Studies of reductive elimination reactions to form carbon-oxygen bonds from Pt(IV) complexes

Article abstract of DOI:10.1021/ja003366k

The platinum(IV) complexes fac-L₂PtMe₃(OR) (L₂ = bis(diphenylphosphino)ethane, o-bis(diphenylphosphino)benzene, R = carboxyl, aryl; L = PMe₃, R = aryl) undergo reductive elimination reactions to form carbon-oxygen bonds and/or carbon-carbon bonds. The carbon-oxygen reductive elimination reaction produces either methyl esters or methyl aryl ethers (anisoles) and L₂PtMe₂, while the carbon-carbon reductive elimination reaction affords ethane and L₂PtMe(OR). Choice of reaction conditions allows the selection of either type of coupling over the other. A detailed mechanistic study of the reductive elimination reactions supports dissociation of the OR^- ligand as the initial step for the C-O bond formation reaction. This is followed by a nucleophilic attack of OR^- upon a methyl group bound to the Pt(IV) cation to produce the products MeOR and L₂PtMe₂. C-C reductive elimination proceeds from L₂PtMe₃(OR) by initial L (L = PMe₃) or OR^- (L₂ = dppe, dppbz) dissociation, followed by C-C coupling from the resulting five-coordinate intermediate. Our studies demonstrate that both C-C and C-O reductive elimination reactions from Pt(IV) are more facile in polar solvents, in the presence of Lewis acids, and for OR^- groups that contain electron withdrawing substituents.

Full text of DOI:10.1021/ja003366k

2576
J. Am. Chem. Soc. 2001, 123, 2576-2587
Studies of Reductive Elimination Reactions To Form
Carbon-Oxygen Bonds from Pt(IV) Complexes
B. Scott Williams and Karen I. Goldberg*
Contribution from the Department of Chemistry, UniVersity of Washington, P.O. Box 351700,
Seattle, Washington 98195-1700
ReceiVed September 13, 2000
Abstract: The platinum(IV) complexes fac-L2PtMe3(OR) (L2 ) bis(diphenylphosphino)ethane, o-bis(diphen-
ylphosphino)benzene, R ) carboxyl, aryl; L ) PMe3, R ) aryl) undergo reductive elimination reactions to
form carbon-oxygen bonds and/or carbon-carbon bonds. The carbon-oxygen reductive elimination reaction
produces either methyl esters or methyl aryl ethers (anisoles) and L2PtMe2, while the carbon-carbon reductive
elimination reaction affords ethane and L2PtMe(OR). Choice of reaction conditions allows the selection of
either type of coupling over the other. A detailed mechanistic study of the reductive elimination reactions
supports dissociation of the OR- ligand as the initial step for the C-O bond formation reaction. This is followed
by a nucleophilic attack of OR- upon a methyl group bound to the Pt(IV) cation to produce the products
MeOR and L2PtMe2. C-C reductive elimination proceeds from L2PtMe3(OR) by initial L (L ) PMe3) or
OR- (L2 ) dppe, dppbz) dissociation, followed by C-C coupling from the resulting five-coordinate intermediate.
Our studies demonstrate that both C-C and C-O reductive elimination reactions from Pt(IV) are more facile
in polar solvents, in the presence of Lewis acids, and for OR- groups that contain electron withdrawing
substituents.
Introduction
a limited number of examples of directly observed reductive
elimination reactions which form C-O bonds from model
complexes are known.7,11-17 In addition, these latter reactions
primarily occur at low-valent d8 metal centers (such as
Pd(II))7,12-14 and/or involve aryl and acyl carbon groups.7,12,13,15
Examples of C-O reductive elimination reactions involving
alkyl carbons and higher oxidation state metals (e.g., Pd(IV),
Pt(IV), Rh(III), and Ir(III)) are quite rare.11,16-18
However, the product release step in platinum-catalyzed
alkane oxidation reactions is proposed to be such a C-O
coupling reaction involving an alkyl group from a high-valent
metal center.3,4,19 Support has been presented for nucleophilic
attack of water on a Pt(IV)-methyl group as the methanol-
Reductive elimination is a key bond-forming and product-
release step in homogeneous catalysis.1 The formation of
carbon-oxygen bonds by reductive elimination is a particularly
powerful reaction as it can result in the production of alcohols,
ethers, and esters. Despite the significance and potential
importance of this reaction in both homogeneous catalysis2-8
and interesting stoichiometric metal-mediated reactions,9,10 only
(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) (a) Shilov, A. E. ActiVation of Saturated Hydrocarbons by Transition
Metal Complexes; D. Riedel: Dordrecht, The Netherlands, 1984. (b) Shilov,
A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 10235.
(3) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed.
1998, 37, 2180 and references therein.
(4) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii,
H. Science 1998, 280, 560.
(5) (a) Sen, A. Acc. Chem. Res. 1988, 21, 421. (b) Sen, A. Acc. Chem.
Res. 1998, 31, 550.
(6) (a) Mann, G.; Hartwig, J. F. Tetrahedron Lett. 1997, 38, 8005. (b)
Mann, G.; Hartwig, J. F. J. Org. Chem. 1997, 62, 5413.
(7) (a) Mann, G.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 13109.
(b) Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2046 and
references therein. (c) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852. (d)
Mann, G.; Incarvito, C.; Rheingold, A. L.; Hartwig, J. F. J. Am. Chem.
Soc. 1999, 121, 3224.
(8) (a) Palucki, M.; Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc.
1996, 118, 10333. (b) Palucki, M.; Wolfe, J. P.; Buchwald, S. L. J. Am.
Chem. Soc. 1997, 119, 3395. (c) Aranyos, A.; Old, D. W.; Kiyomori, A.;
Wolfe, J. P.; Sadighi, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121,
4369.
(9) (a) Davidson, M. F.; Grove, D. M.; van Koten, G.; Spek, A. L. J.
Chem. Soc., Chem. Commun. 1989, 1562. (b) Kapteijn, G. M.; Dervisi, A.;
Verhoef, M. J.; van den Broek, M. A. F. H.; Grove, D.; van Koten, G. J.
Organomet. Chem. 1996, 517, 123.
(10) (a) Bernard, K. A.; Atwood, J. D. Organometallics 1987, 6, 1133.
(b) Bernard, K. A.; Atwood, J. D. Organometallics 1988, 7, 235. (c) Bernard,
K. A.; Atwood, J. D. Organometallics 1989, 8, 795. (d) Bernard, K. A.;
Churchill, M. R.; Janik, T. S.; Atwood, J. D. Organometallics 1990, 9, 12.
(11) Thompson, J. S.; Randall, S. L.; Atwood, J. D. Organometallics
1991, 10, 3906. See ref 7a for more information on this reaction.
(12) (a) Widenhoefer, R. A.; Zhong, H. A.; Buchwald, S. L. J. Am. Chem.
Soc. 1997, 119, 6787. (b) Widenhoefer, R. A.; Buchwald, S. L. J. Am. Chem.
Soc. 1998, 120, 6504 and references therein.
(13) Komiya, S.; Akai, Y.; Tanaka, K.; Yamamoto, T.; Yamamoto, A.
Organometallics 1985, 4, 1130.
(14) (a) Koo, K.; Hillhouse, G. L. Organometallics 1998, 17, 2924. (b)
Han, R.; Hillhouse, G. L.; J. Am. Chem. Soc. 1997, 119, 8135. (c)
Matsunaga, P. T.; Mavropoulos, J. C.; Hillhouse, G. L. Polyhedron 1995,
14, 175. Note that since these reductive elimination reactions from Ni(II)
are promoted by oxidants, Ni(III) is likely to be involved in the reactions.
(15) Haarman, H. F.; Kaagman, J.-W. F.; Smeets, W. J. J.; Spek, A. L.;
Vrieze, K. Inorg. Chim. Acta 1998, 270, 34.
(16) (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.
(17) For a preliminary report on C-O reductive elimination of methyl
esters from Pt(IV), see: Williams, B. S.; Holland, A. W.; Goldberg, K. I.
J. Am. Chem. Soc. 1999, 121, 252.
(18) The reverse of alkyl C-O reductive elimination, alkyl C-O
oxidative addition, has been observed. (a) Ittel, S. D.; Tolman, C. A.;
English, A. D.; Jesson, J. P. J. Am. Chem. Soc. 1978, 100, 7577. (b) Tolman,
C. A.; Ittel, S. D.; English, A. D.; Jesson, J. P. J. Am. Chem. Soc. 1979,
101, 1742. (c) van der Boom, M. E.; Liou, S. Y.; Ben-David, Y.; Vigalok,
A.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 6531.
10.1021/ja003366k CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/23/2001

Formation of C-O Bonds from Pt(IV) Complexes
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2577
Table 1. Selected Bond Lengths and Angles in the Crystal
Structure of 1a
bond lengths (Å)
bond angles (deg)
Pt-C(1)
2.103(6)
2.087(6)
2.080(6)
2.3793(15)
2.3767(16)
2.159(4)
1.268(8)
1.229(8)
1.501(9)
1.842(6)
1.858(6)
1.539(9)
2.7
P(1)-Pt-P(2)
83.67(5)
84.5(3)
95.7(2)
96.02(18)
93.6(2)
92.31(19)
84.4(3)
Pt-C(2)
C(1)-Pt-C(2)
C(1)-Pt-P(2)
C(2)-Pt-P(1)
C(3)-Pt-P(1)
C(3)-Pt-P(2)
C(3)-Pt-C(1)
C(3)-Pt-C(2)
O(1)-Pt-P(1)
O(1)-Pt-P(2)
O(1)-Pt-C(1)
O(1)-Pt-C(2)
Pt-O(1)-C(5)
O-(1)-C(5)-C(4)
O(1)-C(5)-O(2)
O(2)-C(5)-C(4)
C(3)-Pt-O(1)
P(1)-Pt-C(1)
P(2)-Pt-C(2)
Pt-C(3)
Pt-P(1)
Pt-P(2)
Pt-O(1)
O(1)-C(5)
O(2)-C(5)
C(5)-C(4)
C(6)-P(1)
C(7)-P(2)
C(6)-C(7)
O(2)-O(3W)
86.7(3)
96.72(13)
97.01(12)
85.3(2)
84.0(2)
125.3(4)
115.8(6)
124.8(6)
119.4(6)
166.8(2)
178.0(2)
178.95(17)
Figure 1. ORTEP diagram of fac-(dppe)PtMe3OAc (1a). Ellipsoids
are shown at the 50% probablity level. Hydrogens are omitted for
clarity.
) 2.7 Å). A slightly distorted octahedral geometry is observed
about the platinum center, and the Pt(IV) carbon and phosphorus
bond lengths are comparable to those in other reported structures
with similar trans ligands.21 The Pt-O bond length in 1a
(2.159(4) Å) is the longest of the Pt(IV) acetate complexes
which have been structurally characterized, including several
cases in which the acetate is involved in hydrogen bonding.22
In addition, the formal carbon-oxygen single and double bonds
in the acetate fragment have very similar bond lengths
(1.268(8) and 1.229(8) Å), suggesting that the carboxylate
π-bond is rather delocalized and the bonding to platinum fairly
ionic. Two strong carboxylate stretches are observed in the IR
spectrum of 1a (CH2Cl2) at 1614 and 1377 cm-1. The small
difference between the asymmetric and symmetric stretches
(∆ ) 237 cm-1) is also indicative of significant ionic bonding.23
Anhydrous samples of 1a were prepared by recrystallization
from toluene and pentane under air- and moisture-free condi-
tions. The thermolyses of anhydrous samples of 1a at 99 °C
were studied in a variety of solvents. Competitive reductive
elimination was observed, in which either a carbon-oxygen
bond was formed to produce methyl acetate and (dppe)PtMe2
(2a) or a carbon-carbon bond was formed to produce ethane
and (dppe)PtMe(OAc)24 (3a) (Scheme 1). There is a strong
forming step in Shilov’s original Pt-catalyzed oxidations.3,19
Similarly, the reductive elimination of a methyl and a sulfonate
group from Pt(IV) has been proposed as the methylbisulfate
forming step in the recently reported Catalytica methane
oxidation reaction.4 Such carbon-oxygen bond formation to
release the alcohol in a protected form such as an ester or
sulfonate ester has been advanced as a useful strategy to avoid
the overoxidation of the desired alcohol to carbon dioxide and
water, a significant problem in selective alkane oxidation
catalysis.3-5
Detailed investigations of a variety of alkyl C-O reductive
elimination reactions from Pt(IV), including those which form
esters, have now been carried out.17 Thermolyses of the Pt(IV)
complexes, fac-L2PtMe3OR (L2 ) bis(diphenylphosphino)ethane
(dppe), o-bis(diphenylphosphino)benzene (dppbz), R ) car-
boxyl, aryl; L ) PMe3, R ) aryl), under the proper conditions
produce high yields of methyl esters and methyl aryl ethers.
Kinetic and mechanistic studies of these novel C-O reductive
elimination reactions from Pt(IV) have allowed us to identify
trends and requirements for both the neutral ligands on the metal
and the oxygen groups involved in the coupling reactions.
Results
(21) (a) Clark, H. C.; Ferguson, G.; Jain, V. K.; Parvez, M. J. Organomet.
Chem. 1984, 270, 365. (b) O’Reilly, S. A.; White, P. S.; Templeton, J. L.
J. Am. Chem. Soc. 1996, 118, 5684. (c) Redina, L. M.; Vittal, J. J.;
Puddephatt, R. J. Organometallics 1995, 14, 2188. (d) Levy, C. J.; Vittal,
J. J.; Puddephatt, R. J. Organometallics 1996, 15, 2108 (e) Hill, G. S.;
Vittal, J. J.; Puddephatt, R. J. Organometallics 1997, 16, 1209. (f) Schlecht,
S.; Magull, J.; Fenske, D.; Dehnicke, K. Angew. Chem., Int. Ed. Engl. 1997,
36, 1994. (g) Cheetham, A. K.; Puddephatt, R. J.; Zalkin, A.; Templeton,
D. H.; Templeton, L. K. Inorg. Chem. 1976, 15, 2997. (h) Monojlovic-
Muir, L.; Muir, K. W. Croat. Chem. Acta 1984, 57, 587. (k) Ling, S. S.
M.; Jobe, I. R.; Monojlovic-Muir, L.; Muir, K. W.; Puddephatt, R. J.
Organometallics 1985, 4, 1198. (l) Ling, S. S. M.; Payne, N. C.; Puddephatt,
R. J. Organometallics 1985, 4, 1546. (m) Hoover, J. F.; Stryker, J. M.
Organometallics 1989, 8, 2973.
Platinum(IV) Carboxylates. The Pt(IV) acetate complex fac-
(dppe)PtMe3(OAc) (1a) was prepared by reaction of fac-(dppe)-
PtMe3I with AgOAc in toluene at room temperature. Complex
1a has been characterized by spectroscopy, elemental analysis,
and X-ray crystallography. The ORTEP diagram appears in
Figure 1 and selected bond lengths and angles are presented in
Table 1.20 The crystal was grown by slow evaporation of a
toluene solution of 1a in ambient air, and contains a water
molecule hydrogen bound to the acetate moiety (O-O distance
(19) Nucleophilic attack by a heteroatom group (X-) upon a metal-bound
R group is sometimes classified as a reductive elimination as it results in
C-X bond formation and a two oxidation state reduction of the metal. See:
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; pp 279-281. See ref 3 and references
therein for more information about this form of C-O coupling at Pt(IV)
centers.
(20) See Supporting Information for a complete listing of bond lengths
and angles, atomic coordinates, and displacement parameters. Further
refinement has been carried out on the crystal structure data after a
preliminary report in ref 17.
(22) (a) Lee, E. J.; Jun, M.-J.; Lee, S. S.; Sohn, Y. S. Polyhedron 1997,
16, 2421. (b) Kratochwil, N. A.; Zabel, M.; Range, K.-J.; Bednarski, P. J.
J. Med. Chem. 1996, 39, 2499. (c) Rochon, F. D.; Melanson, R.; Macquet,
J.-P.; Belanger-Gariepy, F.; Beauchamp, A. L. Inorg. Chim. Acta 1985,
108, 17. (d) Ellis, L. T.; Er, H. M.; Hambley, T. W. Aust. J. Chem. 1995,
48, 793. (e) Neidle, S.; Snook, C. F.; Murrer, B. A.; Barnard, C. F. J. Acta
Crystallogr., Sect. C 1995, C51, 822. (f) Khokhar, A. R.; Deng, Y. Inorg.
Chim. Acta 1993, 204, 35.
(23) Nakmoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds. Part B: Applications in Coordination, Orga-
nometallic and Bioinorganic Chemistry, 5th ed.; John Wiley and Sons: New
York, 1977; pp 59-62.

2578 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Williams and Goldberg
Table 2. Rate Constants and Product Yields for the Thermolysis of 1a (99 °C)
c
c
solvent
C6D6
THF-d8
acetone-d6
C6D5NO2
polaritya
kobs (105 s-1
)
% C-O
kCO (105 s-1
)
% C-C
kCC (105 s-1
)
0.73
0.84
1.06
1.14
1.25 ( 0.04
1.28 ( 0.03
60-110
88-89 (94-98b)
88 (96b)
1-5
1.17 ( 0.07
1.16 ( 0.07
2-4
2-12
4-12
95-99
99.5+
0.07 ( 0.04
0.12 ( 0.06
60-110
140 ( 10
<0.5
d
140 ( 10
a Relative polarity scale for protio solvents.25 b PVP added to the reaction (see text). c Reference 26. d Insufficient product was generated to
accurately determine the rate constant.
Scheme 1
) (4.0 ( 0.1) × 10-5 s-1).30,31 Under these conditions, ethane
production was completely inhibited and methyl acetate and
(dppe)PtMe2 (2a) were formed in quantitative yield. Thus, the
carbon-oxygen bond formation rate constant in acetone-d6 (4.0
× 10-5 s-1) is approximately 4-fold the C-O coupling rate
constant in the less polar solvents C6D6 and THF-d8 (1.1 ×
10-5 s-1).
To investigate the effect of modification of the ancillary
ligand, L2, the analogous dppbz (o-bis(diphenylphosphinoben-
zene)) complex, fac-(dppbz)PtMe3(OAc) (1b), was prepared and
its thermal reactivity studied. In the presence of PVP in THF-
d8 at 99 °C, 1b undergoes reductive elimination to form
solvent dependence of the product ratios (C-O versus C-C
coupling) and the rate constants for these reactions (Table 2).
In the relatively nonpolar solvents of C6D6 and THF-d8,
carbon-oxygen bond formation is favored with methyl acetate
and 2a as the primary products (88-89%). Even higher yields
of C-O coupling products (94-98%) were observed when 2%
cross-linked 4-polyvinylpyridine (PVP), a known acid scaven-
ger, was added to the reaction. The fluorinated analogue, fac-
(dppe)PtMe3(O2CCF3) (4a), prepared from fac-(dppe)PtMe3I and
AgO2CCF3, undergoes a similar competitive reductive elimina-
tion at 99 °C in C6D6 to form either methyl trifluoroacetate and
2a or ethane and (dppe)PtMe(O2CCF3) (5a) (Scheme 1).
However, the percentage of C-O coupled products in the
presence of PVP in C6D6 was significantly lower for 4a (49 (
2%) than for 1a. Yet, the overall reaction rate was considerably
faster for 4a than for 1a (kobs ) (2.9 ( 0.1) × 10-4 s-1, as
compared to kobs ) (1.25 ( 0.4) × 10-5 s-1 for 1a). From the
ratio of the products (2a to 3a or 5a), it can be determined that
the rate constant for carbon-oxygen reductive elimination is
12 times faster for the more electron withdrawing trifluoroacetate
compound, 4a, than for the acetate complex, 1a.26 The rate
constant for carbon-carbon coupling is 2 orders of magnitude
faster for 4a than for 1a.
In contrast to the reactivity in the relatively nonpolar solvents
C6D6 and THF-d8, in which C-O coupling dominates, ther-
molysis of 1a in the more polar solvents acetone-d6 or
nitrobenzene-d5 favors carbon-carbon coupling (95-99% and
99+%, respectively). The rates of reductive elimination are also
on the order of 2 to 3 orders of magnitude more rapid in these
polar solvents (Table 2). It should also be noted that while the
thermolysis of 1a in acetone-d6 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).27 The exact
cause of this variability is unclear.28,29 However, when a constant
acetate concentration was maintained by the addition of acetate
ions as [N(n-Bu)4]OAc (0.38 or 0.60 M), a reproducible reaction
rate independent of the acetate concentration was observed (kobs
primarily methyl acetate and (dppbz)PtMe2 (2b) (94%) (kobs
)
1.0 × 10-5 s1, kCO ) 0.9 × 10-5 s-1), with carbon-carbon
reductive elimination to form ethane and (dppbz)PtMe(OAc)
(3b) representing a minor reaction (kCC ) ca. 1 × 10-6 s-1
)
(Scheme 1). In the absence of added PVP, higher amounts of
C-C coupling from 1b were observed, as was the case for 1a.
When [N(n-Bu)4]OAc was added to the reaction, no carbon-
carbon coupling was observed. The observed rate of C-O
reductive elimination from 1b was also independent of the
amount of [N(n-Bu)4]OAc that was added to the reaction (20
mM [N(n-Bu)4]OAc, kobs ) kCO ) 1.2 × 10-5 s-1; 48 mM
[N(n-Bu)4]OAc, kobs ) kCO ) 1.1 × 10-5 s-1). It should be
noted that the rate constant for CO coupling for 1b in THF-d8
is virtually identical to that for the dppe analogue 1a
(kCO ) 1.2 × 10-5 s-1).
Effect of Lewis Acids. Consistent with the observation that
the addition of the acid scavenger, PVP, resulted in a retardation
of C-C reductive elimination from the platinum acetate
complexes 1a and 1b, addition of acids to the thermolysis
reactions of 1a caused a striking acceleration of the carbon-
carbon coupling rate. In C6D6, the half-life of thermolysis of
1a is approximately 15 h at 99 °C, forming primarily C-O
products (90%). However, in the presence of even a small
amount of triflic acid (0.1 equiv, 3 mM) or silver triflate (0.1
equiv, 6 mM), a dramatic change in reactivity occurred. Under
these conditions, the C-C coupling products were produced
exclusively at room temperature over the course of a day.
(28) Possible sources of variability in kobs(acetone-d6): (a) Trace amounts
of a Lewis acidic impurity could cause erratic kinetic behavior (see Results
under Effect of Lewis Acids). (b) The acetone reactions may not be strictly
first order. In contrast to thermolysis in less polar solvents, >95% of the
reaction proceeds via C-C reductive elimination, which is inhibited by
acetate ions (see text). The acetate concentration is a complex function of
the concentration of 1a, that of the major product, (dppe)PtMe(OAc) (3a),
and of any acetic acid produced.29
(29) A side reaction between the primary Pt(II) product, (dppe)PtMe-
(OAc) (3), and acetone-d6 to form (dppe)PtMe(CD2COCD3)24 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 Results under Effect of Lewis Acids) rather than simple variability
between experiments.
(24) 3a and (dppe)PtMe(CD2COCD3): Appleton, T. G.; Bennett, M. A.
Inorg. Chem. 1978, 17, 938.
(25) Swain, C. G.; Swain, M. S.; Powell, A. L.; Alunni, S. J. Am. Chem.
Soc. 1983, 105, 502.
(30) 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.
(26) The rate constant for C-O reductive elimination is defined as
follows: kco ) (percent C-O products formed)(kobs)/100. Similarly, kcc
)
(percent C-C products formed)(kobs)/100 so that kobs ) kCO + kCC
.
(31) During the course of the thermolysis reaction, conversion of
[N(n-Bu)4]OAc to (n-Bu)3N and n-BuOAc occurs at a rate approximately
an order of magnitude slower than the Pt(IV) reductive eliminations.
(27) A similar variation in rates was observed at 79 °C (kobs ) (3.4-19)
× 10-5 s-1).

Formation of C-O Bonds from Pt(IV) Complexes
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2579
Figure 3. First-order kinetic plots of the thermolyses of fac-(dppe)-
PtMe3(p-OC6H4Me) (6a) at 120 °C with added aryloxide sources (9
) p-cresol, 2 ) [18-crown-6-potassium][p-cresolate]).
Figure 2. Curved first-order kinetic plots of the thermolysis of fac-
(dppe)PtMe3(p-OC6H4Me) (6a) at 120 °C (9 ) C6D6, 2 ) THF-d8).
Scheme 2
Scheme 3
The addition of acetic rather than triflic acid to a C6D6
solution of 1a afforded a rather different result. The initial ethane
formation rate constant at 99 °C (kCC ) ca. 5 × 10-6 s-1 26
)
was clearly faster than that without added acid (ca. 1 × 10-6
s-1), yet the acceleration was much less pronounced than with
triflic acid. Also unlike the triflic acid reaction, the primary
products were the C-O coupling products, (dppe)PtMe2 (2a)
and methyl acetate (ca. 80%). Thus, the initial rate constant of
methyl acetate production (kCO ) 2.2 × 10-5 s-1) was
approximately double that of the rate constant without acetic
acid. However, as the reaction proceeded, the product 2a itself
reacted with the acetic acid to form (dppe)PtMe(OAc) (3a) and
CH4,32 and the reaction approached its uncatalyzed product ratio
and rate constant (ca. 90% C-O coupling and kobs ) ca. 1.2 ×
10-5 s-1).
Addition of p-cresol (0.6 equiv, 18 mM) to the thermolysis
of 6a in THF-d8 caused a substantial increase in reaction rate,
but also resulted in linear first-order behavior (kobs ) 1.77 ×
10-4 s-1, 85% C-O coupling, kCO ) ca. 1.5 × 10-4 s-1) (Figure
3). Addition of the conjugate base, [18-crown-6-K][p-OC6H4-
Me], also produced linear first-order behavior (kobs ) 3.61 ×
10-5 s-1, 93% yield of methyl acetate) (Figure 3). However, in
this latter thermolysis reaction, other side products were
observed. The first of these side products was indicated by a
broad singlet at ca. 17 ppm which is assigned to the bicresolate
anion H[p-OC6H4Me]2 .
- 33 An additional series of signals, just
1
visible above the baseline of the H and 31P NMR spectra, are
assigned to the species cis-(PMePh2)(P(CHCH2)Ph2)PtMe2 (8),
cis-(P(CHCH2)Ph2)2PtMe2 (9), and cis-(PMePh2)2PtMe2 (10)
(Scheme 3).
Platinum(IV) Aryloxides: (a) Platinum(IV) Aryloxide
Complexes of dppe. The complex fac-(dppe)PtMe3(p-OC6H4-
Me) (6a) was prepared in good yield (81%) by the reaction of
fac-(dppe)PtMe3(O2CCF3) (4a) with potassium p-cresolate in
1
THF at -78 °C and characterized by H, 13C, and 31P NMR
While 8-10 are minor side products when generated by the
addition of a relatively mild base such as cresolate to dppe
complexes, compounds 8-10 can also be quantitatively gener-
ated in THF by the reaction of fac-(dppe)PtMe3(O2CCF3) (4a)
with the strong base KOH. This high-yield production of 8-10
allowed us to identify and characterize these complexes via 1H,
31P{1H}, 1H{31P}, 1H COSY, and 1H/31P HELCO NMR
experiments.34 Complex 8 was the major product at early
reaction times, but the distribution between 8, 9, and 10 reached
a statistical 2:1:1 pattern over the course of the reaction as a
result of phosphine exchange.35 Subsequently, dppe was added
to the reaction mixture, which cleanly generated (dppe)PtMe2
spectroscopy and elemental analysis. Thermolysis of 6a in the
relatively nonpolar solvent THF-d8 at 120 °C afforded primarily
the carbon-oxygen coupling products p-methylanisole and
(dppe)PtMe2 (2a) (ca. 85%). The carbon-carbon coupling
products, ethane and (dppe)PtMe(p-OC6H4Me) (7a), made up
the remaining fraction (ca. 15%) (Scheme 2). However, linear
first-order kinetic behavior was not observed in these thermoly-
ses of 6a in THF-d8 or in similar reactions carried out in C6D6
(Figure 2). Instead, the reactions accelerated as they progressed.
As noted by the curvature in the plots in Figure 2, this
accelerative behavior is more apparent in C6D6 than in THF-
d8. While accurate rate constant data could not be obtained from
these nonlinear plots, the initial observed rate constants in both
(33) The chemical shift of this species varied between 16.5 and 17.5
ppm dependent upon the individual thermolysis reaction and the extent of
solvents were approximately 3 × 10-5 s-1
.
1
reaction within the thermolysis. The same H NMR signal was generated
(32) The reaction between 2a and acetic acid was demonstrated
when p-cresol was added to [18-crown-6-K][p-OC6H4Me].
independently.
(34) HEteronuclear Long-range COrellation.

2580 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Williams and Goldberg
Scheme 4
Thus, the reaction is considerably faster for more electron
withdrawing substituents (kobs ) 5.6 × 10-4 s-1 for p-CN) than
for electron donating substituents (kobs ) 2.1 × 10-5 s-1 for
p-Me).
The dppbz complex fac-(dppbz)PtMe3(p-OC6H4Me) (6b1)
demonstrates similar behavior to that of the dppe analogue 6a
in the presence of Lewis acids. While in the absence of acid,
kobs ) (2.7 ( 0.2) × 10-5 s-1, the addition of p-cresol brings
about a significant acceleration of the reaction (kobs ) 3.2 ×
10-4 s-1, 30 mM, 0.8 equiv). In addition, C-C coupling to
form ethane and (dppbz)PtMe(p-OC6H4Me) (7b1) increases from
15% to 30% of the reaction products (kCO ) 2.2 × 10-4 s-1
and kCC ) 1.0 × 10-4 s-1). Thus, acids have a catalytic effect
on both the C-O and C-C coupling processes. This is strikingly
similar behavior to that observed for the Pt acetate complex
fac-(dppe)PtMe3(OAc) (1a).
Also of note is that the exchange of aryloxides is more rapid
and occurs at lower temperatures than reductive elimination for
these complexes. When 18-crown-6 potassium phenoxide (6.0
mM, 0.21 equiv) was added to a solution of the platinum
p-cresolate complex 6b1 (28.2 mM in THF-d8) and the tube
heated at 79 °C, complete exchange (formation of an equilibrium
mixture of 6b1and 6b2 and the corresponding aryloxide salts)
was observed over the course of 20 h.40 Since exchange is
observed at temperatures 40 °C lower than those at which
reductive elimination occurs at similar rates (120 °C), aryloxide
exchange is considerably faster than the reductive elimination
reactions.
(c) A Monodentate Phosphine Aryloxide Complex of
Pt(IV). The monodentate phosphine complex fac-(PMe3)2PtMe3-
(p-OC6H4Me) (6c) was prepared in high yield (91%) by reaction
of potassium p-cresloate with [PtMe3OTf]4 followed by the
addition of PMe3. When heated at 120 °C in THF-d8, this
complex rapidly undergoes reductive elimination to form ethane
and a mixture of cis- and trans-(PMe3)2PtMe(OAr) (cis- and
trans-7c) in 93% yield (77% cis, 16% trans, Scheme 4). A small
amount (ca. 4%) of the C-O coupling products, p-methylanisole
and cis-(PMe3)2PtMe2 (2c),41 was observed. The reaction was
Figure 4. Hammett plot for the thermolysis of 6b1-6 in THF-d8 at
120 °C in the presence of [18-crown-6-potassium][OAr -].
Table 3. Observed Rate Constants and σ- Values for Thermolyses
of 6b1-6 in THF-d8 at 120 °C in the Presence of
-
[18-Crown-6-potassium][OAr
]
complex
p-X
σ -
kobs (105 s-1
)
6b1
6b2
6b3
6b4
6b5
6b6
Me
H
OMe
Cl
CF3
CN
-0.15
0.00
-0.16
+0.27
+0.56
+0.89
2.05
2.85
2.22
8.92
28.7
56.2
(2a), PPh2Me, and PPh2(CHCH2), all of which had spectral
characteristics that matched those of authentic samples.
(b) Aryloxide Complexes of dppbz. The aryloxide complex
fac-(dppbz)PtMe3(p-OC6H4Me) (6b1, dppbz ) o-bis-(diphenyl-
phosphino)benzene) was prepared by reaction of [PtMe3(OTf)]4
with potassium p-cresolate followed by the addition of dppbz
1
(70% yield). The complex was characterized by H, 13C, and
31P NMR spectroscopy and elemental analysis. When 6b1 was
heated in THF-d8 at 120 °C in the presence of PVP, p-
methylanisole and (dppbz)PtMe2 (2b) were the major products
(92%), with the remainder being the C-C coupling products
ethane and (dppbz)PtMe(p-OC6H4Me) (7b1) (Scheme 2).36 In
contrast to the thermolysis reactions of the dppe analogue, fac-
(dppe)PtMe3(p-OC6H4Me) (6a), the thermolysis of 6b1 obeyed
clean first-order kinetics through greater than three half-lives
(kobs ) (2.7 ( 0.2) × 10-5 s-1). Addition of p-cresolate as the
18-crown-6-potassium salt in various concentrations (9-30 mM)
to the reaction completely inhibited the formation of ethane and
7b1. The rate of C-O coupling in these reactions was
independent of the concentration of added aryloxide (kobs ) kCO
) (2.1 ( 0.2) × 10-5 s-1).
(37) A wide variety (see refs 37a-c, 38, and 40) of values for σ- for
any given substituent is available in the literature. The values reported in
ref 37a were used as this represented the most recent and comprehensive
source, and the values were exclusively determined by the pKa of the
analogous phenol rather than the anilinium cation (which frequently yields
a very different σ- value (ref 37c)). The value for p-CF3 was obtained from
ref 37b due to its exclusion from ref 37a. (a) Fischer, A.; Leary, G. J.;
Topsom, R. D.; Vaughan, J. J. Chem. Soc. B 1967, 846. (b) Liotta, C. L.;
Smith, D. F., Jr. Chem. Commun. 1968, 7, 416. (c) Correlation Analysis in
Chemistry: Recent AdVances; Chapman, N. B., Shorter, J., Eds.; Plenum
Press: New York, 1978.
(38) (a) See the following for an excellent early review of Hammett
relationships: Jaffe´, H. H. Chem. ReV. 1953, 53, 191. (b) Exner, O. In
Correlation Analysis in Chemistry-Recent AdVances; Chapman, N. B.,
Shorter, J., Eds.; Plenum Press: New York, 1978; p 439. (c) AdVances in
Linear Free-Energy Relationships; Chapman, N. B., Shorter, J., Eds.;
Plenum Press: London, 1972.
The analogous compounds fac-(dppbz)PtMe3(p-OC6H4X)
(X ) H (6b2), OMe (6b3), Cl (6b4), CF3 (6b5), CN (6b6)) were
prepared in the same manner as 6b1. Thermolysis of the
complexes 6b1-6 in THF-d8 at 120 °C in the presence of the
appropriate 18-crown-6 potassium aryloxide in each case yielded
the anisole product and 2b in quantitative yield. All thermolyses
followed first-order kinetic behavior and the respective rate
constants are listed in Table 3. Figure 4 shows a Hammett plot
for this reaction, using σ- values.37-39 The Hammett plot
exhibits a good fit with a positive F value of 1.44 (r2 ) 0.986).
(35) (a) Rominger, R. L.; McFarland, J. M.; Jeitler, J. R.; Thompson, J.
S.; Atwood, J. D. J. Coord. Chem. 1994, 31, 7. (b) Morita, D. K.; Stille, J.
K.; Norton, J. R. J. Am. Chem. Soc. 1995, 117, 8576. (c) Garrou, G. E.
AdV. Organomet. Chem 1984, 23, 95 and references therein.
(36) Variable and higher percentages of C-C coupling were observed
in the absence of the acid scavenger, PVP.
(39) Exner, O. Correlation Analysis of Chemical Data; Plenum Press:
New York, 1988; and references therein.
(40) Overlap of the signals in the NMR spectrum prevented facile
quantitative measurement of the equilibrium constant.
(41) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411.

Formation of C-O Bonds from Pt(IV) Complexes
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2581
Scheme 5
reductive elimination, the rate constant for C-C coupling can
be expressed as kCC ) k1k3/(k-1[OR-] + k3).46 Thus, as [OR-]
increases, the rate of C-C reductive elimination will approach
zero. As expected for a mechanistic pathway involving the
generation of ionic intermediates (L2PtMe3+ and OR-), the use
of more polar solvents has a significant accelerative effect on
the rate of C-C coupling from fac-L2PtMe3OR. For example,
the rate constant for this reaction increases by approximately 3
orders of magnitude on going from C6D6 to nitrobenzene-d5
for the thermolysis of fac-(dppe)PtMe3(OAc) (1a) (Table 2).
The addition of Lewis acids, which can coordinate to OR
and assist in OR- dissociation (increasing k1 and decreasing
k-1, Scheme 5), would also be expected to accelerate the C-C
reductive elimination reaction. The ability of even weak acids
to bind to these species is demonstrated by the X-ray crystal
structure of the dppe Pt(IV) acetate complex, 1a, in which a
water molecule is hydrogen bound to the acetate moiety (Figure
1). In agreement with this prediction, the addition of small
amounts of triflic acid or silver triflate to benzene-d6 solutions
of the Pt(IV) trimethyl acetate complex 1a resulted in very high
rates of C-C coupling. The addition of acetic acid produced a
somewhat smaller rate acceleration of the C-C reductive
elimination (ca. 5×) as the effect was attenuated by the
simultaneous addition of the acetate ion. Similar enhancements
of the rate of C-C coupling were observed for the p-cresolate
complexes of dppe (6a1) and dppbz (6b1) when small amounts
of cresol were added to the thermolysis reactions. Finally, it
should be noted that carbon-carbon reductive elimination is 2
orders of magnitude more rapid from the trifluoroacetate
complex 4a than from the acetate complex 1a. As trifluoro-
acetate is a much better leaving group than acetate, this again
supports a mechanism involving dissociation of the OR- group
prior to ethane elimination (Scheme 5).
For fac-L2PtMe3X (X ) halide or methyl) complexes
containing monodentate ligands L, evidence has been provided
for L dissociation as a preliminary step in the mechanism of
C-C reductive elimination.45a,b Even in the case of the bidentate
phosphine complex, (dppe)PtMe4, there is evidence for the
dissociation of one arm of the chelate prior to the C-C coupling
step.44 To probe the importance of preliminary phosphine
dissociation in reductive elimination reactions from fac-L2PtMe3-
OR complexes, the thermal reactivity of the monodentate
phosphine complex fac-(PMe3)2PtMe3(p-OC6H4Me) (6c) was
investigated. Carbon-carbon reductive elimination from 6c is
much faster than from the analogous compounds bearing
bidentate ligands; at 120 °C, thermolysis is nearly complete
within 1 h. Reductive elimination produces predominantly the
C-C coupled products ethane and an equilibrium mixture of
cis- and trans-(PMe3)2PtMe(p-OC6H4Me) (cis- and trans-7c)
(>90%). This reaction was not first order in the absence of
added phosphine. In contrast, addition of ca. 2 equiv of PMe3
greatly inhibits the rate of carbon-carbon reductive elimination
and results in first-order kinetic behavior.47 This inhibition by
added phosphine suggests that PMe3 dissociation precedes C-C
coupling from 6c.
approximately 90% complete in 1 h, but did not exhibit linear
first-order kinetic behavior.
When PMe3 was added to the reaction (2 equiv), the
disappearance of 6c followed first-order behavior (kobs ) 7.5
× 10-7 s-1). More significant is that the rate of ethane formation
was slowed by approximately a factor of a 1000 and C-O
coupling became competitive. Throughout the reaction, the C-O
coupled product p-methylanisole was produced in 40% yield.42
Discussion
Mechanism of Carbon-Carbon Bond Formation. Carbon-
carbon reductive elimination has been observed from a number
of Pt(IV) phosphine alkyl complexes. Mechanistic evidence
suggests that for all sp3-sp3 carbon-carbon couplings from Pt-
(IV), dissociation of a ligand occurs prior to reductive
coupling.43-45 For the complexes fac-L2PtMe3X, where L ) a
monodentate neutral ligand and X ) halide or alkyl group,
preliminary dissociation of L occurs.45a,b For fac-L2PtMe3I where
L2 is the bidentate phosphine dppe, iodide dissociation precedes
C-C coupling.43 Finally, for (dppe)PtMe4, dissociation of one
arm of the phosphine chelate occurs prior to C-C coupling.44
Thus in each case, the C-C coupling reaction takes place from
a five-coordinate intermediate. The results of our study of the
thermal chemistry of fac-L2PtMe3OR complexes point to a
similar mechanism for the C-C reductive elimination reactions
from these compounds.
For fac-L2PtMe3OR (OR ) carboxylate, aryloxide) complexes
with bidentate phosphines, the thermally promoted carbon-
carbon reductive elimination reaction was completely inhibited
by the addition of the conjugate base, OR-. This inhibition
points to a mechanism in which carboxylate or aryloxide
dissociates prior to the C-C bond forming step (Scheme 5).
Ignoring contributions to the rate law from the competitive C-O
(42) Due to overlap of signals, ethane and the platinum products could
not be integrated during the progress of the reaction. Unfortunately, the
long reaction times (>2 months for the reaction to reach completion) and
high temperature (120 °C) caused degradation of the platinum products
over time, preventing clear quantification of the products other than the
anisole, which was consistently produced in 40% yield throughout the
reaction.
(43) (a) Goldberg, K. I.; Yan, J. Y.; Breitung, E. M. J. Am. Chem. Soc.
1995, 117, 6889. (b) Goldberg, K. I.; Yan, J. Y.; Winter, E. L. J. Am. Chem.
Soc. 1994, 116, 1573.
The possibility that similar phosphine dissociation is important
in the thermolysis of the bidentate phosphine complexes fac-
L2PtMe3OR (L2 ) dppe, dppbz) was also considered. However,
(44) Crumpton, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2000, 122,
962 and references therein.
(45) (a) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E. J. Chem. Soc.,
Dalton Trans. 1974, 2457. (b) Roy, S.; Puddephatt, R. J.; Scott, J. D. J.
Chem. Soc., Dalton Trans. 1989, 2121. In the absence of added L, 1-2%
of C-C reductive elimination from L2PtMe4 (L ) MeNC, 2,6-Me2C6H3-
NC) may proceed without ligand dissociation. Significant decomposition
of L under the reaction conditions hinders complete analysis of the data.
(c) Hill, G. S.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1999, 18,
1408. (d) Hill, G. S.; Puddephatt, R. J. Organometallics 1997, 16, 4522.
(46) Note that while the competitive C-O coupling reaction does actually
affect the rate law (k2 appears in the denominator; see Conclusions), this
inverse dependence of kCC upon [OR-] remains valid even when the C-O
coupling reaction is taken into account.
(47) Note that in the absence of PMe3, only small amounts (<5%) of
p-methylanisole and cis-(PMe3)2PtMe2 (2c) are produced, while in the
presence of added PMe3, the percentage of anisole formed rises to 40%

2582 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Williams and Goldberg
Scheme 6
It was recently reported that C-C reductive elimination from
the related bidentate phosphine complexes (dppe)PtMe3R (R
) Me, Et) proceeds via such preliminary phosphine dissociation
at 165 °C.44 In addition, fac- and mer-(dppe)PtMe3Et were
shown to interconvert at 100 °C on a time scale of hours. Since
such isomerizations of Pt(IV) octahedral complexes are generally
proposed to occur via five-coordinate intermediates,48,49 it is
likely that dissociation of one end of the dppe chelate is
kinetically accessible in the 99 °C carboxylate and 120 °C
aryloxide thermolyses. The role of phosphine dissociation (Path
B) in C-O coupling from platinum(IV) was investigated by
the study of the monodentate phosphine complex fac-(PMe3)2-
PtMe3(p-OC6H4Me) (6c). Upon thermolysis, this complex was
observed to rapidly undergo C-C, not C-O reductive elimina-
tion. Appreciable C-O coupling (40%) was observed only when
the phosphine dissociation pathway was blocked by addition
of PMe3. This demonstrates that while C-O coupling can occur
from 6c, phosphine dissociation is not on the reaction coordinate,
and that phosphine loss leads instead to C-C elimination.
Additional evidence against preliminary phosphine dissociation
in these C-O coupling reactions from Pt(IV) is the similarity
in the rate constants of C-O coupling from fac-(dppe)PtMe3-
(OAc) (1a) and fac-(dppbz)PtMe3(OAc) (1b). Both complexes
the fact that C-C coupling is completely inhibited in the
bidentate cases by the addition of OR- argues strongly against
this proposal. While it is conceivable that OR- could coordinate
to the Pt center after phosphine dissociation, it is highly unlikely
that OR- would be able to compete effectively with recoordi-
nation of the pendant phosphine arm. It should be noted that
very small concentrations of OR- were sufficient to completely
inhibit C-C coupling; in the case of fac-(dppbz)PtMe3(p-
OC6H4Me) (6b1), no C-C coupling was observed even when
[OAr-] was as low as 3 mM. Thus, it seems that in these fac-
L2PtMe3(OR) complexes, phosphine dissociation prior to C-C
coupling is the primary mechanism for C-C reductive elimina-
tion from those complexes containing monodentate phosphines.
However, such phosphine dissociation is essentially unimportant
in cases in which the phosphine is bidentate. Instead, OR-
dissociation precedes C-C coupling for these complexes. These
results are similar to those found for C-C reductive elimination
from the complexes fac-L2PtMe3I (L ) PMe2Ph, L2 ) dppe)
in which either monodentate phosphine dissociates or iodide
dissociates prior to C-C coupling.43,45a
Mechanism of C-O Reductive Elimination. While reduc-
tive elimination reactions to form C-C bonds from platinum-
(IV) centers are well-known,43-45 such reactions to form C-O
bonds are extremely rare.17 We have observed and been able to
study in detail the mechanisms of C-O reductive elimination
from several different Pt(IV) alkyl carboxylate and aryloxide
complexes. The C-O reductive elimination reactions from these
various complexes share many key elements. For the complexes
fac-L2PtMe3OR (L2 ) bidentate phosphine; OR ) carboxylate,
aryloxide), the C-O coupling reactions are independent of the
external concentration of OR-, and are accelerated by more
polar solvents and by acids. The carbon-oxygen coupling
reactions were also found to be faster for more electron
withdrawing R groups. These shared features suggest a common
mechanism for C-O reductive elimination from these Pt(IV)
carboxylate and aryloxide complexes.
exhibit a rate constant for C-O reductive elimination of kCO
)
1 × 10-5 s-1. If phosphine dissociation were required to occur
prior to the C-O coupling reaction, one would expect that the
more rigid dppbz complex would react more slowly. This effect
was observed in the investigation of C-C reductive elimination
from L2PtMe4 (L2 ) dppe, dppbz), in which the dppbz complex
underwent C-C coupling approximately 2 orders of magnitude
more slowly than the corresponding dppe complex.44
Exclusion of Path A and Path B leaves us to distinguish
between the single-step, concerted pathway C and the two-step
OR- dissociation/nucleophilic attack pathway D. A substantial
body of evidence suggests that Path D is the more reasonable
mechanism. First, C-O coupling from the acetate 1a is four
times faster in acetone-d6 (4.0 × 10-5 s-1) than in THF-d8 or
benzene-d6 (1.1 × 10-5 s-1). This solvent dependence suggests
an ionic or polar transition state.50 Such a transition state would
be expected for Path D, but not for a concerted elimination such
as Path C. In addition, C-O coupling from the acetate, 1a, is
accelerated by acetic acid and C-O coupling from the p-
cresolate complex 6b1 is accelerated by p-cresol. The assistance
of an acid should promote the dissociation of OR- from fac-
L2PtMe3(OR) (Keq in Path D, Scheme 6). This would increase
the overall reaction rate for Path D, while it is difficult to
reconcile such an acceleration with the mechanism shown in
Path C.
The rate constants for C-O reductive elimination from the
aryloxide complexes 6b1-6 were easily measured in the presence
of added aryloxide. Under these conditions, C-C coupling was
completely inhibited and the rate constants (kCO) were shown
to be independent of the added aryloxide concentration. A
Hammett plot of the rate constants kCO for 6b1-6 exhibited a F
value of 1.44, which demonstrates that the reaction rate increases
with more electron withdrawing aryloxides. Note that a similar
trend is observed in the carboxylate complexes, in which the
platinum(IV) trifluoroacetate complex 4a undergoes C-O
reductive elimination an order of magnitude faster than the
Four different mechanisms for C-O reductive elimination
from fac-L2PtMe3(OR) (L2 ) dppe, dppbz) have been consid-
ered (Scheme 6). The first of these, Path A, involves direct
nucleophilic attack upon the six-coordinate species by OR-.
However, such a reaction path would show a first-order kinetic
dependence upon OR- (kobs ) k1a[OR-]). The C-O coupling
reactions were found to be independent of the concentration of
OR-, and thus Path A can be excluded.
(48) Clark, H. C.; Manzer, L. E. Inorg. Chem. 1973, 12, 362. (b) Crespo,
M.; Puddephatt, R. J. Organometallics 1987, 6, 2548.
(49) Five-coordinate intermediates have also been proposed in the
isomerization of octahedral Ir(III) complexes. Wehman-Ooyevaar, I. C. M.;
Drenth, W.; Grove, D. M.; van Koten, G. Inorg. Chem. 1993, 32, 3347.
(50) Isaacs, N. S. Physical Organic Chemistry; John Wiley and Sons:
New York, 1987; pp 183-184.
The second possible C-O coupling mechanism, Path B,
involves preliminary phosphine dissociation to generate a five-
coordinate intermediate from which C-O coupling takes place.

Formation of C-O Bonds from Pt(IV) Complexes
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2583
acetate analogue, 1a. These observations are more consistent
with Path D, in which dissociation occurs to form a negative
charge at the oxygen, than with Path C.
THF-d8 that the reaction accelerated as time progressed (Figure
2). In addition, the thermolysis of 6a in the presence of 18-
crown-6-potassium aryloxide afforded [18-crown-6-potassium]-
[H(OAr)2] and a mixture of cis-(PMePh2)(P(CHCH2)Ph2)PtMe2
(8), cis-(P(CHCH2)Ph2)2PtMe2 (9), and cis-(PMePh2)2PtMe2 (10)
as minor products (Scheme 3). Similar decompositions of dppe
to vinyl phosphines have been observed in Pd chemistry in the
presence of strong bases.7a,52 When the strong base KOH was
allowed to react with fac-(dppe)PtMe3(O2CCF3) (4a), the same
Pt(II) complexes (8-10) were generated quantitatively.
The absolute magnitude of the F value is difficult to predict
but a F value of 1.44 can be shown to be consistent with Path
D by analysis of the rate law for this mechanism. Since C-C
reductive elimination is completely inhibited in the presence
of added aryloxide, there is no contribution of a C-C coupling
path to the rate expression under these conditions. When a
steady-state concentration of the intermediate five-coordinate
cationic Pt(IV) species (A, Scheme 6) is assumed, a first-order
rate expression with kobs ) k1k2/(k-1 + k2) is derived for Path
D. However, it was shown that the exchange of aryloxides is
significantly more rapid than the C-O reductive elimination
reaction. Complete exchange with added aryloxide was observed
(i.e., equilibrium was reached) at temperatures much lower than
those required for reductive elimination. Thus, the first step of
the mechansim depicted in Path D, dissociation of the aryloxide
group, should be viewed as a preequilibrium. The second step,
nucleophilic attack of the aryloxide on the platinum bound
methyl group of intermediate A, is then rate limiting and the
kinetic expression simplifies to kobs ) (k1/k-1)k2 ) Keqk2. Since
kobs ) Keqk2, Fobs ) Feq + F2.51 These constituent F values can
be crudely estimated from analogous organic reactions. For
dissociation of phenols in water at 25 °C, Feq has been
determined to be 2.113, while nucleophilic attack at sp3 carbon
by aryloxides has been shown to exhibit F values lying between
-0.771 and -0.994.37c Thus, a value of ca. 1.2 could be
predicted for Path D. While this is a Very rough estimate (there
should be substantial solvent effects in these reactions and the
substrates involved are not metal-bound methyl groups), it
suggests that a F value of 1.44 is at least consistent with such
a mechanism.
Thus, the mechanistic evidence provides the strongest support
for Path D, a two-step pathway in which the OR- group
dissociates, and then peforms SN2 attack upon a methyl group
of the five-coordinate cation A. It should also be noted that the
second step in Path D, the nucleophilic attack of an oxygen
center upon an electrophilic Pt(IV) methyl group, bears a strong
resemblance to the proposed final step in the Shilov cycle.2,3
Labinger and Bercaw have provided convincing evidence that
this alcohol formation step proceeds by SN2 attack of water on
a Pt(IV) alkyl ligand.3
An interesting comparison can be made between this mech-
anism for C-O reductive elimination from Pt(IV) and that
proposed for carbon-heteroatom reductive elimination from
Pd(II).7b,c,12 Reductive elimination reactions to form aryl C-O
and C-N bonds serve as the product formation steps in
palladium-catalyzed syntheses of aryl ethers and arylamines,
respectively.6-8,12 In contrast to the Pt(IV) reactions, the rates
of the Pd(II) reactions increase with increasing nucleophilicity
of the heteroatom group. This is because the reactions proceed
via a direct intramolecular nucleophilic attack of the heteroatom
on the electrophilic hydrocarbyl group.7b,c,12 Initial dissociation
of the heteroatom group prior to coupling is not required. Thus,
although directly opposite trends are observed with respect to
the electronics of the heteroatom group in these carbon-
heteroatom couplings from Pt(IV) and Pd(II), the reactions share
a common feature in that the heteroatom group acts as a
nucleophile upon a metal-bound carbon.
Since the most acidic protons on the dppe Pt(IV) complexes
are those in the phosphine ethyl backbone, initial deprotonation
of this site is a likely first step in the degradation. Consistent
with this analysis, the complex analogous to 6a with a phosphine
aryl backbone, fac-(dppbz)PtMe3(p-OC6H4Me) (6b1), did not
display curvature in first-order kinetic plots of its thermolysis
reaction and no signs of phosphine degradation were observed
even in the presence of added aryloxide. The degradation of
the dppe backbone suggests an explanation for the nonlinear
first-order kinetic behavior of fac-(dppe)PtMe3(p-OC6H4Me)
(6a). Even in the absence of added aryloxide, there should be
a small concentration of aryloxide due to dissociation of
aryloxide from 6a. The resultant Pt(IV) cation would be
susceptible to deprotonation by the aryloxide anion. While this
cannot be a major reaction (<3%) because the products are not
1
observed by H or 31P NMR, even a trace amount of phenol
produced in this way would be sufficient to accelerate the
reaction and result in non-first-order behavior. Note that the
addition of a larger amount of p-cresol (0.8 equiv) to the
thermolyses of 6a brought about a much faster reaction, but
also resulted in first-order kinetic behavior. Under these
conditions, the cresol is present under pseudo-first-order condi-
tions (i.e., the generation of trace amounts of cresol would not
change the overall concentration of cresol by an appreciable
amount). Similarly, addition of 18-crown-6-potassium aryloxide
to the thermolysis of 6a also resulted in first-order behavior. In
this case, the cresol produced is trapped in the form of the
bicresolate anion H(OAr)2- (observed by 1H NMR), which is a
much less competent acid catalyst.
Conclusions
The limited number of model complexes for which studies
of C-O reductive elimination involving alkyl carbons from
high-valent metal centers could previously be conducted has
inhibited the development of a mechanistic understanding of
this important C-O coupling reaction. We have observed and
carried out detailed investigations of carbon-oxygen reductive
elimination reactions from a variety of Pt(IV) complexes of the
form fac-L2PtMe3(OR) (R ) acyl, aryl) (Scheme 7). The C-O
reductive elimination reactions to form methyl esters or methyl
aryl ethers from these complexes compete in many cases with
C-C coupling reactions to form ethane. Our evidence supports
two-step mechanisms for both carbon-oxygen and carbon-
carbon reductive elimination reactions from the Pt(IV) com-
plexes fac-L2PtMe3OR. C-O reductive elimination takes place
by preliminary dissociation of OR- followed by nucleophilic
attack of the dissociated OR- anion on a methyl carbon of the
five-coordinate cationic Pt(IV) species. This attack by an oxygen
nucleophile upon an electrophilic Pt(IV)-bound methyl group
is closely analogous to that proposed to be operative in the
Shilov oxidation catalyst.
Origin of Nonlinear Kinetics in the Thermolysis of fac-
(dppe)PtMe3(p-OC6H4Me) (6a). It was noted during the
thermolysis of fac-(dppe)PtMe3(p-OC6H4Me) (6a) at 120 °C in
While C-O coupling proceeds via OR- dissociation and SN2
attack by OR- upon a platinum-bound methyl group, carbon-
(51) Reference 39, p 70.
(52) Zhuravel, M. A.; Glueck, D. S. Personal communication.

2584 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Williams and Goldberg
Scheme 7
tion of OR-, and C-C elimination, which is independent of
the nature and concentration of OR-, selection for C-O
reductive elimination can be favored by the use of nonpolar
solvents, more nucleophilic OR- species, and the addition of
OR- to the reaction. In the presence of added OR-, carbon-
carbon coupling is completely inhibited and the rate of carbon-
oxygen coupling is independent of the concentration of OR-.
Under these conditions, the rate equation simplifies to kobs
kCO ) (k1/k-1)k2 ) Keqk2.
)
Although more nucleophilic OR- groups favor C-O coupling
over C-C coupling, a significant problem was encountered with
the use of basic OR- groups. Since these OR- groups dissociate
from the metal, they can act as competent bases to attack and
degrade the ligand. Dppe complexes of the form fac-(dppe)-
PtMe3(OR) were found to undergo ligand degradation with the
addition of KOH or [18-crown-6-K][OAr] (R ) C(O)CF3, Ar)
or even upon thermolysis without added base (R ) Ar).
However, we have also shown that the judicious choice of a
ligand which lacks acidic protons (e.g. dppbz) can circumvent
the problems posed by basic OR- groups. This aspect of ligand
design must be considered in attempting to use these C-O
coupling reactions in metal-mediated organic transformations.
However, a better reason to avoid the use of highly basic
OR- groups may be that despite the fact that carbon-oxygen
bond formation requires the action of OR- as a nucleophile,
the reaction is actually accelerated by the use of more electron
withdrawing OR- groups. In other words, the weaker OR- is
as a nucleophile, the faster the nucleophilic coupling of R′ and
OR in the Pt(IV)R′(OR) system. While this is somewhat
counterintuitive, it is a direct result of the unusual two-step
mechanism of C-O coupling in these high-valent plati-
num(IV) species. This implies that the most useful coupling
groups in platinum-catalyzed alkane functionalization processes
may be sulfonates, trifluoroacetate, or water, rather than halides,
hydroxides, acetate, or alkoxides, despite the initially attractive
nucleophilicities of the latter species. Indeed, the Shilov catalyst
system forms methanol by nucleophilic attack of water on a
platinum-bound methyl group2,3 and the Catalytica system
couples a methyl group with bisulfate.4 The use of sulfonates,
trifluoroacetate, and bisulfate to form the C-O coupled products
is particularly attractive because these species can protect the
alcohol derivative from overoxidation.3-5 Thus, not only are
these among the easiest protecting groups to remove in post-
catalysis treatment, they may also be the easiest to attach to
the alkyl fragment in the final metal-mediated step.
carbon reductive elimination from Pt(IV) proceeds via prelimi-
nary dissociation of either a neutral phosphine ligand or OR-
anion followed by C-C coupling from the five-coordinate
Pt(IV) intermediate. If phosphine dissociation is limited by the
chelate effect, then both C-O and C-C coupling from fac-L2-
PtMe3OR (R ) acyl, aryl) proceed through the same five-
coordinate intermediate as shown in Scheme 7. The overall rate
expression for the reaction in Scheme 7 (eq 1) predicts that both
C-O and C-C reductive elimination reactions would be
facilitated by factors which enhance dissociation of OR- from
the six-coordinate starting material (increase k1 and decrease
k-1). Thus, the incorporation of electron withdrawing groups
into OR-, the addition of Lewis acids, and the use of more
polar solvents all increase the rates of both reductive elimination
reactions.
k1(k2[OR-] + k3)
(k-1 + k2)[OR-] + k3
kobs
)
(1)
Notably, these factors tend to enhance C-C coupling more
than C-O coupling. For example, the trifluoroacetate complex
4a undergoes C-O coupling an order of magnitude faster than
the acetate complex 1a, but the C-C coupling is 2 orders of
magnitude faster. Likewise, HOAc (3 mM) causes a 2-fold
increase in C-O coupling rate from 1a, but a 5-fold increase
in C-C coupling rate.53 C-O coupling from 1a is four times
faster in the more polar solvent acetone-d6 than in C6D6, but
C-C coupling is faster by a factor of 100. These results are all
consistent with the mechanism shown in Scheme 7. Conditions
which enhance OR- dissociation will have an accelerative effect
on C-C reductive elimination because they act to increase k1
and decrease k-1. In contrast, while these same factors accelerate
the first step of the C-O reductive elimination reaction (increase
k1, decrease k-1), they act to inhibit the second step, nucleophilic
attack by OR- on a Pt(IV) methyl group (decrease k2).
Experimental Section
General Considerations. Unless otherwise noted, all transformations
were carried out under an N2 atmosphere in a drybox, using standard
Schlenk techniques, or under vacuum. THF, diethyl ether, benzene,
and toluene were distilled under N2 from sodium/benzophenone ketyl.
Pentane, methylene chloride, and benzyl ether were distilled from CaH2.
Nitrobenzene-d5 was dried over P2O5 and stored over 4 Å sieves.
Acetone was dried over CaSO4 and stored over 4 Å sieves. Deuterated
solvents were dried by analogous methods and transferred under
vacuum. PMe3 was dried over a sodium mirror. Tetrabutylammonium
acetate (Aldrich) was recrystallized from THF/diethyl ether. KH was
purchased as a suspension in mineral oil, isolated on a fritted glass
funnel, washed repeatedly with hexanes, and dried under vacuum. o-Bis-
(diphenylphosphino)benzene (dppbz) was purchased from Strem and
was either used as provided or treated with KH and recrystallized from
THF/pentane. Other reagents, unless specified, were used as received
from commercial suppliers. NMR spectra were recorded on Bruker
DPX200, AC200, AF300, DRX499, or AM500 spectrometers. All
coupling constants are reported in Hz. 1H and 13C spectra were
referenced by using residual solvent peaks and are reported in ppm
Choice of reaction conditions can be effectively used to select
for either the C-O or C-C coupled product. Since the five-
coordinate intermediate A (Scheme 7) must choose between
C-O coupling, which depends on the nature and the concentra-
(53) (a) The addition of AgOTf and HOTf shows an even more dramatic
trend in acceleration of C-C coupling, but this can be attributed to a special
salt effect, in which the silver or proton abstracts the anion, forming the
unstable complex fac-Pt(dppe)Me3OTf and HOAc or AgOAc. This is
different from the analogous reaction with HOAc, in which the exchange
reaction is nonproductive, and only the hydrogen bonding or Lewis acid
coordination generate reaction acceleration. For more information and
leading references on special salt effects, see: March, J. AdVanced Organic
Chemistry, 4th ed.; John Wiley and Sons: New York, 1992; pp 303-304.

Formation of C-O Bonds from Pt(IV) Complexes
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2585
Table 4. Crystal Structure Information for
downfield of tetramethylsilane. 31P spectra were referenced to external
H3PO4 (0 ppm). 195Pt spectra were referenced to external aqueous K2-
PtCl4. 19F spectra were referenced to external CF3CO2H and are reported
in ppm downfield of CFCl3 (CF3CO2H ) -77 ppm). IR spectra were
recorded on a Perkin-Elmer 1720 Infrared Fourier Transform spec-
trometer at 1 cm-1 resolution. Elemental analyses were performed by
Canadian Microanalytical Laboratory or Atlantic Microlab. [PtMe3I]4
and fac-(dppe)PtMe3I were prepared by established literature proce-
dures.41,54 Cross-linked poly-4-vinylpyridine (PVP, 2%) was purchased
from Aldrich, and dried under high vacuum for 12-36 h at temperatures
between 100 and 120 °C. Celite was dried for >48 h at >150 °C under
vacuum.
fac-(dppe)PtMe3(OAc)‚H2O (1a‚H2O)
empirical formula
formula weight
temperature (K)
wavelength (Å)
crystal system
space group
unit cell dimensions
a (Å)
C31H38O3P2Pt
715.64
183(2)
0.71073
triclinic
P1h (No. 2)
10.048(2)
12.581(3)
b (Å)
c (Å)
13.538(3)
66.15(2)
70.32(2)
78.16(2)
R (deg)
Preparation of fac-(dppe)PtMe3(OAc) (1a). fac-(dppe)PtMe3I
(311.2 mg, 0.407 mmol) was suspended in toluene (25 mL) and AgOAc
(Aldrich, 99.999%, 77 mg, 0.46 mmol) was added slowly while the
solution was stirred vigorously. The colorless solution became yellow
and cloudy within minutes, was isolated from light, and was allowed
to stir for 13 h. At the end of this time, the yellow silver iodide
precipitate was removed by vacuum filtration through Celite on a
medium porosity glass fritted funnel. The solution was evaporated to
dryness and exposed to air.55 Cross-linked poly-4-vinylpyridine (PVP,
2%) was added and allowed to stir with the product for 3.5 h, to remove
any remaining silver salts. The PVP was removed by gravity filtration
through glass wool and the product recrystallized from toluene/n-
heptane (-25 °C) and washed with pentane (2 crops, 184 mg, 64%
â (deg)
γ (deg)
vol (Å3)
1469.1(6)
Z
2
calcd density (Mg m-3
)
1.618
4.914
abs coeff (mm-1
)
F(000)
crystal description
crystal size (mm)
θ range for data collection (deg)
index ranges
712
colorless rhombahedron
0.35 × 0.30 × 0.25
1.72 to 24.97
-11 e h e 11
-14 e k e 14
-15 e l e 16
5444/5119 [Rint ) 0.0394]
99.7%
reflcns collected/unique reflcns
completion to θ ) 24.97
abs correction
1
yield). 31P{1H}NMR (C6D6): δ 23.0 (s w/Pt satellites, JPtP ) 1147).
1H NMR (C6D6): δ 0.05 (t w/Pt satellites, 3H, 2JPtH ) 70, 3JPH ) 7.8,
Ψ scans
2
Pt-CH3 trans to O), 1.84 (t w/Pt satellites, 6H, JPtH ) 57, 3JPH ) 6.8,
refinement method
full-matrix least squares on F2
5119/0/334
1.075
Pt-CH3 cis to O), 1.80 (s, 3H, PtO2CCH3), 2.5, 3.3 (both m, 2H each,
P-CH2-CH2-P), 6.9-7.3 (m, protons of phosphine phenyl rings), 7.7-
7.8 (t, J ) 8, 4H, ortho-protons of phosphine phenyls proximal to
data/restraints/parameters
goodness-of-fit on F2
R1 (I > 2σ(I))
0.0341
0.0995
2.167 and -1.515
wR2a (all data)
acetate). 13C{1H} NMR (C6D6): δ -9.9 (s w/Pt satellites, 1JPtC ) 632,
largest diff peak and hole (e‚Å3)
1
Pt-CH3 trans to O), 8.2 (dd w/Pt satellites, JPtC ) 539, trans-2JPC
)
120, cis-2JPC ) 5, Pt-CH3 cis to O), 23.5 (s w/Pt satellites, 3JPtC ) 13,
PtO2CCH3), 28.4 (complex AA′X pattern, PCH2CH2P), 126-135
(various aromatic signals for phosphine phenyls, not clearly resolved),
a w ) 1/[σ2(Fo ) + (0.0802P)2 + 0.1789P], where P ) (Fo2 + 2Fc2)/
2
3.
2
175.7 (s w/Pt satellites, JPtC ) 9.5, Pt-O2CCH3). 195Pt{1H}NMR
proximal to trifluoroacetate). 13C{1H} NMR (C6D6): δ -8.2 (s w/Pt
1
(C6D6): 817 (t, JPtP ) 1145). IR(CH2Cl2): νCO ) 1377, 1614 cm-1
.
1
satellites, 1JPtC ) 660, Pt-CH3 trans to O), 9.2 (dd w/Pt satellites JPtC
) 530, trans-2JPC ) 114, cis-2JPC ) 5, Pt-CH3 cis to O), 27.1 (complex
Anal. Calcd for C31H36O2P2Pt‚0.5(H2O) (706.70): C, 52.69; H, 5.28.
Found: C, 52.64; H, 5.09.
1
AA′X pattern, PCH2CH2P), 116.1 (q, JCF ) 292, CF3), 129-133
Solution of X-ray Crystal Structure of 1a. Colorless rhombohedric
crystals of 1a‚H2O were grown by slow evaporation from a toluene
solution at ambient temperature on the benchtop. A crystal was mounted
on a glass capillary with epoxy and data were collected on an Enraf-
Nonius CAD4 diffractometer (Table 4).20 Solution and refinement were
carried out employing XPREP SHELXS and SHELXL provided by
Siemens. All hydrogens were located by using a riding model. All non-
hydrogen atoms were refined anisotropically by full-matrix least squares.
Preparation of fac-(dppe)PtMe3(O2CCF3) (4a). fac-(dppe)Pt-
Me3I‚CH2Cl2 (1.131 g, 1.330 mmol) was dissolved in chilled toluene
(110 mL, -35 °C), to which was added Ag(O2CCF3) (Aldrich, 98%
0.299 g, 1.35 mmol). Yellow precipitate (AgI) immediately became
visible, and after ca. 5 min, the reaction mixture was filtered through
a fine porosity fritted funnel layered with Celite. The solution was
subsequently evaporated to dryness under vacuum, while taking care
to isolate the solution from light. The residue was then dissolved in
diethyl ether (150 mL), gravity filtered through glass wool, and layered
with pentane (100 mL). The solution was cooled to -35 °C and white
crystals formed over a 24 h period. The crystals were washed with
cold ether and pentane (972.7 mg, 97% yield, 3 crops). 31P{1H}NMR
(various aromatic signals for phosphine phenyls, not clearly resolved),
2
161.6 (q JCF ) 35, Pt-O2CCF3). 195Pt{1H} NMR (C6D6): δ 895 (t,
1JPtP ) 1125). 19F NMR (C6D6): δ -74 (br s).
Preparation of fac-(dppbz)PtMe3(OAc) (1b). [PtMe3I]4 (295.6 mg,
0.201 mmol) was dissolved in a minimum of THF (25 mL) and AgOAc
(169.8 mg, 1.02 mmol) was added with vigorous stirring. The reaction
was isolated from light and stirred overnight. The next day, the yellow
AgI precipitate was filtered away through Celite on a glass wool pipet.
The solvent was removed from resultant colorless solution under
vacuum. The solid residue was redissolved in THF (10 mL) and dppbz
(380.7 mg, 0.852 mmol) was added. This reaction was allowed to stir
overnight. The volatiles were removed from the yellow reaction mixture
under vacuum and the resulting solids triturated three times with 2 mL
of toluene. The resultant ivory powder was recrystallized from THF/
pentane at -35 °C to afford a white powder that was washed with
pentane (1 crop, 551.6 mg, 84% yield as 1b‚THF). 31P{1H}NMR (THF-
d8): δ 26.4 (s w/Pt satellites, 1JPtP ) 1147). 1H NMR (C6D6): δ -0.55
(t w/Pt satellites, 3H, 2JPtH ) 70.0, 3JPH ) 7.6, Pt-CH3 trans to acetate),
2
3
1.16 (t w/Pt satellites, 6H, JPtH ) 57.2, JPH ) 7.1, Pt-CH3 cis to
acetate), 1.32 (s, 3H, PtO2CCH3), 7.0-8.0 (m, protons phosphine aryl
rings, unresolved). Anal. Calcd for C35H36O3P2Pt‚C4H8O (745.69): C,
57.28; H, 5.42. Found: C, 57.00; H, 5.14.
1
1
(C6D6): δ 20.7 (s w/Pt satellites, JPtP ) 1124). H NMR (C6D6): δ
0.13 (t w/Pt satellites, 3H, JPtH ) 73, 3JPH ) 7.9, Pt-CH3 trans to O),
2
1.80 (t w/Pt satellite, 6H, 2JPtH ) 56, 3JPH ) 6.9, Pt-CH3 cis to O), 2.5,
3.2 (both m, 2H each, P-CH2-CH2-P), 7.0-7.4 (m protons of phosphine
phenyl rings), 7.77 (t, J ) 8, 4H, ortho-protons of phosphine phenyls
Preparation of fac-(dppe)PtMe3(p-OC6H4Me) (6a). In the drybox,
one Schlenk flask was charged with fac-(dppe)PtMe3(O2CCF3) (4a)
(166.1 mg, 0.221 mmol) and THF (5 mL) and a second with potassium
p-cresolate (98.0 mg, 0.670 mmol) and THF (5 mL). The flasks were
capped with septa, brought out of the drybox, and attached to a Schlenk
line. The solution of 4a was cooled to -78 °C, and the potassium salt
solution was added slowly via cannula. The reaction mixture was
allowed to stir overnight and gradually warmed as the dry ice-acetone
bath thawed, warming to room temperature. The solvent was removed
(54) Appleton, T. G.; Bennett, M. A.; Tomkins, B. J. Chem. Soc., Dalton
Trans. 1976, 439.
(55) 1a was prepared both by rigorously anhydrous conditions and as
described here, and no difference was observed in the thermolysis rates or
product ratios. In all cases, 1a was recrystallized from anhydrous toluene/
pentane under N2 before use in kinetics experiments.

2586 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Williams and Goldberg
Table 5. NMR Data for fac-(dppbz)PtMe3(p-OC6H4X) (6b1-6
under vacuum, and the product extracted with 55 mL of toluene in the
drybox. This suspension was filtered and the solution layered with
pentane (65 mL) and cooled to -35 °C. The next day, the crystals
were washed with pentane and isolated as a toluene monosolvate (150.6
mg, 81% yield, 2 crops). 31P{1H} NMR(THF-d8): δ 12.9 (s w/Pt
)
6b1
p-H
6b2
6b3
6b4
6b5
6b6
p-Me p-OMe p-Cl
p-CF3 p-CN
31P NMR δ
23.0
1135
22.5
1139
22.3
1143
23.2
1143
24.3
1140
25.0
1142
1
1
1JPtP
satellites, JPtP ) 1116). H NMR (THF-d8): δ 0.19 (t w/Pt satellites,
2
3
1H NMR
Mea
3H, JPtH ) 68.8, JPH ) 7.2, methyl group trans to O), 1.15 (t w/Pt
satellites, 6H, 2JPtH ) 58.8, 3JPH ) 7.6, methyl groups trans to P), 2.10
(s, 3H, aryloxide methyl group), 2.8 (4H, m, PCH2CH2P), 6.10 (2H,
d, 3JHH ) 8, aryloxide C-H ortho to O), 6.60 (2H, d, 3JHH ) 8, aryloxide
C-H meta to O), 7-7.5 (complex unresolved aryl signals due to dppe
phenyl groups), 7.97 (t, 4H, JPH
δ
-0.27 -0.30 -0.31 -0.25 -0.19 -0.10
70.0
7.0
1.25
59.0
7.6
2JPtH
3JPH
δ
69.5
6.7
1.22
59.0
7.7
6.22
68.5
7.0
1.17
58.0
7.5
6.22
70.4
7.0
1.21
58.8
7.7
6.23
71.2
7.0
1.27
58.8
7.8
6.35
8.6
7.09
n/a
71.4
7.0
1.31
57.5
7.5
6.34
Meb
OAr
2JPtH
3JPH
δ(H2)
3JHH
δ(H3)
3JHH
δ(X)
3
∼
3JHH ) 9, ortho protons on
phosphine phenyls proximal to aryloxide). 13C{1H} (THF-d8): δ -10.3
(s w/Pt satellites, 1JPtC ) 631, methyl trans to O), 6.8 (dd w/Pt satellites,
1JPtC ) 552, trans-2JPC ) 118, cis-2JPC ) 5, methyl groups cis to O),
21.0 (s, aryloxide methyl group), 27 (m, second-order coupling pattern
due to phosphine coupling, PCH2CH2P), 121.6 (s, aryloxide carbon
para to O), 129.0-129.5 (unresolved m, carbons meta to O on
phosphine phenyls and carbons meta to Pt on aryloxide), 131.1
(unresolved s, para carbons on phosphine phenyls), 133.3 (m, phosphine
ortho carbons distal to aryloxide), 135.6 (m, phosphine ortho carbons
6.30
8.5
8.0
8.5
8.8
8.5
6.81
7.80
6.24b
7.55
7.59
7.13
7.33
7.33
7.96
8.6
6.63
8.00
2.11
7.53
7.57
7.11
7.2
7.2
7.99
8.7
6.45
8.50
3.59
7.58
7.58
7.18
7.2
7.2
8.05
9
6.76
9.00
7.14
8.50
n/a
7.59
7.59
7.1
7.33
7.33
7.78
9.2
7.26
7.0
n/a -59.50a
backbone δ (H6)
Ph
Ph+
7.60
7.63
7.16
7.38
7.38
7.95
8.8
7.32
7.0
7.45
7.5
7.59
7.62
7.13
7.36
7.36
7.84
8.9
7.36
n/a
7.41
n/a
δ (H7)
δ (H9)
δ (H10)
δ (H11)
δ (Η13)
Ph-
proximal to aryloxide), 164.3 (s, aryloxide carbon ipso to O). The 13
C
J(H13) triplet
δ (Η14)
J(H14) triplet
δ (Η15)
spectrum was assigned on the basis of DEPT and 2D correlational
spectroscopy. An elemental analysis sample was obtained by grinding
the resultant product and storing the compound under vacuum for one
week. Anal. Calcd for C36H40OP2Pt (745.77): C, 57.98; H, 5.41.
Found: C, 57.61; H, 5.35.
Preparation of [PtMe3(OTf)]4. The literature preparation21f was
modified as follows: [PtMe3I]4 (856.2 mg, 0.583 mmol) was dissolved
in THF (15 mL), to which AgOTf (Aldrich, 99%, 785.4 mg, 3.057
mmol) was added. Yellow precipitate (AgI) formed instantly, and after
five minutes of stirring, the suspension was gravity filtered four times
through glass wool. The solvent was removed under vacuum, and the
product was recrystallized first from THF/pentane, then from THF/
ether. The crystals obtained were washed with THF and ether, and dried
under vacuum overnight (3 crops, 876.8 mg, 97% yield). NMR data
matched those reported in the literature.21f
Preparation of Aryloxide Salts. Aryloxide salts of potassium (p-
KOC6H4X) were obtained by reaction of the appropriate phenol
(purchased from Aldrich and used as received) with KH (in excess,
1.5-3 equiv) overnight in THF. The solution was filtered through glass
wool to remove excess KH, and the potassium salts recrystallized from
THF/diethyl ether at -35 °C. The 18-crown-6-potassium aryloxides
were prepared by reaction of the potassium aryloxide with 18-crown-6
(Aldrich) and KH in THF overnight. After filtering the solutions to
remove the excess KH and evaporating them to dryness, the products
were recrystallized from THF/ether at -35 °C.
Preparation of fac-(dppbz)PtMe3(p-OC6H4X) (6b1-6). All of the
aryloxide complexes of dppbz (X ) Me (6b1), H (6b2), OMe (6b3), Cl
(6b4), CF3 (6b5), CN (6b6)) were prepared in an analogous fashion.
The preparation of the parent phenoxide compound is provided as
typical. [PtMe3(OTf)]4 (125 mg, 0.0807 mmol) was dissolved in THF
(10 mL). KOPh (43.0 mg, 0.325 mmol) was added and a colorless
crystalline material (KOTf) became evident within a few minutes. The
next day, the solution was gravity filtered through Celite on glass wool
to afford a colorless solution. Dppbz (147.6 mg, 0.324 mmol) was then
added, and the solution turned pale yellow. After ca. 24 h, more THF
was added (40 mL total), and the product solution was filtered through
glass wool. The solution was layered with pentane (100 mL) and cooled
to -35 °C. After 4 days, yellowish crystals were collected, washed
with pentane, and dried under vacuum (190.3 mg, 76% yield). These
crystals were recrystallized a second time from toluene/pentane before
use in kinetic studies, at which time white crystals were obtained. NMR
data for all aryloxide compounds are provided in Table 5. NMR signals
were assigned as indicated in Figure 5 on the basis of 2D correlational
spectroscopy. Yields: 6b1, 76%; 6b2, 70%; 6b3, 63%; 6b4, 56%; 6b5,
60%; 6b6, 70%. IR for 6b6: νCN ) 2204 cm-1. Anal. Calcd for C40H40-
OP2Pt‚0.5(C4H8O) (6b1‚0.5THF, 825.83): C, 61.05; H, 5.36. Found:
C, 61.09; H, 5.53. Anal. Calcd for C39H38OP2Pt (6b2, 779.76): C, 60.07;
H, 4.92. Found: C, 60.21; H, 4.92. Anal. Calcd for C39H37ClOP2Pt
(6b4, 814.20): C, 57.53; H, 4.58. Found: C, 57.85; H, 4.71. Anal.
7.26
n/a
7.14
7.3
7.2
n/a
7.31
7
7.2
n/a
7.43
7.2
7.39
7.5
J(H15) triplet
13C NMR
Mea
δ
-6.5
635
6.3
548
113
-6.3
635
7
555
116
5
-7
637
7.1
551
114
4
-6.5
645
6.1
546
115
5
-5.4
714
6.6
549
112
5
-4.9
649
6.4
544
115
1JPtC
Meb
δ
1JPtC
2JPC(trans)
2JPC(cis)
δ(C1)
δ(C2)
δ(C3)
δ(C4)
δ(X)
5
4
OAr
166.4
122.6
128.5
133.3
n/a
142.8
136.6
132.5
133.5
129.0
130.7
136.2
128.9
131.5
164.0 160.0 164.7 170.1
122.5 122.5 122.9 121.7
129.4 114.1 128.0 126.0
121.4 150.1 117.0 114.6c
171.3
122.6
133.7
95.5
20.6
55.5
n/a
n/a
121.8
142.6
136.9
133.2
133.8
129.3
131.3
135.9
129.3
131.8
backbone δ(C5)
Ph
143.2 143.3 142.4 142.7
135.9 136.6 136.1 136.7
132.3 132.4 133.1 133.0
133.2 133.3 133.1 133.7
129.1 128.7 128.5 129.2
131.0 130.6 130.5 131.2
136.4 136.0 135.5 135.9
129.1 128.7 128.5 129.2
131.4 131.0 130.9 131.6
δ(C6)
δ(C7)
Ph+
δ(C9)
δ(C10)
δ(C11)
δ(C13)
δ(C14)
δ(C15)
Ph-
b 3
2
a 19F NMR.
J
) 7.0 Hz. c q, JCF ) 32 Hz.
XH
3
Calcd for C40H37F3OP2Pt (6b5, 847.75): C, 56.67; H, 4.47. Found: C,
56.78; H, 4.40. Anal. Calcd for C40H37 NOP2Pt (6b6, 779.76): C, 59.70;
H, 4.63; N, 1.72. Found: C, 59.37; H, 4.78; N, 1.74.
Preparation of fac-(PMe3)2PtMe3(p-OC6H4Me) (6c). [PtMe3(OTf)]4
(126.5 mg, 0.0813 mmol) and potassium p-cresolate (49.2 mg, 0.336
mmol) were combined in THF (10 mL) and allowed to stir overnight.
The flask was attached to a stopcock adapter, and the volume was
reduced to 5 mL on the vacuum line. PMe3 (1.2 mmol) was added via
a known volume bulb (assuming ideal gas behavior, P ) 125 mmHg).
The solution was allowed to stir for 2 h, and then evaporated to dryness
and returned to the drybox. The product was taken up in 15 mL of
toluene and filtered through Celite on glass wool. This solution was
reduced under vacuum to 5 mL and layered with pentane (15 mL).
White, feathery crystals were obtained by cooling the layered solution
to -35 °C (148.0 mg, 2 crops, 91% yield). 31P{1H} NMR (THF-d8):
1
1
δ -35.3 (s w/Pt satellites, JPtP ) 1290). H NMR (THF-d8): δ 0.24
2
3
(t w/Pt satellites, 3H, JPtC ) 69.5, JPH ) 7.5, methyl group trans to
O), 0.73 (approximate dd w/Pt satellites, 6H, 2JPtH ) 55.0, 3JPH ) 7.5,
3JP′H ) 6.5, methyl groups cis to O), 1.41 (second-order A9XX′ pattern,
major coupling ) 9.0, 18H, P(CH3)3), 2.08 (s, 3H, aryl-CH3), 6.27 (d,
2H, 3JHH ) 8.5), aryl CH ortho to platinum), 6.61 (d, 3JHH ) 8.0, aryl
CH meta to platinum). Anal. Calcd for C16H34OP2Pt (499.47): C, 38.48;
H, 6.86. Found: C, 38.21; H, 6.82.
General Treatment of Kinetics Samples. In the drybox, solid
reagents (e.g., platinum complexes, [NBu4][OAc], PVP, etc.) and benzyl

Formation of C-O Bonds from Pt(IV) Complexes
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2587
collected in another NMR tube, which was then sealed. THF-d8 was
then transferred into the original NMR tube, which was then resealed.
This allowed clear observation of the products. The reaction conducted
in the absence of PMe3 was performed as a typical kinetics experiment
(see above). 31P NMR data for trans-(PMe3)2PtMe(OC6H4Me) (trans-
1
1
16c) (THF-d8): δ -8.9 (s w/Pt satellites, JPtP ) 2888). H NMR: δ
2
3
0.25 (t w/Pt satellites, Pt-CH3, JPtH ) 79.0, JPH ) 6.7), 1.28 (virtual
t w/Pt satellites, P(CH3)3, 2JPH ) 3.8, 3JPtH ) 29.5), 2.09 (s, methyl of
aryloxide), 6.66 (s, aryl protons all chemical shift coincident). 31P NMR
data for cis-(PMe3)2PtMe(OC6H4Me) (cis-16c) (THF-d8): δ -32.9
(s w/Pt satellites, 1JPtP ) 3697, P trans to OAr), -5.9 (s w/Pt satellites,
1JPtP ) 1699, P trans to Me). H NMR: δ 0.41 (dd w/Pt satellites,
1
2
3
3
Pt-CH3, JPtH ) 54.0, JPH ) 6.7, JP′H ) 4.0), 1.34 (d w/Pt satellites,
2
3
P(CH3)3 trans to C, JPH ) 9.0, JPtH ) 15.5), 1.60 (d w/Pt satellites,
2
3
P(CH3)3 trans to O, JPH ) 10.5, JPtH ) 40.5), 2.09 (s, methyl of
aryloxide), 6.53 (d, protons of aryloxide ortho to Pt, 3JHH ) 8.0), 6.64
3
(d, protons of aryloxide meta to Pt, JHH ) 8.5).
Reaction of fac-(dppe)PtMe3(O2CCF3) (4a) with KOH. In the
drybox, a round-bottomed flask was charged with 4a (25.1 mg, 0.0334
mmol), KOH (59.5 mg, 1.06 mmol), and THF (5 mL). An aliquot was
placed in an NMR tube and the reaction was followed by 31P NMR. 8
was produced as the primary initial product, with 9 and 10 gaining
intensity as the reaction progressed, eventually reaching the 2:1:1
statistical distribution. After allowing the reaction to stir for 1 day, the
solvent was removed from the bulk reaction under vacuum, and the
solids were dissolved in C6D6. The products could be unambiguously
assigned as a statistical (2:1:1) mixture of cis-(P(CHCH2)Ph2)(PMePh2)-
PtMe2 (8), cis-(P(CHCH2)Ph2)2PtMe2 (9), and cis-(PMePh2)2PtMe2 (10)
Figure 5. Atom labeling scheme for 6b1-6
.
ether (an internal standard) were loaded into a medium-walled NMR
tube (Wilmad 504) that had been sealed onto a 14/20 joint. The tube
was affixed to a stopcock adaptor and evacuated under high vacuum.
Solvent was then vacuum transferred into the NMR tube, and the tube
was sealed under active vacuum. Nitrobenzene-d5 was transferred at
elevated temperature due to low volatility.
on the basis of a H/1H COSY experiment (200 MHz), H {31P}, 31P-
{1H}, and 1H experiments at both 200 and 500 MHz, and a long-range
(optimized for 7 Hz coupling) 1H/31P correlation experiment (200 MHz).
As further confirmation, dppe was added in large excess to the reaction
products, and both 1H and 31P NMR spectra matched those of authentic
samples of (dppe)PtMe2 (2a), P(CHCH2)Ph2, and PMePh2 in a 1:1:1
mixture.
Aryloxide Exchange Experiment. Both fac-(dppbz)PtMe3(p-OC6H4-
Me) (6b1) (9.4 mg, 0.012 mmol) and 18-crown-6-potassium phenoxide
(1.0 mg, 0.0025 mmol) were weighed into an NMR tube. On a vacuum
line, THF-d8 (420µL) was added by vacuum transfer, and the tube was
sealed. The reaction was heated at 79 °C for 20 h and monitored by
1H and 31P NMR.
1
1
NMR tubes were heated in a Neslab Exacal EX-250 HT elevated
temperature bath, and the reaction was quenched by rapidly cooling
the tube in water (25 °C) or ice water (0 °C). Kinetics tubes were stored
in liquid nitrogen when not being actively monitored. All kinetics were
performed on DPX200 or DRX499 spectrometers, averaging the
integrals of three separate acquisitions for each point. Reactions were
followed by the disappearance of the axial methyl group of the Pt(IV)
fac-L2PtMe3X complex relative to benzyl ether (T1 ) 6 s, pulses spaced
30 s apart), bis-trimethylsilyl ether (T1 ) 8 s, pulses spaced 40 s apart),
or ferrocene (T1 ) 50 s, pulses spaced 300 s apart) as an internal
standard. In all cases, the kinetics samples were followed for 3 half-
lives (ca. 10 points) and a rate constant was obtained by a least-squares
linear regression method. Kinetics tubes were subsequently heated to
completion (>7t1/2) and mass balance was confirmed by integration of
products against the internal standard. In cases in which PVP was used,
the 1H NMR spectrum showed no signs of PVP decomposition or
presence in solution.
Thermolysis of fac-(PMe3)PtMe3(p-OC6H4Me) (6c) in the Pres-
ence of PMe3. A sealable NMR tube was charged with fac-(PMe3)2-
PtMe3(p-OC6H4Me) (5.4 mg, 0.011 mol), THF-d8 (435 µL), and a drop
of benzyl ether as an internal standard. PMe3 (0.022 mmol assuming
ideal gas behavior, P ) 24.0 mmHg) was added by vacuum transfer
using a known volume bulb. The disappearance of the proton signals
for the aryloxide protons ortho to platinum in 6c was monitored by
NMR. However, observation of the platinum-containing products was
complicated by the fact that PMe3 exchanges with some of the Pt(II)
products, causing line broadening. After the kinetics run, the NMR
tube was heated for a further 40 days (70 days (7t1/2) total) at 120 °C.
The NMR tube was then cracked on the vacuum line and the volatiles
Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
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 thank J. Roth and A. W. Holland for preliminary
work on this project and Prof. J. Mayer for helpful discussions.
The X-ray structure determination was performed by Dr. D.
Barnhart, S. Lovell, and Dr. W. Kaminsky (UW).
Supporting Information Available: Tables of atom coor-
dinates, thermal parameters, hydrogen atom parameters, bond
distances, and angles for complex 1a‚H2O (9 pages, print/PDF).
This information is available free of charge via the Internet at
http://pubs.acs.org
JA003366K