Synthesis of Fischer carbene complex of iridium by C-H bond activation of methyl and cyclic ethers: Evidence for reversible α-hydrogen migration

Article abstract of DOI:10.1021/ja953437t

We report here a mild and versatile route to Fischer carbene complexes of iridium via the activation of C-H bonds of methyl and cyclic ethers, along with our preliminary studies of this rare family of carbene complexes. Theoretical studies suggest that α-hydrogen migrations can be kinetically favorable if a coordinatively unsaturated species can be accessed. Thus, the lability of the triflate ligand presumably facilitates this process. Further evidence for the rapidity, as well as reversibility, of this rearrangement was obtained by NMR analysis.

Full text of DOI:10.1021/ja953437t

J. Am. Chem. Soc. 1996, 118, 2517-2518
2517
Scheme 1
Synthesis of Fischer Carbene Complexes of Iridium
by C-H Bond Activation of Methyl and Cyclic
Ethers: Evidence for Reversible r-Hydrogen
Migration
Hans F. Luecke, Bruce A. Arndtsen, Peter Burger, and
Robert G. Bergman*
Chemical Sciences DiVision, Lawrence Berkeley
Laboratory and Department of Chemistry
UniVersity of California, Berkeley, California 94720
ReceiVed October 12, 1995
Since the discovery of the first stable example in 1964,1 the
study and use of transition-metal carbene complexes has
flourished. However, the most intense investigation has been
focused on carbene complexes of metals in groups 5-7, while
less is known about the reactivity of carbene complexes of
groups 8-10, though these late-metal carbene complexes have
been implicated as reactive intermediates in several catalytic
processes.2 This apparent lack of study is perhaps due to the
small number of rational synthetic methods available to generate
these complexes. We report here a mild and versatile route to
Fischer carbene complexes of iridium via the activation of C-H
bonds of methyl and cyclic ethers, along with our preliminary
studies of this rare3-6 family of carbene complexes.
NaB(C6H3(CF3)2)4) with the release of CH4. This reaction is
of additional importance in that it represents the first example
of secondary sp3 C-H bond activation by 1. The 13C NMR
spectrum of 7b contains a low-field resonance at δ 256 ppm,
which is consistent with the R-carbon chemical shifts observed
We demonstrated earlier7 that the Ir(III) complex Cp*-
(PMe3)Ir(Me)(OTf) (1) (Cp* ) η5-C5Me5, OTf ) OSO2CF3)
is capable of activating a variety of alkane and arene C-H
bonds. These studies also revealed that the thermodynamic
product of C-H bond activation often involves subsequent
rearrangement of the initially formed complex, presumably due
to the lability of the triflate ligand and the generation of a
coordinatively unsaturated cationic intermediate. We have now
found that (1) triflate 1 reacts with diethyl ether at 75 °C,
presumably Via methyl-activated intermediate 2, to give methane
and hydrido (methyl vinyl ether) complex 3 in 87% yield and
(2) in contrast, 1 reacts with methyl ethers (ROMe, R ) Me,
tBu) at 25 °C to give cationic hydridocarbene complexes
Cp*(PMe3)Ir(H)(dC(H)(OR))+OTf- (R ) Me (5a), tBu (5c)).
Due to poor crystallinity of the triflate salts, the anion was
exchanged with sodium tetraphenylborate, and the carbene
complex salts 5b,d were isolated in ∼86% yield after recrys-
tallization (Scheme 1). These complexes exhibit characteristic
1
for 5b,d. Additionally, the H NMR spectrum of 7b contains
a hydride resonance at δ -16.4 ppm. A single-crystal X-ray
diffraction study of 7b (see ORTEP diagram in supporting
information, Figure S-1) reveals an Ir-CR bond distance of
1.930(8) Å and planar geometry about CR. A peak in the
difference Fourier map was ascribed to the hydride ligand, but
its position was not refined.
The formation of 5b,d and 7b presumably proceeds by initial
C-H bond activation to form the intermediate cationic iridium
alkyl complexes 4a,c and 6, which subsequently rearrange to
the observed hydridocarbene complexes by a rapid R-hydrogen
migration8-15 (Scheme 1). Theoretical studies suggest that
R-hydrogen migrations can be kinetically favorable if a coor-
dinatively unsaturated species can be accessed.16,17 Thus, the
lability of the triflate ligand presumably facilitates this process.
Further evidence for the rapidity, as well as reversibility, of
this rearrangement was obtained by NMR analysis. Broadening
of the hydride and R-hydrogen resonances was observed in
variable-temperature NMR studies of methoxycarbene complex
5b, but thermal decomposition has prevented the measurement
of an accurate coalescence temperature. We therefore turned
to spin saturation transfer experiments to examine this process.
Upon irradiation of either the hydride or the R-hydrogen
resonance, significant spin saturation transfer (SST) was ob-
1
low-field resonances in both their H (δ ∼13 ppm) and 13C
NMR (δ ∼250 ppm) spectra corresponding to the R-hydrogen
and R-carbon atoms of the carbene ligand, as well as a high-
field resonance at ∼-16 ppm in the 1H NMR spectrum
characteristic of terminal iridium(III) hydrides. The hydride
resonances appear as doublets (2JP-H ≈ 14 Hz) due to coupling
to the 31P nuclei of the trimethylphosphine ligands, but the
R-hydrogen resonances appear as broad singlets with no distinct
three-bond 31P coupling.
(8) Turner, H. W.; Schrock, R. R.; Fellman, J. D.; Holmes, S. J. J. Am.
Chem. Soc. 1983, 105, 4942-4950.
(9) Boutry, O.; Gutierrez, E.; Monge, A.; Nicasio, M. C.; Perez, P. J.;
Carmona, E. J. Am. Chem. Soc. 1992, 114, 7288.
(10) Threlkel, R. S.; Bercaw, J. E. J. Am. Chem. Soc. 1981, 103, 2650-
2659.
This process can be extended to the formation of a cyclic
carbene complex by C-H activation of THF. Thermolysis of
1 in THF at 75 °C for 6 h results in the formation of cyclic
carbene complex 7 (82% yield following anion metathesis with
(11) Osborn, V. A.; Parker, C. A.; Winter, M. J. J. Chem. Soc., Chem.
Commun. 1986, 1185-1189.
(12) Bozec, H. L.; Fillaut, J.-L.; Dixneuf, P. H. J. Chem. Soc., Chem.
Commun. 1986, 1182-1185.
(1) Fischer, E. O.; Maasbo¨l, A. Angew. Chem., Int. Ed. Engl. 1964, 3,
580.
(2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry, 2nd ed.; University
Science Books: Mill Valley, CA, 1987.
(13) Michelin, R. A.; Bertani, R.; Mozzon, M.; Bombieri, G.; Benetollo,
F.; de Silva, M.; Pmbiero, A. J. L. Organometallics 1193, 12, 2372-2376.
(14) Burrell, A. K.; Clark, G. R.; Jeffrey, J. G.; Rickard, C. E. F.; Roper,
W. R. J. Organomet. Chem. 1990, 388, 391.
(3) O’Connor, J. M.; Pu, L. J. Am. Chem. Soc. 1989, 111, 4129.
(4) Klein, D. P.; Bergman, R. G. J. Am. Chem. Soc. 1989, 111, 3079.
(5) A recent example of an iridium(I) methylene complex: Fryzuk, M.
D.; Gao, X. G.; Joshi, K.; MacNeil, P. A.; Massey, R. L. J. Am. Chem.
Soc. 1993, 115, 10581.
(6) Stang, P. J.; Huang, Y. J. Organomet. Chem. 1992, 431, 247.
(7) Burger, P. M.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462.
(15) For a recent study that discusses a kinetic preference for R- over
â-elimination, see: Schrock, R. R.; Shih, K.-Y.; Dobbs, D. A.; Davis, W.
M. J. Am. Chem. Soc 1995, 117, 6609-6610.
(16) Ziegler, T.; Versluis, L.; Tschinke, V. J. Am. Chem. Soc. 1986, 108,
612.
(17) Goddard, R. J.; Hoffman, R.; Temmis, E. D. J. Am. Chem. Soc.
1980, 102, 7667.
0002-7863/96/1518-2517$12.00/0 © 1996 American Chemical Society

2518 J. Am. Chem. Soc., Vol. 118, No. 10, 1996
Communications to the Editor
Scheme 2
Scheme 3
ether by 17O NMR revealed that the 17O label was exclusively
located in the methyl vinyl ether product. This result indicates
that the exchange involves formal transfer of alkoxy groups
between the alkoxycarbene complex and the vinyl ether. To
rule out the possibility that the exchange reaction is catalyzed
by free alcohol, the reaction of 5b and ethyl vinyl ether was
run in the presence of 1 equiv of CH3CD2OH. Analysis of the
served down to -29.2 °C between the two sites, indicating that
these protons are exchanging rapidly (several other reversible
R-H migrations are now known8,10,18,19). The small coupling
constant between the hydride and the R-carbon (3JC-H ≈ 3 Hz)
suggests that there is no unusual one-bond interaction between
these nuclei responsible for the facility of the exchange.
Additional evidence for reversible R-hydrogen migration is
found by trapping the intermediate alkoxymethyl species 4 using
a variety of Lewis bases. The addition of CO, tBuNC, and C2H4
to a CH2Cl2 solution of hydridocarbene complex 5b at 25 °C
resulted in rapid formation of the trapped complexes 8-10
(Scheme 2). Further, thermolysis of 5b in THF at 105 °C for
3 h results in the formation of 7 and dimethyl ether. This is
consistent with reversible generation of unsaturated intermediate
4, which is capable of activating C-H bonds in a manner
analogous to that of 1. However, the addition of trimethylphos-
phine at 0 °C to a CH2Cl2 solution of 5b resulted in the
formation of the trimethylphosphine adduct 11 (Scheme 2).
Thermolysis of kinetic product 11 resulted in rearrangement to
the thermodynamic product of the reaction, 12, which is
analogous to the other Lewis base adducts (8-10). We assume
that this occurs by reversion of 11 to 5b (Scheme 2) at the higher
reaction temperature.
2
carbene product 13 by H NMR revealed that <5% deuterium
was incorporated into the carbene product during the exchange.
The above experiments rule out the conventional mechanisms
that we can conceive of for the vinyl ether exchange. In light
of the rapid reactivity of 5b with π bases such as ethylene, we
suggest that the exchange involves initial formation of an alkene
adduct, which undergoes an intramolecular attack by the pendant
alkoxy group, leading to cyclic intermediate 16, as illustrated
in Scheme 3. The intermediate can then undergo C-O bond
cleavage to form cationic methylene complex 15 and return
along an analogous pathway to accomplish the exchange. This
mechanism is consistent with all observed experimental results
and finds additional precedent in the intermolecular reactivity
of iridium alkoxymethyl complexes with electrophilic carbon
centers.20
In conclusion, we have found a C-H bond activation route
that provides mild and general synthetic access to Fischer
carbene complexes of iridium. The reversible R-hydrogen
migration that these complexes undergo provides an unusual
opportunity for reactivity at both the metal center and the
carbene ligand. We have also discovered an unprecedented
reaction between the cationic carbene complexes and vinyl
ethers that formally involves alkoxy group exchange. Future
studies will focus on additional chemistry of the hydridocarbene
complexes and experiments designed to confirm the proposed
mechanism and test the catalytic utility of the vinyl ether
exchange.
The cationic hydridocarbene complexes 5 undergo unprec-
edented reactions with alkyl vinyl ethers. Addition of 2 equiv
of ethyl vinyl ether to a CD2Cl2 solution of 5b resulted in the
gradual formation of a product exhibiting a new R-hydrogen
resonance at δ 13.88 ppm, indicating the formation of a new
secondary carbene complex. The isolated products of the
Acknowledgment. This research was supported by the Director,
Office of Energy Research, Office of Basic Energy Sciences, Chemical
Sciences Division, U.S. Department of Energy, under Contract No. DE-
AC03-76SF00098. The structural study was carried out by Dr. F. J.
Hollander, director of the U.C. Berkeley X-ray Diffraction facility
(CHEXRAY). The authors also thank Dr. Y. Ma and Dr. G. E. Ball
for assistance with the SST experiments and the Johnson Matthey Aesar/
Alpha Corp. for a generous loan of iridium trichloride.
reaction were determined to be the ethoxy-substituted Fischer
carbene complex 13 (83% yield) and methyl vinyl ether (77%
yield by 1H NMR). To determine whether this reaction involves
cleavage of the carbon-carbon double bond (alkene metathesis),
we examined the reaction of ethoxycarbene complex 13 with
2-methoxypropene. Olefin metathesis product 14 was not
observed; instead, exclusive formation of 5b (96%) and
2-ethoxypropene (93%) was observed. This result requires that
the alkene CdC bond is preserved in the exchange process.
Supporting Information Available: Spectroscopic and analytical
data for 3-13 and X-ray diffraction data (ORTEP diagrams, crystal
and data collection parameters, positional parameters, and intramolecular
distances and angles) for 7b (17 pages). This material is contained in
many libraries on microfiche, immediately follows this article in the
microfilm version of the journal, can be ordered from the ACS, and
can be downloaded from the Internet; see any current masthead page
for ordering information and Internet access instructions.
JA953437T
(18) Green, M. L. H.; Canestrari, M. J. Chem. Soc., Dalton Trans. 1982,
To further probe the mechanism of this reaction, we
synthesized 17O-enriched carbene complex 5a by the reaction
of 1 with CH317OCH3 and treated this labeled material with
ethyl vinyl ether. Analysis of the products 13 and methyl vinyl
1789-1793.
(19) Cooper, N. J.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 1979,
1121.
(20) Thorn, D. L.; Tulip, T. H. J. Am. Chem. Soc. 1981, 103, 5984.