Reversing the role of the metal - oxygen π-bond. Chemoselective catalytic reductions with a rhenium(V)-dioxo complex

Article abstract of DOI:10.1021/ja029498q

The metal-oxygen bond plays a critical role in some of the most important biological and synthetic reactions. However, the majority of these processes result in the oxidation of the target organic substrate; applications of this class of metal complexes to other organic transformations are extremely rare. In this paper, we report a new type of catalytic process in which complexes with metal-oxygen multiple bonds are used as reductants rather than oxidants. The overall reaction provides a highly chemoselective reduction/protection of carbonyl groups. In addition to providing a new way of catalyzing organic reactions, these catalysts can be used in the presence of a wide range of other functional groups such as amino, cyano, nitro, aryl halo, ester, and alkene; unlike many of their late metal relatives, they are inexpensive as well as air and moisture tolerant. Copyright

Full text of DOI:10.1021/ja029498q

Published on Web 03/15/2003
Reversing the Role of the Metal-Oxygen π-Bond. Chemoselective Catalytic
Reductions with a Rhenium(V)-Dioxo Complex
Joshua J. Kennedy-Smith, Kristine A. Nolin, Haluna P. Gunterman, and F. Dean Toste*
Center for New Directions in Organic Synthesis, Department of Chemistry, UniVersity of California,
Berkeley, California 94720
Received November 27, 2002; E-mail: fdtoste@uclink.berkeley.edu
Transition metal complexes with metal-oxygen multiple bonds
have found numerous applications in organic chemistry as catalysts
for oxidation and oxygen atom transfer reactions.1 Aside from
oxidation reactions, applications of this class of metal complexes
to other organic transformations are extremely rare.2 We became
interested in the possibility that these high oxidation metal
complexes could be used as catalysts for other transformations. For
example, hydride reduction of a carbonyl group followed by
protection of the resulting alcohol as a silyl ether is one of the
most prevalent sequences in organic chemistry. Hydrosilylation of
carbonyl compounds represents a one-step alternative to these
procedures, but it is rarely employed because traditional metal
catalysts suffer from air and moisture sensitivity, functional group
incompatibility, and the requirement to use reactive polyhydric
silanes.3 Use of high oxidation state metal complexes could alleviate
many of these drawbacks.
significantly longer reaction times (18 h) to proceed to completion.
A variety of other silanes, including dimethylphenylsilane (DMPS-
H) and diphenylmethylsilane (DPMS-H), participate equally well
in the rhenium(V)-dioxo catalyzed hydrosilylation.8 Notably, 2 mol
% 1 catalyzes the addition of tert-butyldimethylsilane (TBDMS-
H) to afford the TBDMS-protected alcohol (3d) in 93% yield.
However, 1 showed no reactivity toward the sterically encumbered
tri-iso-propylsilane (TIPS-H). Benzene proved to be the most
general solvent, but in many cases the reactions could be performed
in other common solvents such as THF, dioxane, dichloromethane,
and acetonitrile.
With these conditions in hand, we examined the rhenium-dioxo
catalyzed hydrosilylation of a variety of aldehydes (Table 1). The
catalyst showed excellent reactivity toward a wide range of aromatic
(entries 1-5), heteroaromatic (entry 6), and aliphatic aldehydes
(entries 7 and 8), affording the corresponding DMPS ethers in good
to excellent yields. This catalyst system tolerates a wide range of
functional groups including a Lewis-basic tertiary amine (entry 5).
Furthermore, functional groups that are prone to reduction, such
as aryl nitro (entry 1), aryl halo (entry 2), ester (entry 3), and keto
(entry 4) groups, are left untouched. This is further exemplified by
the chemoselective reduction of aldehyde 4i, in the presence of an
aryl iodide and R,â-unsaturated ester, to afford silyl ether 5i in 68%
yield (entry 9).
The Re-dioxo catalyzed reaction was readily extended to the
hydrosilylation of aromatic and aliphatic ketones (Table 2, entries
10-15). The reduction of ketones required slightly increased
catalyst loadings and longer reaction times, but the corresponding
silyl ethers were isolated in good to excellent yields. The reaction
remains highly chemoselective, tolerating functional groups such
as cyano (entry 4) and ester (entry 2). Notably, no products derived
from cyclopropane ring-opening were detected (1H NMR, GCMS)
in the reduction of cyclopropylphenyl ketone (entry 14). This
observation suggests that the reduction is not proceeding through
a mechanism involving electron transfer.9
Generation of metal hydrides from silanes generally involves
an oxidative addition of the silicon-hydrogen bond to the metal
and therefore requires low-valent metal complexes (eq 1). On the
basis of recent reports,4 we envisioned an alternative mechanism
in which the metal hydride is produced by a [2 + 2]-type addition
of the Si-H bond to a metal ligand π-bond (eq 2). The overall
effect of this process is to convert the strongly oxidizing MdO
complex to a silyloxy-substituted metal hydride which could
potentially be employed as a reductant. A variety of complexes
containing metal-ligand multiple bonds were first considered;
however, we chose to examine metal dioxo complexes in the hope
that the reactivity of the metal-oxygen multiple bond would be
enhanced by the presence of a second oxo ligand (spectator oxo
effect5). As such, we chose to begin our study by investigating
rhenium(V)dioxo complexes as catalysts. Herein we describe an
air- and moisture-tolerant catalytic one-step reduction-protection
of carbonyl groups employing rhenium-oxo complexes as catalysts.
This novel reactivity represents a striking reversal of the redox
properties typical of rhenium-oxo complexes (such as MTO6).
We initiated our investigation by employing the readily available
iododioxo(bistriphenylphosphine)rhenium(V) (1)7 as a catalyst. We
were pleased to find that, at 60 °C in benzene, 5 mol % 1 rapidly
(15 min) catalyzed the addition of triethylsilane to p-anisaldehyde
(2) to afford the corresponding triethylsilyl ether (3a) in 91% yield.
Lowering the catalyst loading to 2 mol % 1 required slightly
increased reaction times (2 h), but had no detrimental effects on
the yield of 3a (eq 3). The reaction temperature could be lowered
to 25 °C, but under these conditions the reaction required
We propose the sequence outlined in Scheme 1 as the likely
mechanism of this new type of reaction.10 The first step involves
a formal [2 + 2]-addition4,11 of silane to the RedO bond in 1 to
produce metal hydride 6. In accord with this hypothesis, the reaction
of excess triethylsilane with 1 produced a new rhenium complex,
which we believe to be 6. This complex shows new 1H NMR signals
at 6.60 (triplet, 1H, Re-H), 0.78 (triplet, 9H, OSi(CH2CH3)3), and
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10.1021/ja029498q CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Re-Dioxo Catalyzed Hydrosilylation of Aldehydes and
Ketones
produced. Furthermore, the use of a metal-dioxo complex as a
catalyst for the reduction of organic functional groups represents a
complete reversal from the traditional role of these complexes as
oxidation catalysts. This novel reactivity suggests that metal-ligand
π-bonds may catalyze other types of σ-bond activation reactions.
Further studies on the mechanism and new applications, including
the enantioselective version, of this reaction are ongoing in our
laboratories.
Acknowledgment. We gratefully acknowledge the University
of California, Berkeley, the Camille and Henry Dreyfus Foundation,
Research Corporation (Research Innovation Award), and Merck
Research Laboratories for financial support. We thank Professors
R. H. Grubbs (Caltech) and R. G. Bergman for many helpful
discussions.
Supporting Information Available: Experimental procedures and
compound characterization data (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) For several examples, see: Handbook of Reagents for Organic Synthe-
sis: Oxidizing and Reducing Agents; Burke, S. D., Danheiser, R. L., Eds.;
John Wiley & Sons: New York, 1999. ComprehensiVe Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon Press: New York, 1991; Vol.
7. See also: Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds;
Wiley: New York, 1988.
(2) For a review of hydrosilylation, 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; Chapter 29, Vol. 2.
Carpentier, J.-F.; Bette, V. Curr. Org. Chem. 2002, 6, 913.
Reaction conditions: aldehydes 2 mol % 1, ketones 5 mol % 1, 1.2
equiv of DMPS-H, 1 M (substrate) in C6H6. a Isolated yield after chroma-
tography. b Accompanied by 24% desilylated alcohol. c >25:1 trans:cis.
d DPMS-H used as silane.
(3) For some examples, see: VdO catalyzed acetylation of alcohols. Chen,
C.-T.; Kuo, J.-H.; Li, C.-H.; Barhate, N. B.; Hon, S.-W.; Li, T.-W.; Chao,
S.-D.; Liu, C.-C.; Li, Y.-C.; Chang, I.-H.; Lin, J.-S.; Liu, C.-J.; Chou,
Y.-C. Org. Lett. 2001, 3, 3739. VdO catalyzed Diels-Alder. Togni, A.
Organometallics 1990, 9, 3106. VdO catalyzed rearrangement-aldol of
propargyl alcohols. Trost, B. M.; Oi, S. J. Am. Chem. Soc. 2001, 123,
1230. RedO catalyzed olefination of aldehydes. Ledford, B. E.; Carreira,
E. M. Tetrahedron Lett. 1997, 38, 8125.
Scheme 1. Mechanistic Proposal for Re-Dioxo Catalyzed
Hydrosilylation
(4) Addition of Si-H to TidS. Sweeney, Z. K.; Polse, J. L.; Andersen, R.
A.; Bergman, R. G.; Kubinec, M. G. J. Am. Chem. Soc. 1997, 119, 4543.
Sweeney, Z. K.; Polse, J. L.; Bergman, R. G.; Andersen, R. A.
Organometallics 1999, 18, 5502. Addition of Si-H to TadN. Gountchev,
T. I.; Tilley, T. D. J. Am. Chem. Soc. 1997, 117, 112831.
(5) Rappe´, A. K.; Goddard, W. A., III. J. Am. Chem. Soc. 1982, 104, 448.
(6) For the reaction of MTO with silanes, see: Tan, H.; Yoshikawa, A.;
Gordon, M. S.; Espenson, J. Organometallics 1999, 18, 4753.
(7) 1 is commercially available; however, we generally prepare it according
-0.20 ppm (quartet, 6H, OSi(CH2CH3)3) that were assigned to the
Re-H and OSi(CH2CH3)3 resonances.12,13 This assignment was
confirmed by reaction with 1 with triethysilyldeuteride (TES-D),
which produced an identical complex lacking the signal at 6.60
ppm in the 1H NMR. Addition of 4-nitrobenzaldehyde to 6 afforded
the corresponding silyl ether (5a). However, under catalytic
conditions, monitoring by 1H NMR showed that a different complex
is present until the end of the reaction. This complex also shows
an upfield signal for the silyl group (-0.5 ppm), but it lacks the
signal for the rhenium hydride. Instead, this complex shows a new
signal at 4.10 ppm (2H, Re-OCH2Ph), which we have assigned to
alkoxy-metal intermediate 7 produced by addition of the rhenium
hydride to the carbonyl group.14 Transfer of the silyl group to the
alkoxy ligand, formally a retro-[2 + 2] reaction, produces the silyl
ether product and regenerates dioxo catalyst 1.15
to
a modification (see Supporting Information) of: Ciani, G. F.;
D’Allfonso, G.; Romtti, P. F.; Sironi, A.; Freni, M. Inorg. Chim. Acta
1983, 72, 29.
(8) Competition experiments using 2 mol % 1 and p-nitrobenzaldehyde
revealed the following order of reactivity: DMPS-H > TES-H >
DPMS-H . TBDMS-H.
(9) Copper(+1) catalyzed hydrosilylation of cyclopropylphenyl ketone pro-
duces ring-opened product by a proposed single electron-transfer mech-
anism. Ito, H.; Yamanaka, H.; Ishizuka, T.; Tateiwa, J.; Hosomi, A. Synlett
2000, 479.
(10) We considered the possibility that 1 was reduced by silane to generate a
low oxidation state rhenium catalyst. However, both (Me2PPh)3ReCl3 and
(PPh3)2ReH7 failed to catalyze the hydrosilylation reaction.
(11) A [3 + 2]-type addition of silane to OdRedO is also possible: Collman,
J. P.; Slaughter, L. M.; Eberspacher, T. A.; Strassner, T.; Brauman, J. I.
Inorg. Chem. 2001, 40, 6272.
(12) OdRe-OSiMe3 has been generated by the reaction of OdRedO with
Me3Si-Cl. In this case, the OSiMe3 signal appears between -0.42 and
-0.54 ppm. Paulo, A.; Domingos, Aˆ .; Garcia, R.; Santos, I. Inorg. Chem.
2000, 39, 5669. Reddy, K. R.; Domingos, Aˆ .; Paulo, A.; Santos, I. Inorg.
Chem. 1999, 38, 4278.
(13) The assignment of the signal at 6.60 ppm as a rhenium hydride is consistent
with other rhenium(V)oxo-hydrides. For example, TpReO(H)Cl shows a
1H NMR signal for the Re-H at 6.85 ppm: Matano, Y.; Brown, S. N.;
Northcutt, T. O.; Mayer, J. M. Organometallics 1998, 17, 2939.
(14) Reaction of a Re(V) oxo-hydride with aldehydes, see ref 13.
(15) Alternatively, intermolecular addition of a silane to 7 produces the silyl
ether and regenerates 6.
In summary, we have developed an air- and moisture-tolerant
hydrosilylation of aldehydes and ketones using a rhenium-dioxo
complex as catalyst. The reaction is compatible with a wide range
of functional groups such as amino, cyano, nitro, aryl halo, ester,
and alkene. As such, this reaction provides an efficient and practical
one-step reduction-protection method in which no byproducts are
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