Mechanistic Divergence in the Hydrogenative Synthesis of Furans and Butenolides: Ruthenium Carbenes Formed by gem-Hydrogenation or through Carbophilic Activation of Alkynes

Article abstract of DOI:10.1002/anie.201912161

Enynes with a tethered carbonyl substituent are converted into substituted furan derivatives upon hydrogenation using [Cp*RuCl]₄ as the catalyst. Paradoxically, this transformation can occur along two distinct pathways, each of which proceeds via discrete pianostool ruthenium carbenes. In the first case, hydrogenation and carbene formation are synchronized (“gem-hydrogenation”), whereas the second pathway comprises carbene formation by carbophilic activation of the triple bond, followed by hydrogenative catalyst recycling. Representative carbene intermediates of either route were characterized by X-ray crystallography; the structural data prove that the attack of the carbonyl group on the electrophilic carbene center follows a Bürgi–Dunitz trajectory.

Full text of DOI:10.1002/anie.201912161

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Accepted Article
Title: Mechanistic Divergence in the Hydrogenative Synthesis of
Furans and Butenolides: Ruthenium Carbenes Formed by gem-
Hydrogenation or via Carbophilic Activation of Alkynes
Authors: Alois Fürstner and Sebastian Peil
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912161
Angew. Chem. 10.1002/ange.201912161
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Mechanistic Divergence in the Hydrogenative Synthesis of Furans and Butenolides: Ruthenium
Carbenes Formed by gem-Hydrogenation or via Carbophilic Activation of Alkynes
Sebastian Peil and Alois Fürstner*
Abstract: Enynes with a tethered carbonyl substituent are converted into substituted furan derivatives
upon hydrogenation using [Cp*RuCl]4 as the catalyst. Paradoxically, this transformation can occur
along two distinct pathways, each of which proceeds via discrete pianostool ruthenium carbenes. In
the first case, hydrogenation and carbene formation are synchronized (“gem-hydrogenation”), whereas
the second pathway comprises carbene formation via carbophilic activation of the triple bond, followed
by hydrogenative catalyst recycling. Representative carbene intermediates of either route were
characterized by X-ray crystallography; the structural data prove that the attack of the carbonyl group
onto the electrophilic carbene center follows a Bürgi-Dunitz trajectory.
After a century of intense research by the scientific community on catalytic hydrogenation, our group
has recently been able to identify an entirely new reactivity mode. Specifically, it was shown that
alkynes can undergo gem-hydrogenation, a reaction in which both H-atoms of H2 are delivered to one
and the same acetylenic C-atom while the adjacent position is concomitantly transformed into a
discrete metal carbene.1,2 At the current stage of development, [Cp*RuCl]4 is the precatalyst of choice;
moreover, heteroatom substituents in vicinity of the triple bond are often necessary to render the
reaction efficient. Detailed spectroscopic and computational data indicate that the resulting pianostool
ruthenium complexes basically exhibit a Fischer-carbene character with a certain overtone reminiscent
of Grubbs-type catalysts.3 This view is corroborated by the fact that they participate in either
intramolecular cyclopropanation or metathesis reactions, depending on the chosen substrate.4
*
S. Peil, 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) fort he author(s) of this article can be found
under ###########
1
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Scheme 1. Conceived furan formation by alkyne gem-hydrogenation, emulating known diazo-
decomposition chemistry; Cp = cyclopentadienyl; Cp* = pentamethylcyclopentadienyl
For their largely electrophilic nature, such complexes should be able to participate in various other
catalytic transformations too. If this is the case, gem-hydrogenation might eventually evolve into an
attractive alternative to diazoalkane decomposition, which is arguably the most common gateway to
highly reactive late-transition metal carbenes.5,6 The foray along these lines outlined below was
inspired by a recent publication describing an innovative entry into highly substituted furan derivatives
(Scheme 1).7 Specifically, diazo compounds A were shown to react with catalytic [CpRu(MeCN)3]PF6 to
generate transient cationic ruthenium carbenes B, which get trapped by the tethered ester group to
give the corresponding heterocycle C. We reasoned that this type of transformation might be emulated
by gem-hydrogenation of enyne D, in which the propargylic –OR substituent directs carbene formation
to the distal acetylenic site.1,2 Akin to B, the resulting intermediate E might furnish furan F, even though
E is a neutral rather than cationic entity.
In line with our expectations, product 2 was formed in almost quantitative yield ( 95%) upon stirring
of a solution of 1 and [Cp*RuCl]4 (2 mol%) in CH2Cl2 under an atmosphere of H2 (1 bar) for 3 h at
ambient temperature (Scheme 2). Treatment of the crude material with silica released the
corresponding butenolide 3.8 Alternatively, 2 can be alkylated with allyl iodide to give product 4.9 In
other cases such as 5-7, the furan itself was sufficiently stable for isolation. In line with our previous
investigations,2-4 different propargylic substituents (for example, OR, OSiR3, OMOM) were found
to instigate gem-hydrogenation. Gratifyingly though, even substrates devoid of such directing groups
led to the formation of butenolides 10 and 11;10 in these cases, the ester itself might serve as a directing
group, fostering regioselective carbene formation by gem-hydrogenation.2a This effect is not always
sufficient, however, as illustrated by the formation of alkene 12 via trans-hydrogenation of the
2
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corresponding alkyne substrate.11 This outcome is not overly surprising since trans-hydrogenation has
previously been shown to be a facile process downstream of an initial gem-hydrogenation event (even
though 12 could very well originate from a concerted pathway).1,2,12
Scheme 2. a) [Cp*RuCl]4 (2 mol%), H2 (1 atm), CH2Cl2, RT; b) SiO2; c) allyl iodide, NaI, THF, 100°C; Boc =
tert-butoxycarbonyl; MOM = methoxymethyl; TBS = tert-butyldimethylsilyl
Although these results are fully consistent with the formation of pianostool ruthenium carbenes by
gem-hydrogenation in the first place, we sought to confirm this mechanistic interpretation. To this
end, we resorted to substrate 13, which is expected to undergo regular gem-hydrogenation but should
be resilient to cyclization because heterocycle formation comes at the prize of dearomatization of the
phenyl ring (Scheme 3). Indeed, the reaction stopped at the stage of the pianostool ruthenium carbene
complex 14. The use of para-hydrogen as the reagent leads to a massive amplification of the 1H NMR
signal of the methylene group flanking the carbene center as a result of the PHIP effect (PHIP = p-
hydrogen induced polarization).13,14 This spectroscopic signature provides unambiguous proof that
these two H-atoms originate from the same H2 molecule by pairwise delivery, a characteristic trait of
gem-hydrogenation.2
3
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Scheme 3. Interrupted process: gem-Hydrogenation without subsequent heterocycle formation
Figure 1. Structure of carbene complex 14 in the solid state; the dotted line indicates that the ester
carbonyl oxygen atom O1 adopts a Bürgi-Dunitz trajectory in its approach to the electrophilic carbene
site C1; the O1C1 distance is well below the sum of the van-der-Waals radii
The structure of this sensitive complex in the solid state is highly informative (Figure 1). It suggests that
the directing effect of the methyl ether substituent emanates from its interaction with the Lewis-acidic
Ru center at a distance of 2.18 Å; the actual C1Ru1 carbene bond length (1.883(2) Å) is in the expected
range.2 The arguably most remarkable structural feature, however, is the orientation of the carbonyl
oxygen atom O1 relative to the carbene center C1: the O1C1 distance (2.983(4) Å) is well below the
sum of the van-der-Waals radii of these atoms ( 3.22Å) and the Ru1C1O1 angle of 116.0° shows
that the ester carbonyl approaches the trigonal carbene center along a Bürgi-Dunitz trajectory.15,16 In
view of this ideal geometric predisposition for an outer-sphere attack, it is reasonable to believe that
ring closure stalls because of the unfavorable thermodynamic rendering of this model compound, since
4
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loss of aromaticity of the benzene ring would not be compensated by the enthalpic gain of an emerging
isobenzofuran. The Bürgi-Dunitz angle is well recognized as a fundamental principle of (dynamic)
stereochemistry originating from the particular shape and occupancy of the relevant frontier orbitals.17
Yet, complex 14 seems to be the first case of an electrophilic transition metal carbene and its
nucleophilic reaction partner, for which this prominent effect has been explicitly recognized as a
structure-determining element manifest in the crystallographic data.18
Scheme 4. Extension to the ketone series is accompanied by a switch in mechanism: a) [Cp*RuCl]4 (5
mol%), H2 (1 atm), 1,2-dichloroethane, 70°C, 57% (16), 5% (17), 10% (18)
Next, we attempted to extend the novel hydrogenative furan formation to substrates carrying
nucleophiles other than an ester group. The readily available diketone derivative 15 gave the expected
product 16 but required a reaction temperature of 70°C (Scheme 4). Surprisingly, small amounts of
ketone 18 were also isolated in one run from the crude mixture, which seems to indicate incidential
oxidation of an intermediate of type G featuring the carbene site proximal to the directing –OMe
substituent. Such a regiochemical course violates the mechanistic rationale outlined above for the
ester series and is inconsistent with all other available information on alkyne gem-hydrogenation,1-4
not least with the X-ray structure shown in Figure 1. Formation of G might be explained, however, by
assuming that the -acidic metal fragment activates the substrate to the extent that attack of the
ketone onto the triple bond is faster than binding of H2, as necessary for gem-hydrogenation to occur.
5
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This type of mechanism has ample precedent in the literature for many different carbophilic
catalysts,19 although rigorous proof for the intervention of discrete carbenes is exceedingly rare.20
Figure 2. Structure of the furyl carbene 20 in the solid state; selected bond lengths (Å): Ru1C1
1.890(3), C2C3 1.399(4), C4C5 1.363(4), Ru1C2 2.217(2), Ru1C3 2.330(2)
To test this hypothesis, enyne 15 was reacted with stoichiometric amounts of [Cp*RuCl]4 in the
presence as well as in the absence of H2: either set-up afforded the very same pianostool ruthenium
carbene 20 in less than 30 min reaction time. The structure of this remarkable complex in the solid
state shows that the ruthenium atom does no longer interact with the adjacent OMe group (Figure
2); rather, it is tightly ligated to the electron rich “enol ether” site of the newly formed furan ring as
manifest in the observed distances as well as in an elongated C2-C3 bond. The distinctive up-field shift
of the NMR signals of C2 (C = 132.3 ppm), C3 (C = 89.7 ppm) and the carbene center C1 (C = 266.7
ppm) suggest that this bonding situation persists in CD2Cl2 solution.21 Despite the stabilizing
interaction, 20 is a competent intermediate en route to product and by-product alike. It reacts with H2
at 70°C to give furan 16,22 whereas stirring of a solution in air afforded the furyl ketone 18 (Scheme 4).
Since -bond activation by the carbophilic catalyst does not require assistance by a neighboring
directing group,23 substrates of type 21 (R = alkyl, aryl) devoid of donor substituents at the propargylic
position should react analogously (Scheme 5). As the formation of furans 23a,b and 24 shows, this is
indeed the case as long as the substituent R shields the transient carbene center in 22. If not,
6
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dimerization with formation of a tetrasubstituted alkene 26 becomes competitive. Thus, substrate 21c
(R = Ph) gave a mixture of the monomeric furan 23c and olefin 26 (R = Ph, E/Z  1:1),24 whereas 21d (R
= n-Bu) afforded only traces of the monomeric furan derivative but furnished the product 26 (R = n-
Bu) as the major product in the form of a single isomer. The E-configuration of the central double bond
could only be assigned by crystallographic means.21,25 It is assumed that the triple bond of a second
substrate inserts into the [Ru=C] bond of 22 in an enyne metathesis fashion to furnish a vinylcarbene
25,26 which is then interrcepted by the adjacent ketone to close the second furan ring of the resulting
dimer 26.
Scheme 5. The fate of the pianostool ruthenium carbene formed by carbophilic activation
Insufficient steric shielding of the carbene center can open yet another sidetrack. The NMR data of
carbene 22 (R = tBu, C = 285.3 ppm) correspond to those of 20 and its structure must hence also be
similar,22 but the ruthenium complex derived from enyne 21 (R = Ph) and [Cp*RuCl]4 is strikingly
different. Whereas 20 and 22 (R = tBu) are both rather sensitive burgundy-red compounds, a very
robust green solid material is formed that does not react with H2 even at 70°C, is stable towards air
and moisture for extended periods of time, and does not catalyze furan formation either; it is hence
7
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an off-cycle product. The structure of this complex was determined by X-ray diffraction. As shown in
Figure 3, 27 incorporates a bridging rather than terminal carbene moiety; the dimer persists in solution
as indicated by the markedly upfield-shifted carbene resonance (C = 189.9 ppm).27,28 The distances
between C1 and the two Ru atoms are uneven, as are the distances between the bridging chloride Cl2
and the metal atoms. Ru2 benefits from the same stabilizing interaction with the proximal enol site of
the furan ring discussed above for 20, whereas Ru1 carries an additional terminal chloride ligand.
Figure 3. Top: Structure of complex 27 in the solid state; selected bond lengths (Å): Ru1Ru1 2.822(3),
Ru1C1 2.099(1), Ru2C1 2.118(1), Ru1Cl1 2.448(3), Ru1Cl2 2.456(3), Ru2Cl2 2.393(4), Ru2C2
2.208(1), Ru2C3 2.284(1), C2C3 1.416(2), C3C4 1.452(2), C4C5 1.365(2). Bottom: partial space-
filling representation (except for the furyl ring in the back), showing the immersion of the phenyl group
between the lateral Cp* rings
8
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The static picture of the structure in the solid state, however, provides an incomplete description of
this complex. Rather, the 13C NMR signals of the two Cp* rings show massive line broadening at
ambient temperature; a reversible dyotropic process 27/27’ is the likely cause for the equilibration of
the two different metal subunits,29 a dynamic behavior that can be frozen out by lowering the
temperature to 50°C. At the same time, the rotation of the phenyl substituent comes to a halt.
Inspection of the space-filling model of 27 shows that this phenyl ring is “sandwiched” between the
lateral Cp* rings. Actually, it is plausible that the complex draws some of its remarkable stability from
this peripheral interaction, even though this aspect needs further scrutiny. In any case, only slim
substituents will be able to intercalate analogously. Only two related complexes are known in the
literature: indeed, they carry H/alkenyl and Me/Me substituents at a bridging ruthenium carbene site
between the Cp* ligands.4,30 Sterically demanding branched alkyl residues in lieu of the phenyl group
are unlikely to fit into the groove formed by the Cp* rings and will hence prevent bridging carbene
formation from occurring; the structure of the complex 20 discussed above bears witness of this
notion.
Scheme 6. Conceptual framework featuring two unorthodox roles that hydrogenation can gain in
metal carbene chemistry
The subtlety of the transformations described above is deemed remarkable: hydrogenation of enynes
carrying a tethered carbonyl group with the help of [Cp*RuCl]4 as the catalyst invariably affords highly
substituted furan products (Scheme 6). Depending on the nucleophilicity of the carbonyl substituent,
however, the reaction takes place along two distinctly different pathways, each of which involves
9
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pianostool ruthenium carbenes as the key intermediates. These reactive species evolve, however, on
the opposite ends of the substrate’s central alkyne subunit. In the first case, the -complex initially
formed binds H2, which in turn triggers the unorthodox gem-hydrogenation of the triple bond with
concomitant formation of a ruthenium carbene;1-4 hydrogenation and carbene formation are hence
synchronized. Attack of the tethered ester carbonyl onto the electrophilic carbene center delivers the
heterocyclic product and, at the same time, regenerates the catalyst.
Alternatively, direct attack of a ketone onto the ligated alkyne outperforms gem-hydrogenation.31 The
ensuing five-exo-dig cyclization places the emerging ruthenium carbene away from the incoming
nucleophile. In this -acid catalysis scenario, carbene formation precedes the actual hydrogenation
reaction, which is necessary to regenerate the catalyst and hence ensures turn-over. Insertions of
either free carbenes or metal carbene complexes into H2 are known;32,33 to the best of our knowledge,
however, the furan synthesis described herein is the first example in which carbene hydrogenation is
essential for the release of the desired product and catalyst recovery alike; it therefore keeps the actual
catalytic carbene formation up and running.34 From the conceptual viewpoint, the chemistry described
herein outlines new strategic roles for catalytic hydrogenation chemistry. Ongoing studies in our
laboratory try to leverage some of the opportunities that this unconventional reactivity paradigm may
provide.
Acknowledgements
Generous financial support by the MPG is gratefully acknowledged. We thank Dr. M. Leutzsch for the
PHIP-NMR experiments, Mr. J. Rust, Mr. J. Nöthling and Dr. R. Goddard for solving the X-ray structures,
and all service groups of our Institute for excellent support.
10
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For the Table of Contents
One way or the other: Enynes bearing a tethered carbonyl group are transformed into furans upon
catalytic hydrogenation using a ruthenium catalyst. Strikingly though, the reaction can follow two
distinct pathways, each of which involves discrete metal carbene intermediates. They differ in that the
hydrogenation and carbene formation are either fully synchronized (“gem-hydrogenation) or fully
uncoupled.
Keywords: Bürgi-Dunitz angle  Carbophilic Activation  Furans  Hydrogenation  Metal Carbenes 
Ruthenium
11
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1
A. Fürstner, J. Am. Chem. Soc. 2019, 141, 11-24.
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a) A. Guthertz, M. Leutzsch, L. M. Wolf, P. Gupta, S. M. Rummelt, R. Goddard, C. Farès, W. Thiel,
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Chem. 2019, 131, 8943-8948.
Modern Catalytic Methods for Organic Synthesis with Diazo Compounds: From Cyclopropanes to
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However, anaerobic conditions are needed, otherwise partial oxidation is observed.
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7
8
9
10
11
For the structure of compounds 9 and 11 in the solid state, see the Supporting Information.
a) K. Radkowski, B. Sundararaju, A. Fürstner, Angew. Chem. Int. Ed. 2013, 52, 355-360; Angew.
Chem. 2013, 125, 373-378; b) M. Fuchs, A. Fürstner, Angew. Chem. 2015, 127, 4050-4054;
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12
The concerted pathway is similar to the mechanism of trans-hydrosilylation (stannylation), see:
a) L. W. Chung, Y.-D. Wu, B. M. Trost, Z. T. Ball, J. Am. Chem. Soc. 2003, 125, 11578-11582; b) S.
Ding, L.-J. Song, L. W. Chung, X. Zhang, J. Sun, Y.-D. Wu, J. Am. Chem. Soc. 2013, 135, 13835-
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13842; c) D.-A. Roşca, K. Radkowski, L. M. Wolf, M. Wagh, R. Goddard, W. Thiel, A. Fürstner, J.
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13
14
C. R. Bowers, D. P. Weitekamp, J. Am. Chem. Soc. 1987, 109, 5541-5542.
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15
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H. B. Bürgi, J. D. Dunitz, J. M. Lehn, G. Wipff, Tetrahedron 1974, 30, 1563-1572.
The array in 19 is very similar to that of a substituted benzophenone derivative in which one of
the phenyl rings carries a methyl ester at the ortho-position, see: A. Konosonoks, P. J. Wright,
M.-L. Tsao, J. Pika, K. Novak, S. M. Mandel, J. A. Krause Bauer, C. Bohne, A. D. Gudmundsdottir,
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a) Structure Correlation, (H. B. Bürgi, J. D. Dunitz, Eds.), Wiley-VCH, Weinheim 1994; b) I. Fleming,
Molecular Orbitals and Organic Chemical Reactions, Wiley, New York, 2009.
We found two hits in the Cambridge Crystallographic Database in which a tethered P=O or C=O
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in neither case has this aspect been discussed, see: a) C. Ernst, O. Walter, E. Dinjus, J. Organomet.
Chem. 2001, 627, 249-254; b) B. Weberndörfer, G. Henig, H. Werner, Organometallics 2000, 19,
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19
For leading references, see the following and literature cited therein: a) B. Seiller, C. Bruneau, P.
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20
For Fischer-type (2-furyl) chromiumcarbonyl carbene complexes, see: a) K. Miki, T. Yokoi, F.
Nishino, K. Ohe, S. Uemura, J. Organomet. Chem. 2002, 645, 228-234; for gold and copper
carbenes with an N,N-disubstititued imidazolone ring in lieu of the furan, see: b) J. Wang, X. Cao,
S. Lv, C. Zhang, S. Xu, M. Shi, J. Zhang, Nature Commun. 2017, 8, 14625.
21
22
For further details, see the Supporting Information.
Carbene 22 (R = tBu) was also tested; it reacts more cleanly with H2 to give the furan derivative
23a in 86% yield. In a competition experiment, this reaction showed a kinetic isotope effect of
1.54, which suggests that oxidative insertion of the metal center into the HH bond is product-
determining. Only minute amounts of the cross-over product were observed ( 0.8%) when the
reaction was carried out under a H2/D2 (1:1) atmosphere, thus proving that hydride scrambling
before or during the reaction is marginal; for details, see the Supporting Information.
a) A. Fürstner, P. W. Davies, Angew. Chem. Int. Ed. 2007, 46, 3410-3449; Angew. Chem. 2007,
119, 3478-3519; b) A. Fürstner, Angew. Chem. Int. Ed. 2018, 57, 4215-4233; Angew. Chem. 2018,
130, 4289-4308.
23
24
25
As expected, the ratio 23c:26 is strongly concentration-dependent; for details, see the
Supporting Information.
A closely related compound with R = hexyl has previously been misassigned, see: a) Y. Zhou, J.
Ma, K. Chen, H. Jiang, S. Zhu, Chem. Commun. 2016, 52, 13345-13348. For related dimerization
reactions with formation of olefinic products, see ref. 19h and the following: b) J. González, J.
González, C. Pérez-Calleja, L. A. López, R. Vicente, Angew. Chem. Int. Ed. 2013, 52, 5853-5857; c)
P. Liu, J. Sun, Org. Lett. 2017, 19, 3482-3485.
26
The proposed insertion mirrors the behavior of Grubbs carbenes in enyne metathesis, cf: a) R.
H. Grubbs, D. J. O’Leary (Eds.), Handbook of Metathesis, 2nd Ed., Wiley-VCH, Weinheim, 2015; b)
S. T. Diver, Coord. Chem. Rev. 2007, 251, 671-701; for a pertinent example from this laboratory
see: c) A. Fürstner, P. W. Davies, C. W. Lehmann, Organometallics 2005, 24, 4065-4071.
R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, 4th ed., Wiley, Hoboken,
2005.
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For representative and fully characterized examples, see the following and literature cited
therein: a) X. Dai, T. H. Warren, J. Am. Chem. Soc. 2004, 126, 10085-10094; b) P. Hofmann, I. V.
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10.1002/anie.201912161
Angewandte Chemie International Edition
Shishkov, F. Rominger, Inorg. Chem. 2008, 47, 11755-11762; c) G. Seidel, B. Gabor, R. Goddard,
B. Heggen, W. Thiel, A. Fürstner, Angew. Chem. Int. Ed. 2014, 53, 879-882; Angew. Chem. 2014,
126, 898-901; d) A. G. Tskhovrebov, R. Goddard, A. Fürstner, Angew. Chem. Int. Ed. 2018, 57,
8089-8094; Angew. Chem. 2018, 130, 8221-8226.
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a) M. T. Reetz, Angew. Chem. Int. Ed. 1972, 11, 130-131; b) I. Fernández, F. P. Cossio, M. A. Sierra,
Chem. Rev. 2009, 109, 6687-6711.
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M. R. Gagné, R. H. Grubbs, J. Feldman, J. W. Ziller, Organometallics 1992, 11, 3933-3935.
Ref. 19c,d also found that ketones are better nucleophiles than esters towards alkynes activated
by carbophilic catalysts.
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33
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For reactions of free carbenes with H2, see the following and literature cited therein: a) P. S.
Zuev, R. S. Sheridan, J. Am. Chem. Soc. 2001, 123, 12434-12435; b) G. D. Frey, V. Lavallo, B.
Donnadieu, W. W. Schoeller, G. Bertrand, Science 2007, 316, 439-441; c) S. Henkel, W. Sander,
Angew. Chem. 2015, 127, 4686-4690; Angew. Chem. Int. Ed. 2015, 54, 4603-4607.
For reactions of Grubbs carbenes with H2 (or silanes), see: a) J. Louie, C. W. Bielawski, R. H.
Grubbs, J. Am. Chem. Soc. 2001, 123, 11312-11313; b) S. D. Drouin, G. P. A. Yap, D. E. Fogg, Inorg.
Chem. 2000, 39, 5412-5414; for a review, see: c) B. Alcaide, P. Almendros, A. Luna, Chem. Rev.
2009, 109, 3817-3858.
The arguably closest example is the hydrogenative metathesis recently described by our group
(ref. 4), in which the hydrogenation of the ruthenium carbene is critically important for catalyst
recycling but has nothing to do with product release.
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