknowledged for the generous gift of
molybdenum benzylidyne complex. We are grateful to Elsa
Caytan and the PRISM core facility (Biogenouest©, UMS,
Biosit, Université de Rennes 1) for quantitative 13C NMR
experiences.
Notes and references
1
2
For a recent review, see: A. L. K. Shi Shun and R. R. Tykwinski,
Angew. Chem. Int. Ed., 2006, 45, 1034.
Y. Wang, Q.-F. Liu, J.-J. Xue, Y. Zhou, H.-C. Yu, S.-P. Yang, B.
Zhang, J.-P. Zuo, Y. Li and J.-M. Yue, Org. Lett., 2014, 16,
2062.
3
4
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