Mild and selective H/D exchange at the β position of aromatic α-olefins by N-heterocyclic carbene-hydride-rhodium catalysts

Article abstract of DOI:10.1002/anie.201007238

Pacman bites selectively! Stable rhodium(III)-N-heterocyclic carbene-hydride complexes (Pacman-like catalysts) are highly active and selective catalysts for H/D exchange at the β position of aromatic α-olefins (see picture). The interplay between bulky N-

Full text of DOI:10.1002/anie.201007238

Communications
DOI: 10.1002/anie.201007238
H/D Exchange
Mild and Selective H/D Exchange at the b Position of Aromatic a-
Olefins by N-Heterocyclic Carbene–Hydride–Rhodium Catalysts**
Andrea Di Giuseppe, Ricardo Castarlenas,* Jesffls J. PØrez-Torrente, Fernando J. Lahoz,
Victor Polo, and Luis A. Oro*
One of the major challenges in modern chemistry is the design
nism.[3,4a–e] Despite this observation, the number of isolated
RhIII–NHC–hydride derivatives remains scarce,[7] and all such
compounds known bear two NHC ligands. The presence of
two bulky NHC ligands creates a very crowded environment
around the metal center, thereby reducing the reactivity of
the complex.[6c] In this context, we envisaged the substitution
of one of the quasi-hemispherical NHC ligands by a smaller
planar chelating ligand to facilitate the interaction of the
complex with the substrates while maintaining a high steric
induction.
Herein we report a highly active and selective H/D
exchange at the b position of aromatic a-olefins by a new
rhodium(III)–N-heterocyclic carbene–hydride catalyst that
contains a bulky NHC and a chelating quinolinate ligand.
Thus, we chose 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-car-
À
of improved catalysts for controlled and selective C H
functionalization.[1] In this context, H/D exchange reactions
are valuable transformations for the preparation of isotopi-
cally labeled compounds that have found practical applica-
tions in mechanistic investigations of catalysis, spectroscopic
analysis, or the monitoring of drug metabolism.[2] Organome-
tallic catalysts offer the advantage of mild reaction conditions
with
a
high degree of regio- and chemoselectivity.[3,4]
Although the study of H/D exchange reactions has tended
to concentrate on aromatic[2,3a–i] or aliphatic[2,3a–d,j] com-
pounds, there is an increasing interest in the deuteration of
vinylic derivatives.[4] Moreover, selectivity towards olefinic
versus aromatic protons is an important challenge. Milstein
and co-workers have recently shown the ability of a pincer
RhIII–hydride catalyst to perform selective vinylic deuteration
with moderate activity.[4c]
An electron-rich ligand is often used to increase the
stability of potential RhIII–hydride catalysts, whereas their
selectivity can be modulated by a bulky ligand that exerts
steric pressure on the interaction of substrates with the active
species. N-heterocyclic carbene ligands (NHC) fulfill both
these requirements[5] as they are excellent electron-releasing
groups and provide adequate steric protection in the vicinity
of the metallic center.[6] Hydride ligands are anticipated to
play a fundamental role in the exchange step with the
deuterium source, regardless of the proposed mecha-
bene (IPr) as encumbered NHC ligand and 8-quinolinol as a
[8]
À
suitable hydride source upon oxidative O H addition.
Treatment of the dimer [Rh(m-Cl)(IPr)(coe)]2 (1) (coe =
cyclooctene)[7b] with 8-quinolinol gave rise to the diastereo-
selective
formation
of
the
16-electron
complex
[RhClH(k2O,N-C9H6NO)(IPr)] (2; Scheme 1). The hydride
[*] Dipl.-Chem. A. Di Giuseppe, Dr. R. Castarlenas,
Prof. J. J. Pꢀrez-Torrente, Prof. F. J. Lahoz, Prof. L. A. Oro
Instituto Universitario de Catꢁlisis Homogꢀnea—Instituto de
Ciencia de Materiales de Aragon, Universidad de Zaragoza—CSIC
Pl. S. Francisco S/N, 50009 Zaragoza (Spain)
E-mail: oro@unizar.es
Scheme 1. Synthesis of rhodium–NHC–hydride complexes.
ligand resonates in the 1H NMR spectrum at d = À28.41 ppm
as a doublet with a JH–Rh coupling constant of 46.4 Hz.[9]
Crystallization of 2 from coordinating solvents gave the
saturated species [RhClH(k2O,N-C9H6NO)(IPr)(S)] [S =
methanol (3); acetonitrile (4)]. The structure of 3 was
confirmed by an X-ray diffraction analysis.[10] The complex
can be described as a distorted octahedron with the hydride
trans to the coordinated methanol and the bidentate quino-
linate ligand with its nitrogen atom trans to IPr.
rcastar@unizar.es
Dipl.-Chem. A. Di Giuseppe
Dipartimento di Chimica, Ingegneria Chimica e Materiali
Universitꢂ dell’Aquila, 67100-Coppito (AQ) (Italy)
Dr. V. Polo
Departamento de Quꢃmica Fꢃsica, Universidad de Zaragoza
50009 Zaragoza (Spain)
[**] Financial support from the MICINN of Spain (project numbers
CTQ2009-08089 and CTQ2010-15221), the ARAID Foundation
under the program “jꢄvenes investigadores”, and CONSOLIDER
INGENIO-2010 program, under the projects MULTICAT (CSD2009-
00050) and Factorꢃa de Cristalizaciꢄn (CSD2006-0015) is gratefully
acknowledged. R.C. thanks the CSIC and the European Social Fund
for his Research Contract in the framework of the “Ramꢄn y Cajal”
program.
Complex 2 is an active and selective catalyst for H/D
exchange of a-olefins under mild conditions. High activity
was observed in CD3OD at room temperature with a 2 mol%
catalyst loading. Styrene was deuterated exclusively at the
vinylic positions, with concomitant lack of deuterium at the
phenyl ring. Moreover, a very high selectivity for the
b position was observed, with intriguingly similar rates for
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007238.
both cis and trans protons (Figure 1, TOF1/2 of 192 hÀ1). A
[11]
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Angew. Chem. Int. Ed. 2011, 50, 3938 –3942

6.6 Hz) at d = 0.75 and 0.39 ppm and are coupled to the Rh-
CH protons, which appear as broad signals at d = 5.58 and
6.15 ppm, respectively.[12] Warming the sample to room
temperature results in the formation of a new complex. The
presence of two sets of diastereotopic CH2 protons in the
1H NMR spectrum supports the formation of the linear alkyl
derivative 6 by 1,2-insertion. These experiments suggest that
Markovnikov insertion products are obtained under kinetic
control, whereas anti-Markovnikov products are thermody-
namically favored.[13] Attempts to isolate 6 from the solution
resulted in the recovery of 2, thus showing the reversibility of
the insertion process.
The high selectivity of catalyst 2 for deuteration at the
b position is remarkable. The IPr ligand is a bulky and highly
electron-releasing ligand that may exert electronic and steric
influence on the catalytic activity and selectivity. Its electronic
influence on the selectivity should be related to its ability to
control the type of olefin insertion:[13] 1,2-insertion is respon-
sible for a deuteration whereas 2,1-insertion leads to b H/D
exchange. The linear alkyl derivative is thermodynamically
favored, therefore H/D exchange at the a position should
occur preferently under catalytic conditions. However, the
high selectivity observed for b substitution suggests that steric
factors could have a stronger influence.
Scheme 3 shows the proposed catalytic cycle. The relative
energies of the postulated intermediates were calculated by
DFT (B3LYP, kcalmolÀ1; Figure 2). The first step could be
deuteration of the hydride ligand in 2 by CD3OD to give a;
indeed, 2 readily undergoes H/D exchange in CD3OD at
Figure 1. H/D exchange in styrene catalyzed by 2 at 258C.
95% b H/D exchange with similar selectivity was reached
after 3 h at 258C or 20 min at 508C. The rates observed for 3
and 4 are similar to that of 2, because the lability of the
coordinated solvents on those saturated precursors should
give rise to the same active species. The catalytic species are
stable at room temperature; indeed, styrene was deuterated
with a similar activity and selectivity when a second load of
substrate was added to a completed catalytic mixture (3 h).
The selective replacement of olefinic versus aromatic protons
by deuterium has been reported previously for a RhIII–
hydride catalyst, although this complex shows a lower
catalytic activity (80% conversion after 24 h at 608C) and is
not selective for a or b protons.[4c]
À
room temperature. The steady increase of the O H signal in
Two main mechanisms have been postulated for H/D
exchange on vinylic protons: 1) oxidative addition—H/D
exchange—reductive elimination,[4b,e] or 2) H/D exchange—
the 1H NMR spectrum during catalytic reactions and the
observation of a similar reaction rate when using CH3OD
À
suggest that only the deuterium atoms of the O D group are
migratory insertion—rotation—b elimination.[4a,c,d] The C H
responsible for the exchange. The disposition of the chelating
À
activation pathway (1) is followed by electron-rich complexes,
such as IrI derivatives, whereas steric restrictions favor the
trans vinylic C H activation.[4e] Indeed, aromatic protons can
À
À
only be exchanged in a C H activation mechanism. The
observed selectivity for vinylic protons with similar rates for
both b protons, and the involvement of RhIII–hydride pre-
cursors therefore points to the classical insertion—b elimina-
tion mechanism in our system.
The stoichiometric addition of styrene to 2 was monitored
by NMR spectroscopy. At À308C, the 1H NMR spectrum
shows the formation of a pair of diastereoisomers that bear a
branched alkyl ligand generated by 2,1-insertion of styrene
into the rhodium–hydride bond (5a,b; Scheme 2). The methyl
1
groups appear in the H NMR spectrum as doublets (JH–H
=
Scheme 2. Insertion of styrene into the rhodium–hydride bond moni-
tored by variable-temperature NMR spectroscopy.
Scheme 3. IPr-controlled steric induction for selective H/D exchange.
Angew. Chem. Int. Ed. 2011, 50, 3938 –3942
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3939

Communications
Figure 2. Computed potential-energy profile relative to b (DE, kcal
molÀ1) for the olefin-insertion and alkyl-rotation processes.
Figure 3. H/D exchange in styrene catalyzed by 7 at 258C.
quinolinate ligand directs the coordination of the alkene to
the equatorial position. Two orientations (b and c) are
possible for olefin coordination, with the intermediate that
does not contain eclipsed aromatic rings (b; 0.0) being slightly
more stable than c (1.2). In accordance with the observation
that the Markovnikov insertion is kinetically favored, the
pathway via TSbd (9.2; linear alkyl ligand) is energetically less
favorable than that via TScf (8.0; branched alkyl ligand).
However, the energy values that correspond to the located
minima for the olefin-inserted products are inverted. Thus, in
accordance with the experimental observations, the liner alkyl
species d (À20.6) is more stable than the branched alkyl
derivative f (À13.1).
Catalyst 7 also favors deuteration at the b position but with a
lower selectivity than its NHC counterpart because of the
different steric hindrance exerted by the two ligands.
Although the tricyclohexylphosphine ligand is a bulky
ligand, its substituents are arranged in a conical fashion
pointing out of the equatorial plane of the coordination
sphere. In contrast, IPr adopts an umbrella type arrangement
with the isopropylphenyl substituents pointing toward the
equatorial plane, thereby increasing the steric hindrance.[6]
To elucidate the effect of the bulky electron-donating
ligand on the catalytic results, we tested as catalyst the
hydride species generated in situ by protonation of the dimer
[Rh(m-Cl)(coe)2]2 (8) with HCl. This reaction also resulted in
the exclusive deuteration of vinylic protons, but with a lower
catalytic activity (Table 1, entry 15); no discrimination
between the a and b positions is observed.
The selectivity of the H/D exchange catalyzed by 2 is
retained for a range of aromatic alkenes (Table 1), with the
electron density on the aromatic ring having a major effect.
Thus, introduction of electron-donating groups at the para or
meta positions enhances the catalytic activity (entries 7, 8, and
10), whereas an electron-withdrawing CF3 group reduces it
(entry 11). Increasing the steric bulkiness of the substituents
at the aromatic ring has little impact. Thus, both p-tert-
butylstyrene and o-methylstyrene show slightly lower rates
(entries 6 and 9). 2-Vinylnaphthalene is also deuterated
smoothly (entry 5). Aliphatic alkenes such as 1-pentene
react slowly with 2 and show similar selectivity patterns
(entry 13). Concomitant isomerization to 2-pentene was also
observed, although H/D exchange is 2.5 times faster than
isomerization. tert-Butylethylene reacts slowly at 508C
(entry 14), whereas vinylpyridine, methyl acrylate, and dis-
ubstituted alkenes such as cyclooctene, a-methylstyrene, and
2-pentene do not undergo H/D exchange in the presence of 2.
Actually, stoichiometric addition of these internal alkenes to 2
showed no insertion into the hydride bond, which is probably
due to steric repulsion, thus suggesting that the coordination
“cavity” in 2 accommodates only monosubstituted alkenes.
The use of D2O as deuterium source was also investigated.
Styrene was deuterated with a TOF1/2 of 75 hÀ1 (entry 16) in a
mixture of [D6]acetone/D2O 4:1 at 508C.[14] Another practical
Careful observation of the structure of the alkyl ligands in
d and f shows that a rotation around the C1–C2 axis of the
alkyl ligand is required to exchange the proton and deuterium
positions prior to H/D substitution. In the case of d, the steric
hindrance imposed by the bis-isopropylphenyl substituents of
IPr restricts this rotation because of repulsion with the phenyl
group of the alkyl ligand (e), thus meaning that although a
deuterium atom can enter the benzyl position, the hydride
cannot easily leave. The transition state for this rotation is
highly energetic and could not be determined. However, the
branched insertion process gives rise to a CH2D terminal
group (f) for which rotation around the C1–C2 axis is less
disfavored, thus allowing exchange between proton and
deuterium orientations via TSfg (À9.6), which is only
3.5 kcalmolÀ1 higher in energy than f. Subsequent b elimina-
tion reforms the starting olefin but with a deuterium atom at
the b position in either a cis or trans disposition (cD), thereby
explaining the similar rate observed for H/D exchange for
both cis and trans protons.
A related RhIII–quinolinate–hydride complex that bears
tricyclohexylphosphine was prepared. Although the unsatu-
rated-phosphine analogue of 2 is unstable, we were able to
prepare its acetonitrile adduct [RhClH(k2O,N-C9H6NO)-
(PCy3)(NCCH3)] (7) starting from [RhCl(coe)(PCy3)]2.
Complex 7 also catalyzed the H/D exchange of styrene but
with lower activity and selectivity than 2 or 4. Thus, after 50 h
at 258C the b H/D exchange had reached 66% whereas the
a exchange was only 21% (Figure 3). This decrease in
catalytic activity can be ascribed to the higher electron-
donating ability of the NHC ligand with respect to PCy3.[5]
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Angew. Chem. Int. Ed. 2011, 50, 3938 –3942

Table 1: H/D exchange promoted by RhIII–H catalysts.[a]
Entry Cat Substrate
t [h] T [8C] a-D[b] b-D[b] TOF1/2
[hÀ1 [c]
Experimental Section
Full experimental details and spectroscopic data are available in the
Supporting Information. Coupling constants are given in Hz.
2: A solution of 1 (300 mg, 0.235 mmol) in toluene (10 mL) was
treated with 8-quinolinol (68 mg, 0,470 mmol) and stirred for 45 min
at RT. The solution was concentrated to approximately 1 mL and n-
hexane was added to induce the precipitation of an orange solid,
which was washed and dried under vacuum. Yield: 240 mg (76%).
Elemental analysis calcd (%) for C36H43N3ClORh: C 64.33, H 6.45, N
6.25; found: C 64.14, H 6.19, N 6.24. 1H NMR (400 MHz, C6D6,
298 K): d = 8.89, 7.21, 7.17, 7.05, 6.54, and 6.26 (HQuin), À28.41 ppm (d,
]
1
2
3
4
5
6
7
8
2
2
4
7
2
2
2
2
3
25
3
95
94
93
66
95
90
93
95
192.4
833.4
188.6
2.4
0.3 50
4
2
25
25
25
25
3
50
7
21
2
J
Rh–H = 46.4, 1H, HRh–H). 13C{1H}-APT NMR (100.6 MHz, C6D6,
298 K): d = 179.0 ppm (d, JC–Rh = 49.5, Rh–CIPr).
Catalytic H/D Exchange: In an NMR tube, the catalyst
(0.01 mmol) and the olefin (0.5 mmol) were dissolved in CD3OD
50.0
4
4
140.2
416.6
333.4
1
(0.5 mL). The degree of H/D exchange was quantified by H NMR
integration with respect to hexamethyldisiloxane (2 mL, 0.01 mmol)
as internal standard. Successful deuteration of the olefin was
confirmed by 2H and 13C{1H} NMR spectroscopy.
0.4 25
25
3
1
3
Received: November 17, 2010
Revised: January 27, 2011
Published online: March 25, 2011
9
2
2
2
2.5 25
0.1 25
4
3
4
95
93
90
166.6
930.0[g]
83.3
10
11
Keywords: H/D exchange · hydride ligands ·
.
6
25
N-heterocyclic carbene · rhodium · steric induction
12
2
2
2
24
2
50
50
50
<2
10
9
<2
62
–
13[d]
36.7
5.8
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0.5 50
87
7
86
95
261.0[g]
75.0[g]
+HCl
2
+D2O
16[e]
4
4
50
25
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17[f]
2
98
2
–
[a] CD3OD with 2 mol% of catalyst. [b] (%). [c] H/D exchange at
b position. [d] 28% of 2-pentene. [e] D2O/[D6]acetone 1:4. [f] CH3OH.
[g] Calculated at final conversion.
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deuterated olefins. Thus, treatment of [D8]styrene with 2 in
MeOH (entry 17) gave rise to a-deuterated styrene as a result
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b,b’-deuterated (2), a,b,b’-deuterated (8 + HCl), or a-deuter-
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CH3OH).
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hydride complexes are active and selective catalysts for H/D
exchange reactions. The steric constraints exerted by the
bulky IPr ligand control the rotation of the alkyl ligand, which
in turn determines the selectivity. The design of new catalysts
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www.angewandte.org
3941

Communications
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has been observed in related rhodium–hydride complexes with a
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Angew. Chem. Int. Ed. 2011, 50, 3938 –3942