An efficient catalyst based on manganese salen for hydrosilylation of carbonyl compounds

Article abstract of DOI:10.1021/om400805v

A manganese salen complex is shown to be an effective (pre)catalyst in the hydrosilylation of aldehydes and ketones. The present system features an earth-abundant and inexpensive base metal, low catalyst loading, and functional group tolerance. Mechanistic studies suggest that the reaction likely proceeds through a reduced manganese(III) hydride species that undergoes electrophilic attack by the carbonyl substrates.

Full text of DOI:10.1021/om400805v

Communication
pubs.acs.org/Organometallics
An Efficient Catalyst Based on Manganese Salen for Hydrosilylation
of Carbonyl Compounds
Vamshi K. Chidara and Guodong Du*
Department of Chemistry, University of North Dakota, 151 Cornell Street Stop 9024, Grand Forks, North Dakota 58202, United
States
S
* Supporting Information
ABSTRACT: A manganese salen complex is shown to be an effective (pre)catalyst in
the hydrosilylation of aldehydes and ketones. The present system features an earth-
abundant and inexpensive base metal, low catalyst loading, and functional group
tolerance. Mechanistic studies suggest that the reaction likely proceeds through a
reduced manganese(III) hydride species that undergoes electrophilic attack by the
carbonyl substrates.
ecently, the search for catalysts based on earth-abundant,
inexpensive, and nontoxic metals has seen rapid growth,
reduction reactions.21−23 Although the mechanistic pathways
may vary, catalytic reduction by high-valent transition-metal
complexes has been extended to include ruthenium(VI)24 and
vanadium(V)25 complexes. We became interested in seeing if
high-valent manganese compounds could catalyze such
reductions, given that manganese is abundant and inexpensive
and high-valent manganese compounds are as readily available
as other neighboring metals in groups 6−8.26
R
partially prompted by the rising cost of the commonly used
precious metals in catalysis and concerns over their toxicity in
the final products.1 In particular, catalysis based on iron, one of
the most abundant and readily available metals,2 has been
extremely appealing.3 For example, a number of efficient iron-
based catalysts have been reported for the reduction of carbonyl
compounds by hydrosilanes, or hydrosilylation, an important
transformation carried out conventionally with precious
metals.4 The hydrosilylation reaction is atom economical,
generating silyl ether protected alcohols in a single step, and
can be employed as a convenient alternative to hydrogenation
due to the mild nature and ease of handling of liquid
hydrosilanes.5 Therefore, a wide variety of efficient catalysts
have appeared.6 In addition to iron, notable examples of
reduction catalysts based on non-precious metals include
titanium,7 nickel,8 copper,9 zinc,10 molybdenum and tung-
sten,11 calcium,12 and aluminum.13 Curiously, manganese, with
the third largest global reserve (behind only iron and copper) at
∼630 million metric tons,2 has received much less attention in
these developments. It should be mentioned that manganese
plays an essential role in biological systems, as it is involved in a
number of important oxidation reactions such as oxygen
evolution.14 Perhaps because of this, research on Mn has
focused on its application in oxidation reactions such as alkene
epoxidation.15 However, its application in catalytic reduction
has been much less studied. Only a few low-valent Mn(I)
complexes bearing carbonyl ligands16 and a Mn(III) complex17
have been reported to catalyze hydrosilylation,18 but they offer
little opportunity for ligand modification and fine tuning.
Our interest in Mn-catalyzed reduction originates from the
report that a high-valent compound (ReO2I(PPh3)2) of
rhenium(V), a heavier congener of Mn, catalyzes the
hydrosilylation of aldehydes and ketones.19 This opens up a
new venue for reduction catalysis,20 and various rhenium and
molybdenum complexes bearing multiply bonded oxo or imido
groups have been shown to be effective in a number of
We focused on MnV, which is isoelectronic with MoIV, ReV,
and RuVI, featuring a diamagnetic d2 electron configuration.
[MnN(salen-3,5-tBu2)] (1)27 (Figure 1) was chosen as a model
Figure 1. MnN(salen-3,5-tBu2) (1).
complex on the consideration that it is easily prepared and
soluble in common organic solvents. The catalytic activity of 1
has been examined using benzaldehyde and acetophenone as
representative substrates under various reaction conditions
(Table 1). Initial screening was carried out at ambient
temperature without exclusion of air in a sealed NMR tube.
Reduction of PhCOMe by PhSiH3 was clearly observed;
however, the reaction was marred by a somewhat capricious
induction period. When the reaction was run under nitrogen
and at elevated temperature, the reduction went smoothly.
When PhCHO was employed, an induction period, albeit much
shorter (∼10−20 min), was still notable (see the Supporting
Information). The reaction seemed to proceed similarly in
either polar (CH3CN) or nonpolar (benzene) solvents and
could be conducted under neat conditions without solvent. The
Received: August 9, 2013
Published: September 4, 2013
© 2013 American Chemical Society
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Organometallics
Communication
a
Table 1. Hydrosilylation of Benzaldehyde and Acetophenone
b
entry
substrate
silane (amt (equiv))
solvent
temp (°C)
time (h)
conversn (%)
1
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCOMe
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (0.5)
PhSiH3 (0.5)
PhSiH3 (0.33)
Ph2SiH2 (0.5)
(EtO)3SiH (1.0)
Et3SiH (1.0)
PhSiH3 (0.5)
PhSiH3 (0.5)
C6D6
room temp
47
>9
>97
>97
>97
>97
95
2
C6D6
80
0.33
22
3
CDCl3
CDCl3
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
neat
room temp
4
60
80
80
80
80
80
80
80
1.0
0.33
3.3
3.3
2.5
48
5
6
7
>97
>97
NR
70
8
9
10
11
5.0
CD3CN
2.0
>97
a
b
1
Reaction conditions: substrate (0.4−1.0 mmol), silane, and MnN catalyst (0.5 mol %), under N2. Determined by H NMR on the basis of the
consumption of PhCHO and PhCOMe.
catalyst loading at the 0.5 mol % level works well for both
PhCHO and acetophenone.
achieving full conversion (entry 6). It should be noted that
reduction of an α,β-unsaturated ketone, chalcone (entry 14),
leads to the isolation of 1,3-diphenylpropan-1-one, apparently
via a silyl enol ether intermediate formed by 1,4-addition of
silane.
We started the runs with excess PhSiH3 (1.5 equiv vs
carbonyl substrate) but noted that even with 1.0 equiv there
was still unreacted PhSiH3 after the complete consumption of
PhCHO. A 0.5 equiv amount of PhSiH3 seems to be sufficient
for the reduction (entry 4). Further decrease in PhSiH3
revealed that even 0.33 equiv of PhSiH3 was able to reduce
most of PhCHO (entry 6), at longer reaction times. Obviously,
all of the three Si−H bonds in PhSiH3 are being used for
reduction. These observations prompted us to investigate other
hydrosilanes. Indeed, a secondary silane, Ph2SiH2, and a tertiary
silane, (EtO)3SiH, are both effective in the reaction (entries 7
and 8), and the reduction was complete in less than 4 h for
PhCHO. As expected, they are less reactive than the primary
silane. However, when Et3SiH was employed, no hydro-
silylation was detected after 48 h (entry 9), though a color
change of the reaction mixture can still be observed.
Product analysis of the crude reaction mixture resulting from
PhSiH3 showed that the dialkoxysilanes were the major
products. A small amount of the corresponding alcohols and
mono- and trialkoxysilanes were also detected, on the basis of
the NMR and GC-MS analysis (see the Supporting
Information). With a longer reaction time or higher temper-
ature, the portion of trialkoxysilanes can be prevalent. In one
experiment, the reduction of acetophenone was allowed to run
for a prolonged period of time and the trialkoxysilane
(PhCH(Me)O)3SiPh was isolated as the major product (84%).
Having established the activity of MnN catalyst in hydro-
silylation, we chose 0.5 equiv of PhSiH3 as the reducing reagent
with 0.5 mol % of 1 as catalyst and further investigated the
scope and possible limitations of the present system. A number
of aliphatic and aryl carbonyl compounds, including a series of
benzaldehyde and acetophenone derivatives (Table 2), were
readily reduced under standard conditions. The conversions
were generally complete within 20 min for aldehydes and less
than 3 h for ketones, and high yields of corresponding alcohols
were obtained after workup. Functional groups such as halides,
nitro, and methoxy were tolerated. Cyclopropyl phenyl ketone
was reduced cleanly without formation of ring-opening
products (entry 10). Benzaldehydes with electron-withdrawing
groups at para positions were more readily reduced than those
with electron-donating groups. Particularly, it is observed that
reduction of p-NO2 benzaldehyde was extremely fast, even
without heating. In comparison, the reduction of p-NO2
acetophenone was very sluggish, taking up to 2 days without
Various reaction pathways have been proposed for the high-
valent transition-metal-based reduction catalysts, which may
feature silane activation via 3 + 2 addition toward the metal−
oxo bond or η2-silane σ-adduct20,28 and carbonyl activation by
the Lewis acidic metal center.29 Low-valent rhenium may also
be responsible for hydrosilylation with the oxorhenium(V)
catalysts.30 We are thus interested in gaining more insight into
the reaction mechanisms of the present system. The absence of
any ring-opening product in the reduction of cyclopropyl
phenyl ketone suggests that a radical mechanism is unlikely.31
No reaction was noted between PhCHO and 1, while the
stoichiometric reaction between 1 and PhSiH3 shows a
broadening of the NMR signal, accompanied by a color change
from the green of 1 to reddish brown and then yellow, parallel
to the observation under catalytic conditions. Moreover, a
broad singlet peak between −1 and −3 ppm can be detected,
with its position shifting upfield up to −2.8 ppm over time (see
Figure S2 in the Supporting Information). The exact nature of
such species is unclear at the moment; we tentatively assign it
to a Mn−H species or a Mn−SiH adduct that is apparently in
equilibrium with silane.32 Its appearance seems to correlate
with the induction period: the reduction typically occurs after
the color changed to yellow.
The effect of electronic factor on the rate of reduction was
explored by competition kinetics with a series of p-X-
C6H4CHO derivatives (X = NO2, COOMe, Cl, H, Me,
OMe) using the tertiary silane (EtO)3SiH as the reductant. The
Hammett plot (Figure 2) thus obtained shows a positive slope
(ρ = 0.46), in agreement with the observation that electron-
withdrawing groups accelerate the reaction in the p-X-
C6H4CHO series.
On the basis of these findings, we postulate that the
catalytically active species is reduced Mn, possibly a Mn(III)
hydride (or a silane adduct), that undergoes electrophilic attack
by the carbonyl substrates (Scheme 1). The ρ value is similar to
that observed for a low-valent Re-catalyzed hydrosilylation in
which a Re hydride is identified as the key intermediate.31
However, the nature of the active catalyst has not been
identified, and a few questions remain to be addressed.
Undoubtedly, more specific studies are required to reveal the
mechanistic details.
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Communication
Table 2. Hydrosilylation of Carbonyl Compounds Catalyzed
a
by 1
Figure 2. Hammett plot for benzaldehyde derivatives.
Scheme 1. A Possible Reaction Mechanism
ASSOCIATED CONTENT
* Supporting Information
Text, tables, and figures giving experimental details and
characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*G.D.: tel, +1-701-777-2241; fax, +1-701-777-2331; e-mail,
gdu@chem.und.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
a
This work was supported in part by the ND EPSCoR through
NSF Grant No. EPS-0814442 and the University of North
Dakota.
Reaction conditions: 0.40−1.0 mmol of substrate, 0.5 equiv of
PhSiH3, catalyst 1 (0.5 mol %), under N2, in heated acetonitrile (∼80
b
°C). The conversions are based on NMR integration; isolated yields
c
are in parentheses. Mostly (PhCH(C3H5)O)2SiHPh on the basis of
d
1H NMR. A small amount of 1-indene is also detected after workup.
REFERENCES
■
e
f
Isolated yield of trialkoxysilane PhSi(OCy)3. Yield of isolated 1,3-
(1) (a) Bullock, R. M., Ed. Catalysis without Precious Metals; Wiley-
VCH: Weinheim, Germany, 2010. (b) Chirik, P. J.; Wieghardt, K.
Science 2010, 327, 794−5.
diphenylpropan-1-one. Other products, 1,3-diphenylprop-2-en-1-ol
and 1,3-diphenylprop-2-ene, are observed in small amounts.
(2) Ritter, S. K. Chem. Eng. News 2012, 90, 12−18.
(3) (a) Gopalaiah, K. Chem. Rev. 2013, 113, 3248−3296.
(b) Mancheno, O. G. Angew. Chem., Int. Ed. 2011, 50, 2216−2218.
(c) Sun, C. -L.; Li, B. -J.; Shi, Z. -J. Chem. Rev. 2011, 111, 1293−1314.
(d) Buchwald, S. L.; Bolm, C. Angew. Chem., Int. Ed. 2009, 48, 5586−
5587. (e) Sherry, B. D.; Furstner, A. Acc. Chem. Res. 2008, 41, 1500−
1511. (f) Tondreau, A. M.; Atienza, C. C. H.; Weller, K. J.; Nye, S. A.;
Lewis, K. M.; Delis, G. P.; Chirik, P. J. Science 2012, 335, 567−570.
(g) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816−5817.
(4) (a) Yang, J.; Tilley, T. D. Angew. Chem., Int. Ed. 2010, 49, 10186−
10188. (b) Warratz, S.; Postigo, L.; Royo, B. Organometallics 2013, 32,
893−897. (c) Tondreau, A. M.; Lobkovsky, E.; Chirik, P. J. Org. Lett.
2008, 10, 2789−2792. (d) Langlotz, B. K.; Wadepohl, H.; Gade, L. H.
Angew. Chem., Int. Ed. 2008, 47, 4670−4674. (e) Shaikh, N. S.;
In summary, we have described a manganese−salen-based
system that is effective for hydrosilylation of carbonyl substrates
at low catalyst loadings (0.5 mol %) and tolerates functionalities
such as halide, cyclopropyl, and nitro groups. Remarkably, the
present catalyst features an earth-abundant and inexpensive
first-row transition metal and a salen ligand framework that is
readily tunable. Studies focused on the development of this
type of Mn-based reduction catalysts, particularly for
asymmetric reductions, and an understanding of the mecha-
nism are underway.
5036
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Organometallics
Communication
Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47,
2497−2501.
Omar, M. M. J. Am. Chem. Soc. 2007, 129, 5180−5187. (c) de
Noronha, R. G.; Romao, C. C.; Fernandes, A. C. J. Org. Chem. 2009,
74, 6960−6964. (d) Cabrita, I.; Fernandes, A. C. Tetrahedron 2011, 67,
8183−8186. (e) Nolin, K. A.; Ahn, R. W.; Toste, F. D. J. Am. Chem.
Soc. 2005, 127, 12462−12463. (f) Nolin, K. A.; Ahn, R. W.; Kobayashi,
Y.; Kennedy-Smith, J. J.; Toste, F. D. Chem. Eur. J. 2010, 16, 9555−
9562.
(24) (a) Abbina, S.; Bian, S.; Oian, C.; Du, G. ACS Catal. 2013, 4,
678−684. (b) Truong, T. V.; Kastl, E. A.; Du, G. Tetrahedron Lett.
2011, 52, 1670−1672.
(25) (a) Cabrita, I.; Sousa, S. C. A.; Fernandes, A. C. Tetrahedron
Lett. 2010, 51, 6132−6135. (b) Pehlivan, L.; Metay, E.; Laval, S.;
Dayoub, W.; Delbrayelle, D.; Mignani, G.; Lemaire, M. Eur. J. Org.
Chem. 2011, 7400−7406.
(26) (a) Berry, J. F. Comments Inorg. Chem. 2009, 30, 28−66.
(b) Eikey, R. A.; Abu-Omar, M. M. Coord. Chem. Rev. 2003, 243, 83−
124.
(5) (a) Marciniec, B., Ed. Hydrosilylation: A Comprehensive Review on
Recent Advances; Springer: Berlin, 2010. (b) Ojima, I.; Li, Z.; Zhu, J. In
The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig,
Y., Eds.; Wiley: New York, 1998; Vol. 2, pp 1687−1792. (c) de Vries,
J. G., Elsevier, C. J., Eds. The Handbook of Homogeneous Hydrogenation;
Wiley-VCH: Weinheim, Germany, 2007; Vols. 1−3.
(6) (a) Chakraborty, S.; Guan, H. Dalton Trans. 2010, 39, 7427−
7439. (b) Riant, O.; Mostefai, N.; Courmarcel, J. Synthesis 2004, 18,
2943−2958. (c) Carpentier, J. F.; Bette, V. Curr. Org. Chem. 2002, 6,
913−936.
(7) Yun, J.; Buchwald, S. J. J. Am. Chem. Soc. 1999, 121, 5640−5644.
(8) (a) Zheng, J.; Darcel, C.; Sortais, J.-B. Catal. Sci. Technol. 2013, 3,
81−84. (b) Tran, B. L.; Pink, M.; Mindiola, D. J. Organometallics 2009,
28, 2234−2243.
(9) Diez-Gonzalez, S.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 349−
358.
(27) Yiu, S. M.; Lam, W. W. Y.; Ho, C. M.; Lau, T. C. J. Am. Chem.
Soc. 2007, 129, 803−809.
(10) (a) Das, S.; Wendt, B.; Moller, K.; Junge, K.; Beller, M. Angew.
Chem., Int. Ed. 2012, 51, 1662−1666. (b) Das, S.; Addis, D.; Zhou, S.;
Junge, K.; Beller, M. J. Am. Chem. Soc. 2010, 132, 1770−1771.
(11) Bullock, R. M. Chem. Eur. J. 2004, 10, 2366−2374.
(12) Spielmann, J.; Harder, S. Eur. J. Inorg. Chem. 2008, 1480−1486.
(13) Koller, J.; Bergman, R. G. Organometallics 2012, 31, 2530−2533.
(14) (a) Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.;
Iwata, S. Science 2004, 303, 1831−1838. (b) Wu, A. J.; Penner-Hahn, J.
E.; Pecoraro, V. L. Chem. Rev. 2004, 104, 903−938.
(28) For computational studies on the mechanisms, see: (a) Chung,
L. W.; Lee, H. G.; Lin, Z.; Wu, Y.-D. J. Org. Chem. 2006, 71, 6000−
6009. (b) Calhorda, M. J.; Costa, P. J. Dalton Trans. 2009, 8155−8161.
(c) Drees, M.; Strassner, T. Inorg. Chem. 2007, 46, 10850−10859.
(d) Costa, P. J.; Romao, C. C.; Fernandes, A. C.; Royo, B.; Reis, P. M.;
Calhorda, M. J. Chem. Eur. J. 2007, 13, 3934−3941. (e) Gu, P.; Wang,
W.; Wang, Y.; Wei, H. Organometallics 2013, 32, 47−51.
(29) (a) Shirobokov, O. G.; Kuzmina, L. G.; Nikonov, G. I. J. Am.
Chem. Soc. 2011, 133, 6487−6489. (b) Khalimon, A. Y.; Ignatov, S. K.;
Simionescu, R.; Kuzmina, L. G.; Howard, J. A.; Nikonov, G. I. Inorg.
Chem. 2012, 51, 754−756. (c) Khalimon, A. Y.; Shirobokov, O. G.;
Peterson, E.; Simionescu, R.; Kuzmina, L. G.; Howard, J. A.; Nikonov,
G. I. Inorg. Chem. 2012, 51, 4300−4313.
(15) Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J.
Am. Chem. Soc. 1991, 113, 7063−7064.
(16) (a) Gregg, B. T.; Hanna, P. K.; Crawford, E. J.; Cutler, A. R. J.
Am. Chem. Soc. 1991, 113, 384−385. (b) Mao, Z.; Gregg, B. T.; Cutler,
A. R. J. Am. Chem. Soc. 1995, 117, 10139−10140. (c) Gregg, B. T.;
Cutler, A. R. J. Am. Chem. Soc. 1996, 118, 10069−84. (d) Hanna, P. K.;
Gregg, B. T.; Cutler, A. R. Organometallics 1991, 10, 31−33. (e) Mao,
Z.; Gregg, B. T.; Cutler, A. R. Organometallics 1998, 17, 1993−2002.
(f) Cavanaugh, M. D.; Gregg, B. T.; Cutler, A. R. Organometallics
1996, 15, 2764−2769. (g) Son, S. U.; Paik, S.-J.; Lee, I. S.; Lee, Y. -A.;
Chung, Y. K.; Seok, W. K.; Lee, H. N. Organometallics 1999, 18,
4114−4118. (h) Son, S. U.; Paik, S.-J.; Chung, Y. K. J. Mol. Catal. A:
Chem. 2000, 151, 87−90. (i) Hilal, H. S.; Suleiman, M. A.; Jondi, W. J.;
Khalaf, S.; Masoud, M. M. J. Mol. Catal. A: Chem. 1999, 144, 47−59.
(17) Magnus, P.; Fielding, M. R. Tetrahedron Lett. 2001, 42, 6633−
6636.
(30) Smeltz, J. L.; Boyle, P. D.; Ison, E. A. Organometallics 2012, 31,
5994−5997.
(31) Yang, D.; Tanner, D. D. J. Org. Chem. 1986, 51, 2267−2270.
(32) (a) Sun, J.; Lu, R. S.; Bau, R.; Yang, G. K. Organometallics 1994,
13, 1317−1325. (b) Schubert, U. Adv. Organomet. Chem. 1990, 30,
151−187.
(18) It should be mentioned that reports of Mn-based hydrosilylation
catalysts have appeared very recently in patent literature. (a) Tondreau,
A. M.; Chirik, P. J.; Delis, J. G. P.; Weller, K. J.; Lewis, K. M.; Nye, S.
A. U.S. Pat. Appl. Publ. US 20120130021 A1 20120524, 2012.
(b) Delis, J. G. P.; Nye, S. A.; Lewis, K. M.; Weller, K. J.; Chirik, P. J.;
Tondreau, A. M.; Russell, S. K. U.S. Pat. Appl. Publ. US 20110009573
A1 20110113, 2011.
(19) Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H. P.; Toste, F.
D. J. Am. Chem. Soc. 2003, 125, 4056−4057.
(20) Thiel, W. R. Angew. Chem., Int. Ed. 2003, 42, 5390−5392.
(21) (a) Sousa, S. C. A.; Cabrita, I.; Fernandes, A. C. Chem. Soc. Rev.
2012, 41, 5641−5653. (b) Du, G.; Abu-Omar, M. M. Curr. Org. Chem.
2008, 12, 1185−1198.
(22) For examples of molybdenum-based catalysts, see: (a) Ziegler, J.
E.; Du, G.; Fanwick, P. E.; Abu-Omar, M. M. Inorg. Chem. 2009, 48,
11290−11296. (b) Peterson, E.; Khalimon, A. Y.; Simionescu, R.;
Kuzmina, L. G.; Howard, J. A. K.; Nikonov, G. I. J. Am. Chem. Soc.
2009, 131, 908−909. (c) Maddani, M. R.; Moorthy, S, K.; Prabhu, K.
R. Tetrahedron 2010, 66, 329−333. (d) Fernandes, A. C.; Romao, C.
C. Tetrahedron Lett. 2005, 46, 8881−8883. (e) Shirobokov, O. G.;
Gorelsky, S. I.; Simionescu, R.; Kuzmina, L. G.; Nikonov, G. I. Chem.
Commun. 2010, 46, 7831−7833. (f) Reis, P. M.; Romao, C. C.; Royo,
B. Dalton Trans. 2006, 1842−1846.
(23) For examples of rhenium-based catalysts, see: (a) Nolin, K. A.;
Krumper, J. R.; Pluth, M. D.; Bergman, R. G.; Toste, F. D. J. Am.
Chem. Soc. 2007, 129, 14684−14696. (b) Du, G.; Fanwick, P. E.; Abu-
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