Z-selective, catalytic internal alkyne semihydrogenation under H 2/CO mixtures by a niobium(III) imido complex

Article abstract of DOI:10.1021/ja206016s

The discovery of a Nb(III)-mediated catalytic hydrogenation of internal alkynes to (Z)-alkenes that proceeds through an unprecedented mechanism is reported. The mechanistic proposal involves initial reduction of the alkyne by the Nb(III) complex (BDI)Nb(N^tBu)(CO)₂ to provide a Nb(V) metallacyclopropene, itself capable of σ-bond metathesis reactivity with H₂. The resulting alkenyl hydride species then undergoes reductive elimination to provide the (Z)-alkene product and regenerate a metal complex in the Nb(III) oxidation state. Support for the proposed mechanism is derived from (i) the dependence of the product selectivity on the relative concentrations of CO and H₂, (ii) the isolation of complexes closely related to those proposed to be part of the catalytic cycle, (iii) H/D crossover experiments, and (iv) DFT studies of multiple possible reaction pathways.

Full text of DOI:10.1021/ja206016s

COMMUNICATION
pubs.acs.org/JACS
Z-Selective, Catalytic Internal Alkyne Semihydrogenation under H2/
CO Mixtures by a Niobium(III) Imido Complex
Thomas L. Gianetti, Neil C. Tomson, John Arnold,* and Robert G. Bergman*
Department of Chemistry, University of California, Berkeley, California 94720, United States
S Supporting Information
b
Scheme 1. Monohydride Mechanism (Pathway A) and
Dihydride Mechanism (Pathways B and C)
ABSTRACT: The discovery of a Nb(III)-mediated cataly-
tic hydrogenation of internal alkynes to (Z)-alkenes that
proceeds through an unprecedented mechanism is reported.
The mechanistic proposal involves initial reduction of the
t
alkyne by the Nb(III) complex (BDI)Nb(N Bu)(CO) to
2
provide a Nb(V) metallacyclopropene, itself capable of σ-
bond metathesis reactivity with H . The resulting alkenyl
2
hydride species then undergoes reductive elimination to
provide the (Z)-alkene product and regenerate a metal
complex in the Nb(III) oxidation state. Support for the
proposed mechanism is derived from (i) the dependence of
the product selectivity on the relative concentrations of CO
and H , (ii) the isolation of complexes closely related to
2
those proposed to be part of the catalytic cycle, (iii) H/D
crossover experiments, and (iv) DFT studies of multiple
possible reaction pathways.
Scheme 2. Hydrogenation of 1-Phenyl-1-propyne with 1
(
0.2 equiv) under a H /CO Atmosphere (1:12 Ratio)
2
omogeneous hydrogenation reactions catalyzed by transi-
H
tion-metal complexes are some of the most extensively
studied in organometallic chemistry. Many of these systems
operate via one of two common mechanisms, namely, the
1
monohydride and dihydride mechanisms shown in Scheme 1.
imido-catalyzed reaction has not been reported. We recently
showed that treatment of the Nb(III) dicarbonyl complex
The development of new catalysts that can access alternative
hydrogenation pathways constitutes an attractive area of research
for discovering reagents capable of providing control over
selectivity and substrate scope. The selective conversion of
alkynes to (Z)-alkenes, typically accomplished by the hetero-
t
(
BDI)Nb(N Bu)(CO) (1) with 1-phenyl-1-propyne does not
2
result in alkyneꢀalkyne coupling but instead yields the metalla-
t
2
cyclopropene complex (BDI)Nb(N Bu)(η -MeCtCPh)(CO)
9
(
2). Acidification of 2 in methanol leads to the formation of
2
(Z/E)-β-methylstyrenealongwith the starting alkyne in a 2:1ratio.
The formation of β-methylstyrene focused our attention on the
potential use of 1 as a hydrogenation catalyst.
geneous Lindlar’s catalyst, is difficult to achieve because of E/Z
1
ꢀ3
isomerization and overhydrogenation.
effective molecular catalysts for this transformation remains an
The development of
1,4
Catalytic hydrogenation of 1-phenyl-1-propyne to (Z)-β-
area of intense study with recent notable successes. Here we
report a mechanistic investigation of a selective semihydrogena-
tion reaction catalyzed by a d transition metal under a mixture of
methylstyrene (2 h, 1.0 equiv of H , 75% yield) was achieved
2
2
in the presence of 1 (20 mol %) and an excess of CO (12 equiv)
in benzene at room temperature (Scheme 2). Increasing the H2
loading to 3.0 equiv led to a marginal increase in the yield of
H and CO. Since most of the H produced industrially must be
2
2
separated from CO, the lack of hydroformylation reactivity of the
catalyst presented herein constitutes an interesting step in
designing effective hydrogenation catalysts that are able to
(
Z)-β-methylstyrene (85%) and produced trace amounts of
n
n-propylbenzene ( PrPh, 3%). Higher CO loadings (40 equiv)
led to a drop in catalytic activity but no decrease in selectivity,
providing (Z)-β-methylstyrene in 45% yield after 2 h. Previ-
5
function in the presence of unpurified syngas.
2
Many low-valent early transition metals, such as d metal
ous reports of both CO exchange on 1 and the isolation of
complexes, are known to oxidatively couple alkynes to form
t
3,6
the tris(isocyanide)Nb(III) complex (BDI)Nb(N Bu)(CNXyl)
3
metallacyclopentadiene species. Because of this reactivity, the
9b
2
(Xyl = 2,6-Me C H ) suggest that higher CO loadings result
2 6 3
use of d transition-metal complexes as hydrogenation catalysts is
7
2
8
rare. Only one d system, reported by Boncella and co-workers,
Received: June 28, 2011
Published: August 19, 2011
2
r 2011 American Chemical Society
14904
dx.doi.org/10.1021/ja206016s
|J. Am. Chem. Soc. 2011, 133, 14904–14907

Journal of the American Chemical Society
COMMUNICATION
Scheme 3. Role of CO To Maintain High Reactivity and
Selectivity
Scheme 4. Synthesis of Complexes 3a and 3b
in the formation of the electronically and coordinatively satu-
t
rated tricarbonyl complex (BDI)Nb(N Bu)(CO) .
3
Decreasing the CO loading also resulted in lower yields of (Z)-
β-methylstyrene (∼12%) but concurrently led to the formation
n
of both PrPh (∼16%) and allylbenzene (∼6%). Under these
1
conditions, H NMR monitoring of the reaction mixture indi-
cated that the catalytic activity stopped after 2 h, at which time
the metal complex had been converted into a new, catalytically
Figure 1. Molecular structures of 3a and 3b determined by single-
crystal X-ray diffraction. H atoms have been omitted for clarity; the
thermal ellipsoids are at the 50% probability level.
inactive species, complex 4-d [see the Supporting Information
6
1
2
(SI)]. Analysis using H and H NMR spectroscopy led to the
assignment of 4-d as a C D -coordinated complex (Scheme 3),
6
6 6
based in part on the presence of a characteristic singlet at 3.7
but the high ν
for both complexes [νCO(free CO) =
CO
2
6
ꢀ
1
ppm in the H NMR spectrum. A related η -arene Mo complex
2
143 cm ] suggests little π back-bonding from the metal into
8
was observed by Boncella and co-workers. Over the course of
the π* orbitals of CO, consistent with a high oxidation state at the
metal center. Complexes 2 and 3a are two of the few examples in
the reaction leading to 4-d , the intermediate complex 5 was also
6
0
observed. The latter species displayed three resonances with a
which CO is coordinated to a formally d group 5 transition-
11
1
:2:2 ratio between 3.6 and 4.1 ppm. This pattern is consistent
metalcomplex. Substitution of the CO on 2 by the more strongly
t
with a monosubstituted arene ligand bound to the metal center; 5
σ-donating ligand BuNC yielded the thermally stable complex
t
2
t
was therefore assigned as a catalystꢀproduct π complex
(
BDI)Nb(N Bu)(η -PhCtCMe)(CN Bu) (3b; Scheme 4) in
6
(
Scheme 3), similar to 4-d . The presence of this η -arene
6
52% yield. Similar to complex 3a, weak-to-nonexistent π back-
donation from the metal to the isocyanide in 3b was indicated by
intermediate in hydrogenation mechanisms is not unusual and
8,10
has been observed with several catalysts.
Separate experi-
ꢀ1
t
IR spectroscopy [ν (3a) = 2167 cm , ν (free BuCN) =
CN
CN
ments showed that treatment of 1 with (Z)-β-methylstyrene
provided no observable reaction; thus, the excess CO required
for efficient catalysis may be needed to drive catalyst turnover via
displacement of the product alkene from the catalystꢀproduct π
complex.
ꢀ1
2
125 cm ]. The crystal structures of 3a and 3b exhibit distorted
12
square-pyramidal geometries (Figure 1; τ = 0.32 , τ = 0.33).
3a
3b
In both structures, the C ꢀC bond lengths of the metallacyclo-
1
2
propene unit show significant elongation from those of the
uncoordinated alkyne to values consistent with CꢀC double
bonds [C ꢀC (3a), 1.308(4) Å; C ꢀC (3b), 1.309(3) Å; C-
Monitoring the course of the most-selective and highest-
1
2
1
2
yielding catalytic reaction (12:1 CO/H ) by NMR spectroscopy
2
2
13
2
(
[
(
sp )ꢀC(sp ), 1.31ꢀ1.34 Å].
The NbꢀC distances
allowed us to observe that the metallacyclopropene complex 2
maintained a constant concentration, suggesting that this com-
plex is the resting state of the catalytic cycle. This hypothesis was
supported by a half-turnover experiment wherein treatment of 2
NbꢀC (3a), 2.144(3) Å; NbꢀC (3a), 2.175(3) Å; NbꢀC -
1
2
1
3b), 2.144(2) Å; NbꢀC (3b), 2.143(2) Å] are within the range of
2
14
Nb(V)ꢀC(alkyl) bonds reported previously. Because complexes
3
a and 3b both exhibit considerable metallacyclopropeneꢀNb(V)
with 1.0 equiv of alkyne and 0.5 equiv of H gave 0.5 equiv of (Z)-
2
character, the oxidative addition of H to such formally oxidized
2
β-methylstyrene, 0.5 equiv of alkyne, and 1.0 equiv of 2. We note
that the formation of 2 by treatment of 1 with 1-phenyl-1-
propyne was rapid within the concentration range used during
catalysis and that complex 2 was the only transition-metal-
containing species observed in solution during the course of
the reaction.
complexes (2, 3a, 3b) is unlikely.
Performing the hydrogenation reaction with mixtures of H2
and D under the conditions found to produce (Z)-β-methyl-
2
styrene exclusively (12:1 CO/H ) revealed that only the d and
2
0
d isotopomers were formed, irrespective of the H /D ratio
2
2
2
employed (see the SI). This finding is consistent with a mechan-
ism involving the hydrogenation of one molecule of alkyne by
Obtaining structural information on 2 was hampered by its
thermal instability, but closely related analogues were isolable.
Treatment of 1 with 1,2-diphenylacetylene yielded the metalla-
one molecule of H . This contrasts with many early-transition-
2
metal hydrogenation mechanisms (Scheme 1, pathway A), in
which two separate molecules of H formally provide one H atom
a piece when reducing an unsaturated hydrocarbon by one bond
order. The mechanisms for late-metal-mediated hydrogenation
t
2
cyclopropene complex (BDI)Nb(N Bu)(η -PhCtCPh)(CO)
3a) in 71% yield (Scheme 4). The higher ν absorption
2
(
CO
ꢀ
1
frequency for 3a (2052 cm ) relative to that for 2
ꢀ
1
(2039 cm ) correlates with the π acidity of the two alkynes,
reactions typically involve oxidative addition of H to the metal
2
1
4905
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Journal of the American Chemical Society
COMMUNICATION
Scheme 5. Mechanistic Hypotheses
Figure 2. Enthalpy profile for the hydrogenation mechanism.
center (Scheme 1, pathways B and C). While 1 appears to serve
as a catalyst for the formation of HD from mixtures of H and D2
2
in the absence of an alkyne substrate, no HD was observed during
the course of the hydrogenation reaction under the conditions
for selective alkyne reduction described above. A mechanism
involving initial H oxidative addition to the metal center is
2
15
therefore deemed unlikely under the catalytic conditions. Thus,
both the metal hydride mechanism common to early-metal
hydrogenation catalysts and the oxidative addition mechanisms
common to late-metal catalysts are unlikely in this case.
We propose two possible mechanisms, both of which are
consistent with our data (Scheme 5). In the first pathway
Figure 3. Calculated transition states for the rate-limiting steps of the
two proposed mechanisms.
(Scheme 5, pathway 1), the alkyne substrate oxidizes the metal
center of complex 1 to give the Nb(V) metallacyclopropene
complex 2. Subsequent σ-bond metathesis of one of the NbꢀC
the rate-determining step (rds) of pathway 1, TS-3a-1, which
corresponds to σ-bond metathesis of the NbꢀC
bond
(
α-CH3)
bonds with H forms alkenyl Nb(V) hydride complex A.
2
with H , was favored by ∼30 kcal/mol over the rds of pathway 2,
2
Reductive elimination of the alkene generates the catalystꢀpro-
duct adduct 5, in which external CO may replace the alkene to
regenerate 1. The second proposed mechanism (Scheme 5,
pathway 2) involves the same initial addition of the alkyne to 1
TS-3c, itself comprising the [1,2] addition of H across the
2
t
NbdN Bu bond (Figures 2 and 3). Alternative pathways were
considered, including a five-membered-ring transition state (TS)
formed by interaction of the alkyne and the imido group as well
as a means of generating 2, but H addition in this case occurs
2
as a TS for σ-bond metathesis with the NbꢀC
bond, but
t
(α-Ph)
across the NbdN Bu bond to form amido niobium hydride
they were all found to lie significantly higher in energy than
TS-3a-1 (see the SI). The CO moiety, which during TS-3a-1 was
displaced to a trans position relative to the alkyne to give I-4, was
found to relax to a basal position (I-5) prior to CꢀH reductive
elimination. The proximity of CO and the hydride led to a
nonproductive CO insertion pathway to generate a formyl
alkenyl complex (I-8b). However, the small difference in the
calculated energies of I-5 and I-8a/b suggest that formation of
the latter may be reversible and difficult to detect. Whether CꢀC
reductive elimination from I-8a/b is kinetically or thermodyna-
mically disfavored is not currently known, but we found no
experimental evidence for hydroformylation products under the
studied conditions. Finally, the calculations suggest that the
productꢀmetal adduct I-7 and a hypothetical four-coordinate
4
b
complex B. Insertion of the coordinated alkyne into the newly
formed MꢀH bond yields amino alkenyl complex C, from which
[
1,2]-α-NH elimination results in the same catalystꢀproduct
adduct (complex 5) as described for pathway 1. We currently
favor pathway 1 on the basis of our computational investigations
(
see below) as well as the following precedents: (i) The Nb(V)
t
dimethyl complex (BDI)Nb(N Bu)Me was observed to react
2
rapidly and cleanly with H in THF to give a product whose
2
9b
identity strongly suggested a Nb(III) intermediate. (ii) Reac-
tion-site-selectivity studies on related neutral and cationic alkyl
Nb(V) and alkyl Ta(V) imido complexes indicated a preference
for both polar and nonpolar substrates to undergo reaction at the
16
alkyl group as opposed to the imido group.
0
Density functional theory (DFT) calculations were performed
to probe the potential energy surface describing the interaction
monocarbonyl Nb(III) complex (BDI )Nb(NMe)(CO) (I-9s;
18
S = 0 ) are essentially equienergetic (see the SI).
0
0
of (BDI )Nb(NMe)(CO) (I-1; BDI = HC[C(Me)NPh] )
In summary, we have reported the efficient and selec-
tive catalytic semihydrogenation of 1-phenyl-1-propyne into
2
2
1
7
with MeCtCPh and H (see the SI). At this level of theory,
2
1
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Journal of the American Chemical Society
COMMUNICATION
2
0
(
Z)-β-methylstyrene by a d niobium complex under H /CO
(9) BDI is N,N -bis(2,6-diisopropylphenyl)-β-diketiminate. See: (a)
Tomson, N. C.; Arnold, J.; Bergman, R. G. J. Am. Chem. Soc. 2008,
1
2
mixtures. The experimental data are supported by DFT calcula-
tions, which suggest a novel mechanism for the hydrogenation
reaction that involves coordination of an alkyne to form a
metallacyclopropene Nb(V) complex followed by σ-bond meta-
30, 11262. (b) Tomson, N. C.; Arnold, J.; Bergman, R. G. Organome-
tallics 2010, 29, 5010.
10) (a) Giernoth, R.; Heinrich, H.; Adams, N. J.; Deeth, R. J.;
Bargon, J.; Brown, J. M. J. Am. Chem. Soc. 2000, 122, 12381. (b)
Giernoth, R.; Huebler, P.; Bargon, J. Angew. Chem., Int. Ed. 1998,
(
thesis with H and subsequent reductive elimination to yield
2
the product (Z)-alkene. An excess of CO is required for catalyst
stability and proposed to function as a means of displacing the
product alkene from a Nb(III) intermediate for achieving catalyst
turnover. We are currently performing further synthetic, mech-
anistic, and kinetic studies in order to support our preliminary
results on the mechanism and intermediates involved in this
reaction.
37, 2473.
(
11) (a) Burckhardt, U.; Tilley, T. D. J. Am. Chem. Soc. 1999,
21, 6328. (b) Sabo-Etienne, S.; Rodriguez, V.; Donnadieu, B.; Chau-
dret, B.; el Makarim, H. A.; Barthelat, J. C.; Ulrich, S.; Limbach, H. H.;
Moise, C. New J. Chem. 2001, 25, 55. (c) La Pierre, H. S.; Arnold, J.;
Bergman, R. G.; Toste, F. D. J. Am. Chem. Soc., submitted. (d) An
1
2
alternative description of these complexes would be that of a d metal
center with both the CO and the alkyne as strong π-accepting ligands.
However, a complex with this latter electronic structure would likely
exhibit both shorter CꢀC bonds and more linear RꢀCꢀC (R = Me, Ph)
’
ASSOCIATED CONTENT
angles for the alkyne as well as a lower ν and the presence of a strong
CO
S
Supporting Information. Experimental procedures, ana-
b
π-bonding interaction between CO and the metal center in the occupied
molecular orbitals of the complex I-2a as calculated by DFT (see the SI).
(12) As determined using the continuous symmetry parameter τ =
(α ꢀ β)/60°, where α and β are the largest and second-largest angles
about the metal center, respectively, See: Addison, A. W.; Rao, T. N.;
Reedijk, J.; van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans.
lytical data, NMR spectra, crystal data, CIF files for 3a and 3b,
and DFT methods and results. This material is available free of
charge via the Internet at http://pubs.acs.org.
’
AUTHOR INFORMATION
1
984, 1349.
Corresponding Author
arnold@berkeley.edu; rbergman@berkeley.edu
(13) Modern Physical Organic Chemistry; Anslyn, E. V., Dougherty,
D. A., Eds.; University Science Books: Mill Valley, CA, 2006; p 22.
14) Tomson, N. C.; Arnold, J.; Bergman, R. G. Organometallics
010, 29, 2926.
15) In addition to this result, DFT calculations showed that
dihydride intermediate I-10 is +27.5 kcal/mol from dicarbonyl complex
and thus +3.8 kcal/mol from the σ-bond metathesis TS (see the SI).
16) Tomson, N. C.; Arnold, J.; Bergman, R. G. Dalton Trans. 2011,
40, 7718 .
(17) All structures were fully optimized with Gaussian 09 using the
(
2
(
’
ACKNOWLEDGMENT
We thank the NSF (CHE-0848931 and CHE-0841786) for
1
financial support; Drs. A. DiPasquale, C. Canlas, and J. Krinsky
for experimental assistance; and Dr. G. Nocton and H. S. La
Pierre for helpful discussions. T.L.G. is grateful to the UCB
Department of Chemistry for the Howard W. Crandall
Fellowship.
(
B3LYP hybrid functional. The LANL2DZ basis set was used for the
metal center, and the 6-31G* basis set with a 5d diffusional was used for
the H, C, N, and O atoms. All optimized geometries were compared
using their zero-point energies. For computational expediency, the aryl
groups of the BDI ligand were replaced with phenyl groups and the tBu
imido ligand was replaced by a methylimido group.
’
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(
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(
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2
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1
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