Mechanism for reduction catalysis by metal oxo: Hydrosilation of organic carbonyl groups catalyzed by a rhenium(V) oxo complex

Article abstract of DOI:10.1021/ja055704t

The rhenium oxo complex [Re(O)(hoz)₂][TFPB], 1 (where hoz = 2-(2′-hydroxyphenyl)-2-oxazoline(-) and TFPB = tetrakis(pentafluorophenyl)borate) catalyzes the hydrosilation of aldehydes and ketones under ambient temperature and atmosphere. The major organic product is the protected alcohol as silyl ether. Isolated yields range from 86 to 57%. The reaction requires low catalyst loading (0.1 mol %) and proceeds smoothly in CH₂Cl₂ as well as neat without solvent. In the latter condition, the catalyst precipitates at the end of reaction, allowing easy separation and catalyst recycling. Re(O)(hoz)(H), 3, was prepared, and its involvement in an ionic hydrosilation mechanism was evaluated. Complex 3 was found to be less hydridic than Et₃SiH, refuting its participation in catalysis. A viable mechanism that is consistent with experimental findings, rate measurements, and kinetic isotope effects (Et₃SiH/Et₃SiD = 1.3 and benzaldehyde-H/benzaldehyde-D = 1.0) is proposed. Organosilane is activated via η²-coordination to rhenium, and the organic carbonyl adds across the coordinated Si-H bond [2 + 2] to afford the organic reduction product. Copyright

Full text of DOI:10.1021/ja055704t

Published on Web 10/13/2005
Mechanism for Reduction Catalysis by Metal Oxo: Hydrosilation of Organic
Carbonyl Groups Catalyzed by a Rhenium(V) Oxo Complex
Elon A. Ison, Evan R. Trivedi, Rex A. Corbin, and Mahdi M. Abu-Omar*
Brown Laboratory, Department of Chemistry, Purdue UniVersity, 560 OVal DriVe, West Lafayette, Indiana 47907
Received August 19, 2005; E-mail: mabuomar@purdue.edu
Table 1. Hydrosilation of Organic Carbonyl Compounds Catalyzed
by Oxorhenium 1a
High oxidation state oxo complexes are widely used in catalytic
oxidations and oxygen atom transfer reactions.1 However, the use
of such complexes in catalytic reductions is extremely rare.2 Few
reports have appeared recently describing rhenium(V), rhenium-
(VII), and molybdenum(VI) oxo catalysts for the hydrosilation of
aldehydes and ketones.3 In their mechanistic interpretation of the
rhenium dioxo system, Toste and co-workers proposed Si-H
addition across the RedO multiple bond to give a hydrido-
oxorhenium(V) complex.3a The additional oxo ligand was hypoth-
esized to stabilize the resulting oxo-hydrido, compensating for the
loss of π donation from one of the oxo ligands (eq 1). However,
several mechanistic questions remain, including the role of the
additional oxo ligand and if indeed a dioxo complex is needed.4
Herein, we report a new system for the hydrosilation of aldehydes
and ketones using a monooxorhenium(V) catalyst. The highlights
of our system are the following: (1) it offers mechanistic insight
on activation of organosilanes by oxometalates; (2) the reaction
proceeds efficiently under ambient temperature and atmosphere with
low catalyst loading (0.1 mol %); and (3) the reaction may be
performed without a solvent, wherein the catalyst precipitates at
the end of reaction. This last feature is very attractive because it
limits the amount of organic waste generated and facilitates catalyst
separation.
a Reaction conditions: 0.1 mol % of 1, substrate ) 8-10 mmol, and
1.5 equiv of Et3SiH. b Isolated yields. c NMR yields.
We have reported recently on the hydrolytic oxidation of
organosilanes using the catalyst [Re(O)(hoz)2][TFPB] (hoz ) 2-(2′-
hydroxyphenyl)-2-oxazoline(-), TFPB ) tetrakis(pentafluorophe-
nyl)borate), 1.5 In the presence of an aldehyde or ketone, reduction
of the carbonyl group occurs, yielding the protected alcohol as silyl
ether, eq 2.
The accepted mechanism for metal-catalyzed hydrosilation of
organic carbonyl compounds involves insertion of the carbonyl
group into a metal hydride bond, which is formed by either oxidative
addition (late metals) or σ-bond metathesis (early metals).7 Toste
proposed a new mechanism in which silane adds across a RedO
bond (eq 1).3a However, we have shown through 18O labeling studies
that addition of Si-H across the RedO bond is an unlikely
pathway.5 Moreover, the resulting siloxyrhenium(V) hydride would
not be sufficiently hydridic to reduce organic carbonyl groups,
which is discussed next.8
Another mechanism that has been documented in recent years
by Bullock is ionic hydrogenation. In this mechanism, the proposed
metal hydride complex acts as a hydride donor to an activated
carbonyl compound, bypassing the insertion step.9 To test the
viability of an ionic hydrogenation mechanism, we prepared the
rhenium hydride complex, Re(O)(hoz)2H (3), from the reaction of
Re(O)(hoz)2Cl, 2, with Bu3SnH.6 Complex 3 did not react with
benzaldehyde. However, 3 converted to 1 in the presence of the
silylium adduct of benzaldehyde, which was prepared according
to the methodology of Lambert et al.,10 affording the silyl ether
(eq 3). Furthermore, under steady-state conditions, the experimental
rate law for hydrosilation exhibits first-order dependencies on [1]
and [Et3SiH] and no dependence on [benzaldehyde]: d[PhC-
Representative yields for the reduction of different substrates
are shown in Table 1.6 The reaction may be performed with or
without a solvent. Furthermore, for the solvent-free reaction, the
catalyst precipitates out of solution as the polarity of the solvent
changes from the reactants (aldehyde or ketone and Et3SiH) to the
silyl ether product. The catalyst retains activity after being recycled
(entry 4). Even though preparative reactions were run with Et3-
SiH, other organosilanes, such as PhMe2SiH, work quite well. For
example, NMR scale reaction of acetaphenone with 1.5 equiv of
PhMe2SiH and 5 mol % of 1 in CD3CN afforded PhC(H)(OSiMe2-
Ph)(CH3) in quantitative yields.
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C O M M U N I C A T I O N S
(OSiEt3)H2]/dt ) k[Et3SiH][1].6 Therefore, the reaction described
by eq 3 would involve two steady-state intermediates, and their
reaction must be facile enough to account for the observed rate
law.
We have presented herein a new mechanism for metal-oxo-
catalyzed hydrosilation of organic carbonyl compounds. Like
Bullock’s ionic hydrogenation mechanism, prior coordination of
the aldehyde/ketone and insertion are not required. However,
formation of a metal hydride is not necessary either as the resulting
oxorhenium hydride is not sufficiently hydridic. The featured
catalyst exhibits high activity in comparison to that of other metal
oxo hydrosilation catalysts.3c Further studies on the synthetic scope
of this system and its utility in asymmetric hydrogenation are
ongoing in our laboratory.
To probe the hydricity of 3, we studied its reaction with [Ph3C]-
[B(C6F5)4] (eq 4). The kinetics of hydride abstraction was deter-
mined by stopped-flow following the disappearance of the trityl
cation at 450 nm. Bullock and co-workers have used the reaction
of metal hydrides with trityl cation to establish a hydricity scale
for a series of transition metal carbonyl hydrides.11 To check the
reliability of our stopped-flow kinetics, we reproduced the kinetics
Acknowledgment. Acknowledgment is made to Purdue Uni-
versity and NSF for financial support.
Supporting Information Available: Experimental details and
kinetic data. This material is available free of charge via the Internet
at http://pubs.acs.org.
for the reaction of Et3SiH and [Ph3C][B(C6F5)4] (kEt SiH ) 170 ( 8
3
M-1 s-1), which is in excellent agreement with that observed
previously (150 M-1 s-1).11 In comparison, the rate constant
observed for complex 3 is approximately 70 times less than that
for Et3SiH (kRe-H ) 2.40 ( 0.05 M-1 s-1).12 This positions 3 among
the least hydridic of all the transition metal carbonyl hydrides
studied by Bullock.11 The kinetic data suggest that the ionic
hydrosilation mechanism cannot account for the observed catalysis.
Under steady-state conditions, any silylium benzaldehyde adduct
generated is more likely to react with Et3SiH than complex 3
because (1) Et3SiH is present in higher concentration, and (2) Et3-
SiH is significantly more hydridic than 3. Thus, if silane and 1
produce silylium and complex 3, rhenium would act as a mere
initiator and [Et3Si+] would be the true catalyst. This mechanism,
however, is refuted as it has been shown that the product distribution
for [Et3Si+]-catalyzed hydrosilation reactions is significantly dif-
ferent from what we observed for our system.13 The rhenium
hydride 3 can be reactivated for catalysis by reaction with
[Et3Si+][B(C6F5)4-] to afford Et3SiH and 1. Additionally, a siloxy-
rhenium(V) hydride, produced as a steady-state intermediate from
the addition of silane across the RedO bond, would be less hydridic
than complex 3 due to the absence of π-bonding from a terminal
oxo, providing further evidence against such a mechanism.8
References
(1) (a) Sheldon, R. A.; Kochi, J. Metal-Catalyzed Oxidations of Organic
Compounds; Academic Press: New York, 1981. (b) Nugent, W. A.;
Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley: New York, 1988.
(c) Handbook of Reagents for Organic Synthesis: Oxidizing and Reducing
Agents; Burke, S. D., Danheiser, R. L., Eds.; John Wiley & Sons: New
York, 1999.
(2) Recent examples of metal-catalyzed hydrosilation: (a) Yun, J.; Buchwald,
S. L. J. Am. Chem. Soc. 1999, 121, 5640 (Ti catalyst). (b) Reyes, C.;
Prock, A.; Giering, W. P. Organometallics 2002, 21, 546 (Rh/BINAP
catalyst). (c) For a review on hydrosilation, see: Ojima, I.; Li, Z.; Zhu,
J. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apelog,
Y., Eds.; John Wiley & Sons: New York, 1998.
(3) (a) ReV(O)2 catalyst: Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H.
P.; Toste, F. D. J. Am. Chem. Soc. 2003, 125, 4056. (b) Mo(O)2Cl2
catalyst: Fernandes, A. C.; Fernandes, R.; Romao, C. C.; Royo, B. Chem.
Commun. 2005, 213. (c) Rhenium(V) and (VII) oxo catalysts: Royo, B.;
Roma˜o, C. C. J. Mol. Catal. A: Chem. 2005, 236, 107. Except for Re2O7,
oxorhenium catalysts reported require heating at 80 °C and 5 mol %
catalyst loading. (d) ReV(O) catalyst for reduction of imines: Nolin, K.
A.; Ahn, R. W.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 12462.
(4) Thiel, W. R. Angew. Chem., Int. Ed. 2003, 42, 5390.
(5) Ison, E. A.; Corbin, R. A.; Abu-Omar, M. M. J. Am. Chem. Soc. 2005,
127, 11938. Et3SiH reacts slowly with 1 to give Re(III). [ReIII(hoz)2(CH3-
CN)n]+ does not catalyze the hydrosilation reaction.
(6) See Supporting Information for details.
(7) (a) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16. (b) Chaloner,
P. A.; Esteruelas, M. A.; Jou, F.; Oro, L. A. Homogeneous Hydrogenation;
Kluwer Academic: Boston, 1994.
(8) High valent rhenium polyhydrides, such as (PPh3)2ReH7, fail to catalyze
the hydrosilation reaction and are stable in boiling water (i.e., not hydridic).
See ref 3a.
(9) (a) Bullock, R. M.; Voges, M. H. J. Am. Chem. Soc. 2000, 122, 12594.
(b) Voges, M. H.; Bullock, R. M. J. Chem. Soc., Dalton Trans. 2002,
759. (c) Bullock, R. M. Chem.sEur. J. 2004, 10, 2366.
(10) (a) Lambert, J. B.; Zhang, S. Z.; Ciro, S. M. Organometallics 1994, 13,
2430. (b) Lambert, J. B.; Kania, L.; Schilf, W.; Mcconnell, J. A.
Organometallics 1991, 10, 2578.
(11) Cheng, T. Y.; Brunschwig, B. S.; Bullock, R. M. J. Am. Chem. Soc. 1998,
120, 13121.
(12) Despite using dried CH2Cl2, plots of kψ versus [3] are linear with an
intercept, k2 ) 1.1 × 10-2 s-1. See Supporting Information. This is
attributed to negligible side decomposition of the trityl cation in the
stopped-flow analyzer due to small adventitious moisture. This intercept
is too small to be precisely discernible for Et3SiH because its slope is
significantly larger than k2 (see Figures S4 and S5).
By process of elimination, the most viable mechanism would
involve organosilane activation through formation of η2-Et3SiH
complex (Scheme 1), with the rate-determining step (RDS) being
formation of the organosilane Re adduct.14,15 The observed kinetic
isotope effects from NMR kinetics (Et3SiH/Et3SiD ) 1.3 and
benzaldehyde-H/benzaldehyde-D ) 1.0) are also consistent with
the proposed mechanism.14
Scheme 1
(13) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090.
The silylium aldehyde adduct reacts with Et3SiH to give (Et3Si)2O and
the benzyl carbocation, which abstracts a hydride from Et3SiH to give
ethylbenzene and regenerate the silylium catalyst. For our system, we do
not detect any ethylbenzene in the NMR.
(14) Oxidative addition of Si-H to afford [(hoz)2ReVII(O)(SiEt3)(H)]+ followed
by carbonyl group addition in a [3 + 2] mechanism is also possible.
However, the observed KIE is closer in value to that observed for σ-bond
metathesis. See, for example: Sadow, A. D., Tilley, T. D. J. Am. Chem.
Soc. 2005, 127, 643.
(15) For η2-silane complexes, see: Crabtree, R. H. Angew. Chem., Int. Ed.
Engl. 1993, 32, 789.
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