2580
J. Am. Chem. Soc. 1997, 119, 2580-2581
Scheme 1
The Mechanism of Addition of an Ir-OH bond to
Ethylene. Catalytic Tandem Activation by Two
[η5-Cp*(Ph)IrPMe3]+ Complex Fragments
Joachim C. M. Ritter and Robert G. Bergman*
Department of Chemistry, UniVersity of California
Berkeley, California 94720
Scheme 2
ReceiVed October 23, 1996
The Wacker process is used to make about 4 million tons of
aldehydes from alkenes each year.1,2 The critical step in this
reaction involves the metallohydroxylation of an alkene, al-
though the exact mechanism of this fundamental transformation
has been a matter of controversy.3-6 Other M-OH + alkene
reactions could in principle serve as models for this step.
However, the number of isolable mononuclear organometallic
hydroxides is small, and examples that undergo insertion of
alkenes into the M-OH bond are even more rare.7,8 Because
of this, our recent synthesis of the iridium hydroxide Ph[Ir]OH
(1) ([Ir] ) Cp*IrPMe3),9 and its reaction with ethylene at 25
°C to give the hydroxyethyl complex Ph[Ir]CH2CH2OH (2), was
intriguing, and we felt that this unique process deserved in-
depth mechanistic study (Scheme 1). Additional impetus for
such an investigation was the perplexing observation that
product 2 itself undergoes apparently spontaneous oxidation to
iridium-substituted aldehyde 3. We now report a number of
unexpected characteristics of these transformations, the most
important of which is that the mechanisms of both processes
involve the participation of two metal centers.
(1) No change in the H- or 31P{1H}-NMR chemical shifts, or
in the shapes of any of the signals in the NMR spectra of the
reaction mixture, was observed when Ph[Ir]OH (1) and Ph[Ir]-
OTf (4) were dissolved in C6D6 or THF-d8.11,12 (2) Addition
of 1 equiv of ethylene to these mixtures resulted in the
immediate formation of Ph[Ir]C2H4+OTf- (5) and rapid forma-
tion of Ph[Ir]CH2CH2OH (2) (Scheme 2). This was followed
by steadily decreasing concentrations of olefin complex 5 and
hydroxide 1 and appearance of oxidation product 3. The
cationic olefin complex 5 was synthesized independently from
triflate 4 and ethylene. (3) Addition of 1 equiv of hydroxide 1
to a solution of independently-prepared olefin complex 5 in
THF-d8 also led to the formation of 2 along with an ap-
proximately equimolar amount of 4 (with respect to 1) (Scheme
2). We assume that triflate 4 and ion pair 6 are in rapid
equilibrium. (4) Olefin complex 5 was observed by NMR to
form during the reaction of hydroxide 1 and ethylene in the
presence of catalytic amounts of 4 when more than 0.5% of the
triflate 4 was used.
1
When we initiated our study, we noted large differences in
insertion reactivity using different batches of hydroxide 1.
Concerned that this process was being catalyzed by a small (and
variable) amount of an adventitious impurity, we took advantage
of earlier observations10 and added 2-5 mol % of a phosphine
(PMe3 or PPh3) to the reaction mixtures. The presence of either
phosphine resulted in the dramatic inhibition of insertion rates
of “active” batches of hydroxide 1. Clearly the added phosphine
was sequestering some material capable of activating 1 toward
reaction with ethylene. In light of the fact that the synthetic
precursor to hydroxide 1 was the corresponding triflate Ph[Ir]-
OTf (4) (OTf ) trifluoromethanesulfonate), we considered the
possibility that 4 was the activating species. To test this
hypothesis, we added 0.4-2% of 4 to an “inactive” mixture of
hydroxide 1 and ethylene. This resulted in a reacceleration of
the original ethylene insertion reaction. Subsequent addition
of PPh3 to this activated mixture resulted in conversion of 4 to
Ph[Ir]PPh3+OTf- and termination of the conversion. These
results demonstrate that the apparent insertion reaction can be
“turned on” by small amounts of 4 and “turned off” by added
phosphine, which converts triflate 4 into an inactive cationic
phosphine complex. Clearly “insertion” of ethylene into the
Ir-OH bond of 1 is catalyzed by 4.
To account for these results we postulate the mechanism
outlined in Scheme 3, which involves the cooperative participa-
tion of two Ir centers. The reaction is initiated when the oxygen
atom in the hydroxyl ligand of Ph[Ir]OH (1) attacks the metal-
bound ethylene13 in Ph[Ir]C2H4+OTf- (5); we propose that the
transition state for this entropically-demanding process is rate-
limiting in this reaction. This leads initially to bridged binuclear
intermediate 7, which dissociates by cleavage of the Ir-O bond,
generating the “insertion” product 2 and the Ph[Ir]+ species 6.14
Ion pair 6 reacts rapidly with free ethylene to regenerate 5.
As mentioned (vide supra), in addition to the hydroxyethyl
complex 2 a second product, the formylmethyl complex Ph[Ir]-
CH2CHO (3), is formed during the reaction of hydroxide 1 with
ethylene. This material is formally a dehydrogenation product
of the hydroxyethyl complex 2. The following information has
been obtained on the formation of 3: (1) As 3 is formed the
concentration of hydroxyethyl complex 2 steadily decreases,
indicating that 2 is the precursor of 3. (2) No dihydrogen is
observed during this transformation. Instead, a third product,
Ph[Ir]H (10)15 is identified albeit in 10-20% smaller concentra-
Additional investigation of the role of Ph[Ir]OTf (4) in the
ethylene insertion reaction yielded the following information.
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; John Wiley & Sons, Inc.: New York, 1988
(2) Wittcoff, H. A.; Reuben, B. G. Industrial Organic Chemistry; John
Wiley & Sons, Inc.: New York, 1996.
(3) Majima, T.; Kurosawa, H. J. Chem. Soc., Chem. Commun. 1977,
610.
(11) Bimetallic cationic Cp*Ir-complexes with bridging hydroxy ligands
have been reported and characterized spectroscopically: [Cp*Ir(µ-OH)3-
IrCp*]+OH-, Nutton, A.; Bailey, P. M.; Maitlis, P. M. J. Chem. Soc., Dalton
Trans. 1981, 1997. For other related µ-oxo complexes see also ref 14.
(12) Jacobsen, E. N.; Trost, M. K.; Bergman, R. G. J. Am. Chem. Soc.
1986, 108, 8092.
(13) This presumably occurs by anti-attack. For the mechanism of
nucleophilic addition to olefin complexes, see: Eisenstein, O.; Hoffmann,
R. J. Am. Chem. Soc. 1981, 103, 4308.
(4) Stille, J. K.; Divakaruni, R. J. Am. Chem. Soc. 1978, 102, 1303.
(5) Ba¨ckvall, J.-E.; Akermark, B.; Ljunggren, S. O. J. Am. Chem. Soc.
1979, 101, 2411.
(6) (a) Ba¨ckvall, J.-E.; Bjo¨rkman, E. E.; Pettersson, L.; Siegbahn, P. J.
Am. Chem. Soc. 1984, 106, 4369. (b) For earlier references suggesting a
cis addition mechanism for the Wacker process, see: Henry, P. M. Acc.
Chem. Res. 1973, 6, 16, and references cited therein.
(7) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163.
(8) Bergman, R. G. Polyhedron 1995, 14, 3227.
(14) Iridium-assisted oxygen transfer to phosphines was observed for
Cp*Ir(µ-O)2IrCp*: McGhee, W. D.; Foo, T.; Hollander, F. J.; Bergman,
R. G. J. Am. Chem. Soc. 1988, 110, 8543.
(9) Woerpel, K. A.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 7888.
(10) Butts, M. D.; Bergman, R. G. Organometallics 1994, 13, 2668.
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