A useful method for preparing iridium alkoxides and a study of their catalytic decomposition by iridium cations: A new mode of β-hydride elimination for coordinativeiy saturated metal alkoxides

Full text of DOI:10.1021/ja980672d

6826
J. Am. Chem. Soc. 1998, 120, 6826-6827
Scheme 1
A Useful Method for Preparing Iridium Alkoxides
and a Study of Their Catalytic Decomposition by
Iridium Cations: A New Mode of â-Hydride
Elimination for Coordinatively Saturated Metal
Alkoxides
Joachim C. M. Ritter and Robert G. Bergman*
Department of Chemistry, UniVersity of California
Berkeley, California 94720
ReceiVed March 2, 1998
ity required the development of effective methods for the synthesis
of such alkoxides, so that their decompositions could be studied
directly. We now report the successful preparation of a series of
coordinatively saturated iridium alkoxides and evidence that their
decomposition to aldehydes and iridium hydrides is in fact
catalyzed by a second iridium center.
Compared with late transition metal alkyls, the corresponding
metal alkoxides are difficult to prepare, especially if C-H bonds
at the carbon connected to the oxygen atom are present.1b
Attempts to obtain these compounds by the displacement of
halides or other good leaving groups from transition metal centers
with alkali metal alkoxides (in analogy to the general method
used to prepare metal alkyls) often lead to the corresponding metal
hydrides. In fact, this occurs so frequently that the treatment of
metal halides with alcoholic base is a classical method for
preparing hydrides.2 It is normally assumed, and in some cases
established,3-7 that aldehydes are produced in these reactions. This
provides evidence that metal alkoxides are intermediates, but
undergo rapid â-H elimination (eq 1, X ) halide, OTf).
In accord with the previous experience summarized above, our
initial attempts to prepare alkoxides of the general formula (η5-
C5Me5)(PMe3)Ir(Ph)(OCH2R) (1) (here abbreviated Ph[Ir]OCH2R)
by metathesis of Ph[Ir]X (X ) OTf (2), Cl (3)) with the
corresponding sodium alkoxides in various solvents were unsuc-
cessful. Instead, apparent â-H elimination products were ob-
served. For example, addition of 1 equiv of NaOEt to a solution
of 3 in EtOH led to quantitative formation of Ph[Ir]H (4) and
1
acetaldehyde (90-95% by H NMR).12 Suspecting that traces
of cationic iridium species present in solution were responsible
for catalyzing the decomposition of Ph[Ir]OEt (1a) to hydride 4,
we turned to a hydroxyl/alkoxyl exchange approach in which the
concentration of Ph[Ir]+ ions is deliberately kept as low as
possible. Treating Ph[Ir]OH (5) with a primary alcohol in THF-
d8 results in an equilibrium mixture containing the starting
LnM(X) + NaOCH2R f LnMOCH2R f LnMH + RCHO (1)
As in the related â-H eliminations of metal alkyls, it is normally
assumed that a site of coordinative unsaturation cis to the alkoxo
ligand is required for this process to occur.3,5,8 Kinetically inert
late transition metal complexes with M-O bonds have been
prepared which lack R-oxy hydrogens, do not have an open
coordination site, or have a sterically disfavored transition state
for â-H elimination,9 but for many systems, coordinatively
saturated alkoxides are difficult to prepare or, once generated,
are relatively unstable kinetically.1,10
In a recent study of apparent ethylene insertion into a metal-
hydroxide bond, we proposed the binuclear complex (η5-C5-
Me5)(PMe3)(Ph)Ir-CH2CH2-O-Ir(Ph)(PMe3)(η5-C5Me5) as a cru-
cial intermediate, and obtained evidence that its decomposition
occurred by â-H elimination catalyzed by a third cationic iridium
center.11 Because this intermediate has an Ir-O bond at a
formally coordinatively saturated iridium center, we considered
the possibility that simpler iridium alkoxides might decompose
by analogous metal-catalyzed mechanisms. Testing this possibil-
materials, the alkoxide Ph[Ir]OCH2R (1), and H2O, with Keq
≈
0.1 (Scheme 1). The mixtures are stable for several days at room
temperature, confirming that uncatalyzed â-H elimination is not
a rapid process in this system. Upon addition of standard drying
agents to remove the water13 and drive the equilibrium toward
alkoxide product, clean decomposition to Ph[Ir]H (4) and aldehyde
is observed.14
HoweVer, if the corresponding sodium salt of the alcohol is
used as a drying agent the corresponding alkoxides can be
isolated in 95-100% yields as yellow crystalline compounds
(Scheme 1; R ) CH3, 1a; R ) CMe3, 1b; R ) CH2CMe3, 1c). In
this acid/conjugate base metathesis approach, hydroxide 5 is
treated with 2 equiv of NaOCH2R and 0.2 equiv of RCH2OH in
pentane.15 Analytically pure material can be obtained by crystal-
lization from toluene/pentane mixtures. All of the alkoxides are
stable at room temperature as neat compounds and in solution.
Their 1H NMR spectra show a characteristic pattern for the
diastereotopic R-methylene protons (C6D6; δ 3.5-4.0 ppm)
Single-crystal X-ray diffraction studies of ethoxide 1a and
neopentoxide 1b confirm the covalent nature of their Ir-O bonds
in the solid state. ORTEP diagrams of both compounds are shown
in Figure 1 and details of the structure determinations are provided
as Supporting Information.
(1) (a) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163. (b) Bergman,
R. G. Polyhedron 1995, 14, 3227. (c) Ros, R.; Michelin, R. A.; Bataillard,
R.; Roulet, R. J. Organomet. Chem. 1978, 161, 75. (d) Bryndza, H. E.; Fong,
L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109,
1444. (e) Bennett, M. A. J. Organomet. Chem. 1986, 300, 7 and references
therein. (f) Yoshida, T.; Otsuka, S. J. Am. Chem. Soc. 1977, 99, 2134. (g)
Diamond, S. E.; Mares, F. J. Organomet. Chem. 1977, 142, C55. (h) Milstein,
D.; Calabrese, J. C. J. Am. Chem. Soc. 1982, 104, 3773. (i) Arnold, D. P.;
Bennett, M. A. Inorg. Chem. 1984, 23, 2110. (j) Fernandez, M. J.; Esteruelas,
M. A.; Covarubias, M.; Oro, L. A. J. Organomet. Chem. 1986, 316, 343.
(2) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987; p 90.
To account for the clean formation of 1a-c during the acid/
conjugate base metathesis, we assume that under these basic and
(3) Blum, O.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 4582.
(4) Bryndza, H. E.; Kretchmar S. A.; Tulip, T. H. J. Chem. Soc., Chem.
Commun. 1985, 977.
(12) In the case of [Ir]Cl2 the formation of H[Ir]OEt is observed: Newman,
L. J.; Bergman, R. G. J. Am. Chem. Soc. 1985, 107, 5314.
(13) We employed 4 Å sieves, Al2O3, MgSO4, NaSO4, and azeotropic
distillation.
(14) In contrast Ph[Ir]OPh is formed quantitatively from a 1:1 mixture of
5 and PhOH when mixed in THF, indicating that electron-withdrawing
substituents attached to the oxygen thermodynamically stabilize the M-OPh
complex relative to the hydroxide: see ref 20.
(15) Minimal amounts of alcohol and the use of pentane as a solvent in
which NaOCH2R and NaOH are insoluble are essential to avoid hydride
formation (the iridium hydroxide and alkoxide are both soluble in the pentane/
alcohol mixtures). This contrasts with the synthesis of some platinum alkoxides
where an alcohol/benzene solution of the alkoxide was used for metathesis:
see ref 5.
(5) Bryndza, H. E.; Calabrese, J. C.; Marsi, M.; Roe, D. C.; Tam, W.;
Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 4805.
(6) Bernard, K. A.; Rees, W. M.; Atwood, J. D. Organometallics 1986, 5,
390.
(7) Hoffman, D. M.; Lappas, D.; Wierda, D. A. J. Am. Chem. Soc. 1989,
111, 1531.
(8) Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7010.
(9) See ref 3 above and papers cited.
(10) Glueck, D. S.; Newman-Winslow, L. J.; Bergman, R. G. Organome-
tallics 1991, 10, 1462.
(11) Ritter, J. C. M.; Bergman, R. G. J. Am. Chem. Soc. 1997, 119, 2580.
S0002-7863(98)00672-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/27/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 27, 1998 6827
For example, when a mixture of 1 equiv of EtOH, 1 equiv of
i-PrOH, and 1 equiv of Ph[Ir]OH (5) in benzene or THF is treated
with 0.05 equiv of iridium triflate 2 at room temperature,
quantitative oxidation to acetaldehyde is observed by NMR within
minutes, whereas the amount of i-PrOH is unchanged (in a
separate experiment, no reaction was observed between Ph[Ir]-
OH and excess i-PrOH by NMR). During the reaction Ph[Ir]-
OEt (1a) is observed as an intermediate, with no evidence for
the formation of either Ph[Ir]Oi-Pr or acetone. The catalyst can
easily be deactivated by addition of PPh3.11 The hydride transfer
most likely proceeds via a hydride shift (Scheme 2) or by a SET
mechanism.22 In either case we expect that the C-H bond is
broken as part of the rate determining transition state.
Figure 1. ORTEP diagrams of Ph[Ir]OEt (1a) (top) and Ph[Ir]OCH2-
CMe3 (1b) (bottom).
To gain additional information on the exchange processes
occurring in solution during the hydride transfer reaction, we
conducted double crossover experiments. Addition of 0.5-1
equiv of Ph Ir OTf (2′)20 ( Ir ) (C5Me4Et)Ir(PMe3)) to a solution
of Ph[Ir]OCH2CH2CMe3 (1c) in C6D6 results in equilibration to
give substantial amounts of Ph[Ir]OTf (2) and Ph Ir OCH2CH2-
CMe3 (1c′). The equilibrium was established immediately (t <
1 min, Keq ≈ 1) at room temperature (RT), prior to the appearance
of any hydride products 4 and 4′. Subsequent decomposition to
Ph[Ir]H (4) and Ph Ir H (4′) was complete after 1 h. In addition,
rapid scrambling was observed upon mixing a solution of 1c with
a solution of 1b′ under exclusion of air and moisture without
further reaction at RT. Both experiments show that bond-breaking
and -forming processes between the iridium center and the
alkoxide oxygen are much faster than the H abstraction.18
Comparison of the relative kinetic stability of the alkoxides
Ph[Ir]OCH2R (R ) Me, t-Bu, neopentyl) in the presence of 0.1
equiv Ph[Ir]OTf (2) indicates that the barrier for hydrogen transfer
to the iridium cation is affected mostly by steric factors. Whereas
quantitative decomposition of 1a (R ) Me) occurs immediately
at ambient temperatures, decomposition of 1c (R ) neopentyl)
under the same conditions requires 4 h. The sterically hindered
alkoxide 1b (R ) t-Bu) is stable at RT when treated with 0.1
equiv of 2 but decomposes over the course of 12 h when heated
at 75 °C.
In summary, we have synthesized a series of primary iridium
alkoxides by utilizing a novel acid/conjugate base metathesis
approach and demonstrated that in the case of coordinatively
saturated alkoxides R[Ir]OCH2R (1), decomposition to give the
corresponding iridium hydrides and aldehydes is catalyzed by
iridium cations. This report provides the first direct evidence
that â-H elimination can involve the cooperative participation of
two metal centers. It seems likely that other examples of apparent
mononuclear â-elimination may also proceed by this more
complicated binuclear mechanism. This novel catalytic process
can be used to selectively oxidize primary alcohols to aldehydes
in the presence of secondary alcohols. Further investigation of
the reactivity of these alkoxides is under way.
Scheme 2
relatively nonpolar conditions the formation of iridium cations is
suppressed.16 The unfavorable equilibrium is presumably driven
by a surface reaction of NaOCH2R with the H2O formed to
produce insoluble NaOH and the corresponding alcohol (Scheme
1). Since no overall consumption of alcohol takes place, only
catalytic amounts of RCH2OH are necessary to achieve quantita-
tive conversion. We believe that hydrogen bonding from the
alcohol to the strongly basic oxygen in hydroxide 5 is important
for the facilitation of the metathesis. In the absence of RCH2-
OH the above transformation is very slow or does not occur at
all. Hydrogen bonding in late transition metal alkoxides has been
found to be important for alkoxide exchange reactions.10,17,18
Treating a solution of Ph[Ir]OCH2R (1) with a catalytic amount
of Ph[Ir]OTf (2)19 known to be a source of the corresponding
cation,20 leads to quantitative formation of Ph[Ir]H (4) and
aldehyde (Scheme 2). Dissolved in benzene or THF in the
absence of 2, the alkoxides require temperatures above 60 °C to
undergo slow decomposition to a mixture of â-H elimination and
other unidentified products. We believe that under conditions
where ionization of Ph[Ir]X (X ) OH, OTf, OR, Cl) occurs,
catalytic hydride transfer from the alkoxide 1 to the cationic
species 2 initiates the formal â-H elimination (Scheme 2). This
catalytic cycle constitutes a rare example of bimetallic catalysis
involving cooperative participation of two iridium centers.21
This novel hydride transfer reaction can be used to selectively
oxidize primary alcohols in the presence of secondary alcohols.
Acknowledgment. We acknowledge the National Institutes of Health
(Grant No. GM-25459) for generous financial support of this work.
J.C.M.R. acknowledges support through a NATO Postdoctoral Fellowship
administered through the Deutscher Akademischer Austauschdienst
(DAAD). We would also like to thank Dr. F. J. Hollander and Dr. R. E.
Powers for determination of crystal structures of 1a and 1b.
(16) Alternatively, 1b and 1c can be isolated, only as oils (95% purity),
from mixtures of Ph[Ir]OTf and the corresponding sodium alkoxides in THF/
1-hexene 1:2 (the 1-hexene is added to keep the concentration of iridium
cations as low as possible by complexing to the alkene) or by removing the
solvent (THF/(Me3Si)2O) and water from mixtures of 5 and RCH2OH under
vacuum. In the case of 1a, however, the formation of 10-30% hydride 4 and
acetaldehyde was still observed.
(17) Alsters, P. L.; Baesjou, P. J.; Janssen, M. D.; Kooijman, H.; Sicherer-
Roetman, A.; Spek, A. L.; van Koten, G. Organometallics 1992, 11, 4124
and references therein.
(18) For a system in which alkoxide exchange between metals seems to
be much slower see: Simpson, R. D.; Bergman, R. G. Organometallics 1993,
12, 781.
Supporting Information Available: Spectroscopic and analytic data
for 1a-c and X-ray diffraction data (ORTEP diagrams, crystal and data
collection parameters, positional parameters, and intramolecular distances
and angles) for 1a and 1b (24 pages, print/PDF). See any current
masthead page for ordering information and Web access instructions.
(19) Woerpel, K. A.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 7888.
(20) Veltheer, J. E.; Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1995,
117, 12478.
(21) (a) Hansen, K. B.; Leighton, J. L.; Jacobsen, E. N. J. Am. Chem. Soc.
1996, 118, 10924 and references therein. (b) Nugent, W. A. J. Am. Chem.
Soc. 1992, 114, 2768. (c) Steinhagen, H.; Helmchen, G. Angew. Chem., Int.
Ed. Engl. 1996, 35, 2339 and references therein. (d) Reference 10.
JA980672D
(22) Hydride transfer reactions are frequently observed in organic trans-
formations such as the Meerwein-Ponndorf-Verley reaction where both
classical and SET mechanisms have been discussed: March, J. AdVanced
Organic Chemistry; Wiley & Sons: New York, 1992.