Diverting Hydrogenations with Wilkinson's Catalyst towards Highly Reactive Rhodium(I) Species

Article abstract of DOI:10.1002/anie.201506216

The addition of Barton's base has a dramatic effect on the classic rhodium(III)-mediated hydrogenations promoted by Wilkinson′s catalyst. Following the initial oxidative addition, a barrierless reductive elimination of HCl from the traditional rhodium(III) intermediates instantly produces a rhodium(I) monohydride species, which is remarkably reactive in the hydrogenation of several internal alkynes and functionalized trisubstituted alkenes. The direct formation of this species is unprecedented upon addition of molecular hydrogen and its catalytic potential has been hitherto barely explored.

Full text of DOI:10.1002/anie.201506216

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506216
German Edition: DOI: 10.1002/ange.201506216
Homogeneous Hydrogenation
Diverting Hydrogenations with Wilkinsonꢀs Catalyst towards Highly
Reactive Rhodium(I) Species
Jesus E. Perea-Buceta, Israel Fernµndez, Sami Heikkinen, Kirill Axenov, Alistair W. T. King,
Teemu Niemi, Martin Nieger, Markku Leskelä, and Timo Repo*
Abstract: The addition of Bartonꢀs base has a dramatic effect
on the classic rhodium(III)-mediated hydrogenations pro-
moted by Wilkinson’s catalyst. Following the initial oxidative
addition, a barrierless reductive elimination of HCl from the
traditional rhodium(III) intermediates instantly produces
a rhodium(I) monohydride species, which is remarkably
reactive in the hydrogenation of several internal alkynes and
functionalized trisubstituted alkenes. The direct formation of
this species is unprecedented upon addition of molecular
hydrogen and its catalytic potential has been hitherto barely
explored.
well-known phosphine dissociation/reassociation equili-
bria.[11]
Despite counting with this thoroughly studied mechanism,
and an outstanding performance as the benchmark catalyst in
a number of processes including complex late-stage hydro-
genations,[12] little is known to overcome the well-documented
limitations of 1 without previously modifying its inner
coordination sphere.[1,5] In the course of our ongoing efforts
on small-molecule activation and bifunctional metal-free
hydrogenations,[13] we became interested in the synergetic
catalytic potential of adding an external base to the classic
hydrogenation of apolar multiple bonds catalyzed by 1.
For the study, we bore in mind that the addition of an
external base into a metal-based catalytic hydrogenation
system usually causes the stepwise deprotonation of a weak
M-h2-H2 bonded intermediate, or the corresponding dihy-
dride species.[14] In the case of 1, several structural factors
preclude the stabilization of a M-h2-H2 bond, hence a stronger
base and drastic reaction conditions would be required to
H
ydrogen not only finds ordinary use in innumerable
chemical transformations,[1] but is also the simplest model
molecule to study low-energy catalytic pathways towards the
activation of most inert s bonds.[2] The discovery of Wilkin-
sonꢀs catalyst ([Rh(PPh3)3Cl]; 1)[3] was instrumental in the
fundamental understanding of metal–H interactions and
hydrogen activation.[4] Subsequently, this landmark has trig-
gered a rapid evolution on the rational design of hydro-
genation catalysts,[5] which in turn, have nurtured a burgeon-
À
deprotonate the stronger Rh H bonds featured in the
resulting neutral dihydride intermediates.[15]
ing progress in other key areas such as asymmetric catalysis,[6]
energy storage, and C H activation.
To begin our research, we conceived that the hydro-
genation of a mildly reactive internal alkyne, such as 2a,
would constitute a convenient model system (Table 1). First
we examined the effect of the steric hindrance of the external
base on the catalytic activity of 1. Starting with Et3N, the use
of increasingly strained aliphatic tertiary amines, such as 6–8,
or aromatic ones (9), had negligible effects upon mild reaction
conditions (PhMe, 0.5 bar H2, RT). This observation indicated
that such tertiary amines just play a spectator role.
We next assessed the impact of adding stronger bases.
Polysubstituted guanidines seemed to be obvious candidates
given also their tunable sterics.[16] Whilst TMG (10; pKa of
conjugated acid in MeCN is ca. 23.6) and its proton sponge
derivative (11; pKa ꢀ 25.1) failed to induce any change,
a nearly seven-fold boost of catalytic activity was observed
with 2-tBuTMG (12; pKa ꢀ 26). Moreover, another base of
similar strength to 12, such as phosphazene (13; pKa ꢀ 28),
promoted a similar increase of activity in relation to the
catalyst operating alone.
The promising outcome of the initial screening encour-
aged us to extend the observed catalytic amplification to
a series of alkenes and alkynes (Table 2). We commenced by
optimizing the reaction conditions for the hydrogenation of
2a (see Section S11 in the Supporting Information). Raising
the reaction concentration (PhMe, 0.5m) and H2 pressure
(1.0 bar), and the use of 12 (5 mol%), furnished 3a in an
excellent yield of 90% upon isolation. Conversely, similar
reaction conditions failed to promote an analogous effect on
[7]
À
Mechanistically, the catalytic hydrogenation of olefins
with 1 is the customary textbook example of a two-electron
catalytic redox cycle. First, H2 adds oxidatively to the
rhodium(I) center, thus forming a rhodium(III) dihydride.
Comprehensive studies carried out by Halpern proved that
the 16-electron complex [H2Rh(PPh3)2Cl] is the kinetically
competent rhodium(III) species to which the olefin then
coordinates and undergoes the rate-limiting migratory inser-
tion step.[8] Finally, reductive elimination generates the
hydrogenated product and closes the catalytic cycle.[9] Con-
ventional NMR techniques only allow the detection of off-
cycle species such as [H2Rh(PPh3)3Cl].[10] However, these are
connected with the transient active rhodium(III) species by
[*] Dr. J. E. Perea-Buceta, Dr. S. Heikkinen, Dr. K. Axenov,
Dr. A. W. T. King, T. Niemi, Dr. M. Nieger, Prof. M. Leskelä,
Prof. T. Repo
Department of Chemistry
P.O.Box 55, 00014 University of Helsinki (Finland)
E-mail: timo.repo@helsinki.fi
Dr. I. Fernµndez
Departamento de Química Orgµnica I, Facultad de Ciencias
Químicas, Universidad Complutense de Madrid
Ciudad Universitaria, 28040 Madrid (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506216.
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Table 1: Effect of bases on hydrogenations with Wilkinson’s catalyst.[a]
basic conditions of the benchmark substrate 2g.[18] Indeed,
a boost of yield from 27 to 90% was afforded by merely
adding 5 mol% of 12.
A similar trend was observed in going from 2-cyclo-
hexenone (2e) to the methyl-substituted analogue 2 f. Whilst
a severe decrease on the catalytic activity occurred on
classical hydrogenation conditions (3e: 84% and 3 f: 13%),
the addition of 12 (5 mol%) maintained high product yields
(3e: 87% and 3 f: 79%) by just elevating the H2 pressure
from 0.5 to 1.5 bar, and expanding the reaction time from 8 to
24 hours (Table 2).
Entry
Base
Yield [%][b]
(Z)-4/(E)-5
The catalytic amplification was also remarkable with
a relatively complex substrate, such as (5R)-(À)-carvone (2h),
containing two different double bonds and a chiral center. In
practice, the addition of 12 (10 mol%) to a reaction mixture
with 1 (2 mol%) under 1.5 bar of H2, raised the isolated yield
of the desired hydrogenated product 3h from 3 to 61%.
We then sought to elucidate, through NMR spectroscopy,
the origin of the reactivity promoted by the addition of 12. In
line with the catalytic screening, addition of an excess
(4 equiv) of a hindered tertiary amine such as PhTMP failed
3a
1
2
3
4
5
6
7
8
9
–
12
11
13
14
13
2
15
69
73
22:0.3
21:0.4
22:0.3
23:0.5
22:0.3
10:0.2
19:0.5
12:1.2
12:1.6
Et3N (6)
(iPr)(tBu)NMe (7)
TMP (8)
Ph-TMP (9)
TMG (10)
11
12
13
1
to promote any changes either in the H and 31P{1H} NMR
[a] 2a (1 equiv), 1 (0.01 equiv), base (6–13; 0.02 equiv), PhMe (0.2m),
H2 (0.5 bar), RT, 4 h. [b] Yields determined by GC-MS analysis with
mesitylene as an internal standard. For full details, see the Supporting
Information. TMP=2,2,6,6-tetramethylpiperidine, TMG=tetramethyl-
guanidine.
spectra of 1 (see Sections S1 and S2), or [H2Rh(PPh3)3Cl] (14;
Figure 1a,b).[11]
Identical NMR profiles also arose after adding 4 equiv-
alents of 12 to 1 under Ar (see Section S3), thus indicating no
interaction between the base and the rhodium center prior to
exposure to H2. However, an immediate change of color from
orange to yellow announced rapid reaction (ca. 5–10 s at RT)
after charging 1.0 bar of H2 into this mixture. Indeed, a new
scenario emerged in the 1H NMR spectrum with a broad
signal at d = 10.68 ppm, characteristic of a guanidinium cation
the hydrogenation of 2a with 1 operating alone, in accord with
the classic mechanism where changes in the pressure of H2
have no effect on the rate-limiting step.[8] This result indicates
that the addition of 12 promotes a mechanistic switch
manifested by the key role that the activation of H2 acquires
in the overall reaction kinetic profile. Even more drastic
differences between the classical and new basic reaction
conditions were observed during the hydrogenation of
internal alkynes such as 2b and 2c. Whilst 2b could be fully
converted into 3b under the new base-enhanced reaction
conditions, no reactivity was noticed in the
absence of 12.
À
À
N H, and a Rh H region only featuring a broad singlet at d =
À8.26 ppm (Figure 1c). Moreover, this species (15) undergoes
fast intra/intermolecular ligand exchange equilibria in solu-
tion at room temperature.[19]
We next undertook further spectroscopic studies at low
temperature to study the fluxional regime of 15 and gain
Remarkably, the alkynylaniline 2c was
also only completely hydrogenated under
basic conditions (2 mol% of 1, 10 mol% of
12, 1.5 bar H2) to furnish 3c in 90% yield after
16 hours (Table 2). No signs of the cyclo-
isomerization reactions, which this family of
substrates typically undergoes with a broad
variety of metals including rhodium(I), were
detected.[17] Unfortunately, this functional-
group tolerance was not observed in terminal
alkynes featuring acidic hydrogen atoms as
the alkyne 2d was only converted into the
corresponding alkane 3d under classical reac-
tion conditions. However, negligible catalytic
amplification was observed for challenging
hindered alkenes such as limonene and a-
Figure 1. Diagnostic areas in the 1H and 31P{1H} NMR spectra (RT, [D8]THF, 1.0 bar of
H2) of 14 (a,b), 15 (c,d), and species observed in the low and variable NMR studies (e).
For further details, see the Supporting Information and DFT calculations. For clarity the
pinene. To our surprise, this detrimental effect
of an increased level of olefin substitution did
not appear during the hydrogenation under PPh3 ligands are labeled P.
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Table 2: Impact of the base 12 on the hydrogenations with 1.[a]
prepared by treating 1 with either
an equimolar amount of lithium
dimethylamide and an excess of
dimethylamine,[20] or with an
excess of Et3SiH,[21] but not directly
in the presence of molecular H2.
The complex 17 is produced by
the reaction of H2 with 16 at low
temperature and was identified as
Substrate
Major product
Cat.
(mol%)
P(H2)
[bar]
t
Yield [%][b]
[h] no base 12
2a
3a
1
1
1.0
1.0
4
6
16
97(90)
99(96)
(fac)-[Rh(PPh3)3(H)3]
(Fig-
we
ure 1e).[22]
Furthermore,
observed the gradual conversion of
rhodium(I) complex 16 into
rhodium(III) complex 17 by main-
taining the mixture at 193 K (com-
pletion after 4–6 h; see Figure S6.2).
At such temperatures the mixture is
in a nearly-zero fluxional regime
and thus there is no steric barrier
preventing free H2 from approach-
ing and reacting with the rhodium-
(I) center.
2b
3b
1
2c
2d
3c
3d
2
1
1.5
1.0
16
3
0
96(90)[c,d]
7[e]
To confirm the feasibility of the
dehydrochlorination event and also
preclude alternative heterolytic
pathways compatible with the
origin of the observed rhodium(I)
species, the rhodium(III) complex
14 was preformed instantaneously
under H2 and the resulting solution
was frozen (Figure 2). The H2
atmosphere was replaced by Ar to
avoid equilibration with 1, and an
excess of 12 was added. To our
98
2e
2 f
3e, R=H
3 f, R=Me
1
1
0.5
1.5
8
84
13
87[f]
79
24
2g
3g
1
1.5
8
27
90
delight, we observed how the RhIII
À
H resonances of 14 gradually dis-
I
À
appeared while the Rh H resonan-
ces slowly emerged.[23] Repetition
of the experiment demonstrated
that the rate of the process varied
with the rate of H2 diffusion in the
NMR tube (see Section S8). There-
fore, our experimental findings indi-
cate a detour in the classical hydro-
genation mechanism promoted by
1, which involves the formation of
a rhodium(I) species by reaction of
the rhodium(III) dihydride com-
plex 14 with 12.
2h
3h
2
1.5
20
3
76(61)[d,g]
[a] Unless stated otherwise: 2a–h (1 equiv), 12 (0.05 equiv), PhMe (0.5m), RT. [b] Yields determined by
GC-MS analysis with mesitylene as an internal standard. Yields of the isolated products are given within
parentheses. [c] Reaction with 12 (0.1 equiv) and PhMe (0.25m). [d] Yield determined by H NMR
spectroscopy using mesitylene as an internal standard. [e] No alkane detected. Listed yield corresponds
to the alkene. [f] Reaction with PhMe (0.2m) at 0.5 bar of H2. [g] Reaction with 12 (0.1 equiv). 3h was
isolated in 91–94% purity. (For full details, see Section S12 in the Supporting Information.)
1
further insight into the mechanism. Upon cooling the mixture
1
1
to 193 K, the H, H{31P}, and 31P{1H} NMR spectra showed
that 15 evolved into two other complexes, 16 and 17 (Fig-
ure 1e; see Section S5). The species 16 can be assigned as
[Rh(PPh3)3H] which was first reported by Shriver et al.[20] This
species results from the dehydrochlorination of the rhodium-
(III) species 14 promoted by 12 in the presence of H2. This
assignment was fully confirmed by H–31P and 31P–31P two-
1
dimensional correlation NMR measurements (see Sec-
tion S5). Eventually, a white precipitate was extensively
deposited on the NMR tube, which was later identified as
the hydrochloride salt of 2-tBuTMG (18; see Figure S6.3).
This was an unexpected result since 16 is only known to be
Figure 2. Control NMR studies. The PPh3 groups are labeled P (for full
details see the Supporting Information).
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In addition, the 1H NMR spectra of 15 and [Rh(PPh3)3H]
(16) are quite similar, thus indicating that both species are
strongly related. However, there is a noticeable difference,
4 equivalents of [NBu4]Cl, a solution which also exhibits
À
a similar broadening of the Rh H signal (corresponding
T1 value of 0.92 s; see Section S9).
À
namely a slight broadening and shifting of the Rh H
Finally, we were interested in understanding the key
dehydrochlorination event. Unfortunately, all our attempts to
locate a transition state for the direct conversion of 14 into 15
on the potential energy surface met with no success. This
outcome is very likely because of the steric hindrance in 14
preventing the approach of the bulky base 12. Hence, it can be
suggested that a PPh3 ligand dissociation occurs first in the
RhIII complex 14 (leading to complex [H2Rh(PPh3)2Cl], 19) to
facilitate the reaction with 12. Strikingly, relaxed scans
performed at different Rh···H distances starting from 19
(where Rh-H = 1.6 Š, that is, a similar bond length in
complex 14) nicely show that the abstraction of the hydride
promoted by 12 is essentially a barrierless process (see
Figure S14.1).
resonance in 15 (see Figure S9.4).[24] This change may be
ascribed to the occurrence of a weak interaction between the
hydride and the guanidine proton, an interaction which is
known to provoke a similar effect in related organometallic
systems.[25] Indeed, we found that when a pure sample of
[Rh(PPh3)3H] is treated with 4 equivalents of [H-tBuTMG]-
À
[PF6], a similar broadening of the Rh H resonance is
observed. Furthermore, the corresponding T1 relaxation
time of Rh H measured at 300 K decreases from 0.89 to
0.80 seconds (see Section S9), and is fully compatible with the
occurrence of such weak H···H interaction.
To gain more insight into the nature of 15, we carried out
a DFT study (see Section S14). Our calculations indicate that
indeed a weak Rh-H···HN interaction exists in 15 (Figure 3).
I
À
Furthermore, our calculations also suggest that this
process is assisted by the chloride ligand, which starts to
À
dissociate during the transformation (i.e. the Rh Cl bond
becomes longer and longer as the H abstraction takes place).
À
As a result, the initial Rh Cl bond in 1 can easily dissociate,
thus forming [Rh(PPh3)3H] (16) and the hydrochloride salt 18,
as experimentally observed. Therefore, the diversion in the
classical hydrogenation mechanism can be formally viewed as
a barrierless reductive elimination of HCl from rhodium(III)
species 14 as a result of H abstraction by 12, thus ultimately
giving 15, a complex similar to [Rh(PPh3)3H].
In conclusion, we have discovered an unprecedented,
facile, and efficient manner to amplify the reactivity of the
benchmark catalyst 1 in situ without modifying its inner
coordination sphere. The simple addition of a strong base (12)
in catalytic amounts gives rise to highly reactive rhodium(I)
species for the hydrogenation of a series of alkynes and
functionalized trisubstituted alkenes. Extensive spectroscopic
NMR analyses combined with DFT studies identified a bar-
rierless reductive elimination of HCl, promoted by 12, from
the classic dihydride rhodium(III) species 14 as the key event
leading to the observed catalytic scenario. Ongoing studies
are pursued in our laboratories to further explore prospective
catalytic applications for this type of reactivity.
Figure 3. Reaction profile calculated [PCM(toluene)-M06/def2-SVP//
M06/def2-SVP level] for the formation of 15 from 14 and 12. Bond
distances and relative electronic energies (ZPVE included) and free
energies (within parentheses) are given in Š and kcalmolÀ1, respec-
tively. The Ph rings in the PPh3 ligands are omitted for clarity.
The corresponding computed H···H distance of 1.881 Š
concurs quite well with that computed for related dihydro-
gen-bonded iridium complexes.[25] Despite that, the formation
of 15 from 14 and 12 seems unlikely in view of the
Acknowledgements
The work was supported by the Academy of Finland (139550,
276586) and the Spanish MINECO-FEDER (grant
CTQ2013-44303-P).
endergonicity computed for this reaction.[26] This discovery
I
À
suggests that the broadening of the Rh H resonance may be
caused by a different weak interaction occurring in [Rh-
(PPh3)3H].[27] Indeed, two different species, 15B and 15C,
were located on the potential energy surface and they lie 11.8
and 13.7 kcalmolÀ1, respectively, below the dihydrogen-
bonded species 15 (Figure 3). In these complexes, the weak
interaction is established between the transition metal and the
Cl of the protonated guanidine (computed Rh···Cl distance of
2.593 and 2.752 Š, respectively). Further experimental sup-
port for this Rh···Cl intermolecular interaction is given by the
1H NMR spectrum of a mixture of pure [Rh(PPh3)3H] with
Keywords: density functional calculations ·
homogeneous hydrogenation · reaction mechanisms ·
rhodium(I) · Wilkinson’s catalyst
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14321–14325
Angew. Chem. 2015, 127, 14529–14533
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À
Section S13), and their different Rh H signals (see Section S9).
[25] See, for instance: A. Rossin, E. I. Gutsul, N. V. Belkova, L. M.
Epstein, L. Gonsalvi, A. Lledós, K. A. Lyssenko, M. Peruzzini,
E. S. Shubina, F. Zanobini, Inorg. Chem. 2010, 49, 4343 – 4354.
[26] We also computed the formation of the corresponding dihydro-
gen-bonded cation where the chloride ligand is already dis-
sociated. The 14 + 12![(Ph3P)3RhH···H(tBu)N = C(NMe2)2]+ +
ClÀ process is even more unlikely (DE =+ 47.1 kcalmolÀ1, DG =
+ 55.7 kcalmolÀ1).
[27] A reviewer suggested that the broadening of the Rh-H
resonance might be related with a dynamic equilibrium between
[Rh(PPh3)3H] and small amounts of cluster species in solution, as
it was proposed before: A. J. Sivak, E. L. Muetterties, J. Am.
Chem. Soc. 1979, 101, 4878 – 4887.
[14] a) M. G. Basallote, M. Besora, E. Castillo, M. J. Fernµndez-
Trujillo, A. Lledós, F. Maseras, M. A. MµÇez, J. Am. Chem. Soc.
2007, 129, 6608 – 6618; b) Y.-Y. Jiang, H.-Z. Yu, Y. Fu, Organo-
metallics 2014, 33, 6577 – 6584; c) R. R. Schrock, J. A. Osborn, J.
Am. Chem. Soc. 1976, 98, 2143 – 2147.
Received: July 6, 2015
Revised: August 5, 2015
Published online: October 6, 2015
Angew. Chem. Int. Ed. 2015, 54, 14321 –14325
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 14325