Direct observation of C - O reductive elimination from Pt(IV) [4]

Full text of DOI:10.1021/ja982211y

252
J. Am. Chem. Soc. 1999, 121, 252-253
resulting from C-O reductive elimination; methyl acetate and
dppePtMe2 (2) are formed in g88% yield. Competitive with this
C-O bond forming process is a C-C reductive elimination
yielding ethane and dppePtMe(OAc) (3).13 In the more polar
solvents acetone-d6 and nitrobenzene-d5, the C-C reductive
elimination dominates. Rates of reaction are also substantially
faster in the more polar solvents.
Direct Observation of C-O Reductive Elimination
from Pt(IV)
B. Scott Williams, Andrew W. Holland, and
Karen I. Goldberg*
Department of Chemistry, Box 351700
UniVersity of Washington, Seattle, Washington 98195-1700
Higher percentages of methyl acetate (94-98 vs 88-89%)
were obtained when 2% cross-linked polyvinylpyridine (PVP),
an acid scavenger, was added to the benzene-d6 and THF-d8
thermolysis experiments. This inhibition of ethane elimination
by PVP suggested that the C-C reductive elimination pathway
might be acid-catalyzed. Indeed, the addition of a small amount
of HOTf (0.1 equiv, 3.4 mM) or the Lewis acid, AgOTf (0.1
equiv, 5.6 mM), caused a dramatic increase in the rate of C-C
elimination. Under these conditions, exclusive ethane elimination
proceeded at room temperature. In contrast, the addition of a small
amount of acetic acid (0.1 equiv, 5.3 mM) caused a lesser
enhancement of the initial C-C coupling rate but also increased
the initial rate of C-O reductive elimination (kobs ) 2.7 × 10-5
s-1 at 99 °C vs 1.25 × 10-5 s-1 without added acid; C-O:C-C
) 80:20). Subsequently, the acetic acid was consumed by reaction
with the Pt(II) dimethyl product, 2 (to produce methane (s, δ 0.15)
ReceiVed June 25, 1998
Reductive elimination is a fundamental organometallic reaction
and a critical product-release step in a number of industrially
important catalytic processes.1 In particular, reductive elimination
reactions which form C-O bonds from Pt(IV) have been proposed
as product-forming steps in the functionalization of alkanes by
Pt(II) catalysts (Shilov’s oxidations and the system recently
reported by Catalytica).2-4 Only a limited number of examples
of reductive eliminations which form carbon-heteroatom (C-O,
C-N, C-S) bonds from model complexes are known. These
reactions primarily occur at low valent d8 metal centers (Pd(II),
Ni(II)) and involve aryl and acyl carbon groups.5 Examples of
carbon-heteroatom reductive couplings which occur from high
valent d6 metal centers such as Pd(IV),6 Pt(IV), Rh(III), or Ir-
(III),7 as well as those which involve alkyl carbon groups, are
extremely rare.3,5c,8 This contribution reports the first direct
observation and study of high yield alkyl C-O reductive
elimination from a d6 octahedral Pt(IV) metal center.
The Pt(IV) complex, dppePtMe3OAc (1), was prepared from
dppePtMe3I and AgOAc.9 Crystals of 1 suitable for an X-ray
diffraction study were grown by evaporation from a toluene
solution in air. The ORTEP view is included in the Supporting
Information.10,11 A water molecule is shown to be hydrogen-
bonded to the acetate moiety (O-O distance ) 2.75 Å).
The thermolyses of anhydrous samples of 1 at 99 °C were
studied in a variety of solvents (Table 1). In the relatively nonpolar
solvents benzene-d6 and THF-d8, the primary products are those
and 3).14 The rate of reaction then slowed to a final rate (kobs
)
1.25 × 10-5 s-1) identical to that without added acid and the
C-C coupling dropped to 10%. Thus, the rate and product ratios
observed in the thermolysis of 1 are very sensitive to the presence
of Brønsted and Lewis acids.
Thermolysis of 1 in the more polar solvent acetone-d6 produced
primarily C-C reductive elimination (95-99%). Although these
reactions consistently followed first-order kinetic behavior, a high
variability in the rate constant was observed (at 99 °C, kobs
)
6.2-11 × 10-4 s-1).15 We have not as yet identified the exact
cause of this variability.16,17 However, when a constant acetate
concentration was maintained by the addition of acetate ions
([N(n-Bu)4]OAc, 0.38 or 0.60 M), a reproducible reaction rate
was observed (kobs ) 4.0 (( 0.1) × 10-5 s-1).18,19 Under these
conditions, ethane production was completely inhibited and methyl
acetate and 2 were formed in quantitative yield.
(1) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: The Applications
and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.;
Wiley-Interscience: New York, 1992.
(2) Shilov, A. E. ActiVation of Saturated Hydrocarbons by Transition Metal
Complexes; D. Riedel: Dordrecht, The Netherlands, 1984.
The reaction mechanism shown in Scheme 1 is consistent with
all of our data. In agreement with substantial literature precedent,
C-C reductive elimination from Pt(IV) proceeds via a five-
coordinate intermediate.20,21 Dissociation of acetate from 1 is
supported by the inhibition of ethane elimination in the presence
of a high concentration of acetate ions and also by the dramatic
solvent effect. The C-C coupling rate increases by over 3 orders
of magnitude from benzene-d6 to nitrobenzene-d5 (Table 1).22
Finally, the accelerative effect of added acid on the C-C coupling
reaction can be explained by Lewis acid coordination to the acetate
group (similar to H2O coordination in the crystal structure). This
interaction should promote dissociation of the acetate, increasing
k1. When the counterion is OTf-, k-1[OAc] and k2[OAc] decrease
as the acetate ion is more strongly bound to the acid, further aug-
menting the C-C coupling rate.23a However, when the counterion
is OAc-, the acetate dependent steps are not substantially inhibited
(and may be accelerated) which explains the less dramatic increase
in the C-C coupling rate with with HOAc vs HOTf and AgOTf.
(3) Mechanistic support has been presented for the product forming step
in Shilov’s oxidations being nucleophilic attack at a Pt(IV) carbon group by
an external heteroatom species. This reaction which results in C-X bond
formation and generation of a Pt(II) species is sometimes classified as a
reductive elimination. Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J.
A.; Bercaw, J. E. J. Organomet. Chem. 1995, 504, 75 and references therein.
(4) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii,
H. Science 1998, 280, 560.
(5) See: (a) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046-2067
and references therein. (b) Widenhofer, R. A.; Buchwald, S. L. J. Am. Chem.
Soc. 1998, 120, 6504 and references therein. (c) Han, R.; Hillhouse, G. L. J.
Am. Chem. Soc. 1998, 120, 7657 and references therein. (d) Komiya, S.; Akai,
Y.; Tanaka, K.; Yamamoto, T.; Yamamoto, A. Organometallics 1985, 4, 1130.
(6) (a) Canty, A. J.; Jin, H. J. Organomet. Chem. 1998, 565, 135. (b) Canty,
A. J.; Jin, H.; Skelton, B. W.; White, A. H. Inorg. Chem. 1998, 37, 3975.
(7) (a) Mann, G.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 13109. (b)
Thompson, J. S.; Randall, S. L.; Atwood, J. D. Organometallics 1991, 10,
3906.
(8) The microscopic reverse, oxidative addition of an alkyl C-O bond
(RCO2R′) to form a d6 octahedral compound, has been reported. (a) Ittel, S.
D.; Tolman, C. A.; English, A. D.; Jesson, J. P. J. Am. Chem. Soc. 1978, 100,
7577. (b) Khin-Than, A.; Colpitts, D.; Ferguson, G.; Puddephatt, R. J.
Organometallics 1988, 7, 1454.
(9) See supporting material for characterization of 1, 5 and dppePtMe(O2-
CCF3) (6).
(13) 3 and 4-h5: Appleton, T. G.; Bennett, M. A. Inorg. Chem. 1978, 17,
738.
(10) 1‚H2O (C31H38O3P2Pt), MW ) 715.64, colorless rhombohedron,
triclinic, space group ) P1h, a ) 10.043(2) Å, b ) 12.581(3) Å, c ) 13.538-
(3) Å, R ) 66.15(2)°, â ) 70.32(2)°, γ ) 78.16(2)°, V ) 1468.3(6) Å3, Z )
2, R1 ) 0.0369 [I > 4σ(I)], wR2 ) 0.1179, GOF (on F2) ) 0.695.
(11) Selected bond lengths(Å): Pt-C (trans to OAc) 2.079(7); Pt-C (cis
to OAc) 2.087(7), 2.092(7); Pt-O 2.162(5); O-C 1.272(9); CdO 1.228(9).
Select angles(deg): P-Pt-P 83.62(6); Pt-C-O 125.1(4); O-CdO 124.8-
(7), OdC-C 119.1(7), O-C-C 116.1(7).
(14) The reaction of 2 with HOAc has also been independently demon-
strated.
(15) A similar variation in rates was observed at 79 °C (kobs ) 3.4-19 ×
10-5 s-1).
(16) A side reaction between the primary Pt(II) product, dppePtMe(OAc)
(3) and acetone-d6 to form dppePtMe(CD2COCD3) (4)13 and DOAc further
complicates the analysis. However, production of acetic acid during the reaction
would be expected to cause a deviation from first-order behavior (see text)
rather than simple variability between experiments.
(12) Swain, C. G.; Swain, M. S.; Powell, A. L.; Alunni, S. J. Am. Chem.
Soc. 1983, 105, 502.
10.1021/ja982211y CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/17/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 1, 1999 253
Table 1. Thermolysis Rate Constants and Product Yields for 1 (99 °C)
c
c
solvent
polarity (A + B)a
kobs (105 s-1
)
%C-O
kC-O(105 s-1
)
% C-C
kC-C(105 s-1
)
b
C6D6
0.73
0.84
1.06
1.14
1.25 ( 0.04
1.28 ( 0.03
62-110
88-98
88-96
1-5
1.18 ( 0.05
1.16 ( 0.07
2-4
2-12
4-12
95-99
99.5+
0.07 ( 0.04
0.12 ( 0.06
60-110
b
THF-d8
acetone-d6
C6D5NO2
140 ( 10
<0.5
d
140 ( 10
a Relative polarity scale for protio solvents.12 b Some reactions contained PVP (see text). c Reference 22. d Insufficient product was generated to
accurately determine the rate constant.
Scheme 1
both derivatives of methanol which may be protected from
overoxidation similarly to the methyl bisulfate product in the
Catalytica methane functionalization process.4,25 Investigations of
these and other novel carbon-heteroatom couplings from octa-
hedral d6 metal centers continue in our laboratories.
Acknowledgment is made to the donors of the Petroleum Research
Fund, administered by the ACS, the National Science Foundation, the
Union Carbide Innovation Recognition Program, and the DuPont
Educational Aid program for support of this work. K.I.G. is an Alfred P.
Sloan Research Fellow. We are also grateful for a loan of K2PtCl4 from
Johnson Matthey/Aesar/Alfa and we thank J. Roth for preliminary work
on this project. The X-ray structure determination was performed by Dr.
D. Barnhart and S. Lovell (UW). We also want to thank the reviewers
for insightful comments and suggestions.
The mechanism for C-O reductive elimination suggested in
Scheme 1 is analogous to that previously presented for C-I reduc-
tive elimination from the related compound, dppePtMe3I.20 As
shown by the experiments with added [N(n-Bu)4]OAc, the rate
of methyl acetate production from 1 is independent of the acetate
ion concentration. This is consistent with the path involving ace-
tate dissociation detailed in Scheme 1.23 While a pathway involv-
ing direct reductive elimination of methyl acetate from 1 should
also be independent of acetate ion concentration, the acceleration
of the C-O reductive elimination in the presence of HOAc argues
against direct elimination. In contrast, acetic acid coordination
to the acetate group of 1 should increase k1 in Scheme 1 as de-
scribed above, thereby increasing the overall C-O coupling rate.
To further consider the dissociation of the acetate group as an
integral part of the C-O and C-C coupling reactions, the ther-
molysis of the trifluoroacetate analogue of 1, dppePtMe3(O2CCF3)
(5)9 was studied in benzene-d6 at 99 °C. Thermolysis of 5 in the
presence of PVP proceeds much faster (kobs ) 2.8 ( 0.1 × 10-4
s-1) than that of 1 and results in a much lower C-O coupling
percentage of 49((2)%. The rate of C-O coupling is ap-
proximately an order of magnitude faster, and the rate of C-C
coupling is roughly 100 times that of the acetate analogue, 1.24
Thus, the greater lability of the trifluoroacetate group increases
the rate of both eliminations. In contrast, studies of C-O and
C-N reductive eliminations from Pd(II) in which coupling is
proposed to occur without preliminary heteroatom dissociation
report that the more electron-withdrawing heteroatom groups react
more slowly than their more electron-donating analogues.5a,b
The evidence presented strongly suggests preliminary acetate
dissociation and the involvement of a common five-coordinate
cationic intermediate in both C-C and C-O reductive elimination
reactions from 1. The coupling of an alkyl moiety with an acetate
or trifluoroacetate fragment represents a promising product-release
pathway in alkane functionalization involving high-valent late-
metal complexes.25 Methyl acetate and methyl trifluoroacetate are
Supporting Information Available: Spectroscopic data for com-
pounds 1, 5, and 6 and crystal structure data for 1 (13 pages, print/PDF).
See any current masthead page for ordering instructions and Web access
information.
JA982211Y
(17) Possible sources of variability in kobs (acetone-d6): (a) Trace amounts
of a Lewis acidic impurity could cause erratic kinetic behavior. (b) The acetone
reactions may not be strictly first order. In contrast to thermolysis in less
polar solvents, >95% of the reaction proceeds via a pathway that depends on
the concentration of acetate anions.23a This will be a function of the
concentration of 1, that of the major product, dppePtMe(OAc) (3) and of any
acetic acid16 produced (Scheme 1).
(18) The disappearance of starting material was monitored by 1H NMR
for two half-lives, beyond which, the dynamic range of the NMR spectrometer
hindered accurate data collection.
(19) During the course of the thermolysis reaction, conversion of [(n-Bu)4N]-
OAc to (n-Bu)3N and BuOAc occurs at a rate approximately an order of
magnitude slower than the Pt(IV) reductive eliminations.
(20) Goldberg, K. I.; Yan, J. Y.; Breitung, E. M. J. Am. Chem. Soc. 1995,
117, 6889 and references therein.
(21) Hill, G. S.; Puddephatt, R. J. Organometallics 1997, 16, 4522 and
references therein.
(22) kc-o ) kobs(%C-O/100) and kc-c ) kobs(%C-C/100) were calculated
using the final product ratios as measured by integration of 1H NMR signals
for Pt(II)-Me groups and of methyl acetate against an internal standard.
Product ratios estimated by 31P NMR through the majority of the reaction
were consistent with the final ratios. However, spectra taken at very early re-
action times indicated higher amounts of C-C coupling. The low signal-to-
noise ratios at early reaction times makes quantitation difficult. A trace amount
of acid consumed during the reaction could be responsible for a higher initial
C-C coupling rate as could a variable acetate concentration (see ref 17).
(23) (a) See the Supporting Information for a derivation for the rate law
corresponding to the mechanism in Scheme 1. (b) If (k-1 + k2)[OAc] . k3,
then kC-O ≈ k1k2/(k-1 + k2).
(24) An even faster rate and higher C-C coupling percentage is observed
for the triflate analogue. dppePtMe3(OTf) is reported to react below room
temperature in acetone-d6 to yield exclusively ethane and dppePtMe(OTf).21
(25) This strategy of deactivating the products of alkane functionalization
toward further oxidation via esterification as RO2CCF3 or ROSO3H has been
employed successfully in a number of systems. See references in recent review
article: Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed.
1998, 37, 2180.