Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne Metathesis Catalysts

Article abstract of DOI:10.1002/anie.201908571

A new type of molybdenum alkylidyne catalysts for alkyne metathesis is described, which is distinguished by an unconventional podand topology. These structurally well-defined complexes are easy to make on scale and proved to be tolerant toward numerous functional groups; even certain protic substituents were found to be compatible. The new catalysts were characterized by X-ray crystallography and by spectroscopic means, including ⁹⁵Mo NMR.

Full text of DOI:10.1002/anie.201908571

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Molybdenum Alkylidyne Complexes with Tripodal Silanolate
Ligands: The Next Generation of Alkyne Metathesis Catalysts
Authors: Alois Fürstner, Julius Hillenbrand, and Markus Leutzsch
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201908571
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Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne
Metathesis Catalysts
Julius Hillenbrand, Markus Leutzsch, and Alois Fürstner*
Abstract: A new type of molybdenum alkylidyne catalyst for alkyne metathesis is described, which is
distinguished by an unconventional podand topology. These structurally well-defined complexes are
easy to make on scale and proved to be tolerant toward numerous functional groups; even certain
protic substituents were found to be compatible. The new catalysts were characterized by X-ray
crystallography and by spectroscopic means, including 95Mo NMR.
The discovery that triarylsilanolate ligands synergize exceedingly well with molybdenum alkylidynes
marks a milestone in the development of alkyne metathesis.1,2,3 Catalysts such as 1 or the derived
bench-stable adduct 2 combine excellent activity with user-friendliness; most importantly, they are
distinguished by a remarkable and so far unrivaled functional group tolerance.4,5,6 This auspicious
profile is manifest, inter alia, in the high-yielding formation of numerous poly-functionalized targets;
the examples shown in Figure 1 are representative.7,8,9,10,11,12,13,14 Even protic groups were found to be
compatible in favorable cases, although important limitations still remain:15 the polarization of the
operative [MoCR] unit, which is inherently nucleophilic and basic at carbon, constitutes a potential
Achilles heel of any Schrock alkylidyne in a protic environment.16,17 Moreover, substrates able to entail
ligand exchange will eventually bring productive alkyne metathesis to a hold since catalyst 1 and
congeners draw their excellent performance from a (largely) intact siloxide ligand sphere. In
consideration thereof, it may not come as a surprise that the attempted metathesis reaction of a
substrate as simple as 8 containing an unhindered primary alcohol basically met with failure (Scheme
1).
*
J. Hillenbrand, Dr. M. Leutzsch, Prof. A. Fürstner
Max-Planck-Institut für Kohlenforschung
45470 Mülheim/Ruhr (Germany)
E-mail: fuerstner@kofo.mpg.de
Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found
under ############
1
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Figure 1. Selected examples from the cornucopia of products formed by alkyne metathesis at the
indicated site (scissors) with the aid of molybdenum alkylidynes such as 1 or the derived adduct 2;
tolerated protic groups are marked in yellow
Scheme 1. Previous experimentation with potential podand ligands
2
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We conceived that the use of chelating ligands might allow this issue to be addressed, at least in part,
since the concomitant cleavage of up to three Mo-O-Si bonds is statistically less likely. In a first foray,
the tripodal ligand scaffolds 4 and 5 were prepared (Scheme 1).18 Upon mixing with precatalyst 3,
however, they fail to afford well-defined alkylidyne complexes; rather, the available data suggest that
a mixture is generated as a result of partial cross linking. This aspect notwithstanding, these two-
component catalysts show high activity and, indeed, a gradually improved tolerance toward functional
groups which the parent complex 1 does not allow to handle; the structurally simple but chemically
challenging primary alcohol 8 referred to above is one of them, which afforded the corresponding
metathesis product 9 in up to 72% yield.18,19
Other potentially tripodal ligands were explored by Zhang and coworkers: originally, their design relied
on the use of amine-tethered (nitro)phenol groups 6 (Z = N; X = H, NO2). Although derived podand
complexes were isolated and characterized, only the in situ generated mixture showed appreciable
catalytic activity.20,21 This system suffers from the internal coordination of the basic N-atom onto the
molybdenum center, which compromises the Lewis acidity of the alkylidyne unit.20,22 To remedy the
issue, the ammonium-, silicon- and carbon-tethered variants of 6 (Z = (NMe)+, SiMe, CH) were
prepared, which do indeed entail higher activities.23 For the lack of the anchoring N-atom, however,
more than one active species is generated on reaction with 3; rigorous proof of formation of well-
defined catalysts of podand topology is missing for any of these systems.24,25,26
It is against this backdrop that the current study must be seen. The next-generation catalyst design
should allow the following aspects to be addressed: (i) a tridentate arylsilanolate ligand scaffold
continues to be most desirable given the well-proven synergy with the catalytically active [MoCR]
unit; (ii) in order to reach a well-defined single-component catalyst, the ligand has to be more
preorganized than 4 or 5 to minimize the entropic penalty upon formation of the targeted podand
topology and hence avoid competing oligomerization from occurring; (iii) importantly, the necessary
level of preorganization must not entail an overly rigidified backbone, because any active catalyst has
to support different geometries while passing through the catalytic cycle;1-3 specifically, the ligand
needs to accommodate the tetrahedral geometry of the resting state as well as the trigonal
bipyramidal extreme to be adopted during substrate binding and metallacycle formation/processing;
(iv) this essential compromise between preorganization and flexibility has to be reached without
3
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recourse to a Lewis-basic heteroelement in the tether; (v) ideally, the ligand synthesis is
straightforward, scalable and modular, as deemed necessary for future fine-tuning of the ligand
properties in steric and/or electronic terms; (vi) an additional desideratum is the use of a molybdenum
precatalyst other than 3: although this complex can be prepared on scale, its synthesis mandates
particularly rigorous organometallic techniques and access to an argon (not nitrogen!) Schlenk-
line/glove-box.27
Scheme 2. a) TfOH (neat), 130°C, 59%; b) tBuLi, Et2O, 125°C  RT, then Ar2Si(OMe)2, 74% (12a), 12%
(12b, unoptimized); c) aq. HCl (12 M), THF, quant. (13a), quant. (13b); d) NaOtBu, THF, 83%; e) 13,
toluene, 95% ([16a]2), 76% ([16b]2); f) toluene, 60°C, see Text; g) MeCN, see Text
We thought to meet these stringent criteria by the design shown in Scheme 2. A literature-known
cyclocondensation reaction converts 2-bromoacetophenone (10) into 11 on scale.28,29 Exhaustive
metal/halogen exchange followed by quenching of the lithiated intermediate with Ph2Si(OMe)2
furnishes 12a,30 from which the desired trisilanol 13a is released on treatment with aqueous HCl. This
simple three-step sequence gave > 2 g of 13a (single largest batch); we have no reason to believe that
it would not work similarly well on even larger scale. Moreover, the preparation of the analogous
4
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silanol 13b bearing lateral MeO- groups suggests that the route will qualify for more systematic ligand
variation once the project enters the next stage of development.
Figure 2. Structure of 13aH2O in the solid state; only the H-atoms of the Si-OH groups involved in
hydrogen bonding with the co-crystallized water molecule (disordered over two positions) are shown
for clarity
The structure of 13a in the solid state (Figure 2) shows that all three Si-OH groups are
“upward/inward”-oriented; the NMR data prove that the approximate C3 symmetry is maintained in
solution. Although this conformation is arguably the consequence of a hydrogen-bonding array with a
co-crystallized water molecule, it indicates that the C3-symmetric orientation, as necessary for the
formation of a podand complex, is geometrically and entropically viable.
Next, we addressed the problem of rendering the actual complex formation more practical by avoiding
the use of 3 as the molybdenum source. The readily accessible tribromide [Br3MoCAr(dme)] (14)
might be conceived as the starting point of choice, because its treatment with Ph3SiOM (M = Na, K) is
known to afford 1 as the parent catalyst of this family in high yield.4,5 Adaptation of this procedure to
the formation of the targeted podand complexes, however, is arguably unrewarding: (i) dipole
repulsion in the triple sodium (lithium, potassium) salt derived from 13 almost certainly enforces a
conformational change away from the “all-inward” orientation of the SiO vectors;30 (ii) such triple
5
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salts are poorly soluble in toluene and even THF; (iii) salt metathesis with the Mo–Br of 14 bonds is
irreversible; successful formation of a podand structure, however, seems more likely if the reaction
has the chance to correct eventual “mistakes” such as undesirable cross-linking.
Gratifyingly, interposition of a ligand exchange step with tBuONa provides a very convenient solution
(Scheme 2). Complex 15 thus formed is catalytically inactive31 but the tert-butoxy groups are more
basic than a triarylsilanol; therefore, addition of 13 to a solution of 15 in toluene at ambient
temperature results in quantitative ligand exchange. As an additional advantage we note that the
released tBuOH can be conveniently removed in vacuo to give a new complex in the form of a bright
yellow, gently air-sensitive powder.32 This material is only sparingly soluble in [D8]-toluene: although
the recorded spectra speak for the presence of a well-defined entity rather than an oligomer, the signal
pattern is too complicated for an approximately C3-symmetric target complex of type 16. Importantly,
however, significant spectral changes are observed upon stirring of the mixture at elevated
temperatures: a new species is quantitatively formed after 60 min at 60°C, which seems to correspond
to the expected complex. In consideration of the fact that even the parent catalyst 1 is a non-covalent
dimer (at least) in the solid state held together by multiple /-interactions between the lateral phenyl
rings,5 the observed temperature dependence suggested that the yellow crude product might also be
a self-assembled dimeric or oligomeric entity that converts into the monomeric complex upon gentle
heating. Once formed, the monomer is stable in solution for days; however, it is the aggregate which
precipitates upon increasing the concentration and/or cooling of the mixture.
Although an X-ray structure of this yellow compound has not yet been obtained, the notion of a pre-
equilibrium is supported by DOSY spectra recorded at the point where a  1:1 mixture of two
compounds was present in solution. The observed diffusion constants D = 5.56 1010 m2s1 and D =
3.30 1010 m2s1, respectively, suggest that the two species have considerably different molecular
weights/sizes; actually, the predicted D-values for the monomer 16a and a likely dimeric (or potentially
even trimeric) species  tentatively assigned as [16a]2  nicely match the recorded data.32 From a
solution in benzene/MeCN we were able to isolate dark blue crystals of adduct 16aMeCN suitable for
X-ray analysis. Figure 3 shows the expected monomeric podand-like molybdenum alkylidyne complex
bearing an “end-on” bound MeCN ligand;33 with 163.2(2)°, however, the Mo1N1C69 angle notably
deviates from linearity. Taking acetonitrile as a dummy for an incoming alkyne substrate, this structure
6
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allows the binding site of the catalyst to be assessed: though tripodal in nature, the ancillary ligand
scaffold is obviously flexible enough to open a sizeable bay area that is likely able to accommodate
even reasonably bulky compounds. The first coordination sphere about the metal center is
approximately tetrahedral, reflecting the fact that the binding site trans to the alkylidyne is unoccupied
yet efficiently blocked by the central phenyl ring forming the platform of the tripodal ligand scaffold.
Although the Lewis-acidic MoVI center will sense this -cloud in vicinity, the observed distances (3.5 –
4.0 Å) rule any direct bonding interactions out. With 1.742(2) Å, the length of the MoC alkylidyne is
in the normal range; likewise, the kink in the MoCAr axis (161.4(2)°) has correspondence in the
structures of 1 and analogues.4,5
Of particular interest are the remarkably obtuse SiOMo bond angles (155.8(8), 163.2(2), 169.0(1)°),
enforced by the ligand architecture; for comparison, we refer to the corresponding angles in 1
(141.3(1), 147.7(1), 159.5(1)°). The change in hybridization of a silanol O-atom  on going from a
tetrahedral (sp3) to a linear (sp) geometry  alters the degree of back-donation of the lone pairs into
the empty orbitals of the Mo atom and hence affects the Lewis acidity of the catalytically active
alkylidyne unit.5 It has previously been pointed out that the eminent suppleness of an unrestrained
SiOMo linkage translates into an advantageous electronic adaptiveness: as a result, silanolate
ligands bolster the different electronic optima of an alkyne metathesis catalyst when passing through
the catalytic cycle.5 Under this premise, it is likely that the new podand complex 16 featuring uniformly
stretched MoOSi hinges is not just a tethered variant of the parent complex 1; rather, its electronic
character and, as a consequence thereof, the catalytic performance should differ from that of 1.
7
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Figure 3. Top: structure of complex 16aMeCN in the solid state with the alkylidyne unit disordered
over two positions; bottom: truncated view of the podand core structure (all lateral aryl groups were
removed); hydrogen atoms and one molecule of co-crystallized but unbound MeCN are not shown for
clarity)
8
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Figure 4. 95Mo NMR spectra ([D8]-toluene, 60°C) of the parent complex 1 (top) and the podand
complexes 16a (middle) and 16b (bottom)
95Mo NMR spectroscopy provides qualitative support for this notion. As a spin 5/2 nucleus with low
natural abundance ( 15.9%), low gyromagnetic ratio and a quadrupole moment, the 95Mo isotope is
rarely used for analytical purposes in organometallic chemistry.34 Actually, the only previous
investigation into molybdenum alkylidynes failed to detect the 95Mo NMR signal of [(tBuO)3MoCPh].35
Gratifyingly though, we were able to record good spectra of complexes 1 and 16a,b in [D8]-toluene at
60°C (Figure 4). Although the largely missing precedent makes it currently impossible to interpret these
data by comparison with known compounds, the distinct shift difference between 16a ( = 421 ppm)
and 1 ( = 397 ppm) is taken as an indication for a subtle but noticeable difference in their electronic
character.36 The fact that –OMe substituents on the peripheral phenyl rings of 16b cause a gentle up-
field shift ( = 414 ppm) speaks for a qualitative correlation between the chemical shift and the electron
density at the Mo center: if this is the case, the podand complexes are slightly more Lewis acidic that
the unrestrained parent complex 1.37 Although this conclusion needs further scrutiny, we note that the
alkylidyne C-atoms ([D8]-toluene) of monomeric 16a (C = 310 ppm) and 16b (C = 309 ppm) are also
deshielded relative to that of 1 (C = 301 ppm); the dimeric complex [16a]2, in contrast, resonates at
almost the same shift (C = 298 ppm).
9
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Table 1. Alkyne metathesis reactions catalyzed by the alkylidyne complexes of podand topology.[a]
Entry Catalyst T (°C)
Product
Yield (%)[b]
1
2
3
4
5
16a
16a
16b
16b
16a
RT
60
60
60
60
92 (X = OMe)[c]
90 (X = CF3)
77 (X = CN)[d]
93 (X = OH)[e]
NR (X = NO2)
6
7
16a
16a
RT
RT
91
67
8
16a
16a
16a
RT
RT
RT
91
9
52
10
89[f]
11
12
13
16a
16a
16a
60
RT
RT
71
88
83
14
15
16a
16a
60
RT
95 (n = 1)
85 (n = 3)
16
17
16a
16a
RT
RT
98
98
18
19
16a
16a
RT
RT
9938
8539
20
21
16a
16a
60
7540
110
68[g],19
10
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[a] Unless stated otherwise, all reactions were carried out using 5 mol% of catalyst in toluene at RT (ca. 23°C) in
the presence of powdered 5 Å MS; all substrates were internal alkynes carrying a methyl cap, which release 2-
butyne as by-product of the metathesis reaction; [b] isolated yield of analytically pure material; [c] the required
reaction time was 9 h; for comparison, complex 1 (5 mol%) delivers the product in 82% yield after 2 h; [d] 16a at
60°C gave 78% yield; [e] 16a at RT afforded only 48% of the product together with recovered starting material;
[f] the required reaction time was 6 h; for comparison, complex 1 (5 mol%) delivers the product in 73% yield after
30 min; [g] yield over two steps: in the actual metathesis product, R = 2,2,6,6-tetramethylpiperidin-1-yl; for the
ease of isolation, the crude product was subjected to NO bond cleavage with zinc dust in aq. THF/HOAc to give
the diol product (R = H), see ref. 19 and the SI; NR = no reaction
The key measure for success, however, is the catalytic performance of the new podand complexes
(Table 1). It is perhaps unsurprising that a rigidified backbone with reduced geometrical and electronic
adaptability result in diminished reactivity: 16a,b (5 mol%) convert 1-phenyl-1-propyne into tolane
within  3 h at ambient temperature in the presence of molecular sieves (5 Å) as butyne-sequestering
agent,4 whereas complex 1 requires <10 min (for further direct comparison, see the SI or the footnotes
to entries 1 and 10 in Table 1).41,42 A normalized time scale analysis43 of different conversion/time
diagrams suggests that the reaction is first order in catalyst 16a.44 From the application point of view
it is gratifying that the dimer [16a]2 and the monomeric podand 16a are equally effective.32 As
expected, electron poor substrates are less reactive and mandate higher temperatures; the
corresponding nitro derivative failed to react at 60°C (entry 5).15,45
Since a truly comprehensive study into functional group compatibility is beyond the scope of this initial
foray, we focused on potentially problematic substituents. The high yielding homo-metathesis of a
primary alcohol is significant (entry 6); the obtained yield is actually higher than that of homologous 9
formed with the two-component catalyst system [3+4] (Scheme 1).18 Additional substrates containing
different protic sites (sec-propargylic alcohol, phenol, amide, malonate, fluorenyl group) also gave
appreciable results. Likewise, the compatibility with a secondary as well as tertiary amine (entries 11,
12) is a striking asset for a catalyst containing a high-valent Mo-center, as is the fact that a potentially
chelating Weinreb amide group did not bring the reaction to a halt (entry 9); moreover, the stability of
the elimination-prone primary tosylate is noteworthy (entry 13). The different examples of ring closing
alkyne metathesis reactions under high dilution conditions prove that the new catalysts qualify for this
important transformation, even in challenging cases in which the products carry sensitive functionality,
potential donor sites, or feature strained backbones (entries 14-21).
11
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Overall, we conclude that an important step toward a new generation of structurally well-defined
alkyne metathesis catalysts has been made. The new design largely preserves the virtues of the parent
tris(triarylsilanolate)molybdenum alkylidyne complex 1 but holds the promise of an even better
functional group tolerance. Although it entails slower rates, we hope that full reactivity be restituted
upon systematic variation of the ligand structure by harnessing the modularity of the developed
synthesis route. Investigations along these lines are currently in progress in this laboratory.
Acknowledgements
Generous financial support by the MPG is gratefully acknowledged. We thank Mr. Ch. Wille for
assistance, Dr. J. Novacek for computational support that allowed alternative ligand designs to be
sorted out, Mr. N. Nöthling and Prof. C. W Lehmann for solving the X-ray structures, and Mrs. C. Wirtz
and Mrs. J. Lingnau for excellent NMR assistance.
For the Table of Contents
The canopy: Molybdenum alkylidyne complexes with triarylsilanolate ligands are privileged catalysts
for alkyne metathesis, but tethering of the silyl groups seems to make things even better.
Crystallographic data and a rare case study based on 95Mo NMR spectroscopy provide insights into why
that is so.
Keywords: Alkylidyne Complexes  Alkyne metathesis  Molybdenum  NMR  Podands  Silanolate
Ligands
12
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1
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13
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21
Podand structures of type 7 were proposed but the isolated complexes were dimeric, featuring
the N-atoms cis (not trans) to the alkylidyne; the phenol O-atoms form the bridges between the
Mo-centers, cf. ref. 20,22
22
23
See also: D. W. Paley, D. F. Sedbrook, J. Decatur, F. R. Fischer, M. L. Steigerwald, C. Nuckolls,
Angew. Chem. Int. Ed. 2013, 52, 4591-4594; Angew. Chem. 2013, 125, 4689-4692.
a) K. Jyothish, Q. Wang, W. Zhang, Adv. Synth. Catal. 2012, 354, 2073-2078; b) H. Yang, Z. Liu, W.
Zhang, Adv. Synth. Catal. 2013, 355, 885-890; c) Y. Du, H. Yang, C. Zhu, M. Ortiz, K. D. Okochi, R.
Shoemaker, Y. Jin, W. Zhang, Chem. Eur. J. 2016, 22, 7959-7963.
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For selected applications, see: a) Q. Wang, C. Zhang, B. C. Noll, H. Long, Y. Jin, W. Zhang, Angew.
Chem. Int. Ed. 2014, 53, 10663-10667; Angew. Chem. 2014, 126, 10839-10843; b) Z. Chen, D. W.
Paley, L. Wei, A. L. Weisman, R. A. Friesner, C. Nuckolls, W. Min, J. Am. Chem. Soc. 2014, 136,
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8027-8033; c) Q. Wang, C. Yu, H. Long, Y. Du, Y. Jin, W. Zhang, Angew. Chem. Int. Ed. 2015, 54,
7550-7554; Angew. Chem. 2015, 127, 7660-7664.
25
For yet other catalysts with pincer-type ligand frameworks, see: a) D. E. Bellone, J. Bours, E. H.
Menke, F. R. Fischer, J. Am. Chem. Soc. 2015, 137, 850-856; b) S. S. Nadif, T. Kubo, S. A. Gonsales,
S. VenkatRamani, I. Ghiviriga, B. S. Sumerlinn, A. S. Veige, J. Am. Chem. Soc. 2016, 138, 6408-
6411.
26
For other recent advances in catalyst design, see : a) M. Koy, I. Elser, J. Meisner, W. Frey, K.
Wurst, J. Kästner, M. R. Buchmeiser, Chem. Eur. J. 2017, 23, 15484-15490; b) D. P. Estes, C. P.
Gordon, A. Fedorov, W.-C. Liao, H. Ehrhorn, C. Bittner, M. L. Zier, D. Bockfeld, K. W. Chan, O.
Eisenstein, C. Raynaud, M. Tamm, C. Copéret, J. Am. Chem. Soc. 2017, 139, 17597-17607; c) O.
Arias, H. Ehrhorn, J. Härdter, P. G. Jones, M. Tamm, Organometallics 2018, 37, 4784-4800; d) W.
Bai, W. Wei, H. H. Y. Sung, J. D. Williams, Z. Lin, G. Jia, Organometallics 2018, 37, 559-569.
W. Zhang, Y. Lu, J. S. Moore, Org. Synth. 2007, 84, 163-176.
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X. Feng, J. Wu, V. Enkelmann, K. Müllen, Org. Lett. 2006, 8, 1145-1148.
For a pioneering use of this platform for the design of tripodal ligands, see: E. Y. Tsui, M. W. Day,
T. Agapie, Angew. Chem. Int. Ed. 2011, 50, 1668-1672; b) J. S. Kanady, J. L. Mendoza-Cordes, E.
Y. Tsui, R. J. Nielson, W.A. Goddard, T. Agapie, J. Am. Chem. Soc. 2013, 135, 1073-1082.
The structure of 12a in the solid state is contained in the Supporting Information; in contrast to
13aH2O, the SiOMe groups are not uniformly “upward/inward” oriented.
Compare: a) L. G. McCullough, R. R. Schrock, J. C. Dewan, J. C. Murdzek, J. Am. Chem. Soc. 1985,
107, 5987-5998; b) C. Bittner, H. Ehrhorn, D. Bockfeld, K. Brandhorst, M. Tamm, Organometallics
2017, 36, 3398-3406.
30
31
32
33
For details, see the Supporting Information
The diffusion coefficient of 16aMeCN determined by DOSY spectroscopy is very similar to that
of 16a; this finding confirms the notion that 16a itself is a monomeric species formed by
dissociation of a likely dimeric precursor complex. For details, see the Supporting Information.
D. Rehder, Coord. Chem. Rev. 1991, 110, 161-210.
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35
36
C. G. Young, E. M. Kober, J. H. Enemark, Polyhedron 1987, 6, 255-259.
All 95Mo shifts were indirectly referenced relative to Na2MoO4 (2 M in D2O).
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37
The only recorded molybdenum alkylidynes in ref. 35 carried alkyl substituents as the ancillary
ligands rather than alkoxy (aryloxy, silyloxy) groups; the fact that their 95Mo resonances appear
at very low field ([(Me3SiCH2)3MoCSiMe3] (1845 ppm), [(Me3CCH2)3MoCCMe3] (1400 ppm))
might indicate high Lewis acidity as a consequence of missing pd back donation.
A. Fürstner, O. Guth, A. Rumbo, G. Seidel, J. Am. Chem. Soc. 1999, 121, 11108-11113.
a) A. Fürstner, O. Larionov, S. Flügge, Angew. Chem. 2007, 119, 5641-5644; Angew. Chem. Int.
Ed. 2007, 46, 5545-5548; b) A. Fürstner, S. Flügge, O. Larionov, Y. Takahashi, T. Kubota, J.
Kobayashi, Chem. Eur. J. 2009, 15, 4011-4029.
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a) A. Fürstner, C. Mathes, C. W. Lehmann, Chem. Eur. J. 2001, 7, 5299–5317; b) A. Fürstner, C.
Mathes, K. Grela, Chem. Commun. 2001, 1057-1059.
The time/conversion diagram included in the Supporting Information indicates that 16a is
slightly more reactive than 16b; a more detailed comparison of the reactivity and functional
group tolerance of these and related catalysts are subject to ongoing investigations, cf. ref. 4a.
Attempted metathesis of phenylacetylene led to polymerization; however, it remains to be
elucidated if complexes of type 16 are suitable for cross alkyne metathesis reactions of terminal
alkynes as described with catalyst 1 in ref. 6b.
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43
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J. Burés, Angew. Chem. Int. Ed. 2016, 55, 2028-2031; Angew. Chem. 2016, 128, 2068-2071.
It is pointed out that this order refers to reactions performed at catalyst concentrations in the
range of 0.5-1.4104 M; because of the pre-equilibrium between monomer and dimer
(oligomer), we cannot exclude changes at other concentrations; this aspect is subject to further
studies.
45
Other catalysts such as 1bipy (ref. 18) as well as complexes developed by Zhang (ref. 23) are
able to metathesize 1-nitro-4-(prop-1-yn-1-yl)benzene in appreciable yield.
16
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