Ruthenium-Catalyzed trans-Hydroalkynylation and trans-Chloroalkynylation of Internal Alkynes

Article abstract of DOI:10.1021/jacs.0c08582

[Cp*RuCl]4 catalyzes the addition of iPr3SiCCX (X = H, Cl) across internal alkynes with formation of 1,3-enyne or 1-chloro-1,3-enyne derivatives, respectively; the reaction follows an unorthodox trans-addition mode. The well-balanced affinities of the different reaction partners to the ruthenium catalyst ensure that crossed addition prevails over homodimerization of the individual components, as can be deduced from spectroscopic and crystallographic data of various intermediates; this includes a dinuclear complex in which an internal alkyne bridges two [Cp*RuCl] fragments.

Full text of DOI:10.1021/jacs.0c08582

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Ruthenium-Catalyzed trans-Hydroalkynylation and trans-
Chloroalkynylation of Internal Alkynes
̈
Nagaraju Barsu, Markus Leutzsch, and Alois Furstner*
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ABSTRACT: [Cp*RuCl]4 catalyzes the addition of iPr3SiCCX (X = H, Cl) across internal alkynes with formation of 1,3-enyne
or 1-chloro-1,3-enyne derivatives, respectively; the reaction follows an unorthodox trans-addition mode. The well-balanced affinities
of the different reaction partners to the ruthenium catalyst ensure that crossed addition prevails over homodimerization of the
individual components, as can be deduced from spectroscopic and crystallographic data of various intermediates; this includes a
dinuclear complex in which an internal alkyne bridges two [Cp*RuCl] fragments.
he addition of a terminal alkyne across an internal triple
bond is a conceptually appealing yet highly challenging
arenes by cycloisomerization/cross-coupling (see below)7,8
and the preparation of π-conjugated oligomers with valuable
optoelectronic properties,9 are deemed representative.
Following up on our investigations into ruthenium-catalyzed
trans-hydrogenation10,11 and trans-hydrometalation12−19 cata-
lyzed by {Cp*RuCl]4 or related complexes, we reasoned that
the reactivity pattern manifested in these unorthodox trans-
formations might be further extended.20 For their activated C−
H bonds, terminal alkynes were deemed promising candidates;
the desirable “crossed” addition mode seemed possible because
[Cp*RuCl] readily forms heteroleptic complexes comprising
two different π-ligands.21,22
T
approach to 1,3-enynes (Scheme 1).1,2 For such a hydro-
Scheme 1. Challenge of Crossed Hydro(chloro)alkynylation
To test this hypothesis, various terminal alkynes were
screened (see the SI), but only triisopropylsilylacetylene (1a)
gave good results (Scheme 2).23 In the presence of catalytic
[Cp*RuCl]4, 1a reacts with internal dialkylalkynes to form the
corresponding trans-addition products; the Z:E ratios are
generally excellent. The stereochemistry was assigned by NMR
and confirmed for product 9 by X-ray diffraction (see the SI).
As expected, the functional group tolerance is high, in that
ketones, esters, unprotected alcohols, acetals, aryl and alkyl
halides, as well as cyclopropyl rings, remain intact. Aromatic
substrates, however, react less well, likely because [Cp*Ru]
tends to form kinetically stable η6-arene adducts that may
sequester the catalyst (cf. 6; for further examples, see the SI);
this limitation has precedent in the trans-hydroelementation
reactions cited above.10−20
alkynylation reaction to become useful, competing homodime-
rization, oligomerization, and/or cyclotrimerization of either
partner must be suppressed and regiocontrol be imposed when
working with unsymmetrical substrates (R1 ≠ R2). The
stereochemical course of the reaction is usually less of an
issue in that cis-hydroalkynylation is observed,1,2 except for
special cases: a notable exception employs biased N-sulfonyl
ynamides, which resulted in net trans-hydroalkynylation.3,4
Even more demanding are related halo-alkynylations.5 The fact
that the C−X bond of the resulting haloenyne product might
react with the catalyst used for its preparation poses an
additional challenge; unsurprisingly, perhaps, the few known
examples uniformly follow a cis-addition mode.6
Outlined below are an efficient trans-hydroalkynylation of
unbiased internal alkynes and the first trans-chloroalkynylation
reactions ever. Since 1,3-enynes in general serve as valuable
building blocks,1,2 the new entry is enabling. This is
particularly true for chloroenynes of type A (X = Cl), as
they comprise adjacent electrophilic and nucleophilic sites
amenable to orthogonal activation. Their dual reactivity can be
harnessed in small-molecule synthesis and material science
alike: the benzannulation strategy leading to polysubstituted
Unsymmetrical substrates usually afford mixtures of
regioisomers (see the SI), but propargyl alcohols of type 10
provide a handle to control the outcome (Table 1):
[Cp*RuCl]4 favors “proximal delivery” to give the α-trans
Received: August 10, 2020
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© XXXX American Chemical Society
A

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Scheme 2. trans-Hydroalkynylation
Scheme 3. trans-Chloroalkynylation of Symmetrical
Alkynes
a
a
b
5 mol% of catalyst. NMR yield.
Table 1. Catalyst-Dependent Regioselectivity
a
b
2.5 mol% catalyst, unless stated otherwise. 5 mol% catalyst.
Figure 1. Structure of compound 21 in the solid state. Thermal
ellipsoids at the 50% probability level.27
a
b
In the presence of nBu4NCl (10 mol%). The remainder is the α-cis
isomer.
The scope is significantly broader than that of the trans-
hydroalkynylation in that good results were obtained in many
cases even for aromatic and/or unsymmetrical substrates
(Scheme 4). This is particularly true for propynylated arenes,
which gave excellent yields and notably high E/Z-ratios,
independent of whether electron-withdrawing or -donating
substituents were placed on the aromatic ring. Likewise,
propynylated pyridine or thiophene reacted well despite the
heteroatom donor sites. Tolane, in contrast, was the only
alkyne investigated so far in which cis-chloroalkynylation was
truly competitive (23, E:Z = 45:55). Collectively, these
examples illustrate the scope and notable functional group
compatibility of the reaction, which matches the experiences
previously made with various other ruthenium-catalyzed trans-
addition processes.20
The trans-chloroalkynylation of 3-hexyne was also carried
out on 12.2 mmol scale with a reduced catalyst loading of 1.25
mol%. While the yield of 13 remained unchanged (92%),28 the
E/Z-ratio was slightly improved (≥95:5 versus 93:7 at 2.5 mol
% [Cp*RuCl]4); this observation is consistent with the
mechanistic insights outlined below. Likewise, chloroenyne
addition product, whereas cationic [Cp*Ru(MeCN)3]SbF6
leads to the regio-complementary outcome, although the
overall selectivity is lower. As previously shown for analogous
trans-hydrometalations, proximal delivery is caused by
interligand hydrogen bonding between the [Ru-Cl] group
and the propargylic −OH substituent.18,19 The selectivity can
be further improved by using the bulkier complex 12 in
combination with nBu4NCl,24 even though the reaction
proceeds more slowly. This result holds the promise that
more systematic ligand tuning will allow for further
optimization.
At this point, however, the search for yet other substrates
amenable to trans-addition was given priority. Gratifyingly,
(chloroethynyl)triisopropylsilane (1b) also reacts well, result-
ing in trans-chloroalkynylation of internal alkyne partners
(Scheme 3);25,26 to the best of our knowledge, this
transformation is unprecedented and the selectivity remarkably
high. The stereochemical outcome was ascertained by NMR
(see the SI). The structure of 21 in the solid state confirmed
the assignment (Figure 1).27
B
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formation of 33 represents just one such possibility.32 The
many other ways of engaging a halide into all sorts of cross-
coupling bring innumerous arene derivatives into reach with
substitution patterns that are difficult to make otherwise.33,34
Equally important is the fact that the concept underlying this
new benzannulation is also applicable to the heterocyclic series,
as illustrated by the formation of chlorobenzothiophene 37.
Further flexibility is gained by the possibility of interchanging
the order of cycloisomerization/cross-coupling, as demon-
strated by the two sequences leading to 35. These enabling
virtues are subject to further study.
Scheme 4. trans-Chloroalkynylation of Unsymmetrical
Alkynes
a
The fact that the “crossed” addition prevails over
homodimerization (oligomerization) of either reaction partner
speaks for a well-orchestrated coordination chemistry,
especially since neither substrate has to be used in large
excess. To gain insights, we first studied the interaction of the
individual components with the catalyst (Scheme 6). Addition
Scheme 6. Reactive Intermediates
a
b
Only the major product isomer is shown (isomer ratio). 5 mol%
c
catalyst. NMR yield.
24 was formed on gram scale; after recrystallization, the
material was almost isomerically pure.
The chloroalkenes thus formed are relevant in that they
bring stereodefined tetrasubstituted alkenes into reach, as
illustrated by the iron-catalyzed formation of the polyfunction-
alized product 30 (Scheme 5).29 The π-acid-catalyzed
cycloisomerization of 31 derived from 25c showcases a very
different application: Catalytic PtCl2 affords the corresponding
naphthalene derivative 32, retaining a chloride substituent for
further manipulation;30,31 its iron-catalyzed borylation with
Scheme 5. Downstream Functionalization
of [Cp*RuCl]4 (0.25 equiv) to 1a in CD2Cl2 at −50 °C leads
to a cherry-red solution containing some unbound 1a and a
single new species. Based on the diagnostic deshielding of the
alkyne C-atoms (135.7/137.5 ppm; compare: 85.9/94.8 ppm
in 1a) and the “olefinic” character of the alkyne proton (δH =
8.64 ppm; compare 2.43 ppm in 1a), this species can be safely
assigned as the corresponding π-complex 38.18,19 Its structure
in the solid state (Figure 2) shows the substantial elongation of
the C1−C2 (1.265(3) Å)35 bond, together with the notable
bending of the alkyne away from linearity (H1−C1−C2
144.5(4)°; C1−C2−Si1 153.0(2)°) as the result of substantial
electron back-donation from the filled metal d-orbitals into the
π*-orbitals of the bound alkyne.19 The silyl group is oriented
toward the chlorine ligand, which is favorable on steric as well
as electronic grounds:36 attractive interligand interactions
between a polarized [Ru-Cl] unit and a silyl substituent have
previously been invoked to explain the outcome of various
mechanistically different transformations.18,19,37 The fact that
only a single molecule of 1a is coordinated to the 14-electron
fragment [Cp*RuCl] is of particular relevance, as it leaves a
vacant site for uptake of the reaction partner as necessary for
C
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benzene was met with success. In the resulting dinuclear
complex 43, one massively elongated alkyne (C2−C3 1.332(5)
Å) and the two chlorine atoms bridge the two Ru centers
(Figure 4).43,44
Figure 2. Structure of 38 in the solid state; thermal ellipsoids at the
50% probability level. The dotted green line indicates an attractive
interligand interaction between the [Ru-Cl] unit and the silyl group.36
crossed addition.38 It is here that the size of the TIPS group is
thought to come into play: slim Me3SiCCH in lieu of 1a is
rapidly consumed by homocyclodimerization39 and is therefore
no suitable substrate for trans-hydroalkynylation. Although 1a
will eventually also homodimerize upon warming, the reaction
is slow enough to leave the desired crossed addition time to
proceed.
Figure 4. Structure of complex 43 in the solid state; thermal ellipsoids
at the 50% probability level. Selected bond lengths (Å) and angles
(deg): C2−C3 1.332(5), C1−C2−C3 147.2(4), C2−C3−C4
142.3(4).
Chloroalkyne 1b shows a similar coordination behavior, as
indicated by the massive downfield shifts of the alkyne C-
atoms (141.1/150.6 ppm; compare: 70.9/79.5 ppm in 1b).
Complex 39 also comprises only one alkyne ligand (Figure 3),
With all individual complexes identified, a 1:1:1 mixture of
[Cp*RuCl]4, chloroalkyne 1b, and 3-hexyne was investigated
with the hope of identifying the heteroleptic bis-alkyne
complex resulting in crossed chloroalkynylation. When mixed
at −50 °C in CD2Cl2, the hexyne-derived complexes 40 and 41
were the major species, whereas the chloroalkyne adduct 39
was minor. Upon gradual warming to room temperature, the
speciation changes in that 40 and 41 disappear and 39 is the
only complex left (product formation commences). Signs of a
mixed complex have not be detected at any point. Re-cooling
of the equilibrated sample to −50 °C does not restore the
original product distribution. Therefore, we conclude that
binding of 3-hexyne is kinetically favored, but the chloroalkyne
complex 39 is thermodynamically more stable.
The finding that an ordinary alkyne can bind two catalyst
fragments simultaneously raised the question as to whether
complex 40 or the [2:1] adduct 41 accounts for product
formation. Variable time normalization analysis45 proved that
the formation of the trans-chloroalkynylation product (E)-13 is
first-order in [Ru] (Figure 5, top), whereas the formation of
the minor cis-isomer shows a second-order dependence (see SI
Figure S28).46 The unexpected finding that the trans- and the
cis-addition follow different rate laws readily explains why the
E/Z-ratio depends on the catalyst concentration (Figure 6). In
this context we reiterate the observation made during scale-up
that lowering of the catalyst loading improved the selectivity to
≥95:5; for comparison, the stoichiometric control experiment
furnished 13 with a poor E/Z-ratio of 64:36.
Furthermore, the consumption of 3-hexyne and the
formation of the trans-addition product 13 show first-order
dependence on the concentration of complex 39. Hence, 39
likely represents the resting state of the catalytic process before
the turnover-limiting step (Figure 5, bottom).
Since a “loaded” complex carrying two different alkynes has
not been observed experimentally, we are currently not in the
position to rigorously exclude an outer-sphere process, in
which only the chloroalkyne is activated by coordination to
ruthenium and is then attacked by 3-hexyne. Although indirect
Figure 3. Structure of 39 in the solid state; thermal ellipsoids at the
50% probability level. The dotted line indicates an attractive
interligand interaction.40 Selected bond lengths (Å) and angles
(deg): C1−C2 1.279(2), C1−Cl2 1.70(1), Cl2−C1−C2 141.4(5),
C1−C2−Si1 152.7(1).
featuring the typical signs of partial rehybridization.19,40 When
a solution of this complex in CD2Cl2 is warmed from −50 °C
to room temperature, slow decomposition with formation of
the corresponding conjugated diyne and paramagnetic
41
[Cp*RuCl2]2 is observed.
In contrast, 3-hexyne as prototypical reaction partner for
1a,b leads to two new signal sets when reacted with
[Cp*RuCl]4 (0.25 equiv) at low temperature (Scheme 6).
While one of them certainly corresponds to the corresponding
monoalkyne complex 40, the second species is a [2:1]-adduct
in which two metal fragments ligate the same triple bond.42
Single crystals of putative 41 could not be grown, but
replacement of 3-hexyne by 1-bromo-4-(prop-1-yn-1-yl)-
D
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c08582.
■
sı
Experimental section including characterization data,
NMR spectra of new compounds, and supporting X-ray
crystallographic data (PDF)
X-ray crystallographic data for 9 (CIF)
X-ray crystallographic data for 21 (CIF)
X-ray crystallographic data for 38 (CIF)
X-ray crystallographic data for 39 (CIF)
X-ray crystallographic data for 43 (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Alois Fu
45470 Mu
0098-3417; Email: fuerstner@kofo.mpg.de
̈
rstner − Max-Planck-Institut fu
̈
r Kohlenforschung,
̈
lheim/Ruhr, Germany; orcid.org/0000-0003-
Authors
Nagaraju Barsu − Max-Planck-Institut fu
45470 Mulheim/Ruhr, Germany
Markus Leutzsch − Max-Planck-Institut fu
45470 Mulheim/Ruhr, Germany; orcid.org/0000-0001-
8171-9399
̈
r Kohlenforschung,
̈
̈
r Kohlenforschung,
̈
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c08582
Figure 5. Variable time normalization analysis of NMR data.
Formation of (E)-13 shows first-order dependence in [Ru] (top) as
well as in the chloroalkyne adduct 39 (bottom); in contrast, formation
of (Z)-13 shows second-order dependence in [Ru] (cf. Figure S28).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Generous financial support by the MPG is gratefully
acknowledged. We thank the analytical departments of our
Institute for excellent support, especially J. Rust and Prof. C.
W. Lehmann for solving the X-ray structures, and J. Lingnau, P.
̀
Philipps, M. Kochius, and Dr. C. Fares for help with the
configurational assignment of numerous samples by NMR.
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(11) Guthertz, A.; Leutzsch, M.; Wolf, L. M.; Gupta, P.; Rummelt, S.
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M.; Goddard, R.; Fares, C.; Thiel, W.; Furstner, A. Half-Sandwich
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Ruthenium Carbene Complexes Link trans-Hydrogenation and gem-
Hydrogenation of Internal Alkynes. J. Am. Chem. Soc. 2018, 140,
3156−3159.
(12) For pioneering studies into trans-hydrosilylation, see: (a) Trost,
B. M.; Ball, Z. T. Markovnikov Alkyne Hydrosilylation Catalyzed by
Ruthenium Complexes. J. Am. Chem. Soc. 2001, 123, 12726−12727.
(b) Trost, B. M.; Ball, Z. T. Alkyne Hydrosilylation Catalyzed by a
Cationic Ruthenium Complex: Efficient and General Trans Addition.
J. Am. Chem. Soc. 2005, 127, 17644−17655. (c) Trost, B. M.; Ball, Z.
T. Addition of Metalloid Hydrides to Alkynes: Hydrometallation with
Boron, Silicon, and Tin. Synthesis 2005, 2005, 853−887.
(24) Biberger, T.; Gordon, C.; Leutzsch, M.; Peil, S.; Guthertz, A.;
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Coperet, C.; Furstner, A. Alkyne gem-Hydrogenation: Formation of
́
Pianostool Ruthenium Carbene Complexes and Analysis of Their
Chemical Character. Angew. Chem., Int. Ed. 2019, 58, 8845−8850.
(25) Other chloroalkynes were unsuitable; see the SI. However,
iPr3SiC≡CX (X = Br, I) react but lead to partial halide scrambling
with incorporation of chloride from the catalyst and/or the solvent.
(26) For a remotely related trans-addition of HCl catalyzed by
(13) Lacombe, F.; Radkowski, K.; Seidel, G.; Furstner, A. (E)-
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Cycloalkenes and (E,E)-Cycloalkadienes by Ring Closing Diyne or
Enyne-yne Metathesis/Semi-Reduction. Tetrahedron 2004, 60, 7315−
7324.
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[Cp*Ru] complexes, see: Derien, S.; Klein, H.; Bruneau, C. Selective
Ruthenium-Catalyzed Hydrochlorination of Alkynes: One-Step Syn-
thesis of Vinylchlorides. Angew. Chem., Int. Ed. 2015, 54, 12112−
12115.
(27) Figure 1 unmistakably shows the trans-arrangement of the
chloride and the alkyne substituent on the tetrasubstituted double
bond. Because the sample used for crystallization was prepared using
(bromoethynyl)triisopropylsilane in 1,2-dichloroethane, partial incor-
poration of bromine and chlorine over the same position is observed,
cf. ref 25; only the chlorine-containing product is shown for clarity
(28) Loadings <1 mol% tend to lead to incomplete conversion.
(14) Matsuda, T.; Kadowaki, S.; Yamaguchi, Y.; Murakami, M.
Ruthenium-Catalyzed trans-Hydrogermylation of Alkynes: Formation
of 2,5-Disubstituted Germoles through Double trans-Hydrogermyla-
tion of 1,3-Diynes. Org. Lett. 2010, 12, 1056−1058.
(15) Sundararaju, B.; Fu
Internal Alkynes. Angew. Chem., Int. Ed. 2013, 52, 14050−14054.
(16) Longobardi, L.; Furstner, A. trans-Hydroboration of Propargyl
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rstner, A. A trans-Selective Hydroboration of
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Alcohol Derivatives and Related Substrates. Chem. - Eur. J. 2019, 25,
10063−10068.
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pubs.acs.org/JACS
Communication
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(29) Fu
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rstner, A.; Leitner, A.; Mendez, M.; Krause, H. Iron-
(45) (a) Bures, J. A Simple Graphical Method to Determine the
Catalyzed Cross-Coupling Reactions. J. Am. Chem. Soc. 2002, 124,
13856−13863.
Order in Catalyst. Angew. Chem., Int. Ed. 2016, 55, 2028−2031.
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(b) Bures, J. Variable Time Normalization Analysis: General
Graphical Elucidation of Reaction Orders from Concentration
Profiles. Angew. Chem., Int. Ed. 2016, 55, 16084−16087.
(46) Deviations were observed at low loading (0.625 mol%), which
is attributed to partial degradation of the catalyst. Compare: Martínez-
Carrion, A.; Howlett, M. G.; Alamillo-Ferrer, C.; Clayton, A. D.;
Bourne, R. A.; Codina, A.; Vidal-Ferran, A.; Adams, R. W.; Bures, J.
Angew. Chem., Int. Ed. 2019, 58, 10189−10193. This observation
inspired the variable time normalization analysis with regard to [39]
where all lines (including the one based on 0.625% loading) overlap
when assuming first-order-dependence; see the SI.
(30) Mamane, V.; Hannen, P.; Furstner, A. Synthesis of
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Phenanthrenes and Polycyclic Heteroarenes by Transition-Metal
Catalyzed Cycloisomerization Reactions. Chem. - Eur. J. 2004, 10,
4556−4575.
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(31) Kang, D.; Kim, J.; Oh, S.; Lee, P. H. Synthesis of Naphthalenes
via Platinum-Catalyzed Hydroarylation of Aryl Enynes. Org. Lett.
2012, 14, 5636−5639.
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(32) Yoshida, T.; Ilies, L.; Nakamura, E. Iron-Catalyzed Borylation
of Aryl Chlorides in the Presence of Potassium t-Butoxide. ACS Catal.
2017, 7, 3199−3203.
(47) The fact that a propargylic −OH group exerts a directing effect
most likely via Ru-Cl···HO− interligand hydrogen bonding speaks for
coordination of both reaction partners to the Ru-center, cf. Table 1;
this steering effect mirrors the results of trans-hydrosilylation
(stannylation), which has been shown by a combined experimental
and computational approach to be an inner-sphere process. Cf. refs 18
and 19.
(33) Toyota, S.; Iwanaga, T. Naphthalenes, Anthracenes, 9H-
Fluorenes, and Other Acenes. Science of Synthesis 2010, 45, 745−854.
(34) For a very different recent entry into 1-chloronaphthalene
derivatives and summary of pertinent literature on their functionaliza-
tion and use, see: Moriguchi, K.; Kono, T.; Seko, S.; Tanabe, Y.
Gram-Scale Robust Synthesis of 1-Chloro-2,3-dimethyl-4-phenyl-
naphthalene: A Promising Scaffold with Three Contiguous Reaction
Positions. Synthesis 2020, DOI: 10.1055/s-0040-1706471.
(35) The corresponding bond length in Me3SiCCH is 1.200 Å,
cf.: Bond, A. D.; Davies, J. E. Trimethylsilylacetylene. Acta Crystallogr.,
Sect. E: Struct. Rep. Online 2002, 58, o777.
(36) The Cl1···Si1 (3.53 Å) distance is well below the sum of the
van der Waals radii of these atoms (4.22 Å), and the C−Si−C angles
slightly deviate from the ideal tetrahedral geometry (106.2, 111.1,
112.9°); moreover, the distances between Cl and the H-atoms
directed toward it are notably short (2.7−3.1 Å; sum of van der Waals
radii: 3.6 Å) and are likely stabilizing, too.
(48) For recent forays into trans-carboboration, see the following
and literature cited therein: Jin, H.; Furstner, A. Modular Synthesis of
̈
Furans with up to Four Different Substituents by a trans-
Carboboration Strategy. Angew. Chem., Int. Ed. 2020, 59, 13618−
13622.
(37) Gutsulyak, D. V.; Churakov, A. V.; Kuzmina, L. G.; Howard, J.
A. K.; Nikonov, G. I. Steric and Electronic Effects in Half-Sandwich
Ruthenium Silane σ-Complexes with Si-H and Si-Cl Interligand
Interactions. Organometallics 2009, 28, 2655−2657.
(38) This notion is supported by a crystallographically and
spectroscopically fully characterized complex, in which a
[Cp*Ru(Cl)(η2-alkyne) fragment ligates a tethered alkene at the
former “open” coordination site”; see ref 22.
(39) See the following reference which corrects earlier literature:
́
Gemel, C.; LaPensee, A.; Mauthner, K.; Mereiter, K.; Schmid, R.;
Kirchner, K. The Substitution Chemistry of RuCp*(tmeda)Cl.
Monatsh. Chem. 1997, 128, 1189−1199.
(40) Again, the short contact between the TIPS group and the [Ru-
Cl] unit in 39 speaks for an attractive interligand interaction: as for
complex 38 (see ref 36), the Cl1···Si1 (3.45 Å) distance as well as the
distances between Cl and the H-atoms directed toward it (3.04−3.13
Å) are notably short; in this case, however, the C−Si−C angles
deviate only slightly from the ideal tetrahedral geometry (105.1,
110.1, 110.5°).
(41) Identified by comparison with an authentic sample; for details,
see the SI.
1
(42) This composition was suggested by the integrals over the H
NMR signals; at ≥0 °C, the monomeric and the dimeric complexes
are in exchange on the NMR time scale.
(43) The closest analogue to 43 is a triruthenium cluster, in which
trimethylsilylacetylene bridges two of the three metal atoms in a
similar manner, serving as a 4e-donor ligand. See: Campion, B. K.;
Heyn, R. H.; Tilley, T. D. Reactions of Alkynes with Coordinatively
Unsaturated (η5-C5Me5)Ru Derivatives. X-ray Crystal Structures of
(η5-C5Me5)Cl2Ru(η2:η4-μ2-C4H4)Ru(η5-C5Me5) and (η5-
C5Me5)3Ru3(μ2-Cl)(μ3-Cl)(η2-μ2-HC≡CSiMe3). Organometallics
1990, 9, 1106−1112.
(44) Another binuclear complex was observed, likely as a byproduct,
in trans-additions to 1,3-diyne substrates; in this case, one of the Cp*
rings was transformed into a fulvene by interligand hydride transfer.
See: Mo, X.; Letort, A.; Rosca, D.-A.; Higashida, K.; Furstner, A. Site-
̈
Selective trans-Hydrostannation of 1,3- and 1,n-Diynes: Application
to the Total Synthesis of Typhonosides E and F, and a Fluorinated
Cerebroside Analogue. Chem. - Eur. J. 2018, 24, 9667−9674.
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