Modular Attachment of Appended Boron Lewis Acids to a Ruthenium Pincer Catalyst: Metal-Ligand Cooperativity Enables Selective Alkyne Hydrogenation

Article abstract of DOI:10.1021/jacs.6b03972

A new series of bifunctional Ru complexes with pendent Lewis acidic boranes were prepared by late-stage modification of an active hydrogen-transfer catalyst. The appended boranes modulate the reactivity of a metal hydride as well as catalytic hydrogenations. After installing acidic auxiliary groups, the complexes become multifunctional and catalyze the cis-selective hydrogenation of alkynes with higher rates, conversions, and selectivities compared with the unmodified catalyst.

Full text of DOI:10.1021/jacs.6b03972

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Communication
Modular Attachment of Appended Boron Lewis Acids to a Ruthenium Pincer
Catalyst: Metal–Ligand Cooperativity Enables Selective Alkyne Hydrogenation
Kuei-Nin T. Tseng, Jeff W. Kampf, and Nathaniel K. Szymczak
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b03972 • Publication Date (Web): 29 Jul 2016
Downloaded from http://pubs.acs.org on July 29, 2016
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Journal of the American Chemical Society
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Modular Attachment of Appended Boron Lewis Acids to a Ru-
thenium Pincer Catalyst: Metal–Ligand Cooperativity Enables
Selective Alkyne Hydrogenation
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KueiꢀNin T. Tseng, Jeff W. Kampf, and Nathaniel K. Szymczak
Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, United States
Supporting Information Placeholder
9
active catalyst for alkene hydrogenation. We recently found that
ABSTRACT: A new series of bifunctional Ru complexes with
pendent Lewis acidic boranes was prepared by lateꢀstage modifiꢀ
cation of an active hydrogenꢀtransfer catalyst. The appended boꢀ
ranes modulate the reactivity of a metal hydride as well as catalytꢀ
ic hydrogenations. By installing acidic auxiliary groups, the modiꢀ
fied complexes become multifunctional and catalyze the cisꢀ
selective hydrogenation of alkynes with a higher rate, conversion,
and selectivity when compared to the unmodified catalyst.
modifying this ligand framework by replacing ortho –CH with –
OH units prior to metalation enabled distinct catalytic reactivity:
rapid H–E (H and pinacolborane, HBPin) activation and catalytic
3
2
7d
nitrile hydroboration. To further elucidate the changes in reacꢀ
tivity that can be imparted by appended groups, we have targeted
a ligand variant that replaces the Brønsted acidic –OH group(s)
with a boronꢀbased Lewis acid that importantly can be readily
installed post metalation (Figure 1). These appended groups may
be used to bias selectivity for a given catalytic reaction when unꢀ
selective catalysis is observed for an unmodified variant. In this
Communication, we report the development of a new series of
For homogeneous catalysts, the selection and design of appropriꢀ
ate ancillary ligands serves an important role to control both the
bifunctional Ru complexes with appended BR groups via B–H
2
1
bond activation and demonstrate that the Lewis acidity of the
borane influences the reactivity of the Ru hydride and also proꢀ
motes Zꢀselective semiꢀhydrogenation of alkynes.
activity and the selectivity in subsequent catalytic reactions. Altꢀ
hough the steric and electronic properties of the primary coordinaꢀ
tion sphere are most often modified during catalyst optimization,
secondary groups can also play a key role to promote substrate
2
activation. Elaboration of a catalyst’s secondary structure often
requires extensive synthetic redesign prior to metalation, which
limits rapid evaluation of structure/function details. In contrast,
lateꢀstage modification of an already active catalyst can also be
used to install appended groups and it offers several advantages:
(
1) functionalization of the ligand’s secondary coordination
sphere without perturbing the primary coordination environment,
2) methodical variation of the pendent group(s) for precise conꢀ
Figure 1. Conceptual development of lateꢀstage catalyst redesign
to introduce Lewis acidic sites for metal–ligand cooperativity.
(
trol over the steric and electronic properties, and (3) minimal need
to reꢀoptimize metalation conditions to ensure reaction compatiꢀ
bility (e.g. deleterious interꢀligand acid/base interactions).
To evaluate the strategy of installing appended boronꢀbased
Lewis acids within 1, we assessed the reaction with boranes folꢀ
lowing deprotonation. The addition of catecholborane (HBCat) to
8c
Bifunctional transitionꢀmetal complexes have been shown to
a C H solution of [Ru(CH Mepi)PPh ] (2) resulted in the clean
6
6
2
3 2
1
synergistically activate small molecules (e.g., H ) via a metal–
conversion to HRu(CH BCatMepi)PPh3 (3, Figure 2). The H
2
2
3
ligand cooperative pathway. Although such ligandꢀfacilitated
NMR spectrum confirmed the asymmetry of the appended BCat
unit on the pincer ligand and featured a broad peak for the hydride
reactivity has emerged as a prominent reaction theme within cataꢀ
lysts for alkene, ketone, and imine hydrogenation reactions, highꢀ
ly selective and efficient hydrogenation catalysts that employ
10
11
1
ligand at −8.8 ppm, while the B{ H} NMR spectrum exhibited
a broad resonace at 14.6 ppm. The solidꢀstate structure confirmed
4
Lewis acid–metal cooperativity remain underdeveloped. Comꢀ
a pyramidalized boron atom [∑B = 339.3(3)°], and furthermore,
α
plementary to the role that Brønsted acidic groups can serve in
revealed a distorted octahedral geometry around the Ru center
with the phosphorous and oxygen atoms in pseudoꢀaxial positions
[P1–Ru1–O2: 164.83(7)°] and the hydride ligand (located from
the difference map) trans to the isoindolate nitrogen atom (N3).
5
bifunctional activation/transfer, boronꢀbased Lewis acids can also
6
modulate substrate binding, and promote insertionꢀtype reactions.
Our group is working to evaluate how the precise structural,
electronic, and cooperative modes in the secondary coordination
The reaction between 2 and HBPin afforded a distinct product
that incorporated two BPin units. Ru(CBPin Mepi)PPh (4, Figure
5a,7
sphere can be used to regulate reactivity. We recently reported
2
3
an
N,N,NꢀbMepi
(bMepi
=
1,3ꢀbis(6’ꢀmethylꢀ2’ꢀ
2) was isolated by treating 2 with either 2 or 4 equiv of HBPin.
1
pyridylimino)isoindolate) Ru–H complex (1, HRu(bMepi)(PPh ) )
The H NMR spectrum confirmed the presence of two BPin
3
2
11
1
capable of mediating promoterless dehydrogenation of alcohols,
groups, and in contrast to 3, the B{ H} NMR spectrum exhibited
a broad signal at 28.1 ppm, consistent with minimal pyramidalizaꢀ
tion at both boron centers. The Xꢀray crystal structure confirmed
8
amines, and upgrading ethanol to 1ꢀbutanol. In addition to the
hydrogenation and dehydrogenation of polar bonds, 1 is also an
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Journal of the American Chemical Society
Page 2 of 12
that the appended BPin units retain trigonal planar geometries at
B3 and B4 [∑B3 = 359(1)°, ∑B4 = 360(1)°], and also revealed a
markedly different structure than 3; the Ru resides in an octaheꢀ
dral environment with a bis(borylated) carbon atom (C70) cyꢀ
clometalated trans to the isoindolate nitrogen atom (N8).
Complementary to the solidꢀstate characterization, the solution
1
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6
structure of 5 was investigated using variable temperature NMR
α
α
1
spectroscopy. At 25 °C, the H NMR spectrum exhibited broad
signals in the alkyl region and a broad signal at −3.5 ppm, and the
1
1
1
B{ H} NMR spectrum was featureless. However, upon cooling
1
a CD Cl solution of 5 to −80 °C, the H signals sharpened and a
2
2
11
1
broad signal appeared at −8.5 ppm in the B{ H} NMR spectrum,
indicative of a fluxional structure at room temperature. Moreover,
in the H NMR spectrum at −80 °C, the signal at −3.87 ppm apꢀ
1
peared as a wellꢀresolved doublet with a coupling constant of 16.5
Hz that appeared concomitantly with a doublet at 2.06 ppm with
0
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0
the same coupling constant and a T (min) of 211 ms (−40 °C, 500
1
MHz) (Figure S5). This observation is consistent with an agostic
6e
interaction of a geminal –CH group with Ru. Thus, we propose
2
two coordination modes of the 9ꢀBBN motif to the Ru center (Ru–
2
(
η ꢀB–C) and a C–H agostic interaction, Figure 3).
Figure 2. Synthesis and crystal structures (thermal ellipsoids deꢀ
picted at 50% probability) of 3 and 4. H atoms, except the hyꢀ
dride, and PPh phenyl groups are omitted for clarity.
3
The
stronger
boronꢀbased
Lewis
acid
9ꢀ
1
1
borabicyclo[3.3.1]nonane (9ꢀBBN) afforded a distinct product,
complex 5 (Ru(CH9BBNMepi)PPh ) in 78% yield (Figure 3),
3
when using analogous reaction conditions as those to prepare 3.
Figure 3. (Top) Crystal structures (thermal ellipsoids depicted at
50% probability) of 5 and 6. H atoms, except the hydride, and
The Xꢀray crystal structure revealed a distorted octahedral enviꢀ
2
ronment about the Ru center with a rare Ru–(η ꢀB–C) interaction
PPh phenyl groups are omitted for clarity. Select bond distances
3
that may be viewed in one of two limiting resonance forms of a
borataꢀalkene, analogous to the Dewar–Chatt–Duncanson descripꢀ
for 5 (Å): Ru1–C20 2.521(2), B1–C20 1.661(3). (Bottom) Limitꢀ
ing resonance description for 5 and solution equilibrium process.
12
tion of alkene coordination (Figure 3). This unit results from
To interrogate the capabilities of the pendent 9ꢀBBN Lewis acꢀ
id and Ru in 5 to cooperatively promote H–H activation, we evalꢀ
loss of H from the ligand CH (C20) and the B–H unit and repreꢀ
2
2
sents a form of ligandꢀenabled H elimination that is reminiscent
2
1
3
uated the H reactivity in the presence of a πꢀacidic ligand. The
2
of bifunctional complexes developed by Milstein’s group. In
those cases, bifunctional activation is achieved via aromatization–
dearomatization of the pyridine group concomitant with protonaꢀ
tion–deprotonation of the methylene arm. However, in contrast to
aromatization–dearomatization observed in prior cases, we note
retention of aromaticity in the pyridine ring, based on the normal
C=C and C=N bonds as well as the distance between C19–C20
addition of H (15 psig) and CO (15 psig) to a C H solution of 5
2
6
6
yielded a new orange product, HRu(CH 9BBNMepi)(PPh )CO (6,
2
3
−1
Figure 3). The IR spectrum exhibited a ν band at 1935 cm and
a broad Ru–H–B peak at 1820 cm , which falls within the range
of previously reported complexes. In the H NMR spectrum, the
hydride ligand was visualized as a broad doublet at −9.83 ppm
with a JHP of 97.5 Hz, consistent with a hydride ligand trans to a
phosphine ligand. The Xꢀray crystal structure revealed the prodꢀ
CO
−
1
15
1
(1.490(3) Å), which is consistent with a single bond. Thus, by
tuning the Lewis acidity of a pendent borane (BPin < BCat < 9ꢀ
BBN), a cooperative bifunctional H2 release step is enabled,
which also serves to provide a Lewis acid in close proximity to a
metalꢀcoordinated substrate. Although the degree of pyramidalizaꢀ
ucts of H heterolysis: a Ru–H (located from the difference map),
2
3
and a sp CH unit adjacent to the boron. Similar to 3, the Ru–H
2
unit is capped by the appended borane, forming a Ru–H–B bridge.
Furthermore, the boron atom (B1) in 6 is pyramidalized at boron
tion at boron is considerably high [∑B = 339.2(2)°], the Ru1–B1
α
11
[
∑B = 339.2(3)°], consistent with the B NMR resonance at −6.5
α
distance of 2.592(3) Å is longer than the Ru–B distances (2.093–
14
ppm. The structural characterization of 6 is consistent with H2
heterolysis across the metal–ligand framework promoted either by
the basic methanide moiety (C20), which is similar to Milstein’s
2
.176 Å) found in reported Ru–BR complexes, which suggests
3
a weak Ru→B interaction.
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Journal of the American Chemical Society
4
a
bifunctional complexes, or alternatively, with assistance from
of diphenylacetylene to Z-8 was achieved using either 3 or 5.
Selectivities of 86% and 98% were obtained when 3 and 5, reꢀ
3
e
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
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5
5
5
5
5
5
5
6
the pendent boron Lewis acid in concert with the metal.
1
7
spectively, were used instead of 1 (Table 1, entries 5 and 6).
Furthermore, significantly higher conversion (100%) and reaction
The effect of the varied appended borane groups were evaluatꢀ
ed by examining the reactivity of 3–5 toward H (Figure 4). When
a J. Young tube containing a C D solution of 4 and PPh was
charged with 30 psig of H , the immediate formation of 1 (the
only Ruꢀcontaining product) was detected by H and P NMR
spectroscopy. In contrast to the reactivity observed with 4, 1 was
not observed when allowing 3 or 5 to react with H under identiꢀ
cal conditions even after 48 h, consistent with an equilibrium of
formation strongly favoring 3 or 5. Moreover, these results sugꢀ
gest that both Ru–H and η ꢀH adducts with appended BPin
groups are unstable intermediates and the weak Lewis acidic BPin
group cannot stabilize the Ru–H species analogous to 3.
2
−3
rate (4×, see SI) were found when 5 (2.6(3) × 10 Mꢁmin) was
6
6
3
−4
used instead of 1 (6.5(5) × 10 Mꢁmin). Overall, the reaction
profiles displayed by 1 and 5 for alkyne hydrogenation are disꢀ
tinct. Catalyst 5 consumes the alkyne completely prior to subseꢀ
quent olefin hydrogenation that occurs over longer time periods (8
h), while 1 promotes the hydrogenation of both species simultaneꢀ
2
1
31
2
18
ously. Thus, incorporation of an appended Lewis acidic site,
such as 9ꢀBBN, introduces a dramatic bias for three aspects relatꢀ
ed to alkyne hydrogenation: (1) selectivity for a single olefin steꢀ
reoisomer, (2) selectivity for the reduction of alkynes over alꢀ
kenes, and (3) enhanced reaction rate.
2
0
1
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8
9
0
2
Table 1. Alkyne Semi-Hydrogenation Catalyzed by Bifunc-
tional Ruthenium Complexes
T
(°C)
time conversion
selectivity
entry [Ru]
a
Z-8:E-8:9
b
(h)
24
24
24
2
(%)
12
14
0
(%)
c
1
2
3
3
5
23
23
23
80
80
80
80
80
12:0:0
14:0:0
0:0:0
100
100
0
c
c
1
Figure 4. Influence of appended Lewis acids on the reactivity of
of 3 and 4 toward H and CH Cl .
4
1
65
56
100
50
31:18:16
48:7:1
98:2:0
39:10:1
48
86
98
78
52
2
2
2
5
3
2
The reactivity of the Ru–H unit was significantly suppressed
when intramolecularly coordinated to a borane (Figure 4). H/Cl
exchange has been used to evaluate the nucleophilicity of a given
d
6
7
5
2
e
5
2
−
metal hydride, where facile exchange corresponds to a strong H
f
8
10
2
65
34:21:10
6g
donor. When 1 and 1 equiv of CH Cl or CHCl were allowed to
2
2
3
a
1
b
react in C D , Ru(bMepi)(PPh )Cl (7) was immediately formed in
Conversion versus PhSiMe
3
( H NMR). Selectivity deterꢀ
6
6
3
c
d
quantitative yield. In contrast, no H/Cl exchange was observed
when 3 was used under the same conditions, or in the presence of
mined by conversion of Z-8 per total conversion. 24 h. No
change in the presence of Hg. With 10 mol% NEt .
HRu(b Prpi)(PPh
e
3
f
i
excess PPh . 7 was also generated quantitatively when performing
) .
3 2
3
a control experiment using 1, 1 equiv of (9ꢀBBN)CH CH Ph, and
2
2
Table 2. Catalytic Hydrogenation of Terminal Alkynes
either CH Cl or CHCl , which illustrates that the proximity of the
2
2
3
intramolecular pendent BCat unit plays a critical role in regulating
reactivity. Thus, the Lewis acidic properties of the borane moiety,
when appropriately placed in the secondary coordination sphere
has a significant effect on the reactivity of the hydride; the BCat–
hydride (Lewis acid–base) interaction likely reduces the hydricity
of the Ru–H and thus prevents the substitution reaction.
conversion
selectivity
entry
R
a
11:12
b
(%)
(%)
In addition to the stoichiometric H reactivity, we evaluated the
2
1
2
3
Ph
100
100
55
100:0
100:0
31:14
100
100
69
catalytic activity of 3 and 5 for hydrogen transfer. When a J.
C H
6
13
Young tube containing a C D solution of diphenylacetylene and
6
6
1
mol% of 3 or 5 was charged with H (30 psig) at room temperaꢀ
CH N(CH CH )
2 2 3 2
2
ture for 24 h, cisꢀstilbene (Z-8), was formed in 12% and 14%
yields (Table 1, entries 1 and 2). In contrast, no reaction was obꢀ
served when using 1 under identical conditions, even after a week
4
CH CH CH CN
80
80:0
100
2
2
2
a
1
b
Conversion determined versus PhSiMe
( H NMR). Selectiviꢀ
3
(Table 1, entry 3) and in the presence of 1 equiv of (9ꢀ
ty determined by conversion of 11 per total conversion.
BBN)CH CH Ph. These results suggest that bifunctional catalysis
2
2
The semiꢀhydrogenation of aryl and alkyl terminal alkynes also
afforded high conversions to the corresponding alkenes (Table 2,
entries 1–2). The presence of a strongly Lewis basic amine unit
might be accessed when bMepi is functionalized with a Lewis
1
6
acidic borane in close contact with the metal center.
To examine the extent to which the appended borane groups inꢀ
fluence alkyne hydrogenation, we investigated the selectivity and
reaction rate of diphenylacetylene hydrogenation at 80 °C for 2 h.
When the hydrogenation reaction was performed with 1, dipheꢀ
nylacetylene was converted to a mixture of Z-8 (31%), E-8
(
N,Nꢀdiethylpropargylamine; Table 2, entry 3) decreased both the
conversion (55%) and selectivity (69%). However, the alkyne was
selectively hydrogenated in the presence of another reducible
group possessing diminished Lewis basicity. For example, 5ꢀ
hexynenitrile was converted to 5ꢀhexenenitrile in 80% yield with
(18%), and 9 (16%) with low selectivity (48%) for Z-8 (Table 1,
1
00% selectivity (Table 2, entry 4), which suggests compatibility
entry 4). In contrast, high selectivity for the semiꢀhydrogenation
19
(or reversible binding) of nitriles with the 9ꢀBBN motif in 5.
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Journal of the American Chemical Society
Page 4 of 12
In addition to Lewis acidic character of the appended borane
units, they also impose increased steric profiles, compared to a
(2) (a) Cook, S. A.; Borovik, A. S. Acc. Chem. Res. 2015, 48, 2407. (b)
Crabtree, R. H. New J. Chem. 2011, 35, 18. (c) Grotjahn, D. B. Dalton
Trans. 2008, 6497.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CH unit, and the distinct steric environment may alternatively
3
(3) (a) Khusnutdinova, J. R.; Milstein, D. Angew. Chem., Int. Ed. 2015,
determine selectivity. To evaluate whether a similar steric effect
5
4, 12236. (b) Ikariya, T.; Shibasaki, M. Bifunctional Molecular Catalꢀ
influences the preference for a single stereoisomer, alkyne hydroꢀ
ysis; Springer: New York, 2011. (c) Muniz, K. Angew. Chem., Int. Ed.
2005, 44, 6622. (d) Kuzu, I.; Krummenacher, I.; Meyer, J.; Armbruster,
F.; Breher, F. Dalton Trans. 2008, 5836. (e) Li, Y.; Hou, C.; Jiang, J.;
Zhang, Z.; Zhao, C.; Page, A. J.; Ke, Z. ACS Catal. 2016, 6, 1655.
(4) (a) Lin, T.ꢀP.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 15310. (b)
Boone, M. P.; Stephan, D. W. J. Am. Chem. Soc. 2013, 135, 8508. (c)
Harman, W. H.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 5080.
i
genation was examined using HRu(b Prpi)(PPh ) (10), which
3
2
contains isopropyl groups that are more sterically encumbering
around the Ru center than the orthoꢀsubstituents in 1–7. For diꢀ
phenylacetylene hydrogenation, the product distribution and conꢀ
version were strikingly similar to that of 1 (52% selectivity, 65%
conversion, Table 1, entry 8). In addition to this ligand variation,
the Lewis acidic properties of the borane unit in 5 were effectively
quenched by performing catalytic hydrogenation reactions of
(5) (a) Moore, C. M.; Bark, B.; Szymczak, N. K. ACS Catal. 2016, 6,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
981. (b) Fujita, K.ꢀi.; Tanaka, Y.; Kobayashi, M.; Yamaguchi, R. J.
Am. Chem. Soc. 2014, 136, 4829. (c) Wang, W.ꢀH.; Hull, J. F.; Muckꢀ
erman, J. T.; Fujita, E.; Himeda, Y. Energy Environ. Sci. 2012, 5, 7923.
(6) (a) Braunschweig, H.; Dewhurst, R. D. Dalton Trans. 2011, 40,
549. (b) Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 47, 859.
(c) Alcaraz, G.; Grellier, M.; SaboꢀEtienne, S. Acc. Chem. Res. 2009,
42, 1640. (d) Drover, M. W.; Schafer, L. L.; Love, J. A. Angew. Chem.,
Int. Ed. 2016, 55, 3181. (e) Cassen, A.; Gloaguen, Y.; Vendier, L.;
Duhayon, C.; PobladorꢀBahamonde, A.; Raynaud, C.; Clot, E.; Alcaraz,
G.; SaboꢀEtienne, S. Angew. Chem., Int. Ed. 2014, 53, 7569. (f) Barꢀ
nett, B. R.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S. J. Am.
Chem. Soc. 2014, 136, 10262. (g) Ostapowicz, T. G.; Merkens, C.;
Hoelscher, M.; Klankermayer, J.; Leitner, W. J. Am. Chem. Soc. 2013,
diphenylacetylene in the presence of 10 mol% NEt (Table 1,
3
entry 7). Notably lower conversion (50%) and selectivity (78%)
for Z-8 were obtained, which further highlights the role of the
appended Lewis acid to promote high activity and Zꢀselectivity.
Collectively, these experiments provide clear support that the
origin of selective alkyne reduction arises from the acidic characꢀ
ter of the pendent boranes, rather than an increased steric profile.
In conclusion, we have developed a new class of bifunctional
Ru complexes with appended Lewis acidic BR groups. This work
2
demonstrates that the Lewis acidic properties of the boranes in the
secondary coordination environment can be used to modulate the
reactivity of the Ru–H and turn on metal–ligand cooperativity for
hydrogenation catalysis. Of particular note, higher reaction rate,
conversion, and selectivity were noted for the Zꢀselective semiꢀ
hydrogenation of alkynes when using the bifunctional complex 5
appended with the most Lewis acidic borane. Comparison with
the unfunctionalized complexes containing only inert –CH3
groups illustrates the critical roles of the Lewis acids in the secꢀ
ondary coordination sphere to synergistically mediate and regulate
alkyne hydrogenation by (1) facilitating H–H heterolysis, (2) staꢀ
bilizing the hydride intermediate via the formation of a Ru–H–B
bridge, and (3) selectively reducing alkynes over alkenes. Because
installation of the pendent groups occurred at the last step, this
strategy may be exploited as a versatile protocol to access a large
variety of appended functional groups (Lewis acids and bases)
with different steric and electronic properties. Future efforts will
explore the incorporation of pendent acidic and basic groups to
allow further control over the activity and selectivity of metalꢀ
based catalysis and to activate a variety of small molecules.
1
35, 2104. (h) Masuda, Y.; Hasegawa, M.; Yamashita, M.; Nozaki, K.;
Ishida, N.; Murakami, M. J. Am. Chem. Soc. 2013, 135, 7142. (i)
Podiyanachari, S. K.; Froehlich, R.; Daniliuc, C. G.; Petersen, J. L.;
MueckꢀLichtenfeld, C.; Kehr, G.; Erker, G. Angew. Chem., Int. Ed.
2
012, 51, 8830. (j) Hasegawa, M.; Segawa, Y.; Yamashita, M.; Nozaki,
K. Angew. Chem., Int. Ed. 2012, 51, 6956. (k) Buil, M. L.; Cardo, J. J.
F.; Esteruelas, M. A.; Fernandez, I.; Onate, E. Organometallics 2015,
34, 547. (l) Hesp, K. D.; Kannemann, F. O.; Rankin, M. A.; McDonald,
R.; Ferguson, M. J.; Stradiotto, M. Inorg. Chem. 2011, 50, 2431. (m)
Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2008,
1
30, 11874.
(7) (a) Dahl, E. W.; Szymczak, N. K. Angew. Chem., Int. Ed. 2016, 55,
3101. (b) Moore, C. M.; Szymczak, N. K. Chem. Sci. 2015, 6, 3373. (c)
Geri, J. B.; Szymczak, N. K. J. Am. Chem. Soc. 2015, 137, 12808. (d)
Tutusaus, O.; Ni, C.; Szymczak, N. K. J. Am. Chem. Soc. 2013, 135,
3403.
(8) (a) Tseng, K.ꢀN. T.; Rizzi, A. M.; Szymczak, N. K. J. Am. Chem.
Soc. 2013, 135, 16352. (b) Tseng, K.ꢀN. T.; Lin, S.; Kampf, J. W.;
Szymczak, N. K. Chem. Commun. 2016, 52, 2901. (c) Tseng, K.ꢀN. T.;
Kampf, J. W.; Szymczak, N. K. ACS Catal. 2015, 5, 5468.
(9) See SI for alkene hydrogenation.
(10) We note that H–B coupling can be broad and not always observaꢀ
ble by NMR spectroscopy, see reference 6g.
ASSOCIATED CONTENT
Supporting Information
(
(
11) Sivaev, I. B.; Bregadze, V. I. Coord. Chem. Rev. 2014, 270, 75.
12) (a) Emslie, D. J. H.; Cowie, B. E.; Kolpin, K. B. Dalton Trans.
2
012, 41, 1101. (b) Chiu, C.ꢀW.; Gabbai, F. P. Angew. Chem., Int. Ed.
Experimental procedures and characterization data. This material
is available free of charge at http://pubs.acs.org.
2007, 46, 6878. (c) Cook, K. S.; Piers, W. E.; Hayes, P. G.; Parvez, M.
Organometallics 2002, 21, 2422.
(13) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011, 44, 588.
(14) (a) Crossley, I. R.; Foreman, M. R. S. J.; Hill, A. F.; Owen, G. R.;
AUTHOR INFORMATION
White, A. J. P.; Williams, D. J.; Willis, A. C. Organometallics 2008,
27, 381.
Corresponding Author
(
15) Gloaguen, Y.; BenacꢀLestrille, G.; Vendier, L.; Helmstedt, U.;
Clot, E.; Alcaraz, G.; SaboꢀEtienne, S. Organometallics 2013, 32,
868.
(16) Note that PPh
Email: *nszym@umich.edu
4
ACKNOWLEDGMENT
3
dissociation from 1 is fast, see reference 8c. Thus,
the active catalyst coordination environment is analogous to 5.
(17) The most common semiꢀhydrogenation catalyst is Lindlar’s cataꢀ
lyst, which favors the Zꢀisomer. However, this catalyst also promotes
E/Z isomerization over time, and has considerable variability in the acꢀ
tivity and selectivity between batches. For further discussion, see Ulan,
J. G.; Maier, W. F. J. Org. Chem. 1987, 52, 3132.
Acknowledgment is made to the NSFꢀCAREER (CHEꢀ1350877),
the University of Michigan Department of Chemistry, and the
National Science Foundation (CHEꢀ0840456) for Xꢀray instruꢀ
mentation. N.K.S. is an Alfred P. Sloan Research Fellow and
Camille Dreyfus Scholar.
(18) Competition experiments between 1ꢀoctyne and 1ꢀoctene demonꢀ
strated that 3 favored alkyne insertion while 1 showed no preference for
either substrate, see SI for further details.
(19) We note that the reduction of the nitrile group was observed when
the reaction was allowed to continue for longer than 2 h at 80 °C.
REFERENCES
(1) P. W. N. M. van Leeuwen, P, W. N. M., Chadwick, J. C. Homogeꢀ
neous Catalysts; WileyꢀVCH: Weinheim, Germany, 2011.
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