Formation of Ruthenium Carbenes by gem-Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans-Hydrogenation

Article abstract of DOI:10.1002/anie.201506075

Insights into the mechanism of the unusual trans-hydrogenation of internal alkynes catalyzed by {CpRu} complexes were gained by para-hydrogen (p-H₂) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans-r

Full text of DOI:10.1002/anie.201506075

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506075
German Edition: DOI: 10.1002/ange.201506075
Ruthenium Carbenes Very Important Paper
Formation of Ruthenium Carbenes by gem-Hydrogen Transfer to
Internal Alkynes: Implications for Alkyne trans-Hydrogenation
Markus Leutzsch, Larry M. Wolf, Puneet Gupta, Michael Fuchs, Walter Thiel, Christophe Far›s,
and Alois Fürstner*
Abstract: Insights into the mechanism of the unusual trans-
hydrogenation of internal alkynes catalyzed by {Cp*Ru}
complexes were gained by para-hydrogen (p-H2) induced
polarization (PHIP) transfer NMR spectroscopy. It was found
that the productive trans-reduction competes with a pathway in
which both H atoms of H2 are delivered to a single alkyne
C atom of the substrate while the second alkyne C atom is
converted into a metal carbene. This “geminal hydrogenation”
mode seems unprecedented; it was independently confirmed by
the isolation and structural characterization of a ruthenium
carbene complex stabilized by secondary inter-ligand inter-
actions. A detailed DFT study shows that the trans alkene and
the carbene complex originate from a common metallacyclo-
propene intermediate. Furthermore, the computational analy-
sis and the PHIP NMR data concur in that the metal carbene is
the major gateway to olefin isomerization and over-reduction,
which frequently interfere with regular alkyne trans-hydro-
genation.
catalyzed additions to p-bonds proceed through suprafacial
[11]
À
À
delivery of H2, H BR2, or H ER3 (E = Si, Ge, Sn). The
unorthodox stereochemical course may arise from the
intervention of ruthenacyclopropenes (h2-vinyl complexes)[12]
as suggested by an in-depth computational study for the
hydrosilylation manifold.[13] Although it seems reasonable to
assume that the other transformations mentioned above
follow similar pathways,[14] secured information is largely
missing, and alternative mechanisms cannot be ruled out.
Specifically, scenarios involving more than one metal center
have been proposed in early studies.[15,16]
Catalytic trans-hydrogenation constitutes a favorable
alternative to dissolving metal reductions of alkynes[17] and
as such holds the promise of being highly enabling. Yet, its full
potential can only be harnessed if side reactions, such as olefin
isomerization and over-reduction, are suppressed.[8,9] To this
end, a better understanding of the entire reaction manifold—
including the competing pathways—will be necessary. There-
fore, we embarked on a mechanistic study that commenced
with NMR investigations using para-hydrogen (p-H2) induced
polarization (PHIP) transfer. This hyperpolarization method
has the potential of selectively enhancing the signals origi-
nating from the reacting H2 by up to four orders of magnitude
over the conventional Boltzmann-governed NMR polariza-
tion.[18] This massive effect allows the fate of the hydrogen
nuclei to be surveyed and (fleeting)[19] intermediates to be
detected, characterized, and tracked—provided that the
H atoms are transferred in a pairwise fashion; moreover,
they have to be magnetically inequivalent and mutually
coupled.[20,21] In the present study, the spectra were recorded
at 11.7 T (500 MHz (1H)) using zero-field (ALTADENA)[18b]
and high-field (PASADENA)[18c] experiments with a double-
quantum OPSY filter introduced in all pulse sequences.[22]
As a first foray, we showed that the trans-alkenes formed
upon semi-hydrogenation of a set of representative alkyne
substrates with enriched p-H2 (ca. 5 bar) in the presence of
1 or 2 (ca. 5 mol%) in a high-pressure NMR tube invariably
A
lthough it has been known for some time that the cationic
complex [Cp*Ru(MeCN)3]PF6 (1; Cp* = pentamethylcyclo-
pentadienyl) catalyzes the stereochemically uncommon trans-
hydrosilylation of internal alkynes,[1–3] it was recognized only
recently that the scope of the underlying reactivity mode is
actually much larger. Specifically, this and related catalysts
are also able to effect highly selective trans-hydroboration,[4]
trans-hydrogermylation,[3,5] trans-hydrostannation,[3,6,7] and
even trans-hydrogenation reactions.[8,9] In many cases, the
use of neutral precatalysts, such as [Cp*RuCl(cod)] (2; cod =
cycloocta-1,5-diene) or [{Cp*RuCl}4] (3), in lieu of the
cationic species 1 allows significantly better regioselectivities
to be imposed on such trans-additions when working with
non-symmetric alkyne substrates.[3]
Whereas the preparative significance of these transfor-
mations stems from their stereochemical complementarity to
the existing arsenal,[10] little is known about their mechanism.
They formally violate the reigning paradigm that metal-
1
exhibit PHIP-enhanced olefinic H signals; a representative
example is shown in Figure 1. This result confirms that both
H atoms of a single H2 molecule are transferred pairwise to
the substrate and thus corroborates a conclusion reached for
a different ruthenium source in an earlier spectroscopic
study.[15] In stark contrast to this previous report, however,
which had failed to detect any intermediates, additional
PHIP-enhanced signals were observed in the aliphatic region.
In all cases investigated, these resonances showed large
negative scalar coupling constants in the range of À15 to
À17 Hz, suggesting the presence of geminally coupled
diastereotopic methylene protons; this assignment was con-
[*] M. Sc. M. Leutzsch, Dr. L. M. Wolf, P. Gupta, Dr. M. Fuchs,
Prof. W. Thiel, Dr. C. Far›s, Prof. A. Fürstner
Max-Planck-Institut für Kohlenforschung
45470 Mülheim/Ruhr (Germany)
E-mail: fuerstner@kofo.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506075.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
Angew. Chem. Int. Ed. 2015, 54, 12431 –12436
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12431

.
Angewandte
Communications
Figure 2. Aliphatic region of the 1H-OPSY NMR spectrum showing the
gem-coupled protons that must have been delivered pairwise during
the hydrogenation of 8a (R=H) with p-H2 in the presence of catalytic
amounts of complex 2; characteristic 13C NMR data of the resulting
carbenes 9a (R=H) and 9b (R=Me).
Scheme 1. Formalism underlying the gem-hydrogenation of an alkyne.
a species with 1,2-dicarbene character: One of the vicinal
carbene sites gets trapped by the ruthenium fragment,
À
whereas the other one oxidatively inserts into the H H
bond of the reagent (Scheme 1). The resulting geminal
delivery of H2 to a single carbon center is highly unorthodox
in the realm of organic chemistry,[24] even though it mimics
nothing but the prototype reactivity mode of transition metals
vis-à-vis hydrogen gas.[25]
The long lifetimes of the intermediates observed by NMR
spectroscopy suggested that even their isolation in pure form
might be possible. Our attempts were met with success for the
slightly modified substrate 8b (R = Me) bearing one OH and
one OMe substituent. The structure of the derived carbene 9b
(dC = 340.9 ppm) in the solid state confirms the conclusions
drawn from the NMR data (Figure 3). The carbene nature is
evident from the almost perfectly planar coordination geom-
Figure 1. Top: 1H-OPSY NMR spectrum taken during the catalytic
hydrogenation of 4a (R=H) with p-H2. Bottom: aliphatic region of
a H-OPSY-COSY spectrum, confirming the spin system of the carbene
isomers 6a/7a causing the PHIP-enhanced signals. For the assign-
ment of all visible peaks, see the Supporting Information.
1
1
firmed by H-OPSY-COSY spectra. Interestingly, alkyne 4a
(R = H) led to two sets of such signals (in addition to the
signals of the E-alkene product 5a; Figure 1), the different
multiplicities of which are consistent with the presence of two
regioisomers 6a and 7a.[23] As a flanking OR group (R = H,
Me) seemed to increase the lifetime of such intermediates, we
went on to study sterically encumbered substrates of type 8
with two potentially stabilizing substituents (Figure 2). As
expected, these alkynes gave rise to a single intermediate
each, which was stable enough for full characterization by
conventional NMR spectroscopy. The recorded data left no
doubt that the resulting complexes 9 are ruthenium carbenes
that must have been formed by geminal hydrogenation of the
triple bond, as the PHIP NMR data confirmed that both
transferred H atoms definitely arose from the very same H2
molecule.
À
etry about the C1 center and the short Ru1 C1 bond
(1.883(2) Š), which is only slightly longer than the
=
Ru CHPh bond in prototype Grubbs carbenes (1.79–
1.85 Š).[26,27] Secondary interactions confine the carbene site
of 9b within a cyclic array, which precludes an optimal orbital
À
overlap with either vicinal C H bond and hence renders
a conceivable 1,2-H shift unfavorable. The stabilizing periph-
eral contacts consist of a donor–acceptor bond between the
ether oxygen atom O2 and the Ru center (2.230 Š) and
a hydrogen bond between the OH and the chloride ligand
(3.19 Š).[28] It is interesting to note that the carbene site in 9b
resides next to the alcohol function and distal to the ether;
this observation is in line with our previous conclusion that
inter-ligand hydrogen bonding exerts a massive directing
In a formal sense, a carbene such as 9 can be thought of as
being derived from an alkyne substrate that has reacted as
12432 www.angewandte.org
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 12431 –12436

Angewandte
Chemie
product 10 as well as to alkane 11, which is formed by over-
reduction.[31]
To better understand the role and fate of the carbene
intermediates, a detailed computational study was carried out
at the M06/def2-TZVP/SMD(CH2Cl2)//M06/def2-TZVP level
of theory (for computational details, see the Supporting
Information). Complex 9b served as a calibration point, the
molecular structure and reactivity of which were well
reproduced.[32] After this validation, a more detailed mech-
anistic analysis was carried out for the general 2-butyne
substrate (Scheme 2). Starting from 2 (= A0), a heteroleptic
complex A1 is formed upon substrate binding and coordina-
tion of H2 through its s-bond. This notion is in excellent
accord with our previous experimental finding that the s-
hydrogen complex [Cp*Ru(H2)(cod)]OTf (OTf = trifluoro-
methanesulfonate) is indeed a competent trans-selective
Figure 3. Structure of the ruthenium carbene 9b (R=Me) in the solid
state.
hydrogenation catalyst.[8] Subsequent activation of the H H
À
bond in A1 affords a short-lived dihydride species A2, which
effect on trans-additions to non-symmetric alkynes as long as
transfers one of the H atoms to the alkyne when passing over
[3]
catalysts comprising polarized Ru Cl bonds are used.
the
low-lying
transition
state
TSA2–A3
(DG° =
À
The remarkable stabilities of 6 and 9 raise the question as
to whether these species play any role in the trans-hydro-
genation or whether they are merely thermodynamic sinks off
the catalytic cycle. The latter possibility was ruled out by
additional PHIP NMR experiments with the metastable
+ 2.1 kcalmolÀ1).[33]
At the stage of the resulting h1-vinyl complex A3, the
reaction pathway bifurcates for a first time: An almost
À
barrier-free rotation around the Ca Cb axis (TSA3–E1) leads to
ruthenacyclopropene E1, which evolves, via the stereochem-
istry-determining TSE1–E2, into the final E-2-butene (E2);
product formation is strongly exergonic (by 33.6 kcalmolÀ1)
1
carbene 7b (Figure 4). Specifically, cross peaks in a 2D H-
OPSY-EXSY experiment[29] definitely linked this hyperpolar-
ized species to E-alkene 5b;[30] importantly though, the same
spectrum also unmistakably connected 7b to the isomerized
À
and therefore likely irreversible. Rotation around the Ca Cb
axis in A3 in the opposite direction opens the channel leading
to the undesired Z-2-butene
(Z3). This rotation, however,
is not barrier-free (DG° =+
3.4 kcalmolÀ1). Therefore, we
can safely deduce that very
little of the Z-alkene will
form, which is in accord with
the generally excellent E/Z
ratios observed in Ru-cata-
lyzed trans-hydrogenations of
unstrained
alkyne
sub-
strates.[8,9]
Of particular relevance is
the finding that the ruthenacy-
clopropene E1 cannot only
evolve into the desired product
E2 by the pathway outlined
above, but can also be con-
verted into a true carbene C2
upon rotation about the
À
Ru Ca bond. The decisive
transition state TSE1–C1 is only
1.2 kcalmolÀ1 higher in energy
than TSE1–E2, which leads to E2.
Carbene formation is exer-
gonic by 14.6 kcalmolÀ1; con-
sidering that a flanking OR
substituent may provide an
estimated additional stabiliza-
tion in the order of 7 kcal
Figure 4. Strips of the 2D 1H-OPSY-EXSY spectrum (CD2Cl2, 298 K) showing characteristic cross peaks that
link the carbene 7b with products 5b, 10, and 11.
Angew. Chem. Int. Ed. 2015, 54, 12431 –12436
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 12433

.
Angewandte
Communications
Scheme 2. Free energy profile for the hydrogenation of 2-butyne with complex 2 (=A0) at 298 K; computed structures of pertinent intermediates
(for the full set, see the Supporting Information).
Scheme 3. Computed fate of the carbene formed by geminal hydrogenation upon addition of a second H2 molecule.
molÀ1, it is very plausible that carbenes such as 9 gain lifetimes
long enough for spectroscopic observation or even isola-
tion.[32] Moreover, the computations nicely concur with the
conclusions drawn from the PHIP NMR data that the geminal
protons of the CH2 group flanking the carbene center in C2
indeed derive from a single H2 molecule.
Most relevant for the understanding of the product
spectrum is the finding that the 16-electron carbene species
C2 is capable of binding and activating a second molecule of
H2 (Schemes 3 and 4).[34] The effective barriers associated
with the evolution of the resulting hydrogen adducts are
invariably lower (by ca. 3 kcalmolÀ1) than that of the
unimolecular reverse reaction of C2 to E1 (DG° =
+ 20.9 kcalmolÀ1), thus implying that this reaction channel
constitutes a kinetically favorable outlet for the carbene once
formed.
The product spectrum depends on the side from which H2
approaches the carbene center. If H2 approaches from the
side of the methyl group (path 1), adduct R1 is formed, which
evolves into the ruthenium alkyl complex R3 with an a-
agostic interaction. A subsequent barrier-free rotation about
À
the Ru Ca bond swaps the agostic binding sites and brings the
b-hydrogen atom of the methyl terminus into the Ru
coordination sphere, which opens a gateway (red path in
Schemes 3 and 4) to the isomerized side product 1-butene
(D4). If, instead, the internal b-hydrogen atom engages with
the Ru center through yet another low-energy rotatory
motion (R3!TSR3–B1!B1, DG° =+ 2.1 kcalmolÀ1), a subse-
12434 www.angewandte.org
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 12431 –12436

Angewandte
Chemie
Scheme 4. Free energy profile for path 1 from Scheme 3 at 298 K (for the corresponding profile for path 2, see the Supporting Information).
quent reductive elimination affords the alkane side product
B2 (blue path). Analogously, in case H2 approaches from the
side of the ethyl group (path 2), the resulting hydrogen adduct
R1’ is converted into the desired E-alkene E2 and the alkane
side product B2’ by processes that are very similar to those
described for path 1 (for details, see the Supporting Informa-
tion).
We thus conclude that the carbene C2 is linked to the
desired product; at the same time, this very intermediate also
constitutes the major gateway to the side products. This
interpretation is in excellent accord with the 1H-OPSY-EXSY
experiment shown in Figure 4. The somewhat higher barrier
for the formation of B1 compared with that of D1 suggests
that over-reduction is less of a problem than isomerization,
which is again in qualitative agreement with the available
preparative data.[8,9]
[1] a) B. M. Trost, Z. T. Ball, T. Jçge, J. Am. Chem. Soc. 2002, 124,
7922 – 7923; b) B. M. Trost, Z. T. Ball, J. Am. Chem. Soc. 2005,
127, 17644 – 17655; c) B. M. Trost, Z. T. Ball, Synthesis 2005,
853 – 887.
[2] a) A. Fürstner, K. Radkowski, Chem. Commun. 2002, 2182 –
2183; b) F. Lacombe, K. Radkowski, A. Fürstner, Tetrahedron
2004, 60, 7315 – 7324.
[3] S. M. Rummelt, K. Radkowski, D.-A. Rosca, A. Fürstner, J. Am.
Chem. Soc. 2015, 137, 5506 – 5519.
[4] B. Sundararaju, A. Fürstner, Angew. Chem. Int. Ed. 2013, 52,
14050 – 14054; Angew. Chem. 2013, 125, 14300 – 14304.
[5] T. Matsuda, S. Kadowaki, Y. Yamaguchi, M. Murakami, Org.
Lett. 2010, 12, 1056 – 1058.
[6] S. M. Rummelt, A. Fürstner, Angew. Chem. Int. Ed. 2014, 53,
3626 – 3630; Angew. Chem. 2014, 126, 3700 – 3704.
[7] S. M. Rummelt, J. Preindl, H. Sommer, A. Fürstner, Angew.
Chem. Int. Ed. 2015, 54, 6241 – 6245; Angew. Chem. 2015, 127,
6339 – 6343.
In summary, we have shown that the trans-hydrogenation
of internal alkynes competes with a formal gem-hydrogena-
tion process; this latter reactivity mode has hardly any
precedent in organic chemistry.[24,25] The resulting ruthenium
carbene intermediates seem to be the very origin of the side
reactions that can plague catalytic trans-reductions at this
stage of methodological development. Our ongoing attempts
at optimizing this valuable transformation will be guided by
this insight.
[8] K. Radkowski, B. Sundararaju, A. Fürstner, Angew. Chem. Int.
Ed. 2013, 52, 355 – 360; Angew. Chem. 2013, 125, 373 – 378.
[9] M. Fuchs, A. Fürstner, Angew. Chem. Int. Ed. 2015, 54, 3978 –
3982; Angew. Chem. 2015, 127, 4050 – 4054.
[10] A. P. Dobbs, F. K. I. Chio in Comprehensive Organic Synthesis
II, 2nd ed., Vol. 8 (Eds.: P. Knochel, G. A. Molander), Elsevier,
Amsterdam, 2014, pp. 964 – 998.
[11] J. Hartwig, Organotransition Metal Chemistry. From Bonding to
Catalysis, University Science Books, Mill Valley, CA, 2010.
[12] D. S. Frohnapfel, J. L. Templeton, Coord. Chem. Rev. 2000, 206 –
207, 199 – 235.
[13] a) L. W. Chung, Y.-D. Wu, B. M. Trost, Z. T. Ball, J. Am. Chem.
Soc. 2003, 125, 11578 – 11582; see also: b) S. Ding, L.-J. Song,
L. W. Chung, X. Zhang, J. Sun, Y.-D. Wu, J. Am. Chem. Soc.
2013, 135, 13835 – 13842.
[14] A. Fürstner, Angew. Chem. Int. Ed. 2014, 53, 8587 – 8598; Angew.
Chem. 2014, 126, 8728 – 8740.
Acknowledgements
Generous financial support was provided by the MPG and the
Austrian Science Fund (Erwin Schrçdinger fellowship to
M.F., J3466 N28J). M.L. thanks Prof. Benjamin List for his
support and Dr. K. Münnemann for early advice with PHIP.
We thank J. Rust and Prof. C. W. Lehmann for solving the
X-ray structure and Umicore AG & Co KG, Hanau, for
a generous gift of noble-metal salts.
[15] D. Schleyer, H. G. Niessen, J. Bargon, New J. Chem. 2001, 25,
423 – 426.
[16] See also: R. R. Burch, A. J. Shusterman, E. L. Muetterties, R. G.
Teller, J. M. Williams, J. Am. Chem. Soc. 1983, 105, 3546 – 3556.
[17] D. J. Pasto in Comprehensive Organic Synthesis Vol. 8 (Eds.:
B. M. Trost, I. Fleming), Pergamon, Oxford, 1991, pp. 471 – 488.
[18] a) C. R. Bowers, D. P. Weitekamp, Phys. Rev. Lett. 1986, 57,
2645 – 2648; b) M. G. Pravica, D. P. Weitekamp, Chem. Phys.
Lett. 1988, 145, 255 – 258; c) C. R. Bowers, D. P. Weitekamp, J.
Am. Chem. Soc. 1987, 109, 5541 – 5542.
Keywords: alkynes · carbenes · density functional calculations ·
hydrogenation · NMR spectroscopy · ruthenium
How to cite: Angew. Chem. Int. Ed. 2015, 54, 12431–12436
Angew. Chem. 2015, 127, 12608–12613
[19] It can be estimated that an intermediate must persist for several
tens of milliseconds to be detectable.
[20] a) S. B. Duckett, R. E. Mewis, Acc. Chem. Res. 2012, 45, 1247 –
1257; b) R. Eisenberg, Acc. Chem. Res. 1991, 24, 110 – 116; c) J.
Angew. Chem. Int. Ed. 2015, 54, 12431 –12436
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 12435

.
Angewandte
Communications
Bargon, R. Giernoth, A. Harthun, C. Ulrich in Organic Synthesis
via Organometallics OSM 5 (Eds.: G. Helmchen, J. Dibo, D.
Flubacher, B. Wiese), Vieweg, Wiesbaden, 1997, pp. 147 – 154.
[21] For leading PHIP studies on alkyne semi-hydrogenation to Z-
alkenes and Z/E isomerization, see the following and references
cited therein: a) J. López-Serrano, S. B. Duckett, S. Aiken, K. Q.
Almeida LeÇero, E. Drent, J. P. Dunne, D. Konya, A. C. Whit-
wood, J. Am. Chem. Soc. 2007, 129, 6513 – 6527; b) M. Boutain,
S. B. Duckett, J. P. Dunne, C. Godard, J. M. Hernµndez, A. J.
Holmes, I. G. Khazal, J. López-Serrano, Dalton Trans. 2010, 39,
3495 – 3500.
[25] H. Berke, ChemPhysChem 2010, 11, 1837 – 1849.
[26] A. Fürstner, L. Ackermann, B. Gabor, R. Goddard, C. W.
Lehmann, R. Mynott, F. Stelzer, O. R. Thiel, Chem. Eur. J. 2001,
7, 3236 – 3253.
[27] S. Torker, A. Müller, R. Sigrist, P. Chen, Organometallics 2010,
29, 2735 – 2751.
[28] G. Aullón, D. Bellamy, L. Brammer, E. A. Bruton, A. G. Orpen,
Chem. Commun. 1998, 653 – 654.
[29] a) J. Jeener, B. H. Meier, P. Bachmann, R. R. Ernst, J. Chem.
Phys. 1979, 71, 4546 – 4553; b) B. H. Meier, R. R. Ernst, J. Am.
Chem. Soc. 1979, 101, 6441 – 6442.
[22] a) J. A. Aguilar, P. I. P. Elliott, J. Lopez-Serrano, R. W. Adams,
S. B. Duckett, Chem. Commun. 2007, 1183 – 1185; b) J. A.
Aguilar, R. W. Adams, S. B. Duckett, G. G. R. Green, R.
Kandiah, J. Magn. Reson. 2011, 208, 49 – 57.
[30] EXSY cross peaks to the trace amounts of Z-alkene present in
the mixture were also detected.
[31] An exchange cross peak is also observed to H2 at dH = 4.7 ppm.
[32] The computed pathway for 9 is contained in the Supporting
Information.
[23] The 6a/7a ratio depends on whether 1 (ca. 3:2) or 2 (ca. 10:1) is
chosen as the catalyst, see the Supporting Information.
[24] Free carbenes (but not regular N-heterocyclic carbenes) or
vinylidenes seem to be the only known examples; for pertinent
references see the following and references cited therein:
a) G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller, G.
Bertrand, Science 2007, 316, 439 – 441; b) C. Kçtting, W. Sander,
J. Am. Chem. Soc. 1999, 121, 8891 – 8897; c) P. S. Zuev, R. S.
Sheridan, J. Am. Chem. Soc. 2001, 123, 12434 – 12435; d) S.
Henkel, W. Sander, Angew. Chem. Int. Ed. 2015, 54, 4603 – 4607;
Angew. Chem. 2015, 127, 4686 – 4690.
À
[33] For the cationic complex 1, H2 activation and C H bond
formation are actually concerted.
[34] The direct conversion of C2 into the E-alkene product E2
through a 1,2-H shift is conceivable but is ruled out based on
a relatively high predicted energy barrier; for details, see the
Supporting Information.
Received: July 2, 2015
Published online: August 31, 2015
12436 www.angewandte.org
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 12431 –12436